For some missions, particularly human spaceflight missions, designers wanted a pilot to be able to control the craft so it would land it at a specific spot. Beginning with the Mercury and Gemini spacecraft programs in the late 1950s through the mid-1960s, U.S. aerospace engineers investigated the possibility of providing space capsules with some amount of lift during reentry so their landing could be controlled to some extent. Of course, the most logical method for generating lift is through wings. But wings create a problem for a reentering craft because they have to be very strong to handle the extreme stresses of the wind pushing against them from below at hypersonic speeds (at least five times the speed of sound) during reentry.

In 1962, NASA engineer Dale Reed began urging research on wingless aircraft that could reenter from space and yet still be flown to a landing. He called these wingless airplanes "lifting bodies" because they generated their lift from the body of the aircraft, rather than from wings like in a conventional aircraft. He advocated a design that was essentially a cone cut in half. Many of the people he spoke to were skeptical, so he constructed a model out of balsa wood and threw it off the roof of a building. It proved stable in flight (in other words, it did not tumble uncontrollably but glided through the air) and soon he was towing the model into the air behind a gas-powered model where it was released to glide to a landing. He filmed his experiments and showed them to colleagues at the NASA Flight Research Center (now NASA's Dryden Flight Research Center) at Edwards Air Force Base in California. Soon he headed a small development team that had approval to build a lightweight, full-scale manned aircraft designated the M2-F1, "M" signifying a manned vehicle and "F" designating a flight vehicle. Others soon began calling it the "Flying Bathtub."

Like its tiny model predecessor, the M2-F1 was built of wood. Officially it was only a full-scale wind tunnel model, but Reed and his team members planned to tow it behind a Pontiac Catalina that was souped-up by a hot-rod shop in Long Beach, California, so that it could drive faster than normal. They did this for some initial tests of the aircraft in March 1963, towing it along the desert floor behind the car and then flying it a few feet off the ground. It proved stable and earned them further support.

On August 16, 1963, the M2-F1 was towed into the air behind an R4D (C-47) "Gooney Bird" transport plane. Over the next two years, Reed and his NASA team conducted a number of tests using the aircraft to determine its flying characteristics. These tests showed that the aircraft could indeed fly. Interest in the aircraft grew both in NASA and the U.S. Air Force and NASA approved the development of a heavier aircraft to explore higher speeds. This was designated the M2-F2 and it was equipped with a rocket engine and dropped from a B-52 airplane, like many of the X-plane experimental aircraft.

Engineers at NASA's Langley Research Center in Virginia developed another lifting body design. It was designated the HL-10. The HL-10 design offered a great amount of internal volume for its size, meaning that an operational version would be able to carry a lot of people or cargo inside. It differed somewhat from the M2-F2 by having the rear edges of the fuselage extended outward into angled vertical fins. The M2-F2 and HL-10 would be dropped from the B-52, ignite their rockets to achieve high speeds, and then glide to unpowered, "dead-stick" landings on the dry lakebed at Edwards Air Force Base.

The M2-F2 made its first glide flight on July 12, 1966. The HL-10 made its first glide flight in the same manner on December 22, 1966. But the HL-10 did not fly again for 15 months because those running the program became concerned about its safety. This was prompted in part by a number of near crashes and temporary losses of control with the other lifting bodies such as the M2-F2.

One of the big problems the lifting bodies had was with what is called "flow separation." The airflow over the fuselage became turbulent and did not flow smoothly. The HL-10 designers fixed this by extending the leading edges of the fins (the edge that heads into the airflow) and cambering, or curving, them.

On May 10 1967, the M2-F2 crashed when its pilot, Bruce Peterson, lowered the landing gear half a second too late. It rolled down the dry lakebed and tumbled, severely injuring its pilot. (Footage of this crash was later used in the opening credits of the 1970s TV show The Six-Million Dollar Man.) The M2-F2 was damaged but later rebuilt as the M2-F3, with an additional vertical fin.

The HL-10 was eventually flown to a speed of Mach 1.86. Several powered landings were also conducted with this aircraft, using rocket engines to provide power as the aircraft set down on its landing gear, but these demonstrated that powered landings were little better than unpowered ones. Another aircraft, the X-24A, was also built and tested. In 1972 it was modified into the dart-shaped X-24B, which was used to explore higher speeds. It eventually reached a speed of Mach 1.75. NASA also experimented with the unmanned Hyper III lifting body. Long, thin, and angular, it could not land without help, so a one-piece pivoting wing was developed that deployed just before landing, generating more lift at lower speeds.

People usually think of technology development in terms of producing items that they can use. But often developing technologies can demonstrate limitations and the need for either new technological approaches or the need to accept existing, less-than-ideal solutions. The lifting bodies developed by NASA during the 1960s demonstrated the limitations of aircraft without wings. Their primary limitation was their high landing speeds, which made controlling them difficult and dangerous. As a result of this experience, NASA engineers chose to develop a Space Shuttle that had wings. Although the Space Shuttle still has a high landing speed, it is slower and more maneuverable than a large lifting body design would be. However, another value of the lifting body research was that it proved that unpowered landings could be safe. The space shuttle benefited greatly from this research.

In the early 1990s, Langley Research Center proposed an upgraded HL-10 known as the HL-20 Personnel Launching Vehicle to serve as a "space taxi" to the International Space Station. Although a mockup was built, the program was not approved and no flight tests took place.

Later, in the 1990s, NASA engineers began evaluating a lifting body again as a Crew Return Vehicle (CRV) for the International Space Station. The CRV would serve as a lifeboat, attached to the Space Station and capable of returning the crew to Earth in event of an emergency or injury. It had to be able to decelerate through the atmosphere at a relatively low rate so not to put too much strain on an injured crewmember. It also had to be able to glide a great distance so that the crewmembers could detach from the space station anywhere in their orbit—even over the oceans—and still return to a safe area. A craft that could generate lift during its reentry into the atmosphere was ideal for this because it could detach from the space station far from its landing site and glide for thousands of miles before landing.

Designers chose to develop a lifting body and initiated a testing program using a new vehicle, designated the X-38. The X-38 uses the same exterior configuration as the X-24A, including the rounded bump on the upper fuselage where the X-24A had a cockpit even though the X-38 has none. This was done to be able to use the X-24A's testing data to maximum extent. The one key difference is that the X-38 does not land conventionally on wheels on a runway. Instead, the X-38 deploys a rectangular parachute as it nears the ground and floats to a landing. This method was chosen because NASA learned during its earlier lifting body research that lifting bodies have unsafe landing characteristics.

--Dwayne A. Day

Sources and Further Reading: