Cryogenic fuels (liquid hydrogen, liquid methane)1 and oxidisers (liquid oxygen)2 are the rocket propellants of choice where raw performance is the overriding concern, due to the very high performance (by chemical-rocket standards) produced by rocket engines burning cryogenic propellants. However, they come with the disadvantage that, having boiling points well below room temperature (or Space Coast outdoor temperature, or, for that matter, even Plesetsk-in-winter temperature), they start to evaporate as soon as the rocket is fueled; to keep the tanks from bursting from the accumulation of gas produced thereby, the boiled-off propellant is vented through relief valves, and, thereby, lost to the rocket. Insulating the rocket helps somewhat, especially for liquid hydrogen, which has the lowest boiling point of all cryogenic propellants (hence the ubiquitous orange foam insulation seen on essentially all hydrolox rockets and rocket stages), but the boiloff eventually occurs even with insulated tanks.

As a result, cryogenic rockets are best launched as soon as they’re fully fueled (in order to minimise the amount of propellant that boils off before launch), and cryogenic upper stages generally aren’t good for more than a week or so, tops, in space, before enough of the propellant has boiled off to drop the stage’s Δv budget below that of an upper stage using so-called “storable” propellants3 (or, in some cases, a solid-fuel motor).

On the other hand, it is obviously possible to chill liquid hydrogen and methane and oxygen to well below their boiling points (as evidenced by the fact that you have the cryogenic propellants available in liquid form to put in the rocket in the first place), via one or more refrigeration methods. If one could somehow keep the propellants refrigerated to sub-boiling temperatures even once they’ve been loaded into the rocket, it would be possible to greatly reduce, or even completely eliminate, the problem of propellant boiloff.4 This shouldn’t be too hard for first stages, where, after fueling, the only sitting around they do is on the pad; the fuel and oxidiser tanks could simply be plumbed into fixed refrigeration plants via a couple more umbilicals, with valves being closed to isolate the tanks from the ground infrastructure as part of the ignition sequence, and the umbilicals then separating at liftoff:

Food safety as applied to rocket fuel

This would be harder for upper stages, as, in order to keep their propellants refrigerated until it came time to use them, they would have to carry (potentially-heavy) refrigeration equipment with them (although this could potentially be mitigated to some degree by jettisoning the refrigerators prior to the stage’s final burn, when they’d no longer be needed); where refrigerated propellants would really come into their own in space would be for an orbiting cryogenic propellant depot, as the refrigeration machinery could (again) stay on the piece of infrastructure that isn’t going anywhere (here, the propellant depot), with client spacecraft reaping the benefits of stored cryogenic propellants without having to pay the weight penalties of refrigeration equipment. Admittedly, refrigeration does require an external source of energy, but solar energy is essentially free for an orbiting spacecraft, with rechargeable batteries to cover the periods of shadow time.

What am I missing?

1: Liquid ethane, ethylene, and propane are also cryogenic (although nowhere near as deeply so as liquid methane), but, to the best of my knowledge, have not actually been used in any mass-produced rocket engines.

2: Liquid fluorine is also cryogenic, and is actually a somewhat-better-performance oxidiser than liquid oxygen, but is somewhat more expensive and difficult to handle, results in engines with more spectacular failure modes, and can only realistically be used as an upper-stage oxidiser (due to the vast amounts of hydrogen fluoride it produces with all common rocket fuels); for these reasons, it is not generally used as a rocket propellant nowadays.

3: In practice, this generally means room-temperature hypergolic propellants - generally hydrazine and/or one or more of its derivatives (for the fuel) and dinitrogen tetroxide (for the oxidiser).

4: As well as the related problem of thermal expansion of chilled liquid propellants; all liquid rocket propellants (even those that are liquid all the way up to, and past, room temperature, like RP-1) expand with increasing temperature, which has led some rocket surgeons to chill these propellants almost to their freezing points in order to densify them and let more propellant be packed into the tanks. If the rocket then has to sit around on the pad absorbing heat, the propellants warm back up and expand, and some quantity thereof has to be drawn from the tanks to keep them from bursting, negating the performance advantage of chilled propellants.

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    $\begingroup$ Estimate cost and development time of recovery hardware versus cost of lost propellants? $\endgroup$ – Russell Borogove Jun 5 at 23:03
  • $\begingroup$ @RussellBorogove: What "recovery hardware"? $\endgroup$ – Sean Jun 5 at 23:05
  • $\begingroup$ The return lines in your diagram and whatever they interface with. $\endgroup$ – Russell Borogove Jun 5 at 23:10

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