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I'm designing a single open cycle expander rocket engine using propane. However, I've been unable to find out whether or not the fuel is split into the turbine and combustion chamber before or after it circulates through the cooling loop. I would assume that splitting the fuel before the phase change would make it more efficient as it wouldn't necessitate the use of valves to achieve the correct ration of fuel going through the turbine as well as the fact that because of the smaller amount of fuel (in the RL-10 I believe it's around 3% however I believe this would vary in fuels like cryogenic propane) in the cooling loop, it would concentrate the thermal energy where it is most useful, in the working fluid. However, I'm not sure if this is even possible since there is so little fuel travelling through the cooling loop that it could prove to be insufficient in cooling down the combustion chamber walls as it could be evaporated to quickly and significantly reduce it's cooling ability as it has become less conductive (despite propane's incredible range as a liquid of -188 to -42 Celcius).

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  • $\begingroup$ Do you also want to build and test it? $\endgroup$ – Everyday Astronaut Jul 28 '20 at 12:33
  • $\begingroup$ Well, that's the goal - especially testing it, but it might prove too costly or just too complicated to machine. I assumed that the 'bleed-off' power cycle would be the simplest configuration as well as using graphite throughout the majority of the design as opposed to a cooling loop but I'm not really set up to build much. I'd love to test its reliability and isp however $\endgroup$ – Reuben Farley-Hall Jul 30 '20 at 1:04
  • $\begingroup$ Just as a note: to the best of my knowledge the RL-10 is a closed expander cycle, see figure 16 in this paper $\endgroup$ – Ruben Oct 3 '20 at 11:18
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Although I agree with Organic Marble that open expander cycles should most likely split the fuel before the cooling jacket, there are some examples where this isn't the case. For example in the RD0146 there is a split of the fuel line after the cooling jacket (see image), although this is just to power the fuel boost pump (so in essence it is more a closed expander cyle).

In this paper by the Japanese Space Society they even denote the coolant bleed cycle as splitting at the pumps and partly merging after the coolant jacket again. Coolant bleed cycle and open expander are used interchangeably in literature (although maybe they shouldn't to denote this difference).

The point is: the classification of open expander is just that: a general classification. Real engines will often not fit any of the cycles or fit multiple classifications at the same time.

Image taken from V.Rachuck(2006)

(I would make this a reply to Organic Marble's answer, but it was too long and I don't know how to attach an image to a reply) RD0146 Flow Schematic

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    $\begingroup$ Thanks for posting the schematic! Interesting engine. $\endgroup$ – Organic Marble Oct 3 '20 at 16:16
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The paper Cycles for Earth to Orbit Propulsion shows that for open (bleed) expansion cycles, the flow paths diverge at the pump outlet. Two designs are shown, the second has a split pump.

enter image description here

enter image description here

The wikipedia article says the right thing

only a small portion of the propellant is heated and used to drive the turbine and is then bled off

but its accompanying schematic shows all the flow being heated.

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    $\begingroup$ I actually came across a paper that actually looked at different configurations of bleed cycle engines and quickly discarded the one that splits the flow after the cooling jacket, because it was found to be inefficient. Systems analysis of a future semi-reusable launcher, based on a high thrust bleed cycle rocket engine sci-hub link interesting part on page 4 $\endgroup$ – Ruben Oct 10 '20 at 14:08
  • $\begingroup$ @Ruben thanks, will read the paper with interest. $\endgroup$ – Organic Marble Oct 10 '20 at 14:21

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