Martin Marietta Corp. said it will award a contract, expected to exceed $500 million, to Hercules Aerospace Co. for 15 sets of solid rocket motors to power the Titan 4 launch vehicle. The initial Titan 4 launches will use seven-segment motors that can carry a 10,000 pound satellite to a parking orbit. Hercules will make an upgraded three-segment version that can carry 12,500 pounds. Martin Marietta has an Air Force contract to build and launch 23 Titan 4s--the first in late 1988. The upgraded rocket is expected to be available by late 1990.