Big question of what to build remains after Challenger study. Choice is difficult because much space technology is out of date

Congress has raked over the Challenger investigation findings. But it has left the big questions unanswered. Should NASA build a replacement orbiter or should it make do with three shuttles and go for a ``next generation'' spacecraft?

Should the space station project be slowed down -- even postponed -- or should NASA pursue it vigorously?

Should NASA look to private enterprise to supply a fleet of unmanned launch rockets, or build this itself?

And, most important, how will such things be paid for? Will the civilian space program be given significant new money or will NASA have to cut back severely on space science and other programs to regain manned-spaceflight momentum?

The Presidential Commission of the Space Shuttle Challenger Accident did not go into such matters. But it did point out that the pressures of trying to do too much with too few resources led NASA to cut safety corners. Commission chairman William P. Rogers has warned that the overarching issue of adequately supporting the projects NASA undertakes must be resolved at the same time that the management and engineering flaws that the commission found are corrected.

President Reagan -- who reportedly favors replacing Challenger -- has not announced a decision on what to do. Some of his advisers, especially chief of staff Donald T. Regan, question the wisdom of buying ``obsolete'' technology.

Experts point out that the term ``obsolete'' is misleading. Shuttle technology is old. But the technology to replace it is far from ready.

Joseph Loftus, planning assistant to the director of the Johnson Space Center (JSC) -- lead agency for the space station and shuttle -- says research on advanced materials and other components of next generation spacecraft has lagged for more than a decade. He explains that such advanced research was put off as NASA concentrated on developing the shuttle and, more recently, on space station design.

Mr. Loftus says NASA needs extra money in its budget now ``to start the kind of technology programs so that there are things that are mature enough that you can pursue them in 1995.'' Only then, he adds, would engineers be able to recommend what next-generation manned spacecraft would be like.

The extra money would amount to a few hundred million dollars over 8 to 10 years. That is not much compared with $2 billion to $3 billion for a replacement shuttle. Loftus notes, however, that the new spacecraft models probably would not be operational until the 21st century.

On the other hand, JSC director Jesse W. Moore says a replacement shuttle probably would join the operational fleet about 4 years after it was ordered. That would give NASA a four-orbiter fleet around 1992, when it would start to deploy its space station, if it sticks to the original schedule.

President Reagan has repeated his desire to keep the station on schedule. So, Mr. Moore says, NASA really needs that fourth orbiter. Being restricted to a three-orbiter fleet ``will hurt us in the way of the overall deployment and operation of the station,'' he says.

Thus, waiting for an advanced spacecraft would cost the United States extensive delays in its manned-spaceflight program. It is unclear how much money such delay would save in the long run. Congress had asked the National Aeronautics and Space Administration to consider a three-year phase-in during which the space station would not be permanently manned. NASA now estimates such a stretchout of the station schedule would add $1 billion to initial station costs. It also would cripple plans of prospective users impatient to run extensive materials and biological experiments that require the presence of scientists and research engineers.

Meanwhile, the US has been reminded that its Soviet competitors are gaining the advantage. Editor Reginald Turnill warned in the latest edition of Jane's Spaceflight Directory, published June 17, that the Soviets are so far ahead that ``they are almost out of sight.''

The Soviets are pressing their advantage diplomatically. On June 20, they formally invited other nations to join in a world space organization under United Nations sponsorship. And they have invited Britain to send a quest astronaut to their program.

Gen. Georgy Beregovoy, who heads the Soviet manned-spaceflight program, noted that British astronaut Nigel Wood scheduled to fly on the US shuttle this month would be indefinitely grounded, according to the trade journal Aviation Week.