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In case of a (single) Starship (as a) fuel depot orbiting the Earth, how can we prevent boil off, may be so? Some fuels have lower temperatures such as hydrolox is colder than methalox. Could we may be make the some shield of hydrolox but in gaseous form so it’s still colder than methalox but doesn’t boil off, to prevent methalox boil off as shown in this illustration below? May be we could even have layers of these like an onion if it would help. Please see this image for illustration.

enter image description here

Thanks in advance.

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    $\begingroup$ HydroLox are two different cryogenic liquids with different boiling temperatures that may be mixed only within the rocket engine but not in a tank. The same is true for methalox. Hydrogen is gaseous at the temperature of liquid oxygen. Oxygen is solid at the temperature of liquid hydrogen, $\endgroup$
    – Uwe
    Commented Apr 29 at 7:25
  • $\begingroup$ Thanks. May be there is something in between like slush of hydrolox which could be used. With second flight of Starship there were some concerns of it forming and blocking fuel supply during take off. $\endgroup$
    – estinamir
    Commented Apr 29 at 7:35
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    $\begingroup$ "maybe there is slush" is not the basis of a good Stack Exchange question. So far as I know there is no good reason to premix fuel and oxidizer (if they are even miscible) and a whole bunch of reasons to keep them separate. Hydrogen boils at 20 K and is a real pain to store long term. I would focus your question on storage of methane and oxygen only (bp 112 K and 90 K respectively) which is the most practical combination for long term storage, or just ask about LOX by itself. Everybody likes LOX! $\endgroup$
    – uhoh
    Commented Apr 29 at 8:12
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    $\begingroup$ If you are interested in finding out why folks don't premix fuel and oxidizer, and if anybody has ever, ever tried it (except for solid fuel) then that's an excellent new question. Keep this one about how to keep stuff cold in space only. $\endgroup$
    – uhoh
    Commented Apr 29 at 8:14
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    $\begingroup$ Hydrogen's critical temperature is below methane's freezing temperature. You definitely don't want to make the methane into ice. $\endgroup$
    – Erin Anne
    Commented Apr 29 at 17:38

2 Answers 2

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"Methalox" requires a methane tank and a separate oxygen tank. "Hydrolox" requires a hydrogen tank and a separate tank of, again, oxygen. As liquified propellants absorb heat from their surroundings, their temperature will rise until they reach their respective boiling points. After that, the temperature of the liquid will stop rising but some of it will begin to phase transition into a gas.

If you created a propellant depo with three tanks containing liquid hydrogen, methane, and oxygen, and all three were at their boiling points, then the liquid hydrogen tank would be the coldest. You could take advantage of that and use the hydrogen tank to cool the other two tanks. Then you could keep the temperatures of the liquid methane and liquid oxygen tanks below their boiling points. This way you would only have to deal with boil-off in the liquid hydrogen tank - at least so long as there is still liquid hydrogen in that tank.

To figure out if this would be worthwhile, you need to know the enthalpy of vaporization of the three liquids. Hydrogen is 0.9 kJ/mol, Oxygen is 6.92 kJ/mol, and Methane is 8.17 kJ/mol.

If the liquid hydrogen absorbs 0.9 kJ, then one mol would be converted to H2 gas and so you'd lose about 2 grams.

If the liquid oxygen absorbs 0.9 kJ, then 0.9/6.92=0.13 mols of O2 would be converted to gas, about 4 grams.

If the liquid methane absorbs 0.9 kJ, then 0.9/8.17=0.11 mols of CH4 would be converted to gas, about 1.76 grams.

It might be worthwhile to send up some liquid H2 to keep the liquid O2 below its boiling point, but it probably would not be worthwhile to use liquid H2 to keep liquid CH4 below its boiling point.

If you did send up liquid H2 to cool the liquid O2, it might absorb heat faster since it's a lot colder than liquid oxygen. This would cancel out some of the benefits.

Still, it's an interesting idea...

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    $\begingroup$ I suspect density is important here, not just the energy per Mol - Hydrogen is infamously low density making tanks bigger and heavier than people would like. Since the Hydorgen is not being burned replacing it with something less energetic but with better enthalpy per cubic meter might come out ahead. $\endgroup$
    – GremlinWranger
    Commented Apr 29 at 9:15
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    $\begingroup$ I agree that there could be better choices for the sacrificial boil-off gas than hydrogen. Still, @estinamir might be onto something here. $\endgroup$
    – phil1008
    Commented Apr 29 at 19:32
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This would be a very good idea and almost everyone will be using it if not for a few things that will totally make it impractical:

  1. Hydrogen takes up an enormous volume. Hydrogen itself is not dense and if you were to compare bringing extra Liquid Methane rather than Liquid Hydrogen to cool the methane, it will still be about 10 times less volume.
  2. Liquid Hydrogen is incredibly hard to store. Given that the atoms of liquid Hydrogen are the smallest, it will seep through the tiny gaps in-between atoms of most materials, and stainless steel is no exception. It will just seep through it and leak, that's why Hydrolox rocket stages have tanks made of aluminum lithium alloys to minimize leakage to a minimum. Even so, a significant portion of fuel leaks out of the tanks and is wasted. If this could be prevented, the Delta IV Heavy's center Core could have had around 300m/s more Delta V [citation needed] 3.Liquid Hydrogen has a very low boiling point. With a boiling point of 20K, it would almost instantly turn the liquid Methane and liquid oxygen into a solid, which engines can't use (unless you decided it would be a good idea to make it a Methalox Solid Rocket Motor, which would be a terrible idea). 4.It would require and extra tank and slot if heavy insulation. Liquid Hydrogen Boils off very easily and thus would require insulation as well as an extra tank to contain it. Which will add on a lot of extra weight and make it less efficient.

This is why starship will not use this method. Bringing along extra methane is still easier than trying to prevent boil of with hydrogen.

This is a good idea however, so, I will do some research on the theory of this.

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