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What is Pogo and Why On Earth Would Anyone Want To Suppress It?

The time was almost 30 years ago and it is fair to say I was not quite out of the "still wet behind the ears" stage for an engineer. I had been working in industry for about three years and I was just given a project that would change my career direction and, in fact, my life. The project? I was assigned to evaluate a new concept Pogo suppressor on a cryogenic rocket engine liquid oxygen (LOX) feedline. 

How did I end up getting assigned this project? Well, I had a few things going for me at that stage of my career. Firstly was that my company division was on a massive hiring spree. Since I had been hired three years earlier my division had doubled in size. Fortuitously for me, that meant I was now in the upper 50% of seniority. Secondly, I had just had a performance review and I had casually told my immediate supervisor during the review that I would be interested in learning about waterhammer should the opportunity arise. Thirdly and finally, I had demonstrated a knack for solving tough analytical problems. What that meant is that I was in the enviable position of having my supervisors assign me to any problem that was out-of-the-ordinary and otherwise unusually difficult. The new Pogo suppressor was such a problem.

So what is Pogo? I will quote from a Wikipedia article:

Pogo oscillation is a self-excited vibration in liquid-propellant rocket engines caused by combustion instability. The unstable combustion results in variations of engine thrust, causing variations of acceleration on the vehicle's flexible structure, which in turn cause variations in propellant pressure and flow rate, closing the self-excitation cycle. The name is a metaphor comparing the longitudinal vibration to the bouncing of a pogo stick. Pogo oscillation places stress on the frame of the vehicle, which in severe cases is dangerous.

"Self-excited vibration" means natural frequencies and every young engineer is shown the classic catastrophic engineering failure due to natural frequency response that happened in 1940 at the Tacoma Narrows Bridge. In the case of Pogo, the above technospeak quote means, in slightly more plain English, that the liquid propulsion system frequency response gets in sync with the natural frequency of the rocket structure.

Before I tell you more about the project that changed my life, here are two links for anyone who wants to read about the history of Pogo and how it affected the Saturn rocket and Moon missions and the Space Shuttle:

NASA Experience with Pogo in Human Spaceflight Vehicles

By the time I got introduced to waterhammer and Pogo 30 years ago, aerospace companies and NASA had learned the hard way about waterhammer. And they had learned the hard way about Pogo.

Up to that point I had mostly worked on the commercialization of the Atlas/Centaur rocket. This was happening in the late 1980's in the aftermath of the Space Shuttle Challenger accident. This commercialization effort resulted in the Atlas I rocket and it's larger sibling, the Atlas II. The Atlas I did not need or have a Pogo suppressor. However, it was suspected that the Atlas II did. The Atlas II was a taller rocket than the Atlas I (the engineers referred to it as a "stretched tank") and also had more powerful MA-5A engines built, at the time, by Rocketdyne. For more on this see Atlas (rocket family).

The Dynamics group decided that a Pogo suppressor needed to be added to the LOX feedline of the Atlas booster rocket in order to change the frequency response of the LOX system. At the time I myself knew nothing about Pogo or Pogo suppressors. And I knew nothing about waterhammer. 

My company had a waterhammer software we used and had developed in-house - which was typical in the 1980's. Today they use AFT Impulse but that is a story for another day. The developer of the in-house software was no longer with the company. And now the company had a problem - the in-house software did not have the capability of modeling Pogo suppressors. So I was in the right place at the right time, and my supervisor asked me to add this capability to our in-house software.

I started with Wylie and Streeter's trusty textbook on waterhammer, used it to teach myself waterhammer, learned how the in-house FORTRAN-based software worked, and proceeded to add a new subroutine for a Pogo suppressor. 

A Pogo suppressor is really just a gas accumulator and Wylie and Streeter have a couple pages in their book on gas accumulators plus, as a bonus, some FORTRAN software code. Within a short time I had the gas accumulator code working and I had results. 

Gas accumulators work by introducing a vessel with a pre-charged gas in it which acts kind of like a shock absorber. Some or maybe most gas accumulators today have a bladder to separate the gas from the liquid, and most use air but some use nitrogen. We could not use either. Why? Because the Pogo suppressor was in a LOX line and LOX is cryogenic. Because the LOX was subcooled, air or pure nitrogen would condense at the subcooled LOX temperature. So we firstly needed a gas which remained in gas form at LOX temperatures. Secondly the gas had to play nice with LOX because our gas accumulator would not have a bladder and the gas would be in direct contact with the LOX. That means the accumulator gas had to be inert or at least mostly inert. There are a few options here but the best one and the one we used was helium.

There were two waterhammer cases we needed to consider in the LOX line. The first and most important was the transient that occurred when the two booster engines were shutdown some 180 seconds into flight. This was known as booster engine cutoff or BECO. This happened on every flight and we needed to make sure we did not overpressure the LOX line from the waterhammer transient that occurred at BECO.

The second case was a rare case that could potentially happen called a Hot Abort. This was when the booster engines were ignited at liftoff and the mission was aborted - which meant the booster engines had to be shut down while the rocket was still on the ground (as well as the sustainer engine for the Atlas aficionados out there).

I took my graphs of the LOX pressure predictions at BECO from my new model using a helium-filled gas accumulator to the Dynamics expert Alex. For good measure I made the same model runs without the Pogo suppressor (gas accumulator). Frankly I had no idea what my models should be predicting except that the pressures needed to be below the maximum allowables. 

Alex, however, was not interested in pressures. He was interested in frequencies. At this time I had the barest of understanding of Pogo and I had zero idea of what the Pogo frequencies were on an Atlas rocket. Alex looked at my graphs carefully and said they looked correct to him and the Pogo suppressor was doing what it was supposed to be doing. I asked him to explain to me what that meant. He told me that with no Pogo suppressor the Dynamics group was expecting a frequency response in the LOX line of 17 Hz. That was bad and too near the Atlas II rocket structure natural frequency. What they were hoping to do was shift the LOX line frequency down to 11 Hz. They did that by selecting the proper volume of helium. I knew what the helium volume was supposed to be and had used that in my waterhammer model.

I then looked with renewed curiosity at my own graphs and counted the spacing between the pressure spikes and, sure enough, the model without a Pogo suppressor was responding at 17 Hz and the one with a Pogo suppressor was responding at 11 Hz. Alex looked at me and said, "See, you got this right". I felt extremely proud of myself because I had not known previously what "right" even was.

Eventually I added a lot more code to our in-house waterhammer tool and after a few years came to be regarded as one of the company's waterhammer experts. I wrote an ASME paper on some of my waterhammer work on Atlas rockets that includes Pogo suppressors called "Rocket Propellant Line Waterhammer Transients In a Variable-G Environment".

So how did all of this change my life? Well, at the time I had zero idea I would someday start Applied Flow Technology. When I started AFT I hoped we would survive long enough that I could write a waterhammer software using what I had learned back in my first job. In 1996 I had a chance to do just this and AFT Impulse 1.0 was released. Today AFT Impulse is an internationally recognized waterhammer software supported by a team of software developers and technical support engineers. Who knew?

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Wednesday, 22 March 2023
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