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Hydro/lox is an excellent fuel choice and has been used in many engine/rockets with high ISP requirements, however, the two biggest drawbacks of the propellant are its density and boil-off. Could this be solved by storing the propellant as water at atmospheric pressure, and then using solar panels or even RTG's to generate the power necessary to turn to water into 2H/O through electrolysis. This would have the drawback of having to be burnt Stoichiometrically and only with a large power source relative to the engine, possibly limiting its use to space probes. Could this potentially replace ion engines as a low-cost and high T/W alternative?

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    $\begingroup$ I've left a partial answer but I think that this deserves a more thorough treatment. Good question! $\endgroup$ – uhoh Nov 23 '20 at 3:38
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    $\begingroup$ @uhoh yes thanks for your answer ion engines would probably make such a technology obsolete $\endgroup$ – Reuben Farley-Hall Nov 23 '20 at 3:45
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    $\begingroup$ Could be done, yes. But for this to make sense, you need a very specific scenario: 1) you must need high thrust (which you wouldn't get from ion drives); 2) you must have a very long coasting/parking phase before, e.g. multiple years (storage of hydrogen not possible due to boil-off). Then you could use your electrical power source to efficiently refill the hydrolox tanks from the water tank in advance. In addition for this to make sense, 3) the overall efficiency, cost, and reliability of this system would have to at least match that of SRB's. Maybe there are some military use cases. $\endgroup$ – Everyday Astronaut Nov 23 '20 at 9:49
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    $\begingroup$ If you are using nuclear energy to power the propulsion system, better approach for high ISP is ion propulsion; for high thrust where ISP is less of a concern, directly superheating the propellant. Water might not be the best choice for a storable propellant because it freezes at what might be an inconveniently high temperature. $\endgroup$ – Anthony X Nov 24 '20 at 23:05
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The object of burning chemical propellant is to convert chemical energy to heat, using that heat to accelerate the propellant. If you are starting out with electrical power, you have no reason to limit the energy you put into a given mass of propellant to what you can store in it as chemical energy: just heat water directly, and you can reach temperatures that combustion couldn't achieve, outperforming a hydrolox chemical thruster. Heat it enough and you dissociate the water into hydrogen and oxygen, improving specific impulse further by reducing the average molecular weight. The thruster technology being developed by Momentus is an example of this, using microwave heating: https://momentus.space/

If you need thrust more than propellant efficiency, you could run at a lower temperature and higher mass flow rate. Simple resistance heating might suffice in this case...a resistojet.

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    $\begingroup$ Moreover, water is the chemical product of fully oxidizing (burning) hydrogen. You cannot get more energy out of that reaction than you have to put in to reverse it (second law of thermodynamics), and in practice you have to put in more energy than you can get back. So hydrolyzing water just to burn the resulting hydrogen and oxygen is a losing proposition. $\endgroup$ – John Bollinger Nov 23 '20 at 15:13
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    $\begingroup$ @JohnBollinger You seem to be missing the point - with a satellite in space, that energy would be from sunlight, so the water is acting as a way to convert electricity into thrust, so the efficiency of electrolysis is almost entirely irrelevant. All that matters is whether it is more efficient (in a specific scenario) than other electrically driven thrusters $\endgroup$ – Mike Brockington Nov 23 '20 at 19:52
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    $\begingroup$ @JohnBollinger there's inefficiencies in directly heating the water as well. Generating microwaves is far from 100% efficient, for example. And of course you have to pay the energy cost of splitting the molecules if you want dissociated exhaust...it's just part of the power/propellant efficiency trade. $\endgroup$ – Christopher James Huff Nov 23 '20 at 20:38
  • $\begingroup$ @JohnBollinger The advantage of splitting water is the high power (energy/time) delivered by burning H2/O2, which is sometimes important for orbital maneuvers. The Hydrolox is an immense energy store which can be converted to thrust in very short time. Chemical energy has still the highest energy density (per volume? mass? not sure). Directly "storing" electricity (supercapacitors, batteries) is less efficient (and actually just another flavor of chemical energy anyway!). $\endgroup$ – Peter - Reinstate Monica Nov 24 '20 at 13:29
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    $\begingroup$ @Peter-ReinstateMonica are you suggesting storing hydrogen and oxygen produced over a long period of time and using them in a short burn? This would allow more thrust and take better advantage of the Oberth effect, but you'd be limited to the performance of hydrolox with a stoichiometric mix (or the penalties of carrying around water containing oxygen that gets discarded after electrolysis), and you'd have to carry heavy pressure vessels for gaseous hydrogen and oxygen, which won't actually hold much propellant. (If you could liquefy H2 and O2, you wouldn't be electrolyzing water.) $\endgroup$ – Christopher James Huff Nov 24 '20 at 15:30
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Partial answer:

If one has solar electric power, one can use each kilogram of propellant much more effectively (i.e. higher delta-v through a higher Isp) if it is ionized and accelerated. Electrostatic acceleration can impart roughly 10,000 to 100,000 m/s (or higher potentially (pardon the pun)) velocity, versus circa 4500 m/s from an 2H2 + O2 chemical engine.

So while you could do this (there's no fundamental reason why not) it would be much better to bring some krypton or even iodine to ionize rather than water to split and reform.

Xenon and krypton

Solids like iodine and lithium

Lighter alternatives

Molecular species

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    $\begingroup$ Why these elements particularly? Honest question. $\endgroup$ – Boluc Papuccuoglu Nov 25 '20 at 18:22
  • $\begingroup$ @BolucPapuccuoglu and a good question too! You are right, I'd originally planned on linking to other posts about those, I'll ping you when I do. Short answer is that noble gases like Xenon and Krypton are easy to ionize so for most engines the power supplies and magnets that produce the plasma and contain the electrons can be lighter, but they need to be stored in a heavy high-pressure gas bottle. Iodine can be stored as a solid under modest pressure so it saves weight, but it's still experimental. $\endgroup$ – uhoh Nov 25 '20 at 20:47
  • $\begingroup$ @BolucPapuccuoglu updated $\endgroup$ – uhoh Nov 25 '20 at 22:01
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Yes, and there have been plans for commercial cargo to deliver water to a gateway, where it would be split by electrolysis to create propellant for a reusable descent/ascent vehicle.

While electrolysis is energy intensive, the water can be split over a couple of months and stored, until it is used in minutes, so for a reusable system like this, the mass of the solar panels, electrolysis, refrigeration and storage equipment is a one-off cost for the gateway, not one that needs to be paid for the (relatively) high thrust vehicle.

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  • $\begingroup$ "the water can be split over weeks and stored" only if you both got large tanks for liquid water and gaseous hydrogen and oxygen. Using small and lightweight tanks for liquid gases is difficult over weeks and would require a heavy gas cooler. $\endgroup$ – Uwe Nov 23 '20 at 17:46
  • $\begingroup$ yes @uwe, as explained in the second paragraph, it only makes sense for a refuelling station, not a vehicle. $\endgroup$ – JCRM Nov 24 '20 at 7:03
  • $\begingroup$ You could store the water as ice wrapped in multi-layer insulation. No need for an actual tank. $\endgroup$ – Steve Linton Nov 24 '20 at 19:28
  • $\begingroup$ Again, you can just use water directly by heating it with a nuclear power source. There has been work on nuclear steam landers for the moon. The advantage is that you don't need all the expensive equipment for electrolysis and storing cryogenic fuels. Just build a lander with a big water bladder, force the water into a nuclear powered heating chamber which flashes it to steam and shoots it out a nozzle. It's less efficient in terms of ISP, but TWR is pretty good because of the simplicity. In a 1/6g airleas environment, this works pretty well on paper. $\endgroup$ – Dan Hanson Nov 28 '20 at 20:06
  • $\begingroup$ but does involve a nuclear power source @DanHanson $\endgroup$ – JCRM Nov 28 '20 at 20:39
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Splitting water into hydrogen and oxygen using electric power is very inefficient. The internal resistance of the electrolysis cell is causing a large power loss. The resistance may be decreased by reducing the distance between both electrodes, but you need a minimum distance for the separation of hydrogen and oxygen.

Using electrolysis you get gaseous hydrogen and oxygen. But a hydrolox rocket engine is designed for liquids, not for gases. The neccessary cooling of the combustion chamber is done by liquids only. The turbo pumps don't work with gases.

Liquids are much better for cooling than gases. The VW beetle engine was the only one using air cooling between many other water cooled car engines. No modern car engine uses air cooling.

The exhaust of a hydrolox rocket engine is extreamly hot water vapor, so it is easier and more efficient to use an electrically heated steam rocket engine. But reaching a comparable steam temperature is not easy

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  • $\begingroup$ You don't really care about the separation of hydrogen and oxygen if you're going to burn it directly. You can just pump the mixture to the combustion chamber. Also, the flow rate is going to be quite low so you don't care about turbo pumps or cooling. $\endgroup$ – MSalters Nov 24 '20 at 7:54
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Liquids are much better for cooling than gases. The VW beetle engine was the only one using air cooling between many other water cooled car engines. No modern car engine uses air cooling.

(Sorry, don't have the reputation here to comment). I can think of two other air-cooled auto engines off the top of my head: Porsche and Corvair. Let's not forget all the air-cooled aero engines out there, even today! Air-cooled engines were much lighter and simpler. The only reason you don't see them in cars any more is that it's difficult to control their operating temperature (compared to water-cooled), resulting in much more emission of pollutants.

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  • $\begingroup$ fyi, once you participate for a while you'll have the ability to comment on posts, but trying to use answers as comments just gets them deleted $\endgroup$ – Erin Anne Nov 24 '20 at 23:11
  • $\begingroup$ the coolants used in rocket engine cooling circuits often do evaporate but remain a fluid as the are supercritical $\endgroup$ – Reuben Farley-Hall Nov 25 '20 at 2:34
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    $\begingroup$ My friend had a Corvair once but he told me that he'd read they were dangerous at some speed, but I don't remember which speed it was... I also once owned a Plymouth Satellite and discussed it in this answer but I also discussed Space Exploration related things in that answer. If you'd like to keep this as an answer post (and that could easily be possible) you add something to this somehow to focus on aspects of the question related to spaceflight? Thanks! $\endgroup$ – uhoh Nov 25 '20 at 8:26
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In most scenarios I can't imagine the extra ISP of hydrolox is worth all the extra equipment you will have to carry to pull this off.

However, I do see one mission: Hopping between bodies in the Oort cloud. The key is you don't carry the water, you mine it at each target. Whether there is a mission where they are close enough together to make this practical is quite another matter, though...

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