NASA work towards large area repair capability

by Chris Bergin

Despite the recent run of “clean” flights of the shuttle, NASA are pressing ahead with a plan to add a new on orbit repair capability, as the agency continues a two-pronged effort on both reducing damage from debris strikes and the ability to repair damage to the orbiter in space.

Currently, small holes in the orbiter’s TPS (Thermal Protection System) can be repaired via a number of available techniques. However, NASA is now looking at a patch as big as 12 inches by 12 inches, which would cover a large breach in a RCC panel.

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PRCB Presentation on LAR. Several PRCB presentations on On Orbit TPS repair and more available on L2.

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The panel, which can be shaped to the curve of the leading edge of the orbiter’s wings, would be fastened over the breach with screws, after an astronaut had drilled holes into the Reinforced Carbon Carbon (RCC) panel. It is also capable of being a patch for the nose cap of the orbiter.

The patch is large enough to cover a sizable breach in the TPS, which currently would result in a LON (Launch On Need) requirement being called, with the crew taking up the safe haven option at the International Space Station (ISS), before being ‘rescued’ by another orbiter.

While the 33 page presentation – which was reviewed by the PRCB (Program Control Requirements Board) meeting in Houston – doesn’t directly link this capability to STS-125’s mission to service the Hubble Space Telescope, NASA is looking at a salvo of capabilities in case of a worst case scenario debris event with Atlantis. STS-125 is the only flight on the remaining manifest not to have the option of safe haven at the ISS.

The presentation itself opened with an optimistic tone, pointing to the Large Area Repair (LAR) capability as part of previous proposals that only require refining to become a manifested repair option.

‘With limited Program life remaining we did not start from scratch. Reviewed all proposals received during last 5 years and previous R&D efforts for RCC repair. Did not discourage new proposals – however none received,’ noted the Recommendation to the Space Shuttle Program (SSP) presentation.

‘Determined that LaRC (Langley Research Center) /JSC (Johnson Space Center) LAR provided the best opportunity for success: Most mature. Good successes in previous R&D testing. Capable Team and resources.

‘EA provided initial funding to determine feasibility for implementation. Redirected from Research Approach to Implementation Approach: Focused on refining concepts and improving designs for Large Area Repair (LAR) and fasteners.’

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The increased capability is an improvement on the current ‘plug’ repair, which would be used on small holes, but not those which could occur around interfaces – such as T-seals. LAR already has costings and a level of developmental maturity in place to allow this concept to move forward.

‘EA Recommendation: If RCC Large Area Repair capability is desired, this is the approach to take. Alternate recommendation may be to continue feasibility work on Fastener w/NOAX for MMOD / small hole repair. Work with MV to define project scope and develop detailed cost & schedule.

Concept provides capability over T-seals and other areas not covered by Plug due to interferences. Fastener with NOAX sealant provides small MMOD hole repair capability (Nose Cap and other areas).’

The LAR patch is flexible, allowing for the covering of breaches in the wing leading edge, as well as being able to be placed over small holes – such as MMOD (micrometeoroid/orbiting debris) impacts – on the nose cap.

To install the patch would be no easy task, with spacewalkers being required to drill/tap holes into the RCC around the damaged location. Fasteners and washers would be needed to help secure the patch in place, before being sealed with NOAX.

Testing will continue to evaluate the LAR patches ability to cover such damage on the orbiter. Such testing will help mitigate some of the ‘key issues’ that remain.

‘Need sufficient properties (thermal and mechanical) for repair system component design. Maximum size of LAR cover allowable without buckling during re-entry. Develop technique for locating fasteners within perimeter of LAR cover. Characterize NOAX as edge sealant for LAR system. Plasma resistance of all materials and components. Understand effects of misaligned fasteners on plasma resistance and mechanical behavior & installation.

‘Establish technique for proper alignment of fasteners and threaded holes. On-orbit handling of components and installation of repair system. On-orbit inspection of repair systems after installation. On-orbit thermal cycling of installed repair system.’

Another benefit of developing LAR can also be gained by using the fasteners and washers that will keep LARs in place as repair items themselves. In the case of small holes, likely from a MMOD strike, those items could themselves plug the gap, allowing them to fly on the orbiters before the LAR plate was certified.

‘An inherent benefit of this approach is the ability to use the fasteners or the fasteners and washers in repairs independent of using the LAR cover plate. Items will be qualified early as critical components of the LAR. Minimal stowage requirements Can be flown sooner than whole system,’ continued the presentation.

‘Fasteners & Drill / Tap can be made in more than one size to accommodate holes smaller than the 1.1 in. diameter hole required for a Plug repair Currently assessing nominal 5/16 in. fasteners. Fasteners in at least two sizes provide capability for some MMOD hits to the highly curved Nose (used with NOAX). Fasteners with washers and NOAX may provide additional protection.’

The next major step in bringing LAR into the repair manifest is to procure multiple batches of LAR flexible ceramic sheet, SAR covers, C/SiC threaded fasteners and NOAX sealant – needed to support arcjet tests and thermal/mechanical tests.

LAR will only be certified after extensive thermal and mechanical testing, with no schedule noted at this time. However, with the Shuttle Program due to end in 2010, NASA will want to have this capability assessed and implemented as soon as possible.

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