The Air Force announced yesterday that it has approved a model agreement that will serve as the basis for its support of the fledgling commercial-launch industry.

After the Jan. 28, 1986, Challenger accident, the Reagan administration ordered the National Aeronautics and Space Administration to stop carrying most commercial payloads aboard the space shuttle, as encouragement to the development of a commercial-launch industry. Working out arrangements for commercial launchers to use military launch facilities was a major hurdle.

"This is a highly significant agreement in commercial space support policy," said Edward C. (Pete) Aldridge Jr., secretary of the Air Force. "It is a major step in maintaining space leadership through cooperative effort between the Defense Department and commercial industry."

"We're ready to start negotiating agreements with contractors," said Air Force spokesman Marty Hauser. Details of the plan were not immediately available, he said.

A Martin Marietta spokesman said the company expects to sign a specific agreement soon for the use of Launch Complex 40 at Canaveral Air Force Station, Fla., with the first launch expected in 1989. McDonnell Douglas and General Dynamics also have commercial launch activities under way.

In another development yesterday President Reagan asked Congress for a $28 million down payment on the purchase of two Delta launch vehicles for NASA science payloads. The space agency is seeking approval for four of the rockets.