The fact that pressure fed engines need their tanks to be at an even higher pressure has always confused me a little, but it's got me thinking; does the same logic apply to gas generator engines. If the pressure in the combustion chamber has to be exceeded by the pressure provided by the pumps, is it also exceeded by the pressure in the gas generator. In other words, to achieve a given flow rate of fuel into the combustion chamber, does the force acting on the fuel(and oxidiser), ie. the pressure behind the turbines, have to be greater than that in the combustor, or is there no such direct relation.
"If the pressure in the combustion chamber has to be exceeded by the pressure provided by the pumps, is it also exceeded by the pressure in the gas generator. "
Just to be clear, there's no "if" about this ==> "the pressure in the combustion chamber has to be exceeded by the pressure provided by the pumps"
Addressing "is it [the main combustion chamber pressure] also exceeded by the pressure in the gas generator."
In general, gas generator cycle engines don't have the exhaust from the gas generator flowing into the main combustion chamber, so there is no need for this. Especially since the gas generator exhaust passes through the turbines and in the process loses a great deal of pressure and temperature.
If by "gas generator" you actually mean "preburner" as in a staged combustion engine, then yes, the preburner chamber pressure has to be high enough that even after the flow passes through the turbines, it still has a pressure higher than that in the main combustion chamber.
Gas generator cycle engines:
|Engine||Gas Generator Pc||Main Chamber Pc||Ox Pump Pout||Fu Pump Pout|
Staged Combustion cycle engine (SSME):
|Fu PB Pc||Ox PB Pc||Main Chamber Pc||Ox Pump Pout||Fu Pump Pout|
All pressures in psi
- PB - Preburner
- Pc - Chamber pressure
- Pout - Outlet pressure
Source: F-1 Engine Training Manual
Source: J-2 Engine Fact Book