Orbiter related Ice liberation threat discussed ahead of STS-119

by Chris Bergin

Following a run of “clean” flights during 2008 – thanks to continued progress in reducing foam liberation from the External Tanks – the Space Shuttle Program (SSP) Flight Readiness Review (FRR) focused on the threat of ice liberation during ascent for 2009’s opening mission, STS-119. This follows a near miss with a large piece of ice during STS-126.

An amazing amount of work has taken place with the tanks that are constructed at Lockheed Martin’s Michoud Assembly Facility (MAF) in New Orleans, successfully rising to the challenge of reducing the threat of debris – mainly in the form of foam – shedding from the tanks during ascent, due to the potential of damaging impacts with the orbiter’s Thermal Protection System (TPS).

ET-129, which flew with Endeavour during STS-126’s launch, was the cleanest tank yet, with only a few areas of minor foam loss.

“(4) observations elevated as Post Flight Assessment Reports (PFARs) and assessed as IFA candidates. No new failure modes identified. No constraints or corrective actions required for STS-119/ET-127. No IFAs recommended for STS-126/ET-129.”

All areas of foam loss were deemed as “understood” and relate to either cryopumping during ascent – a known condition, or small areas of manufacturing damage suffered due to “high traffic” of engineers working in certain areas of the tank. Corrective actions have been noted on latter, via the FRR documentation.

However, STS-126 did suffer one debris threat during early first stage flight.

Although ice is a debris threat from the ET, the STS-126 event originated from Endeavour herself, according to ascent imagery. However, the debris failed to impact the vehicle.

“During STS-126, at approximately 26.7 seconds, debris was observed to liberate between the LH2 T-0 umbilical and port OMS pod,” noted one of 43 FRR presentations, available on L2.

“Imagery analysis concluded that liberated ice measured approximately 11.5” x 2”,” noted the presentation on the size of the ice debris. “Imagery and Debris Team trajectory analysis confirmed no contact with the Orbiter.”

The T-0 umbilicals – located on each side of the orbiter – and mated to the orbiter via retractable plates in the Tail Service Masts (TSMs), which can be seen either side of the orbiter’s aft.

As the name suggests, the T-0 umbilcals are retracted just as the vehicle is lift-offs, and it is thought the ice developed on the carrier plate of the LH2 umbilical during tanking. The ice remained with the orbiter before liberating 26 seconds later.

“KSC led resolution team to determine root cause for STS-126 T-0 ice formation,” noted the FRR presentation. “Decision to perform full analysis to assess risk for future flights over range of masses and release times.”

Engineers studied video taken of the T-0 umbilical area during lift-off, and included historical footage all the way back to STS-51L, and reviewed potential impact threats to several areas of the vehicle. Image left from STS-126’s 330mb engineering launch video on L2 – angle from inside the TSM during LH2 T-0 umbilical retraction.

“Produced impact conditions for Orbiter elements: Side Fuselage, OMS Pods, OMS Nozzle, Body Flap, Upper Wing, Elevon, and SSMEs (Space Shuttle Main Engines, plus RSRB (Reusable Solid Rocket Motors) and ET impacts.”

Work is still taking place on threat analysis, following checks of the pad’s GSE (Ground Support Equipment) on the Mobile Launch Platform (MLP), and on the umbilical connection area on Discovery.

“Purge test on MLP 1 LH2 Tail Service Mast (TSM) Umbilical Helium flow test completed,” noted the status of the investigation. “Inspection of T-0 Umbilical Carrier plate perimeter seal interface on OV-103 (Discovery) T-0 Plate completed.

“Debris Transport Analysis (DTA) of ice/frost release in work to characterize risk. Fault Tree Analysis and Block closure is in work. Assessing Ground Support Equipment hardware and process options to prevent ice/frost or have it liberate at/before T-0.”

Preventative actions for future flights will be the only outcome of the investigation, as opposed to being a threat for STS-119’s launch.

The only threat to launch at present relates to the Flow Control Valve (FCV) changeout, which is taking place over the next week.

While the changeout will be performed in time for launch, flight rationale for the spare valves on Discovery – and the LON (Launch On Need) orbiter Endeavour – is still being built.

This issue remains the one constraint for flight as per the SSP FRR last week.

L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.

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