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I understand the Mars ISRU reasoning for SpaceX's Raptor which is also going to be methalox based.

But why would ULA, who is the prime customer for BE-4 besides Blue Origin itself, chose to support a riskier, untried, cryogenic fuel for their next generation launcher? The performance bonus vs RP-1, if any even exists, doesn't seem worth it.

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Reusability is the big factor. Kerosene engines have issues with "coking", where solid carbon is deposited throughout the engine's pipes. (See this dissertation and its supporting research for more details.) This isn't a major issue with expendable engines, but it drives up costs when trying to reuse the engines. The BO New Glenn's first stage is planned to be fully reusable, similar to the Falcon 9; the ULA Vulcan plans to reuse its engines by detaching them in a pod, then catching that pod with a helicopter. Methane burns more cleanly due to its simple chemical structure, so it doesn't have this issue, making it a good choice for a reusable lower-stage engine.

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  • $\begingroup$ Great links! Methane is CH4 while RP-1 is basically CH2-like. It's a poor-mans way to store LH2 (roughly speaking). More hydrogen and no multi-carboned molecules to start with at least. $\endgroup$
    – uhoh
    Oct 12 '16 at 16:11
  • $\begingroup$ I think another point is that BO started designing the engines on their own so reuse was a high priority. ULA possibly buying them for a future rocket is probably a development well after the actual choice of fuel was made. $\endgroup$
    – Evan Steinbrenner
    Oct 12 '16 at 19:27
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Let's look at Wikipedia for possible liquid rocket fuels. The key item is the Exhaust Velocity. Let's remove any that could be toxic, so no Beryllium, Florine, or Boron, despite the fact that each could make a very efficient rocket.

  • $H_2{\space}LOX$- 3816
  • $CH_4{\space}LOX$- 3034
  • $C_2H_6{\space}LOX$- 3006
  • $C_2H_4{\space}LOX$- 3053
  • $\textrm{RP1}{\space}LOX$- 2941

So Methane, Ethane, and Ethylene all perform similar to RP1, and in fact slightly better. None compares, however, to Hydrogen/Oxygen. Still, liquid hydrogen has issues that make it difficult to work with. So, what are some of the side effects of these various fuels?

  • $H_2{\space}LOX$- Difficult to work with, requires larger tanks than traditional fuel. Requires extreme cold.
  • $CH_4{\space}LOX$- Burns clean. Is less energy dense than other fuels, requiring a larger tank.
  • $C_2H_6{\space}LOX$ / $C_2H_4{\space}LOX$ / $\textrm{RP1}{\space}LOX$ Basically these gradually become more energy dense, burn less clean, etc.

Burning clean is important for re-usability, no need to clean the engine periodically. That is probably the major reason why it would be invested for Earth based technologies. They also could be looking for it as a path to Mars, as SpaceX has been.

Furthermore, Elon Musk broke out SpaceX's decision to use Methane.

Of these, only the last doesn't apply. Methane is much cheaper than RP1, hence, if you have a vehicle where most of the cost is the fuel, then Methane makes more sense.

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    $\begingroup$ Performance is mentioned by the author of the question, in your answer except the part "could be looking for it as a path to Mars" which is more like a comment, the other things are for reusability. This answer about reusabilty is given before by DylanSp and more detailed. $\endgroup$
    – D. Miller
    Oct 12 '16 at 15:10

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