Had a thought while playing Space Engineers. So assuming in the context of ship like depicted in CoaDE you wanted to use oxygen and hydrogen as propellant. Either hydrogen Hall effect thrusters, or hydrogen NERVAs with oxygen after-burning, or maybe just straight lox/h chemical rockets for missiles. In specifically the context of a laser-warship with say ~5GW of onboard power supply. Would it not make more sense to store most of the reaction mass as water or ice and then quickly electrolysis the amount of hydrogen you needed for a given burn shortly before doing so? I mean, we have a LOT of power and yet still months of transit time between burns in game.
I could see it being a lot more overt to run the reactors hot and radiate for a while before a burn, but there's still months of transit time. You would need tanks to accommodate two separate storage media but you could have your denser water tank inside your crafts armor and then an external drop-tank for temporary hydrogen storage. You would want some ready hydrogen for combat maneuvers, but you could use a ship with a broad-sider armor profile and turn the external hydrogen tanks away from incoming fire.
You'd have a bit of efficiency loss in that if your thrust was pure hydrogen you would have hundreds of tons more oxygen then you actually need, but you could also have a secondary oxygen hall effect thruster and regain some efficiency, or maybe you could just have a much leaner life support and just vent co2 constantly for cold gas thruster manuvering.
Too much of a Rube Goldberg contraption, I think. Gains will be marginal, and oxygen isn't that great for propulsion. Even warships don't really benefit from high thrust as much as it seems they should. Methane is best, really, unless you absolutely need a ton of dV.
Space Engineers is, quite frankly, ridiculous with its handling of hydrogen. For starters, you can use a hydrogen power generator (one that clearly looks like a combustion engine) to create energy to split water, hydrogen from which powers the generator. They don't put much stock in conservation of energy.
Yeah, I know SE is a mess. I wasn't expecting large gains, maybe just a few percent. Or failing deltaV gains at least finding a kind of redundancy in having kilotons of usable water and oxygen for the crew instead of methane. Or failing the redundancy at least finding a kind of logistical simplicity in that ships wouldn't need separate fuel and water and oxygen supply lines, they would just need ice.