STS-124: Opening TPS inspections begin on Flight Day 2

by Chris Bergin

Discovery and her crew are proceeding through Flight Day 2, following a successful launch on Saturday. FD2’s main objectives include the opening inspections of the orbiter’s Thermal Protection System (TPS).

Due to the payload bay clearances with the giant JEM PM (Japanese Experiment Module Pressurized Module), Discovery’s inspections have been carried out minus the use of the OBSS (Orbiter Boom Sensor System) in these opening evaluations on Flight Day 2.

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**STS-124 News Articles**

STS-124 Latest:

Only two issues with the orbiter have been noted so far, both of which hold no mission impacts as Discovery closes in her arrival at the International Space Station (ISS) on Monday.

The O2 flow meter on Fuel Cell 3 (FC3) was behaving erratically for around 60 minutes after MECO (Main Engine Cut Off) yesterday. However, the problem has not returned since, leading to no work being required.

Also noted, the Left OMS Secondary TVC (or gimbal) positions failed to null in both pitch and yaw, caused by either a failure in the L OMS Secondary TVC power supply, or instrumentation. This is being evaluated by engineers on the ground.

Currently, the forward plan is to utilize remaining on-orbit OMS burns with single engine use of the Right OMS. The deorbit burn – which required both right and left engines – will use the active Left OMS Primary TVC string, as the issue relates only to redundancy.

Flight Day 2 Operations:

Flight Day 2’s activities include rendezvous preparations for Monday’s docking with the ISS, involving centerline camera installation, EMU spacesuit checkouts, docking ring extension and rendezvous tools checkout. However, the primary focus of today will be inspections of Discovery’s TPS.

Evaluations into Discovery’s ascent have been continuing since her launch from the Kennedy Space Center (KSC), with a team of engineers on the ground processing through vast amounts of data and imagery.

This required process for every mission is becoming a fine-tuned exercise, aided by several areas of data gathering that continues through to at least Flight Day 3.

Ascent imagery, radar, and onboard cameras provide the opening salvo of data for engineers to check over, prior to today’s inspections on orbit. Flight Day 3’s RPM (Rbar Pitch Maneuver finalizes the full overview of the orbiter’s TPS health.

Opening evaluations normally include downloaded imagery from the External Tank (ET) umbilical well, located on the belly of the orbiter, though this failed to download via a computer issue onboard Discovery – leading to a request for a second attempt during FD-2.

This is required to gain an insight into ET-128’s performance during ascent, including the pinpointing of where observed foam liberation shed from the tank.

Currently, engineers have no concerns from the initial ET camera views of a couple of debris events – all of which were outside the time of ascent that can cause damage, should they hit the orbiter’s TPS.  

Flight Day 2 TPS Inspections:

While the normal FD-2 TPS inspections will not be possible, a limited check of Discovery’s TPS will be performed using the End Effector (EE) of SRMS. While the use of the SRMS will help provide insight on the health of Discovery’s TPS post-launch, the capability of the SRMS is limited.

A CGI Overview Video of FD-2’s SRMS scans is available on L2.

Due to the same clearance issues that prevent Discovery from carrying an OBSS to orbit, the elbow camera of the SRMS is locked into a position to prevent it from contacting the Kibo module and/or the port radiator during launch, thus making it virtually unusable until the restraint is removed during EVA-1.

‘Flight-specific Starboard and Port wing RCC survey auto-sequences have been developed from existing EE surveys. Starboard survey expanded to include more area on wing glove and wing tip,’ noted an associated presentation.

‘Limited by SRMS reach and clearance to payload bay cargo, so only the upper surface can be scanned. Port survey covers more area on wing tip, but at an increased range than the generic survey. The increased range is primarily due to limited clearance viewing under the payload bay door without the elbow camera.’

In addition to the physical constraints of the SRMS, lighting will play a key role in the timing of the TPS inspections. Unlike the OBSS which can effectively operate during orbital night, the End Effector of the SRMS does not have this capability.

Once Discovery arrives at the Station, preparations will begin for the retrieval of the OBSS – left at the ISS at the end of STS-123 – with the SSRMS grappling the boom shortly after docking.

The next day, two spacewalkers will remove the thermal cover from the sensor packages, physically release the OBSS from the OSE, and disconnect the KAU. Once this is complete, the SSRMS will maneuver the OBSS to a predetermined position where the Shuttle arm will grab hold of the boom.

Following the retrieval of the OBSS, the flight crew will perform a serious of checkout operations on the sensor packages similar to what was seen during the STS-120 mission last fall.

‘Following EVA-4 during STS-120, Imagery Ops requested specific positioning for LCS and IDC sensor checkout prior to Late Inspection. OBSS was unpowered for around nine hours during EVA-4. TPS community has requested checkout similar to that performed during STS-120.’

This checkout will be performed on FD-5 to verify that all sensors are functioning normally ahead of Focused Inspection activities on FD-7 (if required) and Late Inspection operations on FD-11.

The customary Late Inspections operations will see several changes to accommodate the need to conduct a complete scan of the Discovery’s TPS.

‘Inspection procedures will be executed as if they were performed on FD2. Wing glove and crew cabin portions of survey will be performed (usually FD2 only). Crew requested that the post-undock surveys not be referred to as Late Inspection to avoid inadvertently performing callouts to skip steps for ‘Late’ inspection.’

In addition to Late Inspection procedures, an extra flight day has been added to the mission timeline to provide enough time to analyze the OBSS data.

More to follow as updates are available…

L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, updated live.

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