Ignition sequence of five F-1 engines (1/48)
Credit: NASA
Scanning credit to Spacecraft Films
Time until lift-off:

The sequence of pictures are stills from a movie, taken by a camera mounted inside the flame trench of the launch platform. This particular movie is taken during the launch of AS-503 (Apollo 8). Saturn V engine numbers and position labels are added to the picture, shown above, as a reference.

The whole ignition sequence has started at 8.9 seconds before lift-off. At that moment the return of hydraulic fluid of each F-1 engine has switched from ground support equipment to the engine fuel intake ducts. (Hydraulics are used to operate valves and F-1 engines gimbal actuators to control the thrust vector.)

Comment with this picture, time: T- 5.68 seconds.
The ignition sequence has already started.
The four pyrotechnic igniters of each of the five F-1 engines have been activated between 7.5 and 6.7 seconds before lift-off. Two igniters can be seen firing inside the nozzle of engine no. 2, close to the turbine exhaust manifold.

Ignition of the engines will occur in an 1-2-2 sequence with 200 ms intervals to gradually increase the load on the S-IC stage thrust structure:
Engine no.5
Engines no.3 and no.1
Engines no.2 and no.4

The time bar diagram illustrates that the timing of the actual start of each engine has been carefully chosen to allow for the various starting characteristics of each engine. The individually programmed start signals, which were given by an onboard ignition sequencer, were properly timed to obtain the required 1-2-2 ignition sequence in the 200 ms time intervals.

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