At the dawn of the space age, as scientists faced a constellation of technical challenges in launching rockets into the heavens, one question made them especially uneasy:
What would happen to the human beings inside?
This month marks 50 years since the Apollo 11 landing on the moon, and with trips to the International Space Station now a regular occurrence, space travel seems near routine. But long before Neil Armstrong could take his historic steps, scientists began probing the limits of human endurance here on Earth — conducting some of the key early experiments at a circular building in Warminster.
The concrete structure is home to a whirling contraption called the human centrifuge, built by the U.S. Navy in 1950 at its research center in the Bucks County township’s Johnsville neighborhood. Designed to mimic the gut-clenching sensations of riding rockets and fighter jets, the centrifuge’s 50-foot steel arm was capable of exerting 40 Gs — 40 times the pull of gravity on hapless passengers.
None of the astronauts went near that limit, achieving peak accelerations of about 15 Gs. But that was hard enough, Apollo 11 crew member Michael Collins recalled in his 1974 autobiography, Carrying the Fire.
“Diabolical," he wrote. "As if steel bands were tightly encircling your chest.”
For scientists, it was a treasure.
“It was an incredible machine,” said physician and engineer Robert D. Banks, who conducted research at the Warminster facility in the 1990s. “It was the gold standard.”
Power to the device was cut off years ago. The building, now called the Fuge, is in private hands, its various spaces available to rent for businesses and social events.
But it remains open for tours upon request, in collaboration with the Southeastern Pennsylvania Cold War Historical Society. And from noon to 5 p.m. on July 20, the anniversary of the 1969 moon landing, owner Sam Cravero and students of nearby William Tennent High School are hosting a public event to commemorate the occasion.
The steel arm of the centrifuge is housed in a circular chamber measuring 124 feet across, attached to the stout 180-ton motor in the middle.
Test subjects rode in a gondola attached to the arm’s outer end. An early version was shaped like a flying saucer on its side, later replaced by the 10-foot spherical compartment that remains in place today. A simulated fighter-jet cockpit is mounted inside.
Riders climbed aboard from a “flight deck” — a monitoring room built into the surrounding wall. Next to the door leading to the gondola, some long-ago jokester installed a coin slot like the kind used on kiddie department-store rides.
“The government had a sense of humor,” said Cravero, who purchased the facility in 2007.
Said to have been the world’s most powerful centrifuge, the Warminster device had the added advantage that passengers could control it from within, Banks said. (A former flight surgeon for the Royal Canadian Air Force, Banks went on to conduct accident investigations — including the space shuttle Columbia disaster in 2003.)
Controls in the centrifuge cockpit allowed riders to tilt the gondola so they could experience G-forces in various directions.
As scientists would learn from experiments at Warminster and elsewhere, the human body’s tolerance for acceleration depends on the direction of the forces.
If the ball was rotated so the seated rider faced the center of the spinning centrifuge, he or she experienced the sensation of being sucked backward into the seat — like what happens when you hit the gas in a fast car, or when you ride one of those spinning carnival rides in which the floor drops out. With training, physically fit humans can tolerate sustained acceleration in that direction as high as 15 Gs or so, though it is very difficult to breathe, Banks said. (Those carnival rides are fairly tame by comparison, typically hitting just 3 Gs, NASA says.)
But if the ball was tilted sideways so the rider’s head pointed toward the center of the centrifuge, he or she was sucked down into the cockpit seat — like a fighter pilot executing a tight banked turn. (Or, in a much gentler example, like that downward pull you feel at the start of an ascent in a fast elevator.) Unless they are wearing protective equipment and undergo special training, humans cannot long endure more than 4 or 5 Gs in that head-to-toe direction.
That’s because at higher levels of acceleration, the human heart starts to have trouble pumping blood to the eyes and brain. Loss of vision occurs first — a phenomenon called “grayout” — followed eventually by loss of consciousness.
Among the scientists who analyzed the data from Warminster was Alice M. Stoll, whose graph of human response to G-forces would become known as the Stoll curve.
Stoll, who also did pioneering research on fire-retardant fabrics, was in her 30s when she studied the centrifuge. She wanted to ride the device as well, but the initial response to her plea was not encouraging, she told members of the Southeastern Pennsylvania Cold War Historical Society in a video interview.
"Women? Oh no, your breasts are going to get squashed,” other researchers told her.
But Stoll, who died in 2014, found that federal law prohibited scientists from subjecting anyone to an experiment to which they would not subject themselves, she said. Problem solved.
“What could they do? They let me ride the centrifuge,” Stoll said. "And I rode that to grayout. That was about 7.5 Gs. Same as the men.”
A few went much higher. The record was 31 Gs for a period of five seconds, set by another scientist named R. Flanagan Gray, though he took extraordinary protective measures. Gray designed a water-filled aluminum tank in the shape of a seated human being, hung it from the centrifuge arm, and rode inside the tank. Pressure from the water would have counteracted abnormal blood flow, almost like a hand squeezing a tube of toothpaste, Banks said.
From Gray’s account in the American Journal of Cardiology, it is unclear which way he was facing, and thus which way the forces were generated. He described the experience in the third person:
“He had a slight frontal sinus pain during the peak acceleration and noted a few flecks of blood in his handkerchief the following morning.”
But according to lore, Gray experienced additional side effects as he drove home after the experiment, said Eleanor O’Rangers, president of the Cold War historical group.
“Supposedly, he said, ‘Yeah, I’m fine,' ” O’Rangers said. “He got off the base, pulled over, and… blecchh!”
One of the most frequent centrifuge riders did so in more recent times. Steve Cloak, whose official job at the Navy base was as an engineer, eagerly volunteered to ride 135 times from 1990 to 1998. On some occasions, he rode for more than an hour.
When tilted to the side with his head facing the center, he felt like a fighter pilot executing a loop-the-loop, he said.
“That drives the blood out of your brain,” recalled Cloak, 60. “Your motor skills were significantly degraded.”
Asked if he suffered any permanent effects, Cloak said he occasionally suffers memory lapses but cannot say if they were due to the hours he spent on the centrifuge.
The Apollo astronauts rode the Warminster centrifuge years before their historic mission, as part of the preparation for earlier Mercury or Gemini missions, said O’Rangers. They underwent official Apollo training elsewhere.
And in the actual lunar mission, the G-forces were relatively modest, reaching 3.9 Gs on takeoff (in the sucked-backward direction, as they were seated with their backs parallel to the ground for launch) and 6.56 Gs on reentry.
But a chapter in scientific history is secure for the centrifuge — even though the Bucks County landmark is remembered more warmly by scientists than by most who rode it.
John Glenn, who paved the way for the Apollo missions when he became the first American to orbit the Earth in 1962, recalled his Warminster experience in a 2011 Inquirer interview.
“You had to strain every muscle to keep enough blood in your head to keep from passing out,” said Glenn, who died in 2016. “It wasn’t a pleasure trip.”