[Spaceships] Perpetual Motion?

[VariousRen]

I don't see any problem at all, we do the same thing on earth. Oil refineries require some amount of energy (say 1 unit) to produce fuel. That fuel provides enough energy to power the refinery and have some extra left over (for example, the fuel provides 3 units of energy). This means infinite power, up until the point where you run out of raw materials.

For your spaceship you need access to raw fuel components to generate fuel, so really you are turning cargo space and a factory into an inefficent fuel tank.

[Ulzgoroth]

Originally Posted by VariousRen

I don't see any problem at all, we do the same thing on earth. Oil refineries require some amount of energy (say 1 unit) to produce fuel. That fuel provides enough energy to power the refinery and have some extra left over (for example, the fuel provides 3 units of energy). This means infinite power, up until the point where you run out of raw materials.

For your spaceship you need access to raw fuel components to generate fuel, so really you are turning cargo space and a factory into an inefficent fuel tank.

The raw materials in this case don't contribute any energy. A fuel cell runs by combining hydrogen and oxygen to water, and a refinery produces fuel for it by cracking water to hydrogen and oxygen.

You can quite easily run this as a closed cycle with the same water being cracked, recombined, and cracked again. Doing so will require a net energy input, though.

[acrosome]

Originally Posted by Mailanka

You're not creating rocket fuel out of nothing. You're taking ice or water, and refining it into rocket fuel. So you couldn't strictly have a perpetual motion machine, because you'll run out of (usable) water or ice. I suppose in your specific case, you might be able to trap the water being burned by the fuel cell for the purposes of making fuel-cell fuel, and then feed it to the refinery to make more fuel-cell fuel. I suppose that's what you mean by a perpetual motion machine?

Oh, yes, I was being rhetorical in my use of the phrase "perpetual motion."

Originally Posted by VariousRen

I don't see any problem at all, we do the same thing on earth. Oil refineries require some amount of energy (say 1 unit) to produce fuel. That fuel provides enough energy to power the refinery and have some extra left over (for example, the fuel provides 3 units of energy). This means infinite power, up until the point where you run out of raw materials.

You are missing an important point; oil is not a valid comparison.

You have to think of tanks of O2 and H2 as sort of like a battery. It is a way to store energy. You start with the ground state (water) and pump energy into it to electrolyse it into O2 and H2. Later, you burn that in the fuel cell to get that energy back, producing water again. And the Second Law of Thermodynamics says what? That you will lose some energy in this process, so you should never be able to use a fuel cell to produce more fuel than it consumes.

Oil is different because what you are doing is mining a resource that is already rich in chemical energy, and all you have to do is burn it to release it. You aren't making the oil, or converting it into a higher energy state- you're only extracting it. The apt fuel cell comparison would be if we started with graphite and hydrogen, and ran it through a chemical reactor to make gasoline, then burned the gasoline for power. I guarantee you that you would never have a net energy gain in that scenario.

(Also, trapping the water that they produce is exactly what most fuel cells do. They usually include a electrolysis setup to re-crack the water when you give them some power, from a solar cell for instance. I'd bet this is what the ISS does so that it has power when it's orbit takes it to the night side of Earth. Most fuel cells really are a sort of a battery.)

[Flyndaran]

Fuel cells have many wonderful features, but energy density ain't one of them. Hydrogen is also a mega-bear to store unless locked into molecules which messes up its energy density even further.

[RyanW]

Originally Posted by VariousRen

I don't see any problem at all, we do the same thing on earth. Oil refineries require some amount of energy (say 1 unit) to produce fuel. That fuel provides enough energy to power the refinery and have some extra left over (for example, the fuel provides 3 units of energy). This means infinite power, up until the point where you run out of raw materials.

But oil is already a fuel. Refineries just, well, refine it. They turn a messy fuel into a clean one.

This refinery processes water into rocket fuel. A fuel cell produces power by converting rocket fuel to water. Using the original numbers means this setup obviously violates the first law of thermodynamics with the addition of a pipe.

[Anthony]

As a side note, the math from the OP is incorrect (and underestimates the amount of perpetual motion going on). A 1 space SM+5 fuel cell will run for 12 hours on 1 space of fuel (i.e. 4x base endurance) and produces 1 EP. 1 space of fuel at SM +5 is, however, 1.5 tons, not 5 tons.

[VariousRen]

Ah, my bad. I forgot that this was specifically about fuel cells and not fuel in general. Carry on!