tag:blogger.com,1999:blog-5822805028291837738.post586570211600509575..comments2023-12-09T03:51:33.158-05:00Comments on Various Consequences: The Stick Does Not PogoJoshua Stultshttp://www.blogger.com/profile/03506970399027046387noreply@blogger.comBlogger3125tag:blogger.com,1999:blog-5822805028291837738.post-32114088678245393402013-10-08T02:45:18.606-04:002013-10-08T02:45:18.606-04:00This comment has been removed by the author.Anonymoushttps://www.blogger.com/profile/07984069599926145232noreply@blogger.comtag:blogger.com,1999:blog-5822805028291837738.post-72273956545114841772011-05-04T12:49:15.466-04:002011-05-04T12:49:15.466-04:00A version of Titan II was being developed by the A...<i>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. <b>However, male human bodies were another matter, and pogo proved to have serious consequences for testicles</b>. 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-speed motion 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.</i><br /><a href="http://www.me.caltech.edu/centennial/history%22/goldsmith1.htm%22" rel="nofollow">Sheldon Rubin and the Pogo Instability</a>Joshua Stultshttps://www.blogger.com/profile/03506970399027046387noreply@blogger.comtag:blogger.com,1999:blog-5822805028291837738.post-35745834566776160902011-02-09T08:46:15.448-05:002011-02-09T08:46:15.448-05:00The Crucial Role of Error Correlation for Uncertai...<a href="http://www.sti.nasa.gov/Pubs/star/star1102.pdf" rel="nofollow">The Crucial Role of Error Correlation for Uncertainty Modeling of CFD-Based Aerodynamics Increments</a>, <b>Abstract</b>: <i>The Ares I ascent aerodynamics database for Design Cycle 3 (DAC-3) was built from wind-tunnel test results and CFD solutions. The wind tunnel results were used to build the baseline response surfaces for wind-tunnel Reynolds numbers at power-off conditions. The CFD solutions were used to build increments to account for Reynolds number effects. We calculate the validation errors for the primary CFD code results at wind tunnel Reynolds number power-off conditions and would like to be able to use those errors to predict the validation errors for the CFD increments. However, the validation errors are large compared to the increments. We suggest a way forward that is consistent with common practice in wind tunnel testing which is to assume that systematic errors in the measurement process and/or the environment will subtract out when increments are calculated, thus making increments more reliable with smaller uncertainty than absolute values of the aerodynamic coefficients. A similar practice has arisen for the use of CFD to generate aerodynamic database increments. The basis of this practice is the assumption of strong correlation of the systematic errors inherent in each of the results used to generate an increment. The assumption of strong correlation is the inferential link between the observed validation uncertainties at wind-tunnel Reynolds numbers and the uncertainties to be predicted for flight. In this paper, we suggest a way to estimate the correlation coefficient and demonstrate the approach using code-to-code differences that were obtained for quality control purposes during the Ares I CFD campaign. Finally, since we can expect the increments to be relatively small compared to the baseline response surface and to be typically of the order of the baseline uncertainty, we find that it is necessary to be able to show that the correlation coefficients are close to unity to avoid over-inflating the overall database uncertainty with the addition of the increments.</i><br /><br />Haven't had time to read this one in detail, but looks interesting.Joshua Stultshttps://www.blogger.com/profile/03506970399027046387noreply@blogger.com