Booming success for EVA repairs potential

by Chris Bergin

A presentation acquired by has shown that the experimental EVA repairs that were simulated on STS-121 have been deemed successful.

The use of both the SRMS (Shuttle Robotic Arm) and OBSS (Orbiter Boom Sensor System) boom was tested during the mission, with the results vital to approving STS-125’s Hubble mission, with the confidence of on orbit repair capabilities.

Related documentation available on L2: The STS-121 EVA SRMS/OBSS Review Presentation. STS-125 Flight Information Latest. STS-125 HST-SM04 Rescue Presentation and Updates.

The highlight of Flight Day 5 of STS-121 saw astronauts Piers Sellers and Michael Fossum anchored to a foot restraint on the end of the long booms, which were attached together.

With the two arms, extending to around 100 feet in length, the duo tested out movement and stresses for the potential to reach most vital areas of the orbiter, even though the use would only add a small additional capability, given a large amount of the orbiter can be accessed on the end of the Shuttle’s RMS alone.

While no actual repairs were carried out on the orbiter, the six and a half hour spacewalk conducted a variety of tests, including bouncing up and down on the end of the arm, and pushing against the P1 section of the International Space Station (ISS).

Officially, NASA documentation listed the test as ‘Assessment of DTO-849 Results (Repair from Boom) to support current flight operational assessments, and program recommendations on HST repair flight.’

While the full report is not due until the end of November, the general conclusions based on the STS-121 Crew Report found there was a small increased risk (over SRMS/SSRMS alone) of inadvertent contact when working on the boom, but that the crew’s awareness of the platform instability and proximity to fragile, critical structure does mitigate this risk.

Another issue that arose was the need to prevent the safety tether’s attached to spacewalkers from getting tangled, as was seen during the mission. However, simple solutions come to rescue here, involving nothing more than a tie wire.

While the report listed in great detail all related pros and cons of specific repair procedures related to crew members carrying them out on the end of the arms, importantly the report states that ‘In general, based on system dynamic response, the OBSS Boom platform should be considered a viable repair platform.

‘Current DTO team recommendation is to proceed without development of additional worksite stabilization, and revisit if any of the formal assessments indicate that the DTO conclusions to date are invalid or too optimistic. Note: Additional boom modifications may be required to provide access to all vehicle locations (i.e. Worksite interfaces along the circumference of the boom at the mid-point and/or tip locations).’

To back up their conclusions, the crew of STS-121 copied their tests back on Earth, with a trip to NASA’s VR (Virtual Reality) Lab, so that technicians and engineers could cross reference the results gained from the on orbit tests.

During the six day post flight tests, NASA listed their conclusions as positive, noting: ‘If it (also) works in VR, it will work on-orbit.’

The importance of on orbit repair capabilities relate to all missions, but most importantly to the one flight which won’t have the option of the ‘safe haven’ of the ISS – STS-125.

Any unacceptable damage found with Discovery – the orbiter scheduled to carry out the final mission to service Hubble – repair capabilities may prove to be vital, especially if NASA decide against their revolutionary rescue plan to be placed on standby.

Ultimately, should NASA decide next month to officially approve the Hubble servicing mission, it will be based on the conclusion that the recent successes with reducing foam liberation on ascent, thus reducing the threats of damage to the orbiter’s TPS (Thermal Protection System), is the defining factor for confidence in carrying out the mission to Hubble without concerns having a crew stranded in orbit.

Having options to repair any damage will help this mitigate the requirement to have a LON (Launch On Need) mission on standby, although sources claim it is still 50/50 on whether NASA will decide against having an orbiter backing up Discovery.

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