Liquid-propellant Rocket - Engine Cycles

Engine Cycles

For liquid propellant rockets four different ways of powering the injection of the propellant into the chamber are in common use.

Generally speaking, pumping losses are small compared to the heat energy lost in the nozzle. For atmospheric use, high pressure engine cycles are desirable as it improves the efficiency of the nozzle. For vacuum use, pumps aren't usually required.

• pressure fed cycle – the propellants are forced in from pressurised (relatively heavy) tanks. The heavy tanks mean that a relatively low pressure is optimal. The pressurant used is frequently helium due to its lack of reactivity.
• expander cycle – cryogenic fuel is used to cool the walls of the combustion chamber and nozzle. Absorbed heat vaporizes and expands the fuel which is then used to drive the turbopumps before it enters the combustion chamber. No heat or propellant is lost, so efficiency is very high. Pump power and combustion pressure are constrained by available heat transfer.
• gas generator cycle – a small percentage of the propellants are burnt in a preburner to power a turbopump and then exhausted through a separate nozzle, or low down on the main one. This usually gives a small reduction in performance.
• staged combustion cycle – the high pressure outlet from the turbopump is fed back to power a burner which then powers the turbopump in a self-starting cycle. The still high pressure exhaust from the turbine is then fed directly into the main chamber, thus essentially all the energy goes through the nozzle, giving no pumping losses at all, and permitting very high pressures.