EnterpriseSpace Shuttle Enterprise, mated to the heavily-modified Boeing 747 Shuttle Carrier Aircraft (SCA) for the first time, undergoes three “taxi tests” to enusre the structural stability of the two-vehicle combination on the runway before they ever take off. This is the first phase of a series of tests that will culminate, later in 1977, in a series of brief unpowered flights and landing tests using the Enterprise, verifying the shuttle’s gliding aerodynamics.