Commercial crew vehicles may fall short of safety threshold

WASHINGTON — The two companies developing commercial crew vehicles for NASA may not be able to meet a safety threshold specified in their contracts, an agency safety panel found.

At a meeting May 25 of the Aerospace Safety Advisory Panel at NASA’s Marshall Space Flight Center, members said Boeing and SpaceX were making good progress in improving the safety of their vehicles in advance of test flights scheduled to begin within the next year, but have yet to achieve a key requirement in their Commercial Crew Transportation Capability (CCtCap) contracts.

That requirement is known as loss of crew (LOC), a measure of the probability of death or permanent disability of one or more people on a spacecraft during a mission. The CCtCap contracts included a requirement that the spacecraft have a LOC of 1 in 270 or better. The shuttle program, by comparison, had a LOC of 1 in 90 at the time of the program’s retirement in 2011.

“The number one safety-related concern for the program is the current situation with respect to the estimate of loss of crew,” Donald McErlean, a former engineering fellow at L-3 Communications and a member of the panel, said at the meeting. “The threshold values were considered to be challenging, and both contractors currently have a challenge to meet that precise number.”

McErlean didn’t identify specific issues the companies were facing in their efforts to meet that LOC threshold. One factor, though, he said, is how the companies and NASA calculate the risk to the spacecraft from orbital debris and micrometeoroids while in orbit.

“The numbers themselves depend very heavily on the model of orbital debris that one utilizes to develop the risk to the system,” he said. “That is a driving factor in determining the potential for loss of crew.” Those models, he said, “have been validated to some degree, but they are not perfect.”

That assessment matches a February report by the U.S. Government Accountability Office, which…

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