Before John Glenn and his fellow NASA astronauts made history orbiting the earth, they visited the Naval Air Development Center in Bucks County to train for what it would be like in zero gravity.

The base is now gone, turned into an office park.  But the centrifuge still stands, at 780 Falcon Circle, and operates as the Johnsville Centrifuge & Science Museum. Businessman Sam Cravero bought the space in 2007 with plans to preserve the historic site and turn it into a museum and education center.

Back in the 1960s, it was one of the largest human centrifuges in the world, capable of reaching a speed of 178 mph in less than 7 seconds, and generating up to 40 g's in gravitational force.

By comparison, those carnival rides that spin and pin you to the wall typically max out around 3 g's.

"You had to strain every muscle to keep enough blood in your head to keep from passing out," Glenn said in a 2011 phone interview. "It wasn't a pleasure trip. "

Glenn, the last of the original Mercury 7 astronauts, died Thursday at 95.

Back in the 1960s, the spacemen would spend days inside the centrifuge, being spun around inside an enclosed capsule attached to the end of a 50-foot arm.  It was crucial preparation for the rigors of gravitational forces before Glenn's 1962 Mercury mission to orbit the earth.

Up through the 1980s, according to the Johnsville museum, dozens of astronauts subjected themselves to the Warminster spin machine to learn how many g-forces they could withstand.

In 2011, the 50th anniversary of Alan Shepard's becoming the first American in space, Johnsville Centrifuge & Science Museum Inc. made arrangements to bring back the centrifuge 's original gondola (it was replaced in 1964 by the one that is still attached to it today). The museum group found the original gondola at a Smithsonian warehouse in Maryland, and moved it into the space permanently this year.

While it was a bumpy road preserving the historic technology, Cravero has managed to keep the museum afloat with a combination of event-hosting, donations and leasing. Old research labs surrounding the main space were converted into offices in 2011 to offset the costs.