If you had a fusion engine and cryogenic tankage, what' the lightest backup power source?

[SomeGuy12]

Suppose you had a pulsed fusion engine, similar to the one being researched at the University of Washington. You need to crush fuel pellets or controlled puffs of gas, and you collect energy from the resulting alpha and beta particles by using the moving electric charges to induce currents in decelerator coils.

So your engine is also a power generator.

The problem is, to get the ball rolling, in order to have magnets strong enough, they have to be cooled with liquid helium or hydrogen. Your engine has a lot of magnets - probably a hundred tons of more of them, all immersed in cryogenic coolant Fusion doesn't work efficiently small scale.

The coolant system uses droplet radiator arms (it's lighter) so to even cool off at all, pumps have to work, and electrostatic droplet guidance grids have to be charged.

Finally, you would have to have a supercapacitor accumulator bank to supply the energy to pulse your fusion reaction. So the supercaps store the energy to crush 1 fuel pellet, and the fusion reaction releases enough energy to recharge the accumulator bank and crush the next one. So you have to supply enough energy to the bank to make up for leakage as the bank charges.

What's the lightest possible power source you could carry to keep your systems running sufficient for an engine restart and to prevent losing any mass from boiloff if you have to shut the main engine down? As a rough estimate, you probably need at least 100 kilowatts to a megawatt. (it doesn't really matter how big the ship is, if it's bigger you can just start 1 engine at a time)

[AngelLestat]

In that configuration I will said Fuel cells... you will need to provide oxygen.

Or if the period of time is short, then just batteries

In addiction: not sure if you need much supercapacitors to the laser short pulses..

Short pulse Lasers already concentrate energy as it was some kind of capacitor. (or maybe someone can correct me here)

Edited July 29, 2015 by AngelLestat

[Bill Phil]

In that configuration I will said Fuel cells... you will need to provide oxygen.

Or if the period of time is short, then just batteries

Yeah... Batteries are a good idea. I'd make the engine so it can function without providing thrust, so the batteries would only be used in an emergency. Altgough fuel cells and other sources are good for insuring everything is okay.

[SomeGuy12]

That's not a bad idea. Just use bottled oxygen that is part of the life support reserve. I just checked : there are prototype fuel cells that give 1000 watt/kilogram. That's a lot better than anything else, so long as the time duration is limited. After engine restart you just electrolyze the water back to oxygen and recompress it.

That's a comparable power:weight ratio to a jet engine. (in space, if you used a jet turbine, you would have to have a massive heat exchanger to cool the exhaust gas so you don't have to vent it to space) I had no idea that fuel cells were this good..

I like this over batteries because your fusion engine needs large tanks of liquid hydrogen or deuterium (amusingly you can burn the deuterium just the same) for fuel and propellant. So you'd have the hydrogen on hand. Your life support system would need a large stockpile of liquid oxygen, also you'd need tanks of it for lander fuel. So you're already carrying the fuel anyway, the only extra mass is for the fuel cell membranes themselves.

Edited July 29, 2015 by SomeGuy12