Could you do a magnetic cannon to send stuff in orbit

[factotum]

As I understand it, if it's made of carbon nanotubes, the material for the ribbon will be exceedingly light, with a density on the order of a feather.

A feather 71,000km long and whatever cross-section is needed to provide the relevant tensile strength is going to have a *lot* of mass regardless of its low density. Even if the carbon nanotube cable is only a centimetre wide (which is clearly nonsense), we're talking a total mass in excess of seven million tonnes. Sure, it won't all hit at the same time as it falls, but it's still not going to cause no damage at all. Quite apart from the damage inherent in the destruction of a very expensive and presumably important structure in the form of the space elevator itself!

[Chronos]

A space elevator wouldn't wreck whatever it landed on, because it wouldn't land. The most likely failure mode, a cable cut or break near the surface, would leave it drifting away from the Earth, not falling. And even if you somehow got a failure mode that did result in it falling, it'd burn before reaching the ground.

Nor would the loss of the elevator itself be any great devastation. It's nearly the same price to make a dozen elevators as it is to make one. If one out of a dozen is lost, well, you'll just have to make do with 11 for the year or two it would take to replace it.

Phobos, meanwhile, is no problem at all for a Martian elevator. An Earthly elevator would be anchored at the equator, but for Mars, a better anchor point would be the top of Olympus, which would put it well clear of Phobos' nearly-equatorial orbit.

[Tvtyrant]

A space elevator wouldn't wreck whatever it landed on, because it wouldn't land. The most likely failure mode, a cable cut or break near the surface, would leave it drifting away from the Earth, not falling. And even if you somehow got a failure mode that did result in it falling, it'd burn before reaching the ground.

Nor would the loss of the elevator itself be any great devastation. It's nearly the same price to make a dozen elevators as it is to make one. If one out of a dozen is lost, well, you'll just have to make do with 11 for the year or two it would take to replace it.

Phobos, meanwhile, is no problem at all for a Martian elevator. An Earthly elevator would be anchored at the equator, but for Mars, a better anchor point would be the top of Olympus, which would put it well clear of Phobos' nearly-equatorial orbit.

The issue with the Moon or Mars as an elevator site is it fundamentally doesn't help the issue, which is specifically Earth's gravity well. Neither of those locations can sustain a population, so putting them there is akin to putting a subway at burning man, you still have to drive there to get to the train in the first place.

[halfeye]

A space elevator extends beyond geostationary orbit (approximately 2x). On Mars geostationary is 13,634km. Phobos orbits at 9,234km. Eventually the moon and the cable would collide
You could potentially have a problem with Deimos too which orbits at 23,000km (and your elevator should extend to 26,000km)

Phobos weighs about 10,000 billion metric tons and orbits at 2.14km/s (7696km/h) so de-orbiting it is a lot of work.

Why not re-orbit Phobos and tie the elevator to it? You'd have about half the change in orbit that de-orbiting would cost, and a lump at the end of the elevator is one of the ways of doing it that's often discussed.

[Mastikator]

The issue with the Moon or Mars as an elevator site is it fundamentally doesn't help the issue, which is specifically Earth's gravity well. Neither of those locations can sustain a population, so putting them there is akin to putting a subway at burning man, you still have to drive there to get to the train in the first place.

What are we trying to accomplish here? If it's "lift people into orbit" then some kind of mega structure on Earth would be helpful. If it's "visit other planets and make use of resources in the asteroid belt" then a moon base is what we need.

The fact that the only place that can sustain any population is Earth makes me think that we need to accomplish "make use of space resource" before we bother with lifting a significant number of people into space. We are several steps away from colonizing Mars and trying to skip to the Mars colony part is unfortunately a non-starter. We ain't going nowhere.

[Storm_Of_Snow]

A space elevator wouldn't wreck whatever it landed on, because it wouldn't land. The most likely failure mode, a cable cut or break near the surface, would leave it drifting away from the Earth, not falling. And even if you somehow got a failure mode that did result in it falling, it'd burn before reaching the ground.

Agree - the counterweight keeping the cable in tension would pull most of it out of the atmosphere, and you could potentially rig up emergency destruct charges through the structure to break it up into smaller fragments just in case.

[RCgothic]

As had been mentioned, you need a rocket engine to circularise or the orbit always intersects the point of departure. Anything that comes back into contact with the atmosphere once around is in trouble.

That said, it may be possible to build a linear accelerator up mount Chimborazo near the equator with a little tunnelling. The exit point would be at a fraction of the normal atmospheric density and for an 8km accelerator the acceleration would only be 30Gs for a 2km/s exit velocity, which might possibly be survivable by crew or hardened payloads. A sizeable rocket stage would still be required.

[halfeye]

As had been mentioned, you need a rocket engine to circularise or the orbit always intersects the point of departure. Anything that comes back into contact with the atmosphere once around is in trouble.

That said, it may be possible to build a linear accelerator up mount Chimborazo near the equator with a little tunnelling. The exit point would be at a fraction of the normal atmospheric density and for an 8km accelerator the acceleration would only be 30Gs for a 2km/s exit velocity, which might possibly be survivable by crew or hardened payloads. A sizeable rocket stage would still be required.

That's a volcano that last erupted in year 550? That's pretty much still active as volcanoes go. A few degrees off the equator wouldn't matter much for a linear accelerator, unlike a space elevator which has to be within a very few kilometers of the exact equator. So if anyone needs that mountain it's the elevator, and even they would have to have plans for an eruption.

If the mountain is six kilometres high (Chimborazo is 39th highest in the Andes and that high, so finding another mountain should be possible), making the linear accelerator 8 km long would be cutting it very short, 45 degrees would give you 6 * (square root of 2 = 1.4) which is your 8, but no mountain slopes that steeply, you could certainly get 10, probably easily 12, and plausibly 30; certainly 30 if you were prepared to have some sections above ground.

What are we trying to accomplish here? If it's "lift people into orbit" then some kind of mega structure on Earth would be helpful. If it's "visit other planets and make use of resources in the asteroid belt" then a moon base is what we need.

The fact that the only place that can sustain any population is Earth makes me think that we need to accomplish "make use of space resource" before we bother with lifting a significant number of people into space. We are several steps away from colonizing Mars and trying to skip to the Mars colony part is unfortunately a non-starter. We ain't going nowhere.

For myself what I want to see, or see begun, is people living in space, a population of people living in space that don't just go up for a weekend, a fortnight or a few months. The moon would make a fine base to start with, particularly because it's relatively easy to get off it again. I don't see a real need to land on any planet. Making use of resources in space is necessary, you might not need to go as far as the asteroid belt, some of the Apollo third stages didn't re-enter, those things are comparatively huge, and there are nearer asteroids too, though going to them rather than bringing them to Earth orbit would probably be wise.

[Mr Blobby]

For small payloads it's not that bad an idea actually. A lot of a rocket's mass consists of fuel that is needed to lift other fuel up, so if you can instead frontload a lot of that energy in something that never leaves the launchpad you end up with a far lighter vehicle. You'd still need a small rocket on board of the vehicle you launch to circularity it's orbit, but you could get away with a far smaller vehicle.

This would only work for some payloads though, given the massive G-forces involved in accelerating a payload this way. Anything sensitive to getting squished (like, for example, humans), cant be launched in this way.

Now, to take this concept a step further, you could launch a payload using a cannon and then having it be caught by a skyhook and put into orbit to have a launch method that requires no propulsion whatsoever on the object you're launching.

Rail-run launches have a lot of 'kinks' to work out, but from what I can tell there's no *fundamental* tech/science hurdles to clear - the question is simply one of resources to build the thing and deal with said kinks. In fact, it can be argued that we've only gone down to the current system because ultimately, Von Braun and his V-rockets put *everyone* on the same path after WW2 and now we've had perhaps a trillion USD thrown on that system.

Now, it's said that currently we'd need to use booster rockets but I'll argue that it's possible we won't if we use our 'space gun' for more frequent, but lighter launches. Which is perhaps the appeal of a rail-gun; if we can get turnaround down to a week or less, that's perhaps 40-45 launches a year [total payloads 2019, 304].

Yes, but how heavy are these payloads, I hear you ask? Well, average sat weight is around 3,000 kg. For comparison purposes a Challenger tank's main gun is around 20kg, WW2-era heavy howitzers 100kg, a 16-inch gun off USS Iowa 1,200kg (max) and the largest gun ever made - the Schwerer Gustav - is the only pre-existing piece which could move such loads (4,800kg) - though admittedly not far/fast enough to get into space.

Cost-wise, a Falcon 9 launch is about $2.7k per kilo, so our sat will cost around $8m to get there. This cost is so high because the vehicle/payload ratios are really rubbish for rocket launches - 550 tonnes of machine for only 22.8 ton of load (~24:1). Our space gun can do much, much better - for if nothing else, we won't need to launch 72 tonnes of machine/fuel along with our 3 tonnes of payload. (and this is *good*; Ariane 5s are ~48:1, the Shuttle ~73:1 and the old Saturns 28:1).

There's also the issue of carbon weights. It 'costs' perhaps 300 tons to launch a rocket, and there's also the many tons put into construction of the thing - much of it is single-use. A electrical space gun may be able to use most of the power from non-fossil sources, making it much better for the planet (along with the less waste issue).

Next, the increasing problem of space debris - and part of this is bits of old launches. A gun-launch would have much less 'packaging' at worst and at best may even be recoverable. Part of the reason the Space Shuttle got so expensive to run was the fact the ever-increasing level of junk was causing more and more damage to launches.

There is the issue that crewed launches can't be done this way (yet) but in 2019 of 102 launches only 3 were crewed.

The level of benefits I think would justify the expenditure of more resources to explore the idea further.

[DavidSh]

If the mountain is six kilometres high (Chimborazo is 39th highest in the Andes and that high, so finding another mountain should be possible), making the linear accelerator 8 km long would be cutting it very short, 45 degrees would give you 6 * (square root of 2 = 1.4) which is your 8, but no mountain slopes that steeply, you could certainly get 10, probably easily 12, and plausibly 30; certainly 30 if you were prepared to have some sections above ground.

Chimborazo is a stratovolcano, so fairly steep. From photos it looks to be about a 30-degree slope. But that slope isn't continuous all the way down to sea level -- it rises about 2500 m above the surround highlands, so a 10 km accelerator would require either a very long high trestle, a tunnel, or a bit of bending.

[factotum]

Now, it's said that currently we'd need to use booster rockets but I'll argue that it's possible we won't if we use our 'space gun' for more frequent, but lighter launches.

Booster rockets are not optional in any way, shape or form whatever your payload is. All that will happen without the booster is that your payload comes crashing back down to ground again after a single orbit, because it will be in a ballistic trajectory--unless you somehow launch it so fast that it achieves escape velocity, which wouldn't be particularly useful for most microsatellite launches.

[Mr Blobby]

Well, clearly a rail-gun incapable of sending launches into escape velocity would come back to the ground, wouldn't it? And yes, the payload would need some form of internal propulsion to move into position once up there too. The main point - which admittedly I didn't do much on above - is that this tech is 'the path not taken' and thus, has had precious little investment since the early 60s. And it's possible that with 20s tech and a few billion in R&D we might actually get it to work - plus, as pointed out we have different considerations, such as carbon costs and accumilations of space debris.

[factotum]

Well, clearly a rail-gun incapable of sending launches into escape velocity would come back to the ground, wouldn't it?

Escape velocity and orbital velocity are not the same. If you want to put a satellite in orbit you need about 17,000mph, but you still need to circularise the orbit once the satellite gets up there. Escape velocity is more like 25,000mph and that will send your satellite out of the Earth-Moon system entirely, which is what you'd want if you're sending something to visit other planets but not so useful for something you want in Earth orbit.

[Accelerator]

Theoretically, what would limit the power of this magnetic cannon?

We have superconductors, we have nuclear power stations, so other than the unforunate payload being crushed and destroyed by its own immense speed, what else can cause this to fail?

[Mr Blobby]

I think the power of the magnetism would also be a factor. It can't be so strong that it pulls apart the innards of the payload and/or fry the electronics.

[halfeye]

I think the power of the magnetism would also be a factor. It can't be so strong that it pulls apart the innards of the payload and/or fry the electronics.

A Faraday cage would fix that.

[Accelerator]

I think the power of the magnetism would also be a factor. It can't be so strong that it pulls apart the innards of the payload and/or fry the electronics.

Japan already has superconductors that they use to propel the fastest trains on earth. Magnetic shields are not a difficult thing to build. In fact, they're comparatively easy.

[Mr Blobby]

I knew about Farraday cages, but not they could be *that* effective... but how heavy would said cages be?

And the tech developed for high-speed maglevs is one of the reasons I'm quietly optimistic hybrid launches are a viable technology [at least theoretically] - this is tech which didn't exist in the 1960s, and thus was never taken into accounts the last real 'viability studies'.

[Accelerator]

I knew about Farraday cages, but not they could be *that* effective... but how heavy would said cages be?

And the tech developed for high-speed maglevs is one of the reasons I'm quietly optimistic hybrid launches are a viable technology [at least theoretically] - this is tech which didn't exist in the 1960s, and thus was never taken into accounts the last real 'viability studies'.

Well.... Faraday cages can be made from aluminium foil. I say a cm thick should do it, though that's probably overkill.