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Navigating Weightlessness: A Look at NASA’s DC-9 Microgravity Research Flights
A bell rings and a strobe light flashes as the pilot of the DC-9 aircraft sharply raises the nose. Instantly, researchers feel the pull of double gravity, forcing them down towards the cabin floor. As the aircraft reaches its peak and the acceleration starts to diminish, the flight test director announces, “We’re over the top!”
With a sudden drop in pressure, the aircraft plummets into freefall. For the next 20 to 25 seconds, all unrestrained objects—including the researchers themselves—start to float. This brief period of weightlessness allows scientists to quickly attend to their experiments before the bell rings again, signaling the pilot to level the flight and restore Earth’s gravitational conditions.
This technique, flying in a series of parabolic arcs, creates a unique simulated weightless environment. During the 1990s, NASA’s Lewis Research Center, now known as NASA Glenn, utilized DC-9 flights to explore critical scientific phenomena such as fluid dynamics, combustion, and material interactions in microgravity.
The groundwork for this innovative research began in the 1960s when NASA Lewis employed a North American AJ-2 aircraft to investigate the behaviors of liquid propellants under reduced gravity. As the center’s focus expanded to encompass combustion processes and materials testing, it became a key player in the burgeoning field of microgravity research, especially following the introduction of the space shuttle in the early 1980s.
To facilitate terrestrial experiments that would precede the more lengthy missions aboard the shuttle, engineers boldly reconfigured a Learjet to conduct microgravity test flights. This setup allowed for a more intensive investigation of individual experiments in a controlled environment.
By 1990, NASA officials recognized the need for a larger aircraft to support a broader range of experimental designs, including those requiring free-floating conditions. The decision was made to adapt a McDonnell Douglas DC-9 for this purpose, which would be leased from the U.S. Department of Energy.
In the fall of 1993, stakeholders gathered at the Lewis center to discuss the necessary adjustments to the DC-9 for their research endeavors. When the aircraft arrived in October 1994, it was still in a standard passenger configuration. Over the ensuing three months, technicians stripped the interior, reinforcing the structure, and installing advanced power, communications, and guidance systems. A large cargo door was also added to facilitate the loading of oversized equipment.
The introduction of the DC-9 marked a significant milestone for NASA Lewis, solidifying its status as a leading institution for microgravity research. The center had recently enhanced its Space Experiments Division, upgraded facilities such as the 2.2-Second Drop Tower and the Zero Gravity Facility, and established the Space Experiments Laboratory to concentrate its microgravity activities.
Collaborating closely with industry partners and academic institutions, Lewis researchers designed and tested a variety of experiments intended for flight aboard the space shuttle or the International Space Station. The DC-9 was capable of carrying up to eight experiments alongside twenty research personnel during each mission.
The range of experiments conducted included studies on space acceleration, capillary pump loops, bubble dynamics, liquid thin film behavior, material flammability, and flame propagation. Researchers frequently attended these missions, gaining invaluable insights through direct observation, often so engrossed in their tasks that they overlooked their own temporary levitation.
The DC-9 operated bi-weekly, allowing ample time for experiment installation and necessary aircraft maintenance. Based at Cleveland Hopkins International Airport, the aircraft often flew in restricted airspace over northern Michigan, completing up to 40 parabolic arcs in a single mission.
As the late 1990s approached, NASA concluded its lease of the DC-9, returning it to its owner. Between May 18, 1995, and July 11, 1997, the Lewis microgravity flight team successfully logged over 400 hours, executed more than 70 flight trajectories, and carried out 73 diverse research projects—all contributing to the broader understanding of microgravity science.
NASA Glenn continues to be at the forefront of microgravity research, actively supporting experiments aboard the International Space Station aimed at enhancing crew health, spacecraft fire safety, and advancements in propulsion technologies. The center is home to two microgravity drop towers, including NASA’s leading ground-based microgravity research facility.
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