POGO: The Saturn V Problem That Nearly Shook Apollo Apart
How a terrifying resonance phenomenon almost ended the Apollo program before it reached the Moon
On April 4, 1968, the second Saturn V ever launched—Apollo 6, an uncrewed test flight—nearly tore itself apart. About two minutes into flight, the rocket began vibrating so violently that structural panels ripped off the Lunar Module adapter. The oscillations peaked at ±0.6g, meaning the astronauts who would eventually ride this vehicle would have experienced forces slamming them back and forth with enough violence to cause injury or incapacitation.
The problem had a name borrowed from a children’s toy: POGO. And if NASA couldn’t fix it, humans weren’t going to the Moon.
What POGO Actually Is
POGO oscillation is a longitudinal vibration that couples the structural dynamics of a rocket with the propulsion system’s feed dynamics. The name comes from the pogo stick, because the rocket bounces up and down along its long axis like someone on a spring.
The mechanism is a feedback loop with three participants:
1. The structure. A rocket is essentially a long, thin tube filled with liquid. Like any physical structure, it has natural frequencies at which it wants to vibrate. For the Saturn V’s first stage, the relevant structural mode was around 5.25 Hz.
2. The propellant feed system. Liquid oxygen and RP-1 are pumped from the tanks to the engines through long feed lines. These feed lines contain liquid under pressure, and the liquid column has its own natural frequencies. The fluid can slosh and oscillate within the pipes.
3. The engines. The thrust produced by each engine depends on the propellant flow rate and pressure at the injector. If the propellant pressure fluctuates, thrust fluctuates.
Now connect the loop: The structure vibrates, which causes the propellant in the feed lines to oscillate. The oscillating propellant causes thrust variations. The thrust variations feed energy back into the structural vibration. If the frequencies align and the phasing is right, the oscillations grow instead of dying out.
It’s a textbook case of positive feedback, and it can go from imperceptible to structural failure in seconds.
Apollo 6: The Day It Got Real
Apollo 6 was supposed to be the final uncrewed qualification flight of the Saturn V. If it went well, the next Saturn V would carry astronauts. It did not go well.
At approximately T+125 seconds, POGO oscillations began in the S-IC first stage. The vibrations reached amplitudes that would have been dangerous to a crew. Structural panels on the Spacecraft-LM Adapter (SLA), the conical section connecting the third stage to the spacecraft, were ripped away by the oscillations.
But POGO wasn’t done ruining the flight. After first stage separation, the S-II second stage experienced its own version of the problem. Two of the five J-2 engines on the second stage shut down prematurely during the burn—engine 2 at T+412 seconds and engine 3 at T+427 seconds. The remaining three engines burned longer to compensate, but the vehicle reached an orbit significantly different from the planned trajectory.
Then the S-IVB third stage’s single J-2 engine failed to reignite for the simulated Trans-Lunar Injection burn. The mission was effectively a failure.
The engine shutdowns on the S-II stage were later attributed to structural failures in propellant feed lines caused by POGO-induced vibrations. The lines had fractured, leading to propellant leaks and engine shutdowns.
NASA now had a problem that affected multiple stages of the Saturn V, and the next flight was supposed to carry a crew.
The Physics Nobody Wanted to Hear
POGO wasn’t a new phenomenon. It had plagued earlier rockets, including the Titan II used for the Gemini program. Engineers in the liquid propulsion community were aware of the general mechanism. But the Saturn V’s scale made everything harder.
The S-IC first stage was 138 feet tall and 33 feet in diameter. It held 203,000 gallons of RP-1 and 331,000 gallons of liquid oxygen. The propellant feed lines were enormous—the LOX feed line to the center engine was a duct over two feet in diameter.
The structural dynamics of this system were fiendishly complex. As the rocket ascended and propellant was consumed, the mass distribution changed continuously. This meant the structural natural frequencies were constantly shifting. A frequency that was safely separated from the propulsion system’s resonance at T+60 seconds might align perfectly at T+130 seconds.
The POGO coupling was strongest in the center engine’s LOX feed line. The four outboard engines had shorter, angled feed lines that were less susceptible. The center engine’s feed line ran straight down the middle of the stage, creating a long, unbroken column of liquid oxygen that was highly prone to longitudinal oscillation.
The Fix: Injecting Chaos into Order
The solution was conceptually simple and mechanically elegant: break the feedback loop by making the propellant feed system unable to resonate at the same frequency as the structure.
Engineers added a helium gas accumulator to the center engine’s LOX feed line. This was essentially a cavity filled with pressurized helium gas that connected to the LOX duct through a standpipe. The compressible gas bubble acted as a shock absorber for pressure oscillations in the liquid oxygen.
When pressure waves traveled through the LOX feed line, the helium bubble absorbed and damped them. The gas compressed and expanded, soaking up the oscillation energy that would otherwise have created thrust fluctuations. The spring constant of the gas bubble was tuned to ensure the propulsion system’s natural frequency was shifted away from the structural resonance.
The accumulator was not a complex device. It was a can of helium connected to a pipe. But its placement and sizing had to be precisely calculated, and the helium pressure had to be maintained throughout the burn as conditions changed.
For the S-II second stage, a similar approach was used. Helium gas was injected into the LOX suction ducts of all five J-2 engines to provide compliance and break the feedback loop.
Betting the Program on a Can of Gas
The POGO fix was implemented for Apollo 8, which would be the first crewed Saturn V flight—and the first mission to carry humans to the Moon. There was no intermediate uncrewed test. NASA went directly from the POGO-plagued Apollo 6 to putting Frank Borman, Jim Lovell, and Bill Anders on top of the corrected vehicle.
This decision is worth pausing on. The agency had seen POGO nearly destroy an uncrewed vehicle. They had implemented a fix based on ground testing and analysis. And they decided that the fix was validated well enough to put three humans on the next flight.
The confidence came from extensive ground testing of the accumulator system, including hot-fire tests of the S-IC stage at Marshall Space Flight Center’s test stands. The helium accumulators were tested with actual LOX flow at flight-representative conditions. The structural dynamics were modeled and verified against Apollo 6 flight data.
But there was no way to fully replicate the conditions of flight. Ground test stands don’t perfectly simulate the loads, vibrations, and acoustic environment of a Saturn V launch. The POGO fix was validated as thoroughly as ground testing could validate it, and then it was committed to flight.
Apollo 8 launched on December 21, 1968. The S-IC stage burned for 150 seconds. The POGO oscillations were measured at levels so low they were essentially noise. The accumulators worked exactly as designed.
The Apollo 13 Surprise
POGO made one more appearance in the Apollo program, this time in an unexpected place.
During Apollo 13’s launch on April 11, 1970, the center engine of the S-II second stage shut down approximately 132 seconds early. The engine experienced severe POGO oscillations that triggered an automatic shutdown when chamber pressure oscillations exceeded safety limits.
The POGO suppression system on the S-II had been designed to prevent the exact coupling that occurred, but the interaction between the engine and the stage structure at the specific propellant levels during this phase of flight created a resonance that hadn’t been adequately suppressed.
The remaining four engines burned longer to compensate, and the S-IVB third stage also burned longer than planned. Apollo 13 reached a satisfactory orbit, and the early shutdown was not related to the oxygen tank explosion that later crippled the mission.
After Apollo 13, NASA modified the S-II POGO suppression system to prevent recurrence. Subsequent missions experienced no S-II POGO issues.
Engineering Lessons from a Pogo Stick
POGO oscillation teaches several uncomfortable truths about complex systems engineering:
Coupled systems create emergent behaviors. The structure, the propellant, and the engines were each well-understood individually. The dangerous behavior emerged only from their interaction—a classic systems engineering failure mode that can’t be found by analyzing components in isolation.
Scale reveals new problems. POGO was known from smaller rockets. But the Saturn V’s unprecedented size created coupling modes that hadn’t been encountered before. Problems don’t always scale linearly.
Simple fixes can solve complex problems. The helium accumulator was mechanically trivial. Its development and validation were not. Knowing where to put a simple device often matters more than designing a complex one.
Ground testing has limits. You cannot perfectly simulate a Saturn V launch on the ground. At some point, you have to fly the hardware and see if your fixes work. Apollo 8 was that flight, with three humans aboard.
The Saturn V eventually flew 13 times. After the POGO fix, no mission experienced dangerous oscillation levels—except for the S-II anomaly on Apollo 13, which was corrected for subsequent flights. A can of helium and a standpipe had tamed a beast that nearly killed the program.