Turbopumps or Pressure Feed
The pressure inside a combustion chamber can be very high – typically 500 psi or 33 bar. In the vacuum of space, a lower pressure can be used, but the efficiency of any rocket motor is reduced if used in the atmosphere, and one way of increasing the efficiency is by using as high a chamber pressure as possible. The question then is how to feed a large quantity of fuel into the chamber at such high pressure.
There are two options: a pump, or by pressurising the fuel tanks.
Pressurising the tanks had one big drawback: the tank walls had to be strong enough to withstand the pressure, which implies they are also going to be heavy.
The tanks can be pressurised from separate gas bottles, but, for large tanks at high pressure, that has a considerable weight penalty: the gas bottles themselves will be thick-walled and thus heavy. The alternative is a gas generator – two chemicals being mixed to produce large volumes of gas. The French stage of Europa used a gas generator; the German third stage was pressurised by helium in gas bottles. The great advantage of the system is that it is extremely simple and so there is little to go wrong.
A pump has to be driven by something – there needs to be a turbine which is normally driven by fuel from the main tanks. In HTP motors, the kerosene and HTP were well suited to the purpose; the RZ 2 motors in Blue Streak had a turbine which used an excess of kerosene – that is, it burned fuel rich – to keep the temperature down. This can be seen very clearly in Blue Streak launches: the turbines produce bright yellow flames as a result of the excess of carbon.
The great advantage of pump versus pressure is that with a pump, the tanks can be as thin-walled as structurally possible (Atlas and Blue Streak took this rather to extremes). Some small pressure is still needed in the tank for the pump to function, but it is relatively small. One drawback is the extra weight of the turbine and pumps. Another is that the system is relatively complex, and provides another opportunity for something to go wrong.
One of the major problems, particularly with regard to the higher thrust engines, was producing pumps powerful enough to cope with the quantity of propellant at the high pressures needed, as this chart shows:
Flow rate (fuel + oxidant) Combustion chamber pressure