pfdietz
14 hours ago
The article states they are electrolyzing water to make hydrogen and oxygen, then burning these gases in a rocket.
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).
myrmidon
12 hours ago
What exactly prevents you from just heating the steam to like 2K? Would the ISP still be worse, or is that just more of an engineering nightmare?
pfdietz
11 hours ago
The problem is the temperature limit on the heating elements. You could use arc heating, but that's not very efficient and also causes erosion of surfaces (and demands high instantaneous power; this scheme stores energy as chemistry).
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.
myrmidon
2 hours ago
I just realized that heating and expelling something electrically vs just burning in a rocket engine makes absolutely no difference from an ISP perspective. All that matters is exhaust velocity, and that basically only scales with temperature and molecule size (lighter being better), so steam is no good anyway.
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?
pfdietz
14 minutes ago
What matters is the state of the gas as it starts traveling down though the nozzle. All that comes before that doesn't matter.
jordanb
14 hours ago
I think hydrogen it a pretty poor coolant anyway. They could presumably use a mixed fuel or use an inert gas like argon for cooling.
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.
rbanffy
13 hours ago
> The reason this is a dumb idea is because eloctrolysis is the hardest and least efficient way to get hydrogen.
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.
pfdietz
13 hours ago
> I think hydrogen it a pretty poor coolant anyway.
Hydrogen is a pretty good coolant. For example, it's used as a coolant in large generators.
myrmidon
12 hours ago
To expand on this: Turbogenerators use hydrogen because it causes little drag, its cooling properties are good for a gas and it's much cheaper than helium.
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).