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Sept. 23, 2005
Dryden has been the site of key research in the shuttle program

Behind Discovery's landing on Runway 22 at Edwards Air Force Base were several decades of work dedicated to developing the world's first reusable orbital spacecraft. In some ways, building the shuttle presented greater challenges than those of the Apollo program, because a reusable space vehicle embodied technologies far beyond those existing at the shuttle program's outset. Many important elements of this effort took place at Dryden.

Space Shuttle Enterprise prototype separates from the NASA 747 on its first flight without a tailcone, which had been used in earlier flights. Image Right: Space Shuttle Enterprise prototype separates from the NASA 747 on its first flight without a tailcone, which had been used in earlier flights. NASA Photo

In 1954, three years before the launch of the Soviet satellite Sputnik 1, the National Advisory Committee for Aeronautics (NASA's predecessor) began work on the X-15 research aircraft, the first aircraft capable of reaching the edge of space. The X-15 would experience aerodynamic pressures ranging from zero to 2,000 pounds per square foot, and temperatures of some 2,000 degrees Fahrenheit. Piloting the X-15 meant controlling the vehicle under conditions of increased gravitational forces as well as during periods of weightlessness. The data acquired through the X-15 program, and the experience gained during its 199 flights at Dryden, played major roles in designing the space shuttle.

Another important source of information contributing to the shuttle's design was the lifting body program conducted at Dryden from 1963 to 1975.

Rather than deriving aerodynamic lift from wings, lifting bodies generated it from their shape. Dryden engineer R. Dale Reed was among the very first to realize the potential inherent in the lifting body concept. He also knew such an unusual concept would be a hard sell to doubting engineers and program managers.

Beginning with experiments that featured a small balsa-wood-and-tissue-paper model towed aloft by a radio-controlled mothership and then released for a glide landing, Reed was gradually able to gain support for construction of a lightweight, piloted lifting body. The first such vehicle, dubbed the M2-F1, had a plywood fuselage and an internal framework made of metal tubing, and resembled a bathtub on a tricycle. The initial tow tests were done using a souped-up 1963 Pontiac Catalina convertible. Later, the M2-F1 was towed to higher altitudes by a C-47.

This led to construction of a series of heavyweight lifting bodies – the M2-F2/F3, the HL-10 and the X-24A/X-24B. These vehicles were flown at Dryden between 1966 and 1975, and used to test a range of different vehicle configurations. They were launched from what was then the Air Force's NB-52B, reaching speeds approaching Mach 2 and altitudes of up to 90,000 feet. These tests showed that vehicles with these shapes could successfully make a controlled atmospheric re-entry.

Among the most significant contributions derived from lifting body research was the elimination of landing engines from the shuttle. Original shuttle designs called for multiple jet engines that would be started during descent to allow the shuttle to make a powered landing on a runway. This concept was put to the test with the HL-10, which was fitted with several small rocket motors and made powered landings on the dry lakebed. Ironically, these powered landings proved to be much more difficult and risky than a steep-glide approach. As a result, the jet engines were eliminated as a design requirement. This improved safety, simplified the vehicle, and also resulted in a major reduction in the shuttle's liftoff weight.

The CV-990 Landing Systems Research Aircraft engages in a space shuttle tire test. Image Left: The CV-990 Landing Systems Research Aircraft engages in a space shuttle tire test. NASA Photo

Dryden also played a major role in developing key technologies used in the shuttle. The F-8 Digital Fly-By-Wire research aircraft originally featured an Apollo spacecraft computer. The aircraft was then fitted with AP-101 digital computers, which also had been selected for use on the shuttle. The F-8 experience enabled shuttle engineers to find and correct the AP-101s' manufacturing and technical problems at a much earlier stage.

It was at Dryden, where so many revolutionary aerospace vehicles were first flown, that Space Shuttle Enterprise first tested its wings. This took place in the summer and fall of 1977 in the Approach and Landing Test program. The prototype shuttle was carried aloft on the back of a modified 747 airliner. The pair went into a gentle dive, and the shuttle was released. The goal was to test the vehicle's aerodynamics and computer systems in subsonic flight. The first four ALT flights touched down on the lakebed, while the fifth landed on Runway 22. On this final flight, an unexpected problem cropped up. Due to the time lag between a control input by the crew and the response, the vehicle experienced a condition called pilot induced oscillation, or PIO.

To explore the problem, and determine a solution, researchers used the F-8 Digital-Fly-By-Wire aircraft. Various time delays were programmed into the plane's AP-101 computers. Test landings showed that the delay in control surface actuation could cause a pilot to over-control the aircraft, making rapid control inputs and causing a PIO. The solution was to add a software filter, which suppressed the PIO tendency – a solution that was ultimately incorporated into the shuttle's computer system.

Dryden's involvement with the shuttle program did not end with the ALT series. Overlapping the ALT and F-8 flights was the start of drop tests of the solid rocket booster parachute system. A weighted casing was taken aloft by NASA's NB-52B, and then dropped over the National Parachute Test Range, near El Centro, Calif. The first test series involved six drops in 1977 and 1978, and proved the parachute system's viability.

The shuttle was designed as a reusable spacecraft, but the amount of work involved in refurbishing a traditional ablative heat shield – which melted at a controlled rate, to carry off the intense heat of atmospheric re-entry – made this option impractical. A "hot structure" fuselage, built of exotic high-temperature metals also was considered impractical. The solution was to build the shuttle airframe from conventional aluminum, then cover it with ceramic tiles. These were both lightweight and able to sustain multiple re-entries. However, they had to be individually glued to the shuttle.

To test the tiles' ability to remain attached under the aerodynamic loads of flight, an F-104 and F-15 were used in a 1980 test series. Each aircraft was flown with a profile that produced aerodynamic pressures and airflow velocities simulating those of a shuttle flight. Because the tiles were made of materials susceptible to impact damage, there also was concern about the shuttle being launched in the rain. The positioning of the tiles on the aircraft simulated six locations on the shuttle: the forward wing area, vertical tail leading edge, window post area, aft of the wing leading edge, and the elevon trailing edge and hinge areas. The tiles were subjected to speeds of Mach 1.4 and aerodynamic pressures of 1,140 pounds per square foot during 60 flights. As a result of the flight tests, several changes were made to bonding and attachment techniques. The tests also showed that storms would have to be avoided because of the potential for damage that the raindrops would cause.

Over the nearly quarter of a century since the first shuttle was launched into space, development activities have been ongoing at Dryden.

The first of these was a second series of solid rocket booster tests made with the B-52B between 1983 and 1985 and involving eight drops. Among modifications made in the wake of Challenger's loss was the addition of a drag chute to reduce the distance needed to land. A total of eight parachute tests were made with the B-52B at speeds ranging from 160 to 230 miles per hour. These were done on both the lakebed and the concrete Edwards runway during 1990.

Landings at the Kennedy Space Center also revealed problems with tire wear. To address the problem, a CV-990 airliner was modified with shuttle landing gear and a tire in the center fuselage. A hydraulic mounting put stress on the landing gear to simulate the weight of the shuttle and the effects of crosswinds during landing. Crosswind landings impose lateral stress on tires and landing gear beyond the stress caused by forward momentum. A total of 155 landing tests were made between April 1993 and August 1995. As a result of the CV-990 tests, the shuttle tire design was improved, permitting the vehicle to land safely on the Kennedy runway in higher crosswinds.

Dryden's most recent contribution to shuttle development came during the F-15B Lifting Insulating Foam Trajectory, or LIFT flight tests. (See Extra Feature.)

Curtis Peebles
Dryden History Office