Most BBQs use propane. Propane burns quite well and it is quite clean.
So why hasn't anyone use it as a rocket fuel??
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2$\begingroup$ Related (not duplicate) questions: How does liquified natural gas compare to liquid methane? and What was the result of the propellant predictions in the last chapter of “Ignition!”? $\endgroup$– DarkDustMar 1, 2017 at 21:20
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1$\begingroup$ I also found a NASA whitepaper discussing a nitrous oxide / propane (NOP) engine $\endgroup$– DarkDustMar 1, 2017 at 21:28
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15$\begingroup$ It's probably because rockets are not BBQs. $\endgroup$– AlohaMar 2, 2017 at 16:09
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$\begingroup$ I remember reading (but I can't find it anymore, so maybe I'm entirely wrong) that taking both specific impulse and density into account propane was the best hydrocarbon. Better specific impulse than RP-1, and less tankage weight than methane, while providing the same reusability advantages as methane (as long as the propane is pure enough). However the specific impulse differences are quite small. $\endgroup$– JanKanisSep 17, 2017 at 21:20
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1$\begingroup$ The main reason for recent methane rocket development is better reusable engines (compared to RP-1). SpaceX uses pure methane, as that can also be produced on Mars. Blue Origin is not aiming for Mars so it uses natural gas, which also contains ethane, propane and some other compounds. Also, propane needs to be much more low-sulphur than commonly available propane or else it can damage engines (see discussion and linked paper at yarchive.net/space/rocket/fuels/propane.html). $\endgroup$– JanKanisSep 17, 2017 at 21:22
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3 Answers
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If you're going with cryofuels (and don't want to dabble in liquid hydrogen, which opens another can of worms), you're better off with liquid methane for its performance - higher specific impulse thanks to higher hydrogen:carbon ratio.
If you don't want cryofuels, you go with something that stays liquid at ambient temperature: RP-1 kerosene.
Propane, with ~5-7 bar vapor pressure at ambient temperatures would require a pressure tank (not acceptable on a rocket due to mass), or needs to be cooled to at least -42oC. This combines disadvantages of cryofuels and complex hydrocarbons, giving a very small specific impulse rise over RP-1, and while rather large, still insufficient boiling point rise above methane.
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$\begingroup$ It seems like propane might have the advantage of not freezing so easily. I also wonder if it is more stable over long durations -- that property has lead to it being preferred over gasoline for terrestrial engine applications. This might be worthwhile for tiny RCS systems or the like that don't depend so strongly on high dV. $\endgroup$– ikraseAug 21, 2019 at 8:19
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$\begingroup$ @ikrase: These are advantages over kerosene, but not methane. And the property that led to it being preferred over gasoline for terrestrial engine applications is that it's cheaper. $\endgroup$– SF.Aug 21, 2019 at 12:47
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$\begingroup$ I don't think it's actually cheaper. The argument I see used to justify it for terrestrial ICEs is that commercial grade propane in a commercial propane fuel system for an ICE lasts almost indefinitely, but typical commercial gasoline typically suffers several degradation mechanisms at once (hygroscopicity, evaporation of volatile fractions, oxidation, chemical reactions that cause gumminess, etc) that make a "fuel-and-forget" ICE a (literal) non-starter after more than a few months. As to VS methane, propane can be compressed into liquid in a sorta lightweight tank. $\endgroup$– ikraseAug 27, 2019 at 6:27
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1$\begingroup$ @ikrase Current data for Poland, where conversion of cars to LPG is very common - and the motivation is universally the costs. 4,75PLN/l for E95 gasoline, 1,74PLN/l for LPG. Gasoline gives about 125-130% the milleage per liter, for 270% the price. I don't believe the global situation is so vastly different these proportions would reverse. $\endgroup$– SF.Aug 28, 2019 at 11:00
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$\begingroup$ I wonder if this is concerning taxation. In the USA this is not common, though propane is common for fueling generators, forklifts, and other power equipment (especially power equipment that operates indoors). It looks like in the USA the market price by volume of propane and gasoline is similar. When I see people discussing the conversion of generators to propane, the usual assumption is that the cost will be moderately greater, but the shelf-life and cleanliness is worth the cost. $\endgroup$– ikraseAug 30, 2019 at 7:50
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People have used it as rocket fuel, but its density isn't great compared to other hydrocarbons used as rocket fuel, which burn about as well and just as cleanly.
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LOX/Propane is a viable option, especially if the propane is chilled to near freezing point in order to improve its density. Interestingly propane can remain liquid at the temperature of boiling LOX enabling simplified common bulkhead tanks to be used.
With chilled propane, the performance is similar to methane with regards to specific impulse and better when it comes to density impulse.
There is two reasons why propane is not used instead of methane.
Nr 1, propane has a higher risk of undergoing coking under elevated temperature and pressures, conditions which exist in the combustion chamber cooling channels if regenerative cooling is used.
Nr 2, Methane is a smaller molecule and easier to produce on other planets, such as Mars. Propane can be produced from the same basic ingredients as methane (water and carbon oxides), but require further processing and refinement to be usable.
Thus Methane/LNG is favored for any interplanetary missions over Propane.
EDIT: Aerojet Liquid Rocket company performed research for NASA and investigated Propane as a potential fuel. They observed cooking at temperatures as low as 500F. The report is available here albeit in low quality. (https://core.ac.uk/download/pdf/42850833.pdf)
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1$\begingroup$ Do you mean coking? Do you have a reference for any of this? $\endgroup$ May 20, 2021 at 11:54
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1$\begingroup$ @Jeppe Nielsen - your answer is actually the most precise. One startup called Orbex Space aims to use pre-chilled propane because it has similar density as RP-1 but better ISP. $\endgroup$ May 20, 2021 at 21:40
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$\begingroup$ @WOW6EQUJ5 I happen to know about Orbex. I believe Jonas Bjarnø (their rocket engine gure) is aware of the issues with propane, otherwise he should know how to contact me. $\endgroup$ May 25, 2021 at 21:28