Try SCE to AUX: NASA’s Unsung Engineering Save
How a perfect storm of physics nearly doomed a Moon mission, and gave rise to one of the most legendary phrases in the annals of spaceflight.
When you think of NASA engineers saving a space mission through technical know-how, grit, and determination, you probably picture scenes from Apollo 13 — mission control scrambling to make sense of ratty data, engineers jury-rigging solutions to each new problem that the astronauts faced, physicists calculating trajectories and burn times, all spurred on by the decree that “failure is not an option!”
But as dramatic as the saga of Apollo 13 was, it wasn’t the first time something like this had happened. In fact, just one mission earlier, NASA faced an equally dangerous situation— but they had even less time to come up with an answer.
“Three men sit tensely waiting for the dawn
The brilliant burst of fire that will carry them on!”
~Lyrics from the song “Star Fire,” from the album Minus Ten and Counting
Let’s set the scene: It’s 11:22 AM local time on November 14, 1969 at the Kennedy Space Center in Cape Canaveral, Florida. A Saturn V rocket sits on the pad, ready to soar. On board, sitting in the Apollo Command and Service Module (CSM) Yankee Clipper, are three veteran astronauts — Commander Charles “Pete” Conrad, Lunar Module Pilot Alan L. Bean, and Command Module Pilot Richard F. Gordon.
With a roar and and a burst of flame, Apollo 12 lifted off the pad, streaking towards a planned landing on the Mare Cognitum on the lunar surface, where Conrad and Bean would visit the landing site of the Surveyor 3 probe, take parts and samples from the robotic lander, and return home with them.
As the rocket ascended, mission controllers in Johnson Space Center in Houston, Texas kept a watchful eye on their consoles, ensuring that Apollo 12’s onboard systems were functioning normally.
For the first 36 seconds of the flight, everything seemed perfectly normal. Propelled upwards by the laws of physics, the Saturn V ascended through 6,400 feet.
Unbeknownst to anyone, however, the same laws of physics that enabled the Saturn to ascend were about to throw the mission into chaos.
You see, Apollo 12’s launch had taken place at a rather interesting time. In order to allow the spacecraft to land precisely at the chosen site, the mission had to launch during an approximately three-hour window on the morning of November 14.
Unfortunately, the weather in Cape Canaveral that day wasn’t particularly great for launching rockets — the skies were overcast and rainy, and thunderstorms had been reported in the area. However, the weather was still technically within the safe limits for a launch and any delay would mean postponing the launch by over a month. So, the decision to go ahead with the mission was made.
However, this exposed the rocket to a danger that NASA had never fully accounted for before: lightning.
Rockets and lightning aren’t a great mix — millions of volts of electricity plus hundreds of thousands of pounds of volatile fuel isn’t exactly a combination that screams “safety.”
NASA had taken some precautions regarding lightning — launches would be postponed if lightning-producing towering cumulonimbus clouds were detected along the rocket’s trajectory. However, this approach failed to account for something major: rockets are really good lightning rods.
Electricity always takes the path of least resistance towards the ground. This is why tall metal objects like skyscrapers are particularly prone to lightning strikes (the Empire State Building, for example, is struck by lightning around 25 times per year). The Saturn V stood about 363 feet tall, and was made primarily of highly conductive aluminum and titanium. Additionally, the exhaust plume that the five F-1 engines of the Saturn’s first stage created was made largely of water and carbon dioxide — making it conductive as well.
And as Apollo 12 ascended, it passed through cloud layers of differing electrical potentials. Since the Saturn was basically a giant metal tube trailing conductive exhaust, it formed a conductive path between these layers. Essentially, Apollo 12 had become the path of least resistance for a bolt of electricity to pass through.
Thirty-six and a half seconds after liftoff, this quirk of physics reared its ugly head.
A bolt of lightning sent an estimated one hundred million volts of electricity coursing through Apollo 12. The astronauts saw a bright flash of light and felt their rocket shudder.
Almost immediately, a number of electrical systems aboard the CSM began to fail. All three of the fuel cells, which provide power to the command and lunar modules, were knocked out, forcing the CSM to switch to its backup batteries. Nine sensors on the exterior of the CSM were permanently fried by the blast. Every light on the malfunction panel lit up “like a Christmas tree,” in the words of one of the astronauts. In Mission Control, Apollo 12’s telemetry data became nonsensical. Exclamations of surprise flew over the communication channels.
Among the various systems affected by the strike was a small device called the Signal Conditioning Unit, or SCE. The SCE was responsible for collating data from the various sensors on board the rocket, turning this raw data into a standardized format, and then transmitting it to Mission Control. The primary power source for the SCE had been damaged by the strike, causing it to transmit garbled data to controllers on the ground.
As the astronauts and Mission Controllers recovered from the shock, they began to analyze their displays. But things were about to get a lot worse.
Fifty-two seconds after liftoff, and sixteen seconds after the lightning strike, Apollo 12 was struck by lightning a second time.
This strike knocked out the inertial guidance units that fed attitude information to the “eight-ball” artificial horizon displays in the CSM. The displays began to spew incorrect data, showing the rocket tumbling end-over-end.
More systems began to fail. The electrical system began losing power, triggering caution lights. Both main electrical buses failed. Mission Control was still struggling to make sense of the nonsensical data that they were receiving. While the Saturn V’s internal guidance systems were seemingly unaffected by the strike, and Apollo 12 was continuing along its planned trajectory, there was no guarantee that it would stay that way for long.
Something needed to be done — and fast. Otherwise, a “One-Bravo” abort would be have to be called — a rocket motor would pull the Command Module to safety while the rest of the Saturn V was remotely blown up; this risked seriously injuring the three astronauts.
But one man in Mission Control had a plan.
The Electrical, Environmental, and Consumables Manager (EECOM) for the mission was John Aaron, a veteran controller who had served on the earlier Gemini missions, as well as previous Apollo flights.
As he watched the garbage data stream into his console, he was struck by a flash of realization. He’d seen this data pattern before. Roughly a year before Apollo 12 lifted off, Aaron had been present for a ground test at Kennedy Space Center, when this same error had occurred. Intrigued, he traced the error back to the SCE, an obscure system buried deep in the heart of the Apollo CSM. During the test, Aaron noticed that the data issue could be resolved by switching the SCE from its primary power source to an auxiliary source, allowing it to run in a low-voltage environment. He theorized that repeating this trick could save Apollo 12.
Keying his mike, he informed the flight director of this finding,
“Flight, EECOM. Try SCE to AUX.”
Initially, this was met with confusion. Nobody else in Mission Control had even heard of the system. One controller even asked, “What the hell’s that?”
Aaron calmly repeated himself, and the message was passed on to the crew of Apollo 12, albeit with no small amount of trepidation.
By sheer chance, Command Module Pilot Gordon — previously a ground expert on the command module — was also familiar with the system, and directed Lunar Module Pilot Bean to flip the SCE switch to the “AUX” position.
The effects of this were immediate. Telemetry data was restored, and the CSM’s computers rebooted, causing the instrument panel to return to normal. Apollo 12 could continue. The astronauts and Mission Control breathed a sigh of relief.
The rest of Apollo 12’s mission continued uneventfully. After establishing themselves in a parking orbit, the astronauts managed to restore power from the fuel cells, and brought both electrical buses online again. The trans-lunar injection burn went off without a hitch, and the trip to the Moon was utterly routine. Conrad and Bean would spend 31 hours on the lunar surface before rendezvousing with Gordon in lunar orbit. Apollo 12 would splash down in the Pacific Ocean on November 24, 1969, her crew having spent ten days in space.
While the tale of Apollo 12’s wild ride to orbit has largely been forgotten by the general public, largely because it had the misfortune of being sandwiched between the first lunar landing on Apollo 11, and the much more made-for-Hollywood drama of Apollo 13, it still holds a legendary status in NASA tradition. EECOM John Aaron was widely praised for his quick thinking during the crisis, with another controller calling him a “steely-eyed missile man.” To this day, the appellation remains the highest praise a NASA engineer can receive. The command of “try SCE to AUX” became an iconic phrase in the lexicon of spaceflight, coming to represent the extensive knowledge and grace under pressure expected of all ground controllers, and creating a bar that all those who step through Mission Control’s doors aspire to reach, even as they hope that their missions will proceed smoothly, and that such skills will not be necessary.
