authored by Hugo E. Martinez, in collaboration with John Albright, Stephen Damico, and John Brewer of NASA's Johnson Space Center, Engineering (EP4) During ascent of OV-105 during the STS-126 mission, a Main Propulsion System (MPS) engine hydrogen flow control valve (FCV) appeared to transition from the low towards the high flow position without being commanded. The other two valves compensated for the additional flow as expected, and there was no impact to the mission. After landing, an x-ray on the vehicle indicated the cause was a broken poppet. The valve was removed from the system and sent to the vendor for teardown and evaluation. Both the vehicle pressurization system and the valve piece parts were inspected for clues as to the breakage, but no out of print condition, material non-conformance, or evidence of particle impact were found.Structural breakage of metal components is very rare in the MPS, especially during flight. A high speed particle impact causing the poppet to liberate a fragment had already been ruled out, so it was initially believed that the cause would be a defect in the poppet's material (440A steel) or an irregularity in its manufacturing. However, the poppet flange sees no impact load or mechanical stress in service since the high flow and low flow stops are located elsewhere along the poppet and seal.Three FCVs, one per engine, provide Gaseous Hydrogen (GH2) pressurant to the External Tank (ET) hydrogen tank in flight in order to maintain tank pressure as the liquid is consumed. Each valve is commanded on or off by its corresponding ullage pressure signal conditioner. The inlet to each FCV is 3,300 psi hydrogen gas supplied by its corresponding engine. The three outlet flows join via a manifold and are routed to the top of the ET liquid hydrogen (LH2) tank. It was well documented that ET overboard GH2 venting while in flammable atmosphere, below about 150,000 feet or 130 seconds in flight, presents a fire hazard to the vehicle. This would require a much larger particle breakage or multiple poppet breakage to increase flow sufficiently. The other hazardous condition was that of hydrogen leakage into aft compartment if a piece of the poppet were to puncture a pressurant line or bellows assembly. In addition, this would deprive the ET of needed pressurant gas.Once it became clear that a solid root cause would not be found quickly, the approach to finding flight rationale was to be based on the sum of two elements. It was hoped that there would be sufficient flight rationale, even with only a partial contribution from each of these two elements, to support flying STS-119 with an acceptable amount of risk prior to the Soyuz cutoff of mid-March. These two elements were:• Improved confidence of no future poppet breakage • An understanding and/or lessening of the effects of a broken poppetThe MPS community supported extensive ground testing to determine the root cause environmental cause factors prior to proceeding with any design solution for fear of making matters worse. The leadi...
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