J-2 engine ignition sequence

The J-2 rocket engine is a very complex machine with a network of valves, lines, pipes around a thrust chamber and turbo pumps to feed the thrust chamber with liquid oxygen and liquid hydrogen. To bring the J-2 engine through the ignition sequence in which every engine component has to brought on line in a proper order and on the right moment, a sequence controller was used. The F-1 engine ignition sequence, on the contrary, was mainly governed by the interrelated events in the F-1 engine system. Four main steps in the ignition sequence can be distinghuished:
  1. The start of the LOX and the LH2 turbine pumps
  2. The ignition of the Augmented Spark Igniter which acted as an ignition device for the engine combustion chamber
  3. The ignition inside the combustion chamber
  4. Activation of the Propellant Utilization Valve to control the LOX / LH2 mass ratio

J-2 engine

Diagram from J-2 engine fact sheet in "Saturn V News Reference"
Locations of the various J-2 engine components.


This drawing depicts the condition of a J-2 engine prior ignition.
The main fuel lines are filled with propellants. The start tank to start spinning the turbo pumps is pressurized.

The operation of the J-2 engine is controlled by electronics housed in the Electrical Control Package (ECP) mounted against the top of the engine. The Pneumatic Control Package is commanded by the ECP. And the ECP is controlled by the Instrument Unit of the Launch Vehicle. The ECP also receives input from the Primary Instrmentation Packages which monitor 70 different parameters like flow rates, pressures and temperatures. The auxiliairy Instrumentation Package is optional and is meant to measure additional parameters for developing and testing purposes.

The Pneumatic Control Package controles all valves except for the Propellant Utilization Valve. The PCP is supplied from a small high pressure He storage tank.
The Propellant Utilization Valve is controlled by the ECP and is used to control the mass ratio between the LOX and the LH2 being supplied to the thrust chamber. By controlling the ratio the amount of thrust can be controlled and an even depletion of propellant tanks can be achieved to optimize the performance of the stage.

The turbo pumps are driven by a Gas Generator during the mainstage of operation. The Turbine Bypass Valve is meant to obtain a proper balance between the two turbo pump speeds during all stages of operation. When the valve is closed the bypass is closed with exception of a carefully calibrated orifice to attain the proper balance of pump speeds, and therefore a proper propellant mixture ratio, during the main stage of operation.

  1. Pyrotechnic igniters are acvtivated by the Electrical Control Package in the Augmented Spark Igniter and the Gas Generator.
  2. Main Fuel Valve is opened. The LH2 is used to cool the engine nozzle. The LH2 is pre-heated and is injected in the spark igniter and the engine combustion chamber as a gas
  3. Augmented Spark Igniter Valve is opened
  4. Augmented Spark Igniter is ignited and will act as the igniter for the engine thrust chamber.
  5. The Start Tank discharge valve is opened to start spinning the turbo pumps


  1. The main LOx valve is opened. LOx is injected into the combustion chamber.
  2. The already present hydrogen and the torch flame of the spark igniter cause ignition in the combustion chamber.
  3. The turbine bypass valve is opened to depress the acceleration of the LOx turbo pump

  1. The Gas Generator has taken over the propulsion of the turbo pumps from the Start Tank. The turbo pumps are about to attain their normal rotational speed. The LH2 turbo pump operational speed is 27 000 rpm and the operational speed of the LOx turbo pump is 8 600 rpm.
  2. Engine thrust is increasing
  3. The Start Tank is recharged from the J-2 engine fuel supply intake, to make a restart of the engine possible after an engine cut-off.


Main stage of operation achieved

The Propellant Utilization Valve is activated. A LOx flow from the high pressure duct to the low pressure duct is introduced. By controlling the level of this return flow, the ratio between the propellants supplied to the engine can be controlled and as a result of that the engine thrust level can be controlled. The main objective of the PU Valve is to achieve an even depletion of the propellant tanks to optimize the performance of the stage by minimizing its mass at stage burn out.

Site Map |  References |  Change History

Copyright 2012 by   Sander Panhuyzen
All pictures and drawings contained on and through these pages are the author's, unless otherwise noted. No unauthorized reproduction without permission.