How Safe Is Commute To Heavens in World's Most Complex Vehicle?

SIX men who were to fly aboard the shuttle Endeavour earlier this month described their upcoming mission with smiles and comments as crisp as their NASA uniforms. Then came The Question: How do you prepare your families for the possibility that you might not come back?

Ten years after the shuttle Challenger exploded, killing seven astronauts and the myth of routine manned spaceflight, that possibility has grown more remote. Changes made to the world's most complex vehicle and to its launch procedures have significantly cut its risks.

Yet the program faces deep budget cuts and sweeping reorganization. It also faces a demanding schedule as a space ferry for the international space station. Experts inside and outside NASA worry that these pressures may erode the safety gains made since Challenger. The underlying question: Can America's manned space program survive the loss of another orbiter?

Safety improvements in the shuttle program are considerable. A study by the Science Applications International Corporation just after the Challenger disaster concluded that between launch and orbit, NASA could expect to lose an average of one shuttle every 78 missions. Last August, SAIC released a second risk assessment for the shuttle program indicating that NASA could expect to lose one orbiter every 248 missions.

Hardware improvements played a major role in reducing shuttle risks. Following the Challenger explosion, engineers redesigned the joints between segments of the solid-rocket boosters and added heaters to keep O-ring seals from hardening in cool weather. The space agency also accelerated its program to upgrade the shuttles' liquid-fuel main engines. Last year it began using new high-pressure turbopumps to feed the frigid fuel to the combustion chamber. The pumps use half as many rotating parts and require less maintenance. Other engine components have been redesigned to reduce the number of welds they contain. These improvements carry a $1-billion price tag.

''Redesigning the engines and rocket nozzle joints, and adding additional O-rings and leak-check test points were a good investment,'' says Joe Fragola, the study's project manager.

Aside from engines, three of the four orbiters are undergoing other changes to prepare them for space-station work and to cut costs. The changes range from lighter-weight external fuel tanks to navigation equipment that uses global-positioning satellites to more accurately and safely land or dock with other structures on orbit.

Other analysts point to post-Challenger changes in operating rules as factors in reducing risks. ''The safety programs adopted since Challenger are working,'' says a congressional staff member who closely follows the space program. He cites last year's discovery that fiery exhaust gases in the solid-rocket boosters were scorching O-rings in the boosters' nozzles. NASA officials ''found the problem and corrected it. They halted launches indefinitely until the problem was solved.''

Lori Garver, executive director of the National Space Society, an advocacy group based in Washington, adds that the agency has significantly reduced the number of ''criticality 1'' issues, in which the failure of a single component would result in the loss of an orbiter and its crew.

Yet the shuttle program is undergoing wrenching changes that could undercut the safety gains, some analysts say. Over the past three years, NASA has cut 6,000 workers from its shuttle program, says Wayne Littles, NASA's associate director for spaceflight. By 2000, another 7,500 will lose their jobs. At the same time the shuttle program's budget, which reached $4.2 billion in 1992, will fall to $2.8 billion by 2000.

In addition, the agency has selected a prime contractor to take over day-to-day shuttle operations. The move is part of a NASA effort to simplify procedures and cut costs. But some analysts worry that corporate efficiency may take precedence over shuttle safety.

''Downsizing has to be done right. You can't just cut across the board,'' says Frank Manning, executive director of the Aerospace Safety Advisory Panel (ASAP). Prior to Challenger, safety and quality-assurance programs ''got shoved into the background,'' he says. His panel was set up in 1968 as an independent check on NASA's programs following a fire that killed three astronauts during preparations for the Apollo 1 mission. But since Challenger, ''I've got to give the administrator credit,'' Mr. Manning adds. ''He continues to say that safety remains the No. 1'' shuttle priority.

NOT everyone is convinced, however. Last August, Jose Garcia, a shuttle operations manager at the Kennedy Space Center, wrote a three-page letter about what he saw as the harmful effects of downsizing on shuttle safety. He argued that the shuttles remain experimental craft that will grow less reliable as they age. More than half of the work done to turn orbiters around for a new launch is unplanned, he added. ''Our current system of 'checks and balances' is an integral part of safely doing this job. It is not an overlap or duplication of effort as suggested by some,'' he wrote.

But others, such as Ms. Garver, say some shuttle engineers argue that too many layers of redundancy have been added since Challenger. ''Hundreds of people making a decision that can be made by one doesn't necessarily make the decision more safe,'' she says.

''All too often, inspection and maintenance actions are signed off even when they're not completed,'' adds the congressional staffer. Another five signatures on the form are no guarantee the work has been done, he says.

Still, there are hardware concerns. For example, ASAP is keeping a close watch on cannibalizing parts to keep orbiters flying. ''You have to have enough spares readily available,'' Manning says. ''Every time you go in and remove and replace components, you're disturbing a system that works.''

Avoiding those disturbances, and the problems they can cause, becomes more crucial as space-station assembly approaches.

NASA and its international partners have contingency plans for continuing construction even if a shuttle is lost. The question is not whether the shuttle and space-station programs have the technical capability to withstand another orbiter loss. Rather, it is a question of whether the public and politicians would still back NASA if another Challenger-type disaster occurred.

''Whether the agency can withstand losing another orbiter I can't venture to guess,'' Mr. Littles says. ''Obviously, we're going to do everything within our power to prevent that from happening.''