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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. $\endgroup$
    – user10509
    Commented Oct 1, 2018 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
    Commented Oct 1, 2018 at 15:09
  • $\begingroup$ Here's some ethylene waiting for you on Mars! ;-) $\endgroup$
    – uhoh
    Commented Oct 9, 2018 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
    Commented Oct 1, 2018 at 15:11
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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
    Commented Oct 1, 2018 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
    Commented Oct 2, 2018 at 0:30
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    $\begingroup$ I wonder how likely is ethylene to polymerize into polyethylene in standard rocket fuel usage conditions... $\endgroup$
    – SF.
    Commented Oct 3, 2018 at 13:09
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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
    Commented Oct 3, 2018 at 15:15
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It sure is, by some small portion of the industry. I don't think we'll be seeing any big EthyLox (if you'll excuse the neologism) boosters taking off as first stages, though: it's a complicated thing to deal with, Ethylene. it being a room temperature gas means it entails all the hassle methane or hydrogen brings (i.e. you need to chill it down in order to put it in a tank, build pretty good insulation around your tanks, deal with boiloff, etcetera) and since methane has one fewer carbon I'd expect it to get better ISP. What Ethylene is being considered for, however, is as half of a storable bipropellant system: Ethylene self-pressurizes at normal temperatures, so for pressure-fed engines and RCS it's pretty good, and it's supposed to burn nicely when combined with various oxides of nitrogen, which are also self-pressurizing: The combination doesn't get amazing performance, but if proven to work it would be much easier -and cheaper- to handle than the various nasties currently used for reaction control, stationkeeping, auxiliary power units, orbital maneouvering systems and the like: all in all the mixture seems to have the advantage of being a safe (well, about as safe as rocket fuel gets, anyway), storable, and highly energetic bipropellant: and you can use the oxidizer as a monoprop as well. I think it has a decent chance of competing with stuff like hydrazine.

there's a NASA paper on this https://tfaws.nasa.gov/TFAWS06/Proceedings/Aerothermal-Propulsion/Papers/TFAWS06-1026_Paper_Herdy.pdf

I suppose the system has the added benefit that you could use a bit of your fuel for ripening bananas?

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Late answer, but I just stumbled on this. There is a class that is not considered here that is up and to the right of most of the other propellants on the graph. Blends of light hydrocarbons exhibit depressed freezing points, which means things like 50/50 propane/propylene can be chilled down to or even below LOx temperature (90K), which greatly improves both their density and their lubricity. At these temps they give significantly better density impulse that anything on the chart.

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    $\begingroup$ Does this address the question "Why is ethylene not considered as a rocket fuel?" or is it a comment about other potential fuels? $\endgroup$
    – Organic Marble
    Commented Jan 27, 2021 at 19:27

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