How are the engines started prior to liftoff?
The three SSMEs are started by the Orbiters onboard computers at approximately 6.6 seconds prior to liftoff. The Orbiters computers tell the SSME controllers (additional computers in control of each SSME) to start a very complex ignition sequence. The controllers open several liquid propellant valves to allow propellant to ‘fall’ into internal combustion chambers. The controller than turns on several spark ignitors (spark plugs) in each engine which ignite the propellant mixtures in internal combustion chambers. These combustion gases then drive propellant pumps (turbopumps) which continue to feed the overall combustion process. Additional combustion occurs in the ‘main’ combustion chamber and then the gases escape from the nozzle producing the thrust. It is a very convoluted process with lots of feedback paths for liquid and gaseous propellants. The hard part to believe is that it is just as complicated and delicate a process to get the SSMEs shut down safely at the end of their 8 1/2
