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KC-135 Training Aircraft

KC-135 Training Aircraft

Gravity is such a common part of our daily lives that we are rarely conscious of it, even though it affects everything we do. Any time we drop or throw something and watch it fall to the ground, we see gravity at work. Although gravity is a universal force, there are times when it is not useful to conduct activities under its full influence. In these cases, space scientists and engineers perform their work in "microgravity"a condition in which the effects of gravity are greatly decreased.

On Earth, brief periods of microgravity can be achieved by dropping objects from tall towers. Longer periods of microgravity, however, can be created only through the use of airplanes that fly special flight paths. The microgravity research aircraft of the National Aeronautics and Space Administration (NASA) is the KC-135, a four-engine turbojet, similar to the Boeing 707, which has been modified to meet NASA's needs to train astronauts and conduct microgravity research. The KC-135 is part of the space agency's Reduced Gravity Program, which was started in 1959 to expose people and equipment to microgravity. The program is operated from the Lyndon B. Johnson Space Center in Houston where scheduling, test coordination, and in-flight direction of test programs takes place.

Parabolic Maneuvers

To simulate microgravity conditions, the KC-135 is flown through a series of precise parabolic maneuvers in which the plane ascends steeply, levels off, and then begins a dive. Typically, the KC-135 soars over the Gulf of Mexico and levels off at about 8,000 meters (26,250 feet). Then the aircraft climbs rapidly until it is at an approximate 45-degree angle to the horizon. Half a minute later, the pilot pushes the KC-135 "over the top" until the plane points down about 30 degrees. Finally, each parabola is terminated with a 1.8-gravity (1.8-G) pullout as the plane levels off again. These gut-wrenching maneuvers have earned the KC-135 its famous nickname: "The Vomit Comet." Many first-time flyers feel queasy as they experience motion sickness.

The parabolic arc flown by the KC-135 is the key to simulating micro-gravity conditions. As the KC-135 is tracing the parabola, the plane's acceleration matches Earth's acceleration of gravity, making everything inside weightless for up to twenty-five seconds. But these parabolic maneuvers can be modified to simulate different gravity fields and provide any level of microgravity. For example, negative Gs (-0.1 G) can be achieved for approximately fifteen seconds, and a flight profile can be flown to achieve "zero G" for about twenty-five seconds. The pilot of the KC-135 can also follow an arc that produces one-sixth Gthe gravitation force on the lunar surfacefor about forty seconds. "Martian-G" (i.e., one-third G) can also be simulated for about thirty seconds when the KC-135 flies a specific type of parabolic trajectory . These parabolas can be flown in succession (i.e., roller-coaster fashion) or with short breaks between maneuvers to reconfigure test equipment. As many as forty arcs can be flown on a typical flight so that scientists and technicians can conduct several activities or repeat short runs of a single activity many times. A typical mission lasts two to three hours and consists of thirty to forty parabolas.

Specific Uses of the KC-135

Many years ago NASA recognized that short periods of microgravity could be used to conduct basic research, train astronauts, test hardware and experiments destined for space, and evaluate medical protocols that may be used in space. With the coming of age of the space shuttle and the construction of the International Space Station, the KC-135's ability to simulate microgravity conditions remains essential for crew training, experiments, and the development and verification of space hardware. Astronaut candidates are given exposure to the microgravity of spaceflight aboard the KC-135. In addition, the KC-135 provides a unique laboratory for research in which scientists can observe and explore physical events, phenomena, and processes that are normally masked by the effects of Earth's gravity. Russian space officials use a similar type of aircraft to simulate microgravity conditions for training and research.

Student experiments that require microgravity conditions have also been flown aboard the KC-135 as part of NASA's Reduced Gravity Student Flight Opportunities Program. The program offers American college and high school students a unique opportunity to fly with their microgravity experiments aboard the KC-135 aircraft and provides the students a behind-the-scenes look at science and engineering programs and the Johnson Space Center.

To support all of these research and training activities, the KC-135 has a full complement of crew members (pilot, copilot, flight engineer, and two reduced gravity test directors), plus room for technicians, engineers, scientists, and all the necessary equipment and infrastructure . The test area of the KC-135's cargo bay where microgravity activities are carried out is approximately 20 meters (66 feet) long, 3 meters (9.8 feet) wide, and 2 meters (6.6 feet) high. Most of the test equipment is bolted to the floor using 50-centimeter (19.5-inch) tie-down grid attachment points. Electrical power and liquid or gaseous nitrogen are available for experiments or other uses. The aircraft is also equipped with photographic lights to support still and motion picture photography and video.

Since the inception of the Reduced Gravity Program, KC-135 parabolic microgravity missions have been flown in support of the Mercury , Gemini , Apollo , Skylab, space shuttle, and International Space Station programs as well as for general microgravity research. The KC-135 has even played a role in a Hollywood movie. It was used to fly the actors and crew of the 1995 movie Apollo 13 to film scenes about the ill-fated trip to the Moon. However, in the years ahead, the KC-135 will remain an important tool to investigate real-life human and hardware reactions to a microgravity environment.

see also Career Astronauts (volume 1); G Forces (volume 3); Human Factors (volume 3); Medicine (volume 3); Microgravity (volume 2); Zero Gravity (volume 3).

John F. Kross


Compton, William D. Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions. Washington, DC: National Aeronautics and Space Administration, 1989.

Shelton, William R. Man's Conquest of Space. Washington, DC: National Geographic Society, 1975.

Internet Resources

KC-135 Home Page. NASA Johnson Space Center. <>.

NASA Microgravity Research Facilities. National Aeronautics and Space Administration. <>.

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