H-1 engine cut-off sequence

H-1 engines for the S-IB booster stage

The eight H-1 engines at the base of the S-IB stage.
These pictures were taken in the in the final assembly area of Michoud Assembly Facility (MAF) near New Orleans.

credit to NASA
Scanning credit to Kipp Teague

H-1 engine

Technical drawings of the H-1 engine.

The highlighted components are mentioned in the explanation of the H-1 ignition sequence below.
A part of the heat exchanger and two small parts of the turbo pump have been cut away to for illustration purposes.

The H-1 produced about 91 metric Tons of thrust. Eight of these engines were used to lift the 590 metric Tons heavy Saturn IB from the launch pad.

credit to NASA

The cut-off sequence boils down to only one step: close the Main LOX valve to stop the combustion in the thrust chamber and the combustion chamber of the gas generetor (LPGG).

The Main Fuel Valve (MLV) remains open to allow for a fuel rich cut-off.


Cut-off signal is sent to the Conax valve to activate explosive actuators to provide closing pressure to the control port of the Main LOX Valve.


The LOX supply is cut-off first. This allows for a fuel-rich cut-off.

  1. Main LOX Valve (MLV) closes.
  2. Thrust chamber is cut-off from LOX supply.
  3. LPGG control valve and therefore the gas generator (LPGG) is cut-off from LOX supply.
  4. Igniter Fuel Valve (IFV) is mechanically closed by MLV.
  5. Combustion in the thrust chamber is stopped, pressure drops
  6. Combustion in the gas generetor (LPGG) is stopped, pressure drops.
  7. When the thrust chamber pressure falls below 7 atm the LPGG dual control valve is closed. (The LOX valve is closing when the pressure falls below 15 atm)
  8. When the thrust chamber pressure falls below 2 atm the Ignition Monitor Valve (IMV) is closed and the drain port is opened.
    As long as the fuel / hydraulic pressure on the control port of the Main Fuel Valve (MFV) is above 10 atm, the MFV remains open.


The fuel / hydraulic pressure on on the control port of the Main Fuel Valve (MFV) has dropped below 10 atm. The MFV closes.

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Copyright 2021 by   Sander Panhuyzen
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