Titan II and "Pogo"

Ethan Wong

Written throughout 2023, Published with September 20, 2024 edition

Rockets must be extremely reliable and reusable to be successful. If carrying payloads or humans, the durability and flight of the entire rocket must be capable of withstanding external conditions and forces that may affect the vehicle through its launch into space, such as flying debris. However, simple mistakes can sometimes be fatal to the execution of these massive giants as they travel out of Earth’s atmosphere. And while not all of these failures come in forms of explosions or giant catastrophes, the importance of recognizing errors cannot be overlooked on a rocket like the Titan II, which was used as both an ICBM and a vehicle for humans during the Gemini mission.

Pogo was one of the worst issues of the Titan II launch vehicle leading up to the Gemini missions. From an overall standpoint, the consequences of this “Pogo occurrence” could cause sudden engine cut-offs and structural failures, as well as potentially bring harm to the Gemini crew. After testing began on the Titan II rocket for the Gemini Program, the crew noticed a giant increase in vibrations that were coming from vibrations on the first stage–a problem that limited astronauts’ performance in the capsule (vibrations came in around +/- 2.5 g-forces, and the standard was +/- 0.25 g-forces). In December of 1962, The Titan II (N-11) flew with the objective of testing improvements to reduce the Pogo that had been observed on the previous 7 flights. The N-11 launch aimed to fix the oscillations that had been presumed to be caused by the feedlines, which are channels where propellant travels from their tanks to the engine, by installing oxidizer standpipes into the first stage. By installing standpipes near the oxidizer pump with a bit of inert gas, the standpipe could redirect pressure waves. However, the results of the flight after complete stage shutdown showed that the efforts put towards the problem had only made it worse.

After the failure, Sheldon Rubin led a research team that ended up concluding that cavitation bubbles lowered vibrations in the liquid fuel. The team planted oxidizer accumulators, and after multiple tests, decided to place one on each engine fuel line (pipe that carries fuel) to decrease Pogo. Following this information, the rocket was equipped with stronger feedlines and ended up reaching +/- 0.60 g-forces on January 10th, 1963. A series of tests followed this with the idea of using a combination of standpipes and accumulators. While reaching lower vibration levels, there were still failures. For instance, the N-19 Titan II flight on May 29th, 1963 utilized both of these Pogo suppressors; yet, the fuel tank started leaking into the launch, and the rocket caught on fire. After multiple failures, the rocket was installed with improved fuel accumulators to reduce pressure on fuel lines, along with oxidizer standpipes, and flew on November 1st, 1963 (Titan II, N-25) at +/- 0.11 g-forces. This cleared NASA’s requirement for +/- 0.25 g-forces, allowing the Titan II rocket to be used for the Gemini Program. 

The first few Gemini flights went smoothly. The astronauts reported no feelings of vibrations and it looked like the annoying Pogo problem would not be interfering with the Gemini missions. However, the Gemini V flight recorded higher g-forces than +/- 0.25 and the two astronauts had both stated the unpleasant vibrations caused by Pogo. After the mission, it was revealed that the nitrogen gas (inert gas) that was used for the oxidizer standpipes had been injected “too early” before the flight, as the launch had been delayed and the early release of the gas had caused the standpipe to be faulty during the departure.