The first of Enterprise’s five free flights from the NASA 747 Shuttle Carrier Aircraft at Dryden in 1977 were part of the shuttle program approach and landing tests. The tests verified orbiter aerodynamics and handling characteristics in preparation for orbital flights with Columbia. A tail cone over the main engine area of Enterprise smoothed turbulent airflow during flight. It was removed on the last two free flights to enable accurate check of approach and landing characteristics. (NASA Photo) › View Larger Image
Proving Grounds: Enterprise Validated Shuttle Concepts
The space shuttle orbiter was the first spacecraft designed with the aerodynamic characteristics and in-atmosphere handling qualities of a conventional airplane. To evaluate the orbiter's aerodynamic flight control systems and subsonic handling characteristics, Dryden Flight Research Center undertook a series of flight tests, known as the Approach and Landing Test program, at Edwards Air Force Base in 1977.
A full-scale orbiter prototype, named Enterprise, was built for the program. Because the vehicle would not be subjected to reentry heating, Enterprise had no need for a thermal protection system. It was not covered with the space shuttle's reusable surface insulation, but with substitute materials, primarily polyurethane foam and Fiberglas. The flight deck had two crew stations for the commander and pilot. Aerodynamic controls included a body flap at the aft end, elevons and a split rudder that doubled as a speed brake. Reaction control systems, unnecessary at low altitude, were not installed. For the captive flights and the first three free flights, an aerodynamic fairing covered the orbiter's aft end. Three dummy main engines were installed for the final two flights to simulate weight and aerodynamic characteristics of an operational orbiter.
The Enterprise was to be carried aloft by, and eventually released for flight from, a modified Boeing 747. This Shuttle Carrier Aircraft, as it came to be known, had a fuselage strengthened at key stress points, two vertical fins attached to the horizontal stabilizers, and three attach points on top of the fuselage to anchor the orbiter. All original seating except that of the first-class section of the main deck was removed.
NASA selected two two-man orbiter crews for the ALT: Fred W. Haise Jr. (commander) and C. Gordon Fullerton (pilot), and Joe H. Engle (commander) and Richard H. Truly (pilot). Crewmembers for the 747 SCA included pilots Fitzhugh L. Fulton Jr. and Thomas C. McMurtry and flight engineers Victor W. Horton, Thomas E. Guidry Jr., William R. Young and Vincent A. Alvarez.
Wind-tunnel-model tests allayed concerns over the separation characteristics of the two vehicles in flight. Because of the orbiter's positive angle of attack while mated, the Enterprise tended to climb relative to the SCA. Meanwhile, the 747 tended to descend mildly as the crew idled the engines and deployed spoilers, allowing the orbiter to clear the SCA's tail in about 1.5 seconds.
Five captive flights with the inert, unmanned orbiter verified the airworthiness of the 747 as an orbiter transport vehicle and established an operational flight envelope for ALT operations. These were followed by three captive-active flights, with Enterprise powered up and crew in its cockpit to test controls and other functions.
The final phase of the ALT program comprised five free flights during which the orbiter was released from the SCA and glided to a landing at Edwards. Three of these were made with the aerodynamic tailcone on the orbiter, but the last two were made with the tailcone replaced by dummy engines in an effort to replicate actual flight conditions experienced by an orbiter returning from space.
Except on the last free flight, Enterprise landed on Rogers Dry Lake. The final flight ended on the 15,000-foot concrete runway at Edwards, an important demonstration of precision landing capabilities necessary for later operational missions.
At touchdown, the orbiter experienced a pilot-induced oscillation, or PIO, in which the vehicle skipped and bounced down the runway several times before safely coming to a stop. Prior to the start of the shuttle's orbital flight-test program, the PIO issue was corrected through additional research with Dryden's F-8 Digital Fly-By-Wire test bed aircraft, which was equipped with an IBM AP-101 flight control computer identical to that used on the orbiter. Dryden engineers recreated the PIO with the F-8 and developed a software filter to correct for it.
The ALT program demonstrated the orbiter's capability for safe approach and landing after an orbital flight from space. It also validated crucial onboard control systems necessary for the shuttle program's next step: the launch of Columbia, on April 12, 1981.
By Peter Merlin
Dryden History Office