The Pogo Effect

It first showed up in early unmanned flights of the Titan-Gemini flights.

A low frequency vibration of the entire vehicle that observers likened to the motion of a child bouncing on a pogo stick. And it was most certainly not good! This strange phenomenon that was quickly recognized as a dangerous threat to the whole program of manned space flight. There is even some evidence that the phenomenon occurred on the ground in the early 1950s during static firings of the Redstone(The Mercury Program used Redstones) rocket engine.

The Pogo Effect is caused by the pressure difference in the fuel pipe leading to the firing chamber as the Rocket goes higher. Bubbles of Oxygen become trapped and cause a drop in thrust, the bubble clears and thrust returns, and then another bubble forms, and so on.

Rocketdyne Propulsion and Power, a part of The Boeing Company and NASA started on ways to reduce the pogo, at that point it was so severe it would have killed any Astronaut who travelled inside the spacecraft (The g force we are talking about does not seem like much, indeed it is much less than what is experienced at launch. What makes it deadly is the way the force is outputted. In a normal launch the force exerted on the spacecraft is even, constant and fairly gradual. During the pogo effect the normal launch g is present, but also there is extra g pushing down on the ship and then the negative value as it vibrates its way up, this longitudinal vibration is so severe that it would bounce an astronaut in his straps. It would be like shaking a baby, the body just can't handle stress like that.) .

They tried many solutions, a surge-suppression standpipe in the first-stage oxidizer feedline - was installed in the soon to be infamous Missile N-11, the eighth Titan II that the Air Force launched in its missile development program, on 6 December 1962. The supposed cure, far from damping the Pogo effect, it raised it to over a g, and the violent shaking induced the Stage I engines to shut down too soon.

The next Titan II, launched on 19 December, carried no standpipes, but increased fuel-tank pressure, which had shown good results on some earlier flights, again it reduced the Pogo level.

Many tests followed, every Titan II so far flown had displayed Pogo, although the level had varied, reaching a low of just over one-third the force of gravity in the 17th test flight on 13 May 1963. This potential hazard to pilot safety prompted a survey of available data on human tolerance of such vibration, leading MSC (acronym for Manned Spacecraft Center later renamed Johnson Space Center) to conclude that Pogo should be completely eliminated, or at least not allowed to exceed a quarter of gravity.

What had been Project Gemini's greatest concern? Whether Titan II could function as a booster for manned space flight? Titan II Missile N-25 was launched 1 November 1963 from the Atlantic Missile Range, the 23rd in the series of test flights conducted by the Air Force. It furnished the first real proof that Titan II would do for Gemini. Missile N-25 was equipped with the standpipes on its oxidizer lines and mechanical accumulators on its fuel lines that the revised theory had predicted would suppress the severe Pogo that threatened the Titan II role as a manned booster. The November flight proved it worked. The devices installed in fuel and oxidizer feed lines reduced Pogo to the lowest level ever in a Titan II flight, only one-ninth the force of gravity, and for the first time well below the 0.25g that NASA insisted marked the upper limit for pilot safety.

With a few more test flights with the new systems to make sure it wasn't a fluke, Gemini was back on track and would soon be taking Astronauts to the stars. However this was not the last NASA would see of the pogo effect.

9 November 1967 Apollo 4 launched, carried by the new Saturn V rocket, to the horror of NASA engineers who hadn't seen it since Gemini, a clear return of the pogo effect. Having not seen it in the Saturn 1-B they assumed the new F and J class engines would not be cursed by it. Although Apollo 4 did pogo, it was to a lesser degree of any of the Titan II launches. So its appearance during Apollo 6 did not come as a complete surprise although it was still a disappointment. Pogo on Apollo 4 had been measured at one-tenth gravity, much less than the one-fourth gravity set as the upper limit in Gemini. The lower oscillation was probably the result of carrying just "a piece of crap" to simulate Lunar Module weight, on the earlier flight. But a test article flown on Apollo 6 had the shape and weight of a real lander in the adapter. This change in mass distribution coupled back into the fuel system problem and increased the pogo oscillations. The mission analysts later discovered that two of the Saturn V engines had been inadvertently tuned to the same frequency, probably aggravating the problem.

Because of the experience they gained from the pogo problems with Gemini they felt they could get the Apollo pogo problems licked fairly swiftly, the trouble was, the J-2 engine worked in a completely different way from the Titan II predecessor. The fixes they used before didn't work with the Saturn engines, in fact they made the pogo effect worse. NASA was in trouble, if they hoped to keep the target of landing a man on the moon before the end of the decade they would have to move quickly.

It was eventually fixed however. The problem lay in the pre-valves in the liquid-oxygen ducts just above the firing chambers of the five engines. They were used to hold up the flow of oxygen in the fuel lines until late in the countdown, when the fluid was admitted to the main liquid-oxygen valves in preparation for engine ignition. The pre-valves were modified to allow the injection of helium into the cavity about 10 minutes before liftoff, the helium would then serve as a shock absorber against any liquid-oxygen pressure surges. The flow of fuel would now be steady without any pressure build ups. The Saturn V was now on its way to the moon.

Now, This was suppose to be a quick throw away write up like Block I and CSM so my other Apollo stuff can be more complete. All the Hard Links would then go somewhere. The plan was to do a quick one about the Pogo effect before doing Apollo 6 because it was a big deal in that launch. I couldn't however bring myself to put "Pogo makes it bounce, they fixed it". I had to do it justice as a great many people worked very hard to fix it, and without fixing it we would have never gone to the moon. Tragedy! So I apologise for this rather lengthy w-u.

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