# How is the pressure within the gas generator determined?

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.

• Note that the pressure driving the turbines (ie from the gas generator) is not necessarily the pressure the turbines drive the main gas flows at. There's ratios involved based on the turbopump design. Mar 19, 2021 at 4:46
• @throx that’s really interesting is that just a product of the blade geometry in the turbine or can an actual gearing mechanism be used as well Mar 19, 2021 at 5:58
• Typically blade geometry. Gearing might be possible on low end turbopumps, but at the speeds they run at I can't imagine reliability in a gearbox. Turbopump design is crazy complex. Mar 19, 2021 at 7:23

"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.

Examples:

Gas generator cycle engines:

Engine Gas Generator Pc Main Chamber Pc Ox Pump Pout Fu Pump Pout
F-1 980 1125 1602 1870
J-2 667 780 1107 1247

Staged Combustion cycle engine (SSME):

Fu PB Pc Ox PB Pc Main Chamber Pc Ox Pump Pout Fu Pump Pout
4873 4812 2871 5956 4025

All pressures in psi

• PB - Preburner
• Pc - Chamber pressure
• Pout - Outlet pressure

Source: F-1 Engine Training Manual

Source: J-2 Engine Fact Book