Memphet'ran
Looking into the light
A little (or not so little) essay I wrote on what realistic space combat would be like. Thought you guys might find it interesting. Sorry, I admit it IS a bit long, I apologize if it's somewhat intimidating.
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Space battles are ubiquitous in science fiction. Usually it seems to look a lot like some variation on WWII sea battles: fighters whizz around and engage in space dogfights as the great battleships pound each other with death rays. But in fact this is probably a very unrealistic depiction of what a space battle would look like. I’m sure I’m not the only one who’s wondered “so what would a space battle really look like?” In this essay I will attempt to answer that question as best I can. For those who are interested, somebody else has already tackled the question on Strange Horizons, but I believe that essay is flawed in several ways, most notably the conclusion that stealth will be important in space warfare. First, let us take a look at the environment of space and see what considerations it imposes on any attempts to make war in it.
Note: for the purposes of this essay I am assuming only technology theoretically achievable to current science. Devices such as force shields, cloaking devices, FTL drives, reactionless drives, and other such common soft SF tropes are assumed to either not exist or exist in such a matter as to have minimal impacts on tactics (ex. an FTL drive that requires you to exit the solar system before it can be used, like in Larry Niven’s Known Space universe).
I: THE ENVIRONMENT AND ITS IMPLICATIONS
First and foremost, we must realize that space will present a new unique environment with new and unique challenges for any military operations in it. Space warfare will not resemble sea warfare or air warfare; it will be its own thing (this is really where most depictions of space warfare in SF go wrong, from a realism perspective). What are the major environmental factors in space that will influence combat?
Well, the thing you really have to remember about space is that it’s big, dark, cold, and empty, and, paradoxically, you have perfect visibility. This brings us to our first realization: there will be no stealth in space. Any source of radiant energy in space will be very obvious. Since any spacecraft will be emitting a lot of radiant energy (your vessel will usually need to keep its habitation module several hundred degrees warmer than the external environment to prevent your crew from freezing to death for starters) it will stick out from the cold darkness of space like a campfire in the desert at night. Surprise attacks will be rather difficult, to put it mildly, when the enemy can see you coming halfway across the solar system. One possible solution is to try radiating all your heat in the opposite direction from whatever you’re trying to sneak up on. The problem with this is that it can easily be countered by the enemy scattering monitoring stations (basically just satellites with infrared scopes in them) throughout the solar system, something that would cost relatively little and that a militarized spacefaring civilization would be foolish not to do. A better idea is to try storing your waste heat in an internal sink until you’re on top of your target. This might work, but this approach runs into another problem: in order to sneak up on your target you will at some point have to put yourself on an intercept course with it, and when you do so you will reveal your position and your enemy can determine exactly where you’re going and when you’ll get there with a little college level math.
The simple fact is just about every viable space propulsion scheme in existence works by blowing hot gas out the back of your ship, and that’s just not something you can hide. The space shuttle’s main engines could be detected past the orbit of Pluto. The space shuttle’s maneuvering thrusters could be detected from the asteroid belt. Even a puny ion drive with an acceleration of .01 m/s^2 (1/3000th the space shuttle’s acceleration) could be spotted at a distance of 1 AU (the distance of Earth from the sun). And it gets worse when you consider the sort of engines a mature spacefaring society that wants to get around its solar system in less than many months is likely to use – those are likely to be visible from the next solar system, literally! The kind of drives that warships are likely to use will light up the sky like the Fourth of July. And this is all with current off-the-shelf technology; the shipping-monitoring equipment of a militarized spacefaring society, purpose-built with more advanced technology, is likely to be better. The ion drive is the only propulsion system that offers any possibility of moving unobserved whatsoever, and like the directional heat radiator trick that can easily be rendered impossible by stringing a few hundred cheap monitoring platforms on random orbits throughout your solar system. The only drive I can think of that offers the remotest realistic possibility of stealth is a solar sail, and it has its own serious problem: the sails are huge, and the enemy will likely spot them a long way off by the way they reflect sunlight. Incidentally, not only will the enemy be able to see your ships the instant you fire your engines, but they will be able to learn a surprising amount about your ships by studying the exhaust. By running it through a spectrograph they will be able to tell what kind of fuel you’re using. By observing the brightness and temperature of your exhaust plume they will be able to determine your thrust, which they can then compare to your acceleration to determine the mass of the ship. Not only will the enemy be able to know you’re coming and how many ships you have, they’ll instantly know what kinds of ships they are and possibly even what individual ships make up your armada. This means decoys won’t work. In order to be convincing a decoy would have to have the same mass as your ships, in which case you really might as well make them actual ships.
There is one exception I can think of: a Q-ship. You take a merchant ship, fill its hold with missiles, and put launchers and other weaponry under hidden blow-away panels on its hull. Of course, it’ll probably have significantly inferior performance to a real warship, since it has a merchant ship’s engines and hull. And it’ll only work once or twice, until the enemy starts demanding merchant ships submit to inspections before they get within weapons range of their important facilities.
Generally, space warfare will be unprecedented in the degree of battlefield awareness each side will have. Each side will know exactly where the other side is and what he’s doing at all times, save for the signal delays imposed at long distances by the finite speed of light.
The other thing to consider about space is that it’s big. To travel across space in reasonable time frames you have to be moving fast. Really fast. Just to break out of the gravity of Earth you need to be going at 11 kilometers per second. And at speeds like this you’re still stuck puttering around in Hohmann orbits, taking months or years to reach even the nearest planets. To cross the solar system in months you’ll need some kind of high-performance nuclear rocket capable of accelerating for days or weeks on end and getting your ship up to speeds of dozens or hundreds of kilometers per second. This means that in combat your spacecraft will be moving very fast relative to each other. This has serious implications, most notably the fact that a missile travelling at 3 km/s will impact with the equivalent energy to its mass in TNT. With the notable exception of insubstantial directed energy weapons like lasers weapons in space will do a lot of damage. Space combat is will be rather like air combat: largely a matter of one hit kills. This means realistically you’re not likely to see the kind of battleship-style space combat you see in pop SF. The ships slugging it out in Nelsonian broadside exchanges may make great drama and visual effect, but realistically the first hit with a kinetic or a nuclear missile will end the battle.
Finally, the third major environmental factor to be considered is that movement in space will follow different rules from the ones we are accustomed to. We humans have a profoundly distorted intuitive sense of how motion works, as a result of spending our lives in an environment ruled by friction and gravity. In space movement will follow the Newtonian rule that an object in motion will remain in motion unless acted upon by a force counter and equal to the force that set it in motion. What this means, in practical terms, is that slowing down will take every bit as much energy as speeding up. On Earth if you’re in a car and shut the engine off you slow down and stop. In space if you turn the engine off you’ll just keep drifting away at the same speed forever. If you want to stop you have to turn around and accelerate in the opposite direction. Ditto for changing directions. This means that in space there will likely be none of the dogfights and Nelsonian-style slugging matches seen in pop SF like Star Wars. They would require that the combatant ships precisely match speeds, which given the immense speeds at which they are moving will be very difficult to do and could probably only happen by mutual consent (which in practical terms probably means it’ll almost never happen, because the only reason one side would try for it is if it gave them some kind of tactical advantage, which the other side will try to deny them if it has any sense). Instead space battles will be in essence drive-by shootings. The combatants will plunge towards each other at dozens or hundreds of km/s and hit each other as hard as they can as they pass by each other. If both sides are lucky enough to have survivors they may turn back towards each other and try for another pass in a few hours, days, or weeks.
Another thing about motion in space is that changing your ship’s orientation does nothing to your speed and vector unless it is accompanied by firing your main engine, because there is no friction. This means that all those space dogfights where one fighter gets behind the other and the other one has to try and shake it like in air combat are very unrealistic. There’s no comprehensible reason why the pursued pilot can’t just turn his fighter around and blast the bugger. The lack of friction, incidentally, also means there’s no reason for spacecraft to be have clean lines like atmospheric vehicles so realistic ships are more likely to look like this than this.
II: WARSHIP DESIGN
OK, so now that we have that covered, time to design our warship. A single nuke or kinetic will kill it, so it won’t be built like a battleship (unless it’s an Orion, in which case it has to be just to survive the firing of its own engines). There’s no reason to bother with armor, except against lasers (more on that later). Rather, this thing will win or die on speed.
Speed confers two advantages. First, the side with the fastest ships will get to shape the battlefield, determining whether and under what circumstances the fight takes place. Second, missile range decreases against a faster ship. This is because in space the effective range of the missile is the radius within which it can cover the distance to the target ship before the target ship can accelerate to a speed exceeding the delta V of the missile along the most efficient possible “getaway” vector. The faster the acceleration of the target ship, the smaller that radius is.
There are two kinds of speed in space: acceleration and delta V (which basically means the speed the ship will attain if it expends all its propellant, which can be divided in a variety of ways depending on the mission). Our warship will ideally want both high delta V and high acceleration, since both are advantageous. Unfortunately those tend to be mutually exclusive. The rub is that there are basically two ways you can make a rocket go faster: using more propellant or using hotter propellant. The first gives you high thrust but results in very large propellant masses, the second gives you a low mass ratio (ratio of propellant to everything else) but results in very hot engines and hence restricts you to low accelerations. Trying to combine high acceleration with a low mass ratio and a high delta V generally results in a melted engine.
There are only a handful of engines that allow a combination of high thrust and low mass ratio. The most promising are Orion nuclear pulse propulsion and the nuclear salt water rocket. Some nuclear thermal designs also have thrust high enough to possibly be useful, although only for a small ship. The user “RJP” on Spacebattles also suggested something called a fission fragment drive which works by throwing high-velocity fuel fragments out the back of the ship, but other sites I’ve researched suggest it would be a low-thrust high-ISP system more suitable to an explorer than a warship. Orion works by the (seemingly insane, but actually quite effective) method of throwing nuclear bombs behind the spacecraft and having it ride the blasts. The hot gasses from the detonations hit a heavy pusher plate at the back of the ship and drive it forward. NSWR is similar, but it instead uses a solution of fissionables in salt water that spontaneously explodes as it leaves the rocket nozzle. Both systems cleverly shift the propulsive reaction outside the spacecraft, eliminating the need to deal with most of the heat it produces and allowing it to be made much more energetic. NSWR is superior to most Orion designs in terms of exhaust velocity (and hence fuel efficiency), but it has the downside of using fuel that spontaneously explodes outside of a carefully constructed reaction-dampening tank. On a warship this is an obvious liability. Orion also has the advantage of being very efficient for massive spacecraft. Orion driven warships might be large and slow, while NSWR driven warships might be small, fragile, but fast. Whichever one is more desirable will probably depend on the mission profile of the space fleet. Reasonable mass ratios for a warship will probably be between 1-10, with dry weights of several thousand tons. This would translate to delta Vs of several dozen to several hundred km/s for most Orion designs and several hundred to several thousand km/s for NSWR. A 1950s report suggested 4 G of acceleration might be reasonable for a 10,000 ton Orion, so accelerations might be anywhere between around 10 G (limited by the tolerance of the human crew) and less than 1 G. Note that as a high mass-ratio spacecraft burns through its propellant its maximum acceleration will increase; a ship with a full tank might have a maximum acceleration of under 1 G while the same ship with an empty tank might be able to get 4 G. Like WWII bombers, combat spacecraft will also become lighter and hence faster after releasing their munitions.
Since a warship will want the highest acceleration and delta V possible, it will be designed to be as light as possible. Every extra kilogram of payload lowers the delta V, unless you compensate by adding more propellant, which makes your engine work harder and lowers your acceleration. Spacecraft will be engineered like aircraft, not ships. Every effort will be made to eliminate extraneous mass and make the ship as light as possible. As much logistical burden as possible will be shifted from the ship to base. You’re unlikely to see, say, warships with hydroponic gardens. This means that, unlike in many pop SF depictions, warship crews will probably be quite small. Human beings require a lot of supporting mass in supplies, life support, and crew quarters, so spacecraft in general will probably be heavily automated. A warship will probably basically be a can full of weaponry on top of a big fuel tank, with the crew controlling the thing from a small habitat module. The crew will effectively be command crew; there to tell the machines what to do, not to micromanage the operations of the ship. You’ll probably have a small core crew to fly the ship, a few damage control technicians, and maybe a medic or two. Serving on one will be more like serving on a WWII U-boat than anything else.
Note. “Destructionator XIII” on Stardestroyer.net pointed out that if you’re using relatively slow spacecraft (unpowered intercept orbits) acceleration becomes relatively unimportant and the extra weaponry and point defense you can fit on a heavier ship might be more than worth any sacrifice in acceleration. Acceleration only becomes a serious priority when you start using torchships.
III: WEAPONS AND DEFENSE SYSTEMS
In space there are three basic kinds of weaponry available to you: missiles (guided kinetic and explosive weapons), guns (unguided kinetic weapons), and directed energy weapons (lasers and particle beams). Guns will probably be mostly useless: in order to be competitive with lasers and missiles they will need infeasible muzzle velocities of thousands of km/s. That leaves missiles as your most powerful weapons. Missiles are likely to come in two kinds: nuclear and kinetic energy. Nuclear missiles carry nuclear warheads, kinetics dispense with the warheads and use the sheer kinetic energy behind them to achieve their destructive effects. Kinetic missiles are simpler and may have slightly longer range. Nuclear missiles have the advantage of being extremely destructive at both high and low speeds, making them more flexible. Some nuclear missiles may be hybrids, programmed to detonate or not detonate the warhead depending on which will be more effective. Since most nuclear rockets don’t scale down well, missiles are likely to use chemical rockets, meaning they will have high accelerations but low delta Vs, probably around 10 km/s or so. Though missiles will probably have much higher max accelerations than ships they will, in practice, probably be programmed to accelerate just slightly faster than the target ship, because there are few things that suck quite so much as having your missile expend all its delta V in a 10 G burst of acceleration and then having the target ship get away by the simple expedient of breaking to the left when it’s out of fuel.
The great situational awareness and high speeds that characterize space combat would seem to make missiles extremely effective. On the other hand, the same factors make them extremely vulnerable. The most efficient killers of missiles are likely to be directed energy weapons like lasers and particle beams. Particle beams have better penetration but much shorter range, so lasers will probably be the weapon of choice for point defense. The maximum theoretical range of a laser (against a target on an unpredictable course) is around 1 light second (about 300,000 km). Much beyond that and light lag renders effective targeting impossible. In practice a laser’s effective range is likely to be limited by diffusion (a laser, like a flashlight beam, spreads out over distance, making it less powerful the further away you are from it). The diffusion rate of a laser will be a factor of its power, mirror radius, and wavelength. The maximum practical mirror radius for a ship laser is probably around 10 meters, unless you want to make it a spinal mounted weapon. The lasers proposed for real life proposed Star Wars missile defense system are infrared lasers with wavelengths of tens of thousands of nanometers and power levels of single to low double digit megawatts, with ranges in the thousands of kilometers. Infrared lasers have the highest diffusion rates. The best practical laser is probably going to be an ultraviolet laser. SDI estimates suggest that to kill a Soviet ICBM would require 10 kilojoules/cm^2 (100 megajoules/m^2). Missiles designed with laser PD systems in mind will probably be “armored” with a boil-off layer of a substance with a high specific heat and melting point, which may triple or quadruple this. Another neat trick is to spin the missile, so that the laser must heat both sides instead of just one, which should at least double the amount of energy necessary to kill it, depending on the focus of the beam. A missile equipped with these measures may require between 60-80 KJ/cm^2 (600-800 MJ/m^2) to kill. A moderately well-focused 100 MW ultraviolet laser will kill the missile in 2-4 seconds at 10,000 km, 20-40 seconds at 30,000 km, and 70-113 seconds at 50,000 km. The purpose of these countermeasures is not to actually save the missile but to buy time for other missiles to get closer to ship by prolonging the amount of time required to kill each missile. The maximum effective range against hardened targets may be somewhere between 40-100,000 km. As one can infer from looking at the numbers, most missile kills will probably be in the last 10-20,000 kilometers to the ship. The critical limitation on laser effectiveness at short ranges will probably be the time needed to switch from one target to the next. The actual targeting computer will probably be able to do so very quickly, but remember, we’re talking multi-megawatt lasers with 10 meter mirrors and massive cooling systems here. The turrets these things are mounted in will be literally the size of a house, and I doubt they will be able to rotate to a new target with lightening speed. A delay time of at least 2-3 seconds is probably inevitable. Another key limitation may be power and cooling. 100 MW is a lot of energy, and most of the high-performance rocket systems a warship may use don’t really lend themselves to being tapped for that kind of electrical power, meaning the ship will probably have to carry a separate reactor to power the lasers. And lasers are notoriously inefficient; the models currently being tested for the US and Israeli militaries have energy efficiencies of 10%, meaning that a 100 MW laser would be using a gigawatt of electrical power and generating 900 megawatts of waste heat, not counting inefficiencies of the power generator itself. As well as their own reactor, they’ll need massive radiators to get rid of all the waste heat they generate. There may be limits to how long you can those things burning. Current military lasers require minutes of cool-down time after a few seconds of firing time, though future systems will probably be much better.
What we basically have here is a race between missiles and lasers. A cogent point here is that, because of the way momentum works in space, the race is likely to favor lasers at low speeds and missiles at high speeds, because missiles will work better at higher engagement speeds. To illustrate, let’s imagine two scenarios involving combatant ships moving directly toward each other. Both ships have maximum accelerations of 4 G and carry missiles with a delta V of 10 km/s. In the first scenario both ships are moving at 30 km/s, in the second scenario at 1000 km/s, so their combined velocities will be 60 km/s and 2000 km/s (respectively). At 4 G it will take 250 seconds to achieve a speed of 10 km/s along the most efficient breakaway vector (a right angle), so the point at which the missiles will traverse the space between the ships in 250 seconds will mark their maximum effective range. At 60 km/s this gives the missiles an effective range of 15,000 km, at 2000 km/s it gives them an effective range of 500,000 km (greater than the distance between Earth and the moon!). Not only will the missiles have a much longer range in the second scenario, they will spend only 10 seconds crossing the 20,000 km “death zone” where the target ship’s PD lasers can kill one every few seconds, whereas in the first scenario they will launch and spend the entire 250 seconds of flight time well inside the “death zone”. This means that many more missiles will be required for a kill in the first scenario than in the second scenario. It also means that the missiles will strike the target much harder in the second scenario, though this will be mostly academic (because unless your ship is a hollowed-out asteroid being hit by something going at 60 km/s isn’t going to be any more survivable than being hit by something going at 2000 km/s – they’re just two different degrees of brutal overkill). Very significantly, in the second scenario the ships will launch their missiles well outside one another’s effective laser range, whereas in the first scenario they must come deep within it.
To protect the ship against lasers you will probably employ similar techniques to what you use to protect missiles. Most of the ship will probably be covered in a light thermal-protective jacket of a material with a high melting point and specific heat. The glaring exception will be the radiators. By their nature, they are basically impossible to armor, so they will inevitably be very fragile. One possibility is to draw them into your ship and dump your heat in an internal sink, although this will put a sharp limit on the endurance of your lasers and a big folding radiator will probably be an engineering nightmare. Another possibility is to design a segmented radiator, so that if a hole is burned in it only one or two segments will be put out of commission instead of the whole radiator. This can be combined with making the radiator in small readily replaceable sections: when the battle is over the damage control team simply pops out the damaged sections and replaces them with spares. Spinning the ship is likely to greatly increase its effective “toughness”, because it will result in the laser distributing its heat over a much wider area (remember, a ship will have a lot more surface area than a missile). Another good trick is to run chilled coolant through the area being heated by the laser. This will be especially effective in combination with rotating the ship: as the laser’s beam wanders over the ship the cold coolant will rapidly chill all the areas it isn’t actively heating, dramatically slowing down its burn-through if not stopping it altogether. Warships will probably have cooling pipes running all over the hull, buried directly beneath the protective thermal jacket. If all else fails, warships will probably be designed with a fairly high degree of compartmentalization and redundancy to reduce the amount of damage a successful burn-through can do. For instance, the habitat module will probably be divided into a number of airtight compartmentalized sections, so that a laser burning a hole in it will only decompress one section instead of the whole thing.
Point defense may be augmented by rapid-fire short range guns or (more likely) antimissiles. These are (probably) kinetic energy missiles, probably with lower delta V than the offensive missiles so they will mass and cost less per unit. They will allow you to deal with missile volleys you wouldn’t be able to with just lasers for point defense, but they will take up mass and volume that could be used for offensive missiles.
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Space battles are ubiquitous in science fiction. Usually it seems to look a lot like some variation on WWII sea battles: fighters whizz around and engage in space dogfights as the great battleships pound each other with death rays. But in fact this is probably a very unrealistic depiction of what a space battle would look like. I’m sure I’m not the only one who’s wondered “so what would a space battle really look like?” In this essay I will attempt to answer that question as best I can. For those who are interested, somebody else has already tackled the question on Strange Horizons, but I believe that essay is flawed in several ways, most notably the conclusion that stealth will be important in space warfare. First, let us take a look at the environment of space and see what considerations it imposes on any attempts to make war in it.
Note: for the purposes of this essay I am assuming only technology theoretically achievable to current science. Devices such as force shields, cloaking devices, FTL drives, reactionless drives, and other such common soft SF tropes are assumed to either not exist or exist in such a matter as to have minimal impacts on tactics (ex. an FTL drive that requires you to exit the solar system before it can be used, like in Larry Niven’s Known Space universe).
I: THE ENVIRONMENT AND ITS IMPLICATIONS
First and foremost, we must realize that space will present a new unique environment with new and unique challenges for any military operations in it. Space warfare will not resemble sea warfare or air warfare; it will be its own thing (this is really where most depictions of space warfare in SF go wrong, from a realism perspective). What are the major environmental factors in space that will influence combat?
Well, the thing you really have to remember about space is that it’s big, dark, cold, and empty, and, paradoxically, you have perfect visibility. This brings us to our first realization: there will be no stealth in space. Any source of radiant energy in space will be very obvious. Since any spacecraft will be emitting a lot of radiant energy (your vessel will usually need to keep its habitation module several hundred degrees warmer than the external environment to prevent your crew from freezing to death for starters) it will stick out from the cold darkness of space like a campfire in the desert at night. Surprise attacks will be rather difficult, to put it mildly, when the enemy can see you coming halfway across the solar system. One possible solution is to try radiating all your heat in the opposite direction from whatever you’re trying to sneak up on. The problem with this is that it can easily be countered by the enemy scattering monitoring stations (basically just satellites with infrared scopes in them) throughout the solar system, something that would cost relatively little and that a militarized spacefaring civilization would be foolish not to do. A better idea is to try storing your waste heat in an internal sink until you’re on top of your target. This might work, but this approach runs into another problem: in order to sneak up on your target you will at some point have to put yourself on an intercept course with it, and when you do so you will reveal your position and your enemy can determine exactly where you’re going and when you’ll get there with a little college level math.
The simple fact is just about every viable space propulsion scheme in existence works by blowing hot gas out the back of your ship, and that’s just not something you can hide. The space shuttle’s main engines could be detected past the orbit of Pluto. The space shuttle’s maneuvering thrusters could be detected from the asteroid belt. Even a puny ion drive with an acceleration of .01 m/s^2 (1/3000th the space shuttle’s acceleration) could be spotted at a distance of 1 AU (the distance of Earth from the sun). And it gets worse when you consider the sort of engines a mature spacefaring society that wants to get around its solar system in less than many months is likely to use – those are likely to be visible from the next solar system, literally! The kind of drives that warships are likely to use will light up the sky like the Fourth of July. And this is all with current off-the-shelf technology; the shipping-monitoring equipment of a militarized spacefaring society, purpose-built with more advanced technology, is likely to be better. The ion drive is the only propulsion system that offers any possibility of moving unobserved whatsoever, and like the directional heat radiator trick that can easily be rendered impossible by stringing a few hundred cheap monitoring platforms on random orbits throughout your solar system. The only drive I can think of that offers the remotest realistic possibility of stealth is a solar sail, and it has its own serious problem: the sails are huge, and the enemy will likely spot them a long way off by the way they reflect sunlight. Incidentally, not only will the enemy be able to see your ships the instant you fire your engines, but they will be able to learn a surprising amount about your ships by studying the exhaust. By running it through a spectrograph they will be able to tell what kind of fuel you’re using. By observing the brightness and temperature of your exhaust plume they will be able to determine your thrust, which they can then compare to your acceleration to determine the mass of the ship. Not only will the enemy be able to know you’re coming and how many ships you have, they’ll instantly know what kinds of ships they are and possibly even what individual ships make up your armada. This means decoys won’t work. In order to be convincing a decoy would have to have the same mass as your ships, in which case you really might as well make them actual ships.
There is one exception I can think of: a Q-ship. You take a merchant ship, fill its hold with missiles, and put launchers and other weaponry under hidden blow-away panels on its hull. Of course, it’ll probably have significantly inferior performance to a real warship, since it has a merchant ship’s engines and hull. And it’ll only work once or twice, until the enemy starts demanding merchant ships submit to inspections before they get within weapons range of their important facilities.
Generally, space warfare will be unprecedented in the degree of battlefield awareness each side will have. Each side will know exactly where the other side is and what he’s doing at all times, save for the signal delays imposed at long distances by the finite speed of light.
The other thing to consider about space is that it’s big. To travel across space in reasonable time frames you have to be moving fast. Really fast. Just to break out of the gravity of Earth you need to be going at 11 kilometers per second. And at speeds like this you’re still stuck puttering around in Hohmann orbits, taking months or years to reach even the nearest planets. To cross the solar system in months you’ll need some kind of high-performance nuclear rocket capable of accelerating for days or weeks on end and getting your ship up to speeds of dozens or hundreds of kilometers per second. This means that in combat your spacecraft will be moving very fast relative to each other. This has serious implications, most notably the fact that a missile travelling at 3 km/s will impact with the equivalent energy to its mass in TNT. With the notable exception of insubstantial directed energy weapons like lasers weapons in space will do a lot of damage. Space combat is will be rather like air combat: largely a matter of one hit kills. This means realistically you’re not likely to see the kind of battleship-style space combat you see in pop SF. The ships slugging it out in Nelsonian broadside exchanges may make great drama and visual effect, but realistically the first hit with a kinetic or a nuclear missile will end the battle.
Finally, the third major environmental factor to be considered is that movement in space will follow different rules from the ones we are accustomed to. We humans have a profoundly distorted intuitive sense of how motion works, as a result of spending our lives in an environment ruled by friction and gravity. In space movement will follow the Newtonian rule that an object in motion will remain in motion unless acted upon by a force counter and equal to the force that set it in motion. What this means, in practical terms, is that slowing down will take every bit as much energy as speeding up. On Earth if you’re in a car and shut the engine off you slow down and stop. In space if you turn the engine off you’ll just keep drifting away at the same speed forever. If you want to stop you have to turn around and accelerate in the opposite direction. Ditto for changing directions. This means that in space there will likely be none of the dogfights and Nelsonian-style slugging matches seen in pop SF like Star Wars. They would require that the combatant ships precisely match speeds, which given the immense speeds at which they are moving will be very difficult to do and could probably only happen by mutual consent (which in practical terms probably means it’ll almost never happen, because the only reason one side would try for it is if it gave them some kind of tactical advantage, which the other side will try to deny them if it has any sense). Instead space battles will be in essence drive-by shootings. The combatants will plunge towards each other at dozens or hundreds of km/s and hit each other as hard as they can as they pass by each other. If both sides are lucky enough to have survivors they may turn back towards each other and try for another pass in a few hours, days, or weeks.
Another thing about motion in space is that changing your ship’s orientation does nothing to your speed and vector unless it is accompanied by firing your main engine, because there is no friction. This means that all those space dogfights where one fighter gets behind the other and the other one has to try and shake it like in air combat are very unrealistic. There’s no comprehensible reason why the pursued pilot can’t just turn his fighter around and blast the bugger. The lack of friction, incidentally, also means there’s no reason for spacecraft to be have clean lines like atmospheric vehicles so realistic ships are more likely to look like this than this.
II: WARSHIP DESIGN
OK, so now that we have that covered, time to design our warship. A single nuke or kinetic will kill it, so it won’t be built like a battleship (unless it’s an Orion, in which case it has to be just to survive the firing of its own engines). There’s no reason to bother with armor, except against lasers (more on that later). Rather, this thing will win or die on speed.
Speed confers two advantages. First, the side with the fastest ships will get to shape the battlefield, determining whether and under what circumstances the fight takes place. Second, missile range decreases against a faster ship. This is because in space the effective range of the missile is the radius within which it can cover the distance to the target ship before the target ship can accelerate to a speed exceeding the delta V of the missile along the most efficient possible “getaway” vector. The faster the acceleration of the target ship, the smaller that radius is.
There are two kinds of speed in space: acceleration and delta V (which basically means the speed the ship will attain if it expends all its propellant, which can be divided in a variety of ways depending on the mission). Our warship will ideally want both high delta V and high acceleration, since both are advantageous. Unfortunately those tend to be mutually exclusive. The rub is that there are basically two ways you can make a rocket go faster: using more propellant or using hotter propellant. The first gives you high thrust but results in very large propellant masses, the second gives you a low mass ratio (ratio of propellant to everything else) but results in very hot engines and hence restricts you to low accelerations. Trying to combine high acceleration with a low mass ratio and a high delta V generally results in a melted engine.
There are only a handful of engines that allow a combination of high thrust and low mass ratio. The most promising are Orion nuclear pulse propulsion and the nuclear salt water rocket. Some nuclear thermal designs also have thrust high enough to possibly be useful, although only for a small ship. The user “RJP” on Spacebattles also suggested something called a fission fragment drive which works by throwing high-velocity fuel fragments out the back of the ship, but other sites I’ve researched suggest it would be a low-thrust high-ISP system more suitable to an explorer than a warship. Orion works by the (seemingly insane, but actually quite effective) method of throwing nuclear bombs behind the spacecraft and having it ride the blasts. The hot gasses from the detonations hit a heavy pusher plate at the back of the ship and drive it forward. NSWR is similar, but it instead uses a solution of fissionables in salt water that spontaneously explodes as it leaves the rocket nozzle. Both systems cleverly shift the propulsive reaction outside the spacecraft, eliminating the need to deal with most of the heat it produces and allowing it to be made much more energetic. NSWR is superior to most Orion designs in terms of exhaust velocity (and hence fuel efficiency), but it has the downside of using fuel that spontaneously explodes outside of a carefully constructed reaction-dampening tank. On a warship this is an obvious liability. Orion also has the advantage of being very efficient for massive spacecraft. Orion driven warships might be large and slow, while NSWR driven warships might be small, fragile, but fast. Whichever one is more desirable will probably depend on the mission profile of the space fleet. Reasonable mass ratios for a warship will probably be between 1-10, with dry weights of several thousand tons. This would translate to delta Vs of several dozen to several hundred km/s for most Orion designs and several hundred to several thousand km/s for NSWR. A 1950s report suggested 4 G of acceleration might be reasonable for a 10,000 ton Orion, so accelerations might be anywhere between around 10 G (limited by the tolerance of the human crew) and less than 1 G. Note that as a high mass-ratio spacecraft burns through its propellant its maximum acceleration will increase; a ship with a full tank might have a maximum acceleration of under 1 G while the same ship with an empty tank might be able to get 4 G. Like WWII bombers, combat spacecraft will also become lighter and hence faster after releasing their munitions.
Since a warship will want the highest acceleration and delta V possible, it will be designed to be as light as possible. Every extra kilogram of payload lowers the delta V, unless you compensate by adding more propellant, which makes your engine work harder and lowers your acceleration. Spacecraft will be engineered like aircraft, not ships. Every effort will be made to eliminate extraneous mass and make the ship as light as possible. As much logistical burden as possible will be shifted from the ship to base. You’re unlikely to see, say, warships with hydroponic gardens. This means that, unlike in many pop SF depictions, warship crews will probably be quite small. Human beings require a lot of supporting mass in supplies, life support, and crew quarters, so spacecraft in general will probably be heavily automated. A warship will probably basically be a can full of weaponry on top of a big fuel tank, with the crew controlling the thing from a small habitat module. The crew will effectively be command crew; there to tell the machines what to do, not to micromanage the operations of the ship. You’ll probably have a small core crew to fly the ship, a few damage control technicians, and maybe a medic or two. Serving on one will be more like serving on a WWII U-boat than anything else.
Note. “Destructionator XIII” on Stardestroyer.net pointed out that if you’re using relatively slow spacecraft (unpowered intercept orbits) acceleration becomes relatively unimportant and the extra weaponry and point defense you can fit on a heavier ship might be more than worth any sacrifice in acceleration. Acceleration only becomes a serious priority when you start using torchships.
III: WEAPONS AND DEFENSE SYSTEMS
In space there are three basic kinds of weaponry available to you: missiles (guided kinetic and explosive weapons), guns (unguided kinetic weapons), and directed energy weapons (lasers and particle beams). Guns will probably be mostly useless: in order to be competitive with lasers and missiles they will need infeasible muzzle velocities of thousands of km/s. That leaves missiles as your most powerful weapons. Missiles are likely to come in two kinds: nuclear and kinetic energy. Nuclear missiles carry nuclear warheads, kinetics dispense with the warheads and use the sheer kinetic energy behind them to achieve their destructive effects. Kinetic missiles are simpler and may have slightly longer range. Nuclear missiles have the advantage of being extremely destructive at both high and low speeds, making them more flexible. Some nuclear missiles may be hybrids, programmed to detonate or not detonate the warhead depending on which will be more effective. Since most nuclear rockets don’t scale down well, missiles are likely to use chemical rockets, meaning they will have high accelerations but low delta Vs, probably around 10 km/s or so. Though missiles will probably have much higher max accelerations than ships they will, in practice, probably be programmed to accelerate just slightly faster than the target ship, because there are few things that suck quite so much as having your missile expend all its delta V in a 10 G burst of acceleration and then having the target ship get away by the simple expedient of breaking to the left when it’s out of fuel.
The great situational awareness and high speeds that characterize space combat would seem to make missiles extremely effective. On the other hand, the same factors make them extremely vulnerable. The most efficient killers of missiles are likely to be directed energy weapons like lasers and particle beams. Particle beams have better penetration but much shorter range, so lasers will probably be the weapon of choice for point defense. The maximum theoretical range of a laser (against a target on an unpredictable course) is around 1 light second (about 300,000 km). Much beyond that and light lag renders effective targeting impossible. In practice a laser’s effective range is likely to be limited by diffusion (a laser, like a flashlight beam, spreads out over distance, making it less powerful the further away you are from it). The diffusion rate of a laser will be a factor of its power, mirror radius, and wavelength. The maximum practical mirror radius for a ship laser is probably around 10 meters, unless you want to make it a spinal mounted weapon. The lasers proposed for real life proposed Star Wars missile defense system are infrared lasers with wavelengths of tens of thousands of nanometers and power levels of single to low double digit megawatts, with ranges in the thousands of kilometers. Infrared lasers have the highest diffusion rates. The best practical laser is probably going to be an ultraviolet laser. SDI estimates suggest that to kill a Soviet ICBM would require 10 kilojoules/cm^2 (100 megajoules/m^2). Missiles designed with laser PD systems in mind will probably be “armored” with a boil-off layer of a substance with a high specific heat and melting point, which may triple or quadruple this. Another neat trick is to spin the missile, so that the laser must heat both sides instead of just one, which should at least double the amount of energy necessary to kill it, depending on the focus of the beam. A missile equipped with these measures may require between 60-80 KJ/cm^2 (600-800 MJ/m^2) to kill. A moderately well-focused 100 MW ultraviolet laser will kill the missile in 2-4 seconds at 10,000 km, 20-40 seconds at 30,000 km, and 70-113 seconds at 50,000 km. The purpose of these countermeasures is not to actually save the missile but to buy time for other missiles to get closer to ship by prolonging the amount of time required to kill each missile. The maximum effective range against hardened targets may be somewhere between 40-100,000 km. As one can infer from looking at the numbers, most missile kills will probably be in the last 10-20,000 kilometers to the ship. The critical limitation on laser effectiveness at short ranges will probably be the time needed to switch from one target to the next. The actual targeting computer will probably be able to do so very quickly, but remember, we’re talking multi-megawatt lasers with 10 meter mirrors and massive cooling systems here. The turrets these things are mounted in will be literally the size of a house, and I doubt they will be able to rotate to a new target with lightening speed. A delay time of at least 2-3 seconds is probably inevitable. Another key limitation may be power and cooling. 100 MW is a lot of energy, and most of the high-performance rocket systems a warship may use don’t really lend themselves to being tapped for that kind of electrical power, meaning the ship will probably have to carry a separate reactor to power the lasers. And lasers are notoriously inefficient; the models currently being tested for the US and Israeli militaries have energy efficiencies of 10%, meaning that a 100 MW laser would be using a gigawatt of electrical power and generating 900 megawatts of waste heat, not counting inefficiencies of the power generator itself. As well as their own reactor, they’ll need massive radiators to get rid of all the waste heat they generate. There may be limits to how long you can those things burning. Current military lasers require minutes of cool-down time after a few seconds of firing time, though future systems will probably be much better.
What we basically have here is a race between missiles and lasers. A cogent point here is that, because of the way momentum works in space, the race is likely to favor lasers at low speeds and missiles at high speeds, because missiles will work better at higher engagement speeds. To illustrate, let’s imagine two scenarios involving combatant ships moving directly toward each other. Both ships have maximum accelerations of 4 G and carry missiles with a delta V of 10 km/s. In the first scenario both ships are moving at 30 km/s, in the second scenario at 1000 km/s, so their combined velocities will be 60 km/s and 2000 km/s (respectively). At 4 G it will take 250 seconds to achieve a speed of 10 km/s along the most efficient breakaway vector (a right angle), so the point at which the missiles will traverse the space between the ships in 250 seconds will mark their maximum effective range. At 60 km/s this gives the missiles an effective range of 15,000 km, at 2000 km/s it gives them an effective range of 500,000 km (greater than the distance between Earth and the moon!). Not only will the missiles have a much longer range in the second scenario, they will spend only 10 seconds crossing the 20,000 km “death zone” where the target ship’s PD lasers can kill one every few seconds, whereas in the first scenario they will launch and spend the entire 250 seconds of flight time well inside the “death zone”. This means that many more missiles will be required for a kill in the first scenario than in the second scenario. It also means that the missiles will strike the target much harder in the second scenario, though this will be mostly academic (because unless your ship is a hollowed-out asteroid being hit by something going at 60 km/s isn’t going to be any more survivable than being hit by something going at 2000 km/s – they’re just two different degrees of brutal overkill). Very significantly, in the second scenario the ships will launch their missiles well outside one another’s effective laser range, whereas in the first scenario they must come deep within it.
To protect the ship against lasers you will probably employ similar techniques to what you use to protect missiles. Most of the ship will probably be covered in a light thermal-protective jacket of a material with a high melting point and specific heat. The glaring exception will be the radiators. By their nature, they are basically impossible to armor, so they will inevitably be very fragile. One possibility is to draw them into your ship and dump your heat in an internal sink, although this will put a sharp limit on the endurance of your lasers and a big folding radiator will probably be an engineering nightmare. Another possibility is to design a segmented radiator, so that if a hole is burned in it only one or two segments will be put out of commission instead of the whole radiator. This can be combined with making the radiator in small readily replaceable sections: when the battle is over the damage control team simply pops out the damaged sections and replaces them with spares. Spinning the ship is likely to greatly increase its effective “toughness”, because it will result in the laser distributing its heat over a much wider area (remember, a ship will have a lot more surface area than a missile). Another good trick is to run chilled coolant through the area being heated by the laser. This will be especially effective in combination with rotating the ship: as the laser’s beam wanders over the ship the cold coolant will rapidly chill all the areas it isn’t actively heating, dramatically slowing down its burn-through if not stopping it altogether. Warships will probably have cooling pipes running all over the hull, buried directly beneath the protective thermal jacket. If all else fails, warships will probably be designed with a fairly high degree of compartmentalization and redundancy to reduce the amount of damage a successful burn-through can do. For instance, the habitat module will probably be divided into a number of airtight compartmentalized sections, so that a laser burning a hole in it will only decompress one section instead of the whole thing.
Point defense may be augmented by rapid-fire short range guns or (more likely) antimissiles. These are (probably) kinetic energy missiles, probably with lower delta V than the offensive missiles so they will mass and cost less per unit. They will allow you to deal with missile volleys you wouldn’t be able to with just lasers for point defense, but they will take up mass and volume that could be used for offensive missiles.
