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Water based thrusters have been proposed (and possibly tested by now) for use in satellites and other in-space vehicles - see this NASA article. The idea is to perform electrolysis on the water to separate the oxygen and hydrogen into two bladders, then pumping them into a combustion chamber.

For some quick guesstimation math, the density of $H_2O=1 g/cm^3$; density of $H_2=0.07g/cm^3$; density of $O_2=1.14g/cm^3$. Volume needed for 1 g total of propellant is $29.45cm^3$ for $2H_2+O_2$ and $2cm^3$ for $2H_2O$ (take the inverse of each density to get volume per gram, multiply by 1 gram). So storing water as a propellant requires roughly 15 times less volume than an equivalent mass of separated hydrogen and oxygen (not including differences in equipment needed, just the propellants).

My question is this: could water be used as a propellant with an electrolysis engine in a 1st or 2nd stage rocket engine to launch from Earth?

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    $\begingroup$ The density of water is 1g/cm^3, not 2. $\endgroup$
    – Russell Borogove
    Feb 26, 2021 at 21:58
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    $\begingroup$ Cool, I didn't know about that! I've shared your linked article with my social media. Thanks. $\endgroup$
    – Greg
    Feb 26, 2021 at 22:07
  • $\begingroup$ @RussellBorogove you're of course correct, and density does not change with increased portions... I should have been talking volume instead of density. $\endgroup$
    – Chris Collett
    Feb 26, 2021 at 22:18
  • $\begingroup$ @RussellBorogove Just updated my calculations to match the correction, thank you. Let me know if this is wrong. $\endgroup$
    – Chris Collett
    Feb 26, 2021 at 22:30
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    $\begingroup$ Where does the energy (or power) for the electrolysis come from? $\endgroup$
    – Jens
    Feb 28, 2021 at 16:07

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Let's run some numbers: A single Raptor engine consumes about $140 \frac{kg}{s}$ of methane, which is burnt in an oxidizer rich environment, i.e. the methane is burnt completely. Burning a kilogram of methane releases an energy of $55.5 MJ$. As such, a Raptor engine has a chemical power consumption of $7.77 GW$. That's a couple power plants worth of power.

Now, when you do electrolysis to create oxygen & hydrogen from water, you are expending the same amount of energy in the form of electrical power, as you regain as heat when you burn the two gases within a rocket engine. And, to avoid the necessity of large hydrogen/oxygen tanks on ascent, you need to produce your fuel as quickly as you burn it. That is, you'd need an electrical power source as powerful as a dozen of power plants put together right inside your rocket.

I guess, it should be obvious why this is totally infeasible.

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  • $\begingroup$ Excellent break down. The speed at which electrolysis would need to occur is the part that I was missing. Thank you! $\endgroup$
    – Chris Collett
    Feb 28, 2021 at 15:47
  • $\begingroup$ That's 6.4x more power than you need to run a time machine. $\endgroup$
    – Sam
    Feb 28, 2021 at 23:55
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Electrolysis-based propulsion becomes practical only once you've reached orbit, where you can power the electrolysis with solar panels and where you don't need enormous thrust. Whatever you'd use to power electrolysis for a first stage would be much heavier than conventional chemical propulsion.

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    $\begingroup$ @Engineer_Chris A Merlin 1D rocket engine has an effective power output of about 3 GW, and the first stage has 9 of them, for a total exhaust power output beating the Three Gorges Dam. So, no, remote power supply is not viable. $\endgroup$
    – Christopher James Huff
    Feb 26, 2021 at 22:03
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    $\begingroup$ We've built lasers powerful enough, but they can run only for a fraction of a second, lasers.llnl.gov/about/faqs#192_beams_produce. And we sure don't have targets that can absorb that much radiation and convert it into useful power. $\endgroup$
    – Camille Goudeseune
    Feb 26, 2021 at 22:33
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    $\begingroup$ If you had such a laser you'd be better off just using it to boil hydrogen. $\endgroup$
    – ikrase
    Feb 26, 2021 at 23:31
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    $\begingroup$ in addition to remote power being unfavorable to just using the remote power system to heat up the water directly, theres also the option of just keeping the electrolysis unit on the ground and using its outputs in a conventional rocket. You need an extremely mass efficient electrolysis unit for that not to be preferable when cryogenic storage is an option. $\endgroup$
    – user1937198
    Feb 27, 2021 at 15:35
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    $\begingroup$ @user1937198 that also has the advantage that you can use a more-optimal O/F ratio...rockets rarely burn stoichiometric mixes, they usually use more fuel to reduce the average molecular weight in the exhaust and higher exhaust velocities, and to control corrosion of the engine. $\endgroup$
    – Christopher James Huff
    Feb 27, 2021 at 15:48
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I'm pretty sure this wouldn't work as these types of thrusters don't have enough thrust to lift off against Earth's gravity. The HYDROS-C thruster (the focus of the article you linked) has a thrust of >1.2 N, while (to use one example) the Space Shuttle's solid rocket boosters each had 12-15 MN of thrust (depending on the stage of the launch). That's a difference of seven orders of magnitude.

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  • $\begingroup$ The size of the HYDROS thrusters is also significantly smaller than those of the Space Shuttle. The same combustion process is used in both engines, so the thrust could be made comparable (if it were possible to overcome the power and weight issues). $\endgroup$
    – Chris Collett
    Feb 26, 2021 at 21:25
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Water is the low entropy product of combustion. (That's why it drip's out of you car's exhaust pipe, and is the reaction mass which spews out of the Space Shuttle's Main Engines.) Thus, is order to use it as rocket fuel, you must first "uncombust" it, and that takes a lot of external energy.

This is in contrast to regular old chemical engines, which "just" directly combust their fuels.

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The point of electrolysis engines is to get the performance of hydrolox with a space-storable propellant. You don't need space-storable propellants on the pad, even if you could somehow do it why would you?

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    $\begingroup$ One of the biggest reasons is safety. Water is non-combustible, making it far safer for astronauts. $\endgroup$
    – Chris Collett
    Feb 28, 2021 at 15:42
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    $\begingroup$ @Engineer_Chris: Running a multi-GW nuclear reactor on the other hand will make it significantly less safe for the astronauts and everyone within a rather large radius. The fundamental problem is that the launch needs a huge amount of energy, and that amount of energy is going to be dangerous no matter what form it takes. $\endgroup$
    – MSalters
    Mar 1, 2021 at 11:00

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