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rs2excelsior
2014-05-12, 11:44 PM
*NOTE: This thread is in this forum because it is about the underlying physics behind the debate, rather than any particular sci-fi setting or universe.*

Welcome, one and all, to the (in)famous "Purple vs. Green"* debate that has haunted science-fiction communities for years: will laser or kinetic weapons be dominant in space combat?

First off, some background and reference material:
The term "Purple vs. Green" arose from an episode of the sci-fi TV series "Babylon 5," in which members of one alien race chose their leader by combat every few years--and the factions were not based on ideology, but rather what color each individual drew at random. Naturally, these two colors were purple and green. Among sci-fi communities, it has come to mean a debate between two fiercely-entrenched sides, which are largely determined by which of equally-valid initial conditions one assumes. The most prevalent is, by far, whether in a "plausible near-future" laser or kinetic weapons will reign supreme.

For those of you who want a (less than brief) primer on this topic (or if you are interested in space technology with an eye toward realistic sci-fi), go here:
http://www.projectrho.com/public_html/rocket/spacegunconvent.php
http://www.rocketpunk-manifesto.com/2009/09/battle-of-spherical-war-cows-purple-v.html
http://www.rocketpunk-manifesto.com/2009/09/further-battles-of-spherical-war-cows.html

It's long, I know. Go ahead and read through it if you like. I'll wait.

Y'all back? Okay.

So. Before we begin, let's set some ground rules. We're talking "near-future" or "midfuture" technology here. What exactly do those mean? Well, they're a bit slippery. But generally they mean technology at the limit of what is theoretically possible with modern technology. So things that would physically work but are beyond current engineering limits are okay. Things that require a few big advances to modern technology are fine. Things that require a complete technological revolution, or things that do not appear to be even physically possible (such as Faster-than-Light travel, energy shields, and stealth in space) are right out.

I will try to give a summary of the two sides who tl;dr'd the above links (but seriously, they explain things better than I will).

(Spoilered for length)


So, for real-world lasers, the output energy at the target is basically how much energy leaves the laser. It's the spot size--the area over which this energy is deposited--that matters. Smaller spot size=more energy per unit area=more hurt on the target. Spot size, then, depends on the wavelength of the laser, the size of the focusing lens or mirror, and the range to target. So, this implies laser-ships with the shortest possible wavelength lasers (usually short-visible or UV for near-future) and one big honkin' lens rather than a bunch of smaller emitters. That lets the laser-ship start zapping at the maximum range, and lasers are BIG, anyway. Get one heavy-duty one and it'd be more efficient than a bunch of piddly lasers.

Now, this big honkin' lens can be used to zap things. It can also be used as a scanner, with a pixel size about equal to its spot size at the desired range. Which means the laser-ship can find things at longer ranges. Little things. Things like missiles. And whatever it can see through it's lens, it can zap.

So, in addition to zapping enemy vessels, it can zap enemy missiles--so it also serves as the Biggest Point Defense System Ever (TM). And with a maximum missile-zapping range of "ridiculous," the laser-crowd maintains that their ships can zap as many missiles as an enemy can put up. And while a missile can only be fired once, a laser can keep zapping as long as it has power. So once that evil missile ship has dry magazines, all it can do is run from a still-zapping laser ship. And while the laser can be overwhelmed, it wouldn't be cost effective. Not only do you need an absurd number of missiles, but also fuel to get those missiles to the battlefield. And fuel to get fuel for the missiles there. And fuel to get that fuel there (which is why payloads of rocket spaceships decrease exponentially as the fuel requirements go up). And at that point, even if you do kill the laser-ship, what have you gained? You'd have been better off just building a laser-ship or two of your own.


As the laser argument is centered around shooting down droves of missiles, so the missile argument is centered around overwhelming the ability of the laser to shoot down those missiles.

By missiles, we probably mean either smart, powered kinetic shells or dumb, unpowered ones. At interplanetary speeds, the kinetic energy contained in a simple lump of metal makes any warhead--even a mighty nuclear one--redundant. So the missile-ship will start cranking off hordes of missiles and shells toward the laser-ship. And any laser, no matter how powerful, can be overwhelmed.

A salvo of missiles with sufficiently robust drives can spread out a bit before converging back on the ship from multiple directions. It's limited by the iron laws of Newtonian motion (there ain't no friction in space), but even a second or two traverse time could be lethal. A sizable projectile impacting at these kind of velocities is enough to mission-kill the target, almost regardless of any practical armor it might carry. And, that big honkin' lens will be heavier than a smaller one, which means it will traverse more slowly.

Another trick the missile crowd keeps up their sleeve is the missile bus. So a laser-ship thinks it's got a salvo of ten missiles incoming, until (surprise!) each of those missiles splits into ten mini-missiles. So now it has 100 incoming missiles to zap in the same amount of time. And as I said before, it doesn't take much mass at these speeds to do a whole world of hurt on a target. The bus is a cheap way of getting even more little bundles of death hurtling at your enemy.

The missile crowd claims a few more advantages. Missiles can follow their target; a laser can't. Sure, electronic warfare, flares, and the like might sucker off a few missiles, but at a sufficient range small "jinks" and random thrusts can make a laser beam useless. Laser optics are more fragile than missile launchers--and while a laser-ship would do better with one (or two) really big emitters, a missile ship ought to have as many launchers as possible to saturate enemy point defense, so losing one launcher does NOT mean you lose your entire offensive (and defensive) punch. Probably the most damning, though, is the question of heat. Lasers generate a LOT of waste heat. More than missile launchers, so the missile crowd says. And while heat sinks are an option, they can only last so long. And heat radiators are nice, big, unarmored targets just asking to be shot off.

So, according to the missile crowd, a missile-ship can practically overwhelm a laser-ship's ability to defend itself, with a bit of ingenuity.


Now, playgrounders, you've heard the arguments. I'll keep my own thoughts out of it for the moment, but I invite you to think about it, pick a side, and go at it!

Jallorn
2014-05-13, 01:05 AM
An important question: are we talking space ships, or something inside the atmosphere? Because those change the dynamics extensively. Consider that the range of combat in space is likely way way way more than anything possible on a planet, any planet, simply by being bigger than those planets, let alone the atmosphere, curvature, et all getting in the way.

With that in mind, space combat becomes basically rocket tag, possibly literally. Also, at that distance, being able to detect the enemy is vital, and difficult. Space is big, and the bigger the ranges you're dealing with, the more volume in which the enemy might be. Plus, if we're dealing with even low relativistic speeds, the need for effective sensors becomes paramount because that spot you just scanned? They could be there now.

Plus, any scanning moves at light speed at best, which means that in the time it takes you to scan an area and process the data, you could have already been hit, especially when dealing with a laser moving at light speed.

I think there's a few important possibilities: ships will likely have passive sensors so they know when they've been pinged by an active sensor, making laser weapons harder to aim without alerting the enemy to your presence. However, if you can pick them up on passive sensors, and have really good aiming, your laser will be undetected until it hits them.

Passive missiles are similarly hard to detect, since they aren't giving off heat, you need active sensors to detect them. Also, at relativistic speeds, it's unlikely that a fuel pulse is going to be very effective at changing the missile's course, barring some form of acceleration he have yet to discover. It also might not take that many missiles to overwhelm a point defense system, since it takes a lot of energy to deflect a relativistic projectile. In fact, you're better off dodging than spending your energy deflecting projectiles. Using the laser as a point defense is probably not efficient.

Ailurus
2014-05-13, 08:27 AM
With near-future technology, I'd say the answer is orange (non-explosive, unguided kinetic weapons, railguns and the like).

With lasers, there are two main problems - heat generation and power concerns. Heat generation is probably the larger issue, since in space there's nowhere for the heat to dissipate. And if you throw in lots of additional heat generation, you need to add lots of additional cooling in order to keep your ship functional and crew alive (especially since many diode-pumped lasers, which is where we're currently finding the best laser results, tend to be very sensitive to heat fluctuations. And your gun shutting down after a couple shots is not something you want) Additionally, even if we assume we can get cold fusion (or some similar small, high-powered, long-duration power source), lasers still have power concerns other weapons don't since they need to have long-duration shots in order to serve a non-point-defense role. Firing off most other weapons requires a brief, large-capacity burst of energy, which we know how to deal with (batteries and capacitors). But, we can't rely on them for lasers, since they need continuous energy input (and possibly obscene amounts of energy, if the 5 minutes of continuous fire one of your links talks about is to be believed). So, either you need to shut down whole sections of your ship to shoot the lasers (impractical at best), or you need to ship out with a large amount of extra power generation (which means a more expensive vehicle, and one that's harder to orbit and deorbit due to the extra weight to haul around).

Missiles (and this is why I created the orange category) also have problems. First, they lose their big advantage of accuracy with the ranges we're talking in outer space. Trying to pull off any type of rapid course-correction in outer space is both difficult and expensive, so shipping missiles out will require large amounts of extra fuel (see the power generation waste with lasers) for minimal benefit. And all the fancy missile guidance systems while nice are very very expensive, so it would be an expensive waste if we brought them along. Also, while explosive payloads are nice in an atmosphere, they're significantly less-nice in space. First, they're not strictly needed - if you knock a small hole in a building, no big deal. But if you knock a small hole in a spaceship, significantly larger deal. Second, by exploding you're blasting most of your payload off into outer space and you don't have the benefit of atmosphere to help magnify the concussive wave. And, finally, explosive payloads make it that much easier for your shot to get stopped before reaching the target, whether it gets shot by a laser or happens to bump into a micrometeorite or something similar in the way (and also leads to possible issues of missile fratricide depending on how many you're launching and how close they are).

Which leads to non-explosive, unguided weapons. Heat issues still exist (modern railguns, at least, still produce lots of waste heat), but the power concerns are less since slower charging capacitors and batteries can handle the power spikes. The ammunition is much cheaper, easier to store, and safer to store (the current railguns use $25000 slugs, versus missiles costing upwards of a million dollars). And despite what one of your links says, non-explosive ordinance doesn't have much to fear from laser point defense. Sure, the impact will somewhat less damaging than it was before, since the mass is less, but unless you burn away nearly all of the projectile (or deflect it)... to quote Mass Effect "Sir Issac Newton is the deadliest son of a ***** in space." (The US Navy's current railguns can send 18lb projectiles at 5000 mi/hr. Even if you can burn away 90% of the mass with a laser, its still roughly equivalent of an anti-tank gun slamming into your ship. And, again, that's current tech. Going a decade or two ahead you can probably at least multiply the speed by a factor of 10 which means you've got battleship shells hitting your spaceship)

The biggest issue with railguns would be accuracy, but even that is mitigated a good deal by the fact that you can't make (controlled) sharp maneuvers in space. So, the net effectiveness comes down to the speed increases in railgun ammo versus the maneuverability increases in ships, and also the size of the ship (not only are bigger ships easier targets, but they also maneuver even more slowly) . Any modern computer would be capable of predicting where a ship will be in 10, 20, etc. seconds if it stays on its current course, so your chance of survival is directly proportional to how fast you can get out of the way. The flipside of that, though, is that (at least with current technology) even if you did maneuver out of the way you're likely burning up huge amounts of fuel.

Yora
2014-05-13, 09:53 AM
Railguns, just as lasers, suffer from time delay in space. At the speed at which spacecraft move, even fractions of a second in timing can make a difference between hit and miss. Missiles have the huge advantage that they can adjust their course as they get close to the target.

I just had an awesome idea: A frictionless mass driver (like a coilgun), which shots an inactive missile, and when the missiles internal timer indicates close proximity to the target point, it starts is sensors and engines to make the course adjustment for the final split second, to guide it into the targer. Should be very hard to detect or jam.

noparlpf
2014-05-13, 11:40 AM
Pretty sure kinetic weapons will continue to dominate for the foreseeable future.

-Easy, cheap space travel is a ways off. So I'm going to stick to on-Earth combat.
-Last I heard (a few years ago, admittedly) the most compact high-powered lasers still require energy packs roughly the size of a large microwave. That's not going to become portable anytime soon, barring a Tony Stark-esque breakthrough in power sources. Granted, these lasers can shoot down missiles and such, but they won't become portable weapons.
-A low-powered hand-held laser is less likely to cause lethal damage than a handgun. Burn wounds or even neat, clean, cauterised punctures aren't as bad as gaping, messy, bleeding holes.
-Lasers will probably require recharge and cooldown times. A handgun has multiple shots per clip, and reloading takes mere seconds.
-Missiles can course-correct mid-flight. Lasers just go in a straight line, even if they get where they're aimed faster once you fire. Missiles can also, as mentioned, split into multiple smaller missiles.
-Missiles can explode once they get where they're going. I imagine a large explosion (possibly including EM pulses to disable the other side's lasers :smalltongue:) would be more damaging than a neat little burn hole. (A laser might be more specific, though, if you have one target and want to avoid collateral damage.)
-Guns are cheap. Handheld lasers will be very expensive.

the_druid_droid
2014-05-13, 11:47 AM
I think I'm going to have to side with an earlier post and vote orange, although I favor a slightly different shade of the color.

See, the problem for me is that we're thinking about the upper limits of current technology, but we seem to be talking about proper deep space battles (of a Star Wars/Star Trek/Battlestar variety). The issue with this is partly technological, but largely economic - essentially, in order to fight a war in outer space, we have to have a motivation to fight it, and a reason to fight it there rather than on the ground.

Honestly, keeping to the probable upper bound of current tech, I don't see a clear path to making space profitable or habitable enough to want to fight over on that kind of scale. Basically, gravity is too expensive as far as logistics goes, and building the infrastructure to get around those costs is a pretty huge investment so it's going to be a slow-growing industry, if it grows at all. As a result, I can't see interplanetary battles being all that common in any reasonably near- or mid-future world.

However, we are currently seeing a rise in the number of individuals and nations with orbital capability, so I think it's reasonable to suppose that if we fight a war in space in the near future, it will be an orbital one (and if we're very lucky, maybe it'll be in orbit around some other planet) rather than a proper interplanetary one (this also follows from considerations about travel times in the absence of near-luminal transit capability).

My own pet weapon for that kind of war is actually information - essentially, if you know where the enemy is orbiting, the easiest thing to do is simply have one of your weapons satellites release a cloud of small metal beads, with a center-of-mass intercept course for the enemy ship. It's almost impossible to detect, and if you choose wisely, the relative velocities can be on the order of kilometers per second. There might potentially be issues with spread, and it has the downside that you can't clean up after it very easily, so it hurts you in the long run, but if you absolutely have to destroy something in orbit, I think you'd be hard-pressed to find a more energy and cost effective option.

shawnhcorey
2014-05-13, 04:09 PM
How about puce? David Weber, in his Honorverse, has missiles carrying nuclear-pumped X-ray lasers (http://honorverse.wikia.com/wiki/Missile). The missiles get close to their target, aim, and detonate, releasing high-power X-ray beams at the target.

The Random NPC
2014-05-13, 07:46 PM
Don't forget lasers also have blooming (http://en.wikipedia.org/wiki/Blooming_%28laser%29) issues. It's less of a problem in space, but missing a lethal shot because some sand got between you and your target would suck.

Max™
2014-05-13, 08:03 PM
Free-Electron-Lasers for short range/point defense and regular UV/soft X-ray laser powered kinetic buses for long range with full on self-powered kinetic vehicles for excessively long range or planetary bombardment.

shawnhcorey
2014-05-13, 08:19 PM
Free-Electron-Lasers for short range/point defense and regular UV/soft X-ray laser powered kinetic buses for long range with full on self-powered kinetic vehicles for excessively long range or planetary bombardment.

Aren't "self-powered kinetic vehicles" called missiles?

Max™
2014-05-13, 08:41 PM
Aren't "self-powered kinetic vehicles" called missiles?
I generally see a distinction made based on what is being delivered, missiles deliver warheads, kinetic kill vehicles are the warhead, and past a certain point of technological development there is no point putting a warhead on a weapon assuming you can get it going fast enough.

Lasers have the benefit of instant acceleration, making them ideal for short range work, any laser will always beat any missile across any distance, making the actual challenges ones of being able to burn/deflect/discourage the missile and being able to aim the laser quickly enough.

Over long enough distances you would be better served using a laser to push a hunk of metal as fast as you can get it going and simply use that to actually impart the energy into your target.

Having a hunk of tungsten ramped up to high velocities with a laser before any final target correction takes place is a better use of the mass and energy involved. Once you get it within range of a target you could do something silly like toss out casaba-howitzer nukes to provide last minute course adjustments and the bonus of "hiding" the actual payload in a cover of annoyingly bright nuclear flashlights.

Mr. Mask
2014-05-13, 09:26 PM
Someone mentioned you may not see space wars, since there isn't likely to be resources we'd fight over. As they alluded to, however, I think space fighting is very likely in the future for one reason: Orbital Bombardment. Space superiority is a lot scarier than air superiority.

If point-defence really improves, you'll be able to shoot down enemy nukes launches from the ground before they hit your own country. Even with improves point-defence, it's quite likely you'll be able to bombard enemy nations effectively from space (even if not, space vehicles and upper atmospheric vehicles will see increasing usefulness in battle). To prevent one nation becoming master over the others, major powers will develop technology for war in outerspace.


So, in the end, I think upper-atmospheric warfare will be an important point to consider in the future, but I don't think outerspace warfare will be irrelevant.


To half-answer with pink, also on the subject of upper-atmosphere warfare... I wonder if weapons that make use of the earth's magnetosphere might see use.

shawnhcorey
2014-05-14, 08:36 AM
So, in the end, I think upper-atmospheric warfare will be an important point to consider in the future, but I don't think outerspace warfare will be irrelevant.

I think cruise missiles and drones could even the odds.

Chen
2014-05-14, 10:15 AM
It always seemed to me like missiles would have huge problems with fuel. Every course correction you'd need to do would use up valuable fuel. The more fuel you include, the bigger the missile has to be which just requires even more fuel for each course correction.

You'd also need to include some sort of sensors to actually determine how to make your course corrections, or some communication between the main ship and your missiles.

If you're going kinetic, seems like the most effective way would be a shotgun type approach. Many little mass drivers that launch small projectiles in clouds at the enemy. Make the entire area dangerous for the ship to travel through so that they have to waste fuel with large course corrections or run into very difficult to detect "clouds" of metal moving at extremely high speeds (especially relative speeds if they're moving into them). I imagine you'd want one or two big mass drivers that could send a slug at higher relativistic velocities at something too, for when you're surprising them. Detecting a ship is easy due to its waste heat. Detecting a fast moving slug of metal is going to be significantly harder. If they don't know to make a course correction its going to be a big problem for them.

rs2excelsior
2014-05-14, 12:01 PM
Wow, some good responses. Now that I actually have time to read through them and post a reply:


With that in mind, space combat becomes basically rocket tag, possibly literally. Also, at that distance, being able to detect the enemy is vital, and difficult. Space is big, and the bigger the ranges you're dealing with, the more volume in which the enemy might be. Plus, if we're dealing with even low relativistic speeds, the need for effective sensors becomes paramount because that spot you just scanned? They could be there now.

Plus, any scanning moves at light speed at best, which means that in the time it takes you to scan an area and process the data, you could have already been hit, especially when dealing with a laser moving at light speed.

I think there's a few important possibilities: ships will likely have passive sensors so they know when they've been pinged by an active sensor, making laser weapons harder to aim without alerting the enemy to your presence. However, if you can pick them up on passive sensors, and have really good aiming, your laser will be undetected until it hits them.

Passive missiles are similarly hard to detect, since they aren't giving off heat, you need active sensors to detect them. Also, at relativistic speeds, it's unlikely that a fuel pulse is going to be very effective at changing the missile's course, barring some form of acceleration he have yet to discover. It also might not take that many missiles to overwhelm a point defense system, since it takes a lot of energy to deflect a relativistic projectile. In fact, you're better off dodging than spending your energy deflecting projectiles. Using the laser as a point defense is probably not efficient.

There are two competing issues here. One, space is big. Two, spaceships are very, VERY bright. And the background is very dark. Spaceships give off a lot of heat, given that if it matches the ambient radiation of about 3 Kelvin the crew won't survive the experience (nor will much of the equipment). Especially when it makes a burn, the plume will be visible from a LONG way away. Like a nearby star system for a high-powered drive. Practically, actual detection ranges won't be this long, but still quite respectable. What this means is that tactical surprise is impossible in space
(tactical and strategic misdirection is a different story).

The point about lasers is a good one. With light-speed detection apparatus, by the time you see a laser fire, it's already missed (or you've been fried). But when engaging a laser-armed ship, you can keep up a sequence of random small jinks to throw off fire they may be sending your way. Which, of course, burns fuel.

Concerning laser point defense, the amount of energy required to get something up to speeds that count as "hypervelocity" is quite high, enough so that it's probably out of reach of near-future tech. But that's semantics. Against a guided missile, you don't really need to deflect it. You just need to burn the sensors or engine (which are probably rather fragile) and then dodge a bit. Against unguided missiles... I'll get to that in a minute.


With near-future technology, I'd say the answer is orange (non-explosive, unguided kinetic weapons, railguns and the like).

Generally, unguided and unpowered kinetics fall under the "green" side of this debate. I used the term "missile" because unguided weapons have some issues of their own.


With lasers, there are two main problems - heat generation and power concerns.

All very good points. Lasers will be power-hogs, especially if their efficiency cannot be improved. A series of rapid pulses could potentially drill better than a continuous burst and use a bit less energy, but these concerns would still hold.


Missiles (and this is why I created the orange category) also have problems.

Certain types of thrusters do have the acceleration required. Chemical fuel would possibly be the best bet. It'd need a good boost to start with and would burn fuel like nobody's business, but it'd have the acceleration required. And a missile isn't limited to the 2g or so maximum acceleration of a human-crewed spaceship.

Also, missile doesn't necessarily mean warhead. The missiles I was talking about were still kinetic weapons, just ones that could chase a target down. A nuclear warhead gives the missile a bit of standoff range, but it still has to get close. By that point, you might as well just smack it into the enemy ship.


Which leads to non-explosive, unguided weapons.

Dumb-fire weapons do have their proponents. They have definite advantages over lasers or powered projectiles. They also combine many of the disadvantages of both. They cannot react to a target's maneuvers, but still travel slower than light, so they can actually be seen and dodged. They lack some of the responsiveness of lasers, and still require the ship to carry around ammunition. That being said, especially if they carry submunitions, they could potentially saturate the area enough that the target couldn't dodge. But space is a big place.

Also, you don't need to burn up a kinetic shell coming at your ship. A laser zapping it off-center starts vaporizing material, which acts like a thruster. That kicks the shell off course, and since it's unpowered and unguided, it can't get back on. Depending on your laser's power, it could only take a short zap to make the shell harmless.


-Easy, cheap space travel is a ways off. So I'm going to stick to on-Earth combat.

Combat in an atmosphere does change things. Atmosphere tends to scatter a laser more than it would in space. Between that and the horizon, the laser's range changes from "ridiculous" to "line of sight." Which can be rather close. In near-future ground combat, lasers as personal weapons are probably impractical. As heavy weapon emplacements, it could be more useful.


See, the problem for me is that we're thinking about the upper limits of current technology, but we seem to be talking about proper deep space battles (of a Star Wars/Star Trek/Battlestar variety). The issue with this is partly technological, but largely economic - essentially, in order to fight a war in outer space, we have to have a motivation to fight it, and a reason to fight it there rather than on the ground.

That's the thing. In order for sci-fi to have space combat, there have to be things in space worth fighting over. So there are things in space worth fighting over. It's one of those given assumptions--assuming that we do have plausible-future space combat, how will it be fought?


However, we are currently seeing a rise in the number of individuals and nations with orbital capability, so I think it's reasonable to suppose that if we fight a war in space in the near future, it will be an orbital one (and if we're very lucky, maybe it'll be in orbit around some other planet) rather than a proper interplanetary one (this also follows from considerations about travel times in the absence of near-luminal transit capability).

Orbital combat is a bit of a different beast. Ranges are shorter, sight lines can be obstructed by the planet, "orbital fighters" are more practical than "space fighters," and the space is a lot more cluttered.


My own pet weapon for that kind of war is actually information - essentially, if you know where the enemy is orbiting, the easiest thing to do is simply have one of your weapons satellites release a cloud of small metal beads, with a center-of-mass intercept course for the enemy ship. It's almost impossible to detect, and if you choose wisely, the relative velocities can be on the order of kilometers per second. There might potentially be issues with spread, and it has the downside that you can't clean up after it very easily, so it hurts you in the long run, but if you absolutely have to destroy something in orbit, I think you'd be hard-pressed to find a more energy and cost effective option.

These are actually quite lethal space weapons. Of course, you have to cross an enemy's orbit before they cross yours and drop mines in your path.


Don't forget lasers also have blooming (http://en.wikipedia.org/wiki/Blooming_%28laser%29) issues. It's less of a problem in space, but missing a lethal shot because some sand got between you and your target would suck.

The problem is getting enough sand between you and the high-powered laser to disrupt it enough to no longer be harmful. It's nearly impossible, and then you have the problem of getting hit by your own sand particles accelerated by the laser.


Over long enough distances you would be better served using a laser to push a hunk of metal as fast as you can get it going and simply use that to actually impart the energy into your target.

Having a hunk of tungsten ramped up to high velocities with a laser before any final target correction takes place is a better use of the mass and energy involved. Once you get it within range of a target you could do something silly like toss out casaba-howitzer nukes to provide last minute course adjustments and the bonus of "hiding" the actual payload in a cover of annoyingly bright nuclear flashlights.

This is an interesting idea. I don't think I've ever seen anyone suggest a laser-powered missile. The only problems I could see would be avoiding destroying your missile and the limited number of directions you can accelerate it (basically directly away from your ship). That, and it takes a LOT of power to get a piddly amount of acceleration from shooting a laser at something, so possibly not the best for a missile.

A Casaba-Howitzer would probably be better served aimed at the enemy than directing your missile. A jet of nuclear-driven molten death can ruin your enemy's entire day :)


So, in the end, I think upper-atmospheric warfare will be an important point to consider in the future, but I don't think outerspace warfare will be irrelevant.


To half-answer with pink, also on the subject of upper-atmosphere warfare... I wonder if weapons that make use of the earth's magnetosphere might see use.

Again, controlling the orbit of a planet can be bad for the defenders. Though the planet does have some significant advantages. Check this out: http://www.rocketpunk-manifesto.com/2009/06/space-warfare-i-gravity-well.html

A nuclear EMP can be quite devastating, and requires an earth-like magnetic field. It's very far from a precision weapon, though...


If you're going kinetic, seems like the most effective way would be a shotgun type approach. Many little mass drivers that launch small projectiles in clouds at the enemy. Make the entire area dangerous for the ship to travel through so that they have to waste fuel with large course corrections or run into very difficult to detect "clouds" of metal moving at extremely high speeds (especially relative speeds if they're moving into them). I imagine you'd want one or two big mass drivers that could send a slug at higher relativistic velocities at something too, for when you're surprising them. Detecting a ship is easy due to its waste heat. Detecting a fast moving slug of metal is going to be significantly harder. If they don't know to make a course correction its going to be a big problem for them.

A valid point. With enough warning time, though, a ship could dodge even a large spread of kinetics (which depends on your assumptions about drive capabilities). And, again, space is big. It takes a lot of impactors to fill up an area enough that you will hit the enemy ship. But it doesn't take much mass at interplanetary velocities to do a lot of damage, and one little shell can do a world of hurt on an enemy vessel.

AgentPaper
2014-05-14, 02:51 PM
Well, to know what weapons will be used, you have to think about who is fighting, why they're fighting, where they're fighting, and what they have available to them.

Assuming WW3 broke out sometime in the near future, space would likely become a battlefield as each side fought to keep up a GPS and communications network and deny their enemy the same. Actually sending people to space over this is unlikely given the cost, and instead any weapons sent up will be controlled remotely. Rockets will dominate here, simply because projectile weapons will be too slow. A round fired by a main gun from a tank, for example, would take at least 10 minutes to reach it's target, and doesn't have the accuracy to hit anything at that range. A rocket, on the other hand, can adjust itself as it gets closer, and it's explosion can hit it even if it doesn't score a direct hit.

The payload on these weapons is likely to be low, and might not even be explosive. Debris in space is a major concern, and the last thing you want to do is blow up all your enemy's satellites only to have the debris flying around hitting yours. An EMP pulse can fry a satellite into unusability just as well as an explosion, and doesn't leave thousands of tiny potential projectiles behind.

Lasers are another possibility, fired either from the ground or high-flying aircraft, but would be limited to things that flew over them. Satellites in Geosynchronous or highly elliptical orbits would be out of reach, at least until air superiority can be established.

Overall, this "space war" is unlikely to last very long. A matter of hours, maybe, most likely resulting in the destruction of all satellites on both sides. There may be a few "skirmishes" as one side or the other attempts to send up replacements, but these will be expensive and easy to destroy, so it may not even be attempted.


The next stage of space war is likely to be around asteroids, as competing forces fight to take or defend mining installations from each other. Here, missiles start to fail, because they're simply too slow and fragile. With the distances involved, even a simple radar system should be able to spot the rocket hours or even days before it hits the installation. Stealth technology (aka: fancy black paint) might help, but unless a significant advance happens, it won't be enough.

Any deep space installation will by necessity have a system set up to detect and destroy incoming projectiles, even if they don't expect to be attacked. If you make the missile small enough, it might slip under the radar, but at that point it's probably not powerful enough to hit the installation, which is likely well inside the asteroid under a few hundred to a few thousand meters of rock.

A projectile is actually better, since it can be smaller than a missile (doesn't have to hold any propellant), potentially even faster, and doesn't have to worry about it's target moving. But, it's probably still too slow and too weak to break through a large asteroid's surface.

Lasers are even less likely to penetrate the surface, however they would be the preferred method because their speed would allow the ship carrying it to shoot down any "freighter" ships bringing supplies to and material from the installation, essentially laying siege to it. Of course, the installation itself will likely have lasers to defend itself as well, both from attackers and from stray rocks, and they will likely be well-protected, with their power sources far underground and their turrets well-armored.

In this case, the space combat would probably look a lot like a medieval castle siege, with one side trying to starve out the other, without attacking them directly. This would also be ideal for the attackers since they could take the installation mostly in-tact. Missiles might also be used to destroy the installation quickly, if a siege is impossible (ie: the enemy can produce food on site, or it's entirely automated) or undesirable, but only after any point-defense laser systems have been taken out.

Max™
2014-05-16, 02:06 AM
This is an interesting idea. I don't think I've ever seen anyone suggest a laser-powered missile. The only problems I could see would be avoiding destroying your missile and the limited number of directions you can accelerate it (basically directly away from your ship). That, and it takes a LOT of power to get a piddly amount of acceleration from shooting a laser at something, so possibly not the best for a missile.
I've seen a couple of stories use some version of it, I know one of the civilizations in Singularity Sky (Charles Stross) used them.

At a long enough distance you will be able to keep from putting much if any propellant in the missile and don't have to worry about the rocket equation at all.

You could launch it out of a railgun and then keep a laser on it to push it as fast as you can before it hits, though you can tack against a laser and alter the trajectory to some extent after launch.


A Casaba-Howitzer would probably be better served aimed at the enemy than directing your missile. A jet of nuclear-driven molten death can ruin your enemy's entire day :)
They're very short range by comparison, and the amount of energy it would impart is overtaken once the missle gets up to a certain velocity, at which point each additional CH going off would just push the energy delivered to the target up that much more.

You could also ablate an incoming CH with a defensive laser array before it reached effect range. Ablating a hunk of laser and CH accelerated chunk of tungsten is a whole other game, and I can't imagine that looking directly at said chain of explosions would play nicely with sensor systems trying to lock on to the incoming projectile.

Incidentally, once the main projectile is close enough you could dump all the Casaba-Howitzers remaining and set them to fire just as the main body strikes.

rs2excelsior
2014-05-19, 09:58 PM
They're very short range by comparison, and the amount of energy it would impart is overtaken once the missle gets up to a certain velocity, at which point each additional CH going off would just push the energy delivered to the target up that much more.

That's the thing about energy. It doesn't really matter what form it's in. If you have a 100 MJ Casaba-Howitzer charge, setting it off will add 100 MJ of kinetic energy to the projectile it hits. At most. I can't see a jet of plasma being anywhere near 100% efficient at depositing its energy to something in the form of kinetic energy. It also requires that your projectile (which, as I understand it, is an inert hunk of metal) be able to withstand said jet of plasma. Which will probably be mass-intensive.


You could also ablate an incoming CH with a defensive laser array before it reached effect range. Ablating a hunk of laser and CH accelerated chunk of tungsten is a whole other game, and I can't imagine that looking directly at said chain of explosions would play nicely with sensor systems trying to lock on to the incoming projectile.

Incidentally, once the main projectile is close enough you could dump all the Casaba-Howitzers remaining and set them to fire just as the main body strikes.

So do you carry these nuclear charges within the missile, or launch them simultaneously? Either way, it's more mass you're launching, which means more energy. It's basically no different than any other fuel that the missile could carry. I do question it's efficiency, though. Although a kinetic weapon backed up by a salvo of Casaba-Howitzer charges does seem appealing, if perhaps overkill.

With regards to the vulnerability of CH charges, you could have an "ablation plate" over the material that you will accelerate to eat the laser fire before detaching to allow the charge to fire. Which adds extra mass, and doesn't help a bit if the enemy has lasers powerful enough to scrag your charges with the shield. It all depends on your assumptions.

Max™
2014-05-19, 11:18 PM
That's the thing about energy. It doesn't really matter what form it's in. If you have a 100 MJ Casaba-Howitzer charge, setting it off will add 100 MJ of kinetic energy to the projectile it hits. At most. I can't see a jet of plasma being anywhere near 100% efficient at depositing its energy to something in the form of kinetic energy. It also requires that your projectile (which, as I understand it, is an inert hunk of metal) be able to withstand said jet of plasma. Which will probably be mass-intensive.
The efficiency of a CH for propulsion is vastly improved compared to a spherical detonation, which was the reason it was invented.

The projectile doesn't have to worry about shock absorbing so you can just put doubled purpose pusher plate to accelerate with the laser mechanism and then use CH charges tossed behind it to do last minute course changes.



So do you carry these nuclear charges within the missile, or launch them simultaneously? Either way, it's more mass you're launching, which means more energy. It's basically no different than any other fuel that the missile could carry. I do question it's efficiency, though. Although a kinetic weapon backed up by a salvo of Casaba-Howitzer charges does seem appealing, if perhaps overkill.
The fuel carried by the missile is on the ship that launched it, as it was accelerated with a laser.

The CH idea was to counter the need for angular correction closer to the target.


With regards to the vulnerability of CH charges, you could have an "ablation plate" over the material that you will accelerate to eat the laser fire before detaching to allow the charge to fire. Which adds extra mass, and doesn't help a bit if the enemy has lasers powerful enough to scrag your charges with the shield. It all depends on your assumptions.
Well, the tungsten "bus" carrying the CH charges kinda fulfills that role, except in a far better fashion than a shield would, getting rid of several meters of tungsten with a laser is going to be pretty damn difficult.

Radar
2014-05-20, 09:47 AM
I might have missed this in the discussion, but I didn't see anyone mentioning diffraction, when it comes to lasers. It makes them next to useless at large ranges and it's the very reason the kinetic projectiles are much more efficient.

The choice between guided or unguided projectiles mainly depends on the maneuverability of space ships we want to slug. That in turn depends on the expected ranges of said spaceships and the power source used. With current technology any spaceship (even those that would be launched from orbit) would have very little thrust to spare on dodging. If they were build for that, they would in turn have to haul excessive ammount of fuel, which in turn would make them bigger and less then nimble.

At any rate, both types of projectiles can be knocked down by lasers at short ranges (to make it effective for relativistic slugs, you would need a squardon of ships spread in a wide area in order to push the projectile away at an angle instead of straight-on). This means, that whatever we shoot, it should bring pain at distances larger then regular lasers. I see two candidates:

Nuclear-explosion-powered lasers if the energy conversion rate was reasonable and the range would give a fair chance of actually hitting the target. Such constructions have a decent chance of having a larger effective range then the reusable on-ship lasers due to the excessive energy used in one shot.
Antimatter warheads either laced with a dense material to produce a plasma burst or relying on the gamma radiation alone. Like with any explosives, it's more effective to use clusters of smaller payloads then one big warhead. If any weapon can saturate a noticable area, it's this. Antimatter is the most effective source of pure energy there is with every gram of a balanced matter-antimatter mixture being an equivalent to about 21 MT.


Side note: the chaff used for orbital engagements can also be effective in outer space, when you are the one being attacked. It's not a big problem to put a lot of pretty much sand-filled bags around the area you want to defend leaving enough space for safe travel, but not enough when the bags are triggered to burst. Even better: such defences can be kept cold and it means, they are much harder to detect without active sensors and the effectiveness of those can be mitigated by regular stealth technology. It would still require a huge investment of resources to lay a properly large minefield in space, but it is possible to have one. Just be prepared, that once in a while a stray mine will go off-orbit due to slight gravitic interactions between them.

Max™
2014-05-20, 02:52 PM
I did bring up issues of diffraction when trying to justify my answer of "beam accelerated kinetic weaponry", beyond a certain range you need ridiculously large lenses to get sufficient energy delivery to consider something a weapon as opposed to a searchlight.

There are also ways to make use of deliberately ablating portions of a projectile to get very high specific impulse in the range of 1000 to 5000 s, this could also be used with an ablative fuel coated pusher plate and Casaba-Howitzers once you get far enough out that the laser acceleration tapers off or close enough that the instant boost of energy from a CH wins out.

Putting something on the front like a shiny aerogel needle tipped nosecone would make it very annoying to try to alter the trajectory/reduce the velocity/completely ablate an incoming projectile, especially one consisting of large quantities of dense metal with a very high melting point moving at stupid velocities. You gotta deal with the reflectivity at various wavelengths, then you gotta deal with the ridiculous heat capacity, and all you achieve is removing a tiny fraction of the incoming mass?

Regarding antimatter, you get ~43 kt by adding a gram of matter to a gram of antimatter.

You get ~132 kt by hitting something with 1 gram of anything at 0.99 c, or ~1.5 mt if you can ramp it up to 0.9999 c.

A kg at 0.75 c gives ~11 mt, 0.99 c gives... well, you get the idea.

Hitting stuff with antimatter is all well and good, but if you've got some room to work with, you'll get much better results using that antimatter as an energy source rather than a payload.

Radar
2014-05-21, 01:20 AM
Regarding antimatter, you get ~43 kt by adding a gram of matter to a gram of antimatter.

You get ~132 kt by hitting something with 1 gram of anything at 0.99 c, or ~1.5 mt if you can ramp it up to 0.9999 c.

A kg at 0.75 c gives ~11 mt, 0.99 c gives... well, you get the idea.
>.< I think I have mixed up grams with kilograms there. At any rate, it was not the point to reach the same payload as with kinetic projectiles, but to omit the problem of acute targeting by spreading the fun in an area.

As for defending against kinetic projectiles, slowing thm down or ablating is inefficient. You just have to change their direction a bit. A highly reflective surace would actually make it simplier. Granted, changing a path of a projectile going 0.99 c is a problem, but you really need little corrections to make it miss its target. If anything else fails, you can shoot your own kinetic projectile to bounce it off.

Max™
2014-05-21, 12:23 PM
Hitting something which is hauling along at damn near c with something energetic enough for it to notice is no simple task, one would think.

By the time you can see the projectile it's pretty much on top of you already.

Even if you did manage to hit an incoming relativistic projectile, you're changing it from a single object moving at a significant fraction of c... to a cloud of objects moving at a significant fraction of c? Still not fun to be in the way of.

Though, I'm trying to make sense of how relativistic aberration of light could alter the effectiveness of laser ablation and it makes my head want to turn inside out through my ears...

AgentPaper
2014-05-21, 01:18 PM
There's a lot of talk here about objects traveling at near the speed of light, but I think you're grossly underestimating how difficult it is to get something moving that fast, and not just because it takes a lot of energy. The faster it's moving, the less time you have to impact it, so even if you have a battery large enough to hold the required energy, you still need a gun big enough to fire it (http://en.wikipedia.org/wiki/Large_Hadron_Collider). On the other hand, a laser is (relatively) easy to make on a much smaller scale (http://en.wikipedia.org/wiki/Boeing_YAL-1), and always moves at the speed of light by definition, so you can have large and small lasers without issue.

And of course, then you have the issue of aiming the damn things, which again is far easier with lasers. And if you're shooting at something far enough away for diffraction to be an issue, then your .9c projectile is going to fare even worse, as even the difference between .9c and 1c is enough to give your target ample time to move out of the way unless you're basically at point-blank range.

I could see mass drivers being used as a sort of "bunker buster" weapon for taking out stationary targets like planetary defenses or other heavily armored, but stationary targets built to resist normal laser attacks, but not as a primary weapon. If you have the energy to fling things around at relativistic speeds, missiles become basically another form of mass driver, a bullet without the barrel, so to speak, because by the time you speed up that much, you're not going to be able to affect your trajectory enough to truly "home in" on anything.

Radar
2014-05-21, 02:56 PM
Hitting something which is hauling along at damn near c with something energetic enough for it to notice is no simple task, one would think.

By the time you can see the projectile it's pretty much on top of you already.
It's mostly a matter of reaction speed. If accelerating a projectile takes time, then it does so for the enemy as well, which means, you see it that much earlier and you can counteract.


Even if you did manage to hit an incoming relativistic projectile, you're changing it from a single object moving at a significant fraction of c... to a cloud of objects moving at a significant fraction of c? Still not fun to be in the way of.
That's a more difficult question, but the resulting could should disperse in many directions, which lessens the resulting impact and if you hit with something of a similar energy to the original projectile I can venture a guess, that most of the cloud would go in a direction different to the original one. Still, those are just barely educated guesses based on my general understanding of physics.


Though, I'm trying to make sense of how relativistic aberration of light could alter the effectiveness of laser ablation and it makes my head want to turn inside out through my ears...
Which means, your're doing relativity right. :smalltongue:

@ AgentPaper
And lasers are superior on short ranges, but the dispersion makes them less and less effective the further you try to shoot. Kinetic projectiles conserve the energy in one piece, which makes them better suited for hitting far-off targets.

AgentPaper
2014-05-21, 03:32 PM
@ AgentPaper
And lasers are superior on short ranges, but the dispersion makes them less and less effective the further you try to shoot. Kinetic projectiles conserve the energy in one piece, which makes them better suited for hitting far-off targets.

What constitutes "short range" in this context? Because as I said, even at very close to the speed of light, if you're far enough away you'll still have ample warning to get out of the way.

As a simple example, say you're near Earth and trying to shoot someone near Mars. The two planets happen to be close to each other, only 60,000 Mm away. You fire off your Big Gun, which accelerates your 1kg projectile to .9c. Since the speed of light is 300 Mm/s, the light from you firing your gun takes 200 seconds to reach the target. Since your projectile is traveling at 90% of that speed, it impacts 20 seconds later. Unless your target is either very large or not actively looking out for incoming projectiles, 20 seconds is more than enough time for it to thrust and move a few feet to the left, so to speak.

I guess what I'm saying is, Lasers might be limited in range, but mass-driver projectiles are too, though in a different way. You can compensate by speeding the projectile up, for example if you had fired your round at .99c instead of .9c, that would give the target only 2 seconds instead of 20, which would be far more difficult to evade. But then, why aren't you using a laser? Would it really lose that much power over those distances? Maybe I'm underestimating diffraction in space, but it seems like if you could get enough energy to fire off your .99c railgun, you could have just used a fraction of that energy to fire a laser powerful enough to overcome the losses and still destroy the target.

Radar
2014-05-21, 04:30 PM
The dispersion angle can be more or less approximated with arcsin(l/d), where l is the wavelength and d is the aperture diameter. So we start with a spot of width d and after a distance x, we have a spot of width 2x*tn(arcsin(l/d))+d, which for small angles can be approximated by 2xl/d. For the given distance 6*10^10 m and for a 47*10^(-9) m laser with an aperture of 10 m diameter, it would give a spot 292 m in diameter. An aperture of 50 m width would give a spot 106 m wide, which still means a 4 times lower power density. At any rate, spreading the beam in such a wide area is far from optimal. We might do better with other light sources, to achieve shorter wavelengths, but those won't be as high-powered as lasers.

The Glyphstone
2014-05-21, 07:22 PM
I could see mass drivers being used as a sort of "bunker buster" weapon for taking out stationary targets like planetary defenses or other heavily armored, but stationary targets built to resist normal laser attacks, but not as a primary weapon. If you have the energy to fling things around at relativistic speeds, missiles become basically another form of mass driver, a bullet without the barrel, so to speak, because by the time you speed up that much, you're not going to be able to affect your trajectory enough to truly "home in" on anything.

If you've got access to both, the laser would be preferable for the planetary defenses. Against a laser attack, your best defense is going to be rapid rotation combined with ablative armor, spreading the heat such that no one point on your hull suffers critical damage before it cools (obviously, this is dependent on ship size/rotation speed versus laser energy, but the theory remains). A fixed defense like an orbital space station or satellite likely won't be capable of rotating in such a manner. Meanwhile, your ship would have its laser emitters spread out around the hull so that you could continue output on a targeted point while rotating to defend against inbound lasers (though if they're smart, your enemy would too).

A ballistic projectile is damage-on-contact, not damage-over-time, so it'd be slightly more suitable against a defensively rotating target if you can deliver the hit. Assuming lightspeed communication, accuracy with both lasers and ballistics would be predictive-based (guess where they'll be when your projectile hits and aim there), giving laser the advantage with a smaller time-lag between firing, but defensive random jinking would work against both ballistics and laser, while rapid rotation would only protect against laser.

Max™
2014-05-22, 05:43 AM
What constitutes "short range" in this context? Because as I said, even at very close to the speed of light, if you're far enough away you'll still have ample warning to get out of the way.

As a simple example, say you're near Earth and trying to shoot someone near Mars. The two planets happen to be close to each other, only 60,000 Mm away. You fire off your Big Gun, which accelerates your 1kg projectile to .9c. Since the speed of light is 300 Mm/s, the light from you firing your gun takes 200 seconds to reach the target. Since your projectile is traveling at 90% of that speed, it impacts 20 seconds later. Unless your target is either very large or not actively looking out for incoming projectiles, 20 seconds is more than enough time for it to thrust and move a few feet to the left, so to speak.

I guess what I'm saying is, Lasers might be limited in range, but mass-driver projectiles are too, though in a different way. You can compensate by speeding the projectile up, for example if you had fired your round at .99c instead of .9c, that would give the target only 2 seconds instead of 20, which would be far more difficult to evade. But then, why aren't you using a laser? Would it really lose that much power over those distances? Maybe I'm underestimating diffraction in space, but it seems like if you could get enough energy to fire off your .99c railgun, you could have just used a fraction of that energy to fire a laser powerful enough to overcome the losses and still destroy the target.

I was thinking of stuff like putting the sort of laser you're describing as the propulsion for a kinetic projectile, as it mixes ridiculously high specific impulse with the ability to devote all of the projectile weight towards being delivered to the target, rather than using a huge quantity of fuel to send a very small quantity of fast moving pain at something.

AgentPaper
2014-05-22, 10:29 AM
I was thinking of stuff like putting the sort of laser you're describing as the propulsion for a kinetic projectile, as it mixes ridiculously high specific impulse with the ability to devote all of the projectile weight towards being delivered to the target, rather than using a huge quantity of fuel to send a very small quantity of fast moving pain at something.

Do you mean firing the laser at the projectile in order to speed it up? Because if you mean using a giant laser as an oversized rocket engine, that doesn't sound like a very good idea. Sure, the specific impulse might be great, but it's going to take a helluva long time to speed up, so even if it is technically capable of hitting .9c eventually, unless it's target is in another universe I can't see that actually working.

And of course, if you're firing the laser at the projectile, that has it's own problems, most notably the inefficiency of ablated material flying in all directions, the challenge of making the projectile durable enough to withstand it, the challenge of making sure that it keeps going in the right direction, and of course making sure it speeds up quickly enough because firing your big laser for a long time is a good way to say, "Hey look, here I am trying to kill you from the other side of the solar system!"

rs2excelsior
2014-05-22, 04:29 PM
Hitting something which is hauling along at damn near c with something energetic enough for it to notice is no simple task, one would think.

By the time you can see the projectile it's pretty much on top of you already.

If you can get a projectile up to high-relativistic speeds, this is one of their biggest advantages. Aside from the ridiculous amount of energy, of course. The catch is, though, to give a projectile a ridiculous amount of energy, you have to expend a ridiculous amount of energy.


Do you mean firing the laser at the projectile in order to speed it up? Because if you mean using a giant laser as an oversized rocket engine, that doesn't sound like a very good idea. Sure, the specific impulse might be great, but it's going to take a helluva long time to speed up, so even if it is technically capable of hitting .9c eventually, unless it's target is in another universe I can't see that actually working.

Basically it's a photon drive. So yeah, the exhaust velocity is great (the speed of light, to be exact). The problem is they're energy hogs. It takes three hundred freaking megawatts (http://www.projectrho.com/public_html/rocket/enginelist.php#id--Other--Photon) to get a single Newton of thrust. So a 1 kg projectile being driven by a 300 MW laser would accelerate at 1m/s^2, and take 269820000 seconds to reach that speed (about eight and a half years) and it would cover 3.64x10^16 meters (over 3.8 light years). And that's not counting the dispersion of the laser beam as the distance increases. And a 300 MW laser is pretty powerful for near-future tech assumptions.


The dispersion angle can be more or less approximated with arcsin(l/d), where l is the wavelength and d is the aperture diameter. So we start with a spot of width d and after a distance x, we have a spot of width 2x*tn(arcsin(l/d))+d, which for small angles can be approximated by 2xl/d. For the given distance 6*10^10 m and for a 47*10^(-9) m laser with an aperture of 10 m diameter, it would give a spot 292 m in diameter. An aperture of 50 m width would give a spot 106 m wide, which still means a 4 times lower power density. At any rate, spreading the beam in such a wide area is far from optimal. We might do better with other light sources, to achieve shorter wavelengths, but those won't be as high-powered as lasers.

This is the biggest disadvantage of lasers and the biggest limit on their range. It's also the reason it's best to make your lasers in the UV (or shorter) range if you can.


If you've got access to both, the laser would be preferable for the planetary defenses. Against a laser attack, your best defense is going to be rapid rotation combined with ablative armor, spreading the heat such that no one point on your hull suffers critical damage before it cools (obviously, this is dependent on ship size/rotation speed versus laser energy, but the theory remains). A fixed defense like an orbital space station or satellite likely won't be capable of rotating in such a manner. Meanwhile, your ship would have its laser emitters spread out around the hull so that you could continue output on a targeted point while rotating to defend against inbound lasers (though if they're smart, your enemy would too).

A ballistic projectile is damage-on-contact, not damage-over-time, so it'd be slightly more suitable against a defensively rotating target if you can deliver the hit. Assuming lightspeed communication, accuracy with both lasers and ballistics would be predictive-based (guess where they'll be when your projectile hits and aim there), giving laser the advantage with a smaller time-lag between firing, but defensive random jinking would work against both ballistics and laser, while rapid rotation would only protect against laser.

These are actually some good points. So if your drives are good enough that your maneuvering thrusters can spin your ship around, it'd gives kinetics an advantage over lasers.

AgentPaper
2014-05-22, 07:42 PM
These are actually some good points. So if your drives are good enough that your maneuvering thrusters can spin your ship around, it'd gives kinetics an advantage over lasers.

Why spin your whole ship, or use thrusters? Simply have the outer hull not be attached to the rest of the ship, and spin that. Put the crew quarters in there too and bam, artificial gravity (assuming your ship is large enough).

Still, I can't help but think that kinetics would be at a severe disadvantage most of the time. How big of a ship do you need to make to have a mass driver like that, anyways? And how often could it fire? If you need a 10-kilometer long ship to house the gun, not to mention all the fuel required to move it around and house the crew that maintains it, then you're really talking about an orbital installment, not a combat-ready ship. Which is why, as I said, I suspect that mass-driver type tech would be a generally long-range kind of thing. You might have a few large "capital" ships that have them, specifically for fighting other large ships and emplacements, but you're also going to have a lot of smaller ships that use lasers instead, which would make up the bulk of your fleet. Hitting one of those that shoots off a small burst to change it's trajectory randomly every few minutes is going to be close to impossible, and probably a waste of time until the ships start to close in. The mass driver still might be able to get some hits in before the smaller ships get close enough to use their lasers, but with 100:1 odds that will still happen.

Another weapon I could see being used, would be a sort of "torpedo" type missile, which would be fairly small and use stealth tech to look even smaller, such that it's indistinguishable from the existing space debris, plus maybe a bunch of dummy debris that is launched alongside them to make sure there are too many targets to try and shoot them all down. These would travel fairly slowly after being launched, from a long ways away, and only after getting within a certain range would they activate, making a bee-line for the closest ship and blowing up near it. It doesn't need to speed up all too much if it can get within a few kilometers of the target, and while some would be shot down with automated point-defense lasers, with a large enough salvo or some good countermeasures like extra chaff or even more advanced stealth tech, you could get a few good hits in from quite a distance.

warty goblin
2014-05-22, 08:57 PM
Why spin your whole ship, or use thrusters? Simply have the outer hull not be attached to the rest of the ship, and spin that. Put the crew quarters in there too and bam, artificial gravity (assuming your ship is large enough).

If you want the ship to be able to move at all, it need some attachment between the outer hull and the inner, and keeping them connected but with a non-spinning core is probably complicated. Also the angular momentum would make maneuvering substantially more difficult; a big hunk of spinning armor makes a very nice gyroscope. As an additional thought, there may well be tactical advantage to choosing the axis of rotation on a situational basis, which would be either impossible or substantially more difficult with a freespinning outer hull design.


Still, I can't help but think that kinetics would be at a severe disadvantage most of the time. How big of a ship do you need to make to have a mass driver like that, anyways? And how often could it fire? If you need a 10-kilometer long ship to house the gun, not to mention all the fuel required to move it around and house the crew that maintains it, then you're really talking about an orbital installment, not a combat-ready ship. Which is why, as I said, I suspect that mass-driver type tech would be a generally long-range kind of thing. You might have a few large "capital" ships that have them, specifically for fighting other large ships and emplacements, but you're also going to have a lot of smaller ships that use lasers instead, which would make up the bulk of your fleet. Hitting one of those that shoots off a small burst to change it's trajectory randomly every few minutes is going to be close to impossible, and probably a waste of time until the ships start to close in. The mass driver still might be able to get some hits in before the smaller ships get close enough to use their lasers, but with 100:1 odds that will still happen.

The conservation of energy remains, so you need to liberate X joules of potential energy to get a projectile with X joules of energy. Actually it's worse than that due to efficiency, so you need to be able to liberate something like kX, k > 1. Which is to say you need a system capable of releasing a nuclear weapon's worth of energy in a very controlled fashion over an extremely short period of time. In conditions that involve getting shot at, this strikes me as a frankly dangerous state of affairs.

Conservation of momentum also is a solid pain in the ass. If your c-fractional slug has momentum X, then your ship is going to have momentum -X; in other words its going to be moving backwards. Not as fast as the slug to be sure, but it will be moving. In order to hold position you need to throw another -X momentum's worth of stuff out the back. Which is to say the ship's going to need to burn a lot of fuel in addition to shooting the slug, and somehow not melt from all the waste heat.

(I'm ignoring any relativistic effects here, since I'm not familiar with those physics, and rather suspect they don't make much difference here anyway. If they end up goofing things up, please let me know.)

Max™
2014-05-22, 11:26 PM
I was actually thinking of something like this (http://nsstc.uah.edu/essa/docs/iac/Bergstue-Grant-IAC-11.C4.8.2.x11495.pdf) to transfer energy (in pulses according to the paper, allowing you trade off spreading the generation over a large area/time and then focusing it so it arrives at a smaller area in an ultrashort duration "chirped" pulse) so you can ablate the target material in bursts and maximize the exhaust velocity.

Have a ship disperse a ring of satellites which each receive and aim a laser pulse at the projectile as it accelerates away, have the appropriate optics to receive and refocus the pulses onto the propellant, and apparently it is even possible to encrypt this arrangement so anyone trying to ride your beam without you knowing it could be detected and/or deactivate the transmission along that path.

AgentPaper
2014-05-22, 11:39 PM
If you want the ship to be able to move at all, it need some attachment between the outer hull and the inner, and keeping them connected but with a non-spinning core is probably complicated. Also the angular momentum would make maneuvering substantially more difficult; a big hunk of spinning armor makes a very nice gyroscope. As an additional thought, there may well be tactical advantage to choosing the axis of rotation on a situational basis, which would be either impossible or substantially more difficult with a freespinning outer hull design.

You know what's harder to turn than a spinning 80-ton living quarters/armor plating? A 160-ton whole spaceship. And you're always going to be spinning sideways, not end-over-end, because otherwise you're exposing your almost certainly vulnerable engines to enemy fire.


The conservation of energy remains, so you need to liberate X joules of potential energy to get a projectile with X joules of energy. Actually it's worse than that due to efficiency, so you need to be able to liberate something like kX, k > 1. Which is to say you need a system capable of releasing a nuclear weapon's worth of energy in a very controlled fashion over an extremely short period of time. In conditions that involve getting shot at, this strikes me as a frankly dangerous state of affairs.

Conservation of momentum also is a solid pain in the ass. If your c-fractional slug has momentum X, then your ship is going to have momentum -X; in other words its going to be moving backwards. Not as fast as the slug to be sure, but it will be moving. In order to hold position you need to throw another -X momentum's worth of stuff out the back. Which is to say the ship's going to need to burn a lot of fuel in addition to shooting the slug, and somehow not melt from all the waste heat.

(I'm ignoring any relativistic effects here, since I'm not familiar with those physics, and rather suspect they don't make much difference here anyway. If they end up goofing things up, please let me know.)

I doubt that this would be a significant factor, simply because the ship firing the projectile will have to be many magnitudes larger than the projectile itself in order to fire it, so any change in momentum should be negligible, or at least easily compensated for. How much heat is produced depends entirely on how exactly you're firing it, and how efficient that firing mechanism is, but again it doesn't seem like it would be a major issue compared to the others (namely, getting a projectile moving that fast in the first place).

Max™
2014-05-22, 11:55 PM
Chirped pulsing and things like distributed arrays for focusing systems get around a lot of both of those problems, the thrust imparted on the ring of satellites is going to be minor, and the actual "oomph" is going to come from heating and vaporizing the materials chosen/designed for their properties on the projectile itself.

Taking the energy generated locally at the ship, spreading it out over a large area/period of time, and then focusing it into a big whack at one position.

warty goblin
2014-05-23, 12:02 AM
You know what's harder to turn than a spinning 80-ton living quarters/armor plating? A 160-ton whole spaceship. And you're always going to be spinning sideways, not end-over-end, because otherwise you're exposing your almost certainly vulnerable engines to enemy fire.

The gyroscopic effect would be worse, but you don't have to deal with issues involving the rotating section putting a hella lot of torque on the fixed section. That just seems like it could be asking for trouble.




I doubt that this would be a significant factor, simply because the ship firing the projectile will have to be many magnitudes larger than the projectile itself in order to fire it, so any change in momentum should be negligible, or at least easily compensated for. How much heat is produced depends entirely on how exactly you're firing it, and how efficient that firing mechanism is, but again it doesn't seem like it would be a major issue compared to the others (namely, getting a projectile moving that fast in the first place).
I think you're thinking of velocity here, not momentum. Velocity is not a conserved quantity, but but momentum (mass * velocity) is. As a single system, the ship and projectile have some momentum that for convenience can be considered zero - aka the ship is stationary. After firing the ship/bullet system will still have zero momentum, and it follows that whatever momentum has been imparted to the projectile, will also have been imparted to the ship, but along the opposite vector as the projectile. A sufficiently massive ship might have a particularly fast negative velocity, but it will have a negative velocity, specifically vship = -vbullet*massbullet/massship, ignoring relativistic effects. However I'm pretty sure accounting for the relativistic effects just makes the situation worse, not better, since the projectile is more massive thanks to its near light-speed velocity. Again, since momentum is conserved, the only way to push forwards again is by firing enough mass at enough velocity out of its engines to equal the momentum of the shot it took*. Now unless the engines are massively more efficient than the gun - in which case a sensible person would just use the engines as a gun - that's a non-trivial energy expenditure. Both the shot and the subsequent burn to return to heading are also going to create a lot of waste heat, not to mention the severe engineering difficulty in building a ship that can survive firing in the first place.


*Assuming the fuel burned is a negligible portion of the ship's mass. In reality it would not take quite as much, but I don't feel like integrating that out.

AgentPaper
2014-05-23, 12:32 AM
The gyroscopic effect would be worse, but you don't have to deal with issues involving the rotating section putting a hella lot of torque on the fixed section. That just seems like it could be asking for trouble.

Presumably the ship would be designed to handle that torque. Still seems like it would be less of an issue than having twice as much or more mass rotating around resisting further rotation. Just bring the outer section to a stop before you turn around if it's a big turn, which should be relatively easy since it's only a fraction of the ship's weight.


I think you're thinking of velocity here, not momentum.

No, I meant momentum. If you have a 1000 ton ship firing a 1-gram projectile at .9c, it would increase the velocity of the ship by approximately 0.3 m/s in the opposite direction. You'll have to compensate for it eventually, sure, but if moving your spaceship that much is a significant concern, then there's something seriously wrong with your spaceship.

Weight does increase exponentially as you get closer to the speed of light, but that's still not a concern unless you want to get really really close to the speed of light. At .9c, your projectile is about twice as heavy, and at .99c it's about 7 times as heavy, at .999c it's 22 times as heavy, and so on.

However, those aren't even as significant as they sound, since those are only the weights they are at those speeds, not while you're speeding it up to that point. It's much closer to "normal" weight most of the time. I'm sure there's a way to calculate that, but even if it did actually double the amount of momentum involved, that's still negligibly small amounts of momentum for the large ship.

Radar
2014-05-23, 12:59 AM
One thing to consider when it comes to weapon sizes: yes, mass drivers would be big, but so would weapon-grade lasers - keep in mind, that you need to first charge the capacitors big enough to store energy required for a single pulse (more effective then continuous fire), then you need to have the lasing medium and the resonanse chamber big enough to produce the required power output without vaporizing, cooling circuits, optics and so on and so forth. There are limits on the miniaturization.

Max™
2014-05-23, 02:09 AM
One thing to consider when it comes to weapon sizes: yes, mass drivers would be big, but so would weapon-grade lasers - keep in mind, that you need to first charge the capacitors big enough to store energy required for a single pulse (more effective then continuous fire), then you need to have the lasing medium and the resonanse chamber big enough to produce the required power output without vaporizing, cooling circuits, optics and so on and so forth. There are limits on the miniaturization.

This is a big part of why I keep coming back to using the lasers as a propulsion source, spreading them out is more effective, hence the array of drones/microships/satellites/whatever you wanna call them to refocus multiple smaller beams.

You can kinda cheat by cutting the delivery time of the beam energy, and you can cheat even more by creative arrangement of the emitters.

A given amount of energy delivered over 1 second will be far less effective than delivering it over 1 millisecond, or microsecond, or you get the idea.

https://upload.wikimedia.org/wikipedia/en/thumb/a/ae/Chirped_pulse_amplification.png/800px-Chirped_pulse_amplification.png
https://en.wikipedia.org/wiki/Chirped_pulse_amplification

AgentPaper
2014-05-23, 02:05 PM
One thing to consider when it comes to weapon sizes: yes, mass drivers would be big, but so would weapon-grade lasers - keep in mind, that you need to first charge the capacitors big enough to store energy required for a single pulse (more effective then continuous fire), then you need to have the lasing medium and the resonanse chamber big enough to produce the required power output without vaporizing, cooling circuits, optics and so on and so forth. There are limits on the miniaturization.

This isn't necessarily the case, though. The problem with mass drivers is that you can't really scale them down very well. Your projectile needs to be moving at close to the speed of light in order to hit anything, even if that much power isn't necessary to destroy the target. A laser, on the other hand, is relatively easy to scale up and down, simply by making more laser ships. Rather than have one large laser, you can build a large number of small laser ships that spread out and can easily switch between firing at many different targets at the same time or concentrate their fire on a single target.

To be clear, I'm talking about space combat using mostly modern-day technology, taken to their logical conclusion. Obviously there could be some breakthrough that makes the Large Hadron Collider look like a potato gun, but at that point there's not much point to discussion, since it's just a matter of "who can come up with the most pseudo-science words".

Ravens_cry
2014-05-23, 11:10 PM
Lasers don't scale down very well either, at least as a lethal kind of weapon. A blinder could be feasible, but something that kills people better a shaped piece of metal accelerated by the release of gas from the rapid breakdown of intermolecular bonds? Doubtful.

Livius
2014-05-23, 11:47 PM
The problem with mass drivers is that you can't really scale them down very well. Your projectile needs to be moving at close to the speed of light in order to hit anything, even if that much power isn't necessary to destroy the target.

That's only true for direct-fire weapons. Small, low-speed mass drivers are perfectly suited for indirect fire, and lasers can't ever do that.


Obviously there could be some breakthrough that makes the Large Hadron Collider look like a potato gun, but at that point there's not much point to discussion, since it's just a matter of "who can come up with the most pseudo-science words".

That wouldn't change very much.

Any direct-fire weapons all have the same few problems which limit their effectiveness.
1. Obstacles:
You can't shoot over or around things with direct fire. You may be able to shoot through things, depending on the exact circumstances, but in that case you're wasting lots of energy on things not your target.

2. Time delays:
The time delay is 2* distance/(speed of light) + your targeting lag.
It doesn't matter if you have a sight line on your targets if they are maneuvering and far away. Even something as close as 1.25 light-seconds away (rough earth to moon distance) has at least 2 seconds to choose how/if to dodge and if you guess wrong, you will miss by miles.

Ravens_cry
2014-05-24, 12:16 AM
Well, they have to know it's coming, and it's not possible to know a laser is coming before it hits, expect by, say, heat signature from the lasing device as it charges up. A mass driver has an even longer delay until impact, and good enough sensors *can* see it coming.

Livius
2014-05-24, 12:44 AM
Well, they have to know it's coming,

No, they don't. All they need is a reasonable suspicion that they are going to be shot at in the near-ish future.
How do you know they're there? If you use active scanners, they will know that you can see them and can react accordingly. If you use passive scanners, they can probably see you too. Also, if they know they are in hostile territory, they might just randomly dodge around on general principle.

Yes, you can bushwhack someone with a laser if you can see them and track them before they know you exist. But there are lots of ways for it to go wrong, and if it does you lose the ambush benefits.

Ravens_cry
2014-05-24, 01:03 AM
No, they don't. All they need is a reasonable suspicion that they are going to be shot at in the near-ish future.
How do you know they're there? If you use active scanners, they will know that you can see them and can react accordingly. If you use passive scanners, they can probably see you too. Also, if they know they are in hostile territory, they might just randomly dodge around on general principle.

If we are talking light second distances though, obviously we are in space and deaking around costs fuel, which is limited. Not saying they won't do it, but every manoeuvre costs. A long thin ship can move itself around the three axes without theoretically spending fuel, but, depending on beam width, that might not be enough.

Livius
2014-05-24, 06:30 PM
If we are talking light second distances though, obviously we are in space and deaking around costs fuel, which is limited. Not saying they won't do it, but every manoeuvre costs. A long thin ship can move itself around the three axes without theoretically spending fuel, but, depending on beam width, that might not be enough.

Sure, it costs fuel, but if you are too cautious with fuel use and never dodge, direct-fire from lasers or relativistic mass-drivers will make sure that your fuel usage isn't a problem anymore. Besides, there's always the bluff dodge, where your attackers think that you will react, so you dodge their fire by not reacting at all.

Ravens_cry
2014-05-25, 10:04 AM
Sure, it costs fuel, but if you are too cautious with fuel use and never dodge, direct-fire from lasers or relativistic mass-drivers will make sure that your fuel usage isn't a problem anymore. Besides, there's always the bluff dodge, where your attackers think that you will react, so you dodge their fire by not reacting at all.
True, but it doesn't change you can't see it coming and therefore you don't know where to dodge with lasers.
Relativistic mass drivers, unless we are talking really, really close to c you can see coming and dodge based on that. Heck, you don't even need active sensors, just watch the radiation from the impacts with the interplanetary medium. Bluff dodge is a dangerous game, as you can dodge in several directions, but only bluff dodge in place.

Radar
2014-05-25, 03:28 PM
True, but it doesn't change you can't see it coming and therefore you don't know where to dodge with lasers.
The only direction, that is relevant: not where you are right now. There are many place, where you could dodge to, so the enemy has to do the guessing game. There is a slight chance, that you and the enemy will take the same random choice, but it's not very big.

Ravens_cry
2014-05-25, 03:44 PM
The only direction, that is relevant: not where you are right now. There are many place, where you could dodge to, so the enemy has to do the guessing game. There is a slight chance, that you and the enemy will take the same random choice, but it's not very big.
Yeah, and if you decide to anti-dodge, they might decide you were anti-dodging and go for that shot. It's a big ol' guessing game with no real certainty.

Hexapuma
2014-06-27, 08:37 AM
An interesting thread! Let's see...

Near future... I'd probably go with missiles being in use to begin with, then mass drivers and finally energy weapons as we get further and further into the future. Missiles, after all, we can do now. What is a space missile but an Amraam or Sidewinder with directional jets instead of fins and a slight tweak to the software to teach it to stop turning using a balanced thrust rather than just turning the thrust off. Plus we can put all our energy into a bomb which is then carried to the target.

The downside of missiles is that they're fairly obvious with their reaction thrusters and they don't really travel that fast. They're much slower than lightspeed passive or active sensors. Plus they have to be fairly large which limits the number a ship can carry. Not to mention that we already have the CIWS anti-missile system which would make mincemeat of a missile in space and aside from the EMP effects of a nuclear detonation and the radiation spike, most of the effects of a nuke don't propagate well through space so you need a hit or a VERY near miss to kill something. An upside of missiles is that they can fly defensively and dodge as well as tracking an evading target and you can program them to track jammers and evading targets.

Obviously if something like Weber's bomb-pumped x-ray laser missiles became a reality along with ultra-fast drive systems then missiles get a lot more useful, but I think that's not a near future thing.

Relativistic velocity projectiles are dangerous and probably the logical successor to missiles. They can't track or dodge countermeasures but they're pretty stealthy, cheap, small and harder to stop. The downside is that they're still slower than lightspeed detection systems. A lot of people think that a ballistic or relativistic projectile is invisible to passive sensors but in fact it is trackable because of mass in space. Space is not empty. It's filled with dust and firing a slug out of a mass driver at, say, .1C will result in the slug impacting some particles of dust on its way to the target which will probably emit x-rays. If you assume an engagement range of in excess of ten light seconds then an x-ray spike on a particular bearing would indicate an incoming mass driver projctile and offer the chance of evasion (obviously you could fire out of the sun or similar tactics but that doesn't eliminate the flaw.

The downsides to this is the necessity of very precise aiming systems at that range since the projectile can't steer and the fact that a mass driver would require very tight station keeping systems as the firing would throw the attacking ship off course. At short ranges a mass driver with a high rate of fire would be an awesome weapon ship-to-ship.

The most distant future weapon would be directed energy. Mostly because of the problems in focusing at that range. Holding a laser down to a focused beam at 100km is pretty difficult and until we can produce something like a gravitational lens it's going to stay that way. The upsides, of course, are lower ammunition storage requirements and lack of detection by lightspeed sensors.

Space clutter is always a problem with lasers. Dust clouds will attenuate the beam and scatter it. Also a ship might not normally notice it was being shot at without space clutter. It'd be nice to be able to blaze away, knowing that your enemy won't even know you're there until you hit them but the scattering of the beam by a fragment of ice could give you away. Targeting with lasers is much like targeting with mass drivers. If you're not spot-on with your aim, you miss. Upside being that the laser won't push your ship off target with every shot.

Someone mentioned the difficulty of being stealthy in space. You actually can, it just requires some sneakiness. Ships radiate a lot of stuff like heat, but all you need to do is make sure it all radiates away from your target. A ship that radiates all waste heat and energy from its rear 180 degree arc and maintains a blackout from the front 180 degree arc would be functionally invisible to a ship in front of it. Then you have the old pilot tactic of attacking with your rear to a star hiding your emissions and presenting your black, non-emitting, non-reflective front to the enemy. Someone also mentioned exhaust plumes which is a good point, but remember ion-drives. They have minimal ejecta. You can reduce the plume by increasing the exhaust velocity and decreasing the mass.

On the topic of essentially space shotguns. Clouds of ball bearings flying at a ship to decrease the chance of a miss, those would be obvious to active and passive radar systems. How you say, the BBs are so tiny! True, but while they would be invisible to centimeter wave radar they would be obvious to old-fashioned meter-wave radar. The gaps between the BBs would be less than the wavelength of the signals so the cloud of shrapnel would show up a massive return. They'd also block and reflect ambient radio on those frequencies.

The upside of the shotgun effect, of course, you could create a blast so big that a ship would be unable to maneuver to avoid it even if it did detect it.

A thought on the heat generated by lasers and dealing with it in space... How about coupling the laser with a magazine that holds chilled blocks of metal or ice? When the laser fires, the heat is dumped into a block as a heat-sink which is then ejected like rifle brass. Taking the heat with it. It adds extra flaws to the laser system and removed a few advantages but it removes the overheating problem.

Phew! Ok, that's everything for now! I'm sure more crazy ideas will occur to me.