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If I understand correctly, a liquid propellant rocket engine combustion chamber is fed with fuel and oxygen from a turbopump. Those turbopumps are driven by a gas generator. The gas generator is fed with fuel and oxygen; but I cannot find if fuel is burnt in the gas generator, or if it is just decompressed and transformed from liquid to gas. My reflection says it would be useless to mix both fuel and oxygen if it is not to burn the fuel, but I fail to see any ignition mechanism in the (simple) drawing I see on wikipedia.

Is fuel burnt in the gas generator? If so, how is it ignited?

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I've attached the image the Original Questioner referred to below. I can't read the Cyrillic labels, but I can tell you that in this schematic

  • yellow = fuel
  • blue = oxidizer
  • pink/red = hot gas

The object labeled 6 is a gas generator, and one can see that fuel and oxidizer combine / burn there to produce hot gas, which spins the turbine to drive the pumps. enter image description here

As Aeroalias mentions, a number of ignition schemes are possible. He didn't list one important one, though: electrical - as seen in this slide from a Space Shuttle Main Engine orientation.

enter image description here

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In the gas generator cycle, some of the fuel is burnt in the gas generator. The hot gasses are used to run the turbines which power the pumps.

I'm not able to make out anything from the given drawing (its in Russian, for one thing). Hope the diagram below makes things clear.

enter image description here

Source:Wikimedia Commons

As for the ignition system a number of methods can be used like a pyrogen, laser, catalyst etc. For example, the J-2X uses a pyrotechnic ignition system.

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    $\begingroup$ Your answer applies to all rocket engines: not just gas generator cycle engines, but also closed-cycle engines. The only difference is where the exhaust of the gas generator goes. $\endgroup$ – Hobbes Aug 16 '15 at 10:58
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    $\begingroup$ Expander cycle or tapoff? No gas generator on those. $\endgroup$ – Organic Marble Aug 16 '15 at 11:37
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There are multiple different designs, or wildly varying complexity, so to answer your question in one phrase: it varies.

In the most common case there's no gas generator as such. Fuel and oxidizer are injected into the combustion chamber from pressurized tanks, ignited, and the burning process takes care of both turning them from liquid to gas, and then of the propulsive combustion. No gas generator, no turbopumps, just some regulated valves, an igniter and the combustion chamber with nozzle.

Then there are designs of increasing complexity:

  • channeling the fuel and/or the oxidizer through the bell nozzle, both cooling it and turning the two into gas for better mixing.
  • driving the turbopumps with an entirely separate propellant, like hydrazine.
  • driving some exhaust gas off the combustion chamber to propel the turbopumps and/or evaporate the cryofuels.
  • using the channeled, heated gasses to propel a turbopump that pumps cryo-fuels. (which is pretty much the case that you doubted. Actually, quite viable as the fuel/oxidizer after boiling off in the walls of combustion chamber has much more energy, and is well capable of driving the turbopumps.)
  • using pre-burners, where fuel and oxidizer are mixed in very different than optimal ratio, both evaporating the liquids (for more smooth combustion) and providing energy to drive turbopumps.
  • using separate gas generators, where hot combustion products (be it from the nozzle or preburners) boils the cryofuels, and separate pre-burners that provide heat and energy for the turbopumps.
  • ...and any wild combination of the above.

This pretty much culminates with the design of the Space Shuttle Main Engine. The public-domain presentation, page 19, shows its gory details: four turbopumps (each with distinct turbines for propelling them and for pumping), two preburners, a gas generator that evaporates LOX, a bell nozzle cooling grid that boils off LH2, and a byzantine grid of connections that drive the fuel and oxidizer heated, hot partially burned oxygen-rich, cool but evaporated. re-cooled after passing the turbopump, heated but not evaporated after being pushed through turbopump, superheated through pre-burning fuel-rich, re-cooled by passing through another turbopump, injected back into tanks to provide pressure, back and forth through the grid, in a way that makes sense if you really put your mind to it, but is hard on one's sanity.

So... your understanding is incomplete ;) The bottom line is there's a lot of designs. The one in the picture you linked is pretty straightforward: some propellant and oxidizer diverted from the main lines gets burned in a preburner (the "gas generator") providing work for propelling the turbine. It may or may not be used to ignite the proper fuel/oxidizer mix. It may or may not be used to heat the oxidizer. The schematics depicts a generic, pretty straightforward, example middle-range rocket engine, and the exact details in reality may vary wildly.

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  • $\begingroup$ "a gas generator that evaporates LOX"? Are you talking about the heat exchanger? $\endgroup$ – Organic Marble Mar 28 '16 at 16:16
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    $\begingroup$ @OrganicMarble: yes. $\endgroup$ – SF. Mar 28 '16 at 20:08
  • $\begingroup$ The SSME was (is?) an awesome beast. $\endgroup$ – Organic Marble Mar 29 '16 at 1:12
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    $\begingroup$ that too, but seriously the number of detours and back-feeds is staggering. I mean, wouldn't a shaft make more sense? Must every single pump be propelled by a different phase of the propellants, diverged from yet another distinct point in the flow? The propellants being first pressure-fed and then still pumped through two distinct pumps each? Two separate preburners not enough - need to tap into hydrogen heated in the bell nozzle heat exchanger to propel the low-pressure fuel pump? Propelling low pressure oxygen pump with the high-pressure oxygen pump? The engineer in me wants to scream. $\endgroup$ – SF. Mar 29 '16 at 2:38
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    $\begingroup$ The "gas generator that evaporates LOX" is used to operate the pogo accumulator and pressurize the main LOX tanks. The only alternative is to use a lot of helium, this is much better. Both LH and LOX are heated to pressurize the propellant tanks but the LH is heated as part of the process of cooling the nozzle and the MCC. Since this heat source is not available to heat the LOX, a separate LOX heat exchanger is used. There are actually 5 turbopumps, but two of them share a turbine shaft: the HPOTP (4,025 psi) and the preburner oxidizer boost pump (6,939 psi) as described on page 20. $\endgroup$ – user8269 Mar 28 at 2:10

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