John Glenn called it a “dreaded” and “sadistic” part of astronaut training. Apollo 11’s Michael Collins called it “diabolical.” Time magazine referred to it as “a monstrous apparatus,” a “gruesome merry-go-round,” and, less originally, a “torture chamber.”

The Johnsville human centrifuge—the machine everyone loved to hate—was operated by the Navy at its Naval Air Development Center (later the Naval Air Warfare Center) in Warminster, Pennsylvania, just outside Philadelphia. For almost 50 years—it ceased government operation in 1996—the centrifuge was the world’s most powerful and versatile tool for studying the G forces that are an inescapable part of flight.

In his 2006 book Getting off the Planet: Training Astronauts, Randall Chambers notes, “Very early in the space program, amusement park rides were considered as possible research vehicles to study acceleration forces.” But Chambers, the scientist who trained all the early astronauts, soon realized that such machines wouldn’t take the extreme forces and sustained abuse needed to conduct serious studies on humans. A high-performance centrifuge, a machine that could produce high acceleration and thus high G-forces by rapid rotation, was the only solution.

By July 1950, inside a giant round 11,000-square-foot building at its Johnsville facility, the Navy had completed the world’s largest centrifuge, which consisted of a 10- by six-foot oblate sphere steel ball, or gondola, at the end of a 50-foot arm. (The oblate sphere gondola was later replaced with a 10-foot-diameter sphere.) A 4,000-horsepower electric engine at the other end whipped the arm around a huge chamber. The dual-gimballed gondola, mounted to the arm on rotating bearings, allowed the test subject to be oriented in various positions relative to the applied G force. This enabled the centrifuge to be used as a “dynamic flight simulator,” capable of accurately reproducing the sensations experienced by pilots in various flight maneuvers. Researchers reveled in the opportunity to study G forces under controlled conditions at levels previously accessible only in high-performance aircraft. Simply by turning the gondola as it spun about the arm, experimenters could subject pilots to positive Gs (“eyeballs in,” with acceleration in a head-to-foot direction), negative (“eyeballs out,” foot-to-head, similar to a rapidly descending elevator), transverse (chest to back), and practically every other variation that might be experienced during flight.

The Johnsville centrifuge rose to stardom at the beginning of America’s space program. It started operating with the training of pilots for the North American X-15 hypersonic aircraft and hit its stride with the Mercury manned spaceflight program.

“We were really worried about what was going to happen when we started spaceflight,” says Barry Shender, a biomedical engineer and specialist in acceleration stress who worked at Johnsville. The centrifuge’s flight simulation capabilities made it possible to reproduce all the ways various spaceflight scenarios could affect astronaut performance. “We did the early Mercury training of John Glenn and [Wally] Schirra and all the rest of those guys just to learn what happens if we go up to these sorts of accelerations in these different vectors,” Shender says. “We were talking about reentry and during takeoff, long-term exposures. So if we’re going to develop these ballistic profiles, how much can people take? It was a great unknown.”

For the Mercury, Gemini, and Apollo astronauts, the “wheel” was both a rite of passage and an invaluable training tool. “Whirling around at the end of that long arm, I was acting as a guinea pig for what a human being might encounter being launched into space or reentering the atmosphere,” Glenn recalled in John Glenn: A Memoir. “You were straining every muscle of your body to the maximum…if you even thought of easing up, your vision would narrow like a set of blinders and you’d start to black out.”

One objective of such ordeals was to teach an astronaut to counteract the G demons by using breathing techniques and muscle contractions. Michael Collins recalls in his 1974 autobiography Carrying the Fire: “If you breathe normally, you find you can exhale just fine, but when you try to inhale, it’s impossible to reinflate your lungs, just as if steel bands were tightly encircling your chest. So you have to develop an entirely new method, keeping the lungs almost fully inflated at all times, and giving rapid little pants ‘off the top.’ ”

Some people not only tolerated the centrifuge, but strove to test its limits—and their own. “Things were different in the ’50s and ’60s,” Shender says. “You could wake up in the morning and think, Let’s do something crazy today, and then do it.”

In August 1958 Navy Reserve officer Carter C. Collins rode the wheel to more than 20 Gs for a record 54 seconds. Later that day, R. Flanagan Gray, a civilian psychologist, repeated the feat. A year later, Gray would go on to greater fame as the first man to ride the “Iron Maiden,” a project that began with a rather odd idea about counteracting G forces.