Looking for a good realistic drive for a space warship

[Junghalli]

This is a follow-up to my earlier question about calculating the drive output for an ore freighter for a hard SF setting. Well, I eventually settled on a variant of the drive the Daedalus probe would use for that, but while the Daedalus's ICF drive is excellent for civilian heavy-lift ships like freighters it has a very lousy acceleration (.03 G), which makes me think it'll look less attractive for warships.

So, for the scientifically knowledgeable out there, what would be a good realistic drive for a military ship?

I figure main priority for a military ship drive will be high performance, i.e. good delta V and acceleration. Payload will be relatively unimportant; armor and big guns probably won't be too useful in hard SF space combat, ships will mostly be boxes for nuclear missiles and KKVs. I'm looking, if possible, for something that could get payload at least the size of a loaded space shuttle orbiter up to a couple of hundred km/s and can sustain accelerations of 1 G or higher.

Any suggestions? I'm thinking Orion might be a possibility, but can anybody think of anything else?

Yeah, I know, I wasn't sure if this should be SLAM or OSF...

[Nephtys]

It's time for the obligatory Atomic Rockets link, everyone!

[kinnison]

Yes, it has to be an atomic rocket. Just possibly, a nuclear-powered mass driver throwing just about anything out of the back, as it's exhaust velocity that matters.

Leading candidates are an Orion nuclear pulsejet, or possibly a Bussard-style Polywell proton-b11 fusion rocket. For long distance, really high speed work maybe a ramscoop?

At a rather higher tech level, the fuel might be antimatter. Ideally, the reaction mass in this case would be hydrogen.

[Gullible Jones]

Fusion or antimatter rocket. You could also use a combination - antimatter-catalyzed fusion, for instance, or fusion with an antimatter injection "afterburner". Or antimatter with separately variable fuel injectors, so you could change the thrust and the specific impulse - or shoot hot antimatter at someone.

[Patrick Degan]

Well, I've always opted for the antimatter rocket for my harder SF spacecraft designs. Assuming the stuff can be manufactured in bulk and fairly cheaply, of course, it makes a good choice for a SF space vessel: high energy density for a lesser fuel mass and a drive capable of 1g acceleration.

[Junghalli]

It's time for the obligatory Atomic Rockets link, everyone!

Yes, I'm familiar with Atomic Rockets, but I wanted to see if anybody here might have some useful advice.

Well, I've always opted for the antimatter rocket for my harder SF spacecraft designs. Assuming the stuff can be manufactured in bulk and fairly cheaply, of course, it makes a good choice for a SF space vessel: high energy density for a lesser fuel mass and a drive capable of 1g acceleration.

Manufacturing it in quantity is the rub. For every gram of antimatter you create you need a conventional power plant generating twice as much energy as is contained in that gram of antimatter (that's at theoretical maximum efficiency, with technology feasible today it's more like 100 times as much energy). Since the energy density of antimatter is 9 X 10^16 joules per gram that quickly adds up to a ridiculously huge power plant. I haven't actually run the math but we're probably talking Death Star size fusion reactor complexes, or a partial Dyson of solar collectors, to make a large fleet of AM beam ships practical. Since my setting is relatively near future, I don't really see that being one of the better options.

Antimatter-catalyzed fusion, which only uses tiny amounts, is more practical. But if ICAN is any indication an ACF spacecraft will have the same problem as the Daedalus: low acceleration.

[phongn]

Orion is pretty much one of the few drives that combines high thrust and high isp, but go look at the Atomic Rockets site for more details.

[Junghalli]

Right now a nuclear salt water rocket looks like the best option. It's a bit complicated and has a somewhat hair-raising chance of blowing up the ship if something goes wrong with it, but then soldiers volunteer to take risks like that (realistic space combat will probably be sort of like air combat, all about not getting hit, so I don't see its vulnerability to a stray round setting it off as a serious disadvantage - odds are it would totally destroy the ship anyway). It seems to offer the good combination of high delta V and high acceleration I want, while (unlike Orion) having the bonus of not being illegal - though I doubt that treaty would last long when we start running into aliens anyway.

http://www.npl.washington.edu/AV/altvw56.html

[The Duchess of Zeon]

Humans can withstand sustained g-forces in a horizontal, eyeballs-in position considerably in excess of 1g, and can survive without going unconscious forces in such a position in the range of 40g for at least short periods of time, which would certainly require an interesting engine for both sustained 1 - 3g forces and short acceleration sprints of 40g's or so.

[Junghalli]

Humans can withstand sustained g-forces in a horizontal, eyeballs-in position considerably in excess of 1g, and can survive without going unconscious forces in such a position in the range of 40g for at least short periods of time, which would certainly require an interesting engine for both sustained 1 - 3g forces and short acceleration sprints of 40g's or so.

The nuclear salt water rocket gets up to 4 G. Survivable for short burns, though I doubt it would practical to cruise under it for hours on end. For most realistic spacecraft drives even 1 G is problematic for something with much more tonnage and range than the Space Shuttle, so that should be plenty. 4 G is probably around the maximum you can get without magitech (40 G is, needless to say, well into magical dancing panda territory for most halfway realistic drive systems).

[Ford Prefect]

Manufacturing it in quantity is the rub. For every gram of antimatter you create you need a conventional power plant generating twice as much energy as is contained in that gram of antimatter (that's at theoretical maximum efficiency, with technology feasible today it's more like 100 times as much energy). Since the energy density of antimatter is 9 X 10^16 joules per gram that quickly adds up to a ridiculously huge power plant. I haven't actually run the math but we're probably talking Death Star size fusion reactor complexes, or a partial Dyson of solar collectors, to make a large fleet of AM beam ships practical. Since my setting is relatively near future, I don't really see that being one of the better options.

In 2005, Ender mentioned a possible method of antimatter production apart from particle accelerators involving magnetic monopoles (catalytic conversion). The link he provided with me is now dead, which is a pity, because I can't remember what was supposed to happen (it was something to do with decay of some sort). It was all theoretical of course.

[The Duchess of Zeon]

The nuclear salt water rocket gets up to 4 G. Survivable for short burns, though I doubt it would practical to cruise under it for hours on end. For most realistic spacecraft drives even 1 G is problematic for something with much more tonnage and range than the Space Shuttle, so that should be plenty. 4 G is probably around the maximum you can get without magitech (40 G is, needless to say, well into magical dancing panda territory for most halfway realistic drive systems).

Orion can do it; a lot of the design is actually based around mitigating the effect of the nuclear bombs going off in terms of causing to much g-force for the crew. If the ship is laid out so the crew is operating in high-acceleration mode while laying perpendicular to the direction of acceleration (on their backs, facing upward, with "up" being the direction of travel), then instantaneous accelerations during the blast phase of even in excess of 60g's could be handled--the highest acceleration a human is able to survive when in that position relative to the acceleration is up to 100g's.

It's so VERY important to remember that g-tolerance is all about the direction the body is facing in. If you build the ship like a sky-scraper then the crew would be very very dead at 40 g's. If you orient it like a traditional ship, but put everyone in vertical acceleration couches, fixing body movement and locking them forward but leaving their arms free, then they'd be fine. What will black you out when you're standing up relative to the direction of acceleration will not black you out when you're laying down facing the acceleration, in short. Starship design should include these considerations.

[phongn]

Orion can do it; a lot of the design is actually based around mitigating the effect of the nuclear bombs going off in terms of causing to much g-force for the crew.

Even a realistic Orion is probably going to be limited to 4G or so, based on the 1959 report - and that might be rather optimistic (based on a 10kT gross mass ship)