I understand the basic principle of how a rocket engine works, but what I cannot find any info about is how do they initiate the whole process. Can someone please explain to me what gives the turbo-pump the initial "kick" to start the process of pumping the fuel and oxidizer?
- Ariane 5: the Vulcain first stage engine is started using a small solid rocket motor.
- Shuttle SSME: after chilldown, the propellant valve is opened, allowing LH into the engine. This expands, driving the turbopump enough to fill the gas generators. The mixture in the gas generator is ignited and this drives the turbopump to operational speed.
When a start command is received, the MFV is immediately ramped to its full open position in two-thirds of a second (see Figure 9). This enables the LH2 to fill the downstream system and begin to power the high pressure turbines. The latent heat of the hardware imparts enough energy to the hydrogen to operate as an “expander-cycle” engine for the early part of the start sequence. This eliminates the need for any auxiliary power to initiate the start sequence,
- Saturn V F-1: The LOX valve opens, allowing LOX to flow through the turbopump, this starts it spinning. Some of the LOX flows to the gas generator. Then the propellant valve opens, RP-1 flows through the pump to the gas generator, the mixture in the gas generator is ignited and this drives the turbopump to operational speed. So basically this is gravity-fed until the gas generator ignites.
Often there is a starting extension to the gas generator. This extension contains a solid fuel which is ignited. The produced hot gas flows to the turbine and starts rotation. The turbine drives the turbo pumps for fuel and oxidizer. A small part of the fuel and oxidizer is pumped into the gas generator and ignited by the still burning solid fuel. The liquid fuel pumped into the gas generator continues the operation when the solid fuel burns out. The larger part of pumped fuel and oxidizer flows to the combustion chamber. When liquid oxygen and hydrogen or kerosene is used, a precise ignition inside the combustion chamber is necessary. A late ignition may cause a catastrophic destruction of the combustion chamber.