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Why was the V-2 rocket made with ethanol and liquid oxygen, which had a specific impulse of 215 seconds at sea level, when a solid rocket using APCP gets 237 seconds?

In addition, a solid rocket engine would have almost no moving parts, so it would be a lot cheaper and quicker to manufacture.

I'm guessing that what I'm effectively asking is: Why was APCP (or a similar solid fuel) not available or not developed at that time? What is the developmental/scientific history of solid rockets?

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    $\begingroup$ This question has an associated meta question about it. $\endgroup$ – Danu Aug 15 '15 at 8:08
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Unlike the short-range rockets which are all solid fueled, making a solid-fuel long range rocket is very challenging. The main technical problem is that the fuel block which is very large can develop a crack under mechanical and thermal stress. The flame will immediately spread into the crack, and the whole thing will explode.

The second difficult problem is how to cool down the nozzle. In the liquid-propelled rockets it is cooled by the fuel. In short range rockets the engine works very short time and no cooling is needed.

For the history of German rockets, and in general for the early rocket history (till the late 40-s) there is an excellent classical book: Willey Ley, Rockets, Missiles and Space travel. (Strongly recommended). The Russian side of the story is described in the books of Chertok (4 volumes in Russian). He mentions a breakthrough in solid fuel development made in Caltech in the early 50-s, the thing they just could not match in Soviet Union.

All early Soviet long range rockets, even those launched from submarines were liquid fueled. Because the Soviets could not develop solid fuel technology needed for large enough engines. (Just imagine a liquid fuel rocket launched from a submarine!!) Earlier models could not be kept fueled inside in a submarine, so they had to undergo a long and extremely dangerous fueling procedure immediately before launch. If the launch was aborted after the fueling began, the rocket had to be just damped overboard:-)

This shows that the problem is highly non-trivial. Roughly speaking the fuel block must be mechanically strong and elastic. And the nozzle has to withstand enormous temperature for relatively long time.

I am afraid that a complete scientific story of (long range, intercontinental) solid fuel rockets is not written yet: the Russians used liquid fuel rockets until recently, and maybe still using some of them.

EDIT. 1. I should mention that liquid fuel rockets have also some advantages: you can regulate the engine power during the flight. It is much more difficult with solid fuel.

  1. Actually all three currently operational Russian submarine based ICBM use liquid fuel, one of them has liquid and solid on various stages. See Wikipedia article "Submarine-launched ballistic missile".
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    $\begingroup$ +1 for grain cracks, but 2 points: (1) cooling in solid rockets is actually easy because they just add an ablative heat shield inside the engine (e.g., Shuttle SRBs). This is a non-moving part, as opposed to extra plumbing. (2) I've read all 4 volumes of Rockets and People by Boris Chertok and I don't recall him mentioning a breakthrough by Caltech about solid rockets. Can you provide a volume and page number? Also, Soviet Project Gnom (1958 - 1965) used solid rockets for a nuclear missile. en.wikipedia.org/wiki/Air-augmented_rocket and astronautix.com/lvs/gnom.htm $\endgroup$ – DrZ214 Aug 29 '15 at 0:19
  • $\begingroup$ @DrZ214: It is in volume 3, "Hot days of Cold war". Section 1.6, "Correction of the errors of the great ones", not far from the beginning of this section. Cannot tell the page number because I am reading an electronic copy which does not have page numbers. $\endgroup$ – Alexandre Eremenko Aug 29 '15 at 2:13
  • $\begingroup$ @Dr214: About cooling problem, my info is also from Chertok. When answering this question, I just searched on the words "solid fuel" (in Russian, of course) in this file. He writes much about the problem. $\endgroup$ – Alexandre Eremenko Aug 29 '15 at 2:17
  • $\begingroup$ Thank you, I just read that whole chapter and learned more about it. It also describes how they did get a solid rocket ICBM, the RT-2, developed in mid and late '60s. It didn't use APCP, but something else apparently similar that the Russians developed. $\endgroup$ – DrZ214 Aug 30 '15 at 22:23
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    $\begingroup$ Chertok's books have been translated to English and are available e.g. here: nasa.gov/connect/ebooks/rockets_people_vol1_detail.html $\endgroup$ – Hobbes Oct 11 '18 at 7:49

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