Crew 1

Haise had previously flown as the Lunar Module pilot of Apollo 13, and was named as the commander of the original STS-2 mission.^{[note 1]} Fullerton later flew as the pilot of STS-3 and commanded STS-51-F.

Crew 2

This crew later flew on STS-2. Engle was originally a USAF pilot on the X-15, and had already gained Astronaut wings by the time he joined NASA. He flew his second Shuttle mission on STS-51-I.^{[note 2]} Truly flew his second Shuttle mission as commander of STS-8.

Shuttle carrier aircraft

In addition to the two assigned Shuttle crews, who would alternate crewing the orbiter, a single flight crew was attached to the Boeing 747 Shuttle Carrier Aircraft (SCA) for the entire program:

Captive flights

Enterprise during a captive flight. Note high nose position relative to ferry flights.

The captive flight phase of ALT saw the SCA/orbiter combination in flight as a test of the SCA's flying characteristics while mated to the orbiter, and as an initial test of the orbiter systems in flight. This was subdivided into two phases:

Free-flight

Enterprise on its approach during the second free-flight

The final phase of flight testing involved free-flights. These saw Enterprise mated to the SCA and carried to a launch altitude, before being released to glide to a landing on the runways at Edwards AFB. The intention of these flights was to test the flight characteristics of the orbiter itself, on a typical approach and landing profile from orbit.^[5][6][7]

For the approach and landing tests, a nose strut longer than those employed in later ferry flights increased the shuttle's angle of attack relative to the 747. Prior to the orbiter being released, the 747 engines were set to full power and the paired aircraft entered a shallow dive. Increased air speed combined with the shuttle's higher angle of attack generated enough differential lift so that the shuttle was effectively supporting the 747. Load cells on the three attachment points monitored the forces, informing the crew when the attachments were in sufficient tension. The mechanical connection between the two aircraft was then severed by the use of explosive bolts and the shuttle essentially dropped the 747.^[8] The shuttle crew reported feeling an upward lurch on separation. The two aircraft then turned in opposite directions to maximize separation. The shuttle executed some more turns to evaluate its handling and glided to a landing.^[9]

Enterprise on SCA for a ferry flight. Note lower nose position vs. approach and landing tests.

There were a total of five free-flights between August and October; the first three saw Enterprise remain fitted with its aerodynamic tail cone, intended to reduce drag when mounted on the SCA during flight. The final two had the tail cone removed, with the orbiter in its full operational configuration, with dummy main engines and OMS pods.^[10] Enterprise used an air data probe mounted on its nose for these flights. These five flights were to be the only time Enterprise flew alone.^[11][12]

After flying missions on Columbia (STS-2) and Discovery (STS-51-I), Engle reported that the flight and handling characteristics of the operational orbiters were similar to those of Enterprise, except that he had to fly a steeper profile with the prototype, as it was much lighter than the operational spacecraft.^[13]

Ferry flights

Following the free-flight tests, Enterprise was prepared for ferry flight tests, which were intended to ensure that the SCA/orbiter configuration was viable for flights of the duration between landing and launch sites.^[14]