
Propelling deep space flight with new fuel source, Momentus prepares for liftoff - helmsdeep
https://techcrunch.com/2018/07/21/propelling-deep-space-flight-with-a-new-fuel-source-momentus-prepares-for-liftoff/
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Animats
_with water as a power source_

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

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syntaxing
Yup, people do not understand how stable water is (hence the low energy
density). Phase change on the other hand, is a extremely effective method to
harness work from heat.

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Animats
They're going all the way to plasma. The idea is to pump as much energy as
possible into small amounts of reaction mass, to maximize momentum while
minimizing reaction mass consumption. They can get more energy from solar
panels over time, but not more reaction mass.

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

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GW150914
_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. ;)

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lgats
I suppose the rocket is "pushing against" the particles as they are exhausted

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CarVac
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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sp332
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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vpribish
hydrazine works at approximately 0 watts. other than opening a valve it's all
chemistry.

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sp332
I think that part of the article was talking about low-thrust applications,
not actual electricity usage.

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pavel_lishin
Reminds me of some of Isaac Asimov's stories, where all ships are powered by
nuclear reactors that flash water into steam for propulsion.

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nkrisc
That's an interesting approach. Where did the water come from? Surely they
weren't storing it as H20 on the ship? Seems like you'd only want to keep as
much as needed for life support in that state. Stored as separate oxygen and
hydrogen?

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

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.

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scott-smith_us
You have to have water storage if you've got people on board. I'd rather have
one system that stores both fuel and life-support than have to have two
separate systems and then guess exactly what proportions of both I'll need...

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King-Aaron
Water storage could also be used as a way to mitigate the radiation problem in
space. If you had a 'sleeve' inside the hull that contained water, you'd have
both a large storage capacity for fuel as well as inbuilt radiation shielding?

(*Note, I am not an engineer)

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lpaone
I have a friend studying radiology with the hopes of working in the space
industry. From what he tells me, I believe this would work.

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victorbinetruy
Let's say we found a way to make water the fuel of the future (that is used
for all (most) of our needs), should we use it? Isn't water (one of) our most
precious resource? Should mankind really risk using (wasting) our precious
water? Well, we wouldn't loose it if we use it on earth, but space would be a
different story (although, if we only use water as fuel for spacecrafts... it
would take a while before we deplete the earth of it). What are your thoughts?

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marsokod
Water is the space fuel of the future. Well, at least the space oil of the
future - if you start mining asteroid and comets, water is one of the best
component you can get: good for any life around, quite a good solvent if you
need one, and more importantly can be a fuel like this or be transformed into
oxygen and hydrogen, which is rocket fuel as well.

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

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politician
Right, like figuring out why you would want to bother lifting significant
amounts of water mass into space in the first place.

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syntaxing
So is the plasma an effect of the superheated steam? Or is the plasma used to
heat the steam? Or is it used to collimate the steam...Pretty neat
nevertheless. I always had a sweet spot for Ion plasma sources after working
in the Semi industry for a couple years.

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mrfusion
How does this work? Is it just vaporizing the water?

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Rebelgecko
Somewhat misleading title, as water/steam have been used before for propulsion
(although AFAIK not beyond low Earth orbit)

