STS-135: ET-138 Tanking Test reveals SSME Fuel Valve issue
Atlantis’ External Tank (ET-138) was put through a Tanking Test on Wednesday, slightly delayed due to a lightning storm which passed over the the Kennedy Space Center (KSC). The test – which is checking the health of the tank’s LO2 stringers – is showing positive signs, although a Space Shuttle Main Engine (SSME-3) Main Fuel Valve (MFV) leaked during tanking, which will require replacement and launch date assessments.
Preparations in the flow towards the Tanking Test were all on track, ahead of what was set to be a 7am loading, including the final work on a small issues with observed moisture on an orbiter system, requiring the removal and replacement of a Window Cavity Conditioning System (WCCS) dessicant.
“The aft closeout and confidence checks for cryo load were successfully completed. Orbiter Crew Hatch gap measurements were completed, GUCP (Ground Umbilical Carrier Plate) QD (Quick Disconnect) leak checks were performed. Results were within spec,” noted the NASA Test Director (NTD) report (L2). “The WCCS carrier panel closeout was completed.”
PRSD (Power Reactant Storage and Distributation) GO2/GH2 Reactant gas load operations were also completed, followed by OMBUU (Orbiter Mid Body Umbilical Unit) demates.
However, operations took a seven hour hit at the T-14 hour point of the schedule towards the Tanking Test, as a lightning storm passed over the pad just a few hours prior to the scheduled retraction of the Rotating Service Structure (RSS).
The events were not as severe as the end of March lightning storm which passed over Endeavour ahead of STS-134’s launch.
As a result, the schedule for the test slipped to the right, with the start of the Tanking Test moving to NET (No Earlier Than) 11am local time – after the RSS was successfully retracted just after midnight.
“S0037 Tanking Test operations continued working a modified timeline driven by significant weather delays; Chief Engineer tagup – 0800. ET Tanking Weather Brief – 1030EDT. Cryo load is now scheduled to begin NET than Wednesday at 1100 EDT and run through early evening,” added the NTD.
“RSS rotation to park was delayed due to the significant weather that came through the area last evening. The Orbiter Weather Protection (OWP) and RSS have been retracted at this point to support tanking.”
However, just an hour prior to the scheduled start time for tanking, managers decided to delay again to 1pm local, before changing the schedule again to a 12:25 start.
With a GO for tanking from management, engineers pressed on with the start of tanking operations, which began with the use of the pumps to chilldown the transfer lines between the storage tanks and Atlantis’ Main Propulsion System (MPS), which is the path taken by the LOX and LH2. LOX Chilldown began at 12:25.
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This was followed by the main stage of tanking, known as fast fill – ahead of around another two hours of loading to the point the tank reaches topping and stable replenish. Fast Fill on LH2 began at 13:06 local, followed by LOX fast fill at 13:31.
As with all recent tankings, several areas of interest will closely monitored, such as the low level and Engine Cut Off (ECO) sensors, which have performed as advertised since a successful resolution to what was a problem in part of the LH2 Feedthrough connector system, and leak detection monitoring around the GUCP.
However, the main focus will be via the numerous pad cameras, which will focus on the LO2 flange to check for any signs of cracks in the Thermal Protection System (TPS) foam, including infra-red footage which spotted the crack in ET-137’s flange during a GUCP related scrub on STS-133’s opening launch attempt countdown last year.
Once the tanking has been completed, the tank will undergo a pressurization cycle, further testing the strength of the stringer, prior to the call to drainback the tank. Once inert, Non Destructive Evaluations (NDE) will take place when the pad is reopened.
These visual and X-Ray/Backscatter scans of the tank will check the stringers for any hairline cracks on the support beams.
Late on Tuesday and early on Wednesday, the ERB met to discuss several lightning events at the pad during the storm, utilizing data from the array of instrumentation located on and near the pad itself. Thankfully, none of the stack’s systems appeared to suffer from any ill effects.
“Issues: An ERB was held last evening to discuss several close proximity lightning strikes that occurred at approximately 1930 yesterday. The data was gone over and discussed. No issues were identified but another meeting was scheduled for the morning – expected to clear the constraint for tanking test,” confirmed the NTD.
By the time of Wednesday morning’s NTD update, four new Interim Problem Reports (IPRs) were charged against the STS-135 flow, all of which were either successfully troubleshooted during the early hours, or will be worked after the Tanking Test.
“IPR 0045: LO2 17 inch disconnect Open Indicator B failed to pickup when cycled. A second cycle gave the same failure and the data path testing showed good health. The latch was properly configured to support tanking test but will need additional troubleshooting after S0037,” the NTD continued, noting the main three IPRs.
“IPR 0046: During system configuring, 9 fuel cell pump AC phase circuit breakers were found to be out of closed. The breakers were re-configured and none of the pumps were powered or commanded on. No damage to hardware and the constraint was cleared.
“IPR 0048 at approximately 0245EDT Wednesday, Power Feeder 606 to the pad had a failure at the C5 substation. This resulted in numerous losses and redundancy losses to Pad A. Engineering is currently troubleshooting the failure and working to get the power leg restored but it is currently a constraint to Air to GN2 transition. This is currently a constraint to tanking this morning (since cleared).
While the tank is currently showing no ill effects from the Tanking Test, engineering notes showed SSME-3’s MFV suffered from a small leak during tanking, becoming the latest Interim Problem Report (IPR).
“IPR-48: Exceeded Launch Commit Criteria (LCC) SSME MFV limit downstream temp of -130 degF. Indicative of ME3 (Main Engine 3) MPV leakage. MPS/SME isolated/slow leakage by closing the E3 prevalve,” the notes (L2) added.
While the closure of the prevalve allowed for the Tanking Test to continue, this condition on launch day would have resulted in a scrub, as was seen in a similar incident during a STS-73 launch attempt.
Already engineers are calling for the valve to be replaced, which should be complete by either the weekend or early next week. However, the launch date – currently July 8, will be assessed based on schedules related to the re-testing of the valve.
“Downstream issue out of this is that we will have to R&R the E3 MFV prior to STS-135 launch. (Pratt & Whitney) Rocketdyne assessing impact to scheduled launch date, may still be able to meet 7/8 date.”
Notably, Mission Management Team (MMT) chair Mike Moses specifically mentioned in the 134 post-landing presser that they had only “two days contingency” on paper, but that was only minimal work planned for the week following the Tanking Test – and that there’s more wiggle room in reality than on paper.
More will follow.
Meanwhile, following the completion of Aft skirt foaming operations on the Solid Rocket Boosters (SRBs), the curtain closeouts will pick up after the tanking test is complete.
Other notable events in the flow include the completion of Payload Ground Handling Mechanism (PGHM) final hydraulic leak checks, in preparation for payload installation later this week.
Payload delivery is currently planned to take place for late Thursday evening, around 2200EDT. Payload transfer to the PGHM is planned for Friday – however, any SSME related work may impact.
(Images: Via Larry Sullivan/MaxQ Entertainment and NASASpaceflight.com, L2 and nasa.gov. Further articles on STS-135′s status will be provided as information arrives, driven by L2′s new and fast expanding STS-135 Special Section.)