Management's rationale for spacewalk repair
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 2, 2005
The following discussion was uplinked to the Discovery astronauts overnight as part of a lengthy daily "execute package" from mission control. It is fairly technical, but given the recent discussion about protruding gap fillers and how they might affect the shuttle Discovery's aerodynamics during re-entry, it provides insight into how managers reached the decision to order a spacewalk repair. Acronyms are spelled out for clarity, but the reader is assumed to be familiar with the general thrust of the discussion.
The major discussion and decision point from the MMT (mission management team) today was with regard to performing the EVA (spacewalk) task to remove the 2 protruding gap fillers. The MMT decided that the prudent course of action was to add this task to EVA 3 to remove the uncertainty and potential concerns for flying entry with the gap fillers in place. As a result we are sending along more details of the task and timeline, and the overall plan for EVA 3 with this task inserted.
The MMT decision resulted after careful deliberation and extensive discussion on the potential thermal and structural effects of leaving the gap fillers in place and experiencing early boundary layer transition (BLT), as compared to the relative risk aspects of this particular EVA task. The following outlines some of those particulars, and of course we're also sending along the presentation material for your consideration.
The aero/thermal team outlined their approach to predicting the point at which early transition could occur, and they then discussed the resulting potential thermal implications for both the RCC (reinforced carbon carbon leading edge panels) and TPS (thermal protection system) tile. As you know there are many challenges in this area, not the least of which is that the wind tunnel and CFD (computational fluid dynamics) data bases do not cover this Mach regime, and of course the only flight test data we have is that for Shuttle entry, since no other winged vehicles have flown in this regime. We have had 2 documented cases of early BLT at about M (Mach) 18, STS-28 and STS-73.
We have had several cases of early transition and ABLT (asymmetric boundary layer transition) at lower Mach numbers, the majority of them in the M 12-15 region. For reference, transition normally occurs in the M 8-12 region. As a result, the team used this limited flight data for the M 18 early BLT's to extrapolate on the currently accepted and certified model for BLT.
The extrapolation was necessary in order to consider the effects of having early BLT with our flight conditions, primarily because these gap fillers are further forward relative to our data base, and among other things the BL is thinner in this area over these forward nose tiles. The end item answer is that because of the further forward position and the amount of protrusion that we have, approximately 1 inch, the resulting best estimate for early BLT is about M 21.5 (+/- 2.5M).
The team then evaluated the potential effects of an early BLT in this Mach regime, and presented the results for both the RCC and the TPS tile area. It was noted that as it relates to the RCC, we have no documented flight evidence or reason to believe that we have ever had early transition that affected the RCC. Said differently, we have no evidence that we have ever had turbulent flow wash onto the RCC in these regions of higher heating.
However, given an early BLT at M 21.5 and the resulting flow due to these gap fillers being off-centerline, the analyses show that we could have turbulent flow wash onto the RCC. It was noted that these analyses were overall best estimate (conservative in some aspects and best estimate in other aspects). The magnitude of the resultant heating could be on the order of the heating rates that we expect on a TAL (trans-Atlantic landing) for example, where we go to the single use RCC limits of ~ 3250 deg F. This is compared to the nominal EOM (end of mission) multi-flight reuse limits that we design to of about 2950 deg F.
If the early BLT occurred at the high end of the uncertainty at about M 24 the heating rate could be even higher than our TAL limits by about 100 deg F. For the tile areas, the analyses showed that this thermal profile could result in negative margins on the mid-body and aft fuselage structures from the normal 1.4 FS (factor of safety). The results ranged from slightly negative margins to as much as a 30% decrease in the FS (0.98 FS) depending on the exact tile locations.
As a note, the aero and flight control communities also reviewed these conditions and determined that there would be no concerns. As you'll recall the WRAP DAP (digital autopilot software) was designed to handle early transition and ABLT, so these satisfactory results were expected. As such, the MMT opted to not review those specific details, although the technical community did the necessary rigor for completeness.
The team acknowledged that there is high uncertainty in the analyses for determining just how early BLT could occur, as well as for determining the resulting potential thermal implications for the RCC and tile. It is possible that we've flown with these conditions before, however we do not have data to show that we have and therefore we cannot prove that it would be a lower risk than the EVA task.
As such, given the relative risk trade between the potential for these heating conditions as compared to the EVA task to remove the gap fillers, the MMT determined that the EVA was the prudent approach. As for the proposed EVA 3 task, the first and preferred option is to remove the gap fillers by pulling them out with the gloved fingers, or the forceps if required. The downmode will be to cut them off to the lowest level feasible, with the hacksaw or scissors. If they are left in place the desire is to get them to a height of no more than 29 ~ 0.4 ". The general plan would be to pre-position ESP-2 on FD8, and then access the area from the starboard side with an APFR on the SSRMS after completing the ESP-2 installation task.
The current estimates are for a 7:15 duration EVA, with 1:15 dedicated to the gap filler task. We are sending the details of this task as well as the overall EVA 3 plan for your consideration and look forward to your comments/questions.
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