How would a steam powered spaceship work

Could ships in space use a steam engine?

A steam engine in space (assuming you mean a steam rocket engine) is not only feasible, it may even be desirable. The Neofuel location makes a strong case for water being used directly by comets, asteroids, etc., rather than breaking it down into hydrogen and oxygen, mainly because the added mass of equipment needed overwhelms the benefits of a higher ISP.

All you really need is a source of energy to heat the water into steam and off you go. This can range from sun mirrors (a sun moth) to radiant laser or microwave energy to the ship or with the help of a nuclear reactor on board. As long as the water receives enough heat energy, you have steam. Throttling the engine is as easy as adjusting the flow of water through the heat source.

The NEOfuel proposal for a steam rocket

Simplified scheme of the NTR steam rocket.The water bubble (s) would be much, much larger in real life

The downside of a steam rocket is a really terrible ISP of 195, compared to @ 450 for the ideal liquid fuel H2 / O2 engines, or between 800 and 1200 for hydrogen NERVA rocket engines. They would use enormous amounts of water to get from point A to point B, and getting from Earth to the outer planets would take many years at best. The NEOfuel site calculates that a water tanker with a payload of 10,000 tons would carry 325,000 tons of water as the reaction mass.

Final configuration: H2O NTR option

10,000 tons net payload

358,000 tons of water propellant

104 tons of water bladder

208 tons of water bubble armor

656 tons of nuclear thermal rocket engines

122 tons of water sucker

In contrast, the LH2 option works like this:

10,000 tons of payload

(138,000 to 453,000 tons of water for LH2) 15,400 to 50,300 tons of LH2 propellant

154 to 503 tons of armored LH2 tanks

472 to 1536 tons of LH2 NTR engines

48 - 156 tons of water to extract

559 to 10248 tons of electricity for electrolysis

70 to 5027 tons for cryolizer hardware

372 to 10609 tons of electricity for cryolisers

So steam is a means of moving inexpensive payloads around the solar system inexpensively, but you have to accept a low ISP, large amounts of water are used as the reaction mass, and long trigger times.

kubanczyk

I think OP said "generation ship" to give us more information about interstellar travel. Is there enough water (comets) outside the Oort cloud? Anyway, you could edit it to erase the weird picture, as the prediction of thousands of tons of ice available on the lunar surface seriously undermines the credibility of the idea (which strikes me as being suitable for hijacking entire ice comets).

Thucydides

If you read the NEOfuel site, the design is predicted on the availability of large amounts of water at the lunar poles as discovered by the Clementine mission in 1994. NEOfuel is based on the idea that it is practical all over there is water in the solar system.

jamesqf

I don't understand why a NERVA engine with water is different from an H2 / O2 rocket. You both push steam out of your back, no? Assuming that the core motor can reach the same temperature and pressure as the chemical reaction, the result should be the same. And you don't have to fool around with cryogenic liquids.

Thucydides

A Nerva rocket heats and accelerates water with a higher molecular mass than the LH2 of a conventional NERVA. An LH2 / LOX missile runs at a much higher pressure and temperature than the nuclear reactor of a NERVA. It is possible to operate the NERVA at higher temperatures and pressures, but the increased voltage could adversely affect the reactor. By breaking off water, the reactor is also exposed to molecular oxygen.

jamesqf

@Thucydides: I would think higher molecular mass would be good (everything else is the same) as it means less space / mass for refueling. So we see ion motors that use xenon (the heaviest non-radioactive inert gas) instead of helium or neon. And I am assuming a technical development so that the exhaust gas velocity of a NERVA would be at least as high as that of an H2 / O2 rocket ...