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Some propellants have wildly different melting points. The most extreme case is the theoretically fantastic (and nightmarish in practice) Fluorine-Lithium-Hydrogen tripropellant, with liquid hydrogen being very cryogenic while lithium melts at 180°C. So instead of having cryogenic, room-temperature or even heated fuel tanks, have there ever been rocket designs where a liquid propellant is stored in solid form?

This would include melting the propellant with a heat exchanger before pumping it into the chamber, storing it as a powder and moving that instead, or even storing it in a suspension of particles in a liquid. Slush hydrogen and similar is outside of the scope of this question, though.

Note that this isn't about hybrid rockets. In hybrid rockets, a liquid propellant is running against a solid grain, with the reaction happening at the interface. The reaction itself may melt the solid grain, but that's at best a way to improve the local reaction, it isn't expected to flow anywhere. This is instead about liquid-fuelled rockets, where some of the fuel is stored as a solid, but at some point moved around like a liquid.

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    $\begingroup$ It would also be interesting to know what freezing a liquid fuel does to its shelf life. If it lengthens the shelf life, that would be useful for storing the fuel prior to filling the rocket. Not quite what @Eth is proposing, but related. $\endgroup$
    – DrSheldon
    Sep 28, 2018 at 18:41

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Liquid rockets are complex and powerful machines. And typically they complete their job under 30 minutes. It's hard to change phase of propellants from Solid to liquids in 30 minutes considering most of fuels have higher latent of fusion. Some materials may turn into semi-solids instead turning into liquids which has totally different set of properties. Also adds additional complexity of heat exchangers and risk of few solid pellets which are not completely melted may block the fuel/oxidizer pipes or damaging the turbos inflight.

With these risks, complexity and propellant properties with no significant benefits I can assure you that no liquid rocket engine of this kind ever be designed.

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    $\begingroup$ As a rule, don't solid propellants change phase from solid to gas before combusting? It's not the same as solid-to-liquid, but dismissing all phase changes with a "phase changes in rockets will never happen because rockets are complex" might be too quick. Why not choose Solid methane/SOX for example and explain why it clearly could never work, if it indeed could never? $\endgroup$
    – uhoh
    Apr 16, 2019 at 2:32
  • $\begingroup$ Good points, and most probably enough to kill the idea. $\endgroup$
    – Eth
    Apr 17, 2019 at 13:45
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    $\begingroup$ @uhoh , the answer I gave is generic an answer. "Some materials may turn into semi-solids". They may or may not apply for some propellants. For example, consider wax as fuel. It's viscosity changes significantly with temperature under standard pressure. It just don't convert from solid to liquid directly. Typically propellants are stored under pressure for safety reasons (It applies here too). At that pressure, temperature and high propellant flow velocity, designers have to consider new variables like viscosity of propellants and other which are mentioned above. $\endgroup$
    – SRD
    Apr 17, 2019 at 14:07
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    $\begingroup$ @uhoh I used this words to "rockets are complex" to express my reluctance to make rockets complicate further. About phase transformations, there are PhD students just working multi phase combustion, it's a very vast subject. Solid methane/SOx may work but I don't see any compelling reason to do so. $\endgroup$
    – SRD
    Apr 17, 2019 at 14:25
  • $\begingroup$ Okay, sounds good, thanks! $\endgroup$
    – uhoh
    Apr 17, 2019 at 14:36
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Not on purpose, but an answer to this question Is it bad if hydrazine freezes on a spacecraft? Is it always kept as liquid, or can it be safely allowed to freeze and then thawed when needed? states that the fuel of the Olympus satellite froze solid and was successfully thawed out and used later.

I have never heard of such a design being used on purpose, and it is difficult to see what advantage there could be from it.

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    $\begingroup$ That question and its answers are interesting, thanks. The interest would be to use liquid fuel that is solid at room temperature, but hydrazine freezing, even if manageable, was clearly a pain already. $\endgroup$
    – Eth
    Apr 17, 2019 at 13:37

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