A secret military satellite that a civilian expert said would provide instant warning of an enemy missile attack was propelled into orbit from the Kennedy Space Center in Florida by a Titan 34D rocket, scoring its second straight success after being grounded for 18 months. The Air Force did not announce the launch in advance, which has been its practice on military space launches, but 20 minutes after liftoff, it reported on the successful launch. However, the nature of the payload was not disclosed. An on-board motor was to be fired today to guide the craft to a stationary orbit 22,300 miles above the Earth. John E. Pike, a space policy expert for the Washington-based Federation of American Scientists, said the payload was one of a series of Defense Support Program early warning satellites that have been launched over the last decade.