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For long term moon operations, would it be practical to have a jetpack that used steam as a source for EVA on the moon surface and above it in orbit?

I imagine a scenario where lunar ice deposits found in craters could potentially be loaded into a canister and melted in the the lunar surface heat of the sun, putting it under pressure in the canister to be released as steam to allow the user to ascend to heights for exploration in craters, mountains and potentially maybe escape the moons gravity to low moon orbit.

Would that be possible with water/steam setup in a mobile cannister/nozzle scenario on the moon - where the ice could be a resource for this exploration instead of carting fuel to the surface? Or would one have to still use traditional jetpack fuels for this purpose?

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    $\begingroup$ "The ice originally appeared to be mixed in with the lunar regolith (surface rocks, soil, and dust) at low concentrations conservatively estimated at 0.3 to 1 percent." - I don't think they could scoop up a handful of highly abrasive lunar regolith and put it into a jetpack. It would be like putting mildly wet sand into a cup and trying to drink it. Source. The concentration of surface ice on the moon is low comparatively to other substances it is mixxed with. $\endgroup$ – Magic Octopus Urn Apr 23 at 21:59
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    $\begingroup$ What are "traditional jetpack fuels"? The only one I know of uses GN2. $\endgroup$ – Organic Marble Apr 23 at 22:02
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    $\begingroup$ @OrganicMarble youtube.com/watch?v=88RZ79K9Tqg $\endgroup$ – JCRM Apr 23 at 22:25
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    $\begingroup$ a whole lot more dV than 20 m/s though @OrganicMarble $\endgroup$ – JCRM Apr 23 at 23:07
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    $\begingroup$ @OrganicMarble 30 s on Earth. On the moon you'd use smaller engines (~1/6 the thrust) and get ~180 s duration, probably not all at once. Maybe that's OK when you have a specific task to accomplish, probably not OK for more general tasks like area reconnaissance. $\endgroup$ – Tom Spilker Apr 23 at 23:32
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For launch, no. For MMU in orbit, sure.

There have been designs for lunar ascent vehicles that use steam, but they require a nuclear power plant to get a decent TWR (thrust to weight ratio). A backpack simply wouldn't have the energy to heat the water hot enough with any technology we know of.

But in orbit it is different. I can imagine a backpack full of water which is used for drinking, cooling, and thrust. I can also imagine a system that diverts cooling water into a nozzle that flash-heats it into high temperature steam internally, perhaps using lasers or some other really fast energy transfer. The steam is then exhausted out the nozzle for thrust. This isn't competing with chemical rockets, but with the type of cold gas thrusters used for astronaut manoevering.

The system might not be terribly efficient, but that would be made up for by using cheap lunar water resources, and having one liquid that provides drinking, cooling, and propellant needs.

The advantage of this over other cold gas thrusters would be purely economical, since traditional thrusters use chemicals like Xenon and Nitrogen, which are highly depleted on the Moon and woild have to be shipped from Earth at much higher cost. It's a trivial amount for current NASA missions which rarely EVA and which cost huge amounts of money already, but in some future with astronauts doing EVA work in lunar orbit constantly, lunar water might be exactly what they'd want to use, even if it's way less efficient.

Here is an interesting paper showing the results of accelerating water vapor for use in a thruster:

Performance of gas fed pulsed plasma thrusters using water vapor propellant

Processing lunar ice for this use should not be difficult. You would just need to distill it to remove the impurities.

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  • $\begingroup$ I found this research on NASA site about expirements with Water Based Resistojets -- mostly for moving space stations/satellites but also envisioned for low gravity as well $\endgroup$ – Fight Fire With Fire Apr 27 at 3:53
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Unfortunately steam rockets would not be practical because they're not very efficient.

I don't know how much you know about rocket propulsion so I'll begin with a discussion of one of the most important metrics: exhaust velocity, the speed of the gas coming out of the engine's nozzle. The higher that speed, generally the more efficient the engine. There are other considerations, to be sure. For instance, if an engine that produces 500 Newtons (~110 lb) of thrust has a mass of 100 kg, that's not very mass-efficient!

The thrust produced by an engine is directly proportional to the exhaust velocity $V_e$ and the mass flow rate ($\dot{m}$), which is the mass of propellant fed into (and coming out of) the engine per second: $$F=\dot{m}V_e$$

You can see that as $V_e$ increases, the more thrust you get from a given rate of propellant use. Or, for a given thrust you need, such as the thrust required to lift you from the surface, the higher the $V_e$ the smaller the propellant use rate required. And that's important.

(Aside: instead of exhaust velocity rocket engineers usually refer to specific impulse, which is the exhaust velocity divided by the gravitational acceleration at Earth's surface)

By the term "steam" rocket engines I mean rockets expelling steam from their nozzles after heating it by means such as solar heating or electrical heating or some such, not by direct chemical combustion. I make this distinction because what came out of the Space Shuttle's main engines was steam, but it was created by direct chemical combustion in the engines' combustion chambers. Those engines were very efficient, with very high exhaust velocities. Steam rocket engines' exhaust velocities are much lower.

Why were Space Shuttle engines so efficient when steam rocket engines aren't? It's that exhaust velocity! And the exhaust velocity is a direct result of high temperatures in the chamber: for a given gas coming out of the nozzle (steam, in this case), the higher the chamber temperature, the faster you can make the exhaust go. On the moon, with solar concentrators or electric heaters, you might get the steam up to several hundred °C. Space Shuttle main engines ran at ~3,300 °C chamber temperatures! That's why they were so efficient.

Why would efficiency be a concern?

The more efficent, the less propellant you'll use up. This has several advantages, such as: your personal rocket-pack rig will be able to work with smaller tanks, making it less unwieldy; and your water mining operation and production plant can be smaller.

There are multiple engineering issues with steam engines. For one, when you heat the steam to several hundred °C, unless you carry a heat source with you, that steam cools due to heat lost through the tank walls, and it gets less efficient. If you have good insulation that cooling goes slowly; with not-so-good insulation it can go quickly. Heat sources can be heavy and bulky. If you're going to have some fixed infrastructure to heat the steam (call it a charging station?), then once you take off and are separated from that infrastructure, you have a finite amount of time to finish your task and get back to it. If you stay out too long, even sitting on the surface somewhere, the steam could get too cool to produce the needed thrust, and then you're stuck! Another is that the steam engine is an example of what is called a "cold gas thruster", an engine that uses a tank of pressurized gas without any kind of internal combustion to heat it. There are other gases, such as nitrogen or helium, that are much easier to use in a cold gas thruster: they don't have to be really hot because their liquefaction temperatures are really low and condensation in a nozzle won't be a significant problem, they aren't as reactive as water, and so on. (But there isn't much of those on the moon. It might be better to use O$_2$ extracted from lunar rocks, despite its reactivity.)

Efficiency is desirable also because there is a finite amount of water on the moon. Estimates run from half a billion to a couple billion metric tons, which sounds like a lot. But if you use it in ways that disperse it, your water production infrastructure must be sized to replace it, and eventually that resource will be gone. Rocket engines whose exhaust is steam disperse water, the great majority of which is lost to space. If you're going to use water in a way that disperses it, it's best to use it in the most efficient way. At least, until you can mine the asteroid belt for a source of water more efficient than lifting it out of Earth's gravity well.

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    $\begingroup$ To get a sense of that, a billion tonnes of water ice is a cube one kilometer on a side. Which would look like a lot if dropped into Times Square, sure, but would literally be a drop in the ocean if dropped in the ocean; it would be a largish iceberg. $\endgroup$ – Eric Lippert Apr 24 at 17:16
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    $\begingroup$ @EricLippert ...and the Amazon river could drain that in 1 hr 20 min! $\endgroup$ – Tom Spilker Apr 24 at 19:15
  • $\begingroup$ thanks for this! It explained a lot and did some research on the links and tangents you provided. I think in one scope i was thinking about how one would use it in an emergency if one was out of a typical propellant, could one try to cobble together propellant from moon ice and use steam without resorting to using energy to split the h2o which would take more time , energy and equipment in an emergency situation. I was trying to think of a way if one could use existing EVA equipment and adapt it to using water as a propellant, but the numbers dont add up directly for thrust in low gravity $\endgroup$ – Fight Fire With Fire Apr 25 at 4:35
  • $\begingroup$ I'm examining papers here at NASA for water propellant research: ntrs.nasa.gov/… $\endgroup$ – Fight Fire With Fire Apr 25 at 5:00
  • $\begingroup$ One company is experimenting with propulsion using microwave energy to heat water to very high temperatures, but like the electrolysis approach it is rate-limited by the power supply. $\endgroup$ – Tom Spilker Apr 25 at 6:18
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While picking up ice and putting it in one's space suit support system would come in handy as a way to keep a space suit cool (Will suits worn on Mars lose kilograms of “expendable water” each time they are used?) its use as a propellant is pretty limited.

To hover above the surface the force is say 120 kg x 9.8 m/s2 x 1/6 or about 400 Newtons. You get that from $v_{ex} \dot{m}$ and expend say 0.1 kg per second you'd need 4000 m/s exhaust velocity which sees pretty hard to get from a steam engine. It would require a huge amount of power; you probably couldn't store it, you'd have to fly around with a giant solar reflector.

And at 0.1 kg/s you'd only be able to do that for a few minutes.

So no, I don't think that there is a practical solution here using ice as a propellant source for a solar-energized steam-powered jet pack, even though it is cool to think about!

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    $\begingroup$ NIce, thanks for the link on the prospector 1, i read the other questions. Wouldnt the Ice once melted to water , turn to steam as soon as it reached the nozzle on the jetpack? Giving a minute amount of thrust? $\endgroup$ – Fight Fire With Fire Apr 24 at 1:01
  • $\begingroup$ @FightFireWithFire "a minute amount of thrust" won't help you to "ascend to heights". You need roughly 400 Newtons of thrust to get off the surface at all. Anything less than that you'd just remain on the surface but feel a bit lighter. $\endgroup$ – uhoh Apr 24 at 1:04
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    $\begingroup$ I think I understand what you're saying about the water turning to steam when it gets to the nozzle. People hear about room-temperature water boiling when exposed to a vacuum. But what those sources don't mention is that some of the water indeed turns to steam, but most of it just gets colder, enough that it turns to ice. The exhaust from a steam engine operating with 0 °C (or even room temperature) water would be a little low-speed steam and a lot of tiny, low-speed ice crystals. $\endgroup$ – Tom Spilker Apr 24 at 7:37
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    $\begingroup$ I am going over some NASA research on water propellant here: ntrs.nasa.gov/… $\endgroup$ – Fight Fire With Fire Apr 25 at 5:01
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On a more positive note than the other answers, what you could do is make use of the relatively plentiful solar power to melt, then use electrolysis to provide you with hydrogen and oxygen, which is well known and well understood as a rocket fuel.

The biggest fault with that idea is that the majority of your solar power would be used up not in the electrolysis, but in liquefying the products for storage - storing gaseous hydrogen isn't really practical at the moment.

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For high thrust, you have to make a lot of steam in a hurry. Electrical heating is impractical. Chemical heating works: you burn hydrogen to make the steam, as the Space Shuttle main engines did. A really hot, high-powered nuclear reactor might work.

A bunch of people are working on steam for small, low thrust systems. For low thrust, electricity is a good energy source. It's even possible to electrically heat the steam to plasma temperatures to get higher specific impulse. Good for adjusting orbits, but not for thrusting against gravity.

https://spacenews.com/water-propulsion-technologies-picking-up-steam/

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  • $\begingroup$ Welcome to Space! Could you quote the relevant part of the linked article so this is not a link-only post? Thanks. $\endgroup$ – DrSheldon Apr 24 at 21:58
  • $\begingroup$ @DrSheldon The whole article is relevant. $\endgroup$ – John Doty Apr 24 at 22:09
  • $\begingroup$ Hi @JohnDoty I'm sure that's true! However for a good Stack Exchange answer we should include some quotes from the source so that the answer can be stand-alone if/when the link rots or breaks. Without that it's called a "link-only answer" and these are strongly discouraged and sometimes down-voted. Stack Exchange is a little different than many other sites. Thanks, and Welcome to Space! $\endgroup$ – uhoh Apr 25 at 1:20

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