STS-134: Endeavour’s brake fire proving to be a mystery

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As engineers continue to safe Endeavour, following her highly successful final mission, an investigation into the observed brake fire – as the orbiter came to a stop on the Shuttle Landing Facility (SLF) runway – is struggling to find a specific root cause. Elevated to the Flight Readiness Reviews (FRRs), the STS-134 incident is classed as a situation that would not be catastrophic and should not happen again.

STS-134 Post Mission Deservicing Latest:

A large amount of work is being carried out on Endeavour inside her Orbiter Processing Facility (OPF-1), as post mission safing and initial deservicing work is carried out on the now-retired orbiter.

Engineers have already removed the “44 door” for midbody access, removed the Waste Containment System (WCS) and are continuing with the removal of the numerous cameras on the Remote Manipulator System (Shuttle Robotic Arm).

The main focus of work over the next few days revolves around the preparations to remove Endeavour’s Forward Reaction Control System (FRCS) – which will eventually be shipped to White Sands – and the dome heat shields around Endeavour’s three Space Shuttle Main Engines (SSMEs).

“The PLBDs (Payload Bay Doors) were opened earlier in the week. In work with removing the payload at this time,” noted KSC Ground Operations via the latest Shuttle Standup/Integration report (L2). “Working preps for engines. Engines will start coming out late next week.”

First on the list of SSME related tasks was the requirement to position the SSMEs engine bells (nozzles) – and other hydraulic powered systems – to their correct positions for the upcoming work.

“Hydraulic operation to position the SSME’s to null, RSB (Rudder Speed Brake) to closed, elevons to null, body flap down have been completed,” noted the NASA Test Director (NTD) report (L2), adding that technicians took the opportunity to check the SSME controller which suffered from a minor – and short-lived – issue during ascent.

The SSME controller was powered up to troubleshoot a Low Pressure Fuel Turbo Pump (LPFTP) discharge pressure transducer that failed low during ascent. Troubleshooting was unable to recreate the anomaly. The sensor will be removed and taken to the design center for further analysis.”

Pratt & Whitney Rocketdyne/KSC – who care for the engines that may find thsemselves with a role on the Space Launch System (SLS) – also noted that an X-Ray took place on the sensor, which also found nothing amiss.

“No anomalies have been found so far during testing,” confirmed their notes on the Standup report. “The sensor was removed and sent for x-rays. A micro-focus X-ray was performed, and no anomalies were found.”

SSME dome heatshield and body flap carrier panel removal began on Friday, while the heatshield removal will continue through the weekend.

Engineers have also removed the three Flow Control Valves (FCVs) from Endeavour’s Main Propulsion System (MPS), ahead of being shipped to California for inspection.

Although these valves will never be involved in another shuttle launch, the intense inspections – implemented since part of a poppet on a valve liberated during STS-126 – are still being carried out as part of the routine due diligence approach that is commonplace for the Space Shuttle Program (SSP).

“USA Logistics (USA/KSC) In preps to ship the FCV off OV-105 (Endeavour once they are removed. Vacco is ready to receive them and start the inspection, and have data for the Engineering team,” added noted on the Standup report.

STS-134 Specific Articles: http://www.nasaspaceflight.com/tag/sts-134/

“(Managers) added that some might wonder why the FCVs are being looked at since we have the ship set and the spare for OV-104 (Atlantis). Orbiter just wants to follow through and ensure that nothing is discovered from these FCVs that might call into question the flight rationale being used.”

Brake Fire Latest:

Engineers are continuing to evaluate the observations of a small fire – lasting around 40 seconds on one of Endeavour’s Left Main Landing Gear brakes – at the end of the braking period as the orbiter came to a stop on the SLF after landing.

“In response to the very shortlived brake fire observed during landing of STS-134, a Problem Report (PR) was (created). Per the PR, brake samples were taken, the rolling resistance of the left inboard wheel was measured, and the wheel and tire assembly was removed,” noted investigation notes (L2).

With engineers removing the tire/wheel from the left hand inboard landing gear, before performing a thorough inspection, no real indications of a fire have been observed. Even a couple of strips of plastic tape in the location showed no indications of being over heated, which is highly unusual for an incident of this nature.

The next step was to apply hydraulic pressure to the brakes to see if there are any signs of an hydraulic leak. It was expected that a very small hydraulic leak would be observed, leading to a potential root cause. However, the checks came back nominal.

One of the investigation photos of the disassembled wheel show the what the brake assembly looks like after Endeavour returned to her OPF, with an engineering explanation noting the hardware in view consists of rotors and stators, which are discs of the carbon/carbon material.

The stators fix to the axle and the rotors spin on the axle, with the two parts forming a sandwich configuration.

Another photo shows the interior of the wheel, with the torque bars bolted to the wheel. When the wheel is put in place on the axle, these bars engage the slots on the brake rotors, which basically locks the rotor to the wheel.

On the inside end of the axle assembly there is a ring of hydraulic pistons around the entire circumference of the axle, which push against the entire brake assembly forcing the rotors and stators together to result in the braking action.

It was also noted that the amount of dust seen on this wheel was not out of the ordinary, but on the high end of what the engineers normally see.

So far engineers have found no obvious evidence of the fire, and the hydraulic pressurization test of the brake pucks did not show any leakage, meaning the event may be classed as an Unexplained Anomaly (UA).

“On the STS-134 brake fire, the team has been looking at the brake area extensively but has not really found anything yet,” noted the Orbiter Project Office (OPO) on the Standup report.

“They know there are no hydraulic leaks, did not find excessive amounts of brake dust or extra grease. There no evidence of the fire except that one of the heat shields appears to be a little darker than the others. They will continue to examine the area and take samples.”

Classing the incident as an UA is allowed, providing the problem is orbiter specific and unlikely to occur again – as in the case with Endeavour’s incident.

However, it will be reviewed ahead of Atlantis’ mission, per the STS-135 Flight Readiness Reviews (FRRs) – which are starting to ramp up ahead of the SSP FRR on June 21, which is the main review ahead of the launch date-setting Agency FRR one week later.

“They (engineering) are nearing the point that they may not know what caused it, but it is a situation that would not be catastrophic and should not happen again,” added the OPO. “This will be discussed at the FRR.”

(Images L2 Content. Post STS-134 coverage of Endeavour is continuing in L2′s specific section, which will follow Endeavour through post flight all the way to her retirement home in California).

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