Discovery undocks, completes Late Inspections via operational OBSS

by Chris Bergin

Following the closure of hatches between Discovery and the International Space Station (ISS) Sunday evening, Discovery and her crew are on the final legs of their hugely successful STS-120 mission.

Discovery undocked from the ISS at 5:31am Eastern, carried out a flyaround of the station during departure, before completing Late Inspections. Those inspections have been boosted by the good health of the OBSS sensors following Saturday’s EVA-4.

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Ready for undocking:

After a farewell between the two crews at 2:25pm EST, hatches between ISS and Discovery were closed at 2:42pm. This marked the departure of the popular Clay Anderson from his mission on the ISS, returning home on Discovery after being replaced by Dan Tani – who’ll be in action on Thursday with EVA-5, which has since been added to the ISS stage.

Because EVA-5 wasn’t completed by STS-120 – due to the priority spacewalk to repair the P6 4B array on Saturday – PMA-2 cabling has not been disconnected as originally planned.

‘The automatic undocking software for the PMA-2 departure under Russian thruster control will still be used,’ noted Sunday’s On Orbit Status report. ‘The procedure provides for the crewmember to stand by with a stopwatch and to take over the automatic operational attitude control sequence manually if the software does not resume control after the period of free drift a few minutes after physical separation.’

OBSS/Late Inspections: 

Once Discovery has moved away from the ISS, Flight Day 14 will be marked by Late Inspections, which will check over the health of her RCC (reinforced carbon-carbon) and other areas of the orbiter’s TPS (Thermal Protection System).

The leading edge panels will be in focus, following nine indications, observed via the WLE (Wing Leading Edge) sensors, including what is believed to be a MMOD (micrometeoroid/orbiting debris) ‘hit’ to panel IL, which was recorded – according to NASA reports – as the largest in WLE history.

‘Reported to NASA SE&I: Imagery WLE reported a significant hit on the port panel 1. This hit was triple the max ever observed,’ noted official NASA notes during Flight Day 8. The ‘impact’ registered 2.74 GMRS on the sensors, which is understood to be below the point of concern for engineers, which was backed up by NASA claiming all of the WLE indications are ‘within family.’

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While the Late Inspections could have been carried out by cameras on the end of the SRMS (Shuttle Remote Manipulator System) arm, NASA have been buoyed by the survival of three of the four main instruments on the OBSS (Orbiter Boom Sensor System) suite, following Saturday’s EVA-4.

Due to being unpowered for around nine hours – four hours past certification – the main element ‘package 1’ has been checked out as in ‘working order’ by Sunday morning, thus allowing it to be used as part of the Late Inspections, as and when it is required.

The OBSS instrumentation package – which rides on the end of the 50 foot boom – consists mainly of visual imaging equipment, such as the Laser Dynamic Range Imager (LDRI), and the Laser Camera System (LCS). A post-Columbia modification, the sensors can focus to a resolution of few millimeters, and can scan at a rate of about 2.5 inches per second.

The OBSS sensors proved their worth during STS-118, examining a gouge on her belly for detailed evaluations on the ground.

The concern raised by managers was that OBSS capability could be lost for Late Inspections, after the sensor package was left unpowered – and below its thermal limitations – for an extended period of time during EVA-4.

‘Reasonable chance OBSS sensors won’t operate correctly afterwards, because they’ll be cold for longer than certified for,’ noted NASA manager Kim Doering ahead of the spacewalk. This was deemed acceptable, given Discovery could spot damage via the SRMS if required, along with the over-riding priority of the array repair EVA.

However, a series of mitigation efforts – including heating up the sensor package to a point it wouldn’t dip below its thermal limits until later into the spacewalk – appear to have exceeded expectations, with the main package of sensors proven to be operational, despite the concerns prior to the EVA.

‘A concern of ours after the EVA were the unpowered sensors (on the OBSS). Turns out the sensors we need for the Late Inspection are fully operational,’ noted Rick LaBrode, STS-120 Lead Shuttle Flight Director.

‘We use sensor package 1 for our Late Inspections, and that consists of the LDRI and ITVC camera. We checked them out and they performed nominally. Sensor package 2 consists of the LCS and the IDC – which is another digital camera. We got perfect imagery from the IDC – so we’re comfortable it’s operational.

‘While the crew were running the checkout procedure on the LCS, they received an error message when they went to do their first scan. Since we only use the LCS for focused inspections, we opted not to even do any troubleshooting on that.

‘LCS engineers did note that this is a symptom they’ve seen before with ground testing, so at this point they can’t say one way or the other if it sustained any damage due to the thermal (limitations) from the EVA. So three of the four sensors checked out just fine, and the two we require for the Late Inspection are operational.’

This article will be updated during the Flight Day.

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