Fusion Drives (again)

[KerikBalm]

So I wanted your thoughts on this fusion drive system I've added (duplicating stock part models and changing their stats and names).

I was basing it around a dense plasma focus reactor (I think these need more development, they look much more promising than toruses from what I've read).

Anyway, these reactors #1) are approaching the conditions needed for P-B11 fusion, #2) are acheiving fusion rates of toroidal and ICF reactors many times their size, and thus wouldn't need to be scaled up as much to break even, #3) produce an ion beam stream that can be directly used for high ISP thrust, or decelerated against a magnetic field for direct generation of electricity.

So my system has 5 parts:

1) A P-B11 fusion Reactor

2) A High ISP "nozzle" consuming P-B11 fuel

3) A 1/10th ISP high thrust engine consuming P-B11 fuel and Xenon gas

4) A P-B11 fuel container

5) Stock Xenon containers

To make my fusion system work, I've added a resource "IonBeam" generated by the reactor, so you can only run 1 of the 2 engines at once.

As for the stats.... I decided my reactor would be 50% efficient... or rather that it generates 2x as much power as it consumes - so it only has to decelerate the ion stream enough to remove 1/2 its kinetic energy. Given KE = 1/2 mv^2, this means approximately an ISP of approximately 70% the maximum listed over here at atomic rockets:

http://www.projectrho.com/public_html/rocket/enginelist.php#hbfusion

That results in an ISP of 70,000 (!!!)

Next, I decided that a net output of 500 Megawatts was "reasonable", and a reactor weight of 3.5 tons was "reasonable", and calculated how much thrust I could get with that much power, at 70,000 ISP.... Skipping the calculations (I can show if you want)... I arrived at 1,428.6 Newtons... or 1.428 kN -> which I rounded to 1.5 kN.... 3x the thrust of the games Ion Engine (though these ion engines are already unrealistically powerful for gameplay purposes, since you can't time warp with active engines) - with a much much worse T:W ratio.

Going back to 1/2mv^2 ... if you lower your ISP by 10x, you can increase your thrust by 100x... so I figured I'd do that, so I did some technobabble about how a high energy ion beam is used to superheat the Xenon propellant into a plasma...

And I figured this system would be ~80% efficient.... so I made another thruster, this time with only 7,000 ISP (1/10th the max), and instead of 1.5 kN, I multiplied that by 80.... 120 kN of thrust.... Now we're talking:) It uses the Nuclear engine model, and adds another 1 ton (reasonable I think).

You'd be surprised how many Xenon tanks you can go through with such an engine... but it easily achieves SSTO.

It seems a bit OP'd I'm not sure this 1/10th ISP, 80x thrust engine is a good idea.... seems a bit unrealistic... but I did the math... and it seems to me that if one assumes a 500 MW reactor that fuses P-B11 (If you used Helium3-Deuterium, you'd get even higher ISPs!) is reasonable... then it seems this would be the result... maybe I need to crank up the reactor weight......

[KerikBalm]

Hrmmm, lest someone read this and think its realistic, I realized I made a mistake.

Reducing ISp by 10x increases the mass accelerated by 100x, but not the thrust, as you accelerate 100x as much mass per second to only 1/10th the velocity.

The 7,000 ISP thruster would only get ~15 kN of thrust.... You'd need a 5 gigawatt reactor for that amount of thrust, a 500 megawatt one wouldn't be sufficient

To get a mere 120 kN of thrust, it would be only 875 kN - not much better than the nuclear engine we've already got. Doube the thrust, sure (but I've set the weight to be nearly 2x higher for the reactor+ engine), and only ~10% more ISP... ah well, I'll go with it. A 5 gigawatt reactor weighing that little is not very realistic.