CAPE CANAVERAL, NOV. 28 -- A secret military satellite that a civilian expert said would provide instant warning of an enemy missile attack was propelled into orbit tonight by a powerful Titan 34D rocket scoring its second straight success after being grounded for 18 months.

The 16-story-tall rocket blazed away from its launch pad at 10:27 p.m. and was visible for nearly six minutes as it etched a fiery path in a star-filled sky.

The Air Force did not announce the launch in advance, which has been its practice on military space launches for several years. But 20 minutes after liftoff, it released a statement saying the rocket had been launched successfully.

The satellite was placed in an initial elliptical orbit. An onboard motor was to be fired Sunday to arrest it in stationary orbit 22,300 miles above a fixed point on Earth.

Although tonight's payload was not identified by the Air Force, John E. Pike, a space policy expert for the Washington-based Federation of American Scientists, said it was one of a series of Defense Support Program satellites that have been launched over the last decade to provide early warning of a hostile missile attack.

The success was the second straight on the comeback trail for the Titan 34D, the nation's most powerful unmanned rocket, and gave a large boost to Air Force plans to launch a backlog of military satellites that had been grounded because of earlier Titan failures and the explosion of space shuttle Challenger.

A Titan 34D carrying a reconnaissance satellite was launched successfully Oct. 26 from Vandenberg Air Force Base, Calif.

Before the October launch, the Titan 34D had been grounded for 18 months following the explosion of one of the rockets shortly after liftoff in April 1986. That failure followed the explosion of another Titan 34D in August 1985.