The sign accompanying the APS module. It reads
Auxiliary Propulsion System
Displayed at right is an Auxiliary Propulsion System (APS) unit from a Saturn V
rocket. It was one of the smallest of the 83 rocket engines that were
incorporated into a complete Saturn V launch vehicle system.
Two of these modules attached to the lower
skirt of the Saturn V's
third stage. They had two purposes:
Ullage Thrust: Each APS module contained a single, liquid fueled,
70-pound thrust rocket engine called an ullage thruster. During the launch
phase of a Saturn V, the rocket continued to accelerate, except during staging.
During staging, the spent rocket stage shut down, immediately deaccelerating
the entire launch vehicle, and pushing the fuel in the next stage toward the
front of its tank. Before the next stage could fire, the ullage engines in the
APS modules provided a short-four second thrust to reapply some acceleration to
the Saturn V to push the fuel in the next stage back to the bottom of its tank
so the engines on that stage would have fuel in their lines to fire.
Attitude Control: After launch, the Apollo spacecraft, along with
their astronaut crew of three, were initially placed into Earth orbit. During
this time, the Apollo was still attached to the Saturn's S-IVB third stage. While in orbit,
the three liquid fueled attitude control engines in each APS module were used
to properly orient the spacecraft for the engine burn that would send the crew
on the way to the Moon.
In addition, on many of the lunar flights, the S-IVB stage continued to follow the
Apollo to the Moon after it was unstaged from the spacecraft. In an effort to
maximize their scientific studies, several of these third stages were directed
to impact the lunar surface. By doing so, the impact would create seismic
shock waves that could be used by instruments left on the Moon by previous
crews to help determine the geologic makeup of the Moon. The APS modules were
used to properly orient the S-IVB
stage so that it would properly strike the lunar surface.
Actually, the APS modules did not fire during staging; rather, there was a pair
of solid propellant Thiokol TX-280 rocket motors, each rated at 3,390 pounds of
thrust, which fired for four seconds at staging to settle propellants in the
S-IVB stage (although a 1964 document did describe APS modules which included both a 150-pound
and a 1,750-pound ullage engine, the former described as a "GH2
venting ullage engine" and the latter described as a "main ullage engine"; the
1,750-pound ullage engines in the two APS modules were at the time apparently
considered sufficient for the staging ullage maneuver, as at the time there
were no solid ullage motors). In order to minimize the amount of weight
carried the rest of the way to orbit, an ordnance system jettisoned both the
spent ullage engine casings and brackets which attached them to the S-IVB after
The 70-pound ullage engines fired during the transition between the first J-2
engine and the coast phase of flight to prevent undesirable propellant movement within the tanks. Firing continued for approximately 50 seconds until
activation of the stage's LH2 continuous propulsive vent system.
The ullage engines fired again at the end of the coast phase, prior to the
translunar injection burn J-2 engine restart to assure proper propellant
positioning at inlets.
Another time during which the APS system provided attitude control was during
transposition, docking, and extraction, when the command/service module
separated from the S-IVB, the SLA panels were ejected, and the CSM turned
around to remove the lunar module from the base of the SLA.
Each APS module carried about 115 pounds of usable monomethyl hydrazine (fuel)
and 150 pounds of usable nitrogen tetroxide (oxidizer).
And, of course, there weren't "engines" (plural) on the S-IVB stage as
indicated on the sign, but rather a single J-2 engine.