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The KC-135 is a modified Boeing 707 four-engine turbojet that NASA uses to simulate conditions of weightlessness. In a typical flight, it traverses the Gulf of Mexico in a series of large parabolic arcs. Peaking at 32,000 feet, the plane then dives to 24,000 feet, its fuselage pitched down at 40 degrees. At the top of the parabola, passengers lose all sense of gravity and become weightless for a period of roughly 25 seconds. When the airplane comes out of the dive and begins its next ascent, the plane pitches upward at about 50 degrees and passengers on the craft are subjected to forces up to 1.8 times that of gravity. This climbing and diving is repeated thirty times in what might be described as the ultimate roller coaster ride. Flying on the KC-135 nauseates passengers so frequently, however, that the plane has been nicknamed the "Vomit Comet".
For more information on the KC-135, see NASA's Reduced Gravity Student Flight Opportunities Program