I run a bi-weekly sci-fi RPG, with an original backstory and universe. I have been trying to maintain reasonably plausible and consistent technology base, introducing as few "magic-tech" devices as possible and exploring the ramifications of those. The recent threads about Enterprise and Stargate military operations have got me thinking about how to model un space combat in my game at least semi-plausibly. I'd particuarily like Stuart's opinion here.

I assume FTL travel similar to that used in Mote In God's Eye, where a "jump drive" can be activated in certain spots in space to take you to certain other spots in space. Travel between worlds is a matter of flying from jump point to jump point at significantly sublight speeds, with only a few minutes spent FTL in jump space. I assume cheap and easy Helium-3 fusion power, and fusion drives with improbably high ISP and thrust-to-weight. Other than that, technology is not significantly advanced beyond current levels - this may seen odd, but it is due to specific cataclysmic events in the universe backstory. No transporters, shields, FTL sensors or FTL communication, or artificial gravity, sensor and targeting capabilities comparable to what we have today, but nearly unlimited delta-V and electrical power available. I have been considering the existence of energy weapons directly powered by fusion reactions - lasers or relativistic plasma beams.

How should space combat work with these assumptions? It seems to me that with no shields, and weapons that include fusion-bomb-tipped missiles, that combat between well-equipped navies would be a short and violent thing. If one good hit cripples or destroys any ship, targetting, sensors, and stealth become pretty damn important, and carriers hanging back from the combat and deploying small, fast missile-ships to attack the opposing fleet becomes the strategy. On the other hand, any decent size ship should carry antimissile lasers, projectile guns, and missiles, as well as ECM, which would imply that ships would have to get close to actually hit each other.

The players in this game won't be directly taking place in any serious battles - they own a small secondhand cargo ship, but will sometimes get caught in the crossfire when larger powers clash.

I would take a look the Honor Harrington series or the Night's Dawn series for good looks at how space combat might work.

Honor Harrington uses artificial gravity for propulsion, but you can change that to read He-3 and just lower the acceleration curves (HH has missiles accelerating at thousands of gees, and starships at 400+ gees thanks to artifical gravity).

Night's Dawn uses He-3 (and antimatter to a small, illegal, extent), so it's ranges and accelerations are closer to what you're looking for.

In both books, the stress is on missiles, not direct fire weapons, for both offense and defense. The missiles are basically just large fusion/nuclear bomb pumped laser systems. When detonated, the explosion powers one shot lasers/maser/x-ray lasers that then damage the ship. Since ship combat would most likely take place at hundreds of thousands of kilometers, maybe even millions of kilometers, direct fire weapons would suffer both loss of energy due to range, and the increased possibility of missing due to travel time (which lets ECM have more effect).

In HH, some missiles are not equipped with warheads but are instead ECM and ECCM missiles that are designed to make it easier for the other missiles to hit their targets without being shot down. Other missiles are much smaller and more of an area of effect weapon designed to shoot down incoming missiles.

In Night's Dawn, offensive and defensive missiles (called Combat Wasps) are identical. They are basically drones that are controlled from the launching ship. They can be fired like normal missiles, seeded like mines, or launched and told to say near the ship and act as anti-missile missiles. They come in both bomb-pumped laser and standard nuclear bomb varities.

Either way, the key to victory in both series is acceleration. Absolute velocity means nothing in space combat, it's all about acceleration. If you can out accelerate your opponent, then no matter how fast he is going, you can eventually lose him (as long as you survive until then). Higher acceleration for your missiles means less flight time and increased survivability.

Finally, I want to make a note on your lack of shields, good job . If you're going to try to be realsitic then you can never add any form of force field technology. There is nothing in our knowledge of physics that would even allow for the kind of shielding found in Star Trek (and similar shows). We'd have to discover a new fundemental force or make some radical new discovery that changes everything before force fields would be possible.

To put it another way, we have plausable theories on how time travel would work, but nothing for force fields.

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Just a quick post about weapons: Electromagnetic weapons which throw things could be pretty devastating to a ship. Just punch some holes and let the air escape. Of course you would have to be pretty close to accurately fire these weapons. For instance, in Star Trek, they usually get very close to one another when fighting. A large railgun would work wonders there once the shields were down.

Lasers, lasers would be the most probable form of energy weapon and they have the advantage of getting to their target almost instantaneously, no matter the distance. Another plus to them is that you don't run out of ammunition. They simply keep drawing energy off of the core.

The Man-Kzin Wars are also some good books to read. They make the point that a fusion drive makes a mighty fine weapon. -Colt

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I don't think you should ignore Isp--otherwise things get a bit too silly.

I ran through some numbers; i could pretty easily have made a mistake. Anyway, i get an exhaust velocity of around 2.5%c with 3He + D (the site i found seemed to say that the proton carried 14.7 MeV, the 4He carried 3.7 MeV, with another 18.4 MeV presumably being emitted as gammas. I assumed that the drive plopped those 18.4 MeV onto the 4He nucleus). Someone should go over those; there's lots of room for mistakes. If you assume your missile is half fuel, that gives a total delta V of 1.7% c.

Liquid hydrogen has a specific gravity of .07, liquid helium of .125. However, we're dealing with D and 3He, so the densities need to be changed to .14 and .09, respectively. So, 1 ton of D occupies about 7 cubic meters, 1 ton of 3He about 10 cubic meters. So a 4 ton missile, half fuel, is going to take up about 20 cubic meters.

Obviously, an impact from a two ton missile moving at 1.7% c is going to be fairly significant. The best strategy would probably be to accelerate towards the target, and blow the missile to atoms just before impact. At 100 g's, it would take about about 1.5 hours to hit max speed. That's about 20 million kilometers--on the order of a light minute.

There's not an awful lot you can do to defend against a missile of this sort. Jamming the guidance package comes to mind. Anti-missile missiles are probably your best bet. You're going to have to intercept the missile quite a ways out and maneuver to dodge the fragments. Directed energy weapons seem too short range to be useful.

You mentioned stealth. I think you should forget this, unless you're thinking about relativistic planet busters (well, probably county or state busters). Any accelerating ship will stand out like a beacon.

You'll have to decide if ships coming through the transport point have to be moving at a particular velocity or not. If so, it should be pretty simple to set up forts to defend those.

Pretty much, ships coming through have to be travelling slowly relative to the jump point. It's not the physics so much as the need to position the ship very precisely before activating the jump drive - activation the drive in the wrong place is lethally bad. So defending systems by guarding the jump points is practical, giving the advantage to the defender in inter-system wars.

First off, please, please, PLEASE remember that lasers are invisible unless thay pass through some form of cloud.

Also, if you have missiles, you don't really need explosives. Just get them up to a very high speed and slam them into a ship. They'd make a big hole, no explosives required. Think of them as huge, guided bullets.

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Modesitt's Ecolitian (or maybe it's the Distant Earth) series has some good space combat. Also, Elizabeth Moon does it pretty well, although she has less of it.

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A 1 meter diameter missile going at 1%c makes a 1 meter diameter hole in whatever it hits. If your missile blasts itself into a 100 meter wide cloud of vapor before it hits, it makes a 100 meter wide hole. You want the explosive to increase the cross section of the missile, not to impart damage to the bad guys.

And you want to do this while conserving as much of the linear momentum you've got going - it's no good if your scatter blast disperses the missle outside the bounds of the target area, or significantly decellerates a large portion of the projectile, you're undoing the whole premise behind the massive acceleration you've been doing.

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If you aim it right all you need is a 1 metre hole! But I see the advantages of a bigger one... *Evil Grin*

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Punch the 1 meter hole and then explode.

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There you go! Better: puch the hole in something explosive then explode.

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Defense mechanisms might depend on how big the transfer point is, or what happens to the ship in transit if something is occupying the transfer point. Small transfer points limit the size of ships. Having the transfer either not take place or go fatally wrong if there's an object already at the far side of the transfer makes defending the points trivial. If transfer is a two-way street (contents of the transfer points get swapped) a more active defense is required--maybe something like a Lofstrom Launch Loop (think streams of orbital-velocity iron pellets criss-crossing the transfer point). Not very friendly to unannounced visitors. In this case, an attacker would probably send a big rock through first, to knock out the pellets.

Chemical explosives would do an insignificant amount of additional damage, and unless the ships have antimatter somewhere inside, there's probably no way to trigger an atomic explosion that quickly.

Any amount of additional dammage is desirable, no matter how small it may be. It doesn't take much extra effort to aim the missile where you know fuel or munitions are stored. Even if they do not do much dammage, they are still good things to destroy.

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Definitely detonate the warhead before the missile gets to the enemy ship- if you detonate it inside, the momentum could carry the explosion through the ship and out the other side... as the crew should all be suited up, you won't be gaining anything by creating a small hull breach.

And use antimatter whenever you can- even if it is illegal- you do wnt to win, dont you?
Even more illegal weapons include the Strange Matter Virus, singularity projector,and conversion weapons.

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Shoot the missile so it penetrates the hull length-wise and detonate it as soon as it penetrates. If you have a big enough missile, the explosion will move along the length of the ship and gut it.

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Shoot the missile at the command center and then have it explode...no more command crew!

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Too easy. I need a challenge.

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Ok, ummm, then try to take out all the weapons systems and get the hatches/airlocks to stick closed, and disable the engines. But don't do to much damage, we want to be able to salvage the ship!

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Sounds like a plan! But, uh, how are we gonna kill the crew?

Oh, I've got it! We set off a series of explosions around the ship that cause it to spin with so much force that the crew is subjected to 200 gs of force! But I'm not going to clean it up...

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If you're using realisitic space combat, then odds are you'll be lucky to get your missiles within 100 km of your target, and you can forget about hitting it directly.

Extrapolating todays technology means that space combat will take place at long ranges, hundreds of thousands to millions of kilometers. We don't have the armor to survive impacts from high velocity kinetic weapons or nearby atomic ones, at least not with giving the spaceship so much mass that it has almost no acceleration.

So bomb-pumped x-ray lasers would be a very effective weapon when attached to a missile. You just within a light second or two of your target and detonate. The x-rays would cause serious damage to whatever they hit. It's unlikely that a smaller spacecraft would even survive a single hit.

Once acclerations start increasing (and thus ranges as well) projectile weapons will start to disappear. Longer range weapons will be dominated by missiles while shorter ranges will be controlled by high powered masers and x-ray lasers.

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The book War in 2080 by David Langford has a couple of chapters devoted to the physics of space combat, both hand to hand and space ship to space ship. Even though some of the information is out of date, it's still a great book.

Think we could get Humphrey to clean it up?

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Que?

Sorry,

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I had a long think about this. What we basically have is a situation where offensive weaponry equates to one hit = one kill. How that kill gets to happen isn't really relevent to the primary problem. At that point it dawned on me that the situation is actually quite like submarine warfare today. There, any hit is fatal (a torpedoed submarine will sink with total loss of life, even the big Russian boats) so the whole objective is to avoid getting hit. In our other space combat string, the need for dramatic tension is mentioned - the dramatic tension is right there - a target MUST NOT be hit. If it is, it dies. In TOS Balance of Terror , there is a good scene where Enterprise is running from an inbound plasma bolt that WILL destroy the ship if a hit gets scored. The tension on the bridge is palpable.

So lets look at defenses. Basically there are four categories; active, passive, deceptive and evasive.

Active Defenses These rely on active countermeasures to physically destroy the inbound threat. Their nature is hard to guess in this context since we have an undefined inbound threat. Assuming that its some sort of missile, an active defense would consist of a gun or missile based defense system that would isolate an inbound threat, identify it, determine its course and speed and then fire the defensive weapon at it. Such weapons today are the Mark 15 Phalanx or the RIM-116 RAM (the first is a gun, the second a missile). In this context such a system would be automated because humans can't react fast enough. Active systems are countered by the threat using a variety of techniques. One type of modern anti-ship missile is called a "streaker" - these move so fast that the defense system has extreme problems in making an intercept (the Russian P-270 Moskit is an excellent streaker). Other missiles are called "dancers" - these are much slower but perform intricate evasive manoeuvers to avoid the active defenses. Harpoon or Exocet MM-40 Block III are good dancers. Don't underestimate the danger of firing cheap simple and very vulnerable missiles - enough of them and one should get through - and one is all it takes.

Passive defenses Which brings us to passive defenses. These are basically concerned with limiting the effects of a hit. In the old days it was done by using heavy armor. That may not be the best approach here. The objective is to limit the effect of a hit; that could possibly be best achieved by ensuring the threat didn't explode. Its arguable that building the warship extremely lightly so that the resistance caused by a hit does not explode the warhead.. Taking that too extremes, I can see a starship design that is primarily a lightweight latticework separating individual key areas. Fast-acting airtight doors are a must - if hull integrity is breeched, they slam shut (similar doors are in the hangars of US carriers - they have no safety system. If you are in the way when they close, they will kill you. I've seen these doors cut a US$80 million F-14 in half). That means that if one of the essential areas is cut off (physically or methophorically) , it can survive on its own. Now, most radar-based guidance systems see a target as a mass of corner reflectors and use those to construct an image then home in on the geometric center of that target - so a good passive defense would be to distort the radar image so that such a guidance system would have a good chance of passing through the structure without detonating. Passive defenses are countered by spreading damage as widely as possible and trying to get multiple hits.

Deceptive Defenses that, of course brings us to deceptive defenses. This means persuading the threat that the target is either something it is not or somewhere it isn't. Deceptive electronic jamming can be used to create misplaced radar returns (extend to other forms of sensors), decoys can be used to lure threats away, others can be used to create false threats to divert an enemy. A ship can fake critical damage to lure an enemy in or conceal critical damage to trick that enemy into pulling back. A key part of all this is understanding enemy systems so they can be spoofed. That means details of sensor characteristics are critical. That, in turn, means that sensor information is going to be closely guarded and equipment rarely used to its full potential (you would be surprised how rarely an AEGIS ship turns its marvellous radars on). Deceptive defenses are countered by sophisticated sensors and good intelligence. Its critical to know exactly what the capabilities of the threat are. Food for thought, Get a military reference book and read the specs of military equipment there. Almost every single number you read is either wrong, incomplete or so generalized as to be worthless.

Evasive Defenses Evasive defenses means not allowing the enemy to know you are there. If he doesn't know you're around, he can't shoot at you. Earlier in this string there was a comment that stealth wouldn't matter. I profoundly disagree with that; "stealth" (actually reduced signature or RS) is going to be the key parameter. Remember no level of RS is going to hide the target completely; what it will do is to reduce the range at which sighting becomes inevitable. That is immensely worthwhile because he who gets seen first will get shot at first. That means they have a good chance of being hit first and one hit is all it takes. This applies in submarines today; a friend of mine who drives nuclear boats compared a duel between two SSNs of equal standard and crew training to two men fighting a duel with sawed-off shotguns at one pace range. Using evasion for tactics is a key here. For example, we have a defending formation facing an attack down a threat axis. As the threat approaches they gear up to attack. What they don't know is that sitting a little off the threat axis is a single ship with its sensors off and SR operating at max. At the appropriate moment, that ship unveils and rakes the attacking formation with missiles. Sound far-fetched? Been done. In the Norwegian Sea. No missiles were actually fired of course, but the Soviet bomber crews had kittens when they were illuminated at slightly less than point-blank range. If it had been a real shooting war instead of a little Cold War jousting, they would have been slaughtered and they knew it. Evasive defenses are couuntered by reconaissance. Old military maxim - time spent in reconaissance is never wasted.

So putting all this together, we have an environment will be a battle where the opposing fleets are continually probing and manoeuvering, pushing forward and pulling back, each seeking an opening where it can get in a shot with a good chance of succcess - also knowing that if that shot fails, retribution will be certain and final. Coming back to the dramatic tension issue, this is the real thing. A bridge crew fighting an inbound missile knowing if their defenses fail everybody is going to die. The attacking crew fighting to get their shot home, knowing that if they miss, the counterpunch will probably kill them. Beats hell out of hippies fighting with technobabble doesn't it?

Taking this a bit further, its almost certain we would be looking at the primary striking ship being mother-ship carrying large numbers of smaller units. Just so we can use modern vernacular, carriers and a stargroup of starfighters. Reason why is a bit of history. Battleships didn't become obsolete because of torpedoes or aircraft - at least not directly. Battleships became obsolete because the only way they could be destroyed was to risk another asset of equal or greater value. That meant that any conflict involving them was likely to be indecisive (because neither side could afford serious losses) . However, the evolution of the aircraft carrier meant that a battleship could be endangered by the deployment of assets (aircraft) that cost far less than it did yet could carry a weapon that could seriously hurt the battleship (a torpedo). Same applies here. If one side can threaten the other's primary assets with counters that cost far less than the target under threat, then the side under threat has a very serious problem.

Wow, war's quite harsh.

Nathan Bedford Forrest summed it up best. "War means fighting and fighting means killing". The whole concept of warfare is industrialised murder. Colonel Supatra once summarized it by saying that "troops do what it takes to win no more no less" then added "the hardest thing to accept is not what you did to others, it is what you did made you." That's why its so important to be good at it. Nothing is more futile or useless than a second-rate military.

Wow!

That's given me a lot to work with. Thanks.

It looks like the only sure defense it to have the other guy not even realize you're there until it's too late.