Jones: Joe mentioned earlier that another Shuttle loss would result in halting U.S. human spaceflight until motivation and safety caught up with ambition. I agree, and agree with the cautious approach to launching post-Columbia. Griffin’s call was correct, in my view, because we were relatively confident that the crew would survive another foam strike, and that if we hope to use the Shuttle practically, we will need to accept its many designed-in risks.
If we encounter another serious system problem, short of disaster, that will take a year and billions to fix, I would accept the inevitable and ground the Shuttle for good. The space station will have to be finished by other means. The smart move then would be to shift resources to a safer crew vehicle and unmanned heavy lift methods (existing or future boosters) to get remaining space station hardware into orbit. At that point we would also wisely debate what ISS assembly still made sense.
Reichhardt: How likely is it that over the course of 16+ flights we’ll encounter another problem that takes a year and billions to fix? 10 percent chance? 50 percent?
McCurdy: Statistically, I would estimate the probability of a flight-stopping event over the next four years of Shuttle operations to be somewhere between 5 and 25 percent. Stated in its converse form, NASA probably has a 75 to 95 percent chance of completing the planned Shuttle flights. The Shuttle remains a risky technology, but the risk is manageable.
Historically, NASA has gone to a “ground and redesign” situation in its human spaceflight program only three times—after the Apollo 204 fire in 1967 and after the two Shuttle accidents. A catastrophic accident triggered each. Actual experience puts the probability of a catastrophic Shuttle accident at slightly less than two percent per mission (two catastrophic events spread over 115 missions.) NASA officials believe that they have improved those odds through Shuttle redesign and a heightened safety culture. They probably have.
Assume that they have reduced the likelihood of a catastrophic loss from 2 in 115 to something like 1 in 350 flights. The probability of completing 16 consecutive, successful missions under that assumption would be about 95 percent. If the old odds hold, and the accident rate remains high, then the chances of completing sixteen consecutive missions fall to about 75 percent. Clearly, culture and technology matter in determining the odds of success.
I personally doubt that we will ever see another Shuttle “ground and redesign” situation like the one we have just endured. Shuttle managers will probably fly through any remaining problems, making incremental changes as they go. If the problem is big enough to stop flying, the remaining orbiters are more likely to wind up in museums than in the redesign shop.
Rothenberg: Based on Shuttle statistics alone, I would say the chances are less than 10% that there will be a significant problem requiring a major recovery effort. Having said that, I believe in practice there is closer to a 50% probability that the Shuttle will have a problem which costs more time and money to recover from than expected. One example would be a Shuttle main engine shutdown in flight, with the orbiter still reaching orbit. In the past, postfight recovery activities might have been limited to understanding and fixing the problem. Today we would have a lot of external pressure to re-evaluate all options to make it even safer.
Reichhardt: The trick for NASA management will be predicting the downtime (and the related costs) of these problems as they happen. The agency’s track record isn’t good here.
Really, it’s the used-car dilemma. When do you do the repair, and when do you buy a new car? NASA is already making payments on the new car, the CEV. But if they spend too much on repairs, they’ll have to delay buying the new one, or, as we’ve seen, start canceling other projects like the Terrestrial Planet Finder that have great public appeal.