NASA checking into STS-120 ascent events and ET foam loss

by Chris Bergin

Images and video of Discovery’s ride uphill during STS-120’s are being pored over by NASA, as the crew continue their Flight Day 2 inspections of the orbiter’s heat shield.

So far, at least one debris impact was observed during ascent, which is being evaluated along with an off nominal plume during first stage, a slightly protruding gap filler, and what appears to be a clean ET-120 – although one area – a LO2 Ice Frost Ramp – at the top of the ET did suffer a large liberation.

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Tuesday’s launch was placed into doubt when ‘clear’ ice was spotted by the inspection team. The area, which was starting to melt during the countdown, was located on the LH2 Umbilical pyro can closeout.

However, the Mission Management Team (MMT) decided to go ahead with the launch, believing that the ice would fall off at SSME (Space Shuttle Main Engine) ignition, thus removing itself as a potential debris hazard during ascent. They were proven correct.

‘During the final-countdown Ice-Team walk down, clear ice with frost was noted near the LH2 umbilical pyrotechnic canister closeout. The ice was 4-in long by 1.5-in wide by 0.5-0.75-in thick. This ice exceeded the requirements of Launch Commit Criteria (LCC),’ noted information on Wednesday.

‘After assessment, a waiver was taken as the ice was attached to the Kapton purge barrier and it was expected to liberate at Space Shuttle Main Engine (SSME) and/or Solid Rocket Booster (SRB) ignition, based upon Eglin Air Force Base testing that demonstrated that ice does not bond to Kapton.

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The remainder of the ice (approximately 10-percent) was bonded to the fire retardant paint and was expected to remain through ascent based on testing. In the event that the ice did not release as expected, using the bounding ice-on-tile damage model and the Expected/Mean ice-on tile damage map, the ice release was determined not to cause critical TPS damage.

‘Launch video later confirmed that the majority of the ice did fall off at ignition.’

During ascent, a few items of interest were noted by controllers on the ground, specifically with SSME 1 and the Main Propulsion System (MPS).

‘At approximately three minutes after lift-off, the SSME 1 Liquid Oxygen (LO2) inlet temperature transducer dropped to off-scale-high (OSH). The temperature sensor resumed operation briefly after Main Engine Cutoff, but again dropped out approximately 2 min later,’ added information. ‘This condition did not affect the operation of the SSME or impact the ascent.’

Classed as off nominal, a plume was observed on tracking video, which appears to originate from the intertank region of the External Tank – however, no further information has been noted since, and it is unlikely the tank would have been venting during first stage without it registering with controllers on the ground. A similar ‘plume’ was observed during STS-121 and STS-114, and is deemed to be condensation. 

Early imagery acquired last night showed at least one debris strike during ascent, although the time of the strike – well after Solid Rocket Booster (SRB) separation – will add confidence that it would not of caused any TPS (Thermal Protection System) damage on the orbiter.

Initially, it appeared ET-120 performed well, with only a small amount of missing foam, as observed by ET photography, taken by the crew after they separated from the tank at MECO (Main Engine Cut Off). However, sources note one area of liberation is being taken more seriously.

This is understood to be related to the top of the ET, which hi res images showing what appears to be a large foam loss event – although it is not yet known how large the liberation is in mass. 

Strangely, Deputy Shuttle manager John Shannon chose not to even mention this specific area, pointing out one area that was a much smaller liberation event.

Flight Day 2 is seeing the crew carry out inspections of Discovery’s TPS, via the OBSS (Orbiter Boom Sensor System). So far only one protruding gap filler has been flagged – one which is unlikely to be of any concern as it is only seen protruding by a very small amount.

Today’s scans are being sent back to technicians on the ground for a complete checkover, as part of clearing Discovery from the ascent section of her mission.

Another check of the orbiter’s TPS will be conducted on Flight Day 3, when Discovery arrives below the International Space Station (ISS). Before docking, she’ll be ‘back-flipped’ under the station for the Rbar Pitch Maneuver (RPM), which will allow ISS crewmembers to take detailed photography of Discovery’s belly.

The OBBS inspections have now been completed, with the crew finalizing their preparations for Thursday’s docking with the ISS.

Hatch opening is expected at 10:33am, followed by a safety briefing, OBSS handoff from SSRMS (Space Station Remote Manipulator System) to SRMS (Shuttle RMS) at 11:23am.

Soyuz seat liner transfer (for the Tani/Anderson exchange), and preparations for the first spacewalk, EVA-1, by EV1 Parazynski and EV2 Wheelock, on October 26, preceded by their overnight Campout in the Airlock (A/L) for denitrogenation/pre-breathe.

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