The F1 engine start sequence begins with the gas generator. An electrical signal fires the ignitors (4 per engine). This in turn burns ignitor links to trigger an electrical signal to the start solenoid of a control valve. The control valve directs hydraulic pressure to open the main LOX valves admitting LOX into the engine thrust chamber. The control valve also opens the gas generator ball valve admitting propellant, under tank pressure, into the gas generator where it is ignited by the ignitors. The combustion gasses from the gas generator begin to accelerate the turbopumps, increasing the propellant and LOX pressures. As the fuel pressure increases to 375 PSIG it ruptures a hypergol cartidge. The hypergolic fluid is forced into the thrust chamber where it mixes with the LOX and ignites. When the pressure in the thrust chamber reaches 20 PSIG the ignition monitor valve directs hydraulic pressure to the main fuel valves. The propellant then enters the thrust chamber and ignites to transitions the engine to mainstage.