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Originally Posted by Brutha  You need as much energy to split H2O into H2 and O2 as you get from "burning" H2. You lose a bit of energy along the way as heat but you won't get additional energy in the process.
If you want to lunch a spaceshuttel the price of the energy that you need to split the H2O is pretty cheap compared to the other costs that you have when you want to build spaceshuttels. That doesn't mean that it's a cheap way to fuel other vehicals.
Compresed air doesn't pollute either directly.
Lithium Ion batteries also don't pollute as long as they get recycled afterwards.
Neither compressed air nor lithium ion batteries have a energy denseity that make them a viable choice for spaceshuttels.
Finding something that works for spaceshuttels is a different problem then the problem of finding a good fuel for vehicals. |
Yes, you need that amount of energy and you have losses.
But think of this...
You may find problems to store lots of electric energy because of the limitation of design of batteries, while if you separate H2 and O2 you have the energy being stored.
H2+O2 is a very high octane fuel because of its specific impulse.
Shuttle uses is not because of its cost (cheap or expensive) but because of its specific impulse.
Other fuels like JP4 or Kerosene provide lower thrust because they have a lower specific impulse.
Kerosene is a hydrocarbon which requires oil. Electrolysis requires water. With increasing oil prices even solar energy is now profitable.
The problem of H2+O2 is that the optimal performance takes place if bipropellant is very cold, but even if not cold the reaction is still very powerful.
Electrolysis is used to produce oxygen inside the shuttle, and you get a remnant of H2.
Using Sabatier reaction you can use H2 as carbon dioxide absorber. So if you ask me if it is too energy demanding or expensive, I would say that if it was that case, they would not use it. NASA has a very tight budget, about 1/4 of what it had during Apollo program, once you remove inflation. So they need to make the best use of their tight budget.
I recall that during the first shuttle missions they used lithium hydroxide canisters which were disposable. That CO2 absortion method is far less efficient than Sabatier reaction for it requires more mass.