BOSTON — SITTING on a launch pad at the Kennedy Space Center, Discovery is poised for the first of seven space-shuttle missions planned for 1991. But once again, an unexpected technical problem threatens a tightly paced launch schedule. That schedule includes three of the 10 missions originally planned for last year. The delay to hunt down elusive hydrogen fuel leaks bumped them into 1991.
Now engineers are assessing cracks found in the hinges of flapper doors on both Discovery and Columbia, but not on the third orbiter, Atlantis.
These are four-foot-square doors in an orbiter's belly - two per space shuttle - that close automatically when the external fuel tank falls away during launch. They must be tightly sealed for safe reentry.
Columbia, the oldest orbiter, has fewer and smaller hinge cracks than does Discovery.
Space Center spokesman Bruce Buckingham explains that Columbia is being used ``as a test bed of sorts to simulate the problem on Discovery.''
Shuttle officials are reviewing the findings and should soon decide on what to do.
Mr. Buckingham says that, if Discovery can fly ``as is'' or be repaired on the pad, it should be able to carry out its Defense Department mission as scheduled in the first half of March. But, if Discovery has to come off the pad to get new hinges, he says that mission would likely be delayed for several weeks or more.
Whatever happens, this incident reinforces the concern felt throughout the United States space community that the shuttle is not a robust launch system.
The National Aeronautics and Space Administration (NASA) advisory committee, chaired by Martin Marietta chief executive officer Norman Augustine, stressed this point in reviewing the space program last autumn. It warned that it is unwise to continue to rely heavily on the shuttle, especially for missions that just carry a satellite or space station material into orbit.
The lack of robustness is not only a hardware problem. It also reflects the fact that safety now has priority over scheduling.
NASA officials stressed this when they delayed missions last summer to fix tiny fuel leaks they might have ignored before the Challenger accident. Vice President Dan Quayle reiterated the policy when he visited the Kennedy Space Center last week.
``We are certainly in no rush to launch this [Discovery] payload. We will never sacrifice safety to meet a schedule,'' he said.
However, this does put a question mark over NASA's mission planning. The latest update envisions eight shuttle missions in 1992 and 11 in 1993 when Challenger's replacement Endeavor joins the fleet.
A year ago, the plans set 11 missions in 1992 and 14 in '93.
President Bush is asking for a total of $350 million for NASA and the Defense Department in 1992 to start work on a heavy lift rocket that the Augustine committee recommends as an alternative to the shuttle. This would enable NASA to use shuttles only when astronauts are essential.