Two shapes generally come to mind when we think about spacecraft: capsule-style, like the Apollo spacecraft, and airplane-inspired like the space shuttle. But in the 1960s and 1970s, NASA explored a third and very different type of spacecraft: lifting bodies. Although these wingless aircraft might look like potatoes with fins, many engineers thought they'd make the perfect spacecraft.
The M2-F1 during an air tow test in 1964
Discovering Lifting Bodies
The lifting bodies story begins at the Ames Research Center in 1957, before the Soviet Union launched Sputnik and kicked of the space age. Engineers with NASA’s predecessor organization, the National Advisory Committee for Aeronautics (NACA), were working out how to prevent missiles' nose cones from overheating as they reentered the Earth’s atmosphere. Missiles needed to detonate on impact, not from friction with their air on their way towards a target. Research found that a blunt nose cone with a slightly rounded bottom was the ideal shape.
Dr. Alfred J. Eggers Jr., then assistant director for Research and Development Analysis and Planning at the Ames, took the missile nose cone idea a step farther. He realized that by slightly modifying the shape, he could create something capable of generating aerodynamic lift. The vehicle could survive reentry through the Earth’s atmosphere and glide smoothly back to Earth. It wouldn't fall uncontrolled like a typical missile.
With the help of his Ames colleagues, Eggers developed the M-2 lifting body shape. It was a modified half-cone, rounded on the bottom and flat on top, with a blunt, rounded nose and twin tail fins. Its shape would allow a pilot to use the aerodynamic forces of reentry to his advantage, maneuvering the vehicle to a suitable landing area to land horizontally on a conventional runway. In theory, the idea was sound, but it would be a while before Eggers’ idea would fly.
The lifting bodies idea resurfaced in the early 1960s. This time the driving force was Dale Reed, an NASA engineer with the Dryden Flight Research center at Edwards Air Force Base in California. Reed built a prototype after Eggers’ original design in the form of a small tissue paper and balsa wood model. He hand-launched it into tall grass and saw that it was actually stable in flight. Reed then tried hand-launching the model off rooftops at Edwards. It was still stable.
It was clear to Reed that a full-scale manned version was the next step, and what he needed was a willing pilot to help him sell the idea to NASA. He found one in Milton Thompson, a test pilot flying for NASA who was equally eager to test a lifting body. The pair pitched their idea, and eventually got a green light from Paul Bikle, the director of NASA’s Dryden Flight Research Center, in 1962.
HL-10 pilots "assisting" with a pilot's cockpit entry
The Unpowered Lightweights
The first lifting body, a full sized M2 called M2-F1 (M for manned and F for flight), was finished in 1963. Like Reed’s small scale model, it was made of extremely lightweight material: a plywood shell around a steel frame.
As was common at Edwards, new vehicles were first tested in ground tow tests. A vehicle pulled behind a truck or a car would generate enough lift to give the pilot of sense of how the vehicle handled. The faster the two car or truck drove, the more lift the test plane would get.
For the M2-F1‘s ground tow tests, Thompson convinced Bikle to buy NASA a Pontiac convertible. The pilot argued that he in the M2-F1 and the car’s driver would need a clear view of each other during the test. Bikle bought the car. The engine was replaced with a General Motors 421 cubic inch triple-carburetor Tripower engine like the cars in the Daytona 500 used that year. The car was then hollowed out, the passenger seat turned around, and the body painted with high-visibility yellow paint.
Thompson flew the wooden M2-F1 on the first series of tow-tests behind the convertible. He found, like Reed had with the small models, that the wingless aircraft was stable enough to fly. His confidence growing, Thompson graduated to air tow tests. The M2-F1 was towed behind a C-47 cargo plane up to launch altitude, at which point the pilot would sever the tow line and leave Thompson to fly the aircraft to a landing on the dry lakebed at Edwards.
Across the board, the M2-F1 tests were successful; the vehicle made nearly 500 air and ground tows before it was retired. But the real flying was yet to come.
The Powered Heavyweights
The success of the lightweight lifting body prompted NASA to fund development of heavier versions, made of aluminum, that could withstand not only powered flight but supersonic flight as well. A heavier M2-F1 was built and called the M2-F2, but it wasn't the only lifting body shape test for powered flight.
Another Ames designed lifting body called the HL-10 (HL for horizontal landing) was built. This was a 21 foot long aircraft curved longitudinally along the bottom with a similarly curved top and an overall triangular shape. Three vertical fins at the back plus two canted outward from the main body gave the design directional control. It also had a hydraulic-powered flight control system that enabled the HL-10 to fly faster than the speed of sound.
The X-24A was also built for powered and supersonic flight. This was a bulbous aircraft with three vertical fins at the rear for directional control measuring 24 feet long and nearly 14 feet wide. The X-24A’s airframe was eventually converted to a straight shape and renamed the X-24B.
All three of the vehicles were fitted with XLR-11 engines recovered from museums. One engine was taken from the Smithsonian Air and Space Museum where it was still mounted in the X-1 Chuck Yeager flew through the sound barrier in 1947. One came from an X-1B aircraft on that was then display in an Air Force Museum. And the third engine came from the Dryden Flight Research Center where the lifting bodies were tested; it was also on display, mounted in an X-1E.
All three of these heavyweights, the M2-F2, the HL-10, and the X-24A/B, traced the same flight path. They were carried to an altitude of around 45,000 feet nestled under the wing of a converted B-52 bomber. At launch altitude, the B-52's pilot would release the lifting body. The first tests were glide flights during which the pilot would just control the aircraft to a runway landing. For powered flights, he would ignite his engine soon after separating from the B-52. He’d fly until his fuel was depleted, reaching altitudes up to 70,000 feet and speeds in excess of 1,000 miles per hour. With his fuel gone, the pilot would fly the lifting body to a smooth landing at about 200 miles per hour.
The X-24B landing in 1975
The Six Million Dollar Man
The most notorious lifting body flight is probably the last unpowered flight of the whole program. Bruce Peterson was at the controls of the M2-F1 on May 10, 1967. His aircraft separated from the B-52 perfectly at launch altitude, and he began a smooth glide towards the dry lake bed. But as he started lining up for landing, the M2-F2’s nose suddenly veered sharply to the right. Peterson instinctively tried to counter the motion, swinging the nose to the left, but it didn't work. Instead, the vehicle started rolling violently from side to side. Nearly out of control, he hit the lake bed hard with the landing gear still retracted.
The M2-F2 was light enough that it bounced back up. Realizing he was airborne, Peterson ignited his engine in an attempt to buy enough time to lower the landing gear before he hit the lake bed again. This also didn't work. He landed with his landing gear partially deployed. The aircraft skidded, turned sideways, and rolled end over end six times. The force of the crash tore the landing gear, one rear fin, and the cockpit canopy from the vehicle, exposing Peterson’s face to the desert floor. It finally stopped, upside down, resting on the pilot’s seat support and remaining rear fin.
Footage from this crash is featured in the opening sequence of the TV series The Six Million Dollar Man. But unlike the fictional TV pilot, Peterson didn't need extensive surgery. He escaped the accident with scrapes on his face, a result of the canopy being ripped off. The loss of skin cost him his right eyelid, and a staph infection eventually took the sight in that eye. But he was otherwise unharmed. The M2-F2 had a steel casing around the pilot’s seat: designed to help balance the vehicle in flight, it saved Peterson’s life.
The M2-F2 after the crash
The Current State of Lifting Bodies
All of the lifting bodies that flew at Edwards were Earthbound flight demonstrators rather than true spacecraft. But the test program was so successful many engineers became advocates of using lifting bodies in spaceflight. Some went so far as to suggest that NASA adopt the unconventional vehicle for its post-Apollo program; a lifting body would function very well as a space shuttle. But NASA wasn't calling the shots about the space shuttle’s dimensions. The Department of Defense was. The DOD stipulated the shuttle have a payload bay 60 by 15 feet so it could launch military and government satellites, and arrangement that would help cover the cost of the shuttle program. The DOD also stipulated that the payload bay be a straight rectangle. This made a wing-and-fuselage configuration more feasible than a lifting body configuration, and that’s the design we recognize today.
Lifting bodies have reappeared from time to time over the years. NASA considered using a version of the X-24A as an emergency return vehicle for astronauts aboard the International Space Station. The X-33 was another lifting body, a prototype single-stage-to-orbit vehicle built by Lockheed Martin that also never flew. And there’s one current lifting body program. Sierra Nevada’s Dream Chaser, one of the vehicles in NASA’s Commercial Crew Program, is also a lifting body design. Currently, it’s on track to fly sometime around 2016 or 2017.
Further Reading: Flying Without Wings by Milt Thompson with Curtis Peebles