I understand the bleed air is directed to the starter valve, however what does the starter do to turn the engine and which part of the engine turns first? (N1,compressor blade?)
Would appreciate any further input, thanks again Newpic
The starter air spins a small turbine in the starter which typically drives the N2/N3 through a gearbox. The idea is to spin up the last compressor rotor spool just fast enough that it can pressurize the burner and still have enough inertia in the rotor to overcome the back pressure from the burner lighting off. Too little speed and the lightoff goes very badly and the rotor just hangs, never accelerating as the back-end temperature climbs.