 Gas turbines are usually used when electricity or steam is not available and place or weight restrictions permit the use of more efficient sources of mechanical energy.One of such cases are rocket engines, which need to pump fuel and oxidizer into their combustion chamber.This is necessary for large liquid rockets, since forcing the fluids or gases to flow by simple pressurizing of the tanks is often not feasible; the high pressure needed for the required flow rates would need strong and heavy tanks.