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I am designing a liquid fueled amateur rocket with my brother, and settled on a gas generator engine. We understand the concept and process, however we are still confused on how the engine is initially ignited. Does a battery/other means of energy kick-start the turbopumps? Thank you for the help

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    $\begingroup$ One solution is the use of a solid fuel gas generator used to start the turbine and the turbo pumps. The solid fuel is only used for start, but the pressure and gas flow should continue when the solid fuel burns out and the liquid fuel takes over. $\endgroup$ – Uwe Aug 7 '17 at 10:16
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    $\begingroup$ I don't think the answer is in Scott Manley's Rocket Engine Plumbing but you may find it interesting or helpful if you haven't seen it already; youtu.be/4QXZ2RzN_Oo also possibly this answer. $\endgroup$ – uhoh Aug 7 '17 at 13:01
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One common solution is what is referred to as a "start bottle" (see the J-2 flow schematic diagram below -- look for the item labeled "start tank") -- a small tank of highly compressed gas (e.g., helium or hydrogen) with either a nozzle that impinges directly on the turbine of the turbopump or a line that pressurizes the gas generator exhaust lines to spin the turbopump up to pump the propellants into the gas generator. When you hear of an engine that cannot be restarted or can only be restarted once or twice, this is a likely reason why -- one start bottle per start. When it's used up, that's it.

J-2 engine flow schematic

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  • $\begingroup$ What engine is this diagram from? $\endgroup$ – Organic Marble Aug 7 '17 at 15:54
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    $\begingroup$ Edited post to clarify -- thought I had typed it in there already. It's the J-2. $\endgroup$ – Tristan Aug 7 '17 at 15:56
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    $\begingroup$ For first stage engines, this start tank may be on the launch pad, connected to the stage via an umbilical. $\endgroup$ – Hobbes Aug 7 '17 at 16:39
  • $\begingroup$ This is true. For the J-2, in fact, the start bottle was in the stage's hydrogen tank and not on the engine. $\endgroup$ – Tristan Aug 7 '17 at 17:31
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    $\begingroup$ If you are happy with the answer, please accept it by clicking on the gray check-mark. $\endgroup$ – Organic Marble Aug 8 '17 at 11:32

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