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In the question Can I make my own RP1? the comments talk about using ethanol and water as rocket fuel. That made me wonder how well easily availible ethanol fuel for cars such as E85 (85% ethanol 15% gasoline) would work. Would such a rocket run into the same problems as a pure kerosen rocket or would it work well for a simple launcher?

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  • $\begingroup$ Ethanol has a higher vapor pressure than refined kerosene, so just like as another comment from the other answer said, regeneratively cooling the engine could become problematic as the vapors of ethanol go in. $\endgroup$ – Jake Blocker Jul 9 '17 at 16:05
  • $\begingroup$ I was thinking more along the lines of would it work well in similar contexts as ethanol and water would, or is the water an important part of that fuel mixture? Hmm wondet if that should be its own question? $\endgroup$ – lijat Jul 9 '17 at 21:05
  • $\begingroup$ If small amounts of water are added to the combustion chamber (in this case it's inside with the ethanol) it produce more thrust, but I believe it slightly drops the Isp, so the water is useful in some ways, but not completely necessary. $\endgroup$ – Jake Blocker Jul 9 '17 at 21:10
  • $\begingroup$ @JakeBlocker: In V-2 water was added because ethanol combustion was producing temperatures the engine was unable to withstand, even with regenerative cooling. In such case the loss of performance is preferable to RUD. $\endgroup$ – SF. Jul 9 '17 at 23:45
  • $\begingroup$ @SF. Very true, I was referring to Tom Mueller's wiki page when saying it produced more thrust because of higher mass flow en.m.wikipedia.org/wiki/Tom_Mueller $\endgroup$ – Jake Blocker Jul 9 '17 at 23:47
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Which one? Which supplier, which brand, which set of additives, what content of impurities?

That matters. This is why rockets fly on RP-1, not on JP-1 kerosene - because the standard for JP-1 is crap, and JP-1 can be pretty much anything, while RP-1 restricts contents of substances that adversely affect combustion stability and regulates contents of substances that affect combustion process, so that it's repeatable.

Ethanol is not a very desirable fuel due to high combustion temperature accompanied by moderate pressure of the exhaust - the nozzle runs very hot while not producing all that much thrust. This can be managed by other additives and E85 could be better than pure ethanol (though likely worse than RP-1) - providing it's standarized to a regime as tight as RP-1 kerosene; it doesn't matter so much what the standard is, only that it's tight so that if a test article engine is made to work reliably on one batch on the fuel, the production engine will work exactly the same on another batch.

In short, with existing E-85 you'll run into the same kind of problems as the early rocket engineers ran into with JP-1 jet fuel. There are detergents, there are denaturing additives, pigment, all that crap that's mostly neutral for a car engine, but your rocket - even if you don't clog the injectors or regenerative cooling pipes - will simply vary in parameters from batch to batch. And so, even if your rocket flies, you'll have a lot of headaches assuring it flies where you want it to!

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It could work, but for commercial purposes, it's not really economical. The energy density of E85 is much lower (34.7% lower per unit volume, and 26.9% lower per unit mass) than diesel (which is basically kerosene, which is basically RP-1....sorry I couldn't find reliable energy numbers for RP-1). Which basically means that you need a larger (and therefore heavier) launch vehicle to carry the extra fuel you'll need to get the needed delta-V for orbit. Engineers continually aim for the best tradeoff of energy density vs. reliability, and complexity of the engine, tanks, and plumbing.

And even though ethanol burns hot (1920°C), the previous answer's assertion that it's hotter than RP-1 is incorrect based on my research, since the RP-1 combustion temperature is 3396°C; which is actually cooler than hydrogen at stochiometric mixture -- 2800°C.

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  • $\begingroup$ Sure, for commercial rockets yes. My intent with the question was more for an amature rocket context, it was expressed in the tags and by virtue of the linked question. Do you think I should clarify that somehow? $\endgroup$ – lijat Jul 18 '17 at 18:12
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    $\begingroup$ Well that seems to be at odds with the previous answer though as well, which is talking about impurities and cleaning additives to the fuel, which for amateur rocketry is really down in the noise. Performance parameters for orbital-class rockets are critical to nail down to ensure you make it to orbit, but for amateur rocketry you're not looking at a "will or won't work" scenario depending on which batch of fuel you get, but rather will you get to 90% or 110% of your predicted altitude (arbitrary numbers). The title of the question is probably what threw me off. $\endgroup$ – ereisch Jul 18 '17 at 18:17
  • $\begingroup$ @ereisch: For amateur rocketry more than one liquid is too many liquids. $\endgroup$ – SF. Jul 18 '17 at 21:20

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