Atlantis is in the final stages of processing inside her Orbiter Processing Facility (OPF), ahead of a targeted rollover to the Vehicle Assembly Building (VAB) on October 6. One item remains under evaluation, specifically a fleet-wide issue with actuators on the Fill And Drain Valves. A problem with an actuator on Atlantis was found after inspections were called for due to the issue during Discovery’s STS-128 countdown.
Atlantis – now reconfigured for ISS missions following her STS-125 servicing mission to the Hubble Space Telescope – is all-but ready for rollover, following the closure of her Payload Bay Doors (PLBD) and successful leak checks.
“OV-104 (STS-129): The landing gear functional tests were completed earlier last week. Aft closeout work completed (over the) weekend. The GN2 decay checks are wrapping up, which will keep the vehicle on schedule to rollover to the VAB on October 6,” noted Ground Operations (L2). “In the VAB, the ET-SRB was mated Monday, September 21. Closeout work continues.”
Two sets of leak checks are conducted on the orbiters ahead of their rollover to the VAB, with the second, longer check around half way through its cycle.
“Orbiter closeouts supporting rollover to the VAB continue; rollover is planned for 10/6,” added NTD processing information on L2. “ECL 3-day GN2 long-term decay test requiring continuous vehicle power was completed; the unpowered 10-day decay check was initiated last Thursday.
“The Payload Bay Doors (PLBDs) are closed and strongbacks removed for rollover. Weekend Work: TPS (Thermal Protection System) work including waterproofing of the PLBD hinge line, configure the orbiter hatch for closure, complete forward/aft/wing closeouts, temporarily install Door 45, and install the GSE (Ground Support Equipment) 50-1/2 doors.”
Work is also expected to take place ahead of rollover- though it can take place inside the VAB or even at the Pad if required – on the changeout of an actuator on one of the four Fill and Drain Valves on Atlantis.
These valves are located inside the MPS (Main Propulsion System), with the LO2 system housing the PV9 LO2 Outboard Fill/Drain Valve and PV10 LO2 Inboard Fill/drain. The LH2 System houses the PV11 LH2 Outboard Fill/Drain and the failed PV12 LH2 Inboard Fill/Drain.
The requirement for the nominal operation and cycling of the valve mainly relates to the ability to offload propellant in the event of – for instance – a pad abort, where detanking needs to take place in a timely manner. Offloading could still occur via the alternate PV11 route – in the event of a PV12 issue – but would take twice as long.
These 8-inch internal diameter Bi-Stable Pneumatic valves are classed as Criticality 1 hardware.
Inspections on both Atlantis and Endeavour were called after the PV12 valve on Discovery suffered from false – lack of a closed valve – indication during tanking ahead of STS-128’s launch. The valve performed without issue during the second countdown and is being changed out – as previously scheduled – ahead of her STS-131 mission.
However, there now appears to be a fleet-wide issue with the hardware, following the inspections – some of which are ongoing – specific to the actuator on the valves.
“Fill-and-drain valve issue will be discussed. These have been inspected on OV-104 (Atlantis), with two being okay, except for needing an actuator changed,” added the Orbiter Project at the Johnson Space Center (JSC). “The third valve is still being inspected.
“Fleetwide, there is a great deal of damage in the actuator, specifically on the arms that trip the sensors. Methods to mitigate this problem for the future are being analyzed.”
Given the association with the sensors, this could turn out to be a root cause for Discovery’s problems, as the false indication – or lack of a closed indication – via the sensor on Discovery’s PV12, caused the scrub during the opening STS-128 countdown.
It had already been noted that a possible failure scenario relating to galling of the ‘Rack and Pinion’ actuator gearing inside the Al Alloy body of the valve was a known candidate, via cyclic fatigue/wear and tear over many cycles.
The main candidate via STS-128 countdown data pointed specifically to a valve position indicator (VPI) failure.
“STS-128 Data Supporting VPI Anomaly: Limited set of vehicle instrumentation available to demonstrate the valve successfully cycled fully closed. Upstream (Orbiter inlet) and downstream (LH2 manifold) pressures,” noted the main MMT presentation (L2) during the troubleshooting.
“LH2 manifold temperature. MPS 750 pneumatic reg pressure responses to demand. Haz Gas aft helium concentrations. Valve response timing (initial move time),”
With the discovery of damaged actuators, evaluations will attempt to ascertain the cause of the damage past nominal and expected wear and tear, in order to align potential replacement work while the orbiters are undergoing their nominal flows inside the OPFs.
Numerous failsafes are built into the data gathering during countdown via the LCC (Launch Commit Criteria), in case a repeat issue is noted during Atlantis’ tanking.
However, thanks to the issue on Discovery, managers will be armed with a mitigation plan, based on the STS-128 data, which may allow for waivers to the LCC during the count.
L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.