NASA Space Technology Report: Pogo in Rockets and Launch Vehicles - Pogo Suppression, Experience with Gemini/Titan, Saturn V and Potential Catastrophic Structural Failure on Apollo 13 Mission

Two reports from NASA describe the "pogo" problem found in liquid-propellant rockets and launch vehicles: Prevention of Coupled Structure-Propulsion Instability (Pogo) - NASA Space Vehicle Design Criteria (Structures), NASA Experience with Pogo in Human Spaceflight Vehicles.

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.

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.

Apollo 13 launched on April 11, 1970. During the second stage burn, two episodes of pogo occurred on the center J-2 engine as expected from previous missions, but the third occurrence diverged severely and acceleration at the engine attachment reached an estimated 34 g's (the accelerometer went out of range) before the engine's combustion chamber low-level pressure sensor commanded a shut down. It was estimated in the post-flight investigation that only one more cycle of amplitude growth could have been sustained without catastrophic structural failure.

Prevention of Coupled Structure-Propulsion Instability (Pogo) - 1. INTRODUCTION * 2. STATE OF THE ART * 2.1 Mathematical Models * 2.1.1 Structural Modeling * 2.1.2 Propulsion-System Modeling * 2.2 Stability Analysis * 2.3 Corrective Devices or Modifications * 2.4 Flight Evaluation * 3. CRITERIA * 3.1 Mathematical Models * 3.2 Preflight Tests * 3.3 Stability Analysis * 3.4 Corrective Devices or Modifications * 3.5 Flight Evaluation * 4. RECOMMENDED PRACTICES * 4.1 Mathematical Models * 4.1.1 Structural Modeling * 4.1.2 Propulsion-System Modeling * 4.2 Preflight Tests * 4.2.1 Structural Testing * 4.2.2 Normal Propulsion-Development Testing * 4.2.3 Special Propulsion Testing * 4.3 Stability Analysis * 4.4 Corrective Devices or Modifications * 4.5 Flight Evaluation * APPENDIX * Assessment of Significance of Structural Modes *REFERENCES

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