A series of documents acquired by NASASpaceflight.com explain the split of opinion that Shuttle manager Wayne Hale referred to during Friday’s Shuttle update press conference.
That split came during the PRCB (Program Requirements Control Board) meeting just a day previous, with the rationale to hold Discovery’s STS-121 mission until further modifications are made on the ice/frost ramps, outlined in a series of presentations.
The following information was collated from several documents, dated the 26th and 27th of April. Selected scans are available (click images below – full documents available on L2)
At the conclusion of last week’s PRCB it was decided to fly STS-121 with ET-119 ‘as is.’ That went against a large section of the ET community, backed by the Marshall Space Flight Center (MSFC), who made the presentations to show why further work should be carried out on the ETs. The fly ‘as is’ only affects ET-119, given Hale noted that work will be on-going on the tanks throughout the remainder of the STS program – which stretches through to 2010. However, focusing on STS-121, documents clearly show that MSFC are against what was agreed on during the PRCB, noted as ‘Option C’.
Three options were presented to the PRCB. Option A involved a further re-shape of ice/frost ramps, option B would involve scalloping and venting of the ramps – and option C, fly ‘as is’.
Option A, already dismissed by Shuttle managers, offers most promising approach to elimination of IFR TPS debris, alleviating cracking and delamination which has been seen in wind tunnel testing on the new ice/frost ramps. A re-shape would see another large section of foam being removed from the ramp.
No TPS loss would be expected with option B’s scalloping and venting, which also offers improvements in aerodynamic and void/delta-p performance. That option was understood to be still open to engineers, up to the point Discovery is mated with the ET and twin Solid Rocket Boosters, mid May.
Option C, which has been decided on, has a list of risks, with expected delamination and foam loss through divoting in the ramp.
Documents show that wind tunnel testing resulted in a 0.2 inch divot at the base of the test article ice/frost ramp (inboard edge at bondline).
While that shouldn’t pose a major threat to the orbiter, the divot was the start of a major structural failure of the ramp at higher aerodynamic pressures, as seen in the wind tunnel test image on the left.
The Low Q mission profile (no higher the 735 psf) will go a long way to ensuring structural failure of the ramp simply won’t happen, although it is noted that the divoting occurred at 735 psf during test runs at the wind tunnel.
While schedule pressure is a dirty word in NASA circles, option A would have been ‘critical’ to the July window Discovery is currently set to launch in. Option B, however, held the potential to be completed – and still allow Discovery to launch in July. It is not fully understood why this option wasn’t taken, as scalloped/vented and fly-as-is options can support the STS-121 schedule, provided the remaining testing and analysis is successful.
On the rationale to go with option C and fly ‘as is’ – as recommended by the ET project team with the Space Shuttle Program, engineers note they have a basic understanding of release mechanism for the cryopumped and noncryo-pumped scenarios and their time of release. The risk to other elements for these scenarios is well within the total risk of the flight.
However, further tests will be carried out by the team of engineers tasked with the ice/frost ramp re-design, to verify the analysis performed for these release scenarios before the first flight. Such information is expected to be presented to Shuttle managers at the May 4th PRCB meeting.
Engineers do, however, have work ahead of them in evaluating the ‘unknown risk’ an unknown release mechanism for the horizontal cracks (as seen in the ice/frost ramp divot), although it currently appears to be within the risk of the void delta P risk. Still, engineers will ‘aggressively pursue design options’ to find a way to mitigate horizontal cracks via the fly ‘as is’ option.
As Hale also noted, managers wish to fly the tank without making yet more changes to the current modifications, due to so little maturity in the design. Managers don’t wish to lose focus on the need to see how the tank reacts in a real ascent situation, without the PAL ramp being in place for the first time.
Added to the rationale was data that noted the risk factor for a catastrophic impact is ‘low’, given the expected release of foam from the ramps – added to the Low Q mission profile, to be no more than small divot-type releases.
However, MSFC’s Engineering Team’s support for Option A was almost insistent, with opening rationale entitled ‘Primary rationale is mitigation of TPS losses and failure mechanisms through redesign’ – with the primary goal of still supporting a July launch window, despite a document a day previously (one day before the PRCB) claiming option A would place July in jeopardy.
In support of option A, MSFC engineers noted the mitigation of 4 of 4 failure mechanisms, notably ‘Delam/cryo-driven losses from body and acreage minimized due to thermal crack/delam reductions. Void delta pressure from body of ramp less due to thinner foam, smaller surface area, and finger removal. Aero losses from body of ramp eliminated from top half of ramp. Delaminations mitigated.’
Classing option C’s fly ‘as is’ as ‘Minimally acceptable, high risk approach,’ engineers listed a large amount of expected risks for STS-121’s flight.
They summarised: ‘If launch in July is pursued and complete redesign implementation for ET119 does not support that schedule, then: Fly as-is rationale for one flight is available. If PRA shows risk is acceptable. High risk for debris. Requires CSCS/safe haven capability.’
The language of MSFC engineers was even stronger in a document dated April 26, one day before the PRCB and used as rationale for their position against the decision that was finally taken by Shuttle managers.
A very strongly worded presentation opened with a ‘Risk Statement’ on page one. ‘Given each design option, there is a potential for foam and/or ice debris form the ET IFR that could strike the Orbiter resulting in a catastrophic failure. These failures are documented in the ET Hazard Reports T.02 (Loss of Thermal Protection System) and E.04 (ET Ice Debris).’
Their position only supported option A, with the S&MA recommendation noting ‘Implement Reshaped-Reshaped design prior to STS-121.
”USE AS IS’ Design is unacceptable for flight.
”USE AS IS Vented Scalloped” does not address key issue of cracking and delamination and is unacceptable for flight
Such was the serious tone of their rationale to support only option A, last Thursday’s PRCB was almost split right down the middle on which option to support. Fly ‘as is’ will allow launch processing to continue to flow towards the July window, but sources are claiming a number of engineers are continuing to put together rationale to change that very decision.
Space Shuttle 25th Anniversary T-Shirt. Use discount code ‘nasaspaceflight’ when ordering from Countdown Creations & receive 5% off! It should be remembered that such differences of opinion, and the open process (the information gained by this site above was not export controlled) surrounding the decision, highlight NASA’s willingness to listen to both sides of the argument when it comes to flight safety.
Shuttle managers, such as vastly experienced Wayne Hale, are tasked with balancing out all sides off the argument, while drawing a line under how far they go with modifying the tanks. Such debates between NASA departments and contractors are now routine.
More articles will follow this coming week
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