The Vomit Comet is an aircraft used to train astronauts.
It was originally going to be how Gravity was filmed.
How it works
What remains is a micro-g environment moving in free fall, i.e. there are no forces other than gravity acting on the people or objects in this environment. To prevent air drag making the free fall less perfect, objects and people can free-fall in a capsule that itself, while not necessarily itself in free fall, is accelerated as in free fall. This can be done by applying a force to compensate for air drag. Alternatively free fall can be carried out in space, or in a vacuum tower or shaft. The two cases that can be distinguished are that where the situation is only temporary because after some time the Earth's surface is or would be reached, and the case where the situation can go on indefinitely. A temporary micro-g environment exists in a drop tube (in a tower or shaft), a sub-orbital spaceflight, e.g. with a sounding rocket, and in an airplane such as used by NASA's Reduced Gravity Research Program, aka the Vomit Comet, and by the Zero Gravity Corporation. A temporary micro-g environment is applied for training of astronauts, for some experiments, for filming movies, and for fun. A micro-g environment for an indefinite time, while also possible in a spaceship going to infinity in a parabolic or hyperbolic orbit, is most practical in an Earth orbit. This is the environment commonly experienced in the International Space Station, Space Shuttle, etc. While this scenario is the most suitable for scientific experimentation and commercial exploitation, it is still quite expensive to operate in, mostly due to launch costs. Objects in orbit are not perfectly weightless due to several effects: Effects depending on relative position in the spacecraft: In Low Earth orbit (LEO), the force of gravity decreases upward by 0.33 μg/m. Objects which have a non-zero size will be subjected to a tidal force, or a differential pull, between the high and low ends of the object. (An extreme version of this effect is spaghettification.) In a spacecraft in LEO, the centrifugal force is greater on the side of the spacecraft furthest from the Earth. This is also a tidal force, adding 0.17 μg/m to the first-mentioned effect. "Floating" objects in a spacecraft in LEO are actually in independent orbits around the Earth. If two objects are placed side-by-side (relative to their direction of motion) they will be orbiting the Earth in different orbital planes. Since all orbital planes pass through the center of the earth, any two orbital planes intersect along a line. Therefore two objects placed side-by-side (at any distance apart) will come together after one quarter of a revolution. If they are placed so they miss each other, they will oscillate past each other, with the same period as the orbit. This corresponds to an inward acceleration of 0.17 μg per meter horizontal distance from the center. If they are placed one ahead of the other in the same orbital plane, they will maintain their separation. If they are placed one above the other (at different radii from the center of the earth) they will have different potential energies, so the size, eccentricity, and period of their orbits will be different, causing them to move in a complex looping pattern relative to each other. Gravity between the spacecraft and an object within it may make the object slowly "fall" toward a more massive part of it. The acceleration is 0.007 μg for 1000 kg at 1 m distance. Uniform effects (which could be compensated): Though very thin, there is some air at orbital altitudes of 185 to 1,000 km. This atmosphere causes deceleration due to friction. This could be compensated by a small continuous thrust, but in practice the deceleration is only compensated from time to time, so the small g-force of this effect is not eliminated. The effects of the solar wind and radiation pressure are similar, but directed away from the Sun. Unlike the effect of the atmosphere it does not reduce with altitude.
The aircraft gives its occupants the sensation of weightlessness by following an (approximately parabolic) elliptic flight path relative to the center of the Earth. While following this path, the aircraft and its payload are in free fall at certain points of its flight path. The aircraft is used in this way to demonstrate to astronauts what it is like to orbit the Earth. During this time the aircraft does not exert any ground reaction force on its contents, causing the sensation of weightlessness. Initially, the aircraft climbs with a pitch angle of 45 degrees using engine thrust and elevator controls. The sensation of weightlessness is achieved by reducing thrust and lowering the nose to maintain a neutral, or "straight and level" configuration (0 degree angle of attack). Weightlessness begins while ascending and lasts all the way "up-and-over the hump", until the craft reaches a downward pitch angle of 30 degrees. At this point, the craft is pointed downward at high speed, and must begin to pull back into the nose-up attitude to repeat the maneuver. The forces are then roughly twice that of gravity on the way down, at the bottom, and up again. This lasts all the way until the aircraft is again halfway up its upward trajectory, and the pilot again reduces the thrust and lowers the nose. This aircraft is used to train astronauts in zero-g maneuvers, giving them about 25 seconds of weightlessness out of 65 seconds of flight in each parabola. During such training the airplane typically flies about 40–60 parabolic maneuvers. In about two thirds of the passengers, these flights produce nausea due to airsickness, giving the plane its nickname "vomit comet".
Use by NASA
NASA has flown zero gravity flights on various aircraft for many years. In 1959, Project Mercury astronauts trained in a C-131 Samaritan aircraft, which was dubbed the "vomit comet". Twin KC-135 Stratotankers were used until December 2004 but have since been retired. One, a KC-135A known as NASA 930, was also used by Universal Pictures and Imagine Entertainment for filming scenes involving weightlessness in the movie Apollo 13; that aircraft was retired in 2000 and is now on display at Ellington Field, near the Johnson Space Center. The KC-135A is estimated to have flown over 58,000 parabolas. The other (N931NA or NASA 931) made its final flight on October 29, 2004, and is permanently stored in the Pima Air & Space Museum in Tucson, Arizona. In 2005, NASA replaced the aircraft with a McDonnell Douglas C-9B Skytrain II (N932NA) that was formerly owned by KLM Royal Dutch Airlines and the United States Navy. NASA currently has a microgravity services contract with Zero Gravity Corporation (ZERO-G) and uses its aircraft, G-FORCE ONE, a modified Boeing 727-200.