Ministry of Misinformation

Sulphur on 26/8/2010 at 20:33

@catbarf: You're assuming a 100 km distance. And also assuming relative velocities for both vehicles to be the same. If your craft's travelling faster than the one you're targeting, the margin of error reduces in that order. There's plenty more irritating math involved in space, but it's ballistics and tracking at the end of the day.

catbarf on 26/8/2010 at 22:23

Quote Posted by Sulphur

@catbarf: You're assuming a 100 km distance.

The effective range of a laser or missile is far greater. Does it make any difference if we assume 50km, giving them 12.25km of dodge room?

Quote Posted by Sulphur

And also assuming relative velocities for both vehicles to be the same. If your craft's travelling faster than the one you're targeting, the margin of error reduces in that order.

Now you run into another problem- if you're moving away from the target at a rate equal or greater than your muzzle velocity, you can't hit them. This also applies if the target is moving laterally. So the only way you can hit is pretty much on a dead-center approach going straight towards them. If they have greater thrust, they will be able to leave your zone of fire with ease, since all they have to do is get 1km/s relative velocity away from you.

Edit: And that means that all increasing your relative velocity will do is decrease time to contact. If they know you're coming (practically guaranteed), all they have to do is point their ship perpendicular to yours and start accelerating.

I can see projectile weapons being used by terrorists or pirates for subduing civilians, but not as any serious military weapon.

CCCToad on 26/8/2010 at 22:44

Quote Posted by Sulphur

Eh? This is assuming small, mobile targets, I suppose?

Not at all. Even earthbound military technology has munitions that can strike at a much greater range than that with a probable error of only three meters. In space, conditions are almost always going to be standardized, unlike in Terrestrial Ballistics requires you to calculate for things like weather conditions, the rotation of the earth, varying chemical composition of different lots of propellant, to name a few factors. The main factor affecting accuracy would be any evasive manuevers on the part of the target, which are somewhat less effective due to the lack of any air resistance to work with. As long as any space-fired projectiles have an integrated guidance system that doesn't get jammed, an impact on target would be almost guaranteed.

And for an idea what that kind of space combat would be like? play EVE online.

Sulphur on 26/8/2010 at 22:47

ninja'd - CCCToad, that's not quite why I asked that question, but yep, integrated guidance is a fair point.

catbarf: Well, first things first, if you're attacking a target you wouldn't want to be moving away from them while they're also moving away from you, you'd want to be moving towards, so that relative velocities add up.

And anyhow, spacecraft move in the order of many kilometres/sec, so a slug's muzzle velocity of 1000m/sec isn't going to realistically be slower than the spacecraft's - nice image, though it might be, of a slug standing stock still in the velveteen blackness as soon as it's fired. Head-on is the ideal approach regardless of whether it's a laser or projectile, anyhow.

Anyway, if your ship is moving faster than the target, the projectile fired from your ship will simply be muzzle velocity plus the difference in velocity between your ship and the target's. It isn't going to be a plain 1000m/sec in a relativistic universe. Yeah, it's a lot harder if it's not head-on. However, given your evasive manoeuvring thing, you can also take into account inertia and momentum and how much of a time delay a change in the target's direction actually incurs since you're not momentumless in space and can't turn on a dime, so you can lead your target to an extent, yadda yadda yadda.

It's probably a whole lot more complicated than that. Man, I'm glad I'm not a rocket scientist.

catbarf on 26/8/2010 at 23:05

Quote Posted by Sulphur

catbarf: Well, first things first, if you're attacking a target you wouldn't want to be moving away from them while they're also moving away from you, you'd want to be moving towards, so that relative velocities add up.

I was assuming they were moving towards each other.

Quote Posted by Sulphur

And anyhow, spacecraft move in the order of many kilometres/sec, so a slug's muzzle velocity of 1000m/sec isn't going to realistically be slower than the spacecraft's - nice image, though it might be, of a slug standing stock still in the velveteen blackness as soon as it's fired. Head-on is the ideal approach regardless of whether it's a laser or projectile, anyhow.

If you are approaching your enemy head-on, and he knows he has greater thrust than you, all he has to do is turn perpendicular and start firing his rockets. The instant he hits a relative 1km/s perpendicular to you, you can't hit him. Your projectile will not be able to catch up. All he has to do is maintain that speed difference and he cannot be harmed by your weapons. That's a pretty glaring weakness.

Quote Posted by Sulphur

However, given your evasive manoeuvring thing, you can also take into account inertia and momentum and how much of a time delay a change in the target's direction actually incurs since you're not momentumless in space and can't turn on a dime, so you can lead your target to an extent, yadda yadda yadda.

This is not terribly important on the timeframe we're discussing.

CCCToad on 26/8/2010 at 23:10

Quote Posted by Sulphur

Anyway, if your ship is moving faster than the target, the projectile fired from your ship will simply be muzzle velocity plus the difference in velocity between your ship and the target's. It isn't going to be a plain 1000m/sec in a relativistic universe.

Actually, 1000m/sec is an optimistic estimate for any projectile without its own propulsion. A modern howitzer loaded with the most powerful propellant possible(Charge 8 Red Bag) is only going to have a muzzle velocity of just over 900 m/s. Realistically, any space-fired kinetic munitions would either be guided missiles or would use railgun technology. In addition, recoil would make firing such munitions tough.

Sulphur on 26/8/2010 at 23:18

Quote Posted by catbarf

I was assuming they were moving towards each other.

So why, then, would you be moving away from the target, as per your example at the top of the page?

Quote:

If you are approaching your enemy head-on, and he knows he has greater thrust than you, all he has to do is turn perpendicular and start firing his rockets.

What I'm saying is, consider if you have greater velocity than he does.

Quote:

This is not terribly important on the timeframe we're discussing.

And what is the timeframe, then? Again, you can't assume that you can simply change thrust into a direction completely perpendicular in a matter of seconds. It won't happen. You'll be moving laterally along a combination of two axes thanks to inertia, and that's easy enough to lead IF you're travelling faster than he is.

DDL on 26/8/2010 at 23:48

Peter F. Hamilton's combat wasps.

Guided missiles with solid state AI: while you're always going to be limited in your acceleration (and by extension, change of direction) by..well, "keeping forces beneath those which tear you apart", they are not: twenty G acc followed by a 10 G hairpin? Fine and dandy for them, less so for you.

Stick some antimatter in the warhead and wheee...

Though presumably a super efficient laser defense could work..unless they painted them silver. :)

(jncidentally, this is a fun conversation: physics + death = win)

catbarf on 26/8/2010 at 23:48

Quote Posted by Sulphur

So why, then, would you be moving away from the target, as per your example at the top of the page?

It's just a possible scenario. If your opponent is bringing slugthrowers you can just stay 1km/s faster than him and zap him to bits with lasers without him ever being able to hit you. Or just keep some reasonable distance between the two of you, since your lasers and missiles outrange his guns by a factor of literally 100 or more.

Quote Posted by Sulphur

What I'm saying is, consider if you have greater velocity than he does.

A weapon that requires you to be faster than your enemy is somewhat limited in its tactical usefulness. Even if you're faster, they can still apply a full burn, and zap you with lasers while you struggle to catch up with your now much smaller relative acceleration.

Quote Posted by Sulphur

The timeframe is based on relative speeds of 1-10km/s versus laser and missile effective range of 10,000+km. At a relative 10km/s, it will take seventeen minutes for them to reach each other. If one is 'stationary' and the other is applying a continuous 1G burn, starting from dead stop at 10,000km, it will take 24 minutes for them to reach each other. Turning perpendicular shouldn't take very long at all, and once perpendicular, no turning is required to apply random burn lengths to dodge projectiles.

And all of this is still ignoring the fact that when you have a minute of waiting before the shell gets near the target, it is trivial for the target to detect it and apply just enough thrust to dodge.

As one more note, keep in mind that the exhaust of a nuclear thermal rocket or similar will be quite intense. At the effective range of a slugthrower, just a few dozen kilometers, your enemy can point his engine at you and bathe your ship in several-thousand-degree highly radioactive exhaust. He can also do this while running away to stay fast enough to avoid your projectiles.

catbarf on 26/8/2010 at 23:55

Quote Posted by DDL

To be fair, to get twenty gees of thrust you would need a ridiculously powerful engine, beyond even what's considered speculative. But the basic premise holds true. Although there's little reason to pull 10G hairpin turns when there's nothing in between you and the target.