STS-134: Endeavour closing in on S0007 – ALCA-2 root cause update

by Chris Bergin

Endeavour is on track to head back into Launch Countdown (S0007) operations on Friday, as her engineers complete tasks associated with reconnecting the ordnance to the stack and closeouts. All retests on the Aft Load Controller Assembly (ALCA-2) are complete and nominal, with a potential lead on the root cause of the April 29 launch scrub failure on Endeavour’s Auxiliary Power Unit (APU-1) heater system.

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With the pad flow back into the business end of preparations for the launch countdown, engineers will begin their complex ballet of operations with Call To Stations (CTS) at 6:30am on Friday morning, with the countdown clock ticking down from T-43 hours just 30 minutes later.

Because of the April 29 scrub, the return to a nominal flow has included a few turnaround related changes, albeit minor, such as with an experiment on the middeck, along with one of the batteries on the Solid Rocket Boosters (SRBs).

“The Payload Bay temperature was adjusted and increased as requested by the Payload community to help preserve science on the STP H3 USAF experiment,” noted the NASA Test Director (NTD) processing update (L2). “SRB Element has requested to change out one of the two EDAS (Enhanced Data Acquisition System) batteries in the SRB forward skirts (completed).”

Checks on the boosters and the Range Safety system are complete, with included a recharge of the batteries which power the cameras on the SRBs. These cameras provide the footage of both ascent and the return to Earth after separating, downloaded and sent to imagery experts once the boosters are returned to Port Canaveral.

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S5009 tasks relating to the final Ordnance Installation/Connection operations were also successfully completed, as closeouts on the vehicle were all-but finalized, with the flight door now installed on the orbiter during late Wednesday pad processing. The External Tank Vent line has also been closed out, allowing for the retraction of the access arm.

One minor element of troubleshooting, relating to a Mission Event Timer display issue, has also been resolved, allowing for a fully functional set of flight deck displays to be used by the STS-134 crew during their mission.

In fact, bar the troubleshooting work on the ALCA-2 fault, relating to the APU 1 String B Heater Failure during the late April countdown, Endeavour has behaved well during her extended stay at the pad, with only one new Interim Problem Report (IPR) of note being charged to her flow. Ironically, the new issue listed another LCA item of interest.

“New IPR 0057 was picked up during the SRB ignition/sep and launch release PIC test. The LCA 3 interrupt box failed its PIC test which constrained continuing. During troubleshooting, the test/active switch was cycled 3-5 times, cleaning the contacts, resolving the issue,” noted the NTD overview.

“Checked the circuit with a resistance test, which passed, and successfully re-ran the SRB ignition/sep and launch release PIC test. The IPR will close.”

Notably, this since-cleared issue has no relation to the LCA-2 issue, given the LCA 3 interrupt box is part of the Ground Support Equipment (GSE) – otherwise characterized as the “ground side” – and not part of the orbiter.

On the Aft LCA-2 issue itself, retests of the system since the replacement of the box – along with the installation of a new set of wiring and thermostats along the path to the heater – have been completed, with no issues noted. This completes a great effort by the technicians working in the tight confinements of Endeavour’s aft.

“On Friday, WSB (Water Spray Boiler) vent and FES (Flash Evaporator) checkout were completed and the Orbiter was powered down,” listed an overview of work (L2) which was undertaken on retesting the numerous systems associated with ALCA-2.

“On Saturday, completed installing and routing the new heater wiring and APU replaced the two overtemp thermostats. On Saturday night, the Orbiter was powered back up and completed heater testing. All heaters reported nominal current. ALCA2 connector 55P320 was then remated to support APU retest.

“APU controller and heater retest began late Saturday night and was completed by Sunday night. All retest was nominal. APU pre-door confidence test was completed. Avionics Bays 4 and 5 are closed back out for flight. APU heater/thermostat insulation installation completed. Successfully performed ammonia system B ALCA2 driver logic verification, completing all retest associated with the LCA R&R.”

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Though officially still tagged as an Unexplained Anomaly (UA) – which satisifies Flight Rationale given all potential suspects for the blown A49 Type III Hybrid Driver have been replaced on the vehicle – a potential root cause is being traced back to work on Endeavour when she was inside her Orbiter Processing Facility (OPF) last year.

Related to OPF work to overheat an overtemp thermostat on the system – in order to test it would cut out as designed – a data review showed a spike of current just prior to when the circuit was cut.

“We were testing thermostats back in June (involving the) heating up of overtemp thermostat (in order to show) it really does cut out and stop the circuit,” noted Mission Management Team (MMT) chairman and Space Shuttle Launch Integration manager Mike Moses.

“It did what it was supposed to do, but if you look really close at the (high speed) data, there was a spike of current right before that happened – much higher than we’ve ever seen before.”

While Mr Moses noted the finding – which was “late breaking news” on Monday, as he addressed the STS-134 status to the media – was preliminary, additional information later in the week continued to focus on the OPF work related to what is known as the S12B thermostat.

With the overtemp thermostat in question now removed from Endeavour, engineers have noticed a bare conductor on one of the leads near the head, leading to a theory that the metal tip on the heat gun – used by the technician to overheat the thermostat – may have accidentally brushed the aforementioned exposed/bare wire, causing the momentary short.

It’s also possible the metal tape which surrounds the thermostat may have also come into contact with the exposed wire, given the requirement to temporarily remove the tape in order to carry out the test. If confirmed, the root cause would then be related to that specific area of the heater leg, and the likely cause of the short in the ACLA-2 when it was powered up on launch day.

If so, given the replacement of the thermostat and associated wiring, Endeavour will not suffer from a similar incident on May 16.

The effort to get Endeavour back into a launch stance also earned praise from Space Shuttle Program (SSP) manager John Shannon, who also noted his admiration for the calm decision to scrub the launch when the Launch Commit Criteria (LCC) was breached by the APU-1 heater fault.

“The team has done a really nice job of going through the troubleshooting and identifying where the issue is. (Mr. Shannon) thanked the team for their work on launch day, and not letting the pressure of VIPs affect them,” noted the closing remarks on the latest Shuttle Standup/Integration report (L2).

“It was clear that once there was an issue with the B-heaters, that there was a violation of launch commit criteria, and when working through troubleshooting, it was clear that there was a bigger electrical problem. The team did a really nice job of agreeing unanimously that we were not ready to fly and needed to step into some significant troubleshooting.”

(All images via L2. Extensive coverage is being provided on the news site and forum, driven by the L2 special sections Рwhich includes a large APU-1/LCA troubleshooting update area, from which this article was created from).

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