Thirty-nine years ago this week, the Space Shuttle Orbiter Enterprise successfully completed the first free flight of the Approach and Landing Tests (ALT) Program. NASA Astronauts Fred W. Haise, Jr. and Charles G. “Gordon” Fullerton were at the controls of the pathfinder orbiter vehicle (OV-101).
Developers of the Space Shuttle Orbiter faced the challenge of designing a vehicle capable of flight from 17,500 mph (Mach 28) at entry interface (400,000 ft) to 220 mph (Mach 0.3) at landing. Complicating this task was the fact that the Orbiter flew an unpowered, lifting entry that covered a distance of more than 4,400 nm. Once at the landing site, Shuttle pilots had a single opportunity to land the winged ship.
An Orbiter’s approach to the landing field is quite steep compared to that of a commercial airliner. Whereas the glide slope of the latter is around 2-3 degrees, the Orbiter’s flight path during approach is about 22 degrees below the horizon. Falling like a rock is an apt description of its flight state.
The Space Shuttle Approach and Landing Tests (ALT) involved a series of flight tests intended to verify the subsonic airworthiness and handling qualities of the Orbiter. Conducted at Edwards Air Force Base between February and October 1977, the ALT employed a modified Boeing 747 known as the Shuttle Carrier Aircraft (SCA). The Orbiter Enterprise was attached atop the SCA to hitch a ride to altitude.
The Shuttle ALT consisted of a total of thirteen (13) flight tests; five (5) Captive-Inactive (CI) tests, five (5) Captive-Active (CA) tests and three (3) Free Flight (FF) tests. CI testing was aimed at verifying the handling qualities of the SCA-Orbiter combination in flight. There was no crew was onboard the Orbiter for these tests. CA testing focused on preparing for the upcoming free flight series. A crew flew onboard the Orbiter which remained mated to the SCA.
The ALT Free Flights were where the rubber met the road so to speak. The Enterprise and her crew separated from the SCA at altitudes ranging from between 19,000 and 26,000 ft to test the Orbiter in free flight. Landings were made initially on Rogers Dry Lake (Runway 17) and ultimately on Edwards’ 15,000-ft concrete runway (Runway 22).
The Enterprise was flown in two (2) different configurations. The first involved the use of a tailcone fairing which streamlined the base region of the Orbiter. This increased the Orbiter’s lift-to-drag ratio which decreased the vehicle’s rate of descent. It also reduced the level of buffeting experienced by the SCA’s empennage while the Orbiter rode atop the carrier aircraft.
The second Enterprise configuration flown involved removal of the tailcone. This significantly reduced the Orbiter’s lift-to-drag ratio and correspondingly increased the rate of sink. Indeed, the Orbiter’s descent rate without the tailcone was roughly twice as high as that with the tailcone. Removal of the tailcone also markedly increased the buffet loads sustained by the SCA’s empennage.
ALT Free Flight No. 1 took place on Friday, 12 August 1977. With Fitzhugh L. Fulton, Jr. and Thomas C. McMurtry flying the SCA (N905NA), the Enterprise and her crew of Haise and Fullerton was carried to an altitude of 24,100 ft. At a speed of 310 mph in a slight dive, the big glider cleanly separated from the SCA. Just 321 seconds later, the Orbiter touched-down on Rogers Dry Lake at 213 mph.
ALT Free Flights No. 2-5 were successfully conducted over the next several months. Astronauts Joseph H. Engle and Richard H. Truly flew Enterprise on the second and fourth free flights while Haise and Fullerton manned the Orbiter’s cockpit on the third and fifth missions.
Enterprise flew without the tailcone during the last two ALT flights. As expected, the trip downstairs was rapid. Time of descent from 22,400 ft for Free Flight No. 4 was 154 seconds with a landing speed of 230 mph. Free Flight No. 5 took only 121 seconds to descend 19,000 ft and landed at 219 mph.
ALT Free Flight No. 5 was notable in that (1) the Enterprise made its first landing on concrete and (2) a Pilot-Induced Oscillation (PIO) occurred at initial touchdown. For a few tense moments Command Pilot Haise struggled to keep his skittish steed on the ground. Following several disturbing skips and bounces, the Enterprise finally settled down and rolled to a stop.
The Space Shuttle Approach and Landing Tests (ALT) were a necessary prelude to space for the Orbiter. Indeed, the ALT flights represent the first time that NASA’s new winged reentry vehicle took to the air. Having successfully demonstrated the ability to safely land an Orbiter, the next flight in the Space Shuttle Program would be STS-1 in April 1981. Interestingly, that 2-day mission would come to a successful conclusion when the Columbia landed on Rogers Dry Lake back at Edwards Air Force Base, California.