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Decision to add spacewalk highlights shuttle risk trades BY WILLIAM HARWOOD STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION Posted: December 17, 2006 Faced with an unforgiving space station assembly sequence, NASA managers were forced to shoehorn an additional spacewalk into Discovery's mission to complete the retraction of a recalcitrant solar array. The spacewalk came at the expense of a landing weather contingency day but it was either that or give up a post-undocking heat shield inspection, an option many shuttle engineers were reluctant to consider. The issue illustrates the difficulty NASA managers may face balancing flight safety with schedule pressure resulting from the Bush administration's 2010 deadline for completing the international space station and retiring the shuttle. That somewhat arbitrary deadline leaves NASA little maneuvering room when it comes to dealing with unexpected problems like the partially retracted solar array that has disrupted the Discovery crew's schedule. In this case, NASA's options are limited by the amount of hydrogen and oxygen the shuttle can carry to generate electricity and a pre-launch decision to add a docked day to the flight because of the mission's complexity and the possibility a day would be needed to correct problems activating the lab's permanent power system. As such, Discovery's flight is classified as a 12-plus-zero-plus-two-day mission, i.e., a 12-day flight with two weather contingency days. There is not be enough hydrogen and oxygen on board to extend the mission itself beyond 12 days and still preserve two backup landing days. NASA went into Discovery's flight with an understanding that the post-undocking heat shield checkout could be taken off the table if the astronauts had problems with the space station electrical work that might require an additional spacewalk. Late inspection, in fact, did not even show up on a list of pre-flight mission priorities. This was not an issue under the original flight plan, which called for undocking Monday, late inspection Tuesday and a landing Thursday at the Kennedy Space Center, with Friday and Saturday as weather backups. But the flight plan was thrown into disarray when the astronauts ran into problems retracting a solar array on the international space station. NASA engineers concluded the P6-4B solar array could be safely left in its partially deployed state for several months, but it had to be dealt with eventually. The P6 array, which provided interim power during the lab's initial assembly, is scheduled to be moved to the left end of the station's main solar power truss in September and it cannot be moved with either of its two wings even partially extended. Only three options were available:
Adding a spacewalk to Discovery's mission, it was believed, would force NASA to give up the late heat shield inspection. That was the going-in position when agency managers approved the 12-0-2 mission plan. But it soon became clear many in the post-Columbia shuttle program viewed the late inspection as a high priority regardless of where it ranked on NASA's official list. At the same time, station managers were not keen to add a spacewalk to the three-person Expedition 14 crew's already busy schedule. Spacewalks use the buddy system and a single astronaut would be required to stay inside and operate the retraction system as well as the space station's big robot arm. "We're choreographed pretty tightly now to get from here to assembly complete," station Program Manager Mike Suffredini told reporters Saturday night. "And the next step for us as a program is to go into this stage where (the Expedition 14 crew has) three EVAs where we have to finish the reconfigurations ... and hook up the lab to the central cooling system." Adding a solar array repair spacewalk would require "arm operations, two crew outside, commanding the solar array," Suffredini said. "That right there is at least four crew and you still haven't talked about your (spacewalk coordinator) crew person we normally have for each EVA." Station planners also were reluctant to add solar array repair work to the next shuttle assembly mission scheduled for March. During that flight, the other wing of the P6 array must be retracted and a new set of arrays installed. In the end, the MMT, chaired by John Shannon, decided Saturday to add a spacewalk to Discovery's mission to resolve the matter sooner rather than later. To the surprise of many shuttle observers, however, the MMT opted to retain the late inspection, giving up one of the two available landing weather contingency day instead. NASA has three shuttle landing sites in the United States: The Kennedy Space Center in Florida, Edwards Air Force Base, Calif., and Northrup Strip in White Sands, N.M. Landings at Edwards add more than a week to a shuttle's turnaround time. Only one shuttle mission ever ended at Northrup, in 1983, and it's not clear how much time would be needed to get Discovery back from New Mexico. As it turns out, the landing site is not as much of an issue for Discovery as it might normally be. Whenever the orbiter gets back to Kennedy, it will be taken off flight status for a major inspection and overhaul. The shuttle Endeavour, coming out of its own orbiter maintenance and down period, or OMDP, will serve as the emergency rescue vehicle for the March shuttle flight. But Discovery's next flight is STS-122, a high-priority mission scheduled for launch next October to carry the European Space Agency's Columbus research module into orbit. While NASA likely could make up any lost time getting Discovery back to Florida, the schedule is tight and program managers don't like to lose time on the front end of such an important processing flow. In any case, when only two landing days are available NASA flight rules require all three landing sites to be staffed and for the shuttle to come home on the first available opportunity. "With that day (Friday) being the day before the last opportunity - we have to be on the ground by Saturday - that means we're going to call up all three landing sites, Edwards, KSC and Northrup, and our intent will be to put the vehicle on the ground somewhere that day at one of those sites," said Phil Engelauf, a senior manager in the mission operations directorate at the Johnson Space Center. Engelauf said the unusual decision to give up a landing day turned on the importance many engineers attach to the late inspection, regardless of how it is officially classified. At orbital velocities, a 0.05-inch-wide piece of aluminum carries the kinetic energy of a 22-caliber long rifle bullet. The overall threat of a potentially catastrophic micrometeoroid strike, or an impact with space debris, is believed to be on the order of 1-in-250. The heat shield is photographed during launch and inspected in excruciating detail before docking. The late inspection is designed to spot any damage that might have occurred after that point, during the normal course of the mission. "It was a very complicated discussion and there were a lot of issues put on the table," Engelauf said. "Since return to flight, we've spent a lot of time talking about MMOD (micrometeoroid/orbital debris), which is one of the top risks in the program. We have wing leading edge sensors and we've signed up to add late inspection to the missions. We do go through a process before the mission, placing the mission activities in relative priority and the late inspection is usually placed below those activities that would cost us reflight." That means, he said, that NASA would give up late inspection in cases where a spacewalk or some other crew action could resolve a situation that otherwise might require another shuttle mission. That logic also would apply for an event that "might have a significant impact on the assembly sequence such that the next mission could not happen in its proper order or that we would have to change the sequence of the assembly because we had to redo an EVA task." "Trading that particular element, the importance of MMOD risk and the importance of the inspection and the relative newness of that topic in the program ... folks still believe that the priority to do that is extremely high," Engelauf said. He agreed that NASA has never before "given up a landing opportunity for routine payload operations or other objectives." "But this is not a routine objective," he said. "This is a fairly significant impact to the subsequent flights that puts it back up into that regime of whether or not it falls above late inspection to do the solar array work. When we added all of those things up, MMOD risk is eventually a safety discussion. And talking about the probability of safely getting the vehicle back on the ground. "When you compare that to the landing-site-opportunity discussion, it's a schedule risk issue, whether or not you're going to get to a preferred landing site and whether there's going to be a schedule implication. The program managers spent a lot of time thinking real hard about this decision and the ultimate conclusion was we wanted to protect the safety issue of late inspection over the schedule issue." Against that backdrop is the overriding drive to complete the station by 2010. Given a tight schedule and limited resources, NASA does not have the leeway it once did to handle unexpected problems. But Engelauf said safety would always trump any perceived schedule pressure. "There is a constant awareness and people are continuously on the lookout for situations where we could feel like there's pressure to do something that might be not in the best interest of safety of the program," he said. "I think in this particular case, all of those decisions were, in fact, put on the table, all of those decisions were talked about and there was pretty unanimous support for the decision we came up with. I don't recall anybody bringing up a dissenting opinion." As for the 2010 deadline, "that's not worth debating from my perspective," he said. "It is what it is. We are charged with executing this program with the tools that we have and I think the community is really doing an outstanding job of vigilance, of making good, safe and well-considered decisions independent of the other deadline issues." In the end, he said, "I think we've got a good workable plan, we think we've minimized the risk to both programs. We have incurred, I think, some programmatic schedule risk that we're going to put the orbiter at a landing site that might cost us a little bit more turnaround time. But we think the bigger picture trade warrants that, given the significance of getting this array retracted for station and what that means to the rest of the sequence."
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