By Joey Roulette and Steve Gorman CAPE CANAVERAL, Florida (Reuters) -The U.S. military’s secretive X-37B robot spaceplane blasted off from Florida on Thursday night on its seventh mission, the first launched atop a SpaceX Falcon Heavy rocket capable of delivering it to a higher orbit than ever before. The Falcon Heavy, composed of three rocket cores strapped together, roared off its launch pad from NASA’s Kennedy Space Center at Cape Canaveral in a spectacular nighttime liftoff carried live on a SpaceX webcast. The launch followed more than two weeks of false starts and delays. Three earlier c…