Defective Solenoid Raises Concerns for STS-92 Launch
During acceptance for the PDU prior to its reuse, one of the actuator bypass solenoid valves failed. When the component was disassembled it was found to have a damaged O-ring with pieces of the O-ring distributed inside the component so as to prevent it from rotating properly. Further disassembly of the component found similar damage in other O-rings. The damaged O-ring and debris were removed, a new O-ring was installed, and the component passed inspection. The actuator on Discovery has been certified for flight since: (a) no damage was found; (b) the actuator passed all required tests; and (c) failure of this solenoid valve to operate (energize) during launch can be compensated for with back up systems and procedures. This issue will be discussed at the STS-92 FRR (Flight Readiness Review) later this week at which time Space Shuttle Discovery is expected to be certified for launch on the STS-92 mission on 5 October 2000.
Earlier this year, in preparation for STS-101, NASA replaced a defective Power Drive Unit (PDU #006) in Space Shuttle Atlantis with one taken from Columbia which was undergoing maintenance in Palmdale, California. The PDU is used to operate the rudder/speed brake located in the Shuttle’s vertical stabilizer.
As NASA checked for other possible uses of this specific component, it was found that the yaw actuator for the center SSME (Space Shuttle Main Engine) on Space Shuttle Discovery also includes the same O-ring design. The design of this component was changed in 1979 and is apparently in use throughout the Discovery – with this one exception. The O-rings within the actuator in Discovery were examined and found to be in acceptable working order.
Background Information
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Observation:
During the acceptance testing of the Rudder/Speed Brake Hydraulic Valve Module at Moog, the Rudder channel D bypass solenoid valve failed to energize
Concern:Inability to bypass a failed servo valve in flight
Discussion:
- RSB HVM S/N006 was sent to Moog for repair due to power spool stop being out of its end cap restricting movement of the power valve
- Unit was repaired by replacing the old stops with vented stops with tightest fit stop/end cap
- S/N006 HVM Rudder channel D bypass solenoid valve failed the pull-in test requirement of the ATP
Acceptable for STS-92 Flight:
- Servo valve bypass solenoid valve criticality is 1R3
- All OV-103 actuators incorporate the new design with the o-ring relief/groove, except for engine #1 Yaw actuator
- OV-103 has successfully passed all OMRSD testing which include the servo bypass solenoid check
- This O-ring installation is unique to the bypass solenoid valve and is not found anywhere else in the actuators
