STS-124: FRR debate outstanding issues – faulty MDM removed

by Chris Bergin

Discovery’s STS-124 mission remains on track, pending the satisfactory outcome of four issues – all of which are heading to the agency Flight Readiness Review (FRR) next Monday.

A successful re-test of Discovery’s General Purpose Computers (GPCs), final confirmation that elbow camera clearances are within spec, a satisfactory status update of the Russian Soyuz investigation, and the on-time delivery of a CO2 scrubber are required to remain on track for May 31’s launch dates.

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Pad processing is continuing on schedule, with the next major milestone relating to orbiter aft closeouts, which begin on Friday 2nd shift. The only new issue noted on Thursday processing reports relates to power supplies, which is undergoing an engineering evaluation.

‘The 1D100 and 1D500 power supplies failed to return to zero volts indication following vehicle power down,’ noted the latest processing report. ‘The constraint right now is paper only. Awaiting engineering evaluation.’

With the SSP (Space Shuttle Program) FRR concluding this week, the launch date will be set by the upcoming Agency FRR – set to meet on Monday.

The two issues relating to Soyuz’s ability as a ‘solid rescue vehicle’ at the International Space Station (ISS), and the on-time delivery requirement of the CO2 scrubber have previously been reported. Below is an expansion on the other two items of interest.

Discovery’s GPC Update:

A subject of FRR interest is the other issue noted during the pad flow, specifically the cause of an off-nominal 2-1-1 spilt of the GPCs – believed to have been caused by a faulty MDM (Multiplexer/Demultiplexer) card (FA2).

Following approval by Wednesday’s Noon PRCB (Program Requirements Control Board) to carry out the removal and replacement of the faulty hardware from Discovery’s aft, engineers completed the removal task overnight.

Re-testing will take place over the next 24-48 hours, though an item of interest was found during the removal of the faulty MDM – which was subsequently replaced on Thursday afternoon.

‘The go to R&R MDM FA2 was given yesterday afternoon and the R&R was worked overnight,’ confirmed Thursday processing information. ‘After MDM removal, a scratch was found on the cold plate. Mold impressions were made and are under evaluation.’

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The cold plates – which transport the heat away from the electronics boxes – have had scratches observed in previous flows. However, despite the ability of scratches to change the heat transfer characteristics, the scratch observed is understood to be too small to have any relation to the MDM failure.

‘S0024, pre-launch propellant servicing, will pick up with fuel loading operations after the portion of the retest to support S0024 operations is complete,’ added processing information. ‘Further MDM retesting requirements will continue through Saturday including Orbiter and SRB hydraulic operations.’

While the re-test is almost certain to correct the issue with the GPCs, the root cause is the main focus of attention that Monday’s FRR will be tasked with debating.

Engineers are certain the now-replaced MDM caused the problem, backed up by the findings from the retests performed overnight. However, what caused the MDM to fail – and the subsequent issues with the GPCs, remains unknown.

The fault tree shows that the GPCs – which read data and send commands to the MDMs – first encountered the problem when GPC 4 received erroneous (non-universal I/O Errors) data, which did not match the other GPCs in the redundant set. As programmed, GPC 4 was voted out.

Three seconds later, the same thing happened with GPC 2. At that time GPCs 2 and 4 were powered down and an MDM FA2 BITE Status Register (BSR) read was performed on MDM FA2 port 1, causing GPC 3 to fail out of set with GPC 1. This is known as a ‘2-1-1 Split’.

At that point, GPC1 was operated in simplex mode to obtain more data. Engineers terminated the work in process and powered down the vehicle for the changeout of the suspect MDM.

When re-testing is completed at the weekend, the engineers will present their full overview to the FRR, who will ultimately decide if they are happy to proceed with the May 31st launch – even without root cause. This is deemed likely, given the MDM is classed as the problem, which has since been removed from the fault tree.

Elbow Camera Clearances:

This issue is more related to double checking the clearances of the elbow camera on the end of the SRMS (Shuttle Remote Manipulator System) arm and the giant Japanese payload.

“Got some pictures of elbow camera installation and the strap. KSC GO was also in the process of taking measurements for us,” noted the Orbiter Project. “Based on the initial picture, we were concerned that the camera might be in a little different than the 0 degrees position, so had a telecon.”

“Asked the Cape to take an additional measurement. Based on this additional measurement and photo analysis, we determined that the camera is 5 or 5.5 degrees off of the 0 based on the pictures, or based on the one measurement – 3 degrees off.

“All of these are within the allowable 6 degrees it could be in and still maintain positive clearance. Some of the guys are looking at the strap, but based on where the camera is, Orbiter does not have a problem with clearance for ascent.”

The camera has a strap and a wedge to hold it in place – and away from contacting the JAXA Pressurized Module (JEM) – during the ride uphill. There isn’t a plan to reopen the Payload Bay doors, unless the FRR requests it.

“On the elbow camera, need to remember this is the tightest clearance for any cameras for any of the flights we’ve ever flown. Had to build a special GFE (General Flight Equipment) wedge to move the camera away from the JEM.

“Want to understand the strap configuration, and make sure that has not changed the dynamics of the situation. Will get the team together to make sure have a good story going forward to the FRR.

“There is no plan to open the doors, but we have worked with KSC and there is an opportunity we need to get in there and reposition that camera.”

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