CAPE CANAVERAL, NOV. 12 -- A Titan 4 rocket, the nation's most powerful unmanned launcher, blasted into space with a secret military cargo today after a delay of nearly two months.

The 20-story-tall rocket ignited the sky as it thundered toward orbit at 7:37 p.m. The Air Force tried in September to launch the unmanned Titan, but had to delay the flight due to undisclosed technical problems.

NASA's two shuttle launch pads a few miles away -- one occupied by Atlantis and the other by Columbia -- were illuminated as the Titan roared into a starry sky and headed out over the Atlantic Ocean. Atlantis is awaiting a Thursday liftoff, also with a classified Defense Department satellite.

It was the Air Force's third launch of the Titan 4, valued at $173 million. The first one was launched in June 1989, and the second one last June.

Air Force officials refused to discuss or even acknowledge the launch before liftoff and would not divulge the nature of the payload, but civilian experts said they believed the rocket was carrying a $180 million advanced missile warning satellite.

A spacecraft of this type, called a Defense Support Program satellite, could be used to instantly detect the launch of Iraqi missiles against sites in the Middle East, said John Pike, director of the Federation of American Scientists' space policy project.

The Titan 4 has been on the launch pad since June.

Martin Marietta Space Launch Systems of Denver has a $7.1 billion contract with the Air Force to supply 41 Titan 4 rockets, with an option for eight more. The Air Force wants 75 by the end of 1997.