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.

“I think it started when somebody spun a fish and didn’t notice anything irregular about the fish because of the high Gs,” says Stephen Cloak, a Navy research engineer and veteran centrifuge jockey. “So they postulated that if we put a human encased in water, it would dissipate the G forces and they could take high G.” The Maiden was an aluminum capsule designed by Gray, sculpted roughly in the shape of a seated human, that could be filled with water. Gray stayed alert throughout the 25-second run up to 32 Gs, suffering only mild sinus pain. “He was another one of these late ’50s, early ’60s guys that just kind of kicked the tires and went at it,” says Cloak. Gray wanted to go to the full 40-G capability of the centrifuge, but the Maiden was too big to fit inside the gondola and so had to be mounted farther inward along the arm, where 32 Gs was the maximum acceleration possible.

In the late 1950s, two scientists, Carl Clark and James Hardy, had a more daring idea. Physics dictated that if a spacecraft could be steadily accelerated at 2 Gs, it could reach the moon or Mars in days or even hours. But could a human being survive the constant acceleration? Clark used the centrifuge to find out.

“He essentially moved into the cab, brought his La-Z-Boy from home, and stayed in there at 2 G for 24 hours,” says Shender. Clark slept, ate, worked, and lived at two Gs for a full day under constant medical surveillance. He suffered nothing more than fatigue. Further marathon rides were planned, but more immediate space missions loomed and the idea was set aside.

One factor that eventually discouraged the sportier research projects was the mounting evidence of all that could happen to the body under acceleration. Under high Gs, Cloak explains, “you’re insulting the brain with a lack of oxygen in the blood. Each person’s brain is a little different, so you don’t know what’s going to happen.” Aside from G-LOC (for “G loss of consciousness”), possible effects included motion sickness, disorientation, anxiety, euphoria, and confusion. Cloak adds, “You get swelling of the feet and ankles, ruptured blood vessels in the groin area, blood clots, temporary change in blood-flow patterns in the lungs, possible collapsing of the lungs, fractured ribs, chest pain. For your heart it’s entirely possible to have arrhythmias, transient electrical changes, myocardial infarctions, interesting little things like that.”

Most of these effects were transient and fairly rare, but they were not to be dismissed. “We had to go through a battery of exams,” Cloak recalls, “because one of the major risks is sudden death. No matter how well they screen you, you just don’t know when you get in there if a 9-G ischemic insult to your system is gonna kill you or not.” Then there are the mild phenomena, such as petechial hemorrhaging. “You actually look like you’ve got measles—at high Gs, blood leaks through the blood vessels and you get little pinpoints all over. It’s kind of interesting, especially the first time you see it.”

Cloak rode the centrifuge routinely throughout his career at Warminster as an acceleration researcher. “I used to tell everybody it broke the week up,” he says with a laugh. He adapted quickly: “135 rides later, it was just like getting up and walking around. You get so used to it.” He became such an expert rider that he ended up teaching anti-G techniques to Navy fighter pilots.

Not everyone was a “G monster” like Cloak. For Barry Shender, one go-round was enough, a routine familiarization ride that didn’t exceed a mild 3 Gs. “I’m not the roller-coaster-ride type,” he admits. Centrifuge engineer Bill Daymon was another one-timer, although in his case the purpose of the ride wasn’t familiarization but basic troubleshooting. “People were hearing noises, and I took a 3-G ride to listen to it,” he recalls. “That was my only ride. The year before I had had bypass surgery, and they were rather reluctant to let me ride it again.”

Subjects generally rode the centrifuge in one of two modes: closed-loop, or dynamic flight simulation, in which the rider had full control over the movements of the centrifuge; and open-loop, or “meat in a seat,” in which the rider was essentially a lab rat at the mercy of the researchers. Riders were usually given various tasks to perform under the G stresses, such as flying simulated combat missions and other activities demanding certain cognitive or motor skills. Doctors monitored all test subjects at every moment, and both the subject and the doctors had the capability to immediately stop the ride. It’s a testament to the Johnsville researchers that no one was ever seriously injured riding the centrifuge.

Despite the discomfort and dangers, willing volunteers were never in short supply. “You have to give a lot of credit to the folks that volunteer to do it,” Shender says, “because basically we beat them up every day, and they come back.” So why did they clamber into the belly of the beast? “Motivations like I want to see what I can do physically. I want to do something that would make good stories. I want to do something that’ll get me out of the office today.” Subjects could also score a souvenir. “If they like, we give them a video of their experiences in the centrifuge so they can show their family and friends when they lose consciousness and how silly they looked.”

The centrifuge research has had a lasting impact on the training of military pilots, the development of anti-G suits and techniques, and the design of aircraft and spacecraft systems. Aside from the biomedical effects of high Gs, the Johnsville researchers investigated practical problems, including the disorientation of Navy pilots following night catapult launches from a carrier, and spin recovery techniques in fighter aircraft such as the F-4B Phantom and F-14 Tomcat. Such projects used the centrifuge’s flight simulation capabilities to full effect. Sometimes the centrifuge was used to re-create the conditions of puzzling crashes that might indicate aircraft design flaws.

The last decade of operations at Johnsville saw one of the centrifuge’s most important contributions. “Back in the ’90s there was a mandate from Congress that everybody should be able to go into the tactical cockpit, boys and girls, small people, big people,” says Shender. “We developed what we called the Gender Neutral Study, where we wanted to ask the question: What happens if you’re a small female and you get put into one of these high-performance jets? Can you fly? Can you eject? Can you hold your head up?” As it turned out, women can more than hold their own against the flyboys. “We established that they can certainly fly high-G maneuvers without any difficulty, and certainly [have] comparable acceleration tolerance with the men,” says Shender. “These female subjects had a good time doing it. And they didn’t complain nearly as much as the male subjects do.”

In 1996 the Warminster base fell victim to the Base Realignment and Closure Act, and the Naval Air Warfare Center moved to the Navy’s Patuxent River facility in Maryland, leaving the centrifuge behind. Veridian Corporation, a private contractor, kept it spinning for mostly Navy programs for a while, but by 1999 mounting costs forced the wheel into retirement. Although centrifuge work continues at other military and NASA centers, “we’re sort of gearing down,” Shender says regretfully. The center of the action appears to be shifting overseas, with new centrifuges in Sweden and Japan. None measures up to Johnsville in capabilities or sheer engineering chutzpah.

As for the Johnsville centrifuge, proposals for its future use range from the sedate, such as turning the facility into a museum, to the outlandish, such as turning it into a thrill ride—an unlikely scenario, given that the deaths of two riders on Epcot Center’s “Mission: Space” simulator were linked to G-induced stresses. Shender and Cloak continue their work in acceleration science at the Naval Air Warfare Center at Patuxent River, while veterans of the center like Bill Daymon meet at reunions to trade war stories. Regardless of whether the Johnsville centrifuge ever spins again, its legacy in aerospace history—and in the memories of all who rode it—is secure.

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