A detailed description of the engine start sequence for each stage of the Saturn V rocket. Journey to the moon with us as we break down the complex engine start sequences of the mighty Saturn V rocket! From the thunderous F-1 engines of the first stage to the intricate dance of electrical and pneumatic systems in the upper stages, we’ll explore the engineering marvels that launched humanity to the stars!
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S-IC Stage
The engine start sequence for the S-IC, the first stage of the Saturn V, is controlled by the ignition sequencer and begins with the checkout valve moving to the engine return position. An electrical signal then fires four igniters in each engine, burning for approximately six seconds. This triggers an electrical signal to open the 4-way control valve, which allows ground support equipment (GSE) hydraulic pressure to open the main LOX valves.
The main LOX valves open, allowing liquid oxygen to flow into the thrust chamber. Fuel pressure then builds and ruptures the hypergol cartridge at approximately 375 psi. The hypergolic fluid and fuel are forced into the thrust chamber where they mix with the LOX and ignite.
Ignition increases the combustion zone pressure, which causes the ignition monitor valve to direct fluid pressure to open the main fuel valves. Once fuel enters the thrust chamber, the transition to mainstage is accomplished as pressure continues to build.
Finally, the thrust OK pressure switch senses fuel injection pressure at approximately 1060 psi and sends a signal to the Instrument Unit (IU).
S-II Stage
The S-II stage J-2 engine start sequence begins when the IU issues a start command to the S-II switch selector, which relays the command to the electrical control package. The electrical control package then provides energy to ignite the gas generator (GG) and augmented spark igniter (ASI) spark plugs.
At the same time, the helium control and ignition phase control valves are energized, allowing helium to flow from the helium tank through the pneumatic regulator to the pneumatic control system.
This regulated helium fills a pneumatic accumulator, closes the propellant bleed valves, and purges the oxidizer dome and gas generator oxidizer injector manifold.
The mainstage control valve opens the purge control valve for purging and keeps the main oxidizer valve closed. It also provides opening control pressure to the oxidizer turbine bypass valve.
An ignition phase control valve then opens, allowing LH2 to flow into the engine thrust chamber and ASI. The ASI oxidizer valve also opens, allowing LOX to flow into the ASI. The spark plugs then ignite the propellants in the ASI.
Simultaneously, a stage-supplied mainstage enable signal initiates the discharge of GH2 from the start tank. This accelerates both the LOX and LH2 turbopumps to appropriate operating levels. The LOX turbopump bypass valve opens to control pump speed.
The main oxidizer valve then opens, allowing LOX to be injected into the thrust chamber. Propellants are admitted into the GG by the GG valves, and ignition causes a pressure build-up. The OK pressure switches send a mainstage OK signal to the Command Module (CM). The engine then reaches and maintains 90% thrust or more.
S-IVB Stage
The S-IVB stage J-2 engine start sequence is similar to that of the S-II stage, with the main difference being the source of the signal that triggers the start tank discharge valve. In the S-IVB, the start tank discharge valve opens upon receipt of a fuel injection temperature OK signal, rather than a stage-supplied mainstage enable signal.