Pressure-fed liquid fuel rocket engines use pressurized propellant tanks to deliver propellant to the combustion chamber, rather than pumps. This eliminates the mass, cost and complexity of the gas generator and turbopumps. Pressure-feed rockets have been used successfully by:
- SpaceX Kestrel engine, upper stage Falcon 1
- Apollo Lunar Module descent engine
- Space Shuttle Orbital Maneuvering engines
Pressure feed can potentially reduce mass, cost and complexity. However, this design approach has fundamental challenges:
- Propellant tank pressure must be higher than combustion chamber pressure to ensure propellant flow. A Falcon 9 Merlin turbopump engine has combustion chamber pressure of 1,410 psi. https://en.wikipedia.org/wiki/SpaceX_Merlin . Attaining this pressure with pressure-feed means heavy tanks.
- If the tank is pressurized with a fixed quantity of gas, the ullage volume is “wasted” tank volume. In OTRAG rockets, 1/3 of tank volume was ullage.
- As the ullage space expands, tank pressure drops. Example: OTRAG tanks dropped from 600psi to 200psi as ullage expanded 3:1 during a burn
- Cryogenic propellants boil off during a burn, lowering the temperature of the ullage gas and further lowering tank pressure
Some of these disadvantages can be overcome with autogenous pressurization such as helium pressurization systems, at the expense of, well, expense. And complexity. The Kestrel engine required a heat exchanger in the combustion chamber to heat the helium. The engine achieved an impressive specific impulse of 317sec.
As an alternative autogenous pressurization system, why not combust propellants inside the tanks? A small burner jet feeding oxidizer into the fuel tank (and fuel into the oxidized tank) could keep pre-launch ullage space small, but maintain constant ullage pressure during main engine burn. The feed pressure of the oxidizer (or fuel) to this burner could be maintained just above the desired ullage pressure, so the burn would be largely self-regulating. The size of the feed orifice would set a fail-safe upper limit on burn rate.