In all expander cycles, fuel is used as the working fluid to generate the pressure difference necessary to drive the pumps. This makes sense in a closed expander cycle as it'll eventually be used in combustion, but with an open expander cycle, this is not the case. My question is, would it make more sense to use a more optimised working fluid with a larger difference between liquid density and gas density or higher heat transfer? What sort of performance/reduction of mass could be achieved? would this help in cooling(most open expander cycles introduce the fuel back into the nozzle in film/curtain cooling-inert gasses could aid this)?

  • $\begingroup$ Are you suggesting using a third fluid rather than the fuel or oxidizer? $\endgroup$ Nov 25 '20 at 4:17
  • $\begingroup$ @OrganicMarble that was the idea it would come with the complexity of another pump/pressure vessel and the need to precisely meter it throughout the flight- could be worth the performance gain-if there is any to be had $\endgroup$ Nov 25 '20 at 4:20
  • $\begingroup$ How interesting... so there'd be a closed pump-driving/chamber-cooling fluid system? $\endgroup$ Nov 25 '20 at 5:27
  • $\begingroup$ Reaction Engines' SABRE engine design uses helium as a working fluid, but that's in a closed loop. $\endgroup$
    – user20636
    Nov 25 '20 at 11:29
  • 1
    $\begingroup$ yes @ReubenFarley-Hall, the heat is dumped into the fuel (LH2), and while the heat source is the intake air this is topped up with a preburner. The reason for using helium is to avoid problems with hydrogen embrittlement, I don't know if it's more appropriate for turbines. $\endgroup$
    – user20636
    Nov 25 '20 at 13:28

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