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Is it possible and worth it to use liquid propellants like liquid oxygen and liquid hydrogen in amateur rockets? What are the other types of liquid fuel used?

Moderator's Note: Remember discussion of homemade engines or propellants is explicitly disallowed here, see this meta FAQ. Answers should address solutions that are commercially available for hobby rocket kits.

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    $\begingroup$ There are two books you should have a look at: "Design of Liquid Propellant Rocket Engines" by Huzel and Huang and "Rocket Propulsion Elements" by Sutton. These two books answer your questions far better than Stackexchange can. Both can legaly be read for free online. $\endgroup$ – Rikki-Tikki-Tavi May 25 '18 at 6:03
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    $\begingroup$ This is going to be very dangerous. You should not try this. Rocket engine debugging often starts with a series of unexpected fires and explosions. $\endgroup$ – uhoh Jun 25 '18 at 15:52
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    $\begingroup$ @uhoh Our rule was regarding making your own engine or propellant; it was explicitly not meant to exclude using commercially available hobby rocket products. There are hybrid and liquid engines and propellants commercially available, so I'm tempted to leave this one open as long as we restrict the discussion to prefab engines/propellants. $\endgroup$ – called2voyage Feb 21 at 14:08
  • $\begingroup$ @called2voyage I didn't realize such things existed, thanks. I've retracted my close vote. $\endgroup$ – uhoh Feb 21 at 14:33
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Liquid propellants like kerosene and hydrogen peroxide - that's risky, though doable.

Cryofuels like liquid oxygen and hydrogen - no. Even if you manage to develop an amateur rocket motor that's capable of running on these, the infrastructure for storage and manipulating them is out of range of amateurs. Even professionals like SpaceX avoid LH2 and run on kerosene because liquid hydrogen is so ungrateful substance to handle they are better off going with slightly lower efficiency and not needing to handle it.

You may achieve limited success with "high-temperature" cryogenic liquid propellants - LPG, nitrous oxide; stuff that stays liquid above -100C and is not extremely aggressive. It's still a headache not really worth the extra effort. Just getting a stable combustion and not destroying the test setup in the process is a great success. With air moisture forming frost everywhere, boil-off products creating risk of explosion, and need to handle everything in thick gloves or through prongs to avoid frostbite, racing against the clock when boil-off begins, fumes being not really health-neutral etc, you'll have your hands so full there won't be enough left to actually get any work on the engine done.

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    $\begingroup$ Cryogenic-capable valves alone are well out of reach of any but the most affluent enthusiasts. $\endgroup$ – Rikki-Tikki-Tavi May 25 '18 at 6:20
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    $\begingroup$ @Rikki-Tikki-Tavi. That's when you want to go the semi-professional way. But the way of amateur rocketry often is "if it leaks, place a bucket beneath. If it doesn't want to move, hit it with a hammer." Every valve is cryogenic-capable as long as it doesn't shatter if you apply sufficient force and permit for sufficient leakage. $\endgroup$ – SF. May 25 '18 at 8:11
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    $\begingroup$ Leaking hydrogen is a good way to off youself real quick. And a valve that doesn't open because it's frozen isn't worth much. $\endgroup$ – Rikki-Tikki-Tavi May 25 '18 at 11:33
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    $\begingroup$ @Rikki-Tikki-Tavi: Let's assume we're working with liquid nitrous oxide instead. At least despite the frostbites, bruises and burns you'd retain high spirits. $\endgroup$ – SF. May 25 '18 at 11:44
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    $\begingroup$ When you order N20, you get it in a metal cyllinder, which can withstand way more than 50 bar. You can keep it in there indefinitely without cooling. When you fill the rocket, you use a pressure regulator. It boils off a little, thus cooling it, but not by much. Whether or not you can use a particular piece of plastic seems to depend on the exact formulation. NBR is sometimes classified as resistant, but YMMV, as we learnt. Once the fuel is in the tank, you wait until either the sun or your heater has raised the pressure in the tank so much that it can be fired using its own pressure. $\endgroup$ – Rikki-Tikki-Tavi May 26 '18 at 9:03

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