Propelling deep space flight with new fuel source, Momentus prepares for liftoff

Re:Propelling deep space flight with new fuel source, Moment... by CarVac on Monday July 23, 2018 @12:36PM (#17593852)

180 seconds is a fairly terrible specific impulse...

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

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Re:Propelling deep space flight with new fuel source, Moment... by sp332 on Monday July 23, 2018 @12:45PM (#17593923)

Sure but can you refuel with more hydrazine on the Moon? With this new engine, just purify some water and you have more propellant ready to go.

Also, is hydrazine so efficient below 30 watts?

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Re:Propelling deep space flight with new fuel source, Moment... by kanox on Monday July 23, 2018 @12:59PM (#17594028)

> Sure but can you refuel with more hydrazine on the Moon?

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?

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Re:Propelling deep space flight with new fuel source, Moment... by CarVac on Monday July 23, 2018 @01:35PM (#17594305)

I was referring to the use for GTO-GSO orbit raising, where it's less mass efficient than both hydrazine and ion engine propulsion.

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.

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Re:Propelling deep space flight with new fuel source, Moment... by vpribish on Monday July 23, 2018 @01:24PM (#17594224)

hydrazine works at approximately 0 watts. other than opening a valve it's all chemistry.

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Re:Propelling deep space flight with new fuel source, Moment... by sp332 on Monday July 23, 2018 @01:25PM (#17594237)

I think that part of the article was talking about low-thrust applications, not actual electricity usage.

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Re:Propelling deep space flight with new fuel source, Moment... by sandworm101 on Monday July 23, 2018 @03:11PM (#17595080)

At an ISP of <200 I'm surprised they are bothering with the microwave tech. It would be more efficient to separate the water into H2/O2 and then burn it in the traditional manner.

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Re:Propelling deep space flight with new fuel source, Moment... by blacksmith_tb on Monday July 23, 2018 @03:42PM (#17595351)

It turns out that electrolysis in zero-g is tricky, the bubbles tend to stick to the electrodes. I believe there are some nanomaterials in development now to prevent that, but it makes it more complex than you would initially think.

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Re:Propelling deep space flight with new fuel source, Moment... by sandworm101 on Monday July 23, 2018 @03:56PM (#17595449)

Then don't do it in zero g. Spin the chamber and siphon off the combined gasses from the center. It might not even need 'moving' parts. The contents of the tank could be stirred with magnets.

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