http://spaceflightnow.com/shuttle/sts115/060815frrbegins/
Shuttle readiness review begins; update on bolts BY WILLIAM HARWOOD STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION Posted: August 15, 2006 http://spaceflightnow.com/shuttle/sts115/060815frrbegins/ NASA managers kicked off a two-day engineering review today to assess the shuttle Atlantis' readiness to blast off Aug. 27 on a long-awaited flight to restart space station assembly. Along with discussing the shuttle's external tank and the ongoing threat of falling foam insulation, engineers and managers also will discuss what to do about bolts holding the shuttle's KU-band antenna in place on the forward right wall of the ship's payload bay. A recent engineering review indicates two of the four bolts holding the KU antenna support box in place are too short. Engineers cannot directly inspect the bolts at the launch pad, but a paperwork review shows the bolts in question may be engaged by less than 2.4 threads. The requirement is 8.4 threads engaged. While the bolts were properly torqued, and while Atlantis has flown 26 flights in the current configuration, engineers cannot prove the bolts have not, or will not, back out at some point, allowing the antenna box to break free. Should it break loose during launch, it could fall the length of the shuttle's six-story cargo bay and cause catastrophic damage. Gaining access to the KU antenna at the launch pad is extremely difficult, but sources say the engineering community wants to replace the bolts rather than launch Atlantis as is. Kennedy Space Center engineers have developed a plan to make the unprecedented repairs, but it's not yet clear what impact such work might have on Atlantis' launch processing schedule. Assuming managers at the flight readiness review approve the current launch target, Atlantis countdown would begin Aug. 24 for a liftoff at 4:30 p.m. on Aug. 27. The launch window closes Sept. 7. The current plan permits up to seven launch attempts within that window. The KU bolt fix, if approved, would require engineers to extend an access platform into the open cargo bay from the payload change out room at pad 39B. A second platform then would be erected on the first to reach the antenna box. Safety nets would be required because the work would take place at a height of nearly 60 feet above the shuttle's aft bulkhead. In addition, the shuttle's robot arm and a 50-foot-long boom used to inspect the ship's heat shield would have to be moved to provide access. A technician, resting on his side in very close quarters beside Atlantis' external airlock, would then remove and replace the two bolts in question. A major concern with working in such tight quarters is inadvertent damage to nearby equipment, but engineers believe they can safely do the work. A variety of other technical issues will be discussed at the flight readiness review, including the status of Atlantis external fuel tank. Atlantis will fly with an external tank that is virtually identical to the one used to launch the shuttle Discovery last month. During the FRR before that flight, NASA's top safety manager and chief engineer voted against launching because so-called ice-frost ramps on the external tank, made up of hand-crafted foam insulation, were formally classified as "probable/catastrophic." That classification means that over the original 100-flight life of a space shuttle, there is a 50-50 chance that a piece of foam large enough to cause catastrophic damage would break away from an ice-frost ramp and hit the spacecraft. The ice-frost ramps received that classification because they are made up of foam that is manually applied on top of other foam. Recent engineering studies show such foam-on-foam buildups can experience cracks because of temperature extremes and pressure changes during fueling cycles. Such cracks, in turn, can lead to foam shedding. Under NASA's existing FRR procedures, the "probable/catastrophic" classification left safety director Bryan O'Connor and chief engineer Chris Scolese with no choice but to vote "no-go" at the end of Discovery's readiness review. NASA Administrator Mike Griffin ultimately approved Discovery's launch over their formal objections, saying he didn't believe the probable/catastrophic classification was justified and that the benefits of keeping the shuttle on the ground until the ramps were redesigned were outweighed by the programmatic need to restart station assembly. NASA currently is in the process of redesigning the ice-frost ramps, but no major changes have been implemented since Discovery's flight and it is assumed O'Connor and Scolese will vote the same way they did last June. During an interview shortly before Discovery took off July 4, Griffin explained his earlier decision in detail. "In order to get this to be a hazard, we have to assume that this flight is going to release the maximum amount of mass we've ever seen," he told CBS News. "Probably that's not going to happen, right? But that's what we assume. Then we have to assume the mass comes off in chunks that are bigger than we've ever seen. The biggest chunk we've ever seen come off with this mechanism is .084 pounds. So we set the limit on this thing at .25 pounds, which is the biggest chunk of mass that you could get off of an ice-frost ramp because that's what the whole thing weighs. We've never actually seen that, but we assume that could happen. "Then we have to assume that all the mass comes off and it comes off in big chunks and it comes off at the worst possible time, which is about a seven or eight or 10 second window in there ... And then we have to say it hits the worst place on the orbiter. So if all that happens, then we have a hazard that we think rises to a level of real concern. "Now, we're not ignoring it, we're going to redesign the ice-frost ramps and we're going to fix it. But I ask myself as an engineer, what are the odds that all those four bad things are going to come true over the next two or three flights while we do a redesign? And I say to myself, not very high. And in fact, when we go and analytically study that, the odds come down to like 1-in-400 or 500, which is well less than many other risks we're taking on the orbiter." Griffin agreed with the observation that complex systems fail in complex ways and worst-case aviation failures do, in fact, occur. But he said the risk posed by the ice-frost ramps is less, in his view, than the overall risk of flying the shuttle at all. "Rotating machinery would be well ahead on my list," he said, referring to the shuttle's main engines. "Our micrometeorite and orbital debris hazard is 1-in-200. I remember flight readiness reviews from my youth where every moment was soaked up in turbopump seals. There are other hazards on this machine that we accept in order to fly it. "Now that's not good, either, and we're trying to transition to a new system ... which will be at least 10 times safer than the shuttle on its best day. We can do better and we will do better. ... But if I can get individual risks like foam debris down to one in many hundreds, then that's very acceptable in comparison to the other risks which we accept to fly this machine at all. And that's where I think we are. "Everyone says why'd you overrule your chief engineer and your safety guy? Well, that's why. Because I believe when we analyze this thing in terms of what is likely to happen as opposed to a worst on worst on worst, the risk is very acceptable." Meanwhile, NASA flight planners have updated the timeline for shuttle mission STS-115, moving space station undocking up one day and deferring late inspection of Atlantis' nose cap and wing leading edges until the day after separation. -------------------------------------------------------------------- mail2web - Check your email from the web at http://mail2web.com/ . _______________________________________________ Medianews mailing list [email protected] http://twiar.org/mailman/listinfo/medianews_twiar.org
