[Spaceships] Reasonable numbers for Refinery Systems / 'Artificial' Fuel?

[Sunrunners_Fire]

Originally Posted by vicky_molokh

I'm assuming that a there's some waste of energy in the process.

O2 + 2×H2 -> 2×H2O + energy
energy + 2×H2O -> 2×H2 + O2

Isn't it?

(Note that I am postulating the use of inert, ubiquitous components, not something that already stores a significant amount of chemical energy. A typical space-operatic example would be mining ice asteroids and turning them into rocket fuel.)

2 H2O -> 2 H2 + O2
Water (liquid) to Hydrogen (gas) + Oxygen (gas)
2 (-285.8 kj/mol) -> 2 (0 kj/mol) + (0 kj/mol)

Costs no less than 285.8 kj per 18 grams of liquid water reacted, producing 2 grams of hydrogen gas and 16 grams of oxygen gas.

Reversing the reaction gives you no more than 285.8 kj, though the usual efficiency is about 50% and so you'd only get 142.9 kj.

0.5 tons is 454,000 grams, and so should cost no less than 7,208 mj to react. Burning the hydrogen and oxygen to produce should give around 3,604 kj.

Assuming I'm not messing up the physics or my math, of course.