When answering How exactly did the separation between Saturn V stage 3 and the Command / Lunar Module work? I first wrote that the retrograde dump after LM extraction happened "through the engine".

But upon reflection and some searching, I could not confirm that the dump happened through the engine (although I suspect it did).

So, how were the systems in the S-IVB stage configured to create "Thrust from available propellants in the launch vehicle auxiliary propulsion system and from main propulsion system venting"? Were valves opened in the J-2 engine and the tanks pressurized, or was it done through a vent system? And was the auxiliary propulsion system fired simo?


According to various bits of information from Apollo Flight Journals (Apollo- 8, 10 to 13 and 15):

  • LH2 was dumped propulsively through Continuous Vent System (very small delta-v).
  • LOX and pressurizing helium were propulsively dumped through the J-2 engine bell.
  • in addition to the above, Auxiliary Propulsion System (two ullage motors) were mentioned to be fired in some of the missions.

Some quotes from the respective Apollo Flight Journals:


...remaining LOX in the tank will be vented through the engine bell. This dump will begin at 005:07:56. Additionally, the APS thrusters will fire until depletion and the various bottles and spheres of pressurising gases will be vented. Overall the S-IVB's velocity will be changed by 41.9 metres/second.

Mission Control are about to start venting the remains of the liquid hydrogen in the S-IVB's fuel tank. This vent will make little difference to the stage's trajectory.

005:06:48 Borman: The S-IVB has started [to] dump. [Long pause.]

[As well as the LOX being dumped from the S-IVB, the remaining contents of the start bottle on the J-2 engine are also dumped at this time.]

005:25:32 the ullage motors within the APS engines at the base of the S-IVB begin firing to add their remaining impulse to the stage's slingshot maneuver. The engines will burn for just over 12 minutes until their propellant is depleted.


 This will be a 19.7 feet per second burn with the service propulsion system, the big engine in the service module, retrograde burn. It will put the S-IVB out ahead of the Apollo 10 spacecraft, the dock configuration. Then shortly after the burn the remaining propellants in the S-IVB will be vented through the engine, be a propulsive vent.


005:13:51 McCandless: Roger. They're continuing with the non-propulsive vent from the liquid oxygen tank. It would be radially opposite, there. And Booster tells me it's the continuous vent system. They're also dumping a small amount of fuel at this time. We've got about 23½ minutes or so until the APS burn. Over.


Twelve minutes after ejection the S-IVB will perform an evasive maneuver using its auxiliary propulsion system, about a 10 foot per second maneuver, essentially retrograde. 

004:26:53 Carr: Ullage motors are on. [Pause.] ...

004:28:13 Carr: 12, Houston. The APS maneuver is complete.

The S-IVB sling shot maneuver was scheduled for 4 hours, 48 minutes elapsed time. The present elapsed time is 4 hours, 39 minutes. During this maneuver, the remaining liquid oxygen in the propellant tanks of the S-IVB is dumped. Though the engine bell does provide small amount of energy, it will cause the S-IVB to go around the trailing edge of the moon and then into a solar orbit.


[This is Apollo Control at 4 hours, 37 minutes. We're now about 3 minutes away from the scheduled LOX dump, the propulsive vent of liquid oxygen through the engine nozzle of the S-IVB. The Saturn Instrument Unit will command this maneuver or command the LOX dump and the LOX will flow out the engine bell for 48 seconds or until the tank is empty whichever comes first. The preliminary plan would be for the LOX dump to produce a change in velocity of about 28 feet per second [8.5 m/s] which would contribute to the trajectory change placing the S-IVB on an impacting trajectory with the Moon and subsequent midcourse corrections using the two 70-pound thrust [311-newton] Auxiliary Propulsion System units on the S-IVB would be intended to correct this trajectory and bring it into the precisely preplanned limits.]

This is Apollo Control at 5 hours, 45 minutes. Our Booster systems engineers just reported that the first midcourse correction opportunity with the S-IVB, the midcourse correction designed to put the S-IVB on the proper impacting trajectory with the Moon, will be required and it is expected that this maneuver will be a 217-second burn of the two Auxiliary Propulsion System modules on the S-IVB.


[The S-IVB is maneuvering to the propellant dump attitude. The thrust from this dump of propellant and pressurising helium gives a planned change in velocity of 10 metres per second.]

[The S-IVB has begun venting LH2 via the CVS (Continuous Vent System), imparting a slight Delta-V of 0.42 m/s.]

[The LOX dump is initiated simply by opening the engine's main LOX valve and allowing it to vent through the single J-2 rocket motor. The greater atomic weight of the venting oxygen helps to impart a Delta-V of 9.14 m/s. The dumping of helium from the high pressure storage spheres adds another 1.37 m/s alteration to the S-IVB's trajectory. Both of these accelerations help to target the stage to its planned impact site.]

Emphasis mine.

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