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RTO-MP-AVT-152 5 - 1 NASA Experience with Pogo in Human Spaceflight Vehicles Dr. Curtis E. Larsen NASA Technical Fellow for Loads and Dynamics NASA Engineering and Safety Center Mail Code WE Johnson Space Center Houston, TX 77058 United States of America curtis.e.larsen@nasa.gov ABSTRACT An overview of more than 45 years of NASA human spaceflight experience is presented with respect to the thrust axis vibration response of liquid fueled rockets known as pogo. A coupled structure and propulsion system instability, pogo can result in the impairment of the astronaut crew, an unplanned engine shutdown, loss of mission, or structural failure. The NASA history begins with the Gemini Program and adaptation of the USAF Titan II ballistic missile as a spacecraft launch vehicle. It continues with the pogo experienced on several Apollo-Saturn flights in both the first and second stages of flight. The defining moment for NASA’s subsequent treatment of pogo occurred with the near failure of the second stage on the ascent of the Apollo 13 mission. Since that time NASA has had a strict “no pogo” philosophy that was applied to the development of the Space Shuttle. The “no pogo” philosophy lead to the first vehicle designed to be pogo-free from the beginning and the first development of an engine with an integral pogo suppression system. Now, more than 30 years later, NASA is developing two new launch vehicles, the Ares I crew launch vehicle propelling the Orion crew excursion vehicle, and the Ares V cargo launch vehicle. A new generation of engineers must again exercise NASA’s system engineering method for pogo mitigation during design, development and verification. GEMINI – TITAN II EXPERIENCE NASA first identified pogo as a threat to spaceflight vehicles and their crews in the early 1960’s during the Gemini-Titan II program. The Gemini spacecraft was to be a two-person vehicle with significant improvements in spacecraft design over that of the Mercury spacecraft, principally for simplified systems check-out and operations, and increased crew piloting functions. In particular, the Gemini project manager considered the event sequencing for the Mercury escape system as “…one of the major problem areas in Mercury in all its aspects - its mechanical aspects in the first part of the program, and the electronic aspects later." Thus the new design of the Gemini spacecraft eliminated the escape rocket tower used in Mercury and put the crew in ejection seats. 1 The change from escape rocket to ejections seats was thought to exclude use of the Atlas booster (used for Mercury orbital flights) because its liquid oxygen and RP-1 propellant combination were considered too highly explosive for an ejection seat to respond quickly enough to save the crew. However, the USAF was developing a new missile, the Titan II, which would use storable hypergolic propellants: a blend of hydrazine and unsymmetrical dimethyl hydrazine (UDMH) as fuel with nitrogen tetroxide as oxidizer. These