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In an answer to this question, the brilliant diagram below was given: enter image description here

The chart makes chilled ethylene look like a very appealing choice, yet I don't believe it is ever used. Why not?

Edit: I just checked the wonderful "Ignition" by John D Clark and while ethylene oxide and many other compounds are mentioned ethylene gets only one mention:

Alcohol, ammonia, and JP-4 or RP-I were the fuels usually burned with LOX, but practically every other inflammable liquid available has been tried experimentally at one time or another. RMI tried, for instance, cyclopropane, ethylene, methyl acetylene, and methyl amine. None of these was any particular improvement on the usual fuels.

(this was around 1950 and their concern was mainly missile boosters, not upper stages).

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    $\begingroup$ Your reference to 'this question' does not exist. Also in your related question. $\endgroup$ – Jan Doggen Oct 1 '18 at 11:05
  • $\begingroup$ It could be something like the reason we don't use aerospike engines or thorium reactors for civilian power... just because. $\endgroup$ – uhoh Oct 1 '18 at 15:09
  • $\begingroup$ Here's some ethylene waiting for you on Mars! ;-) $\endgroup$ – uhoh Oct 9 '18 at 14:07
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For one, you get basically the same $I_{sp}$ to density ratio as RP-1, except with two penalties: lower density, which means bigger tanks and a heavier rocket, and the need to chill the ethylene. Compared to RP-1, you also lose the synergy of being able to use the fuel as a lubricant and hydraulic fluid.

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    $\begingroup$ Wouldn't this also be the reason why methane is not considered a rocket fuel either? (if that were the case, which it isn't of course) $\endgroup$ – uhoh Oct 1 '18 at 15:11
  • $\begingroup$ My thoughts exactly @uhoh! I'm not sure why that ratio would be an appropriate figure to take. The high absolute Isp of ethene suggests it might be a very good choice for an upper stage fuel that didn't have all the issues of liquid hydrogen. $\endgroup$ – Steve Linton Oct 1 '18 at 15:15
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    $\begingroup$ Methane has the advantages of being really cheap (which affects the launch price floor for reusables) (particularly if you can accept common natural gas instead of pure methane) and potentially easily produced on Mars. $\endgroup$ – Russell Borogove Oct 1 '18 at 15:47
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    $\begingroup$ @SteveLinton When you're trying to push your marginal launch cost down to the cost of fuel, that's a significant difference. $\endgroup$ – Russell Borogove Oct 2 '18 at 0:30
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    $\begingroup$ @SteveLinton Not a chemistry guy, especially not an OChem guy, but I did find this: chemistry.wustl.edu/~edudev/Designer/session2.html. It looks like aluminum/titanium can catalyse a polymerization reaction at pressures and temperatures you'd reasonably see in a propulsion feedline system, so that may indeed be a concern. $\endgroup$ – Tristan Oct 3 '18 at 15:15

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