I was surprised to read that Redstone used 75% ethyl alcohol as a fuel. Looks like this idea was abandoned later, and modern rockets use rectified petroleum or liquid hydrogen instead for burning with LOX. I think it would be really strange for somebody to think about the alcohol before trying the usual jet fuel, kerosene, first. For which reasons the alcohol has been initially selected?
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2$\begingroup$ related: youtube.com/watch?v=Eauxlp1wN8Q quick answer: alcohol is easily watered down to reduce engine temperature, which is important for the cooling technology at the time. $\endgroup$– user3528438Commented Jul 16, 2019 at 19:23
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$\begingroup$ The very first American space rocket was Bumper 5 using a German V-2/A-4 rocket as first stage and an American WAC Corporal as second stage. No orbit but a maximum height of 393 km, well above Karman line. 75% ethyl alcohol and LOX used for the first stage. $\endgroup$– UweCommented Jul 27, 2019 at 17:36
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The Redstone's engine was based heavily on the engine of the German V-2, which also used 75% ethyl alcohol fuel with liquid oxygen.
Robert Goddard's first liquid fueled rocket experiments used gasoline with liquid oxygen, which burned extremely hot -- Goddard burned out several nozzles and combustion chambers on his early flights. Figuring out how to cool down the combustion chamber and nozzle became the big barrier to developing more powerful engines.
One route was regenerative cooling: running the fuel through tubes around the engine and nozzle to pull heat before spraying the fuel into the combustion chamber. This was complex, and when jet-grade kerosene (which is made of a mix of different hydrocarbons) was used, some of the fuel would vaporize, making gas bubbles that didn't carry much heat, and some of the fuel would polymerize or "coke", clogging the cooling tubes. Ethyl alcohol, on the other hand, would vaporize but not coke, and diluting it with water kept the temperatures down enough that vaporization in the cooling tubes wasn't an insurmountable problem. The water absorbed a great deal of heat in the combustion chamber and was expelled as steam, providing reaction mass to produce thrust, without contributing to combustion.
As a side note, Bacardi 151 or similar high-proof liquor is, essentially, 75% ethyl alcohol, 25% water. If you've ever had a flambé shot or dessert, that's basically Redstone rocket fuel -- but burning it with LOX (a little denser than water) instead of atmospheric oxygen (only about 20% of the air around us) makes for a much hotter and faster burn.
answered Jul 16, 2019 at 20:11
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1$\begingroup$ IIRC, the reason why A-4/V-2 used ethanol instead of kerosene was that the later was deemed war-essential: the Nazis needed it for the planes and supply was limited. So von Braun and his team had to use a different propellant. Ethanol was available and easy to handle. $\endgroup$– DarkDustCommented Jul 16, 2019 at 20:38
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1$\begingroup$ @DarkDust Military airplanes during WWII needed gasoline, not kerosene. Jet engines were in development but not in mass production. But the supply of all mineral oil products was very limited anyway, coal liquefaction was used instead. The ethyl alcohol was mixed with water to simplify cooling problems. $\endgroup$– UweCommented Jul 16, 2019 at 21:12
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$\begingroup$ @DarkDust Gasoline for piston engines could have been mixed with 5 to 10 % ethyl alcohol as done today. During the war, potatos used for production of ethyl alcohol were also of limited supply. $\endgroup$– UweCommented Jul 16, 2019 at 21:44
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3$\begingroup$ @Uwe: You're right. I just found this in "Ignition!": "Late in 1931 Klaus Riedel of the VfR designed a motor for a new combination, and it was fired early in 1932. It used liquid oxygen, as usual, but the fuel, conceived by Riedel and Willy Ley, was a 60-40 mixture of ethyl alcohol and water. The performance was somewhat below that of gasoline, but the flame temperature was much lower, cooling was simpler, and the hardware lasted longer. This was the VfR's major contribution to propellant technology, leading in a straight line to the A-4 (or V-2) and it was its last." $\endgroup$– DarkDustCommented Jul 17, 2019 at 6:23
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$\begingroup$ Not only the A-4/V-2 used ethyl alcohol and LOX, also the predecessors A-1, A-2, A-3 and A-5. $\endgroup$– UweCommented Jul 17, 2019 at 8:25