first time. Enterprise underwent four more free flights where the craft separated from the SCA and was landed under astronaut control. These tests verified the flight characteristics of the orbiter design and were carried out under several aerodynamic and weight configurations. The first three flights were flown with a tailcone placed at the end of Enterprise aft fuselage, which reduced drag and turbulence when mated to the SCA. The final two flights saw the tailcone removed and mockup main engines installed. On the fifth and final glider flight, pilot-induced oscillation problems were revealed, which had to be addressed before the first orbital launch occurred. Mated Vertical Ground Vibration Test (MGVT) Following the conclusion of the ALT test flight program, on March 13, 1978, Enterprise was flown once again, but this time halfway across the country to NASA's Marshall Space Flight Center (MSFC) in Alabama for the Mated Vertical Ground Vibration Testing (MGVT). The orbiter was lifted up on a sling very similar to the one used at Kennedy Space Center and placed inside the Dynamic Test Stand building, and mated to the Vertical Mate Ground Vibration Test tank (VMGVT-ET), which in turn was attached to a set of inert Solid Rocket Boosters (SRB) to form a complete shuttle launch stack, and marked the first time in the program's history that all Space Shuttle elements, an Orbiter, an External Tank (ET), and two SRBs, were mated together. During the course of the program, Enterprise and the rest of the launch stack would be exposed to a punishing series of vibration tests simulating as closely as possible those expected during various phases of launch, some tests with and others without the SRBs in place. Planned preparations for spaceflight At the conclusion of this testing, Enterprise was due to be taken back to Palmdale for retrofitting as a fully spaceflight capable vehicle. Under this arrangement, Enterprise would be launched on its maiden spaceflight in July 1981 to launch a communications satellite and retrieve the Long Duration Exposure Facility, then planned for a 1980 release on the first operational orbiter, Columbia. Afterward, Enterprise would conduct two Spacelab missions. However, in the period between the rollout of Enterprise and the rollout of Columbia, a number of significant design changes had taken place, particularly with regard to the weight of the fuselage and wings. This meant that retrofitting the prototype would have been a much more