ELIMINATION OF THE POGO INSTABILITY FROM THE GEMINI LAUNCH VEHICLE

Publisher Summary Virtually every large liquid rocket developed by the United States has experienced some form of vibrational instability. However, the unexpected occurrence in 1962 of a self-excited longitudinal instability on the Titan II was of particular concern because of its possible effect on the country's space flight program. In this case, a strong axial vibration (POGO) of the Titan IPs entire structure occurred late in first-stage flight. The vibration was apparently caused by a regenerative feedback interaction between the vehicle's propulsion system and structure. This specific instability, however, had an impact on the space program because of the planned use of Titan II as the Gemini Launch Vehicle. For the Gemini program, the oscillation was critical because it would seriously degrade the astronaut's ability to perform his functions and could lead to a physical injury. As a result, the problem was intensively studied by those concerned with the success of the Gemini program. This chapter describes a part of the analytical POGO study conducted for the Gemini program by the Martin-Baltimore Division and the fixes that resulted from that study. The analytical representation made use of an analog computer simulation and incorporated many of the nonlinearities that are believed to occur in flight. Consideration was given to time-varying structural properties, the propagation of pressure pulses through the propellant feed system, the nonlinear performance characteristics of the pumps, and the combustion alteration as a result of mixture ratio changes.