всё. Я разобрался. Азот для продувки. Всем спасибо!
Nitrogen injected downstream of the preburner (between it and the turbine) would result in the turbines spinning and pumping the same volume of propellant, but that propellant not having anywhere to go. Nitrogen gas cannot be injected prior to the pumps, as you would then be injecting gas into a fluid flow and initiate cavitation in the pump.