Sheldon Rubin and the Pogo Instability

by Martin Goldsmith (MS 52 ME, PhD 55 ME)

An example of a major dispute, over a very important problem, surfaced sometime in 1963. A version of Titan II was being developed by the Air Force for use as the booster for NASA's Gemini (two-man) spacecraft. As flight testing proceeded, a problem of what was called ‘pogo' instability developed. It turns out pogo had always been present in the ICBM version to some degree, but those longitudinal vibrations were simply integrated by the guidance system and didn't affect the payload or other mechanical components. However, male human bodies were another matter, and pogo proved to have serious consequences for testicles. The problem had to be fixed, and there was only one or two remaining unmanned test flights. One of the Aerospace analysts, Sheldon Rubin, a Caltech Ph.D. (BS '53 ME, MS '54 ME, PhD '56 ME), did an absolutely fantastic job of tracking down the problem, and prepared a first class mathematical model. The vibrations depended on a feedback with the first stage liquid rocket engines, and Aerojet was able to take high-speedmotion pictures of transparent pump volutes that showed a clear cavitation problem in the eye of the pump which would account for a feedback mechanism. The cure for the problem, absent a new pump design which was out of the question in a schedule sense, was to increase the propellant tank pressure. This was not without a performance penalty, but the overall performance remained within the envelope. So the fix was to go into the next flight. About this time, an senior experienced bozo who ran the Aerospace flight test operation facility down at Canaveral came up with some crazy theory for the problem which was supported by nothing. He insisted that Rubin's solution would make the problem worse, and the Gemini program would crash and burn. Because this fellow was an old acquaintance of the company president, the Engineering Division took his warning seriously and a major in-house review meeting was called to decide whether the scheduled flight test should go. Because we had a very similar configuration, and might expect to have the same problem, I was delegated to be the TitanIII rep at the meeting. I went over the data carefully, and reviewed the Aerojet information, which was, after all, my own technical bailiwick. Then I read the other fellow's paper. My conclusion that Rubin was right on, and this other guy was just nuts. So the meeting went along, with Barlow and all the major actors present. When it was my turn, I came down very emphatically for going forward with the test flight, and stated all reasons. I was not very kind to the Florida thesis. The decision was made to go forward, but on the way out Barlow asked me "Do you really think it is that black-and-white?" He was somewhat taken aback when I sort of snarled, "Hell, yes!" Ed was the finest kind of gentleman, and very even-handed, and I think he had some trouble with strongly stated opinions. In this case, the opinions were justified. The flight test went off and the pogo problem was fixed.

Last updated 3/26/07.

Christopher E. Brennen