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As I understand it, liquid-fueled rockets typically burn some amount of their fuel in a separate low-pressure combustion chamber, and then use the combustion products produced to drive the turbopump. High-efficiency engines then pump this exhaust into the rocket's combustion chamber to ensure complete combustion and get more efficiency from the engine.

Would it be feasible to instead use some of the high-pressure gas from the rockets main combustion chamber to drive the turbopump, having passed it through a nozzle to reduce the pressure?

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    $\begingroup$ See my answer below, but another reason to not do this is that any mass flow you tap off from the combustion chamber to drive a turbine is a net efficiency loss because that mass flow doesn't get expelled through the nozzle. The whole reason for going to staged combustion engines with all their complexity and high pressure is to ensure that all the propellants flow through the nozzle. $\endgroup$
    – Organic Marble
    Dec 4, 2015 at 15:43
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    $\begingroup$ I believe he is proposing this as an alternative to a gas generator cycle rocket engine, as described in his first sentence. His second sentence talks about a different type of rocket engine, the closed cycle type. The purpose of his question is to eliminate the need for a preburner(s). $\endgroup$
    – user8269
    Apr 5, 2018 at 7:57

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You've got it slightly incorrect. Staged combustion engines pre-burn the propellants at a higher, not lower pressure than the main chamber. The exhaust from the preburner isn't pumped into the main chamber but flows through the turbine, dropping in pressure there and in the ducting before it enters the main chamber. The preburners generally run at a lower mixture ratio than the main chamber to keep things cooler for the turbine blades.

edit: I extracted this schematic with example pressure and temperature numbers from this link

enter image description here

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    $\begingroup$ OMG. These engines serious dump a ton of fuel every two seconds? Furthermore, I always thought these engines just dumped Oxygen and Hydrogen (or a hypergolic fuel) into the combustion chamber and let it rip. I suppose this is why SpaceX was so proud of their newest super-efficient and overly-complicated-looking engine. $\endgroup$
    – iAdjunct
    Dec 7, 2015 at 14:49
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    $\begingroup$ @iAdjunct You've got to get the fuel into the combustion chamber, which if it's going to do its job needs to be at a pretty high pressure to force the exhaust out through the nozzle. The pumps to do that will consume a lot of power, and (except for some very small examples and one recent startup) the only place to get that power from is your rocket fuel. You also need to use the fuel to cool various parts of the engine to stop it melting, which means pumping it around and dealing with the hot fuel coming back. $\endgroup$
    – Steve Linton
    May 22, 2019 at 12:56
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The main reason for this is the temperature. Depending on the actual fuel the temperature in the main combustion chamber can be above 3000K. However, in the gas generator the temperature is kept below 1400-1600K. These lower gas temperatures allow uncooled chamber construction and prevent melting or limit the erosion of turbine blades.

Source: Sutton - Rocket Propulsion Elements

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  • $\begingroup$ Wouldn't the addition of a nozzle between the combustion chamber and turbopump reduce the temperature? $\endgroup$
    – Witnaaay
    Dec 4, 2015 at 8:14
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    $\begingroup$ The purpose of a nozzle is usually to reduce pressure. You could, however, run the tapped off combustion chamber gases through a heat exchanger cooled by rocket propellant (LOX or LH2) to keep the turbine blades from melting. $\endgroup$
    – user8269
    Apr 5, 2018 at 8:00
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There is one flight-tested engine that uses the design you're suggesting: the Blue Origin BE-3. It is a liguid hydrogen/liguid oxygen fueled engine that produces 110,000 lbf of thrust (throttleable to 20,000 lbf). It powers the New Shepard suborbital launch vehicle, which was first flown on April 29, 2015, and then contributed in the first ever soft vertical landing of a rocket returning from space on November 23, 2015. It is also being considered for use on the ACES upper stage for ULA's Vulcan rocket.

From Blue Origin's website:

The BE-3 is the first tapoff engine to fly. We’ve designed a simple rocket engine, where hot gasses from combustion are tapped from the main combustion chambers and fed back to spin the turbopumps in flight. Having only one combustion chamber with a single ignition event enhances reliability.

I have no information on how they achieved this, as the high temperatures of the gasses in a combustion chamber (6,000+ºF) make it a challenge, like other people have mentioned here. Blue Origin is a notoriously secretive company and hardly releases any information about their operations, let alone information about the design of their engine.

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    $\begingroup$ That's great info. When I was writing my answer, I thought I remember a tapoff cycle, but a cursory search could find no information on it in my textbooks or online. Thanks for confirming that I remembered it correctly! I would sure love to see a schematic of that engine. Thanks for posting. $\endgroup$
    – Organic Marble
    Dec 10, 2015 at 2:15
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    $\begingroup$ Something to keep in mind: The New Shepard doesn't have anything like the brutal mass ratio of an orbital craft. This means they would be more willing to trade a bit of fuel for a simpler, more reliable engine. While I don't know anything about it's efficiency I'm simply pointing out their use of it doesn't prove it's a good idea for going to orbit. $\endgroup$
    – Loren Pechtel
    Dec 10, 2015 at 5:43
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    $\begingroup$ The way to deal with the high temperatures of the gasses tapped off from the combustion chamber is the same way the combustion chamber itself survives the high temperatures: the tapoff pipe gets Regenerative cooling for a certain distance from the combustion chamber, a distance long enough that the gasses in the pipe have cooled off enough to avoid melting the pipe. $\endgroup$
    – user8269
    Apr 18, 2018 at 1:12
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I believe you are describing the combustion tap-off cycle.

The J-2S engine from the Apollo era did this (but was never used), as well as the BE-3 mentioned earlier.

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