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Amaril
2017-04-07, 09:44 PM
(Might be better off in Mad Science and Grumpy Technology, but I'm asking for fictional purposes rather than real ones, so I figured here would be best.)

Let's say you have a city-sized space station with people living on it, which simulates gravity through rotation; think of the Citadel in Mass Effect, for example. The station has to support a spaceport for incoming and outgoing traffic. Where would be the best place for that spaceport to be? I'd assume you'd want it to be in a part of the station that doesn't rotate, to save ships the trouble of going in and out of gravity, but that raises the question, where on such a station does one begin to experience the simulated gravity? As a related issue, how would a station like this sustain atmosphere? Would the entire thing have to be sealed, or would the rotation allow an atmosphere to stay in place naturally?

Darth Ultron
2017-04-07, 09:57 PM
The easy answer is in the middle. The middle of your space station has no or little gravity as it does not spin much. So ships would match rotation with the space station to spin as it is, fly to the middle, and then land.

The Babylon 5 Space Station, from the TV show Babylon 5 is a great example.

You can't really avoid ships going ''in and out of gravity''.....unless you park the ships near the space station and shuttle things over and between them.

The station would need to be sealed for a whole bunch of reasons, but mostly to keep in the air, heat and pressure. And the all important shield from radiation and other harmful energy.

CharonsHelper
2017-04-07, 09:58 PM
If there is no artificial gravity (which is inherent to needing space stations to spin), the ship would want to take advantage of the station's spinning and dock on the outside of the station.

Trekkin
2017-04-08, 12:22 AM
We had a thread like this on Mad Science and Grumpy Technology a while back, at least about the docking thing.

There are three basic options for docking a ship to a rotating space station of this size, which you might see referred to as an O'Neill Cylinder.
Option A is an axial docking port at one end of the cylinder, where the angular speed of the docking hardware is lowest. Your incoming ships need to spin up to match the station's speed to be able to interface with it, but that's simply a matter for the reaction control system.
Option B is to have a nonrotating section of the station, which might surround the rotating section or might be attached to the axis. This is easiest on ships, but will drain the station's angular momentum fastest, so the station will need more intense spin maintenance.
Option C, which I like, is to build a track around the circumference of the station and put your docking hardware on a sort of train, one capable of reaching the station's angular velocity (but in reverse) and thus allowing you to put a stationary docking port anywhere on the periphery -- but then shut it down when there's no incoming ships and avoid frictional losses of spin. This has the added advantage of being able to fling ships into higher and lower orbits by moving the train faster or slower.

On such a station, one begins to feel the pseudogravity as soon as one enters the rotating section off-axis; the rotation induces pseudogravity via centrifugal* force, so it's zero exactly at the axis of spin and gets stronger as you move toward the rim. Specifically, gravity at a distance r from the center is equal to r(2pi/T)^2, where T is the rotational period of the station.

The question of atmosphere is a tricky one. Small stations need to be sealed, yes; about the minimum size where it's practical to just build a retaining wall is 2000 km across, I believe. It's called a Bishop Ring. Of course, there will still be negligible atmospheric losses, but these could easily be as negligible as those experienced by a sealed habitat of the same surface area.

*Yes, centrifugal force, which everyone loves to parrot does not exist; I'm working from the rotating reference frame of a station occupant here, so it does exist, albeit as an inertial force.

Beneath
2017-04-08, 12:49 AM
Most likely in the middle, not outside. The latter makes launching easy since you can just fly off tangent, at the expense of making landing more difficult than it needs to be.

The dock at the middle can be available all the time instead of you needing to hit your exact docking berth at the exact right (probably more precise than) second or else at best need to make another approach, expending an enormous amount of delta-V to do so, or at worst collide with another docked ship.

Also, docking in the middle means docked ships will cause the least change to the station's center of mass, which is good because changing the center of mass of a spinning object is going to mess with its rotation (moving people or even things up to the size of like, trucks around in a station the size of a city probably won't have too much effect. a large spaceship that is one, larger than anything in the city and two, further from the center than anything in the city might). You can kinda correct by moving counterweights around but that's added mass on the station.

You can work out the rim velocity of your station for a given radius and gravity strength pretty simply; the amount of centripetal acceleration you need to stay moving in a circle (which is therefore the amount of centrifugal force you feel standing on the rim) is equal to the square of the rim velocity divided by the radius (a = v^2/r); the equation is unit-independent as long as you're consistent (you can use a in meters per second per second, v in meters per second, and r in meters, or do it in feet provided you have feet in all three terms instead of meters, or in furlongs per fortnight as long as a is in furlongs per fortnight per fortnight). v/r is the rotational speed in radians/second (or whatever time unit you have in velocity); divide that by tau (roughly 6.28) and that'll give you the rotational speed in hertz (if you're using seconds).

So for a worked example, let's say our city has a radius of one kilometer and a gravity of 10 m/s (earth, to one significant figure). 10 m/s^2 = v^2/1000 m, so v^2 = 10,000 (m/s)^2; square root is 100 m/s, angular speed is 0.1 radian/s or 0.02 (to one sig fig) revolutions/sec. With these numbers it takes exactly 10*tau seconds to complete a revolution, which is about a minute.

A ship meaning to dock with the center would merely have to match course with the station (plus a small difference to approach) and apply a one revolution/minute rotation to itself to dock. Any accidental collisions that happen will be at the small approach speed, which at least parts of the station's docking apparatus are designed to soak already (since you have to bring the ship to a stop relative to the station once it's in the dock, and you can't use a rocket engine that close to sensitive equipment).

A ship trying to dock with the rim would have to fly toward the station at a relative velocity of 100 m/s (360 km/h or 200 mph; fairly slow by space standards but very high for possible vehicular collisions with populated areas) and hit its docking berth exactly both in terms of time (the docking berth is also moving at 100 m/s; a 50 meter freighter that is half a second off has missed its entire berth) and angle (whatever the length of whatever arresting equipment they have to attach your ship to the station is, you have to hit it from many kilometers away on an exact course. Too close to the station and you've rammed the bottom of a populated city. Think an airliner the size of a container ship crashing into a bridge at rush hour. Very bad. Too far away and you've failed to dock and have to expend 200 m/s of delta-V to make another pass). Not impossible, but closer to a moving aircraft carrier than a ground-based runway.

For an alternate station to get a feel for the math, raising the radius to 10 km, you get v^2 = 100,000 (m/s)^2 so v = 300 m/s. The angular velocity is smaller in this case (300/10,000 = 0.03 radians/second = 0.005 revolutions/second, roughly 200 seconds to complete a revolution). Relative to its size it rotates slower, but you need to ram your ship against it at higher relative velocity, with a smaller window to hit your docking berth, and a higher delta-V cost to make an additional attempt.

I haven't seen a worked, rendered design that docks on the outside, probably for this reason. The station in 2001: A Space Odyssey docked in the middle, the Citadel docks in the middle (kinda, and it has magic gravity machines). The closest is Ringworld, which wasn't designed to dock spaceships very often IIRC and either lands them on the inside surface or on the walls (it contains its atmosphere by using high walls and centrifugal force, allowing pressure to drop off the way it does on planetary atmospheres, rather than having a pressurized upper roof), and the retrieval mechanism for the ships landing on it wasn't, to my knowledge, fully explained (though I imagine a ship could fly parallel to the high walls for quite some distance 'cause of how huge it is. or maybe they always landed on the inside surface using aerobraking? it's been an age since I've read any Niven).

Docks beneath the station provide a useful place from which to deploy craft that need to be deployed rapidly and do not need to be retrieved, such as missiles, or for which it's ok if retrieval is a huge pain in the neck, but are inconvenient for landing day-to-day cargo craft. As an aside, lowering craft from a dock in the center of the station to the underside to give them an extra launch kick just transfers angular momentum from the station. possibly linear momentum too if you don't launch counterweighted pairs. It can be economical if it's much cheaper to put momentum on your station than on the spacecraft, but otherwise don't, just catapult-launch them from the center. Otherwise, it's a basic engineering principle that when you can get rid of moving parts without impacting performance, you should. The moving part is the part most likely to cause problems.

----

In response to your second question (about when you'd begin to experience the simulated gravity) that's much simpler. There are two possibilites. Either the entire station rotates, or the spaceport in the center doesn't rotate and is on some kind of bearing system and the outside rolls around it. Getting the frictional losses in this system down low enough to be economical is considered essentially impossible, and also moving parts wear out faster than non-moving parts do. Then you'd need an elevator system where your elevator car can be transferred between moving and non-moving sections, essentially an elevator that can transfer between a building and a moving train. Possibly, given the requirements for low friction and high velocities, a maglev bullet train. Sure, an elevator that can transfer you onto a maglev train running at full speed sounds awesome, but the accountant who had to find money in your budget to actually build it hates you for it, and, when it breaks, which will be often, so will everyone else even if the breakage just means an out-of-order sign and not mass death (hope the accountant found a budget for safety features!), when they could have just made ships match rotation with the station to dock and used a conventional elevator.

If you do have such a system, you'd start feeling gravity when the elevator makes a loud "clunk" and transfers to the rotating part of the station.

If you do not, you'd start feeling gravity gradually. The above-calculated angular speed is constant throughout the station (or any rigid body). Putting the acceleration equation (a=v^2/r) in terms of angular speed (w=v/r) gives a = w^2*r. In this case a is the acceleration you feel, r is your distance from the center, and w is the angular speed the station's rotating at, which is constant. So at the center you experience no gravity, at the rim you experience the full gravity the station is designed for, a tenth of the way down from the center you experience 1/10 gravity, halfway down you experience half gravity, and so on.

If you do have one of the clunky moving-part systems above, that relation holds as long as you're on a rotating part of the station. In a non-rotating part of the station gravity would be zero regardless of distance from the center since you're not rotating.

For sustaining atmosphere, you'd probably have to seal it. Ringworld got away with having extremely high walls (we're talking hundreds of kilometers. Ringworld was a full astronomical unit in radius, for comparison) so that the atmosphere at the top of the walls was very low pressure. With Earth, losing gas to the solar medium is a matter of energy (a certain amount of kinetic energy is needed to escape the earth's gravity. statistical mechanics guarantees that some fraction of molecules in a gas, which can be fairly significant when you get hot enough, will have this much energy, but is still going to be small. Also I imagine there's some capture from the solar system medium and there's definitely chemical exchange of gas between the atmosphere and the surface; if I had to speculate I'd say the earth's atmosphere is an equilibrium between all of these factors rather than simply something held on by gravity); with a walled ring, it's a matter of position; going high enough to get the pressure low enough to be comparable to the medium is impractical even for a ringworld (which is already an exercise in impracticality where it isn't physically impossible) and if there is a pressure difference there will be bulk fluid flow, which is way faster than the statistical mechanics effects that dominate atmosphere loss from Earth.

Trekkin
2017-04-08, 01:16 AM
I haven't seen a worked, rendered design that docks on the outside, probably for this reason. The station in 2001: A Space Odyssey docked in the middle, the Citadel docks in the middle (kinda, and it has magic gravity machines). The closest is Ringworld, which wasn't designed to dock spaceships very often IIRC and either lands them on the inside surface or on the walls (it contains its atmosphere by using high walls and centrifugal force, allowing pressure to drop off the way it does on planetary atmospheres, rather than having a pressurized upper roof), and the retrieval mechanism for the ships landing on it wasn't, to my knowledge, fully explained (though I imagine a ship could fly parallel to the high walls for quite some distance 'cause of how huge it is. or maybe they always landed on the inside surface using aerobraking? it's been an age since I've read any Niven).

Yeah, the closest to stationary docking ports on a rotating station I've seen proposed has been for transit shuttles between pairs of contrarotating O'Neill cylinders, and those had guide cables.

Also, regarding momentum exchange shenanigans: depending on where you put your station, it might be possible to economize on fuel for re-spinning and reaction control somewhat through electromagnetic tethers, since the station itself is big enough to stabilize the tethers. Alternatively, spin-stabilized solar sails can be an option if the local magnetosphere is weak/nonexistent.

Really, the main advantage of docking to the rim rather than axially is throughput; you can have a maximum of two docking complexes if you insist that they be axial, and it may be the case that the station needs to handle more traffic than that. At minimum you would probably want your escape ships (presuming you're orbiting something habitable) to be around the rim.

EDIT: come to think of it, would the loss of energy from joining rotating and nonrotating components be that significant? It might be easiest to just sheath the rotating pressurized component in a tube of meteor shielding and zero-g facilities and just run mass-balanced pairs of shuttles between them. Spinning up the shielding might be more expensive over the expected lifetime of the station than just running inductors around the spinning and nonspinning sections and levitating the one into the middle of the other.

Berenger
2017-04-08, 04:18 AM
I haven't seen a worked, rendered design that docks on the outside, probably for this reason.

Homeworld 2 has a vaygr space station that has the transporters docking on the outside of the station.

Yeah, it looks like a giant USB hub.

Edit: On a second thought, stuff in Homeworld doesn't rotate, so either they have another source of artifical gravity or just have none.

https://s-media-cache-ak0.pinimg.com/originals/39/9d/ec/399dece9bd835310b52331387de898c2.jpg

Mastikator
2017-04-08, 05:35 AM
Once you get to a certain level of scale it becomes easier to make your own resources than to import it. Which means you need a HUGE energy source.

The station would either be close to a star and collect a ginormous amount of its energy or have its own fusion reactor, in which case it needs a steady supply of fusible material. If that's the case then it should orbit closely to a gas giant and skim hydrogen.

It would not just be out in interstellar space. Interstellar space is empty and dead.

Most of the space station area should be used for hydroponics, btw, since it takes 20 vegetable calories to make 1 animal product calorie they're not making animal products. Every single person living on that station is a vegan, and there are no pets. Nobody who contributes nothing is allowed to be there. There is likely more work needed than there is manpower so everybody is probably working overtime and unemployment is illegal. On the Earth we're supported by a massive ecosystem, in space humans will have to pick up all of that slack, even with the help of technology it will be very difficult.

CharonsHelper
2017-04-08, 09:28 AM
Once you get to a certain level of scale it becomes easier to make your own resources than to import it. Which means you need a HUGE energy source.

The station would either be close to a star and collect a ginormous amount of its energy or have its own fusion reactor, in which case it needs a steady supply of fusible material. If that's the case then it should orbit closely to a gas giant and skim hydrogen.

It would not just be out in interstellar space. Interstellar space is empty and dead.

Most of the space station area should be used for hydroponics, btw, since it takes 20 vegetable calories to make 1 animal product calorie they're not making animal products. Every single person living on that station is a vegan, and there are no pets. Nobody who contributes nothing is allowed to be there. There is likely more work needed than there is manpower so everybody is probably working overtime and unemployment is illegal. On the Earth we're supported by a massive ecosystem, in space humans will have to pick up all of that slack, even with the help of technology it will be very difficult.

A lot of that is assuming that tech is such that space travel is pretty slow and the station needs to be self sufficient. Heck - modern cities aren't self sufficient. There are entire states which aren't self sufficient. So long as a station is within a day or two of a planet I don't see that as being necessary.

Anonymouswizard
2017-04-08, 10:14 AM
(Not focusing on the original questions because I think they've been answered satisfactorily)

The logistics depend on everything from how big the station is, how big spaceships are, how spaceships are propelled, how populated the station is, how populated nearby planets are, automation on the station, the station's purpose, and so on.

For one, automation, this will vastly change how much work people have to do. If most repairs can be handed off to bots, as well as possibly a decent amount of farming and medical work, I can see a station with a decent population not working significantly longer than those on a planet. I suspect working overtime might be more likely as problems arise and tasks get pushed back but nobody wants to leave them to avoid disaster, but it'll depend.

Okay, size of the station is obvious, and how big spaceships are is basically after spaceships hit a certain size they aren't docking with you but rather transporting people and cargo by shuttle. This essentially puts an upper limit on the amount of stuff you can feasibly import, but also on the amount of stuff you need.

The station's purpose will however massively affect what they need to import. A residential station might be self sufficient in most things, but a science station not only probably isn't growing it's own food but also needs raw materials from planets to conduct experiments.

For how spaceships are propelled, essentially how easy it is to get things off of a planet will affect how you meet the demand. If getting stuff into orbit is time consuming or takes a lot of fuel you're not going to be dealing with that as much as possible, I see some stations getting an initial shipment of food, seeds, and fertiliser just so they can produce food for later stations, as well as occasionally scavenging of unused satellites. Of course they'd still have to get some stuff from planets, but I can see a station that's able to produce enough vegetables to feed others that food doesn't have to be shipped in and of itself.

Now, for the physical location I'd argue that unless you're certain in your radiation shielding that you should probably go for the gas giant over sitting near the sun. Less harmful radiation to irradiate you. You'd likely want radiators so you wouldn't save a ton on the 'fixing stuff sticking out' front, and should probably pack a fission generator and some radioactive material just in case (or a spare fusion reactor, if you've got enough drones for skimming that you're unlikely to run out before getting new ones), but I think the need to skim for hydrogen/helium is outweighed by the entire 'less radiation' thing.

Reboot
2017-04-08, 11:52 AM
Really, the main advantage of docking to the rim rather than axially is throughput; you can have a maximum of two docking complexes if you insist that they be axial, and it may be the case that the station needs to handle more traffic than that. At minimum you would probably want your escape ships (presuming you're orbiting something habitable) to be around the rim.

Even if you're not orbiting something habitable (say, an interplanetary ring ship rather than a ring station), you probably want your lifeboats to be on the rim. If you set it up right, it gives them additional launch speed - preferably in the direction most likely to have help, if enough attitude control remains to allow that - and if you genuinely need to evacuate, there's probably a pressing desire to get away from the mothership.

Of course, redocking them would be somewhere between difficult and impossible, but the likelihood that lifeboats would be able to go any significant distance under their own power is pretty low - some ship-to-ship shuttlecraft might be repurposed for escape, but lifeboats would probably be 99% life support. Barely more than a distress beacon, some sort of hibernation system to elongate lifespan and the absolute bare necessities beyond that.

sktarq
2017-04-08, 01:06 PM
Okay, The station/space city need rotaional gravity simulation. The space port does not.

So keep the separate as possible. - Sure you need to get back and forth but what if you just keep that as minimal as you can.

So you have spoke wheel with an axial stick as a metaphor here. The wheel rim holds the city, the spokes hold the hold thing together and deal with the near 0g to near 1g region that is otherwise just awkward to deal with. Having as small a hub as possible is important for friction issues. Then a long axial 0g zone for the spaceport itself. Which would make speed matching, loading issues, etc all much easier. Sure people couldn't live there full time but that is what the rim city is for.

In the above scenario gravity would get stronger as you move down/out (as "gravity" is in the out direction is it also "down").

Trekkin
2017-04-08, 01:15 PM
You need significant radiation shielding near gas giants too; Jupiter's radiation belts are among the most irradiated volumes in the solar system.
Of course, it's also feasible to just set up a cycler to harvest fusible gases from distant locations and return them to a station in some less hazardous place. This takes time (which only really affects the responsiveness of the system to changes in fuel demand), but need not require exorbitant amounts of delta-v.

Regarding lifeboats: if the station isn't orbiting a habitable body, don't bother. Surviving for any length of time in space demands radiation shielding, micrometeor shielding, thermal control, atmosphere scrubbing -- really, a lifeboat capable of supporting the entire population of a ship for long enough to be dramatically interesting is going to be that ship in its own right, less the propulsive bus. At that point it becomes easier to have redundant life-support systems on board and eject whatever is going to make the ship uninhabitable; even the most dramatically improbable reactor explosion is much more survivable from thousands of kilometers away, more so when it happens on the other side of the rad shielding. Keeping everyone in one place also simplifies rescue efforts.
Orbiting habitable planets changes the mathematics somewhat, since you have infinite life support within minimal delta-v (assuming aerobraking) at all times; it's no longer a matter of surviving until help arrives, but rather of surviving the fixed length of time before arriving planetside.

As far as the logistical demands of space: if you want to posit a space station that is a Malthusian Darwinian dystopia where everyone in your chronically overworked populace must continually justify their right to keep breathing and everyone you don't think is worth keeping is chucked out the airlock and there are neither hamburgers nor kittens anywhere within a hundred kilometers, that's your story to tell, but it's not necessitated by the realities of living in space at this scale.
At least for the inner solar system, sunlight is plentiful and plants don't seem to mind growing in microgravity, so there's no need to take up habitable 1g surface area with hydroponics bays when microgravity cluster habitats will manage perfectly well. Further, you'd want a signficant excess of oxygen-producing capacity anyway in case of disaster, and I see no reason not to let pets take some of the load off whatever's producing enough CO2 to keep that excess going. Making animal products at any significant scale is probably superfluous, though; you'd need at the very least enough capacity to meet the scientific and medical need for FBS and so forth, but that can all be done in vitro. As for overwork...robots are so much better suited to working in space than we are, and the jobs we must still do are so dependent on our having a clear head, that it would seem uniquely counterproductive to me to overwork people.

Anonymouswizard
2017-04-08, 01:51 PM
You need significant radiation shielding near gas giants too; Jupiter's radiation belts are among the most irradiated volumes in the solar system.
Of course, it's also feasible to just set up a cycler to harvest fusible gases from distant locations and return them to a station in some less hazardous place. This takes time (which only really affects the responsiveness of the system to changes in fuel demand), but need not require exorbitant amounts of delta-v.

Huh, that's interesting. I should read up on that, most of my (limited) knowledge of radiation in the solar system relates to Earth Orbit because I ideally want to apply my degree to working on satellites if I can't manage to get into artificial intelligence.


Regarding lifeboats: if the station isn't orbiting a habitable body, don't bother. Surviving for any length of time in space demands radiation shielding, micrometeor shielding, thermal control, atmosphere scrubbing -- really, a lifeboat capable of supporting the entire population of a ship for long enough to be dramatically interesting is going to be that ship in its own right, less the propulsive bus. At that point it becomes easier to have redundant life-support systems on board and eject whatever is going to make the ship uninhabitable; even the most dramatically improbable reactor explosion is much more survivable from thousands of kilometers away, more so when it happens on the other side of the rad shielding. Keeping everyone in one place also simplifies rescue efforts.
Orbiting habitable planets changes the mathematics somewhat, since you have infinite life support within minimal delta-v (assuming aerobraking) at all times; it's no longer a matter of surviving until help arrives, but rather of surviving the fixed length of time before arriving planetside.

I think the one time I've seen science fiction lifeboats be near reasonable was in Lensman, where they carried life support for a couple of days and could get to another solar system within a few hours if you knew where to go. Even then it didn't help for most of the people who used them.


As far as the logistical demands of space: if you want to posit a space station that is a Malthusian Darwinian dystopia where everyone in your chronically overworked populace must continually justify their right to keep breathing and everyone you don't think is worth keeping is chucked out the airlock and there are neither hamburgers nor kittens anywhere within a hundred kilometers, that's your story to tell, but it's not necessitated by the realities of living in space at this scale.
At least for the inner solar system, sunlight is plentiful and plants don't seem to mind growing in microgravity, so there's no need to take up habitable 1g surface area with hydroponics bays when microgravity cluster habitats will manage perfectly well. Further, you'd want a signficant excess of oxygen-producing capacity anyway in case of disaster, and I see no reason not to let pets take some of the load off whatever's producing enough CO2 to keep that excess going. Making animal products at any significant scale is probably superfluous, though; you'd need at the very least enough capacity to meet the scientific and medical need for FBS and so forth, but that can all be done in vitro. As for overwork...robots are so much better suited to working in space than we are, and the jobs we must still do are so dependent on our having a clear head, that it would seem uniquely counterproductive to me to overwork people.

I think this is key, especially the robots. I suspect that there will be a decent amount of work fixing the robots and watching over them in case something goes wrong, but unless we have the space luddites I suspect most colonists won't end up with significantly more work.

I do suspect that if there's a danger of decompression or some other disaster there will be people pulling overtime to sort this out now, but that's not all day every day.

Gildedragon
2017-04-08, 07:37 PM
Rotating a big enough metal mass is good against radiation

Trekkin
2017-04-09, 12:20 AM
Rotating a big enough metal mass is good against radiation

This is true for electromagnetic radiation only. You will also need shielding against particles, for which metal is entirely unsuitable; bremsstrahlung kills just as readily as cosmic X-rays, only now it starts from inside the shield.

Water ice is a workable particle shield, though; given annual outside radiation of 740 mSv and a desire to bring that down to 3 mSv/yr (so you can have the talking heads say "it's actually less radiation than on Earth's surface" over and over until scientifically illiterate people stop running in terror from their bananas), you'd need something like 22 cm of titanium under 165 cm of ice. Depending on how big you want your space station, that is going to be very heavy.

Mastikator
2017-04-09, 12:27 PM
A lot of that is assuming that tech is such that space travel is pretty slow and the station needs to be self sufficient. Heck - modern cities aren't self sufficient. There are entire states which aren't self sufficient. So long as a station is within a day or two of a planet I don't see that as being necessary.

Cities are surrounded by infrastructure and a massive complex ecosystem to sustain them. Space aint got none of that. So if you want to keep a space station alive you need to feed it with LOTS of resources and energy.

And what I'm saying is that once you get to a certain big enough scale it's easier to manufacture the resources than it is to transport them. OP asked about structure and logistics so I think it's safe to assume it's based on more hard sci-fi than just some wishy washy star wars space magic.

CharonsHelper
2017-04-09, 12:30 PM
And what I'm saying is that once you get to a certain big enough scale it's easier to manufacture the resources than it is to transport them.

That depends upon the cost of transporting them.

Telok
2017-04-09, 01:07 PM
That depends upon the cost of transporting them.

It also depends on what the cost it. Once in space transport is fuel cheap if you take a long time. It's time cheap only if you burn massive amounts of fuel. It can be materials cheap if you just need to push rocks and ice around without enclosing or preprocessing.

Trekkin
2017-04-09, 01:25 PM
And what I'm saying is that once you get to a certain big enough scale it's easier to manufacture the resources than it is to transport them. OP asked about structure and logistics so I think it's safe to assume it's based on more hard sci-fi than just some wishy washy star wars space magic.

That depends on what you call a resource, doesn't it? I mean, hydrogen is a resource (if nothing else, you can make water with it) but it's unlikely to be easier to produce the necessary particles and manufacture hydrogen ex nihilo rather than simply collecting it.

Transport costs in space, particularly in vacuum (as opposed to, say, the near-vacuum of the Jovian plasma torus), are governed far more by mass than by volume. Depending on what you need, it can certainly be more economical to, say, ship metal around rather than ore and save the fuel you'd otherwise spend accelerating the future dross around -- no wishy washy space magic required, just the Tsiolkovsky rocket equation.

Then, too, once you've committed to smelting in situ, you may as well form it into useful shapes while you've got it molten; even if they're bulky, it doesn't matter until it starts negatively impacting the transit ship's rotational inertia.

Jay R
2017-04-09, 02:30 PM
If the ship takes of from and lands on planets, then it already knows how to match speeds to land on a spinning object.

Mastikator
2017-04-09, 02:52 PM
That depends on what you call a resource, doesn't it? I mean, hydrogen is a resource (if nothing else, you can make water with it) but it's unlikely to be easier to produce the necessary particles and manufacture hydrogen ex nihilo rather than simply collecting it.

Transport costs in space, particularly in vacuum (as opposed to, say, the near-vacuum of the Jovian plasma torus), are governed far more by mass than by volume. Depending on what you need, it can certainly be more economical to, say, ship metal around rather than ore and save the fuel you'd otherwise spend accelerating the future dross around -- no wishy washy space magic required, just the Tsiolkovsky rocket equation.

Then, too, once you've committed to smelting in situ, you may as well form it into useful shapes while you've got it molten; even if they're bulky, it doesn't matter until it starts negatively impacting the transit ship's rotational inertia.

Life support. Food, water, oxygen. Components for the station. You know, stuff humans need to merely live that are not found anywhere in space.

hamishspence
2017-04-09, 03:07 PM
Water and oxygen can be gotten from icy asteroids, at least.

If your spaceship is basically a tunnelled-out asteroid, a thick layer of rock/ice may provide enough radiation shielding.

Trekkin
2017-04-09, 03:35 PM
Life support. Food, water, oxygen. Components for the station. You know, stuff humans need to merely live that are not found anywhere in space.

That's what I mean, though. I agree with you that water is vital to human survival in space, but it can't be manufactured in any real sense. Yes, you can make it out of hydrogen and oxygen, but both of those are harder to transport than water since they need cryogenic pressure tanks (and hydrogen can leak through the tank walls). So long as the inhabitants need water -- and they will, always, because of inefficiencies in recycling if nothing else -- there must be a way to ship water in, ideally with a minimal delta-v cost.

The same infrastructure in place to guide in the water supply ships can also guide in supplies of metal and other materials.

My larger point, though, is that the physics of space travel make a distributed manufacturing network economical for a certain level of mass loss (or savings in ullage and container weight) between stages in production. So yes, it may be cheaper to make new hull plates in space, but the station itself is still going to receive shipments of hull plates -- they're just coming from refineries in the asteroid belt, for example, rather than Earth-to-orbit cargo rockets, because shipping raw ore with that much delta-v is less economical than refining it on site. Cramming a space station full of factories and demanding that everyone be overworked to the point of exhaustion is neither logistically necessary nor maximally efficient; rather, it makes sense to put automated factories near the most massive raw materials and remote-monitor them from the space station and stratify the manufacturing network like that.

Apart from improvements in propellant expenditure per unit product, this also allows for savings in radiator area (since one can let your forges run as hot as the forge components can take without cooking the operators) and energy (since one can use solar for your low-energy steps).

Anonymouswizard
2017-04-09, 03:48 PM
That depends upon the cost of transporting them.

This is an important point, if you can get it without going out of a gravity well then you probably don't want to be shipping from a planet (unless you have a way to get off then with little energy).

This can even apply when shipping across systems, I have a setting where all Interstellar trade is basically controlled by groups of AIs who load up a ship with no FTL and simply drive to another system at 0.9c (the setting including specific engine that accelerate to it almost instantly for relatively little energy), and then trade it at the other end for more goods, fuel, and computer parts. It can occasionally be easier to resupply every few years as various spots come in.

CharonsHelper
2017-04-09, 04:13 PM
This is an important point, if you can get it without going out of a gravity well then you probably don't want to be shipping from a planet (unless you have a way to get off then with little energy).

If it still takes significant energy to fly in & out of a gravity well, I expect for planets much involved in space travel to have space elevators.

Anonymouswizard
2017-04-09, 04:35 PM
If it still takes significant energy to fly in & out of a gravity well, I expect for planets much involved in space travel to have space elevators.

If we can build them in this setting, that's not actually certain (higher tech might mean we have the required materials, but it might not).

Trekkin
2017-04-09, 04:55 PM
If we can build them in this setting, that's not actually certain (higher tech might mean we have the required materials, but it might not).

In which case a Lofstrom loop or space fountain could significantly reduce the strength requirement; they're only big and complicated, much like the habitat they're notionally intended to support.

Anonymouswizard
2017-04-09, 05:56 PM
In which case a Lofstrom loop or space fountain could significantly reduce the strength requirement; they're only big and complicated, much like the habitat they're notionally intended to support.

True, there's solutions, and really I think a massive problem here is that we're all assuming slightly different tech levels and baselines, I agree that there's almost certainly a way to make it work with shipping stuff in from a planet, I personally just think it would likely be less resource intensive to gather/fire as many resources as possible in space (although once it's there are it's built depends on the item and what tools the place has).

Tvtyrant
2017-04-09, 06:03 PM
Stupid question on my part, but shouldn't a ship be able to just fly at the same velocity as the ship is rotating and dock on the outside? They have the same velocity at the time of latching, and then the ship can simply power down its engine and use the wheel/station to keep at that momentum.

Amaril
2017-04-09, 06:18 PM
Thanks for the answers so far, folks. I feel like the conversation passed the point of "no one can clearly agree on a realistic way to handle this, so just do whatever you want from here" a while ago, so with the answer I needed in hand, I'll leave you all to your debate :smallsmile:

Mastikator
2017-04-10, 12:59 AM
Stupid question on my part, but shouldn't a ship be able to just fly at the same velocity as the ship is rotating and dock on the outside? They have the same velocity at the time of latching, and then the ship can simply power down its engine and use the wheel/station to keep at that momentum.

If the space station is sufficiently big then yeah. Ultimately I think "where to dock" is one of the least difficult to solve logistics problems.

Kane0
2017-04-10, 02:24 AM
Maybe this (http://www.sarna.net/wiki/Argo_(DropShip_class))would provide some insight? It's not world-class science, but the devs clearly put some thought into it.

Storm_Of_Snow
2017-04-10, 02:52 AM
Stupid question on my part, but shouldn't a ship be able to just fly at the same velocity as the ship is rotating and dock on the outside? They have the same velocity at the time of latching, and then the ship can simply power down its engine and use the wheel/station to keep at that momentum.

Yes, but unless the station is massive enough to have sufficient gravity for you to enter orbit, or it's got some form of gravity generation that can act like a tractor beam (and if it does, you probably don't need to spin the thing in the first place) you're using a lot of thruster fuel to keep curving your flight while you maintain position over the docking port as you're approaching it (which could potentially take multiple orbits) - plus there's a constant stress on the docking port when you're connected.

Jay R
2017-04-10, 09:02 AM
Stupid question on my part, but shouldn't a ship be able to just fly at the same velocity as the ship is rotating and dock on the outside? They have the same velocity at the time of latching, and then the ship can simply power down its engine and use the wheel/station to keep at that momentum.

This has the same problem of a lumberjack just trying to walk at the same speed as a spinning log in the river. With nothing holding him there, he'll fall off as soon as he isn't concentrating.


Thanks for the answers so far, folks. I feel like the conversation passed the point of "no one can clearly agree on a realistic way to handle this, so just do whatever you want from here" a while ago, so with the answer I needed in hand, I'll leave you all to your debate :smallsmile:

Not at all. There are lots of ways to do it, and each has specific issues that must be addressed. You can pick any of these ways, but in each case, you can't do whatever you want with it. They all have consequences for you.

Mastikator
2017-04-10, 02:06 PM
This has the same problem of a lumberjack just trying to walk at the same speed as a spinning log in the river. With nothing holding him there, he'll fall off as soon as he isn't concentrating.

A smart lumberjack would tie himself to the log so that he couldn't be separated from it even if he pushed himself away.

Trekkin
2017-04-11, 10:56 AM
A smart lumberjack would tie himself to the log so that he couldn't be separated from it even if he pushed himself away.

So a smart lumberjack would drown when the log rolls and pulls them under?

Anonymouswizard
2017-04-11, 11:39 AM
So a smart lumberjack would drown when the log rolls and pulls them under?

I think today's moral is 'smart lumberjacks work in scuba gear'.

Mastikator
2017-04-11, 02:03 PM
So a smart lumberjack would drown when the log rolls and pulls them under?

What in space is that an analogy for?

Trekkin
2017-04-11, 05:04 PM
What in space is that an analogy for?

It was meant to be an admittedly somewhat obscure way of drawing attention to an erroneous assumption we've been implicitly making. A lumberjack tying themselves to a large log is going to follow the log's original trajectory almost exactly, rotation and all; the same is true of a spaceship docking to a rotating space station, only far more so.

This has not been true of everything we have docked together thus far. When, for example, the Shuttle Orbiter was still flying, it had an operating mass around 70,000 kg; when it docked to the International Space Station' 400,000 kg, the center of mass of the station-shuttle pair was noticeably different from the station alone. Were the ISS intended to spin, it would want the Shuttle to dock along its axis.

Now consider the classical cylindrical space station, the O'Neill Cylinder. (it is also called Island Three). As originally described by O'Neill's team of students, one cylinder would be 8 km wide and 20 km long. Using the radiation shielding design I outlined earlier, the mass of the shielding alone -- a mass utterly insensitive to any "sufficiently advanced" structural materials anyone might care to postulate out of their hat -- is ~1.68e12 kg, for an approximate angular momentum of ~1.28e12 kg*m2/s assuming a rotational period of 127 seconds (and treating a 2m-thick, 8000m-wide shell as thin-walled).

Docking the Shuttle Orbiter to such a construction would shift its center of mass by 0.033 cm off-axis. Let us say we instead dock the "Midrange" Orion to it, a 2000 metric ton spacecraft. The center of mass will shift by 0.95 cm. If we were to instead dock all 8,000,000,000 kg of the 400-m wide "Super" Orion, itself easily interstellar ark-sized, we would see a more appreciable change of 37.96 m -- 0.4% of the station's diameter. That, I suppose, might be a problem; perhaps we should include an axial docking port for city-sized spaceships, but anything less just isn't going to shift us much.

Bearing in mind that these are incredibly optimistic numbers that totally disregard the mass of everything else on the station, I don't think we need to concern ourselves with the change in the center of mass produced by docking spaceships to the rim. Much like our hypothetical lumberjack, a ship docked to the rim of a rotating space station isn't going to make it wobble all that much.

Anonymouswizard
2017-04-11, 05:56 PM
It's never a good sign when I see someone working in petagrams, I know that means the GM won't let me get my hands on it just in case I want to ram someone with a space colony. On the plus side it really does give you a sense of the scale.

Yes, I use the full prefix scale in real life as well, megameter is just fun to say.

Telok
2017-04-12, 01:03 AM
One important thing to think about is the power of the engines that drive the ships and the amount of regulation in your shipping system.

If your ships run on antimatter powered fusion drives then there's either extremely strict regulation on ships (use, control, ownership, maintenance, inspection, etc.) or there's no docking or landing that thing in any civilized system, it's shuttles all the way. If anything goes wrong the ship is a fusion cutting torch attached to an antimatter bomb. And 'wrong with the ship' includes everything from mechanical issues to computer glitches to sabotage to piracy.

It's not just the antimatter powered fusion drive ships either. Even an chemical rocket engine is a serious issue. Tramp freighters with dodgy registration and poor maintenance sound good in a space opera, but nobody who has difficulty breathing in vacuum sleeps well at night when there are 2000 ton objects capable of accelerating to 3 km/sec and more being piloted by unknown people who may or may not have properly functioning computers or unmaintained seals and valves on their fuel tanks.

Trekkin
2017-04-12, 01:28 AM
antimatter powered fusion drives

What on earth is an antimatter powered fusion drive? I've heard of antimatter rockets, fusion rockets, and antimatter-catalyzed fusion rockets, but "antimatter powered fusion drive" seems like a contradiction in terms when antimatter annihilates rather than fuses with matter.

Anonymouswizard
2017-04-12, 04:03 AM
What on earth is an antimatter powered fusion drive? I've heard of antimatter rockets, fusion rockets, and antimatter-catalyzed fusion rockets, but "antimatter powered fusion drive" seems like a contradiction in terms when antimatter annihilates rather than fuses with matter.

True, I can see antimatter drives and I can see fission drives, but I can't see them being combined.

A side note is why the heck ships with ion pulse drives or similar are landing on planets. Taking off again is too difficult. Now if we're doing with a side colony I can see it, but honestly we might just be using the strategy of 'get into a close enough orbit, use other ships to shuttle goods'.

Storm_Of_Snow
2017-04-12, 05:48 AM
Were the ISS intended to spin, it would want the Shuttle to dock along its axis.
It would also want the Shuttle to have the docking port on an axis through the exact centre of it's mass, and you could only really get one on either end of the station's axis - ideally, you'd probably want a stationary central core with balanced sets of contra-rotating arms.

And depending on your structure, you may need to pump stored water or some other fluid around to balance any craft docking onto part of the spun habitat (whether an external airlock or an internal bay) - 3mm over 8km may not sound like a lot, but that kind of movement can quickly damage bearings (say if you've got paired arms or a Babylon 5-tyle station with a spinning habitat attached to a stationary structure).

Lord Torath
2017-04-12, 08:12 AM
It's never a good sign when I see someone working in petagramsI first read this as pentagrams. "What does demon-summoning have to do with space stations?" Maxwell's demon, maybe.

Would an electro-magnetic drive (think maglev train in a circle) allow you to avoid having a frictional force between a rotating cylinder and a non-rotating central hub? You'd still need to have some kind of vacuum-proof seal, or else airlocks on both sides of any connection points.

LordCdrMilitant
2017-04-12, 10:40 AM
So, I'm an aerospace engineer, and at least am supposed to know more than zero about orbital maneuvers and space structures.

At least on first glance, I'd rather dock spacecraft to the space station's central hub [I'm imagining a fairly small ring-like space station here], so the ship's axis of rotation lines up with the station's axis if you can. It's easier to spin up the spacecraft using it's stability control system than it is to try to grab the spinning edge of the ring, and doesn't really risk destabilizing the ring [which is a bit of a hazard].

However, from a space structures standpoint, docking at the center of your ring does present different engineering challenges, since you actually have to have a docking port there now.



With regards to docking at the perimeter, consider the following:

The incoming spacecraft that desires to dock has a velocity Vs and an angular velocity as. It's equipped with a rcs system and main engine that allows it to change as and Vs. In order to dock to the space station's perimeter docking port, Vs and as need to equal Vd and ad at all times. The problem is, that the docking port also is accelerating in the stationary reference frame of the station's center. The station hub is exerting a force on the outer docking port to keep it moving in a circle, but there's no such force on the spacecraft. It will have to provide it with its engines. It's not impossible, but it would take a lot of fuel.

Anonymouswizard
2017-04-12, 10:58 AM
I first read this as pentagrams. "What does demon-summoning have to do with space stations?" Maxwell's demon, maybe.

Would an electro-magnetic drive (think maglev train in a circle) allow you to avoid having a frictional force between a rotating cylinder and a non-rotating central hub? You'd still need to have some kind of vacuum-proof seal, or else airlocks on both sides of any connection points.

On pentagrams and space stations, I've been considering creating a steampunk science fantasy setting for a while, where enchanted zeppelins carry Space MarinesRoyal Explorers to other worlds in order to prepare them for colonisation. Bound spirits/demons essentially replace AI in the setting, so your space station might have pentagrams in it to keep the demon bound.

EM drives are going to be annoying, you're going to have at least a small amount of acceleration based on wear you're pointing the darn field, but a 360 degree field of a low magnitude will likely reduce the friction. You'll still want airlocks to be safe.

LordCdrMilitant
2017-04-12, 11:18 AM
I first read this as pentagrams. "What does demon-summoning have to do with space stations?" Maxwell's demon, maybe.

Would an electro-magnetic drive (think maglev train in a circle) allow you to avoid having a frictional force between a rotating cylinder and a non-rotating central hub? You'd still need to have some kind of vacuum-proof seal, or else airlocks on both sides of any connection points.

It would eliminate the frictional force, but magnets aren't magic, and equal and opposite forces still apply.


As far as pentagrams on a space station, a station a DH party visited had lots of octograms on it :P. Almost everywhere an Ordo Hereticus or Malleus party goes will have 8-pointed stars.

DigoDragon
2017-04-12, 01:04 PM
(so you can have the talking heads say "it's actually less radiation than on Earth's surface" over and over until scientifically illiterate people stop running in terror from their bananas)

Heh, yeah that was a great article (http://www.bbc.com/news/magazine-15288975). :smallbiggrin:


On rotating stations--
You don't have to rotate the entire station for your artificial gravity. Rather, just rotate the section inside where most of the crew quarters would be, while other sections like the docking bay would be 'static', making it easier for ships to connect to on the outside. A small scale example of such a plan is the Discovery ship from the movie 2001. Inside the front bulb section, there is a ring-shape compartment that rotates (http://68.media.tumblr.com/4bfeac1e43a521e26a5f0a013b52d0d5/tumblr_odzy3l5Mxw1sxm4gzo2_1280.jpg) and provides gravity for the crew, while other sections like the bay with the pods do not rotate at all.

LordCdrMilitant
2017-04-12, 01:57 PM
Heh, yeah that was a great article (http://www.bbc.com/news/magazine-15288975). :smallbiggrin:


On rotating stations--
You don't have to rotate the entire station for your artificial gravity. Rather, just rotate the section inside where most of the crew quarters would be, while other sections like the docking bay would be 'static', making it easier for ships to connect to on the outside. A small scale example of such a plan is the Discovery ship from the movie 2001. Inside the front bulb section, there is a ring-shape compartment that rotates (http://68.media.tumblr.com/4bfeac1e43a521e26a5f0a013b52d0d5/tumblr_odzy3l5Mxw1sxm4gzo2_1280.jpg) and provides gravity for the crew, while other sections like the bay with the pods do not rotate at all.

Well, you can. I'm not sure I would. I would also make the spinning ring much larger than 8m in radius, because it has to spin almost 10 times a minutes to keep 9.81m/s/s along it, not to mention would result in a fairly significant difference in gravity between the head and the feet.

In addition, having a big rotating part would do weird things to your spacecraft's dynamics, but that goes without saying.

Telok
2017-04-12, 02:10 PM
What on earth is an antimatter powered fusion drive? I've heard of antimatter rockets, fusion rockets, and antimatter-catalyzed fusion rockets, but "antimatter powered fusion drive" seems like a contradiction in terms when antimatter annihilates rather than fuses with matter.

Yeah, my bad. Posting too late at night for clarity. I think I meant a fusion drive and an antimatter power plant. Maybe. Too late at night.

CharonsHelper
2017-04-12, 02:18 PM
With all of the talking past each-other that has been done in this thread - I think that it's safe to say that it largely depends upon the technology involved and/or the lack thereof.

LordCdrMilitant
2017-04-12, 02:35 PM
Yeah, my bad. Posting too late at night for clarity. I think I meant a fusion drive and an antimatter power plant. Maybe. Too late at night.

That's about as meaningful as the original statement, really.

How about: "the spacecraft contains elements that could lead to catastrophic destruction of craft and station were an error in docking to occur."

sktarq
2017-04-12, 03:04 PM
Well since we are talking about a city sized space station the idea that only the outer part spins makes sense. You really only need gravity where the people are. Everywhere else is a problem, from speed matching, logistics, keeping sensors and comm arrays aimed, etc etc.

If you really worried about stabily issues then three stacked rings spinning in an a-b-a pattern (with b ring twice the mass) would give even out the forces and moments just dandy.

DigoDragon
2017-04-12, 03:19 PM
Well, you can. I'm not sure I would. I would also make the spinning ring much larger than 8m in radius, because it has to spin almost 10 times a minutes to keep 9.81m/s/s along it, not to mention would result in a fairly significant difference in gravity between the head and the feet.

In addition, having a big rotating part would do weird things to your spacecraft's dynamics, but that goes without saying.

Naturally the station ring(s) would be much larger to accommodate a city-sized population. How much the dynamics of a partial rotating system matters depends on how hard the OP wants to take the science and engineering aspects of this station.

Beleriphon
2017-04-12, 03:38 PM
So there are two sci-fi videogame answers, both from Mass Effect. The Citadel rotates for gravity effects, and the habitation is on the outer rings. Docking seems to run into the centre of the station and then the ship slow navigates into position at a docking platform. Keeping mind that Mass Effect has artificial gravity, but the Citadel is so old that it isn't equipped for it. It isn't self sufficient either, but it is the hub of galactic government so imports are not exactly a problem.

Mass Effect: Andromeda has The Nexus. Without spoiling the game too much its a sad state of repairs when you first find it and building colony outposts help build more of the space station as well supply it with well supplies. One of the colonies is on an ice world (like Hoth) and sends ice to provide water.

Segev
2017-04-12, 04:43 PM
The discussion of robots being better at things in space than we are, and the logistical needs of humans in space, reminds me of a half-justified use for giant mecha that I came up with about a year ago: glorified spaceship/suits. With cabins that are essentially efficiency apartments or maybe built like campers (the kind you put in the back of a pickup to be even smaller than a camper trailer), the torso would accommodate that and power. Inside would also be a harness and rig for reactive controls - move your arm, the robot moves an arm; move your finger in the glove, the robot does the same. The humanoid design of the 'bot is more to accommodate the expectations of the humanoid pilot controlling it. Legs are, admittedly, kind-of useless even here, but having the kick-off potential might be worth it.

The normal square-cube law problems are reduced in microgravity; this thing is only swinging around against its own mass, not trying to support itself on the ground. Comfortable enough for extended stay in space, it becomes a giant space suit for working with your environment. Just design all the user-serviceable parts on your space ships and stations to work with enormous hands. A logistical concern, sure, but potentially feasible. I mean, we design our screws and such to be used by human-scale hands and tools because that's what we have.

Now, I'm sure better, more efficient designs than humanoid robots can be come up with, but it's a fun idea for a fictional setting, I think, at least.

As to the docking problem, I think you're best served by having literal docking rings in the middle of your O'Neill cylinder, with elevator-tethers going from them down to the surface. Allow for airplane-like shuttles to also transit back and forth. If you want greater throughput, break your cylinder up into multiple rings, and require transit between them to go up to the docking ring level. You can then bring ships in between the rings, and allow them to match rotation in there.

You can also allow smaller ships not to worry about matching rotation. Once in the atmosphere, people disembark and travel to the rotating rings and just catch hold. Linear velocities can be pretty small on the internal rings, so catching hold won't hurt. Then take the elevators down.



An alternative design might involve three concentric structures, with one of them being (again) really multiple structures.

The outer shell is not rotating, and serves for radiation shielding and to really contain the atmosphere. Airlocks and docking mechanisms exist all along it. Smaller vessels can actually enter through a sufficiently-sized airlock and then traverse "up" to the central docking station.

The central docking station also doesn't rotate. It may be one or more structures, but does the job of handling transit from medium-large vessels that come in along the axis to the rings.

The structure in between is a segmented cylinder that serves as the multiple habitable areas. These rotate all at the same speed, acting much like a unified element. They may or may not be permanently linked by large "bridges" between them to maintain constant relative angular speed, but it's important that these "bridges" be spaced quite far apart with enormous gaps, and probably have "drawbridge" capability to pull back entirely. The gaps between them, again, help with throughput. Smaller vessels that dock with the outer shell and come in through its airlocks fly through these gaps. As long as the bridges are far enough apart that, say, it takes a third of a day for one to cross where another had been, there's little need to match speed. Just have some basic traffic control to keep ships from coming in anywhere near them.

The ships fly through those gaps, appearing from the "ground" to be zipping by very fast (and seeing the "ground" zip by very fast, too), and head up to the docking rings, where speeds are much slower. Or, if they're really small enough, they might even just get up high enough to make a conventional "airplane" style landing on a runway, aerobreaking as needs be.


Now, there are two major problems I can see with this design. The first is suggested by what I just discussed: relative air speed. Is the air inside moving with the rotating "ground," or is it stationary wrt the outer shell? In the former case, anything entering from the shell will hit enormous-speed winds as soon as it gets into the air stationary wrt the "ground." The sudden smack of air could wreak havoc with navigation and lead to catastrophic crashes. In the latter case, winds on the "ground" are going to be punishing. So some solution to this would need to be found.

Either way, the friction between the air, the rotating cylinders, and the non-rotating outer shell will be significant drag on one or the other. So while we avoid the large bearing issues that the fixed structures "rolling" against each other would need, we still have to keep transferring angular momentum back from the non-rotating structures (especially the outer one) to the rotating structures (the habitation rings). Which leads us to the second major problem:

With no physical components anchoring the three structural elements together (outer shell, habitation cylinder(s), and central docking station), we avoid those "moving parts" issues, but we have to be very careful to keep the individual components stationary relative to each other (aside from rotation). Careful inertia-matching can be done to start this off, but each internal component likely will require stabilization mechanisms to adjust themselves should anything drive them to move slightly.

I suppose we could make the inner docking spine anchor to the outer shell; that avoids having to keep it adjusting. But we'd still need to keep active stabilization of the habitation cylinder(s).

Edit to add another thought:

Actually, that's another way you could do the exterior landing problem: enormous runways.

Rather than my last suggestion, you have a fairly conventional cylinder. Inside is the habitat. Maybe docking rings in the middle for the large, axial ports.

The outside of it, however, still looks like it's got a great many indentations. Wide, generously-sized indentations that you can fly reasonably-sized transport craft into. Probably with structural trusses outside of them, because once inside the indentations, it becomes clear that there is a significant "lip" rather than a hard wall with berths. This huge "lip" serves as a station-circumference runway. You don't have to match speed perfectly, just well enough that you can land and not crumple from the sudden acceleration. It's like landing a high-speed airplane. You can't miss your berth, you just have to be careful to be applying your inward thrust properly. Ability to temporarily apply sufficient thrust to hover your craft at 1g may be needed. Maybe less than 1g if the "docking runways" are further in. This may be assisted by magnetics or something.

But we at least avoid the need to perfectly match speed or come back. When you've actually "landed," you slow the same way any vehicle does, and you just drive up to an airlock.

Trekkin
2017-04-12, 07:03 PM
Edit to add another thought:

Actually, that's another way you could do the exterior landing problem: enormous runways.


This is functionally similar to my docking-ports-on-the-backs-of-trains idea; in both cases we're wrapping something around the circumference of the cylinder to allow a ship to match speed with the station's center of mass (and therefore dock with it) and then gradually match speed with the station rim, which will be moving ~200 m/s relative to the ship at that point for an Island Three-sized cylinder. You have the ships exert a force on the runway; I have the docking trains exert an upward force on their tracks. Yours does have the advantage of not relying on the station's own power to allow ships to dock; mine doesn't need the ships to have landing gear or fit inside the runways.

Actually, perhaps a combined approach would serve the needs of our hypothetical station best.

I like the idea of separating the radiation shield from a rotating pressure hull; given that much of that hull is in tension, its absolute mass is more variable with respect to advances in materials science than the shielding, but we can confidently assume it will weigh less than the shield. The shield can also support the mirror arrays for lighting the habitat interior and the radiators for keeping it cool -- and, if we orient one end of our cylinder sunward, we can use the former to shade the latter. (conversely, if we put the mirrors on the far end, it becomes slightly easier to maneuver the station by using the mirrors as a solar sail)

Neither necessarily needs a mechanical connection to the habitat either; photons care little if the lenses they're going through are spinning. Cooling is harder, but theoretically we could integrate a droplet radiator into the shield and spray warm fluid from the floor of our habitat. Depending on the angle of spray (and yes, this will need energy input), the habitat will either speed up or slow down its rotation relative to the shield. It may be worthless, but if used in concert with the maglev system already described it at least has the benefit of a certain elegant simplicity.

Regardless, the interstices between the shield, the habitat, the support maglev rings and any heat/spin water cannons as described above would seem an ideal place to put a series of variable-spin annular (or balanced-subsections-of-annular) rings for communicating between the docking ports on the shield and the ones on the hab floor. They would be naturally volume limited, but for water resupply, passenger transport, or other containerizable shipping this would not be a major limitation when we can just add shuttle trains ad nauseam.

That said, one of the major benefits of Island Three is that is permits shirtsleeve microgravity work. We could concievably integrate an airlock into the far end of the habitat from the sun to permit microgravity shirtsleeve ship construction, for example, or just the docking of very large ships. This probably should not be more than a few hundred meters across, but it would suffice for bringing in anything large enough to disturb the station's rotation.

In this way, we would have one system for bringing in all the water and carbon and iron and spanners and puppies and tourists and anything else we can fit in a standard shipping container all day long while minimally disturbing the station's rotation, and another for bringing prefabricated buildings in or out. Neither need limit the size of the ship to dock, nor subject them to any rotational stresses beyond that imposed by the atmosphere in the axial airlock.

Of course, given all the energy used for respin, this station would necessarily need to be one of a contrarotating pair linked by their shields. This also makes it possible to rotate the whole assembly, as in O'Neill's original design; if we wanted, putting two such pairs at right angles to each other would allow us to point the station differently up and down, too.

It would also have a total 1g living area of 2412 square kilometers, which is kind of fun; I'd be happy to work this example more fully if anyone feels like using it in their game or something.

LordCdrMilitant
2017-04-12, 07:22 PM
I'm not sure why, but something doesn't sit with me about the "docking ports on trains" idea.

I [obviously] haven't done any of the work associated with it, though, co I can't say for sure.

With an addendum to docking at the interior, there is a point where the r becomes big enough that the w becomes small enough that a sufficiently hardy spacecraft could dock easily without requiring anything weird.

Telok
2017-04-12, 11:23 PM
How about: "the spacecraft contains elements that could lead to catastrophic destruction of craft and station were an error in docking to occur."

Well that's all spacecraft in general unless you're going with complete handwavium engines, shields, artificial gravity, and physics in general.

LordCdrMilitant
2017-04-13, 12:01 AM
Well that's all spacecraft in general unless you're going with complete handwavium engines, shields, artificial gravity, and physics in general.

Yeah, but "antimatter power plant" and "fusion drive" is pretty much pointless technobabble and handwavium, intended to convey the idea that the craft blows up really big if something bad happens.

Storm_Of_Snow
2017-04-13, 02:49 AM
On pentagrams and space stations, I've been considering creating a steampunk science fantasy setting for a while, where enchanted zeppelins carry Space MarinesRoyal Explorers to other worlds in order to prepare them for colonisation. Bound spirits/demons essentially replace AI in the setting, so your space station might have pentagrams in it to keep the demon bound.
Sounds something like GDW's Space 1889.

Anonymouswizard
2017-04-13, 10:03 AM
Sounds something like GDW's Space 1889.

A bit, although I plan for a more 'realistic' setting and am on the fence about including the magic in the core part. It's an alternative version of a steampunk system/setting I'm writing which explores the idea of 'what if a society with science discovers relatively easy repeatable magic' (essentially combine it with their technology, causing them to gain spaceflight early).

Segev
2017-04-13, 10:30 AM
This is functionally similar to my docking-ports-on-the-backs-of-trains idea; in both cases we're wrapping something around the circumference of the cylinder to allow a ship to match speed with the station's center of mass (and therefore dock with it) and then gradually match speed with the station rim, which will be moving ~200 m/s relative to the ship at that point for an Island Three-sized cylinder. You have the ships exert a force on the runway; I have the docking trains exert an upward force on their tracks. Yours does have the advantage of not relying on the station's own power to allow ships to dock; mine doesn't need the ships to have landing gear or fit inside the runways.


I'm not sure why, but something doesn't sit with me about the "docking ports on trains" idea.The issue for me is that you have to deliberately match docking-train speed with the incoming spacecraft, which brings us back to "if you miss your docking port, you have to do an expensive delta-V turnaround."

The "Runways," you can't miss by having incorrectly-matched speed. You also have the opportunity to try to pull some of the spin-based g-force into your own thrust as you come in.

With the linear delta-v of a stationary-wrt-the-station (but not its shell) ship to the runway, just "touching down" is dangerous because it could rip apart your landing gear. If you somehow have sufficiently robust landing gear and bearings to rotate fast enough that you can make a smooth contact, though, landing is really quite easy. Spin up your landing gear to match its linear velocity at the outer rim of the wheels with the linear velocity of the runway wrt your ship. Touch, and start applying brakes to your landing gear.

I imagine it's a lot easier to match speed of landing gear to the runway than to match speed of your ship to a moving dock.

As you "slow down" wrt the runway, the centrifugal force naturally kicks in and "gravity" starts to apply at as leisurely a pace as you can afford to keep going in a straight line on the runway.

All that said, I suppose the real way the "docking trains" would work is to have the docking vessel approach the station, and then the trains match velocity with it. It just seems like it's requiring a lot more precision speed-matching and a lot more energy to move the "docking train" than it is to spin up "landing gear."


With an addendum to docking at the exterior, there is a point where the r becomes big enough that the w becomes small enough that a sufficiently hardy spacecraft could dock easily without requiring anything weird.While angular velocity (ω) gets lower as you increase r for the same centrifugal g-force, linear velocity of the outer rim remains quite high. g = r(2pi/T)^2, where T is period and r is the radius of the rotating ring, and r(2pi/T) is actually the linear velocity; for constant g, T increases as the square root of r's increase. T is getting proportionally smaller than r. Unless I'm screwing up my math (which I could be, as I'm doing this half in my head), the linear velocity is actually increasing at the outer rim (where we're maintaining 9.8 m/s^2 g-force) the greater r gets.



That said, one of the major benefits of Island Three is that is permits shirtsleeve microgravity work. We could concievably integrate an airlock into the far end of the habitat from the sun to permit microgravity shirtsleeve ship construction, for example, or just the docking of very large ships. This probably should not be more than a few hundred meters across, but it would suffice for bringing in anything large enough to disturb the station's rotation. A quirk of spin-induced gravity is that you can actually decrease or increase your personal apparent-gravity by moving quickly against or with the spin of the station. Run (or drive) exactly at the speed it's spinning in the opposite direction of the spin, and you achieve weightlessness. (This actually means that your docking trains would not be exerting "hanging" tension on the outer hull while moving at the right speed to match the non-rotating vessel with which they intend to dock.)

To illustrate this for those to whom this seems counter-intuitive, imagine a passenger on a large vessel which came in through an axial docking bay. He decides to do some micro-gravity tourism using his personal air-jet, and steps out into the open air at the middle of the station. All around him, he sees the "ground" rotating grandly. Possibly dizzyingly, given its speed and distance. If he picks what is, to him, a straight line to putter towards this ground, he still isn't getting any heavier unless he tries to match spin with the ground. Instead, he remains weightless while seeing a rapidly-moving ground to one side of him. He could come as close to the ground as he dared (considering things like buildings, trees, and people whipping by at high speed) and he would still float in mid air.

It's very like what something moving at orbital speeds does around a planet: it appears to be flying by very rapidly but never falling towards the ground. In fact, hovering at the dead center of the axis of rotation is equivalent to geosynchronous orbit!


Of course, given all the energy used for respin, this station would necessarily need to be one of a contrarotating pair linked by their shields. This also makes it possible to rotate the whole assembly, as in O'Neill's original design; if we wanted, putting two such pairs at right angles to each other would allow us to point the station differently up and down, too.

It would also have a total 1g living area of 2412 square kilometers, which is kind of fun; I'd be happy to work this example more fully if anyone feels like using it in their game or something.

I'm not sure why we need so much more energy for respin, nor how the counter-rotating components are helpful. Especially with the design that uses a non-rotating outer shell. Spin can be maintained for the habitation surface with simple jet propulsion if needs be. ...though I suppose the non-rotating outer shell, to maintain a non-rotating nature, might need to combat the air friction from the inner shell. I'm still quite concerned with the wind speeds, too. In the design where the outer shell is spinning, the air gets picked up and spun and pressed "downwards" in a way that naturally reduces relative wind.

In more traditional stations and ships, that don't have open-air "sky," the air is pushed by the walls of the individual rooms to keep it moving along "stationary" relative to said rooms. We could remove the "outdoors" feel to the station and do it that way. But then there still would be need for internal airlocks, if only to break up the wind differentials between rotating and non-rotating parts of the station. Also, especially for fictional purposes (but also an impact on psychology), the "open air" design is more impressive and gives a sense of freedom.

If we could contrive a way to have the outer shell be transparent enough that those looking up can see the starscape beyond the islands of habitation, a 1 km radius station in orbit around a planet would have the planet (and its star) complete one apparent orbit around the station in about 63 and a half minutes, which would be somewhat majestic. And let's be honest, from a fictional standpoint, the majesty and wow factor of the station is important! (In both directions: if it's there, it says something about the wonder of space travel; if it isn't, it says something about the grittiness of space travel.)


Thinking further about it, I still think the runway design is superior to the docking-train design for two reasons:

1) By limiting the size of the landing ships, it almost inherently limits the weight. It also allows a much sturdier, non-moving-parts design for the load-bearing element that is taking the weight of the docking vessel as it spins up to match.
1a) Just thought of this, but it also means that the ships are going to be "inside" the 1g ring, rather than "hanging" outside of it, so the people on board aren't going to be getting suddenly HEAVIER than they will be on the station.

2) Again, fewer moving parts and less energy required to "spin down" the docking ship's landing gear than to "spin up" the docking train.


And you wouldn't need all the ships to have landing gear; you could have docking tugs which basically latch on to the docking ships which do not and provide the landing gear. Those launch out and meet leisurely with the docking vessel, and then they come in and spin up their landing gear to make the contact.

LordCdrMilitant
2017-04-13, 01:06 PM
The issue for me is that you have to deliberately match docking-train speed with the incoming spacecraft, which brings us back to "if you miss your docking port, you have to do an expensive delta-V turnaround."

The "Runways," you can't miss by having incorrectly-matched speed. You also have the opportunity to try to pull some of the spin-based g-force into your own thrust as you come in.

With the linear delta-v of a stationary-wrt-the-station (but not its shell) ship to the runway, just "touching down" is dangerous because it could rip apart your landing gear. If you somehow have sufficiently robust landing gear and bearings to rotate fast enough that you can make a smooth contact, though, landing is really quite easy. Spin up your landing gear to match its linear velocity at the outer rim of the wheels with the linear velocity of the runway wrt your ship. Touch, and start applying brakes to your landing gear.

I imagine it's a lot easier to match speed of landing gear to the runway than to match speed of your ship to a moving dock.

As you "slow down" wrt the runway, the centrifugal force naturally kicks in and "gravity" starts to apply at as leisurely a pace as you can afford to keep going in a straight line on the runway.

All that said, I suppose the real way the "docking trains" would work is to have the docking vessel approach the station, and then the trains match velocity with it. It just seems like it's requiring a lot more precision speed-matching and a lot more energy to move the "docking train" than it is to spin up "landing gear."

While angular velocity (ω) gets lower as you increase r for the same centrifugal g-force, linear velocity of the outer rim remains quite high. g = r(2pi/T)^2, where T is period and r is the radius of the rotating ring, and r(2pi/T) is actually the linear velocity; for constant g, T increases as the square root of r's increase. T is getting proportionally smaller than r. Unless I'm screwing up my math (which I could be, as I'm doing this half in my head), the linear velocity is actually increasing at the outer rim (where we're maintaining 9.8 m/s^2 g-force) the greater r gets.



Oh, sorry, I meant interior. But you are correct.

Tangential velocity can be computed as the square root of the product of the centripetal acceleration and the radius. [v=sqrt(a*r)] Linear velocity increases with radius. However, the spacecraft attempting to dock still has to gain the constant acceleration of 9.81m/s/s when it tries to dock, regardless of how fast the station spins or how big it is.

I'm not particularly concerned about expensive delta-v turnarounds, though, because 1: missing your docking at the velocities involved could, and not unlikely, would, be instantaneously catastrophic making the turnaround unnecessary, and 2: if we've managed to construct a city-sized space station, it's fairly easy to assume the ship that wants to dock has the delta-v to turnaround and re-align for another go.

But, as for docking at the central hub along the station's axis, if the station was rotating with a small enough w, then the spacecraft only has to be spun up a little bit to make the docking, and the station could potentially just grab it with a docking clamp if the spacecraft was small compared to the station and the delta-w required was very small. This would entirely eliminate the need for assemblies of bearings and drives, at the expense of only allowing 2 ships to simultaneously be connected to the station.

N810
2017-04-13, 01:31 PM
Here are some classic examples.

https://dncache-mauganscorp.netdna-ssl.com/thumbseg/922/922962-bigthumbnail.jpg
https://s-media-cache-ak0.pinimg.com/originals/a8/05/ea/a805ea419effa444219959626ae479f9.jpg

Telok
2017-04-13, 01:31 PM
Yeah, but "antimatter power plant" and "fusion drive" is pretty much pointless technobabble and handwavium, intended to convey the idea that the craft blows up really big if something bad happens.

You're probably right on the power plant but fusion torch ships are actually a serious concern with real science and research behind them. That's because they're one of the best answers to the rocket equation.

LordCdrMilitant
2017-04-13, 01:59 PM
You're probably right on the power plant but fusion torch ships are actually a serious concern with real science and research behind them. That's because they're one of the best answers to the rocket equation.

I assume you're talking about the Project Orion system.

I'm not concerned about a FDR or VaSIMR system doing anything worse than the effects of high-speed collision.

Segev
2017-04-13, 02:39 PM
I dunno; you could easily miss a docking and simply not make contact when it's clear you're missing it, and still have an expensive delta-v turnaround. Now, both the docking train and the runway concept obviate the need for the approaching vessel to make the careful velocity-match; that's handled by the train moving to counter-act the spin of the station, or by the landing gear spinning at near-matched velocity so that it catches gently. The ship, in either case, is docking at a leisurely pace as it would with a non-rotating station, at least until "deceleration" of the landed ship or the docking train begins, which starts spinning up the docking ship to match the speed of the station's rotation to which it's docked (and "turns on" gravity for the docking ship's occupants in the process).


Honestly, "docking trains" probably would be full-circumference rings that spin up and down on a schedule. To use the 63-and-a-half minute rotation of a 1 km radius station, the circumference would be about 6.3 km. Which means that the outer radius is moving about 6 km/hour. That's actually not that fast. Which sounds very wrong...


...ah, I'd made the unit error before of saying it was a period of 63.469 minutes. That should have been seconds.

About 99 m/s is pretty darned fast; it's 221.4 mph.

For that stately 1 hour period you'd need approximately a 3250 km radius. That's nearly twice the radius of the Moon.

So let's dismiss that as impractical for now.

Alright.
So the correct rotational period is just over a minute for a 1 km radius cylinder providing 1g, and its outer shell linear velocity is about 100 m/s. That will take you only about 10s at 1g acceleration to reach, though, which amounts to about 2g by the very end if you have a constant 1g for the linear acceleration adding on to the increasing centrifugal acceleration of the artificial gravity.

A more complicated arrangement would attempt to maintain a constant feeling of 1g, which involves an initial spin up to 1g that starts almost immediately dropping off as the centripetal acceleration picks up, so the apparent direction shifts from, say, behind your back to beneath your seat as you spin up to speed. It would kind-of feel like being in a chair that is on its back and is rotating to an upright position.

Centrifugal velocity being given by v = sqrt(g*r), we calculate g = v2/r.

r = 1000 m
v(t) = a(t)*t; where a(t) is the linear acceleration.
g(t) = (a(t)*t)2/1000

We want to maintain sqrt(a2(t)+g2(t)) = 10 m/s2 at all times on this spin-up, with a(0) = 10 m/s2 and a(tfinal) = 0 m/s2.

i.e. a2(t)+g2(t)=100 m2/s4


This essentially describes a circle for the joint values of a(t) and g(t). It must take longer than 10s and less than 20s for this smooth path to be traced, and I really don't feel like digging deep enough into my calculus and trigonometry to find the precise integrals required, so I'm going to roughly estimate 15s.


So, for a ride on the train that starts weightless, then immediately presses you with about 1g into the back of the seat (making you feel like you're in a chair that's laying on its back) and gives you a sensation that the chair starts to rotate from its back to an upright position, you'll spend about 15s.

That's not bad at all.

If you have a number of these "docking trains" ringing the station, you could have them on 15 minute cycles pretty easily. 14 min., 45s "weightless" going at speed to allow ships to link up and either unload to the train or secure for gravity; 15s of turning on gravity, 10 min. to disembark/unload (and for new passengers to get on and into the waiting ships) to the station (possibly using cargo elevators); 4 min. for them to all catapult off the station in balanced pairs designed to launch into polar orbits of the planetary body, for eventual flight out to their destinations; 15s for bringing the docking train back to weightlessness (probably with nearly nobody on board), and restart cycle. Half an hour per cycle. If that's not enough time, some of them could be on hour-long cycles, instead.

The runway version would allow for a bit more flexibility in initial timing, though probably would require spinning the debarking ships up to "weightless" rather than catapult-launching them, since the timing on the "drop" out of the runway from station-stationary would be trickier and could result in destabilization. Then again, the runways are for less massive ships.

Segev
2017-04-13, 02:49 PM
But, as for docking at the central hub along the station's axis, if the station was rotating with a small enough w, then the spacecraft only has to be spun up a little bit to make the docking, and the station could potentially just grab it with a docking clamp if the spacecraft was small compared to the station and the delta-w required was very small. This would entirely eliminate the need for assemblies of bearings and drives, at the expense of only allowing 2 ships to simultaneously be connected to the station.

The cost for only 2 docks is high, but not quite that high. For small ships, they could be allowed to fly in without having to dock right at the axial entrance. Let them fly inside the station, with an airlock, and maintain in the weightless axial spine region at the "highest" point of the station, and they can handle their own minute roll to "join up" with numerous docking inner-rings. Or just dock with hovering non-rotating sections and transit to relatively slowly-rotating rings (they're moving about as fast as a second-hand on a clock, after all) for their passengers disembarking or their cargo being unloaded.

There can be two massive docks for truly huge ships to actually match rotation and join, but most hopefully will fit inside, either through runways, docking trains (which still have the highest cost in moving parts, I think), and open passageways.



The airlock mechanism I'm envisioning for the two axial openings is a massive air-tight cylinder that pistons in with a complement of small vessels seeking entry, then moves along the spinal track, sealed at both ends, letting air in until it can break open to let everybody fly out. It's parts move back along an outer track (through atmosphere) and then out an air-tight clamp to re-assemble in vacuum and accept a new set of ships in. A series of these are meant to keep the traffic flow relatively constant.

There are numerous problems with that, but it's a first-pass design I just thought up, so I will keep ironing it out for a while, most likely.

Trekkin
2017-04-13, 02:51 PM
I'm not sure why we need so much more energy for respin, nor how the counter-rotating components are helpful. Especially with the design that uses a non-rotating outer shell. Spin can be maintained for the habitation surface with simple jet propulsion if needs be. ...though I suppose the non-rotating outer shell, to maintain a non-rotating nature, might need to combat the air friction from the inner shell. I'm still quite concerned with the wind speeds, too. In the design where the outer shell is spinning, the air gets picked up and spun and pressed "downwards" in a way that naturally reduces relative wind.


My comment on respin was unintentionally vague; I meant respinning the water despun in the droplet radiator, which would impart spin to the shield. Having two contrarotating habitat cylinders in rigidly linked shields would balance that force, and of course you need contrarotation anyway to precess in step with the station's orbit and keep the mirrors oriented sunward.

My main argument in favor of trains over runways is one of safety. I don't quite understand where everyone is seeing a high delta-v cost for turning around; the train is stationary relative to the center of mass of the station and thus an incoming ship, so theoretically misses are no more expensive than they are when docking to a stationary object. The same is true of the runway approach, except that if something goes wrong (say a mismatch in the tire speed relative to the rim speed) there is a risk of high-velocity debris bouncing around the runway chamber -- and a certainty for RCS thrust in certain directions. I'd worry about running over a patch of hydrazine at 200 m/s. The train involves more moving parts, but if those parts suddenly stop moving (relative to the rim), they're going to go out into empty space. Then, too, as long as we're going to have rapidly spinning wheels, it might be more stable to have them constantly on the same track throughout the entire process rather than trying to keep tyres running straight on a flat surface. Whether that justifies the additional energetic expense of spinning up the train, I don't know. Certainly, were I on the station, I would rather trust my train over some random incoming ship's docking linkages with my tugs, given that worse comes to worst I can drop the ship off the train and into space.

Incidentally, depending on the size of the ship, the additional acceleration would hardly be noticeable. Given the 8 km wide station, a 100 m long ship docked to the rim at its nose would experience an acceleration of 10.045 m/s at its tail, relative to 9.8 m/s at the station floor. Is 1.025 g that bad?

As for how to keep all of space visible even through the shielding, I'm working on it.

Segev
2017-04-13, 03:19 PM
My main argument in favor of trains over runways is one of safety. I don't quite understand where everyone is seeing a high delta-v cost for turning around; the train is stationary relative to the center of mass of the station and thus an incoming ship, so theoretically misses are no more expensive than they are when docking to a stationary object. The same is true of the runway approach, except that if something goes wrong (say a mismatch in the tire speed relative to the rim speed) there is a risk of high-velocity debris bouncing around the runway chamber -- and a certainty for RCS thrust in certain directions. I'd worry about running over a patch of hydrazine at 200 m/s. The train involves more moving parts, but if those parts suddenly stop moving (relative to the rim), they're going to go out into empty space. Then, too, as long as we're going to have rapidly spinning wheels, it might be more stable to have them constantly on the same track throughout the entire process rather than trying to keep tyres running straight on a flat surface. Whether that justifies the additional energetic expense of spinning up the train, I don't know. Certainly, were I on the station, I would rather trust my train over some random incoming ship's docking linkages with my tugs, given that worse comes to worst I can drop the ship off the train and into space.My concern on the trains is one of mechanical cost and wear and tear. The runways' moving parts are modular, and one going out of commission doesn't stop others from using it. Break down a ring-train, and that's a significant percentage of the docking capacity out of commission. Safety-wise, too, I think you're underestimating the damage from a docking train suddenly breaking down. Train crashes are pretty spectacular IRL, and they go significantly slower than 221 mph. A train "crash" that causes sudden unexpected, non-uniform deceleration (or acceleration, depending on your PoV) when the train's natural falling direction is up and thus to rip through anchors... it would potentially destabilize the entire spinning cylinder.

A crashed landing "plane" would do damage to a runway and its docking bays, but would not involve the huge forces and momenta of the docking trains.




Incidentally, depending on the size of the ship, the additional acceleration would hardly be noticeable. Given the 8 km wide station, a 100 m long ship docked at its nose would experience an acceleration of 10.045 m/s docked at its tail, relative to 9.8 m/s at the station floor. Is 1.025 g that bad?By 8 km wide, do you mean that's the diameter of the spinning cylinder? I'd been assuming only 2 km.


As for how to keep all of space visible even through the shielding, I'm working on it.

At a 1 km radius, I wouldn't bother, having redone my math and realized that we're watching the planet whiz by every minute. Though I suppose a second-hand isn't dizzying, so...maybe?

LordCdrMilitant
2017-04-13, 03:34 PM
-long post-



With regards to docking via train, I think I figured out why it doesn't sit well with me: the trains have to be moving incredibly fast along the perimeter of the station during the docking procedure, and it introduces a lot of systems to be potentially failure prone. There's also going to be a fairly massive loading on them and, more importantly, the wheels and axles interfacing with the rails they move along.

As for smooth acceleration:

a(t) = sqrt((v(t)^2/r)^2+(d/dt(v(t)))^2)

is your differential equation. I'm not going to do it.


As far as safety goes, you neither want to have a ship crash in the runway dock or jettison a docking train at several hundred m/s.

Trekkin
2017-04-13, 03:39 PM
My concern on the trains is one of mechanical cost and wear and tear. The runways' moving parts are modular, and one going out of commission doesn't stop others from using it.

Perhaps I am misunderstanding your runway design, then. Do you not intend for an incoming ship to make multiple trips around the circumference of the station while slowing down, and thereby coming into contact with debris/damaged sections of runway anywhere along that circumference?

And yes, I have been making optimistic assumptions about the capacity of the trains, which I had thought of as relatively short, to disengage from the tracks and fall away from the station gracefully -- and modularly -- in the event that accelerating the ship exceeds the train's stress tolerances, at which point we can simply bring a new train out of storage and/or repair the track. Perhaps too optimistic, given the significant stresses involved.

In any event, yes, I have been matching the dimensions of O'Neill's Island Three design: 8 km in diameter, 32 km long. This also means the station has a period of 127 seconds, which might be less than dizzying.

LordCdrMilitant
2017-04-13, 03:50 PM
Perhaps I am misunderstanding your runway design, then. Do you not intend for an incoming ship to make multiple trips around the circumference of the station while slowing down, and thereby coming into contact with debris/damaged sections of runway anywhere along that circumference?

And yes, I have been making optimistic assumptions about the capacity of the trains, which I had thought of as relatively short, to disengage from the tracks and fall away from the station gracefully -- and modularly -- in the event that accelerating the ship exceeds the train's stress tolerances, at which point we can simply bring a new train out of storage and/or repair the track. Perhaps too optimistic, given the significant stresses involved.

In any event, yes, I have been matching the dimensions of O'Neill's Island Three design: 8 km in diameter, 32 km long. This also means the station has a period of 127 seconds, which might be less than dizzying.

Keep in mind, during docking, the train is moving at 300m/s.

I can't really think of a good way to jettison the train, though. Simultaneously blowing all the couplers and track "spikes" with explosive bolts would probably be the "best" solution for clearing the thing from the station, but it will still create a disk of lethal debris around the station.

Trekkin
2017-04-13, 03:56 PM
Where do you get 300 from? I get a rim velocity of 197.9 m/s, and thus a train velocity of ~200 m/s -- and while that's terribly fast, maglev trains have reached 167.5 m/s, and those had to run in air.

Admittedly, they have not done so while temporarily holding on to several tons of spacecraft. That might make things more difficult.

LordCdrMilitant
2017-04-13, 04:02 PM
Where do you get 300 from? I get a rim velocity of 197.9 m/s, and thus a train velocity of ~200 m/s -- and while that's terribly fast, maglev trains have reached 167.5 m/s, and those had to run in air.

Admittedly, they have not done so while temporarily holding on to several tons of spacecraft. That might make things more difficult.

9.81m/s/s = v^2/8000m
v = 280m/s

The train can go that fast, I don't doubt, but failure of any component [such as a frozen bearing] would be instantaneously catastrophic.

Several hundred tons of spacecraft, if not more.

Trekkin
2017-04-13, 04:06 PM
Ah, we're all using different dimensions. I've been using 8000m as the diameter, so 9.8 m/s2 = v2/4000m, thus v = 197.98 m/s.

You are right about any failure being catastrophic, though. Especially if we use wheels.

LordCdrMilitant
2017-04-13, 04:12 PM
Ah, we're all using different dimensions. I've been using 8000m as the diameter, so 9.81 m/s2 = v2/4000m, thus v = 197.98 m/s.

You are right about any failure being catastrophic, though. Especially if we use wheels.

Ah, my bad. I used 8000 for the radius.

Anyway, you want to clear the thing away from the station as fast as possible if something goes wrong, whether the spacecraft turns out to be too heavy or a bearing freezes or a coupling fails.

As I've expressed, I'd rather dock spacecraft nearer the center, where tangential velocity is much lower.

There are other bonuses to docking near the center, in that the pseudogravity is lower, allowing cargo to be merely floated aboard the station.

Segev
2017-04-13, 04:19 PM
Maglev trains also aren't trying to hold themselves "down" wrt the ground. Remember that your trains, when not going at full 'negate spin' speeds, are effectively hanging off of the station. While you can probably do maglev designed for that, it gets trickier when you realize that the train is deliberately going to reduce the hanging force the magnets have to resist. For this design, you really want physical track-wheels. And that's a lot of friction.

This, and balancing the load the train represents (especially with docked ships), is why I pictured your "trains" more as full rings around the station. When they're matching speed with the spinning station, they are partially self-supporting; they're not "hanging" off the station. When they're going at "full stop" (so they can dock with incoming ships), there's obviously no force, but we have to be cautious of friction and the like.

Given the periods of rotation and times involved, though, I'm picturing significantly less than 1 full trip around the perimeter of the station to come to stop wrt the station's spin upon landing. With the 1000m radius design, I see ~15s (while I thank LordCdrMilitant for the differential equation, I don't feel like plugging in numbers right now) to come to a stop. The period is about 4x that. I'd have to do some math again to figure out how long it takes to come to stop/speed on the 8 km one. Regardless, I doubt that one's a full period, either.

Debris would be a serious problem. They are on real runways, too, IRL. Measures would be taken to prevent them. The thing about mis-spinning gear is that even if it's slightly off, it probably won't be more than 1 m/s difference, and releasing the motor drive on the wheels before contact will mean that the natural friction will keep it from being too bad. Consider how landing planes don't have powered-spin on their landing gear at all, and that has to thus start rolling from a "stop" to match ground-speed of the plane as it's dropping.



Honestly, for a fictional station, I could see all three docking schemes being used in tandem. Smaller vessels (especially the pricey private transports) would likely use the runways, maybe the axial docking ports when nothing huge is coming in. Larger vessels would use the docking ring-trains, putting up with more delays for greater bulk transfer. Truly huge things would use axial docking, coming in through massive doorways to rest in the air high "above" the station floor in its central section, using in-air docking rings to let people on and off and match rotation. Any thing too big for that has to use shuttle-transfer.

Trekkin
2017-04-13, 04:26 PM
Ah, my bad. I used 8000 for the radius.

Anyway, you want to clear the thing away from the station as fast as possible if something goes wrong, whether the spacecraft turns out to be too heavy or a bearing freezes or a coupling fails.

As I've expressed, I'd rather dock spacecraft nearer the center, where tangential velocity is much lower.

There are other bonuses to docking near the center, in that the pseudogravity is lower, allowing cargo to be merely floated aboard the station.

If only inverted maglev (that is, magnetically attracting the train to the module "ceiling") were not, at best, metastable, it would be perfect. Alas and alack.

That said, I agree that this all pales in energy efficiency compared to docking near the center; the whole rim docking idea was predicated on the station needing to move more cargo than could be transferred through the poles, which was a pessimistic assumption of mine. I'll see if I can determine how much ice and air would actually be needed per unit time to replace that lost to outgassing.

EDIT: I see your point, Segev. For all they're heavier, full rings are probably the least ludicrous way of handling docking trains -- which would lead to the lethal ring of debris problem.

Segev
2017-04-13, 04:40 PM
From, again, a fictional standpoint, however, a sufficiently large station could have docking trains be used for the purpose of telling a western style narrative about space bandits.

Telok
2017-04-13, 05:15 PM
I assume you're talking about the Project Orion system.

I'm not concerned about a FDR or VaSIMR system doing anything worse than the effects of high-speed collision.

I'm not talking about any of the Orions. It's the fact that anything capable of getting a spacecraft up to a useful velocity in a useful amount of time is going to have enough power to be dangerous if anything goes wrong. High-speed collisions are just things going wrong after some portion of the power has been converted into kinetic energy.

The more I think about it the more I stop thinking about even having docking for full sized spaceships. Just make them park off in a zone and use smaller local tenders and shuttles. Unless the station authority can rely on all ships to be safe and properly run.

LordCdrMilitant
2017-04-13, 05:24 PM
It may be better from the docking train perspective to have a "docking spire" protruding along the axis of rotation, with the spinning docking rings mounted along it. They have to move much more slowly and sustain much less force, and you can just haul cargo along the spire to the ring hub and then lower it down the shaft to the main ring.

Also, with regards to the spacecraft landing on a runway, a modern airplane can land at 50-100m/s. The wheels go from 0 to fast when it touches the ground. I suspect we can land our spacecraft freely on the runway without significant difficulty from the gear.


I'm not talking about any of the Orions. It's the fact that anything capable of getting a spacecraft up to a useful velocity in a useful amount of time is going to have enough power to be dangerous if anything goes wrong. High-speed collisions are just things going wrong after some portion of the power has been converted into kinetic energy.

The more I think about it the more I stop thinking about even having docking for full sized spaceships. Just make them park off in a zone and use smaller local tenders and shuttles. Unless the station authority can rely on all ships to be safe and properly run.

So, you're using "Fusion Drive" as technobabble for "big rocket engine". There's no specific property of a FDR or VaSIMR system that makes it more dangerous to the station than a H2-O2 liquid fuel engine.

Also "useful velocity in a useful amount of time" may be a wee bit inaccurate. The most efficient transfers tend to have really long, low impulse burns.

Telok
2017-04-14, 01:31 AM
Also "useful velocity in a useful amount of time" may be a wee bit inaccurate. The most efficient transfers tend to have really long, low impulse burns.

"useful amount of time"

Those efficient transfers is the reason it takes our probes ten plus years to reach the outer system. Do you want your personal round trip to the Europa research station to take twenty years?

CharonsHelper
2017-04-14, 09:41 AM
From, again, a fictional standpoint, however, a sufficiently large station could have docking trains be used for the purpose of telling a western style narrative about space bandits.

Since they'd have ships, wouldn't they be pirates?

Segev
2017-04-14, 09:42 AM
At least as much as fuel economy, a consideration for any manned space travel is how fast we want to get to the cruising speed we want to use. How much "gravity" can you tolerate? For how long? Barring gravitic control, is it better to maintain a constant 1g acceleration to the half-way mark and then reverse to maintain a constant 1g deceleration until you reach the destination, or is it better to have a much higher, temporary g-force to get up to a high cruising velocity and rely on rotational gravity?

Humans can take more gs for short persiods. But can we take enough gs to get us to a high enough velocity fast enough that we save fuel and time by cruising at high speed with a rotating "gravity" generator, or will survivability constraints limit the usefulness of that scheme such that we're not really saving time over the constant acceleration/deceleration method?


As a point of interest, if you could do a dumb-fire straight line approach (you can't), traveling from Earth to Saturn at its nearest point using 1g acceleration/deceleration the whole way (and ignoring any delta-v between Earth and Saturn) would get you up to 0.0035c at your turn-around point and, from the traveler's perspective, would take almost 35 hours. Travel to Saturn from Earth at its farthest point would take 59 hours and 16 minutes. The time dilation effects even if the ENTIRE trip were taken at 0.0035c would be less than 1.5s.

The trips likely take longer what with the parabolic paths around stars and other planets that must be calculated, if one were to actually execute them, however.

LordCdrMilitant
2017-04-14, 09:52 AM
"useful amount of time"

Those efficient transfers is the reason it takes our probes ten plus years to reach the outer system. Do you want your personal round trip to the Europa research station to take twenty years?

A hohmann transfer [a traditional and fairly simple transfer] will take just as long [not to mention is pretty much entirely independent of how big the motor is. When we do the math on transfers like that, we assume all the delta-v is gained instantaneously]

Getting there faster and cheaper isn't about burning really hard for a short amount of time to "get up to speed". You're going to burn for the entire transfer, and you'll get there faster with an engine with lower thrust but far lighter fuel than a really big engine that takes lots of fuel.


As a point of interest, if you could do a dumb-fire straight line approach (you can't), traveling from Earth to Saturn at its nearest point using 1g acceleration/deceleration the whole way (and ignoring any delta-v between Earth and Saturn) would get you up to 0.0035c at your turn-around point and, from the traveler's perspective, would take almost 35 hours. Travel to Saturn from Earth at its farthest point would take 59 hours and 16 minutes. The time dilation effects even if the ENTIRE trip were taken at 0.0035c would be less than 1.5s.

The trips likely take longer what with the parabolic paths around stars and other planets that must be calculated, if one were to actually execute them, however.

We can work out the transfer with some numbers and some integrals [and MATLAB]. I'll take a crack at it over the weekend.

Segev
2017-04-14, 11:18 AM
I have matlab, but lack the interest to do more than order-of-magnitude estimates at the moment. That said, I'll be interested in your results. I just found it interesting that the "accelerate at 1g, then turn around and decelerate at 1g" solution, at least just for straight-line distances, can get you to and from Saturn comfortably and with less than a week of travel time. Possibly less than 4 days' travel time, even on the round trip. Even with the parabolics, I imagine it won't add more than a couple extra days.

Also, given the need to swing in arcs, you could probably accommodate the turn-around such that you never have to have even a "stop, be weightless while we turn" transition. You could maintain 1g acceleration the whole route, even as the actual direction of acceleration shifts from speeding you towards your destination to slowing you to a stop for arrival at the destination.

Thinking further, the longest extension of the trip would be when Saturn is at its furthest, simply because then you want to make sure to arc far enough around the Sun.

LordCdrMilitant
2017-04-14, 11:48 AM
I have matlab, but lack the interest to do more than order-of-magnitude estimates at the moment. That said, I'll be interested in your results. I just found it interesting that the "accelerate at 1g, then turn around and decelerate at 1g" solution, at least just for straight-line distances, can get you to and from Saturn comfortably and with less than a week of travel time. Possibly less than 4 days' travel time, even on the round trip. Even with the parabolics, I imagine it won't add more than a couple extra days.

Also, given the need to swing in arcs, you could probably accommodate the turn-around such that you never have to have even a "stop, be weightless while we turn" transition. You could maintain 1g acceleration the whole route, even as the actual direction of acceleration shifts from speeding you towards your destination to slowing you to a stop for arrival at the destination.

Thinking further, the longest extension of the trip would be when Saturn is at its furthest, simply because then you want to make sure to arc far enough around the Sun.

You wouldn't want to burn, then turn around and burn to slow down. It wastes fuel. It's better if your craft spends the whole time speeding up to reach it's new orbital speed.

Anyway, what I intend to do is to assume that Earth and Saturn are massless and travelling in coplanar circular orbits, and compute the minimum-fuel and minimum-time transfers for that case. It gives a "general idea".

I could also write a program that sums the forces acting on the spacecraft at each moment in time, and then fish around with values until I get the most efficient path with an encounter, but that would also be a lot of work.

Segev
2017-04-14, 11:50 AM
You wouldn't want to burn, then turn around and burn to slow down. It wastes fuel. It's better if your craft spends the whole time speeding up to reach it's new orbital speed.

Good point. The (very bad) estimate I was using was that the endpoints were stationary wrt each other. This obviously isn't true. Calculating the delta-v would allow you to figure out how to burn for optimal fuel, but may not allow you to maintain 1g unless you can arc it just right. Or you rely on a certain amount of spin combined with the thrust, angling the habitation zone such that the thrust+centrifugal force yield a net 1g vector towards the "floor."

LordCdrMilitant
2017-04-14, 12:12 PM
Good point. The (very bad) estimate I was using was that the endpoints were stationary wrt each other. This obviously isn't true. Calculating the delta-v would allow you to figure out how to burn for optimal fuel, but may not allow you to maintain 1g unless you can arc it just right. Or you rely on a certain amount of spin combined with the thrust, angling the habitation zone such that the thrust+centrifugal force yield a net 1g vector towards the "floor."

The thing is, that approximation doesn't work anyway because it's not just getting out to the orbital radius of Saturn, you also have to match Saturn's orbital velocity at that point. So even if you could go fast enough that Saturn doesn't move appreciably along it's orbit, you'd have to turn and burn retrograde for 20km/s [Saturn actually moves with a lower orbital velocity than earth].

I just plugged some orbital data into the delta-v equation in my textbook and got 20.1233 km/s of delta-v required, accounting for Saturn's inclination, for what my textbook terms the "Optimal Quasi-Circular Orbit Transfer", so that checks out.

The velocity profile for this given transfer would be computed by V(t) = (v02-2v0Ttcos(B0)+T2t2)1/2 where T is the thrust acceleration and B thust yaw angle [Equation from page 145 of Orbital Mechanics by Prussing and Conway].

I must have made a mistake somewhere, because if I plug in 9.81 for thrust acceleration I get a time of 1 hr for the transfer, which is way too small.

Segev
2017-04-14, 01:27 PM
The thing is, that approximation doesn't work anyway because it's not just getting out to the orbital radius of Saturn, you also have to match Saturn's orbital velocity at that point. So even if you could go fast enough that Saturn doesn't move appreciably along it's orbit, you'd have to turn and burn retrograde for 20km/s [Saturn actually moves with a lower orbital velocity than earth].

I just plugged some orbital data into the delta-v equation in my textbook and got 20.1233 km/s of delta-v required, accounting for Saturn's inclination, for what my textbook terms the "Optimal Quasi-Circular Orbit Transfer", so that checks out.

The velocity profile for this given transfer would be computed by V(t) = (v02-2v0Ttcos(B0)+T2t2)1/2 where T is the thrust acceleration and B thust yaw angle [Equation from page 145 of Orbital Mechanics by Prussing and Conway].

I must have made a mistake somewhere, because if I plug in 9.81 for thrust acceleration I get a time of 1 hr for the transfer, which is way too small.

The problem is that you're just calculating the time it takes, at 1g acceleration, to change from matching Earth's velocity to Saturn's velocity. i.e. the time, at 1g acceleration, it takes to change from Earth's to Saturn's reference frame.

You're not accounting for the time it takes to travel between 77 million and 222 million miles.

Given that you're also already matching Saturn's velocity at the end of that hour, you're also not moving any closer to it if you stop thrusting at that point.



What you HAVE calculated is that, using the "go 9.8 m/s2 halfway there, and turn around and do it backwards for the rest of it" trip is actually only an hour shorter or longer than it otherwise would be, to account for the delta-v between the start and stop locations.

And...actually, this strongly suggests that, despite it being most fuel-efficient to launch a shuttle that would accelerate from 0 to 20.1233 m/s over a distance of 1.2e12 m...

1,200,000,000,000 m = T*t2
20,123.3 m/s = T*t

T = (1,200,000,000,000 m) / (t2) = (20,123.3 m/s) / t

t = (1,200,000,000,000 m) / (20,123.3 m/s) = 59,632,000 s = 1 year, 10 months, and approximately 20 days (depending on the month and minor delays in docking/landing) ... on average, which is going to have a huge variance depending on real distance to Saturn. But this works for rough estimations.

This would be done at an acceleration of T = (20,123.3 m/s) / (59,632,000 s) = 0.00033746 m/s2, or three tenths of a millimeter per second per second.

That's three milli-Gs. I don't think that quite qualifies as "micro-gravity," but a 250-lb man would weigh only a little over a tenth of an ounce in that environment. It'd be, MAYBE, enough to keep a sense of a "down" by observing objects very gently falling through the pressurized atmosphere.

It's also a very long trip, time-wise. Probably extremely fuel efficient, but given that maintaining a comfortable 9.8 m/s2 shortens it to less than a week, you'll probably find people seeking compromise of comfort and convenience vs. price.

LordCdrMilitant
2017-04-14, 02:26 PM
The problem is that you're just calculating the time it takes, at 1g acceleration, to change from matching Earth's velocity to Saturn's velocity. i.e. the time, at 1g acceleration, it takes to change from Earth's to Saturn's reference frame.

You're not accounting for the time it takes to travel between 77 million and 222 million miles.

Given that you're also already matching Saturn's velocity at the end of that hour, you're also not moving any closer to it if you stop thrusting at that point.



What you HAVE calculated is that, using the "go 9.8 m/s2 halfway there, and turn around and do it backwards for the rest of it" trip is actually only an hour shorter or longer than it otherwise would be, to account for the delta-v between the start and stop locations.

And...actually, this strongly suggests that, despite it being most fuel-efficient to launch a shuttle that would accelerate from 0 to 20.1233 m/s over a distance of 1.2e12 m...

1,200,000,000,000 m = T*t2
20,123.3 m/s = T*t

T = (1,200,000,000,000 m) / (t2) = (20,123.3 m/s) / t

t = (1,200,000,000,000 m) / (20,123.3 m/s) = 59,632,000 s = 1 year, 10 months, and approximately 20 days (depending on the month and minor delays in docking/landing) ... on average, which is going to have a huge variance depending on real distance to Saturn. But this works for rough estimations.

This would be done at an acceleration of T = (20,123.3 m/s) / (59,632,000 s) = 0.00033746 m/s2, or three tenths of a millimeter per second per second.

That's three milli-Gs. I don't think that quite qualifies as "micro-gravity," but a 250-lb man would weigh only a little over a tenth of an ounce in that environment. It'd be, MAYBE, enough to keep a sense of a "down" by observing objects very gently falling through the pressurized atmosphere.

It's also a very long trip, time-wise. Probably extremely fuel efficient, but given that maintaining a comfortable 9.8 m/s2 shortens it to less than a week, you'll probably find people seeking compromise of comfort and convenience vs. price.

Hmm.

I checked the equation for time for continuous burn transfer between two circular orbits [8.23 in Prussing and Conway] of radius a0 and af,

tf=sqrt(Gms)/T * (a0-1/2-af-1/2)

with 6.67*10-11m3kg-1s-2 = G, 1.9891*1030kg = ms, 9.81m/s/s = T, 149*109m = a0, and 1.4*1012m = af, I get 2049s. Which is way too small, like half and hour, but it checks out with the delta-v equation's results.

I'm confused, because when I work through the example the book gives going from LEO to GEO at 10-2 m/s/s by hand, I get the correct [and reasonable] answer of 5.5-days.

I'm not sure why the Earth-Saturn transfer is coming up to be such a small timeframe. If I make the LEO to GEO burn at 9.81m/s/s, I get 475s. If I make the Earth-Saturn burn at 10-2m/s/s, I get 23.26 days, which seems too small.

Is 1g Acceleration just that damn fast?

The Hohmann Transfer for Earth to Saturn comes out to 5.9 years, and for the LEO to GEO transfer to 5.7 hours, which both make sense.

I might have to see what computer simulations have to say about this.

[For the record, I'm teaching this to myself as we go. My orbits class is on impulsive transfers right now; we haven't got to continuous burn transfers yet.]

Segev
2017-04-14, 02:49 PM
What is ms in your formula? I'm not parsing that one.

I do see where you're accounting for the radial orbits. That, though, might only work for the shortest possible transfer distance between Earth and Saturn. Which would be very near the shortest distance they ever come to each other.

Even so, that seems awfully fast, considering that the straight-up speed to simply travel, linearly, from Earth-orbital to Saturn-orbital while burning continuously (admittedly to slow down for half the trip) is 35 hours. Using 1g as the acceleration/deceleration value.

In the last response I had that you quoted, I did the math for travelling that distance at an acceleration that would cover the velocity difference. That came out to almost 2 years.

I still suspect you're still missing something that properly accounts for the time spent travelling the actual distance, rather than simply coming up to speed. However, I haven't parsed the equations you're using closely enough to be sure, yet.

LordCdrMilitant
2017-04-14, 03:01 PM
What is ms in your formula? I'm not parsing that one.

I do see where you're accounting for the radial orbits. That, though, might only work for the shortest possible transfer distance between Earth and Saturn. Which would be very near the shortest distance they ever come to each other.

Even so, that seems awfully fast, considering that the straight-up speed to simply travel, linearly, from Earth-orbital to Saturn-orbital while burning continuously (admittedly to slow down for half the trip) is 35 hours. Using 1g as the acceleration/deceleration value.

In the last response I had that you quoted, I did the math for travelling that distance at an acceleration that would cover the velocity difference. That came out to almost 2 years.

I still suspect you're still missing something that properly accounts for the time spent travelling the actual distance, rather than simply coming up to speed. However, I haven't parsed the equations you're using closely enough to be sure, yet.

ms is the mass of the central body around which all the things orbit, the sun in the case of the earth-saturn transfer, the earth in the case of LEO-GEO transfer.

And that's why I'm confused. 2 years makes sense, half and hour doesn't.

Segev
2017-04-14, 03:07 PM
Re-reading your math post... am I reading this right? Are you doing the Earth-Saturn burn at 1 cm/s2? If so, why that particular acceleration?

LordCdrMilitant
2017-04-14, 03:11 PM
Re-reading your math post... am I reading this right? Are you doing the Earth-Saturn burn at 1 cm/s2? If so, why that particular acceleration?

1 milligee was the acceleration value for the LEO-GEO book problem. I did the Earth-Saturn transfer to see what came up as a sort of reality check, because perhaps 9.81 was just a ridiculous acceleration value for a continuous burn.

Segev
2017-04-14, 03:24 PM
Of interest to you may be that my calculations for constant 9.8 m/s2 acceleration get you halfway there in 23.5425 ... hours. Well, halfway to the average between nearest and farthest distance.

That that's so close to the 23.26 value you get for days is...probably a coincidence.



I only have an MS in physics, and it's been about a decade since my last physics class, with general relativity being the closest I came to astrophysics and orbital mechanics. I'm doing most of this off of kinematic equations and general math.

LordCdrMilitant
2017-04-14, 03:32 PM
Of interest to you may be that my calculations for constant 9.8 m/s2 acceleration get you halfway there in 23.5425 ... hours. Well, halfway to the average between nearest and farthest distance.

That that's so close to the 23.26 value you get for days is...probably a coincidence.



I only have an MS in physics, and it's been about a decade since my last physics class, with general relativity being the closest I came to astrophysics and orbital mechanics. I'm doing most of this off of kinematic equations and general math.

I worked out the units and the units line up too, and I've made myself confused. I've actually got Orbits in half an hour, so I'll ask the professor about it then. The hours/days similarity is probably a coincidence.

I'm working a BS in Aerospace Engineering, Astronautics concentration . Currently my second year, but I'm also a semester ahead, so I'm in 3rd year classes. Physics isn't a unrelated field, though.


On a completely other note, I find it strange to be discussing this on a forum for Roleplaying games. It's kind of funny, really. I play D&D to [I]not have to think about ellipses 24-7, and here I am.

Segev
2017-04-14, 03:50 PM
Heh. Well, it started with somebody talking about fictional space stations and the real considerations one might need to make in designing them vs. what one can get away with given, well, fiction. So it came here naturally. I'm quite enjoying it, though. :smallsmile:

LordCdrMilitant
2017-04-14, 03:53 PM
Heh. Well, it started with somebody talking about fictional space stations and the real considerations one might need to make in designing them vs. what one can get away with given, well, fiction. So it came here naturally. I'm quite enjoying it, though. :smallsmile:

Me too. :) It's fun. Anyway, going to class now. Will follow up on this discussion afterwords.

Segev
2017-04-14, 03:56 PM
It doesn't hurt that I've had the ambition, ever since I was 12 or so, to build giant habitable space stations around the Earth. And participate in the invention of warp drive. Sadly, that isn't the field I'm in. But this remains fun.

LordCdrMilitant
2017-04-14, 04:02 PM
It doesn't hurt that I've had the ambition, ever since I was 12 or so, to build giant habitable space stations around the Earth. And participate in the invention of warp drive. Sadly, that isn't the field I'm in. But this remains fun.

Warp Drives are totally your field, as a Physicist. Me and my peers will build the spacecraft powered by them, but it's up to you to invent them!

Yeah, my parents worked on Cassini, Galileo, and Voyager [they met each other at the Jet Propulsion Laboratory], so ever since I was knee high I wanted to work on space things. I think it shows, because when I GM, I invariably run Traveller or Dark Heresy, and my characters when I play tend to push the bounds of what's considered fantasy and what's considered sci-fi. I've never run a "realistic" sci-fi setting, though, my players would probably throw something at me.

Segev
2017-04-14, 04:28 PM
I'm actually working in computer science, right now. My Ph.D. shifted over to computer engineering with an emphasis on computational intelligence as I developed an interest in nanites. Sadly, jobs in the area I was interested in...didn't exist. But, I do interesting enough stuff here, and I can engage with a few other ideas as hobbies until one or more spark something I can career shift into. I'm currently looking at a rather mad sciency idea involving seemingly unrelated fields of neurology, torpedo rays (and other electric fish), and wireless power. I have a letter I keep putting off writing that I should get to to clarify the limitations of WiTricity's products.

LordCdrMilitant
2017-04-14, 07:53 PM
I'm actually working in computer science, right now. My Ph.D. shifted over to computer engineering with an emphasis on computational intelligence as I developed an interest in nanites. Sadly, jobs in the area I was interested in...didn't exist. But, I do interesting enough stuff here, and I can engage with a few other ideas as hobbies until one or more spark something I can career shift into. I'm currently looking at a rather mad sciency idea involving seemingly unrelated fields of neurology, torpedo rays (and other electric fish), and wireless power. I have a letter I keep putting off writing that I should get to to clarify the limitations of WiTricity's products.

So, I asked my professor about it. He was kinda busy, since he had a meeting right after about meeting the August launch window for a satellite they're building, but he said my computations sounded about right, and that the weirdness is probably because it's almost impossible to continuously burn at 9.81m/s/s all the way to Saturn, and an actual long-duration engine would have far, far lower thrust [and that if I wanted him to look through it in more detail, to come back to office hours next week, because the meeting is sort of important, and nothing bad is going to happen over the weekend if I've made a mistake elsewhere in the problem]

Half an hour still sounds insanely small, though, but since the math give reasonable numbers for other scenarios, I guess I'll trust it. Also, I get the same number for both the delta-v analysis and the radius-to-radius analysis.

I suspect I made a mistake somewhere else in the problem, not the direct computation, and will read through the textbook, and maybe check with a computer simulation, to see what comes up.