STS-133: TCDT completed – Engineers troubleshooting leaky flight cap
Another milestone was passed on Friday, after Discovery, along with her astronauts and controllers, successfully completed the Terminal Countdown Demonstration Test (TCDT). With the dress rehearsal completed, engineers are pushing forward with troubleshooting on a Air Half Coupling (AHC) flight cap, a Quick Disconnect (QD) which is leaking small amounts of hypergolic vapors.
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The TCDT allows the full launch team and astronauts to practise the countdown, which resulted in a nominal T-0 at 11:00am local time. The operations also include the routine Flight/Ground Crew Emergency Egress Training, known as S1025, allowing the teams to practise evacuations procedures.
The business end of the TCDT works from the perspective of a nominal countdown, similar to the previous pre-STS-133 sims, known as the Countdown Simulation (S0044), which took place a month ago. However, S0044 runs are designed to throw numerous problems at the controllers, allowing them to work on mitigation procedures. The TCDT is a more nominal event.
For the TCDT, the STS-133 astronauts also receive a briefing from the engineers, outlining the work that has been carried out on their orbiter during her processing flow to be presented. A 23 page TCDT presentation (L2) also notes Discovery’s modifications for what will be her final mission, and provides the crew with a list of potential “messages” they may receive – and at what times during the count – once they ingress the orbiter on launch day.
Discovery remains on track via her pad flow milestones, with the current plan showing her Payload Bay Doors (PLBDs) will be closed on Saturday, ahead of ordnance connection operations next Monday night.
“OV-103 (STS-133) Got the payload installed in the vehicle and completed the interface verification testing. Got the Orbiter Midbody Umbilical Unit (OMBUU) mated. (Worked) the terminal countdown demonstration test (TCDT),” noted KSC Ground Operations (L2). “This weekend, the PLBDs will be closed on Saturday. Next Monday night, will pick up with ordnance connects.”
In order to allow for the closure of the PLBDs, Discovery’s payload has been undergoing closeout operations. This task – known as S08133 operations – successfully negotiated an Interim Processing Report (IPR) earlier in the week.
“IPR-46: During PMM (Permanent Multi-Purpose Module) 28VDC shell heater verification, 2 of 22 heaters did not cycle on. Affected heaters are FCON 1 and 3. The current should be 1.4 amps, is 0.0 amps. The troubleshooting plan is to increase the PCR/payload bay purge flow rates and lower the purge temps to see if the 2 heaters will kick on,” noted the NASA Test Director (NTD) processing reports (L2).
Engineers cold soaked the Payload Bay to 65 deg F, allowing for the heaters to be retested. which showed nominal results.
“S08133 Payload Closeouts: Payload closeout walkdowns, inspections, and photos will be performed tonight/tomorrow morning,” added Friday NASA Test Director (NTD) processing information. “Payload bay door closure for flight is planned for 1200 tomorrow (Saturday).”
This issue – related to a Quick Disconnect (QD) or Flight Cap – remains outstanding, as engineers work through one of several potential troubleshooting paths. First word of an issue came on Tuesday night, when pad engineers smelt a fishy smell coming from the aft of Discovery – a sign of fuel vapor in the air. (Fuel smells like fish, oxidizer smells like bleach).
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“On Tuesday night, IPR 47 was picked up on fuel vapors on the RH (Right Hand) doghouse door on the right OMS (Orbital Maneuvering System) pod,” noted Ground Operations. “They went in and looked, and there is a suspect QD issue there on a low point bleed QD used on the pod R&R. They will go in Friday after the TCDT is completed and hopefully clean that QD up and solve that problem.”
“New IPR 0047 taken by OMS/RCS (Reaction Control System). Small hyper fuel leak detected in the vehicle aft: At about 1745 EDT Tuesday techs at the pad reported smelling small whiffs of fuel vapors around the aft end of the vehicle near the milk stools,” added the NTD report.
“Safety eventually entered the aft to find a leak source on the right hand cross feed line interface near the right doghouse door. The leak appears to be very small. OMS/RCS engineering is investigating. The area remains cleared of personnel.”
The suspected QD/Cap is located in the area of the crossfeed system, located between the two OMS Pods – with the crossfeed lines allowing for propellant to flow between the two OMS tanks. The area can be accessed by engineers via the 59-64 door – a triangular door located on the aft bulkhead of the orbiter.
“The 59-64 door was removed to investigate the elevated toxic vapors detected from the right hand doghouse door seam. Elevated source readings were found at Air Half Coupling (AHC)-679 with no visible liquid,” added an NTD update. “Engineering suspects that the poppet opened when a vacuum was pulled on the cross feed line in preparation for fill and may not have re-seated properly.
“Going forward, the plan is to bleed the AHC, remove the cap, clean the AHC and install a new cap. A new cap is on order. The planned timeframe is post-S0017.”
The AHC is only brought into play during Orbiter Processing Facility (OPF) flows, for the purpose of crossfeed line deservicing/purging/leak checks for OMS Pod removal and installation. It is specifically located on the low-point drain line, which – when the orbiter is positioned in the vertical – is more or less in the same horizontal plane as the crossfeed line itself.
Interestingly, Discovery’s Right Hand Orbital Maneuvering System (ROMS) Pod underwent a demate and remate to allow for repairs in the Hypergolic Maintenance Facility (HMF) over the summer. These repairs were called for due to the need to replace a faulty valve – noticed ahead of STS-131’s launch – on the Right Reaction Control System (RRCS), which could only be access via the OMS Pod being demated from the orbiter.
A troubleshooting plan was put into effect late this week, which will eventually call for the increase of pressure in the fuel crossfeed line by turning on heaters for the line, and pressure should reseat the AHC poppet. Engineers would then replace the flight cap. However, If the AHC fails to reseat then a replacement of the AHC would be required.
If the AHC requires replacement, the process would entail a large amount of work, revolving around the deservicing, repair and retest procedures. While such work would impact the flow, Discovery has around four days of contingency based on the November 1 launch date.
First on the list of troubleshooting tasks was the bleed of the flight cap, allowing for its removal and preparations for the checks on the AHC.
“MD679 flt cap bleed is complete. No bubbles noted and the center of the cap moves freely (no trapped pressure),” noted an engineering update. “Flight cap will be removed after TCDT. We will attempt to clean the AHC to flight requirement and install a new cap for flight.
“If the AHC does not clean up, we will install a temp flight cap and pressurize the crossfeed using aft and pod purge, and repeat the process.”
The final update came via the NTD ahead of the post-TCDT work, which noted the current path the troubleshooting will take until the replacement of the AHC would be required.
“Troubleshooting continues. Bleed of the MD679 flight cap was performed yesterday. No bubbles were noted and the center of the cap moves freely indicating no trapped pressure,” the NTD confirmed on Friday. “Going forward, the flight cap will be removed today after TCDT.
“The AHC will be cleaned to flight requirement and a new flight cap will be installed for flight. If the AHC does not clean up, a temporary flight cap will be installed and the cross feed pressurized using aft and pod purge, followed by another bleed check.”
(UPDATE: Repair failed. See new article)
The work also gained a mention in the latest Shuttle Standup/Integration report (L2), which provided Mission Management Team (MMT) manager LeRoy Cain an opportunity to praise the teams working on the problem.
“There is good weather here in Houston. Not sure about the weather in Florida, but there are couple more weeks before we really have to worry about it. The crew is in Florida for the TCDT. The team is doing an outstanding job working through the late breaking issue with the QD leak, ensuring that we have our arms around the problem and have a forward plan,” noted Mr Cain.
“The mitigation plan for being able to deal with it during the TCDT operations is good, and it is really good to see the team working through whatever problems we have as we lead up to launch.”
Mr Cain also pre-empted the Agency Flight Readiness Review (FRR), which has been delayed until just days prior to the current launch date. Despite this, Mr Cain noted all departments remains on track to complete the “paper” element to the pre-launch preparations and confirm November 1 – pending successful resolution of the QD/Cap issue,.
“There is a lot of activity going on around the Agency, and within Shuttle Program obviously there is a lot of activity. It looks like we are on track to spend another week in preparation before we go to the Agency FRR,” Mr Cain added. “By all indications we are where we want to be.
“There will be other things for us to deal with between now and then, certainly between now and T-0. But the team is poised and ready to deal with those things, as always.”
(Lead Photograph: Larry Sullivan, NASASpaceflight.com and MaxQ Entertainment. Graphics and other photos via L2)