The "sim:" Apollo astronauts survive a training gauntlet

Before they ventured beyond earth’s atmosphere, Apollo’s astronauts spent just shy of 30,000 hours training. According to NASA, 80 percent of that time was spent in simulators. The teams needed to practice, over and over, both standard operating procedures as well as dealing with hypothetical problems. Here’s Apollo 11 Command Module Pilot Michael Collins in the Command Module simulator, June 19, 1969. Also don't miss the unidentified man next to him in a dress shirt and tie.

The majority of these exercises occurred in two simulators: the Command Module simulator and the Lunar Module simulator. A big hurdle to running the simulations was computing power. The earliest simulators, some of which were adapted from Gemini equipment, were run on analog computers. It was a big milestone when they had expanded the operation to 10 computers, and another feat when they switched to digital. Here’s two of the simulators on March 1, 1968.

Here, Neil Armstrong participates in EVA (Extra-Vehicular Activity) training with the Lunar Module Simulator to his left and NASA officials watching from the back. April 17, 1969.

The two module simulators weren’t the only training rigs for the astronauts. Perhaps the least favorite apparatus of the group was the Centrifuge, also known as the “G Machine,” used to simulate the forces of acceleration a body would have to withstand while traveling by spacecraft. Astronauts strapped into the steel ball, which would then spin around on its 50-foot arm like some sort of intense amusement park ride. This tested how the forces would impact a body, for instance, whether a person can breathe (they can, but with great difficulty), as well as how much the astronauts can maneuver under the physical pressure.

Finally, the engineers at NASA figured out a few different ways to simulate low gravity for the astronauts. Here, Apollo 11 Lunar Module Pilot Buzz Aldrin is suited up aboard a U.S. Air Force KC-135 on July 10, 1969. The big plane, normally used as an aerial refueling aircraft, would fly in specific loopy patterns that would cause its cargo to feel “weightless."

Another alternate-gravity simulator was called the Six Degrees of Freedom Simulator, named for the total number of ways an object can move or rotate in space. Here’s astronaut Jim Lovell, Apollo 13 commander, during a lunar surface simulation in January 1970. He’s holding an Apollo Lunar Hand Tools Carrier.

The Lunar Landing Walking simulator tipped participants on their side to simulate low gravity. With a “floor” at the correct angle, it’s a surprisingly effective way to reduce a person’s perceived gravity by one-sixth, just like the Moon. At NASA’s Langley research center, this simulator allowed NASA to test the movement capabilities of moon-bound. They learned how high they might be able to jump, and how easily they could get up after falling. In a 1965 demonstration video, the suit-wearer shows off the ease of doing acrobatics in low gravity, but as far as we know, no astronauts attempted such feats while on the Moon.