The launch of an Atlas-Centaur rocket that was to carry a Navy communications satellite into orbit on Aug. 28 has been postponed indefinitely, National Aeronautics and Space Administration officials said yesterday.

This is the second delay of the unmanned Atlas, the only remaining major American rocket system that has not been grounded by an in-flight accident this year.

The launch was postponed after engineers "found problems with some of the electronic parts" in the plant that manufactures them, NASA spokeswoman Marilyn Edwards said at Lewis Research Center in Cleveland, which manages the Atlas-Centaur program.

The parts at the plant "were from the same lots as those on the vehicle scheduled for launch and they need to go back and check them out," Edwards said, adding that no further details were available.

NASA spokesman Richard Young at Kennedy Space Center, where the launch was to take place, said the delay is "primarily a reassurance measure, rather than examining any known problem." He said a new target launch date may be set by early August.

The Atlas-Centaur (using an Atlas first stage and a Centaur upper stage) was originally scheduled for launch May 22, but that flight was delayed after an unmanned Delta rocket, with an engine similar to those in the Atlas, exploded shortly after launch on May 3. The Delta rocket, carrying a weather satellite, was blown up by safety officers after its main engine shut down prematurely.

Defense Department officials said the latest delay of the Atlas-Centaur launch poses "no significant problem" yet because there are communications satellites in orbit that are still "healthy" and can do the work of the one to be launched on the Atlas-Centaur.

The relatively small Atlas and the tiny Scout are currently the country's only means of space launch.

The space shuttle and Titan launch systems are the only ones powerful enough to carry the military's heaviest national security satellites aloft. The space shuttle program is grounded until at least early 1988, in the wake of the Jan. 28 Challenger accident that killed the crew of seven; the unmanned Air Force Titan 34D is grounded until at least early next year because one exploded in April, the rocket's second consecutive failure.

The Defense Department is buying new unmanned launch vehicles to carry military payloads, including Titan IVs and a medium launch vehicle, but they will not begin operation until at least 1989.