The importance of a ‘clean’ STS-121 is more than obvious for the return to regular flights of the Shuttle, yet NASA isn’t taking any chances in case of the unlikely event that a major foam loss on ascent requires a repair – with a series of repair techniques available to the crew of Discovery.
However, documents acquired by NASASpaceflight.com show that STS-121 will launch without the full range of developed repair capabilities, as the agency continues to evaluate other available on orbit repair options – ahead of the “no safe haven” flight to service Hubble. As a result of the Columbia Accident Investigation Board (CAIB) findings and recommendations, the inspection of TPS (Thermal Protection System) damage on orbit was vastly improved, with the option to then attempt repairs to the vehicle, or take up the ‘safe haven’ of the International Space Station to await another Shuttle to rescue the crew – as opposed to having to risk re-entry.
The resulting information was collated from information presented to the Shuttle PRCB meeting, which is available in full document form on L2. To join L2, click the advert —>
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The whole requirement of having a level of capability for repair options came about following the loss of Columbia on STS-107, doomed less than two minutes into her 16 day mission after a piece of foam punched a hole in the leading edge of her left wing – a breach which allowed super-hot gases of re-entry into the vehicle and ripped her apart.
Some complained that the repair options made available were insufficient, although the likelihood of such a repair taking place was highly unlikely, given NASA’s primary goal is to eradicate the damage occurring in the first place. Still, NASA pressed ahead with a number of experiments during STS-114 last year.
The results proved useful, with the elimination of the CIPAA (cure in place ablator applicator) back pack system – added to a better understanding of what are viable options to add to the manifest of repair techniques, should they be required.
As a result of the Columbia Accident Investigation Board (CAIB) findings and recommendations, the inspection of TPS (Thermal Protection System) damage on orbit was vastly improved, with the option to then attempt repairs to the vehicle, or take up the ‘safe haven’ of the International Space Station to await another Shuttle to rescue the crew – as opposed to having to risk re-entry.
Currently, NASA has a few options available for STS-121, although new techniques are still in works. However, they will not be available to the crew of STS-121, even though some new options may still fly for potential experimentation on this summer’s mission.
‘Plug: Preliminary data looks good, greatest limitations are hole size and excluded areas where interference may preclude a fit,’ noted the document. ‘NOAX: High confidence for use in minor cracks. Not viable for severe cracks w/offset; may be application sensitive. No data yet which address capability for small holes. Overlay: Significant development testing remains as forward work. Technical communityâ€™s confidence does not provide a repair capability yet.’
The Plug option is still in the evaluation stages and would only be used in certain scenarios. The plan is to have ’13 Plugs, 4 attachment sets on ISS,’ with the concept ‘in fabrication (best case delivery by July).’ This concept would be used for small gaps in the TPS, as ‘Arc jet testing has shown plug design capable of repairing holesat BP 5505 (2960Â°F). With gaps (Plug IML to RCC OML at edge) up to 0.020 inches.’
The concept also requires proper testing, with the application of the plugs involving an EVA (Spacewalk) that would undertake drilling around the damaged area of the orbiter’s tiles, with the placement of the plugs then covering the gap created by the damage.
The one option that is the ‘only current capability for STS-121’ is ‘Emittance Wash: High confidence in minor repairs; mostly adds margin on close calls. Does not protect tile out or severe damage.’
The document also notes that there is ‘no coverage for critical MLG door seal areas.’ The door seals have been noted as a problem area for a number of months, with a solution to hand, ironically taken from the scrapped CIPAA option.
The option uses the ablator material ‘STA-54’ as a sealant for the landing gear doors – a major weak point in the orbiter’s TPS, with the document noting NASA is ‘evaluating STA-54 as Door Seal & Penetration repair for tile,’ with the note ‘recommend STA-54 with new applicator (T-RAD) for Door Seal & penetrations.’
A full seperate document on the T-RAD concept is available on L2.
‘STA-54 material selected as leading candidate for this type repair due to: TPS and Aero-Thermal NSEâ€™s confidence in material for this application. Maturity and amount of material testing. Operations experience and development work.’
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One clear note in the document on the repair options is focusing on the one mission that doesn’t have the major back-up of the ISS Safe Haven – the Hubble Space Telescope Servicing Mission.
STS-125, due to launch no earlier than April 11, 2008, is the highlight mission of the remaining 17 flight Shuttle manifest. NASA is aiming to develop most of its on orbit repair capabilities for this mission – ‘Develop HST support plan â€“ considering all methods and capabilities.’
The need for repair options couldn’t be clearer when presented with data from Shuttle manager Wayne Hale himself, with the document noting that the latest data shows the total mission risk of a orbiter and her crew being lost by damage to the TPS is 52 per cent, by far the highest risk issue known to the orbiters.
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