Essay on Realistic Space Combat I Wrote

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.

IV: COMBAT

OK, so we’ve got the technical stuff worked out, so what will a real space battle probably look like? Well, you’ve probably got a fairly good idea already, but let me summarize it for you. The best terrestrial analogy for space warfare would probably be a battle on a perfectly plain at night, fought between sports cars painted with phosphorescent paint, with machine guns mounted on their hoods. All sides will be aware of the movements of the other. The battles will likely consist of long periods of boredom while the ships chase each other, accelerate towards each other, or vie for an intercept that favors them, punctuated by a few minutes of terror as they scream past each other at many kilometers per second and fire away. The primary weapon will probably be missiles, which will be fired in huge volleys. Depending on the vessels’ relative speeds, a warship will probably need to shoot dozens or hundreds of missiles to be sure of one getting through. In such an exchange, the winner is likely to be the ship with the heaviest missile throw weight, best PD, or both. Victory and defeat will be a question of cold arithmetic: can you can kill all the other guy’s missiles before they reach you and visa versa. Mutual kills will probably be quite common. As the ships pass within a few tens of thousands of kilometers of each other they may fire lasers at each other. The lasers will be relatively weak weapons, and the aim will probably mostly be to damage the enemy’s sensors, laser turrets, radiators, and other sensitive equipment. A laser battle will be a race to see which ship can cripple the other first, and will be won by the side with the best lasers or best design redundancy. At low engagement speeds the most common strategy is probably going to hamstring the enemy ship as badly as you can with your lasers and then finish it off with missiles. If the engagement leaves survivors on both sides and one side wants to press the attack they will have to do so by burning off their momentum and putting themselves back on an intercept course, basically reversing their present course.

Fans of David Webber may notice this sounds a little like how space combat works in his Honor Harrington novels. They wouldn’t be wrong. Despite its large amounts of rubber science technology and deliberate resemblance to eighteenth century naval warfare HH actually has one of the more realistic depictions of space combat in SF, and probably one of the most realistic of any universe that isn’t deliberately trying to be hard SF.

 
V: DEFENSE

Defensive policy in a mature militarized spacefaring civilization will have to deal with the fact that the sort of drives such a civilization is likely to use will run full-tilt into Jon’s Law: any drive powerful enough to be interesting is powerful enough to be a weapon of mass destruction. Let’s say you have a nuclear salt water rocket propelled warship with a mass ratio of 5, which will give it a delta V of 8,421 km/s, and you want to bomb New York. If you expend all your propellant in a single burn in the direction of Earth and then launch a 1 ton missile into New York City it will impact with the energy equal to an 8.48 megaton bomb. Consider that to be able to saturate the PD of enemy warships your ship will probably carry several hundred such missiles. In other words, forget about New York, you can singlehandedly devastate the entire United States. And this is peanuts compared to what you can do if you really want to make a splash: crash your ship into the planet along with your missiles. If your ship weighs 10,000 tons it will impact with a force of 84.8 gigatons. Assuming the blast is similar to an equivalent size nuclear explosion that will cause widespread destruction for hundreds of kilometers around and third degree burns more than a thousand kilometers from ground zero. Essentially, it will wipe out the entire state of New Jersey, and turn the entire northeastern United States into a disaster area. Basically anybody willing to fly his ship into a planet can singlehandedly kill tens of millions of people. And there’s no reason to demand human crews consign themselves to riding a giant kinetic missile into a planet to do this either. You can just take the propellant tanks and engine of a warship, mount a simple targeting computer on it in place of the weaponry and habitat modules, and create a similarly powerful kinetic missile with no need for a crew of would-be suicide bombers.

As if this isn’t bad enough it’s not just enemy states you have to worry about. If your warships have high performance engines odds are so will a lot of your merchant ships, which will basically turn them into the ultimate mad bomber’s dream. Imagine a twenty-fourth century Mohammad Atta riding a hijacked merchant freighter into downtown Manhattan at a few hundred kilometers per second. It’s enough to make one nostalgic for the height of the Cold War!

What all this means is that the orbital space around a planet will be very tightly watched and defended. In pop SF fixed defenses usually take the form of armed space stations bristling with weaponry like Medieval castles. Realistically, it’ll probably be a bit different. The largely one-shot one-kill nature of space combat will tend to push defensive installations away from reliance on a handful of hardened installations and toward reliance on a large number of dispersed expendable platforms. There will be military space stations, but they’ll be refueling and servicing facilities for the warships. Defensive firepower will likely take the form of a network of laser and missile satellites and possibly bomb-pumped X-ray laser “mines”, rather like the Star Wars program proposed under the Reagan administration but built around threats from above rather than below. This will undoubtedly be augmented by extensive ground-based antimissile systems to catch any missiles that slip through the net in orbit.

Incidentally, this probably means you won’t get any Han Solo type free traders in a realistic universe. Even assuming spacecraft are cheap enough for small-time operators to afford (unlikely), even a very slow small craft that putters around in Hohmann orbits will be a potential multi-kiloton kinetic energy missile, and the sort of ship that will get anywhere in less than six months has a destructive potential in the wrong hands that doesn’t bear thinking about. High speed ships will be treated with the same respect that we treat nuclear reactors; they will not be handed out like sporting yachts to anybody who can afford them, even if it’s economically feasible for your average citizen to own one.

 
VI: THE MYTH OF THE SPACE FIGHTER

This one is pretty much beating a dead horse, but I feel the need to include it for completeness. The notion of a space fighter arises simply from overstretching the analogy between space combat and sea warfare. Aircraft came to dominate over battleships on Earth because they enjoyed speed advantages of orders of magnitude and could move in three dimensions, while surface ships were restricted to two. True, large spacecraft will probably have lower accelerations than small ones, but let’s consider what a space fighter will do for a moment. It would be a small craft designed to deliver missiles to a target out of range of the main fleet and then return to a carrier ship. But why bother returning to the carrier? The fighter is probably nothing more than a glorified chemical-fueled missile anyway, no more sophisticated in principle than the missiles you already expend by the dozen. You can at least double its effective range by replacing the pilot with a computer and turning it into a disposable missile bus (I say at least because the computer will probably mass a lot less than the pilot and the life support systems necessary to sustain him). It may be able to accelerate faster too, since it’s now freed from the restriction of having to not kill the pilot with bone-crushing sustained G forces.

The closest thing one is likely to see to a space fighter in a realistic universe is something like the X-15 DynaSoar. It’s basically a very small one-man armed space shuttle, designed to be lifted into orbit on top of a disposal rocket booster, where it could attack enemy space infrastructure or drop bombs on the planet below and then land like a normal aircraft once it had finished its mission. Such a craft would probably have utility very early in the history of a spacefaring civilization, when it was just starting out.

 
VII: THE MYTH OF THE SPACE PIRATE

A less oft-discussed counterpart of the space fighter is the space pirate. Like the space fighter, the space pirate is a result of too-close analogizing between space and the sea, in this case the age of sail rather than the sea battles of WWII. The problem is the buccaneer, like the fighter plane, is a concept that does not translate well to the environment of space.

The reason why can be demonstrated best by a little example. Let’s say our swashbuckling space pirate sets his sight on a merchant ship full of some valuable high-cost low-bulk booty (platinum or fissionables, perhaps). With his fast raider he easily intercepts the lumbering craft, bullies the crew into letting him dock, takes the booty, and heads back to his secret pirate base out in the Kuiper belt. He’s just gotten home and is considering how to unload his ill-gotten gains when one of his lieutenants brings him some bad news: there’s a couple of warships headed straight for him at 1.5 Gs. You see the ever-watchful electronic eyes of the United Nations’ Space Command’s observation platforms have seen the whole thing, and his secret pirate base stopped being secret the instant the light from his rendezvous burns reached them. Ouch! Quite simply, piracy only works if the pirates can disappear when real warships come looking for them, and there’s no disappearing when you light up every sensor in the neighborhood every time you fire your engines.

Defenders of the space pirate may say that, with FTL, the concept may work. After all, as big as a solar system is, interstellar space is much, much bigger. Trying to patrol it will be an incredible challenge, and simply outright impossible with most imaginable tech bases. I hadn’t really intended to discuss FTL here, because the rules for it are so variable (on account of nobody having any idea how it would actually work – if it could work at all, that is), but I really feel the need to point out that even with most imaginable FTL schemes space piracy is a no go. The only FTL schemes where space piracy works are those where ships can see and fight while in FTL mode. With hyperdrive that may be possible, depending on the nature of the hyperspace. With warp drive it may be possible with sufficient magitech, but it’s a pretty tall order requiring at least one, probably several extra pieces of technobabble. With jump drive, wormholes, and Krasnikov tubes it just doesn’t work.

Even if you have the right pieces of technology, there are other problems with it. First, it requires that FTL ships be cheap enough that criminals can acquire them. This is another area in which the analogy between the age of sail and the space age breaks down. Sailing ships were skill-intensive but materially cheap. You had to have people with the right skills, but once you did all you needed was wood, rope, and cloth. But spacecraft are going to follow a post-industrial revolution paradigm of being materially expensive as well as skill-intensive. They are likely to require sophisticated, precision-manufactured components and expensive fuels like helium 3, fissionables, or antimatter. Imagine Captain Jack Sparrow commanding a nuclear-powered aircraft carrier and you’ll get an idea of the kind of difference we’re talking about. And even if you have cheap ships, they’ll still be treated with all the caution and respect one reserves for potential WMDs. Remember what I said earlier about Jon’s Law and the consequences it’ll have for free traders? It’ll have similar consequences for would-be pirates. A criminal organization getting its hands on a starship will be a feat of similar magnitude to a criminal organization getting its hands on nuclear fuel today. It’s theoretically possible, but it won’t be easy, and if you want to make a quick buck there are going to be a thousand more profitable and much less risky ways to do it (not to mention that any criminals that do pull it off will probably make a lot more money selling it to would-be terrorists than using it to go around stealing stuff from cargo ships). Basically, to have viable space piracy you need the right kind of FTL drive, FTL sensors, FTL weaponry, cheap ships, and technobabble defense systems capable of deflecting gigaton range kinetic energy weapons (so starships aren’t treated as WMDs anymore).

The closest plausible thing to space pirates are commerce raiders. These are warships used like German U-boats in WWII: to disrupt enemy shipping by destroying as many enemy merchant ships as possible. Unlike space pirates they need not keep their movements secret, they are operated by exactly the sort of organizations that would logically own high-speed spacecraft (national governments), and they need not turn a profit.

I agree with pretty much everything except the idea of kinetic kill missiles if space fights occur at the speeds you're suggesting. It seems like it would make a lot more sense to use either nukes or nuke dispersing warheads for proximity effect. Hell, wouldn't just flying through the plasma of a nuclear blast that occurred 20 seconds ago fuck a ship over?

No, because in vacuum there is NO blast........

And IIRC even to do real damage with a 1 Mt nuke you still need to detonate it as close as 1 km to your target. Given the distances involved you might as well go for a contact kill rather than a proximity one.

What about ground based weapons. Concerns about power-plant mass and heat dissipation become essentially moot there. Couldn't you defend a planet with fields of lasers even the largest ships couldn't hope to have?


And while not cost effective, surface launched missiles could be militarily effective.


Remember, there's no stealth in space, but there most certainly is stealth in an atmosphere.

Atmosphere is not your laser's friend, and firing a missile from surface to space is a bitch. You're going to need a booster and expend large amounts of fuel just to climb out of the gravity well, and then you need to have a regular missile on top of that. Moon-based lasers and maybe even missiles might be more forgivable, because you could have big heat sinks and have no atmosphere to worry about, but then you have the problem of being a big, huge target for people to sling difficult-to-deflect rocks at.

Correct. Relative to the distances we're talking about here a nuke in space might as well be a contact weapon. They would be most useful if you're trying to achieve mass destruction when you've got relatively low velocities involved, such as an orbiting warship bombarding a planet. A nuclear warhead wrapped in a heat shield with guidance fins and just enough fuel to deorbit itself would do a lot more damage than an orbit-dropped kinetic rod of the same mass.

As has been said, surface-launched weapons are problematic. A surface-launched missile, for instance, would probably have to be 15-20 times larger than an orbit-to-space missile. You're probably better off putting most of your defense weapons in orbit. A civilization like we're talking about should have fairly impressive STO payload capabilities, and an orbiting weapon can still be as big as you want since it doesn't have to move.

True. Although you would reveal your position the instant you fire. One interesting possibility I can think of is orbital weapons mounted on a submarine: I doubt an orbiting spacecraft could scan through water.

What about nuclear bomb powered X-ray lasers? It's a missile that doesn't have to hit.

The proposed SDI bomb pumped designs were very inefficient, but I don't know what kind of theoretical efficiencies these devices can possibly have.

Even if their effective range is less than point defence range, you reduce the time defenders have to shoot down your missile.

Ofcourse, you'd need to use frequencies that penetrate the atmosphere. The much increased power would help. And as said, atmosphereless planets los pose a large advantage.

The missile thing is mroe a concern of "why let them just poind you with asteroids, you should do something. As a last resort weapon, they'd be foolkishly expensive, but impossible to detect, and would give yo a fighting chance at targets outside of any laser range.

Depends. They might have a somewhat better hit rate than missiles, but they'd only be making holes in enemy ships instead of blowing them up.

They might be particularly effective against ships that use volatile fuel, like NSWR. Program them to shoot for the fuel tanks.

To me would seems best to specialize warship designs and have them operate in a combined arms fashion like naval battle groups than to have the multi-capability ones that are often discussed. If you build a warship with a spinal laser, a moderate missile payload, and a massive amounts of point defense lasers it will be compromised. However, a dedicated destroyer blistering with PDLs and no other weapons while using its payload capacity to carry the maximum amount of coolant is going to make a better anti-missile platform since it can fire more PDLs for longer periods of time. A dedicated missile cruiser can its maximum payload capacity to carry the most number of missiles possible and if designed right fire the most number of missiles possible. And a battleship would be just like the destroyer, except of course it has one big spinal cannon instead of numerous small ones, in that its payload is dedicated to coolant.

On the issue of point defense laser issues with target switching time and cooling could not the problem be somewhat be alleviated by increasing the number of PDLs. You could stager your fire so that you have some PDLs firing while others are seeking and the rest are cooling and seeking to get a high overall rate of fire. Yes you still have to deal with the total heat but you now you do not have to wait for the last turret fired to cool down before taking the next shot. I wounder if it might be possible to transfer heat from fired PDLs to unfired ones that are in unusable firing arcs.

Another approach to making PDLs manageable might be instead of taking one size fits all approach make some of them large (10m) for long range, some of them medium sized (5m) medium range, and the last small (2.5m) for short range. The idea being that the short and medium ranged ones being less massive would take less time to move and stabilize. Additionally you can fit more small and medium turrets on the same foot print than you can fit a large ones.

On the issue of particle beam I am not sure when atomics rockets refers to its rang if it is talking about hard kill range or soft kill range.

You still have the big problem of RADIATING the heat away. Simply increasing the number of lasers and firing them less often, does not change that. The waste heat generated is still in your ship, and must be radiated away.

Also, I am skeptical as to the amount of damage that could be caused on the laser mirrors by microasteroids impacting it. This is posible, considering that we are talking anout mirrors of a 10m diameter. Something like that could seriously impair the laser efficiency.

Keep them covered while not in use, it's a simple enough solution....

I just had an idea, and I wonder how feasible this would be.

Have your ship equipped with some kind of scoop to catch interstellar dust and gas and such. Now, I assume that stuff would be fairly cold. Collect it in an internal container and use it as a heat sink.

When it gets too hot, jettison and repeat.

Or you could y'know, use water, liquid coolants, any number of other forms of internal heat sink and external radiator.

On laser mirrors: remember that the mirror size can be reduced by making the wavelength of the laser smaller. My original calculations were for a 400 nanometer UV laser, with a 200 nanometer laser you can reduce the mirror size to 5 meters and have the same effective range. This lets you have a smaller, easier to rotate turret, which is very good because at close range where lasers will be most effective that's the main limitation on how many missiles you can kill.

And some sort of covering for the laser aperture would probably be a good idea, I think.

You're describing a Bussard ramjet, but using the stuff you collect as coolant instead of fuel. It might be doable but unfortunately, aside from the problems associated with trying to do H-H fusion, it has all the same problems as a Bussard ramjet. You need a magnetic scoop thousands of kilometers across, and the ship needs to be moving at between 1-6% c (3000-18,000 km/s), before it starts working, so your warship will require a lot of fuel. It doesn't really seem worth it.

If you intend to store the coolant you also have to deal with the fact that slowing down all that gas will create drag which will reduce your engine performance (this is what basically killed the classical Bussard ramjet as a propulsion concept; above .12 c the drag is greater than the engine thrust). You can, however, get around this in the same way modern theoretical Bussard ramjet-derivative propulsion systems do: by letting the gas travel through the middle of your ship at high speed. The collection process will still create drag but it is reduced. Note that transferring heat to this stuff will accelerate it, so your cooling system here will double as a low-power engine.

The problem with water and internal heat sinks is that eventually they'll have too much heat in them to function as a heat sink, limiting a ship's endurance. That was my point. A constantly renewing heat sink.

The problem with external radiators is that in a fight they're a big honkin' target.

Isn't some of the problem with the need for a big scoop size alleviated by the fact that hopefully you're not needing new coolant as often as you're needing more fuel? Wouldn't even a very low collection over a long period of time give you enough replacement coolant?

If you're using a RAIR-type design where you don't slow down the coolant you'll need a big scoop; the gas shoots through the ship at thousands of km/s and cannot be stored. A classical Bussard ramjet is more feasible if you're using it to collect coolant instead of fuel but you will still need to use a large scoop if you want to be able to use it tactically in the way you envision. With a small scoop it'll take a long time to refill the coolant tank; much longer than you're likely to have during the battle.

Note that if you're using a RAIR-type design you really should have it double as an engine, because you have almost all the machinery you need to make a true RAIR.: just throw a little lithium or boron into the slipstream when you're not using it for coolant and you have a low-grade fusion drive. Since you're probably going to have to be travelling over 10,000 km/s to use this thing at all your warship will need all the help it can get reducing its mass ratio.

Yes I acknowledged that with

I never said anything about firing them less often. To the contrary at that point I was discussing maximizing the total rate of fire. Radiating your heat away is going to be moot if an enemy missile gets through.

Where I did go into cooling was in two sections.

and

And I did not mean in a 1:1 ratio.

Could a segmented mirror work like the ones on solar towers along with the already mentioned protective covers?

Oh, I don't mean using it during battle, but while traveling basically, as a way to improve the logistical endurance of the ship, because replacing dust is easier than replacing water (if that was what you were using for a heat sink).

Which is why you would use both internal heat sinks and external radiators. Its not like you wont be able see when enemy ships and missiles are getting close enough to pose a danger to your radiators. You should have more than enough time retract radiators. The actual shooting portion of the fight should not last long enough to make maxing out your internal heat sink an issue even after you have retracted your radiators. After the missile exchange is over extend your radiators back out until you get into cannon range and repeat. The cannon fight should be even shorter.

Also every once in a while someone brings up the idea of dumping your heat into your exhaust but I am not familiar of the merits of that.

Let me put it this way.

1 PDL produces X heat per "shot".

If you fire the same PDL 5 times, or fire 5 seperate PDLs you are still going to get 5X heat.

The difference is that with one PDL you have to wait until the mirror has cooled down enough so it will not be destroyed with the next "shot", which will also give you some time to radiate the waste heat generated due to the inneficiency of the laser, while with 5 PDLs you don't have to wait, but produce the same amount of heat in a smaller time span.

And yes, if a missile goes through the defences you are dead, but you are also dead if you are cooked by your own waste heat.


And Memphet'ran, just curious, what effective ranges are we discussing here? And how much power do the lasers generate? And the efficiency?

If that's the plan you're much better off using a conventional heat sink and radiator arrangement with retractable radiators. It'll be much less of a headache than the massive and complicated equipment required for a Bussard-style collector. You also don't need to be moving at very high speeds to use it.

My calculations were for a 400 nanometer UV laser with a power level of 100 MW and a mirror 10 meters across (or a 200 nanometer laser with a mirror 5 meters across). Here are the kill figures I got. They assume a rotating missile with a protective thermal jacket, for the kinds of missiles used today divide the times by six. Effective ranges against other warships will probably be much lower, because they can run coolant through the effected area and dump slowly accumulating heat elsewhere on their vastly larger surface area.

At 10,000 km -- 4 seconds for thermal kill
At 20,000 km -- 17 seconds
At 30,000 km -- 40 seconds
At 40,000 km -- 71 seconds
At 50,000 km -- 113 seconds

Effective range for a laser capable of continuous firing is difficult to quantify, it will depend on the "toughness" of the target and the relative speeds involved, and is more a subjective calculation of "is the effort worth it" than an absolute limit. For a laser with limited firing time effective range is more sharply limited. Modern military lasers can only fire for a few seconds and require minutes of cool-down time but they're the laser equivalent of the muzzle-loading arquebus.

Modern military lasers have a 10% efficiency so a 100 MW laser will require 1 GW of power and create 900 MW of waste heat. Free electron lasers have a maximum theoretical efficiency of 65%, with most others it's much lower, 1/10-1/3 is probably a sane practical efficiency range for a laser. Of course for waste heat you've also got to factor in the waste heat of the reactor itself, which at typical efficiencies will probably be another few hundred MW.

I found this essay to be quite informative. I am a new member to this forum and I'm going through the different posts to compare what the different members have to say about space combat and I find this to be very useful. Although I happen to think star fighters are anything but a myth this is still an excellent and intellectual essay.