Retire the Space Shuttles Now

Last week, NASA announced that the same kind of fuel-sensor problem that delayed last summer's flight has cropped up again. Program managers decided this time to replace all four sensors with new ones, a process that will take three weeks and delay the next flight until sometime in July. It was originally planned for May. This is both good news and bad news.

The good news is that NASA managers are finally showing some conservatism in their approach to potentially catastrophic problems. The fuel sensors monitor fuel levels in the external tank, telling the engines to cut off before the liquid-hydrogen fuel runs out. If the fuel tank ran dry while the engines were still operating, the resulting oxygen-rich mixture could cause severe corrosion and damage to the engines. Under normal operation the sensors are not needed, but if two or more sensors gave a false "empty" reading, the resulting engine shutdown could force an emergency landing or even cause a crash. So NASA is showing wisdom in replacing all the sensors before attempting another launch.

The bad news is that once again, NASA is going into space with a flying antique. Major elements of the space shuttle design are now over thirty years old. NASA engineers routinely comb the web for surplus sales of outmoded electronic components to use for repairs on the shuttle. I own a pickup truck that was built in 1981, the year after the first shuttle flew. I still drive it around town, but I must confess I'm somewhat reluctant to take it on a 35-mile trip to Austin and back for fear of a breakdown or worse. Granted that the shuttle fleet has received a great deal more attention and refurbishing than my truck, the fact remains that for every year the existing shuttles are kept in operation, maintenance and operating costs rise and the chances of failure from a hitherto unexpected cause grow greater.

Every reliability engineer is familiar with the "bathtub curve" that shows rates of failure in a collection of components over time. Suppose you buy a thousand new light bulbs for a large institution such as a school or hospital, install them, and keep track of when they fail. A small number will blow out within a few hours of first being turned on. This is called "infant mortality" and is due to defects that did not show up at the factory's inspection. This is the downward-sloping end of the bathtub curve. Then for a long time, you will see a very low rate of failure, one or two every month, perhaps. This is the bottom of the bathtub. Finally, as the usual failure mechanisms start to act, the failure rate will rise toward the end of the rated lifetimes of the bulbs. This is the rising slope of the bathtub, and continues until virtually all the bulbs fail.

The shuttles have literally thousands of components, each with a particular lifetime. No doubt NASA reliability engineers have studied the problem extensively, and the fact that the remaining shuttles still work is mute testimony that the engineers have done something right. But as time goes on and numerous components are used far beyond their expected lifetimes, unusual and undocumented failure modes can start to show up. It's not normal for a car's wheel to fall off, but when I pushed the mileage of an old car past the 200,000 mile point, that's almost what happened. Every successful launch moves the shuttles closer to the next failure, and as time goes on, it will be harder and harder to predict what the failure might be. From an engineering perspective, the only sensible thing to do with such antiquated hardware is to retire it. But politics plays as much a role in what NASA does as engineering, if not more.

No one likes to kick an organization when it's down, so ironically, the 2003 Discovery disaster probably kept President Bush from doing the sensible thing and terminating the shuttle program in a timely way. But who knows how many more astronauts will die between now and 2010 when the program is scheduled to end?

Space is billed as the last great frontier, and no one pretends that space exploration is without its hazards. The Apollo program cost the lives of three astronauts in a 1967 launchpad fire. The accident investigation wrapped up in three months, the program continued, and we landed on the moon two years later. No great achievement is without risks, and the consensus at the time was that the risks were worth it.

No such consensus exists today. The primary mission for the shuttles these days is to support the international space station, which is itself an enterprise of dubious utility, plagued by cost overruns, equipment problems, and a signal lack of clarity in its goals and mission. Some continued presence of man in space is probably worth while. But the numerous recent successes in privately funded space efforts indicate that private enterprise can do everything NASA is doing with the shuttle at less cost, more safely, if private firms are given some good ground rules and sufficient funding to make a fresh start. If the U. S. government had taken the same attitude toward air travel that it has taken toward manned space flight, we would still be watching a few highly trained NASA aeronauts fly across the Atlantic in single-engined Spirits of St. Louis, if that much. Shut down the shuttle, open up the field to private competition, and let the idealism of a new generation of space explorers come up with something that old institutions cannot even conceive.

Sources: For more details on the Shuttle's external tank, see http://en.wikipedia.org/wiki/Space_Shuttle_external_tank