Military Shuttles.
'Like the shuttle it has delta-shaped wings which would seem to offer the same "cross-range" capability that the US military insisted upon for the bigger spaceplane: a capability intended for top-secret intelligence missions which would be extremely difficult for other nations to monitor – though in the event the Shuttle never actually flew such missions.'
The Soviet Union, which would have been the target of such missions, would have had no trouble monitoring polar launches of the Shuttle. They had tracking stations across the globe and even on ships designed for nothing else.
The military launches of the Shuttle were killed by the Shuttle's continued failure to achieve the number of launches that had been expected, its unreliability in meeting launch windows and finally by the explosion of Challenger caused by SRBs which were considerably stronger than those needed to launch out of Vandenberg.