A Titan rocket Friday propelled into space a secret military payload that a civilian expert identified as a satellite capable of eavesdropping on Soviet military and diplomatic communications.

The powerful, 16-story-tall Titan 34D, scoring its third straight success after being grounded for 18 months, thundered from its launch pad at 8:05 a.m.The Air Force did not announce the launch in advance, which has been its practice on military space flights, and afterward said only that the Titan 34D, "carrying a classified military payload," was successfully launched.

John E. Pike, a space policy expert for the Washington-based Federation of American Scientists, said last month the payload would be a satellite to monitor Soviet missile tests and radio, telephone, radar and other electronic military and diplomatic communications.

He said the payload carried an antenna built to unfold to the size of a baseball field.

Pike said the satellite project originally was code-named Chalet but was changed to Vortex when Chalet became known. Vortex became compromised some time ago and yet another name may have been substituted, he said.