Air Force managers Saturday cleared the first of 20 new Delta 2 rockets for blastoff Sunday on a maiden voyage marking the birth of a new era in America's resurgent military space program.

The 128-foot rocket, the first of a new breed of unmanned boosters ordered in the wake of the Challenger disaster, was scheduled to take off at 1:38 p.m. Sunday from launch complex 17A at the Cape Canaveral Air Force Station adjacent to the Kennedy Space Center.Air Force mission managers said Friday they were concerned about possible bad weather. "We're still go," said an Air Force spokesman.

The goal of the first major American spaceflight of 1989 was to place the first of a new breed of $65 million Global Positioning System -!G1/4S - navigation satellites into orbit.

The mission marks the rebirth of the nation's military space launch program and the debut of the Delta 2, one of three new unmanned rocket systems ordered in the wake of the 1986 shuttle disaster to provide independent access to space for high-priority military payloads.

McDonnell Douglas Space Systems Co., of Huntington Beach, Calif., was awarded a $316.5 million contract Jan. 21, 1987, to build seven Delta 2s. The contract included options, however, for 13 additional vehicles for a total contract value of $669 million.

All 20 Delta 2s, upgraded versions of the company's workhorse Delta rockets launched for years by NASA, will be used to launch a fleet of Global Positioning System satellites.

Built by Rockwell International's Satellite Systems Division in Seal Beach, Calif., GPS satellites will provide military forces around with world with altitude, longitude and latitude information with an accuracy of aout 50 feet.

The satellites originally were scheduled to be launched by NASA's manned space shuttle as were all other major military payloads because of decisions in the 1970s to phase out unmanned rockets and to make the shuttle the nation's premier launch vehicle.

But even before the loss of Challenger, the Pentagon ordered 10 shuttle-class Titan 4 rockets from Martin Marietta Astronautics Group of Denver to serve as backups to the shuttle in case of an accident or problem that might ground the complex orbiter.