Boeing C-135 707 NASA Vomit Comet airplane model. At NASA's Johnson Space Center, there is a microgravity research aircraft, a KC-135 Low-G Flight Research aircraft (a predecessor of the Boeing 707) which is typically used for refueling military aircraft. The four-engine turbojet has been modified to meet NASA's needs to train astronaut and conduct microgravity research. The biggest difference (in addition to removal of the fueling probe), is the way that it is used to fly parabolas to create 20-25 seconds of weightlessness so that the astronauts can experience, and researchers can investigate the effects of "zero" gravity. A typical mission is 2 to 3 hours long and consists of 30 to 40 parabolas. These parabolas can be flown in succession or with short breaks between maneuvers to reconfigure test equipment. The gut wrenching effects have earned this aircraft its famous nickname: "The Vomit Comet."