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So i was trying to understand the working of the Full Flow Staged Combustion cycle(FFSC) and was comparing it to the Staged Combustion Cycle(SC) and Closed Combustion Cycle(CCC) a question came up in my mind.

Lets assume 5 units of both propellants (dummy number) are combusted to turn each turbo pump in a FFSC cycle. So a total of 10 units propellant is spent on running the pumps.

In SC , we would spend an extra 5 units propellant , that is the fuel to keep the pre-burner fuel rich.
So my question is that because the SC has a common shaft for both the pumps , its 10 units of net propellant combusted in turning the turbines is distributed over 2 pumps.
So does this mean that we draw double propellant than if we were on a single pump , or half?

In relation with FFSC
Is the 10 units of propellant spent in turning individual turbines ending up drawing more fuel in FFSC than the 10 units of propellant spend in SC?
...
Is this the reason why FFSC has more inherent Mass-flow than traditional SC ?

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  • $\begingroup$ Are you asking about "Difference between 2 pumps on same shaft run by a turbo pump and 2 pumps run on separate turbopumps", or the difference between FFSC and CCC? You speak as if the number of turbopumps are determined by the combustion type, which it is not. Either design can have 1 or 2 pump combustion, 1 or 2 shaft, for the needed 2 pumps. Everyday astronaut has a very nice writeup of various cycles here: everydayastronaut.com/raptor-engine $\endgroup$ Sep 9 at 11:49
  • $\begingroup$ @PcMan I was trying to differentiate between FFSC and CS , and my understanding came down to "Difference between 2 pumps on same shaft run by a turbo pump and 2 pumps run on separate turbopumps" by that I mean , if my hypothesis was right , then I understood the FFSC , otherwise not. I understand that the cycle itself is independent of number of pumps. $\endgroup$ Sep 9 at 12:15
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No. There's no inherent difference due to the design used. The differences are only coming from technological margins.

Imagine your turbine attached to the preburner requires 5 units of bipropellant mix per second to produce enough torque+RPM to propel the pump to transfer 100 units of propellant (any, or mix, whatever you run through the pump) per second. You also add in extra 5 units of fuel to keep it fuel-rich, or 5 units of oxidizer to keep it oxygen-rich, whichever you prefer.

If you run two pumps from the same turbine, same shaft, and you want each to push 100 units, you'll need to put 10 units of bipropellant into the turbine, for a total 200 units transferred. You'll also add 10 units of fuel, to keep it fuel-rich, or 10 units of oxidizer.

You'll need a beefy, strong turbine capable of handling the 20 units worth of flow - about as beefy and strong as technology allows - and you'll need either quite fancy plumbing - "bypasses" wasting some pressure in fuel or oxidizer or constricting the high-pressure flows (and pipes to withstand the increased pressure) - or you accept the fixed ratio of whatever each pump can push through, not able to fine-tune the mix on the fly. If your engine runs optimally at 40:60 oxidizer-fuel ratio on full thrust, but would perform much better at 50:50 when throttled 50%, sorry, the ratio is hard-wired into the engine.

Now take FFSC with two turbine-pump setups. You still want 100 units per second? You can get it from two much smaller, more manageable turbines, 5 units each, (+ 5 fuel, or +5 oxidizer, or one this, one that) that don't try to rip themselves apart when operating at full power. Decrease the feed to one preburner or the other and you'll throttle one flow or the other so you can optimize the mix on the fly. Or use two beefy turbines from the single-stage engine, and double your propellant flow! They now have only one heavy-weight pump to handle each, they can still deliver the work needed - and so you have higher mass-flow.

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  • $\begingroup$ If im getting you correctly in P3 , you are saying in SC , you'd need double bipropellant mix to draw the same 100 units from both the pumps .. right? 10 + 10 + 10? $\endgroup$ Sep 8 at 11:30
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    $\begingroup$ @SHikhaMittal Yes - twice the torque, twice the work, twice the fuel, twice the size and durability of the turbine. And the last one doesn't scale linearly. $\endgroup$
    – SF.
    Sep 8 at 11:32
  • $\begingroup$ Also if I'm getting you right in P5 , the increased massflow is due to us not wasting those extra fuel , and we instead of using that extra mass flow , actually can choose of running the engine on lower temps and pressure while producing the same thrust.? $\endgroup$ Sep 8 at 11:34
  • $\begingroup$ @SHikhaMittal No, the increased mass flow is due to splitting the workload between two turbines, and so being able to provide more work total to the pumps. Adjusting the mix on the fly is a benefit unrelated to the total maximum mass flow. Imagine you're comparing a normal electric car with one electric engine driving two rear wheels to one with two engines, same as the first, each engine driving one rear wheel. $\endgroup$
    – SF.
    Sep 8 at 11:40
  • $\begingroup$ Okay , let me reframe , We have more Mass flow , because we split our work among 2 turbopumps , and they are able to out perform a single pump right? And they outperform because each turbine has less work to do individually. AND we don't waste fuel in the process.... $\endgroup$ Sep 8 at 11:48

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