Concerns have been raised over the need to re-certify elements of the Shuttle’s roll program, following evidence that a Solid Rocket Booster attach strut neared its maximum design limits during Columbia’s roll program on the ill-fated STS-107 mission.
A presentation, dated April 19, collated by members of Boeing, the United Space Alliance and NASA, didn’t spare the dramatics, noting the risk of not certifying new design environments for the roll program “has catastrophic consequences”.
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Such consequences are obvious. A failure of an attach strut would ultimatly lead to a loss of the vehicle. There are no abort options available during the roll program stage of the ascent.
Importantly, the issues raised shouldn’t have any impact to the busy Shuttle manifest, with the ‘use of controlled integrated hazard approach’ being ‘adequate’ – while the document added such an approach to evaluations is suitable ‘only for the short term.’ It’ll take 18 months, and ‘in excess of $100 million,’ to fully complete the certification process.
The recommendations also note that ‘SE&I should complete updates to Roll Manoeuvre environments and provide Orbiter with balanced loads cases and aero database updates perform certification rigor analysis of this environment.’
Little is said on the mechanical information of strut that links the SRBs to the External Tank, although the data that has raised the concern was not expected, as ‘the flight was reasonably nominal.’
However, one consistent pattern in the document dismisses any structural issue with the strut itself, as it concentrates almost exclusively on the environment surrounding the Shuttle’s roll program. This is backed up by ‘SE&I determined that wind dispersions should be added to the roll manoeuvre cases,’ as another element of recommendation.
The new data was gained from wind tunnel testing – although no mention is made on when these tests were conducted, or if they have resulted from on-going wind tunnel tests on the ET modifications. However, the new data is a result of ‘an expanded aero database’.
‘As the vehicle is moving slowly during roll,’ noted the document, ‘winds of moderate magnitude can cause relatively large alpha/beta excursions.’ The new data also identified that ‘wing stall was identified at an alpha of 19 deg,’ thus concluding that ‘flights will be restricted to max alpha of 19 deg to preclude this condition.’
Another addition, noted under the header ‘Description of Roll Manoeuvre Changes,’ claimed that ‘Trajectory constraints (i.e., Orbiter q-planes) have been extended down from Mach 0.6 to Mach 0.2. Constraints are based upon estimated capabilities of Orbiter as described by load indicators.’
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While the tone of the document at times is decidedly reluctant to recommend anything other than full re-certification, it concedes that ‘this process is best possible short term solution for RTF (Return to Flight).’
‘It is considered adequate as flight rationale, but does carry some level of risk compared to the normal process for accepting new environments/certification, (given) this is only an evaluation of load indicators. The roll manoeuvre indicator data cannot be ‘incorporated’ as an environment.
‘This can be done only if indicators have already been validated and results are within certification experience. Only thrust structure indicators meet that criteria.’