No, no. Water is just the reaction mass. The power source is solar. This is a steam rocket.

Remember, not only do rockets need something to push against, you have to bring that something with you.

Rockets do not need to push against anything... they expel reaction mass out one end ensuring an equal and opposite reaction. Hence the term “reaction mass” in the first place. Rockets would work just as well in a vacuum as in atmosphere, but it is true that you have to carry your fuel with you.

Downvote if you must, but Newton’s third law still exists. ;)

To answer your question, I would guess that a microwave can get the reaction mass hotter than just a resistive heating element, and thus get higher exhaust velocity.

Hydrazine monopropellant has something like 220 seconds, and the new safer monopropellants are comparable (less specific impulse but denser for less tank mass).

Also, is hydrazine so efficient below 30 watts?

Water might be present on the moon in always-shadowed craters and it is not 100% proven. Those same craters might also contain other volatiles that could maybe be used to synthesize hydrazine or methane.

If water is found on the moon it could definitely be used to produce hydrolox which has amazing isp, ~450.

> Also, is hydrazine so efficient below 30 watts?

Hydrazine monopropellant is decomposed with a catalyst, it doesn't need external power.

If you need an external power source then you're competing against various ion thrusters which can get 10x the Isp. Maybe it's main advantage is extremely small size, or perhaps it gets higher thrust?

But the Isp might be so low only because of scaling issues on a device that has to fit in a microsatellite.

If they do meet their target of 900 second specific impulse on the larger models, then it might indeed be very useful.

Filling the tanks with Earth-sourced water would mean Earth bacteria, tardigrades, etc. The bacteria that stuck to the side of the tank, instead of being pulled out with the water-fuel, would reproduce once the tank is refilled from extraterrestrial (sterile) ice & water sources.

Could this be a cheap and dirty way of kickstarting terraforming? By landing and taking off on a planet like Mars with this engine, the steam exhaust would contain some of the bacteria and other denizens of the tank, some of which could be anaerobic. And this mixture could easily be intentional. The engine's steam exhaust could condense into a cloud of bacteria, and even precipitate to the surface.

Or the space ship could just have a sticker on the water-fuel tank door that says: Only use (sterile) space ice. No planetary ice.

Space is pretty big, I don't think anyone's ever considered it a problem there.

And re: reproducing in the tank, the temperatures generated would absolutely sterilize everything that was coming out as propellant. You could shove a zoo in there, and it would come out as fine particulate mist of mostly water and maybe some small burnt carbon particulate matter.

There are probably easier ways of spreading bacteria than via clouds, but it's actually an interesting idea - the bacteria could be used as the condensation locus, and cause rain that brings them to the surface. Just do a pass through the upper atmosphere, farting the whole way...

Why not? It's not as if water is flammable, toxic, or highly corrosive - attributes most other fuel sources do have at least one of.

(*Note, I am not an engineer)

Good question! His story "The Martian Way" addresses this. I don't really want to spoil it from you, but basically: where you'd expect.

> Surely they weren't storing it as H20 on the ship?

Why not? It's stable, it's useful for other reasons, and when you've got free energy and easy propulsion, there's no reason not to. (There probably would be some issues with sloshing, actually, but he didn't get into it. Asimov wrote a lot of stories that could be considered Hard Science, but I don't think he'd consider himself an engineer.)

In space, life support is a closed loop system so a lot of water is recycled anyway and with aeroponics, agriculture requires 80-95% less water. Any industrial uses of water will also likely have very efficient recycling (for example, Intel's Arizona plant recycles at least 60% of their water and their company goal is to get to 100% before mid-century). I believe even now, industrial water usage is far bigger than agricultural or personal.

I'd be far more worried about the gold, platinum and other expensive stuff that satellites are built from. We aren't getting those back nearly as quickly.

Also, good luck depleting the water on Earth with space travel. There would be other issues way before this happens.

https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elem...

Potable water is precious. Water in general is not.