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Consider this quote about early designs for the Saturn upper stages:

Formalized as Advanced Research Projects Agency (ARPA) on February 7, 1958, the group examined the DoD launcher requirements and compared the various approaches that were currently available. [...] They had already accepted Krafft Ehricke's arguments that hydrogen was the only practical fuel for upper stages, and started the Centaur project based on the strength of these arguments.

https://en.wikipedia.org/wiki/Saturn_(rocket_family)

Was Krafft Ehricke correct: Is hydrogen the only practical fuel for upper stages? I mean in general, not just the Saturn or Centaur rockets. If upper stage design would be different for 1950s technology versus modern technology, please state which context applies to your answer.

Related: Which is overall more expensive to operate for a final rocket stage? LH2/LOX or hypergolics like UDMH/N2O4?

Related, unanswered: Advantages of a Solid Fueled Upper Stage

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    $\begingroup$ Clearly it is not, because existing, successful upper stages use other propellants. Perhaps it is not clear what you mean by "practical". $\endgroup$ – Organic Marble Jul 12 '18 at 20:58
  • $\begingroup$ If this would be true no transfer of astronauts to the ISS would be possible. First satellite in orbit and first man in orbit was done without hydrogen fuel. $\endgroup$ – Uwe Jul 12 '18 at 21:19
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    $\begingroup$ The argument was that hydrogen was the only practical fuel for the DoD launcher requirements, not the general case. As other rockets exist it's clearly not the only practical answer for the general case. $\endgroup$ – JCRM Jul 12 '18 at 22:15
  • $\begingroup$ Your question is problematic as currently phrased because it mixes "is" and "was". The block quote from Wikipedia uses "was" referencing an argument made in 1958 (or earlier). However, you use "is" in your title, and "Was Krafft Ehricke correct: Is..." switches tense as well. For future reference, a better question would have been "Why was X thought to be the only practical Y circa 1958?" because it addresses what was thought to be practical in the 1950's without conflagrating it with what is currently practical. $\endgroup$ – uhoh Jul 12 '18 at 23:20
  • $\begingroup$ Related: space.stackexchange.com/questions/18251/… $\endgroup$ – Russell Borogove Jul 13 '18 at 21:07
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It was the most practical for that system at that time. LH2/LOX has a number of advantages over other fuel/oxidizer combinations:

  • High performance - LH2/LOX results in high Isp. A given mass of hydrogen will result in more ΔV than an equivalent mass of RP-1 or other fuel. For a stack as heavy as Apollo, you wanted every possible second of Isp you could get. Only reason it wasn't used for the first stage is that because of its low density, hydrogen doesn't give a lot of thrust. That's why the Saturn first stage was kerosene, and why the Shuttle and Ariane use solid boosters on launch.

  • Relatively safe to handle - LH2/LOX aren't immediately toxic in themselves, exhaust products are not toxic or corrosive.

  • Relatively cheap to manufacture (if not to store). At the time that particular trade study was conducted, I'm not sure if hypergolics were that cheap yet.

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  • $\begingroup$ A that time there was very few experience with liquid hydrogen as rocket fuel. LH2 is much colder than LOX. To build reliable valves for LH2 was very difficult. $\endgroup$ – Uwe Jul 17 '18 at 8:54
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No.

Currently operating launchers using other fuels for their upper stages include Falcon 9 (RP-1), Soyuz (RG-1 for the 2nd stage, UDMH/N2O4 for the Fregat upper stage), Vega (solid fuels for 2nd and 3rd stages, UDMH/N2O4 for the AVUM 4th stage), and Electron (RP-1).

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  • $\begingroup$ What is the difference between RG-1 and RP-1? You don't need to explain RocketPetrol-1. $\endgroup$ – Uwe Jul 12 '18 at 21:25
  • $\begingroup$ No idea. They're both refined forms of kerosene, that's about the limit of my knowledge of the topic. $\endgroup$ – djr Jul 12 '18 at 21:34
  • $\begingroup$ From Wikipedia RG-1 is the Russian RP-1 and differences are small. Density is a little bit higher. $\endgroup$ – Uwe Jul 12 '18 at 22:13

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