A Boeing KC-135 aircraft equipped to conduct experiments simulating the zero-gravity environment of spaceflight.

The nickname “Vomit Comet” stems from the fact that many experimenters feel motion sickness during the maneuvers required to go in and out of the zero-gravity environment. A predecessor of the Boeing 707 airliner, the KC-135 Stratotanker was originally designed for refueling military aircraft in flight. Built in Seattle, the first KC-135A entered the U.S. Air Force fleet in 1957, and the last was delivered in 1965. About 550 of the 732 Stratotankers built remain in service.

The National Aeronautics and Space Administration’s (NASA’s) KC-135A Reduced Gravity Flight Laboratory is powered by four turbojet engines. It has a 60-foot-long, 10-foot-wide, 7-foot-high padded cargo bay, equipped with electrical power outlets, compressed gas sources, an overboard vent system, and photo lights with power receptacles and attachment points at which to mount experiments. Experimenters sit in the aft cabin during takeoffs and landings but move up into the padded area during the maneuvering parts of the flight.

The Vomit Comet is generally based at Ellington Field, near the Johnson Space Center in Houston, Texas. It also operates out of the NASA Glenn Research Center in Cleveland, Ohio, for several weeks each year to support the center’s microgravity research.

Users of the KC-135A include astronauts training with experiment hardware prior to space shuttle missions, researchers conducting initial stages of experiments destined for space, and, since 1997, undergraduates and high school students who design and conduct experiments in reduced gravity. The current Vomit Comet is the third in a series of airplanes used for microgravity experiments.

Although the KC-135A has earned the “Vomit Comet” nickname through a long history of usage, other aircraft are also used for similar experiments. NASA Glenn Research Center, formerly called the Lewis Research Center, has operated a McDonnell-Douglas/Boeing DC-9 aircraft and a Learjet Model 25 for weightlessness research. Weightlessness experiments are also performed by the European Space Agency (ESA) and the Russian space agencies using other kinds of airplanes. ESA uses an Airbus A300 airplane. Russia uses an Ilyushin Il-76 aircraft, operated out of the Yuri Gagarin Cosmonaut Training Center in Star City.

During a typical flight, the Vomit Comet flies out over the sea, climbing to around 26,000 feet above sea level and reaching 350 knots indicated airspeed. While the yaw and roll are controlled by autopilot, the pilot steadily pulls the craft’s nose up to a 45- to 50-degree angle. The airspeed drops off, and the aircraft experiences a maximum acceleration of 1.8 times the acceleration of gravity (1.8 g), as the aircraft climbs steeply. The pilot then pushes the stick forward and reduces thrust, while using two of the engines to control the forward acceleration to zero. The aircraft executes a parabolic arc for about twenty-five seconds. At the top of the parabola, the aircraft reaches an altitude of 36,000 feet and a speed of about 160 knots. The pilot then pushes the stick forward to send the plane to a 45-degree, nose-down attitude and increases the thrust. When the speed again reaches 350 knots, the pilot pulls back on the stick, with the aircraft experiencing an acceleration of up to 1.8 g’s.

This maneuver is typically repeated forty times during a two-hour flight, making up to one hundred parabolas on some flights. During the twenty-five-second segment in which the plane goes over the top of the parabola, the occupants experience weightlessness and begin conducting experiments, practicing chores, or executing maneuvers to test movement in weightless conditions.

The abbreviation “g” refers to the acceleration of gravity. Sitting still on Earth, one experiences an acceleration of 1 g, or a gravitational force of 1, the normal sensation of gravity. During periods of changing acceleration, such as a banking turn in an airplane, the so-called g-loading will change.

The duration of altered gravity depends on the types of conditions sought. Approximate values are as follows: Negative g (minus .1 g): fifteen seconds; 0 g: twenty-five seconds; lunar g (.16 g): approximately forty seconds; Martian g (.33 g): approximately thirty seconds. Atmospheric turbulence and other problems prevent the KC-135A from holding a truly zero-gravity level of acceleration for any length of time. The acceleration value is displayed inside the cabin on a large electronic display, which is updated three times per second and typically shows values between plus .03 g and minus .03 g, with the frequency of fluctuations depending on the atmospheric conditions. The target g-level is around .01 g on calm days.

Those who fly experiments on the KC-135A must obtain a U.S. Air Force Class III medical certificate with NASA physiological training, which consists of an eight-hour training course and a high-altitude chamber run in which experimenters are trained to recognize symptoms of hypoxia, or oxygen deficiency, in case of a loss of aircraft pressure.

On the day before a flight in the KC-135A, a test readiness review is conducted, in which safety experts review the test documentation and discuss the experiment in detail with the experimenters. Approved experiments are mounted inside the padded cabin, fastened to strong points, and powered if necessary with the 120-volt AC power supply inside the cabin.

On the day of the flight, a preflight safety video reminds participants about safety equipment and procedures. The occupants wear flight suits and are given antinausea capsules and a cellophane bag to use in case of motion sickness.

Drop towers, facilities from which objects are dropped, fall freely, and are decelerated at the bottom using stacks of cushioning material, allow about two seconds of microgravity. Some drop towers have had the air evacuated from them in order to reduce air resistance and come closer to providing a true zero-gravity environment. The disadvantages of this type of microgravity environment are the short duration of free fall and the relatively sharp deceleration at the end.

Weightlessness experiments are also performed in water tanks with astronauts wearing suits that provide enough buoyancy to make them feel weightless. Although this method allows for a longer duration of weightlessness, water poses considerable resistance to movement.

Sounding rockets, which climb for several miles and then drop their payloads in free fall, offer up to eight minutes of microgravity, after which a parachute opens and brings the payload to Earth at a safe speed. However, this method of acheiving weightlessness is more expensive than are airplane experiments, and does not always allow the recovery of the payload.

• Jenks, Ken. “KC-135, Zero Gravity Trainer.” (www .microgravity.grc.nasa.gov/kjenks/kc-135.htm) A Web site by a scientist at NASA Glenn Research Center, giving a personal description of a flight aboard the Vomit Comet, with photographs, illustrations, and links to technical sources.
• Johnson Space Center. (www.jsc.nasa.gov) The on-line gateway to NASA’s Johnson Space Center, with links to the center’s various activities, including the Vomit Comet program.
• “KC-135 Student Flight Opportunities.” (www.jsc.nasa .gov/coop/kc135/kc135.html) A Web site hosted by NASA’s Johnson Space Center, detailing the process by which students may apply to participate in flight experiments aboard the Vomit Comet.

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