he thrust chamber of the F-1 engine is fed through two fuel ducts and two LOX ducts, each equipped with a valve. In the drawings only one main fuel line and one main LOX line are depicted.

The check-out valve and the 4 pyrotechnic igniters are activated by an ignition sequencer. The next sequence of events are interrelated. The control valve, for example, is activated when the igniters have burned through electrical links.

1. With the check-out valve, the hydraulic return is switched from Ground Source Equipment (GSE) to F-1 engine fuel inlet. The hydraulic pressure is however still provided by the GSE.
2. The control valve provides hydraulic closing pressure to the control ports of the the main fuel valves, main LOX valves and the Gas Generator valve.
3. Start of 6-second burn of 4 engine igniters.
   2 Igniters are located inside the combustion chamber of the gas generator.
   The gas generator produces a high-pressure gas to drive the turbopump. The purpose of the turbopump is to pump propellants under high pressure (129 bar, 129 times atmospheric pressure) into the F-1 engine combustion chamber.
   The other 2 igniters are located in the turbine exhaust inside the F-1 engine nozzle, and their purpose is to ignite the fuel-rich turbine exhaust gases.

The control valve is activated, and hydraulic pressure is applied to open the main LOX valves and the gas generator propellant supply valve.

1. When the pyro igniters have burned through electrical links, the control valve is activated: the hydraulic closing pressure is released, and hydraulic opening pressure is provided.
2. Main LOX valves and the Gas Generator valve are opened.
3. LOX starts to flow via the turbopump into the F-1 engine thrust chamber. The flow of the LOX makes the turbopump spin.
   Combustion has not started yet, so LOX exits the F-1 engine as a dense white cloud.
4. Propellants (LOx and RP-1) start to flow into the combustion chamber of the gas generator.
5. Propellants are ignited in the gas generator combustion chamber by the 2 igniters.
6. Combustion gas, produced by the gas generator, passes through the turbopump, heat exchanger, exhaust manifold and nozzle extension.
7. The fuel-rich combustion gas from the gas generator is ignited in the turbine exhaust manifold by the 2 igniters in the exhaust manifold.
8. Combustion gas accelerates the turbopump, causing the pump discharge pressure to increase.
9. Because of the increasing fuel discharge pressure, the igniter fuel valve opens, allowing fuel pressure to be applied on the burst diaphragm of the hypergol cartridge.
10. LOX is starting to flow under discharge pressure through the F-1 engine thrust chamber, but the fuel valve is still closed.

1. The engine fuel pressure has increased above the ground-supplied hydraulic pressure. The swing check valve switches the intake of hydraulic pressure from GSE to theengine fuel high-pressure duct.
2. When fuel pressure increases to approximately 26 times atmospheric pressure, the fuel ruptures the hypergolic cartridge.
3. The hypergolic fluid and the fuel are forced through the cartridge holder into the thrust chamber, where they mix with the LOX to cause ignition.
4. When the hypergolic cartridge is ruptured, hydraulic inlet pressure is made available for the Ignition Monitor Valve.

1. Ignition causes the pressure inside the combustion chamber to increase.
2. The engine thrust chamber pressure is sensed by the ignition monitor valve through its control port. As the pressure inside the thrust chamber is 1.4 atm., hydraulic opening pressure is provided to the control ports of the main fuel valves.
3. Main fuel valves are opened.
4. Fuel enters thrust chamber.
5. Pressure inside the thrust chamber increases; transition to mainstage is accomplished.
6. If the "Thrust OK" pressure switch senses a fuel injection pressure of 73 atm., a THRUST OK signal is sent to the IU (the Instrument Unit, the electronic heart of the launch vehicle).