There’s a second hero on the launch team of Apollo 12’s Mission Control.
Apollo 12 was struck by lightning 37 seconds after liftoff. That you know. EECOM John Aaron makes the famous “Try to SCE to AUX” recommendation. That you also know.
In the “Trench,” a second flight controller made another important call, just in case there was a more serious issue with the Saturn V itself. Apollo 12 was not out of the woods with the SCE-to-Aux call. And there’s a mystery left for us to solve once you read this story.
While John Aaron was conditionally satisfied with his domain after the Great Switch Flip (as EECOM, Aaron monitored power and life support systems of the Command and Service Modules), Apollo 12’s flight dynamics officer (FDO), on Flight Director Gerry Griffin’s Gold Team, was not as happy.
During launch, FDO monitors the Saturn V’s launch trajectory and vehicle performance. Controlling the Saturn V was the Instrument Unit (IU), the brains of the Saturn V from launch to the CSM and LM separation from the S-IVB third stage (where the IU resides). Among other things, the IU contains a digital and analog computer, accelerometers and rate gyros. So far, the Saturn V stayed on course. Aaron’s call restored the ground’s telemetry from the Saturn.
FDO knew, from Conrad’s messages, that the CSM’s Inertial Measurement Unit (IMU) was still offline. The IMU is the gyroscopic-based guidance platform in the CM, used in tandem with the Apollo Guidance Computer, to orient and fly the SM, controlling the reaction-control jets (RCS) and the Service Propulsion System (SPS), the large Service Module engine. It’s the thing you don’t want to go into gimbal lock. Right now, that platform didn’t know where it was in space. Conrad’s 8-ball indicator was spinning wildly by his account.
The IMU and IU platforms worked in tandem for guidance during launch. One was offline.
Most of all, FDO also considered how the event affected the Emergency Detection System (EDS).
For the first 2 minutes 41 seconds, the S-IC first stage burns. During this time, the EDS uses the rate gyros and other instruments to monitor the flight path and the vehicle’s structural integrity, using three cables that run the length of each stage as well as the “Q-ball”, a device at the tip of the launch escape tower. At lift off, the EDS inhibits abort modes that would shut down of the F-1 engines in the first 30 seconds to keep a Saturn gone awry from falling back to the launch pad. After 30 seconds, the EDS watched the IU gyros, the Q-ball and three cables. If two of these three cables de-energize or the gyros or Q-ball detect things are off-course, the IU concludes that the Saturn is lost and the EDS automatically aborts the mission, firing the Launch Escape System’s tower to rip the Command Module away from the launch vehicle during Mode I abort mode.
Pete Conrad and crew had managed to stabilize enough of the Command Module’s systems just before the second stage ignited. By that time, Conrad had switched the EDS to manual; he alone could use the LES for abort and the IU EDS could not. Soon after staging, the LES tower was jettisoned
Under S-II power, for a few minutes, Apollo 12 was in Mode II. In case the S-II stage fails, the SM engine would pull the CSM away for a quick suborbital abort. There was not yet enough altitude and speed to use the S-IVB’s single engine.
After a few minutes, Apollo 12 moved to Mode III, and that’s where FDO made a call to the Flight Director:
“Flight, be advised that we will not have an S-IVB to COI call,” he said.
“Why?” asked Griffin.
“No plat, Flight.”
In a Mode III abort, also called S-IVB to COI (Contingency Orbit Insertion) capability, should the S-II fail later on in its operation, the S-IVB would ignite with enough power, velocity and fuel to get itself into Earth orbit. However, Mode III kills the lunar mission because the bulk of the third stage’s fuel would be needed for reaching orbit, leaving too little for trans-lunar injection. Mode III was the counterpart to the Space Shuttle abort mode ATO, or “abort-to-orbit”. Since the Shuttle always went to Earth orbit, most of its mission would not be hampered unless they were carrying payloads that required a certain orbit that the larger Orbiter Maneuvering System engines could not reach from their lower orbital insertion.
FDO believed that the IU’s automatic abort modes with the S-IVB were unreliable. If a S-II failure occurred, the IU should command separation and ignition of the S-IVB. But what if the IU were damaged by the lightning strike? A broken IU in a Mode III abort could fail to separate the S-IVB, or, worse, fail to separate the second stage and ignite the S-IVB engine with the stages still attached.
In short, so long as the vehicle behaved as the flight plan prescribed after the lightning strike, FIDO was okay. But with one offline platform, FIDO’s decision prevented another possible calamity: Not just the loss of the mission, but a botched abort if circumstances called for it, and loss of the crew.
CAPCOM relayed the news. “12, Houston. We won’t be sending you an S-IVB to COI call.”
He added, “Pete. And if you do a mode 4, it’ll be on the backup.”
Conrad radioed back, “Yes, no sweat. I got a good SCS.” SCS was the Stabilization and Control System, which provided Conrad the ability to control and monitor the Service Module’s reaction control thrusters and the SPS main engine, especially in a Mode III or IV abort. Mode IV separated the CSM from the S-IVB while it was under power. SCS was independent of (and the backup to) the Command Module’s IMU. Conrad would fly manually.
So FDO conditionally averted a possible disaster on imminent abort, directing the crew to trust neither platform and fly the Service Module manually to get away at any time for the rest of the launch if things went pear-shaped.
Thankfully, no abort of any kind was needed. The IU put the S-IVB in the right orbit. Apollo 12 was able to realign its platform, verify no damage to the spacecraft before TLI, and the rest is history.
Well, almost history. My Google-fu is insufficient and cannot yet find the name of the Gold Team FDO on-station during the Apollo 12 launch. If you know of the man or know a resource to find his name, please say so. Apollo 12’s launch FIDO is way overdue for a party and, perhaps, given his own appellation as a steely-eyed missile-man.
Apollo 12’s Flight Director’s Loop at Launch with animation (by YouTube channel “lunarmodule5”)