Propelling deep space flight with new fuel source, Momentus prepares for liftoff | Hacker News Reader

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

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43 pointshelmsdeep47 comments

with water as a power source

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

Why use a microwave? Why not just a heating element?

Probably mode of energy transfer. Microwaves are extremely efficient with transferring energy to water since you do not need to have something with higher temperature to produce heat flow. Also, you can heat up a contained area (especially water vapor) easily where as heating elements are limited to surface area (and the heat flow will be extremely low since the mass is low).

Their Zeal thruster uses a heating element: http://momentus.space/services/

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.

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.

Yeah, I call water hydrogen ash.

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.

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. ;)

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

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).

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?

> 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?

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

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

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.

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.

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.

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.

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

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?

> 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.

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...

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)

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.

Without revealing too much (and possibly this qualifies as spoilers anyway), in Seveneves (Stephenson, not Asimov), it is a comet that, in a sense, 'becomes' the ship.

Why would you want to store your propellant water separately as O2 and H rather than as water? You'd have to react it to turn it into water, water is pretty inert, there's no mass advantage (more of a problem than volume). What am I missing?

I was thinking of volume, but if the craft doesn't need to ever be in an atmosphere then that wouldn't be an issue as I had first thought.

A bigger volume might even be an advantage - easier to turn your ship if you can mount engines far from the center of mass. Plus, built in shielding.

> Where did the water come from?

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.)

Fair enough, it is just fiction after all. I was thinking that storing it as H20 would be a challenge as storing all the water you might need would take a large amount of space. But I suppose if it was for a craft that would never enter an atmosphere the volume wouldn't really be an issue.

Would this increase the humidity of the local biome? Should we avoid using it in deserts?

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.

> Would this increase the humidity of the local biome? Should we avoid using it in deserts?

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...

the small vehicles in the expanse are also propelled by steam.

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?

At the time we start using a lot of it (enough to matter), we would also be capable to mine it from comets or moons.

Let's imagine that we have a new fuel source that works with dirt and sand. Whenever you can excavate, you can use it for fuel. How long until we run out of "land"? Same concept. Except that we have more ocean surface than we have landmasses.

Potable water is precious. Water in general is not.

You should read Isaac Asimov's "The Martian Way".

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.

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

First, it's fresh, drinkable water, delivered to people who need is, is precious. Not just "water". Second, 7 billions of people who drink and bathe and eat water-dependent food every day and a space mission with a few tons of distilled water every now and then are on completely different scales.

The only way water would be fuel would be via fusion of the hydrogen portion. Of that it is only currently realistic to use the portion of water that contains deuterium which is a relatively small portion of all water on earth. Also fusion would produce a large amount of power from a small amount of hydrogen, so it would have a small impact on earths water supply. So yes we should use it if we can.

Fresh water is a precious resource here on Earth because it is so energy intensive to desalinate ocean water. Once we are capable of mining other bodies in the solar system, there is plenty of water to extractt from other planets, moons, and comets. Getting there and back has proven to be by far the hardest part so far. Either way, water and methane will be the primary hydrogen source for liquid propellant, ion engines, and nuclear rockets.

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.

There's actually quite a lot of potential water available in the Solar System.

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

On any given day tiny comets and asteroid deliver to earth many tons of water. The big ones we see a shooting stars. The giant ones are called asteroids. Only the real monsters actually touch the ground. Most burn up in the atmosphere, delivering their water as steam. So, no. Powering all of our rockets with water will not deplete the planet.

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.

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.

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

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

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