The Air Force will have to build a $40 million wind screen around its space shuttle launch pad at California's Vandenberg Air Force Base to protect the shuttle while it is being fitted with the huge external fuel tank it carries on its way into orbit.

The Air Force is constructing a second launch complex in California to put the shuttle into the North-South, or polar, orbit. That orbit, combined with the Earth's rotation, will carry it over the entire surface of the Earth. Military reconnaissance satellites use this orbit, as do many weather and scientific satellites.

The shuttle cannot go into polar orbit from Cape Canaveral because of safety considerations: it would have to fly over some of the largest cities on the East Coast to do so. From Vandenberg, it would be launched over the Pacific Ocean.

Hans Mark, deputy administrator of the National Aeronautics and Space Administration, told the House Committee on Science and Technology yesterday that the Vandenberg wind screen will be a massive three-sided building of sheet metal surrounding the launch pad.

A little shorter than the 200-foot-tall tower, the wind screen will be fastened to the tower to shield it from the gusts that could sway it while the fuel tank is being fastened to the shuttle.

"It is essential if we are to provide an environmental shelter for mating the external tank to the shuttle," Mark testified at the committee's first day of hearings on the NASA budget.

NASA Administrator James M. Beggs told the committee that the space agency has firm plans to buy four space shuttle orbiters, and will make a decision soon on whether to purchase a fifth to use as a backup.

"It is possible we'll ding one of these machines up from time to time," Beggs said, the first time he has mentioned such a possibility in public. "It's very important that we have a backup ready in case we have an accident. I hope to God we never have an accident, but there's always the chance that one will happen."

NASA officials have always worried that a shuttle craft will damage a nose wheel on landing, which would force it to make a crash landing that would seriously damage the electronics in the belly and nose of the craft. Such an accident could put a shuttle out of operation for as long as a year.

According to the shuttle environmental impact statement, winds coming off the Pacific Ocean at Point Arguello, north of Santa Barbara, where Vandenberg is located, average almost 9 mph year round.

Gusts of 47 mph are routine during January, February and March. That would be more than enough to break the connections between the shuttle and the tank and perhaps topple the tank onto the ground.

"There is a tolerance of 31 one-thousandths of an inch betwen the wall on the external tank and the socket on the shuttle orbiter during the mating procedure," a spokesman for the Air Force Systems Command said. "Nobody knew this tolerance even existed before the first shuttle launch from Cape Canaveral last year."

There is no need for a wind screen at Cape Canaveral, because the shuttle is mated to its fuel tank inside the giant Vehicle Assembly Building. There is no such building at Vandenberg.

The Air Force said construction of the massive wind screen is expected to begin early next year, pending congressional approval. Construction is to be completed in late 1984, and the first West Coast shuttle launch is scheduled for 1985.