Space company is betting its future on a propellant available everywhere
arstechnica.com
arstechnica.com
Normally, a rocket will have higher Isp if fuel is present in somewhat higher amounts than is needed to react with all the oxidizer. This is true in H2/O2 engines as well as hydrocarbon/O2 engines. This is because otherwise the combustion temperature is so high that some dissociation occurs, which robs some heat. Adding extra fuel makes it cooler so combustion is more complete, but also reduces the average molecular weight of the gas, for a net increase in Isp.
Here, I'd worry the combustion temperature would be too high. They might avoid problems by operating at very low chamber pressure, but that makes dissociation even more of a problem.
One might ask why not just heat the water directly. The problem there is that at reasonable temperature for a heater making steam, Isp is be very low. You'd be lucky to get much above 200 seconds, less than half the Isp of a H2/O2 rocket. Ammonia or methanol are better choices that that situation (they can decompose to hydrogen-rich gas, improving Isp at a given temperature).
The temperature of gas in a conventional rocket engine is high enough to melt or vaporize the thrust chamber; only cooling and limits on heat transfer to the walls keeps that from happening. In the SpaceX Raptor engine, for example, the gas reaches a temperature of at least 3500 C. In comparison, the boiling point of iron at 1 bar is 2870 C.
Hypothetically though, if you would just pressurize e.g. H2, then cool it, then pressurize it further-- should that not get you higher exhaust velocity that just heating/burning? why does that scheme not work?
If they wanted to be "fuel rich" they could just release some excess oxygen as process waste.
So that's not why this is a dumb idea
The reason this is a dumb idea is because eloctrolysis is the hardest and least efficient way to get hydrogen.
And also because hydrogen is a really hard gas to store and handle, which is why every hydrogen effort going back to the Bush administration has failed.
That's irrelevant in this case - power comes from the solar panels, electrolysis breaks enough water for the next engine firing, and the result is they get higher thrust than they would with an electric engine for the same total mass. Water is just a dense way of packing hydrogen and oxygen to run the chemical rocket, without requiring cryogenic storage, and it also works as shielding and heat transfer for on-board electronics.
Hydrogen is a pretty good coolant. For example, it's used as a coolant in large generators.
Liquid cooling would be better but cooling pipes are not very amenable to spinning around at 3000 RPM and you have to keep the rotor inside the generator as compact as possible so centrifugal force won't rip things apart.
The non-rotating part of the generator is typically water cooled in bigger generators by piping the water straight through the hollow copper windings themselves (in slower rotating generator types, water cooling is also used for the spinning rotor).