Pressure-fed engine

A separate gas supply, usually helium, pressurizes the propellant tanks to force fuel and oxidizer to the combustion chamber.

Care must be taken, especially during long burns, to avoid excessive cooling of the pressurizing gas due to adiabatic expansion.

Cold helium won't liquify, but it could freeze a propellant, decrease tank pressures, or damage components not designed for low temperatures.

The Apollo Lunar Module Descent Propulsion System was unusual in storing its helium in a supercritical but very cold state.

These include the Aerojet AJ10 and TRW TR-201 used in the second stage of Delta II launch vehicle, and the Kestrel engine of the Falcon 1 by SpaceX.

Pressure-fed rocket cycle. Propellant tanks are pressurized to directly supply fuel and oxidizer to the engine, eliminating the need for turbopumps .
This is a diagram of the pressure fed, reusable Orbital Manouevering System pod, of which there were two on either side of the shuttle’s stabiliser . It was used on the Space Shuttle orbiter (or simply Space Shuttle) for orbital insertion , manoeuvring the orbiter in space, and the deorbit burn. The AJ10-190 engines could be reused for up to 100 missions.
Diagram of an RS-25 (or Space Shuttle Main Engine) , that used a twin shaft staged combustion cycle . There were three of these on the back of the orbiter. Comparing the diagram of the RS-25 to that of the Orbital Manoeuvring System (OMS), it is clear that the RS-25 engine is far more complex. The record for the most space shuttle missions an individual RS-25 engine has been used on is 19.