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Sept. 23, 2005
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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.
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.
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
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| Divot tests give return to flight a LIFT
A double sonic boom from the predawn sky announced Discovery's arrival over Edwards Air Force Base. The successful landing of the STS-114 mission came two years, seven months and eight days after Columbia was lost during re-entry. The damage that caused Columbia's destruction had been inflicted during launch, when a large piece of foam from the vehicle's external fuel tank struck a wing leading edge.
Part of the return-to-flight effort involved trying to understand the behavior of such small foam fragments, called "divots."
Divoting occurs when air pockets trapped beneath the foam expand as the shuttle ascends. The air expands because of decreasing ambient pressure (it was trapped at sea level) and heat transfer. The expanding air causes the adhesive to fail, resulting in a section of foam popping off the tank.
Engineers working to solve the foam problem sought to understand the behavior of the divots as they were caught in this airflow. Did divots break up under the dynamic pressure, did they tumble, or did they "trim" and begin to "fly" in a stable condition? Whichever of these conditions prevailed would determine the degree to which the pieces were slowed by air drag. The amount of kinetic energy of a divot, and, in turn, the amount of damage divots could inflict, is determined by their impact speed relative to that of the shuttle. Understanding divot behavior also was critical to developing computational fluid dynamics programs to assess the risk of impacts.
Image Above: Dryden's F-15B testbed aircraft flies one of the Lifting Insulating Foam Trajectory, or LIFT, research flights. NASA Photo by Carla Thomas
To provide the needed data on divot behavior, NASA's Space Shuttle Engineering and Integration Office funded the Lifting Insulating Foam Trajectory, or LIFT flight tests.
Dryden's two-seat F-15B research aircraft was selected as the testbed. Initial planning for the project began in July 2004, with ground tests continuing into late September of that year. Development work on the divot ejection system, as well as on the high-speed cameras utilized on the aircraft as part of the foam issue resolution began in late September and was completed a month later.
Then in November 2004, the Space Shuttle Engineering and Integration Office got the go-ahead to start preparation work for the LIFT flight-test phase.
A pylon, called the Aerodynamic Flight Test Fixture, was attached to the underside of the F-15B. Small aluminum panels covered with BX-265 tank insulation were mounted on the pylon's side. A gas pressure ejection system was used to separate the divots in flight. A high-speed digital video system was mounted under the wing. The system was capable of capturing up to 10,000 frames per second of the divots as airflow carried them away from the pylon.
The first ground tests of the divot ejection system were successfully completed on Jan. 6, 2005. The four initial LIFT flight tests with the F-15B were made in late February. Divots were ejected at speeds up to Mach 1.4. The LIFT flights resumed in early March, with the final five in the series being made before the project was completed by month's end.
A total of 38 divots were separated at subsonic and supersonic speeds during the nine-flight test series, with test points simulating the shuttle's ascent profile. The top speed was about Mach 2, while the maximum dynamic pressure exerted was 850 pounds per square foot. All of the divot ejections and their trajectories were successfully photographed using the digital cameras. A video clip of one test showed a divot resembling a small dinner plate being ejected from the pylon, then stabilizing in the airflow with a back-and-forth wobble before being carried out of the video frame. As a check on the program's accuracy scientists from NASA Ames Research Center, Moffett Field, Calif., compared their computer predictions of the divots' trajectories with those shown in the videos.
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