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For example, a rocket engine which can use both liquid methane and liquid hydrogen.

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    $\begingroup$ It would be quite possible to build such an engine. But you would need to over-build all aspects of it to an extreme level, and use mechanisms, especially pumps and injectors, that function over a much wider range of conditions than normal. The end result will be many times the mass, and produce less efficient thrust, than a dedicated engine. Or even a cluster of dedicated engines strapped together, each optimized for its specific fuel! $\endgroup$ Jul 15 '21 at 10:36
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    $\begingroup$ @PcMan something close being the RD-701, which was intended to operate from mostly kerosene within the atmosphere and mostly hydrogen later in flight. It was practically two engines mashed together, with separate preburners/turbopumps for LOX/kerosene and LOX/LH2. Even then, the LOX/LH2 preburner ran from kerosene early in flight and required a small amount of kerosene to continue operating with LH2 fuel after the switch. $\endgroup$ Jul 15 '21 at 16:37
  • $\begingroup$ @ChristopherJamesHuff that was a cool engine. $\endgroup$ Jul 15 '21 at 22:42
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The RL10 has been experimentally fired on methane and propane as well as hydrogen. This did entail modifications to several components of the engine; for the methane version:

Fuel component modifications were unique and were accomplished under the contract; assemblies that were changed were the pump, turbine, thrust control, injector, and thrust chamber.

So it’s not exactly the same engine.

https://yarchive.net/space/rocket/rl10.html https://en.m.wikipedia.org/wiki/RL10

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    $\begingroup$ If pump, turbine, thrust control, injector, and thrust chamber were modified, only the nozzle was not modified? $\endgroup$
    – Uwe
    Jul 15 '21 at 15:51

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