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
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.