NASA and the Pentagon are trying to assess the specific impact of this month's rocket fuel plant explosion in Nevada that threatens space agency plans for nine shuttle flights next year, officials say.

According to Rear Adm. Richard Truly, associate NASA administrator for space flight, the May 4 explosion is not a problem in the near term for either his agency or the Department of Defense."We're in good shape in both the shuttle program and also the DOD programs in the immediate future because naturally there's a supply that's already been delivered," Truly told a news conference Thursday.

"The issue is going to first come probably in 1989," he said. "But I can't say there is even going to be an impact in 1989 until we've had the time with the DOD to totally match requirements and availability.

"We're working on all parts of that problem - the possibility of increased production . . . new capabilities in plant production for the future. It is a very serious problem and we're working hard on it."

At issue is the availability of a chemical called ammonium perchlorate, which makes up about 70 percent of the solid propellant used by shuttle boosters. It also is used in a variety of military weapons systems.

A series of explosions May 4 destroyed an ammonium perchlorate production facility run by Pacific Engineering and Production Co. in Henderson, Nev. The plant was one of only two, both in Henderson, that can produce large amounts of the chemical known as AP.

NASA, with two shuttle flights planned for 1988 and nine for 1989, has only enough AP at the moment for five launches - and that includes 1.7 million pounds not yet delivered from the other AP plant, operated by Kerr-McGee Corp.

The second facility has a maximum production capacity of 36 million pounds, officials said. When the explosions leveled the Pacific Engineering factory, the Kerr-McGee plant was operating at 32 million pounds a year.

The rest of the Western world's AP production combined amounts to about 9.1 million pounds a year. The plant that was destroyed could produce 40 million pounds a year, although it was operating at about 20 million pounds a year at the time of the accident, officials said.