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 the shuttle's aerodynamics and handling characteristics in preparation for orbital flights with Columbia. (NASA Photo) › View Larger Image
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 undertook a series of flight tests, known as the Approach and Landing Test, or ALT, 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 didn't have a thermal protection system. The flight deck had two crew stations for the commander and pilot.
Flight crewmembers of Enterprise and the host NASA 747 SCA included, from left, Fitz Fulton, Gordon Fullerton, Vic Horton, Fred Haise, Vincent Alvarez and Tom McMurtry. (NASA Photo) › View Larger Image Aerodynamic controls included a body flap at the aft end, elevons and a split rudder that doubled as a speed brake. 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 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 about 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 parameters for ALT operations. Three captive-active flights followed, with Enterprise powered up and crew in its cockpit to test controls and other functions.
The final phase of the ALT program included five free flights during which the orbiter was released from the SCA and glided to a landing at Edwards.
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 a 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 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 launch of Columbia on April 12, 1981.