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I know I'm not the only one who has this question, but I still have trouble wrapping my head around how a rocket engine preburner works.

I understand that the preburner is what gets the rocket's turbopumps to run, and that's how the fuel enters the combustion chamber. That's not the difficult part. I also understand how the preburner is fed. (It basically feeds itself, as a portion of the fuel and oxidizer is fed into the preburner, which turns a turbine, and that turbine moves the turbopumps.) Finally, I do know that the rocket's tanks are pressurized by heating an inert gas, and then feeding that gas into the tank.

However, how does the preburner begin running? As I mentioned above, I understand how the preburner can keep itself running, but how does mission control start the preburner?

If anyone can answer my question, it'd be extremely appreciated.

Also: If I stated something incorrectly, please correct me. I don't want to misunderstand anything about these complex machines.

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  • $\begingroup$ You may be confusing two seperate designs for the pressurization - you can either release an inert gas such as helium into the tank without heating, or you can warm up eg liquid hydrogen fuel to get hydrogen gas with a much greater volume. $\endgroup$
    – Quentin Clarkson
    Commented Jan 17, 2017 at 0:22
  • $\begingroup$ One possiblity to start the turbopumps is a small solid fueled gas generator producing hot gas to start the turbine. This gas generator is used only for a short time until the preburner is ready to operate the turbine and turbopumps. For several ignitions more than one solid fuel charges are necessary. $\endgroup$
    – Uwe
    Commented Jan 17, 2017 at 12:01

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"However, how does the preburner begin running?"

Refer to this simplified schematic from the crew checklist for the Shuttle system I'm about to describe.

enter image description here

Head and ullage pressure in the propellant tanks is the initial impetus for propellant to flow into the preburner(s). At least in the Shuttle, the tanks were pressurized on the pad by helium supplied by the ground, after their respective vent valves closed. The O2 tank was way above the engines at the tip of the ET so there was significant head pressure too. Not so much head for the LH2, it does not have the density for it. Then when the valves in the plumbing between the tanks and the engines opened, and the engine valves did too, prop flowed into the preburner driven by tank outlet pressure. Then the electrical igniters kicked on, the preburners ignited, the turbines began to spin, etc, as you have described. (reference the Space Shuttle News Reference Manual, page 185 of the pdf)

There is info about the start sequence in the answer to this question but it does not specifically talk about what drives the flow initially.

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  • $\begingroup$ If the propellants flowed into the preburner driven by tank outlet and head pressure only, how could the pressure in the preburner be sufficent for the turbines to reach nominal speed? $\endgroup$
    – Uwe
    Commented Jan 18, 2017 at 16:54
  • $\begingroup$ After ignition, the chemical energy of the propellants is converted into kinetic energy through the process of combustion. See my answer to this question. space.stackexchange.com/questions/19647/… An extremely complicated boot-strapping process then occurs as the turbines accelerate, driving the pumps, flowing more prop into the preburner, etc. Getting this correct is very difficult. @theradicalmoderate's answer to the question linked in my answer links out to a detailed explanation of the SSME start sequence $\endgroup$
    – Organic Marble
    Commented Jan 18, 2017 at 17:03
  • $\begingroup$ Are nonreturn valves used between propellant tanks and preburner to prevent a flow of hot gases back into tacks when the pressure inside the preburner is higher than the pressure of the pumps for a short time?To get a continuos flow of propellants during the boot-strap process seems very difficult. The pressure from the pumps should be higher than the pressure inside the preburner at any time of the bootstrapping. But the rotating mass of the turbine and the pumps may cause a delayed pressure rise of the pumps. $\endgroup$
    – Uwe
    Commented Jan 19, 2017 at 9:41
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The preburner is essentially a standard turbine engine, which is a little easier to find information on; you need compressed air from an external source, or a separate motor to rotate the compressor, and a spark plug to ignite the pressurized fuel/air mix.

The details will vary between engine designs, but a preburner can be simpler than a full turbine engine: if the fuel and oxidizer both come from sufficiently pressurized tanks, you can leave off the compressor stage and the spark plug alone is enough to start the engine.

SpaceX Raptor Preburner at Stennis.

Raptor's Pre-burner]([![Oxyidizer-rich exhaust from aSpaceX Raptor Pre-burner]1

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