Читать книгу Judgment Calls - Thomas H. Davenport - Страница 18

The problem that faced the engineers and scientists who took part in the FRR for STS-119 came to light during the previous shuttle mission, STS-126. Shortly after that spacecraft, Endeavor, lifted off from Kennedy Space Center on November 14, 2008, flight controllers noticed an unexpected hydrogen flow increase related to one of the shuttle's three main engines. Because three control valves work together to maintain proper pressure in the hydrogen tank, the other valves compensated for the malfunction and the flight proceeded safely. But before another mission could fly, the shuttle team would need to understand why and how the problem occurred, whether it was likely to happen again, and just how dangerous a recurrence might be.

Bad weather in Florida forced Endeavor to land in California on November 30 and the shuttle was not returned to Kennedy until December 12, delaying examination of the faulty valve by almost two weeks. X-rays showed that a fragment of the valve's poppet (a tapered plug that moves up and down to regulate flow) had broken off. So the risks engineers had to consider included not only the kind of hydrogen flow anomaly they had seen on STS-126, but the possibility that a poppet fragment racing through propellant lines might rupture one of them. The level of risk depends on two factors: the likelihood of a problem happening and the seriousness of the consequences if it does. The consequences of a ruptured line would be disastrous, so the likelihood had to be extremely low to make the risk acceptable. The necessary technical analysis would have to have two major components: studying the valve to determine why the poppet broke, as a way of understanding the probability of a similar failure; and figuring out whether a poppet fragment was at all likely to breach the propellant system.

Because the valve is part of a system that included the shuttle, the main engines, and the external fuel tank, responsibility for understanding its failure lay with teams at the Johnson Space Center in Houston, the Marshall Space Flight Center in Huntsville, Alabama, and several NASA contractors, including a division of Boeing. They began work on these issues. The process proved challenging.

The first flight readiness review for STS-119 took place on February 3. It quickly became apparent that the technical teams did not yet understand the problems well enough to certify that the next shuttle spacecraft for this mission, Discovery, was ready to fly. Steve Altemus, director of engineering at Johnson Space Center, said, “We showed up at the first FRR and we're saying, ‘We don't have a clear understanding of the flow environment, so therefore we can't tell you what the likelihood of having this poppet piece come off will be. We have to get a better handle on the consequences of a particle release.’ ” The launch was rescheduled for February 22—overoptimistically, as it turned out—and the technical teams kept working.

They faced tricky problems. X-ray analysis had determined that the poppet failed because of high-cycle fatigue—that is, damage caused by repeated use. Unfortunately, these components were no longer manufactured and were in short supply, so the option of acquiring new, unstressed poppets did not exist. Given that fact, a reasonable approach could be to examine poppets for cracks that might indicate potential weakness; a poppet with no cracks seemed extremely unlikely to fail. But even electron microscopes could not reliably locate tiny cracks unless the poppets were polished first, and polishing subtly changed the hardware, invalidating its flight certification.

Trying to determine whether a poppet fragment might puncture a fuel line was made even more difficult because of the complexity of the fluid dynamics analysis necessary to determine the velocity, spin, and probable path of fragments of different sizes. The behavior of rapidly moving fluid is notoriously hard to predict. NASA's Glenn Research Center, Stennis Space Center, and the White Sands Test Facility began impact testing to simulate and try to understand the problem.

A second FRR was scheduled for February 20. Scott Johnson, chief safety officer for the shuttle, noted that “the majority of the safety community was concerned about the amount of open work in front of us. As a result, I recommended that we delay the FRR. We still had a lot of analysis work going on. We weren't really that close to being able to quantify the risks.”