Constraints for "realistic" space battles

CynicalRyan · Sep 14, 2009

I am in the world building stage of the background for future WIPs (novels, short stories, games..).

In an attempt to make the universe as realistic as possible, and me having to break as few theories of physics as possible (I can't break the *laws* of physics, like the Laws of Thermodynamics), I have arrived at certain constraints, which I want to share in three posts: General constraints, propulsion constraints, and weapon constraints. These figure relatively high on Moh's Scale.

If possible, and / or desired, I'll cook up some half-way realistic ways to circumvent some of these constraints, without having Einstein rotate in his grave.

Caveat emptor: This list is based on my understanding of things, and I'm seriously lacking in advanced physics. This list is very probably very incomplete (I'm hitting the high notes, as it were, ignoring things like FTL, economics, and the maths behind the physics, just to name a few). So, proceed at your own risk.
This is also not an Imperial Decree telling you How To Write Your Novel Correctly. Or even at all.

Table of Contents
General constraints
Propulsion constraints
Weapon constraints

Going into medias res:

General Constraints

Detection range is vastly higher than engagement range
Everything is a weapon at non-trivial speeds (F = ma)
Battles have to be short and fast
The defense wins

Expanding and providing a rationale for these points:

(1) There is no stealth in space. The energy a space ship produces has to go somewhere, especially the excess energy (aka "heat"), since to power generator *or* consumer can be 100% effective, so a target can be seen very, very soon. In fact, as soon as the excess heat's radiation hits the passive sensors, the (past) location of any ship is known.
Additionally, due to the vast distances necessarily involved if you want to have at least some action in space, you are looking at the past (if a ship is merely one AU away from my sensors, it takes 8 seconds for any radiation to reach the sensors), and it is trivial to zig-zag in 3 dimensions (with the right propulsion systems). Ergo, you can't hit anything at long distances, even if the theoretical range of weapons is "unlimited".

(2) With speeds that enable story-telling in a space opera-ish universe, absolutely everything becomes a weapon, simply due to impact at high speeds. No need for asteroids, simply have you biggest ship accelerate as fast and as long as possible, and you have you own planet-killer, and that isn't even taking into account the propulsion system!

(3) Any interesting weapon system (except missiles) has an enormous energy hunger. But radiators for the excess heat produced in the process are big, fat targets. So, in the *ahem* heat of battle, radiators will be protected somehow, the easiest way is to limit their exposure to enemy fire. This means that a ship cannot "bleed off" all the energy it produces. So, to prevent the crew from slowly being roasted, ships, or even fleets engaging each other, will hit as fast and as hard as they can, before they have to disengage to get rid of the excess heat.

(4) Evey weapon has countermeasures. Some active, some passive. Kinetic weapons can be countered by Whipple shields. ECM and point defense takes care of missiles, and simply rotating makes lasers useless. Unless you put an obscene amount of energy into any weapon system (except missiles, which'll have to get smarter by themselves, or being slaved to a platform with bigger and better sensors), that is, in which case you run into constraints #3.

Next: Propulsion constraints

CynicalRyan · Sep 14, 2009

Propulsion Constraints

Reaction drives need fuel
Too much mass, and a ship will not be able to accelerate
Reaction mass is volatile
Reaction-less drives aren't
Powerful engines make for powerful weapons

(1) To get anywhere with thrusters, you need something to work against so you can be propelled in the first place. In short, you need reaction mass. You also need some sort of reaction to convert the "inert" energy into kinetic energy. However, fuel storage is a limited resources, competing with all the other necessities and bells and whistles of a starship.

(2) If a ship becomes too large, no matter how much energy you put into it, inertia will keep it stay put where it is (or accelerate so slowly, that you might not even bother). This puts limits on starship designs, and introduces a lot of trade offs. Look at the development of ships in the wet navies (cruisers, battlecruisers, battleships) to see the trade offs that need to be made.

(3) Unless you simply pump out dust, for example, to propel you forward, the reaction mass itself is volatile. That means it is dangerous to handle, and shouldn't be shot at, preferably. The more mass has to be moved, the higher the amount of energy you need, and, probably, the more volatile the fuel becomes, unless you increase the number of thrusters.

(4) Newton's Laws of Motion state that for every force, there exists an equal and opposite force acting against it. Thus even a reaction-less drive produces some sort of effect(s) that can be felt and measured.

(5) This is most obvious with reaction drives: The engines produce a lot of energy, and a lot is wasted in heat. That can make for interesting accidents if ships cross each other, limiting the angles of attack (you cannot attack the undefended area of the engine's exhaust, for example). Reaction less drives are more complex. But let's assume a ship moves by creating a gravitational well in the direction it wants to move to. The gravitational forces created to make a starship of considerable mass move would, very likely, destroy a ship that is too close to this gravitational well. And ships with such a drive near a planet will play havoc with tides and climate.

CynicalRyan · Sep 14, 2009

Weapon Constraints

Kinetic weapons need energy, and lots of it
Missiles are cheap, smart, dangerous. Pick two.
Lasers need time, however short it is, to "burn" through armor
Defenses are cheap

(1) To accelerate even a small projectile to a speed that turns it into a weapon, you need an enormous amount of energy and a bit of room to have the projectile accelerate. That puts a limit on "caliber" and terminal velocity for such a weapon, and, of course, is limited by the power a ship can produce.

(2) The more advanced the sensor package the missiles carry, the more expensive they are to produce. If they aren't smart, they are easily defended against with countermeasures, like ECM, or simple point defense. And, last but not least, the more explosives (or whatever is required to punch a hole into the enemy's ship) it carries, the more dangerous it is.
All of this has to fit into a smaller body than a starship's, if the ship is supposed to have endurance. Thus, unless the starship's propulsion system can be miniaturized (and cheaply enough produced) to fit into a missile, you are stuck with reaction drives, which are very limited in acceleration.

(3) Laser's deliver their payload by focusing beams on a very, very small target. Smaller even than a starship. That makes them difficult to aim (nothing technology can't overcome, however). But as high as the energy is that they deliver, they need time to "burn" through armor. Without going into too much detail, the armor has to be heated enough to vaporize, or at least to become brittle, and that layer for layer. The time needed is increased by armor thickness, and by presenting a larger target (for example by rotating the starship).

(4) Almost every spaceship comes pre-armored, for example. It will be compartmentalized to limit the effect accidents have, and simply bolting on an outer hull negates kinetic weapons (but not relativistic weapons). Putting the ship into a rotation is accomplished y the maneuvering thrusters the ship has anyway to be able to dock, and a white noise generator for almost all possible frequencies is easily installed. The distances of space make even small evasive maneuvers have a large effect, and make TV guided missiles an unsure bet at best.

Dommo · Sep 14, 2009

Check out the realistic space warfare thread. It's quite a few pages long and covers a lot of the topics you're interested in.

Lhun · Sep 14, 2009

CynicalRyan said:
General Constraints

Detection range is vastly higher than engagement range

Everything is a weapon at non-trivial speeds (F = ma)

Battles have to be short and fast

The defense wins

Expanding and providing a rationale for these points:

Heh. This is like blood in piranha water.

Well, i'll comment on a few things.

(1) There is no stealth in space. The energy a space ship produces has to go somewhere, especially the excess energy (aka "heat"), since to power generator *or* consumer can be 100% effective, so a target can be seen very, very soon. In fact, as soon as the excess heat's radiation hits the passive sensors, the (past) location of any ship is known.
Additionally, due to the vast distances necessarily involved if you want to have at least some action in space, you are looking at the past (if a ship is merely one AU away from my sensors, it takes 8 seconds for any radiation to reach the sensors), and it is trivial to zig-zag in 3 dimensions (with the right propulsion systems). Ergo, you can't hit anything at long distances, even if the theoretical range of weapons is "unlimited".

Pretty much spot on. A minor point is that you'll want to fly a randomized spiral as an evasive pattern, not zig-zag. And i'll write more on the chances to hit later.

(2) With speeds that enable story-telling in a space opera-ish universe, absolutely everything becomes a weapon, simply due to impact at high speeds. No need for asteroids, simply have you biggest ship accelerate as fast and as long as possible, and you have you own planet-killer, and that isn't even taking into account the propulsion system!

The best speed for a kinetic weapon is at twice it's drives exhaust velocity. Any more, and you actually lose energy, since the increase in speed doesn't make up for the loss of mass.

(3) Any interesting weapon system (except missiles) has an enormous energy hunger. But radiators for the excess heat produced in the process are big, fat targets. So, in the *ahem* heat of battle, radiators will be protected somehow, the easiest way is to limit their exposure to enemy fire. This means that a ship cannot "bleed off" all the energy it produces. So, to prevent the crew from slowly being roasted, ships, or even fleets engaging each other, will hit as fast and as hard as they can, before they have to disengage to get rid of the excess heat.

Uhm, no. I have to disagree with premise (partly) and conclusion.
Excess heat is not necessarily a problem. At least for weapons. There are various ways to design weapons that produce no, or negligible excess heat. A chemical laser for example can simply vent the gas, including excess heat after each shot. (Thoug this will of course mean you need akind of ammunition for the laser)
A coilgun using superconductive coils will produce much less heat than a railgun. Etc.
Fast and hard engagements are something you'd expect if visibilty was a lot worse. Since you can see ships over very long distances you can start shooting when your chances to hit are still very low. But you'll not hold your fire until you're at knife-fight distance. Much too (incalculable) risky.

(4) Evey weapon has countermeasures. Some active, some passive. Kinetic weapons can be countered by Whipple shields.

You can (heh) whittle down a whipple shield. Shoot shrapnel with your coilguns. Use sandcasters in the missiles. Your chances to hit will improve greatly, and thus your engagement distance. And while a hit will not be devastating, each grain of sand that hits will punch a hole in the whipple shield, and sooner or later you're going to start punching holes in the hull.

ECM and point defense takes care of missiles,

ECM is pretty useless against missiles in such a sensor friendly environment. Like you can't jam a HARM missile. The only useful defense (besides PD) are decoys, but those are extremely expensive, energy-wise.
Active measures against missiles are extremely varied though, and allow for a very huge amount of different strategies. (And so do the various payloads the missiles could carry)

and simply rotating makes lasers useless.

Noone is going to use a laser that pathetically weak, except for burning out sensors.

Unless you put an obscene amount of energy into any weapon system (except missiles, which'll have to get smarter by themselves, or being slaved to a platform with bigger and better sensors), that is, in which case you run into constraints #3.

Missiles need obscene amounts of energy as well. The only viable payload that does not require the missile to be a kinetic kill missile, is a bomb-pumped laser (graser, really) warhead, but even those missiles still have to be extremely fast, or they're easy targets for the point defense.

Lhun · Sep 14, 2009

CynicalRyan said:
(2) If a ship becomes too large, no matter how much energy you put into it, inertia will keep it stay put where it is (or accelerate so slowly, that you might not even bother). This puts limits on starship designs, and introduces a lot of trade offs. Look at the development of ships in the wet navies (cruisers, battlecruisers, battleships) to see the trade offs that need to be made.

Ehh.
No.
Just, no. There is absolutely not mass limit to spaceship design. The limits to ships in wet navies comes from air and water friction. No such thing in space.
The one thing that could limit a ships size in space is the fact that your drive will have to push the mass forward, and that force could crush the structural elements. But if you still want to build bigger, you can just build flat ships with drives distributed over the surface.

(3) Unless you simply pump out dust, for example, to propel you forward, the reaction mass itself is volatile.

Also no. Unless you use a chemical drive or nuclear salt water rockets (which are made of awesome. Highly radioactive, dangerous awesome) there is no reason to have a volatile propulsion mass. Whatever your energy generator uses as fuel might be though.

The more mass has to be moved, the higher the amount of energy you need, and, probably, the more volatile the fuel becomes, unless you increase the number of thrusters.

How volatile your fuel can be is a pretty hard limit. We're already using the best that chemical rockets have to offer, and nuclear rockets don't go much further either. More thrusters are inevitable as well, or bigger thrusters. The only way to get more thrust, which you need for bigger ships.

(4) Newton's Laws of Motion state that for every force, there exists an equal and opposite force acting against it. Thus even a reaction-less drive produces some sort of effect(s) that can be felt and measured.

Reactionless drives break conservation of energy. There is no such thing in reality, and if you use them in a story, it's in the same inexplicable category as FTL drives.

(5) This is most obvious with reaction drives: The engines produce a lot of energy, and a lot is wasted in heat. That can make for interesting accidents if ships cross each other, limiting the angles of attack (you cannot attack the undefended area of the engine's exhaust, for example).

Depends a lot on the kind of attack. Drive exhaust is extremely energetic (more so than the weapons probably) but generally poorly collimated. Thus the dangerous range is quite small. Still a very important thing when docking for example. Or in very crowded orbits.

Reaction less drives are more complex.

No such animal. If you violate the first law of thermodynamics, just go with bolognium wherever that drive is involved, since there's no realism left to be saved anyway. Just make up whatever physics best suit your world.

Lhun · Sep 14, 2009

CynicalRyan said:
(1) To accelerate even a small projectile to a speed that turns it into a weapon, you need an enormous amount of energy and a bit of room to have the projectile accelerate. That puts a limit on "caliber" and terminal velocity for such a weapon, and, of course, is limited by the power a ship can produce.

But this is no different to any other weapon. In fact, kinetic weapons are some of the most efficient in space, you have to spend a lot more power for every joule of a laser that makes it to the target, than for every joule of a slugthrower.

(2) The more advanced the sensor package the missiles carry, the more expensive they are to produce. If they aren't smart, they are easily defended against with countermeasures, like ECM, or simple point defense.

Sensors are trivially cheap in space, at least the kind you'd use on missiles. Starships are extremely high contrast targets on a extremely low contrast background. And you don't need extremely advanced electronics either, even today we have microchips available that are not costly, but vastly more powerful than any simple ballistics program would need. The kind of calculations needed for controlling weapons of all sorts, are ridiculously easy compared to some of the day-to-day applications that are used in every office.

And, last but not least, the more explosives (or whatever is required to punch a hole into the enemy's ship) it carries, the more dangerous it is.

The only explosive you carry on a spaceborn missile is a nuke. A big one. Anything else is a waste of mass, use lead instead. A ridiculously low 2km/s gives you one rick, so all missiles besides bomb-pumped laser missiles, are used for kinetic kill.

All of this has to fit into a smaller body than a starship's, if the ship is supposed to have endurance. Thus, unless the starship's propulsion system can be miniaturized (and cheaply enough produced) to fit into a missile, you are stuck with reaction drives, which are very limited in acceleration.

No such thing as a reactionless drive.

(3) Laser's deliver their payload by focusing beams on a very, very small target.

Uh, no. Lasers deliver the energy through a coherent beam of radiation. In space, expect a laser cross-section of several meters at the target. Better focusing is extremely hard to achieve.

Smaller even than a starship. That makes them difficult to aim (nothing technology can't overcome, however). But as high as the energy is that they deliver, they need time to "burn" through armor. Without going into too much detail, the armor has to be heated enough to vaporize, or at least to become brittle, and that layer for layer. The time needed is increased by armor thickness, and by presenting a larger target (for example by rotating the starship).

Also no. You don't use a laser that's weak enough to actually heat, melt, vaporize armor. A useful laser will have enough amplitude or short enough wavelength to ionize whatever material is hit, immediately turning it into energetic plasma. Producing a shockwave in the material beyond the layer that was hit, and various other effects.

(4) Almost every spaceship comes pre-armored, for example. It will be compartmentalized to limit the effect accidents have, and simply bolting on an outer hull negates kinetic weapons (but not relativistic weapons).

The word negate is a bit too strong. At best, the kinetic weapon will punch a whole in the other hull instead of the inner hull. Which will change with repeated hits. At worst, the kinetic projectile is of relatively large caliber and relatively slow, which will allow it to punch through the outer hull without being significantly weakened. A relaitivistic weapon is actually a worse choice against a whipple shield, since the projectile will turn into a plasma cloud, which will dissipate and hit the inner shield over a much wieder area. A multilayered whipple shield, possible even with filled spaces (use some light material that also absorbs lasers pretty well) will turn a hit from a high velocity projectile into a short, conical hole. The good thing about whipple shields is that you can increase the effectiveness by simply increasing the spacing, which does not require additional mass.

Putting the ship into a rotation is accomplished y the maneuvering thrusters the ship has anyway to be able to dock, and a white noise generator for almost all possible frequencies is easily installed. The distances of space make even small evasive maneuvers have a large effect, and make TV guided missiles an unsure bet at best.

While you are right that defenses are cheap, rotating as defense against lasers and white noise generators as defense against missiles are not effective. And while countermeasures are not more expensive than weapons, you can also employ various effective counter-countermeasures. A big part of the strategic part of a space battle will be to try and anticipate what countermeasures your target will employ, and to counter them.

efkelley · Sep 14, 2009

Lhun said:
The one thing that could limit a ships size in space is the fact that your drive will have to push the mass forward, and that force could crush the structural elements. But if you still want to build bigger, you can just build flat ships with drives distributed over the surface.

The mass issue is pretty significant. It's true that a ship can be as massive as you want, but how quickly do you want it to accelerate and change course? In theory we could strap a big enough engine to the moon and move it around, but is that at all practical?

Lhun said:
Also no. Unless you use a chemical drive or nuclear salt water rockets (which are made of awesome. Highly radioactive, dangerous awesome) there is no reason to have a volatile propulsion mass. Whatever your energy generator uses as fuel might be though.

The fuel could very well be volatile, but I agree there's no particular reason for the propellant to be exceptionally dangerous.

Dommo · Sep 14, 2009

I'm skeptical about the effectiveness of whipple shields against large slugs. While I'm positive they'd offer good protection against something small(sand casters), against a tungsten ball that's got a mass of a few kilograms I don't think they'd offer much protection.

Where I see sandcasters being insanely effective is in destroying enemy sensors(since they're likely going to be exposed), and in the fact that it's an absurdly economical way to do damage to your opponent. I agree with Lhun though on pretty much all of his points.

One thing that I'm curious about is how Metamaterials might impact space combat. Could it be possible to cloak a ship(or have like a cloaking "shield" like a movable meta material shape) that could cloak a ship from laser attack? If that's possible then it basically means that lasers drop a peg farther on the usefulness scale.

Dommo · Sep 14, 2009

The reason the mass is trivial, is because in a manned ship, accelerations that could actually be useful to evade an incoming attack can't be used unless you want to pulverize the crew. Frankly it just means that if you want to accelerate your ship more rapidly, that you've got to use a thrust mechanism that can lay down the oomph(a.k.a. the Orion Drive, or nuclear pulse).

benbradley · Sep 14, 2009

CynicalRyan said:
...
(3) Laser's deliver their payload by focusing beams on a very, very small target. Smaller even than a starship. That makes them difficult to aim (nothing technology can't overcome, however). But as high as the energy is that they deliver, they need time to "burn" through armor. Without going into too much detail, the armor has to be heated enough to vaporize, or at least to become brittle, and that layer for layer. The time needed is increased by armor thickness, and by presenting a larger target (for example by rotating the starship).

There are a lot of different kinds of lasers currently made, and some of the pulsed ones (the ones likely used in space warfare) have hugely short pulses with hugely high power, basically a lot of energy in a very "small" package. An object would have to rotate at relativistic velocities to spread out the incident radiation over a larger area.

Dommo · Sep 14, 2009

As was mentioned in the other warfare thread, the big limiter to lasers is the heat they put out.

You have a choice.

You can either have infinite ammo, but have serious heat problems(e.g. solid state), or you can have finite ammo, but be able to lase to your heart's content(you use chemicals and jettison them as you use them up).

Ideally you'd want both on your ship, but both have drawbacks. The solid state can literally cook everyone on your ship, and can make your ship shine like a beacon from the heat radiating off of it. The chemical has the disadvantage of costing you mass, and only provides a few shots.

efkelley · Sep 14, 2009

Dommo said:
... or you can have finite ammo, but be able to lase to your heart's content(you use chemicals and jettison them as you use them up).

Why would chemical reactants produce less heat? You're still energizing a medium to produce visible light at energy levels powerful enough to cause damage at a significant range. The medium is going to get hot. The chamber will get hot. The emitter will even get hot.

Lhun · Sep 14, 2009

efkelley said:
Why would chemical reactants produce less heat? You're still energizing a medium to produce visible light at energy levels powerful enough to cause damage at a significant range. The medium is going to get hot. The chamber will get hot. The emitter will even get hot.

You vent the lasing medium into space after a shot. Nearly all heat gets produced in the lasing medium, so if you vent it (quickly) you dump most of the heat before it can dissipate into the ship's structure.

Lhun · Sep 14, 2009

Dommo said:
You can either have infinite ammo, but have serious heat problems(e.g. solid state), or you can have finite ammo, but be able to lase to your heart's content(you use chemicals and jettison them as you use them up).

Basically, you use a chemical laser only for shooting at other ships, and a solid state laser for point defense. Since you can adjust the power output (and thus the heat generated) of a solid state laser though, you can use them both ways, you just have to take care you don't overheat them so they need time to cool down when you can't afford to wait.
You could use a chemical laser (or any gas laser) at a lower power too, but that'd be a waste of ammunition.

benbradley · Sep 14, 2009

The main thing I was thinking of was the x-ray laser as envisioned in the Reagan's "Star Wars" Strategic Defense Initiative. It's a one-shot device because it's powered by a nuclear bomb. The SDI goal was to destroy ICBM's after they were launched but before they get close to their destination.

Of course as part of a space warship, you also want such devices to be deployed well away from your ship before they're fired off.

Yeah, nukes and other one-shot devices are expensive, but this is war we're talking about. The cost of not spending enough is losing, a much greater perceived cost than spending too much.

On a more general idea about lasers, you don't necessarily use visible light. Before firing, you might shoot a flash (white light, and infrared and UV, whatever to cover all the wavelengths of lasers you have) through a telescope at the target and see what parts of the spectrum reflect back. You'd want to use the laser that would get reflected the least, thus absorbed the most by the target.

Lhun · Sep 14, 2009

Dommo said:
I'm skeptical about the effectiveness of whipple shields against large slugs. While I'm positive they'd offer good protection against something small(sand casters), against a tungsten ball that's got a mass of a few kilograms I don't think they'd offer much protection.

A few kilograms is pretty big already. That's in the range of one-hit-kill weapons. And needs a lot of energy to accelerate. But even then, a whipple shield is generally effective as long as the projectile carries enough energy to get vaporized when it hits the whipple shield. The best way to defeat a whipple shield is to shoot slow projectiles, though that obviously doesn't work well in space because of the ranges.

Where I see sandcasters being insanely effective is in destroying enemy sensors(since they're likely going to be exposed), and in the fact that it's an absurdly economical way to do damage to your opponent. I agree with Lhun though on pretty much all of his points.

Even with a sandcaster, you shouldn't think of it as a shotgun, or something that does similarly low damage. A kinetic missile that reaches low relativistic velocities with a sandcaster will seriously damage the armor (or whipple shield) of a ship. You don't have to hit sensors to do significant damage, which has an extremely low chance to work anyway. Using a sandcaster offensively has to reasons. For one, it is a great way to strip the whipple shield(s) from a target. Sooner or later, it's going to be so full of holes that projectiles start hitting those holes, instead of the whipple shield. The second is, that a big cloud of sand has a much better chance of hitting an evading target at long ranges, than a dense chunk of iron. Sure, a hit with the iron chunk might destroy it in one shot, but when your chance to hit is one in hundred, it's better to use a hundred shot to slowly wear the target down, than shooting a hundred times until you finally hit. Because in the former case you already reduce his capacity to fight back with each shot. And you might get lucky and hit something critical. And defending against a cloud of sand is harder than against a single projectile. While only a single grain of sand might hit from each shot, it's a harder to calculate which one, and intercept it. Not to mention that all those other grains act as decoys.

One thing that I'm curious about is how Metamaterials might impact space combat. Could it be possible to cloak a ship(or have like a cloaking "shield" like a movable meta material shape) that could cloak a ship from laser attack? If that's possible then it basically means that lasers drop a peg farther on the usefulness scale.

There are two problems with all reflective armor. The first is that really good mirrors (and you want an extremely good one) operate only with a very limited wavelength. The second is that beams of sufficiently short wavelength just don't get reflected, since they tend to simply ionize whatever atom or molecule they hit. This is what makes bomb pumped laser interesting. They're the only good way we currently know to create a graser, but there's not a whole lot of materials that will bend gamma rays by even a few degrees. Metamaterials only have exotic refraction/reflection indexes, their other basic properties aren't much different. That a metamaterial lens could theoretically bend the laser backwards doesn't help much if it still just vaporizes when hit.

Lhun · Sep 14, 2009

efkelley said:
The mass issue is pretty significant. It's true that a ship can be as massive as you want, but how quickly do you want it to accelerate and change course? In theory we could strap a big enough engine to the moon and move it around, but is that at all practical?

You're still thinking in terms of friction. Well, you're maybe not thinking of friction but of size in way that only matter when you have friction.
In a vacuum, if you take a ship, and build a copy at twice the size. Guns twice as big, sensors twice as big, missiles twice as big, crew twice as big (yeah gonna need genetical engineering for that one) and drive twice as big, it will be exactly as fast as the smaller version. And it will accelerate exactly as fast as the smaller version. Because the only two things that matter for the speed of the ship are the efficiency of the drive, and the mass percentage of the ship that is used for the drive. This is what also kills fighters in space.
How big a ship you can afford is an economical and a political question. (no point in building one big ship if you need to be in five places at once) But technologically, bigger is always better. Better the sensors that are twice as big are better than twice as sensitive. And the missiles that are twice as big have a bigger range, which is something the smaller ships can never compensate for, since they're not faster.

efkelley · Sep 14, 2009

Lhun said:
You vent the lasing medium into space after a shot. Nearly all heat gets produced in the lasing medium, so if you vent it (quickly) you dump most of the heat before it can dissipate into the ship's structure.

Gotcha. Thanks!

Lhun said:
In a vacuum, if you take a ship, and build a copy at twice the size. Guns twice as big, sensors twice as big, missiles twice as big, crew twice as big (yeah gonna need genetical engineering for that one) and drive twice as big, it will be exactly as fast as the smaller version. And it will accelerate exactly as fast as the smaller version. Because the only two things that matter for the speed of the ship are the efficiency of the drive, and the mass percentage of the ship that is used for the drive. This is what also kills fighters in space.

I agree on all points. I'm not thinking of friction in any way though. Take the smaller ship's engine and put it on the bigger ship, and it will have less acceleration. The issues at hand are exactly what you mentioned in this post that the efficiency of the drive and mass percentage are the only things that matter.

Lhun · Sep 14, 2009

efkelley said:
I agree on all points. I'm not thinking of friction in any way though. Take the smaller ship's engine and put it on the bigger ship, and it will have less acceleration. The issues at hand are exactly what you mentioned in this post that the efficiency of the drive and mass percentage are the only things that matter.

Well, you're not going to put in the smaller ship's engine, you're going to use one twice as big of course. With most types of engines, increasing the size will actually increase efficiency, and for the engines where that's not the case, you can just use multiple small ones. Efficiency is more a question of the type of drive. I.e. a ship with an IC-fusion drive will be faster than a ship with a chemical drive, even if the second ship has a much higher drive/payload ratio.

FOTSGreg · Sep 15, 2009

Hey, Lhun. I've got a question for ya' (seriously, I know we might have had differences in the past, but this is a serious question and it relates to the thread's topic).

Say I have ships moving at 20% c and missile payloads moving at 40% c. Each missile packs a payload of around a thousand projectiles each the size and weight of a 5.56mm bullet (roughly 0.009 lbs, I think - I might be missing a zero in there somewhere).

What is the best firing solution for these missiles if the payload deployment range is somewhere around 30 light seconds (or is there an ideal deployment range or firing solution possible)?

How would you deploy such a weapon? And when?

Salis · Sep 15, 2009

Really, space battle is a huge problem. I mean, to even make it remotely possible at all, you have to suspend a lot of physical facts. I'm not sure how to approach this, really, other than the space opera tangent, at which point you might as well just make up whatever you want, because space opera is so divorced from reality it might as well be science fantasy.

Dommo · Sep 15, 2009

It's perfectly possible, however it's not exactly like what we've seen.

The battles might take weeks, but the actual "fighting" part might only take a few seconds. In fact there could well be situations where you know your ship is boned(e.g. your point defense lasers are down), and you get to wait a half hour for the incoming KE missile to vaporize you.

The other big difference is that the terrain is literally as open as you can imagine. There's not really any element of surprise(at least not at the tactical level), and the scale of things is really, really huge.

Salis · Sep 15, 2009

Maybe I just have a really terrible imagination/no knowledge, but I have a hard time imagining how a hard-sci space battle would happen in wide open space, if someone saw you coming.

Okay, so we assume that basically the fastest method of propulsion is used for space ship engines, right? Then how is any projectile that uses the same propulsion going to reach someone who decides "fuck it, just going to run"?

Maybe light-based weaponry, but the effective range on that is hella short anyway.

Dommo · Sep 16, 2009

Well how long do you have to accelerate in order to change your course and what's your top speed?

I mean honestly if you're at some fraction of the speed of light with a manned crew, you're going to take at least a few weeks to slow to a stop if you want to completely change direction(humans can't take a 40g decel without turning into goo). This means ships which are hauling ass around are relatively unable to change course significantly. They can jink around a bit, but there's not going to be any sort of U-turns or things of that sort.
This means that any ship that is spotted is constantly jinking around trying to shake off possible shooting solutions from opposing KE weapons. As the ships close with one another, the ability to "jink" is basically reduced and the accelerations get more violent. At close range the accelerations are lethal to humans, and hence why you've got point defense.

Constraints for "realistic" space battles

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