Calculator Smuggled to Apollo 13

The Calculator That Saved Apollo 13

On April 13, 1970, the Apollo 13 spacecraft was 200,000 miles from Earth when an oxygen tank exploded. The command module was losing power. The astronauts, Jim Lovell, Jack Swigert, and Fred Haise, had to move into the lunar module. The lunar module was designed to land on the moon. It was not designed to keep three men alive for four days. It was not designed to navigate back to Earth. The computers on board were not powerful enough to calculate the trajectory they needed. The astronauts had to do the calculations by hand.

They had a slide rule. It was a Pickett N600-ES, a simple device that could multiply and divide, that could calculate sines and cosines, that could do the math that the computers could not. The slide rule had not been authorized. It was not on the official equipment list. It had been smuggled on board by a young engineer named Jack Garman, who thought that the astronauts might need it. He was right.

What Everyone Knows

Apollo 13 is famous as the mission that almost ended in disaster. The explosion, the scramble to power down the command module, the use of the lunar module as a lifeboat, the manual course corrections, the safe return. The story has been told in books, in films, in documentaries. It is a story of ingenuity, of teamwork, of the ability to solve problems under pressure.

What is less often emphasized is that the astronauts did not have a computer that could do the calculations. They had a slide rule. They had to do the math themselves. They had to aim the spacecraft by hand. They had to fire the engines at the right time, for the right duration, in the right direction. They did it. They came home.

What History Actually Shows

The explosion occurred at 9:08 PM on April 13. The astronauts were in the command module, Odyssey. The oxygen tank exploded. The command module was damaged. The fuel cells were dead. The power was failing. The astronauts moved into the lunar module, Aquarius. Aquarius was designed to support two men for two days on the moon. It was now supporting three men for four days in space. The carbon dioxide was building up. The water was running out. The power was limited.

The ground control team in Houston had to figure out a way to get them home. The trajectory they needed was not the trajectory they had planned. They had to use the moon's gravity to slingshot them back to Earth. They had to fire the lunar module's engine at the right time, for the right length of time, to get them on the right course. The calculations were complex. The computer on board was not designed for them. The astronauts did the calculations on a slide rule.

The slide rule was a Pickett N600-ES. It was a simple device, a plastic ruler with a sliding center, used by engineers to do multiplication, division, trigonometry. It had been smuggled on board by Jack Garman, a young engineer who was working on the Apollo program. He had been told that the astronauts would not need a slide rule. He put one in the bag of spare parts anyway. He was glad he did.

The Part That Got Buried

The slide rule was not the only thing that saved Apollo 13. The ground control team in Houston was the reason the astronauts came home. They worked around the clock. They improvised solutions to problems that had never been anticipated. They figured out how to use the lunar module's engine to steer the spacecraft. They figured out how to use the command module's batteries to power the re-entry. They figured out how to get the astronauts home. The slide rule was a tool. The men who used it were the reason it worked.

The astronauts used the slide rule to calculate the burns that would bring them back to Earth. They had to fire the engine at the right time. They had to fire it for the right number of seconds. They had to point the spacecraft in the right direction. They did the calculations. They did them by hand. They did them with a slide rule. They did them correctly. The burns worked. The spacecraft was on course. The astronauts came home.

The Ripple Effect

Apollo 13 splashed down on April 17, 1970. The astronauts were safe. The mission was a failure. It was also a success. The failure was that they did not land on the moon. The success was that they came home. The story of Apollo 13 became the story of the mission that was saved by ingenuity, by teamwork, by the ability to solve problems that had never been solved before. The slide rule that was used to calculate the burns was a small part of that story. It was a part that was not told for years.

The slide rule is now in a museum. It is a reminder of the mission that almost ended in disaster. It is a reminder of the men who did the math by hand. It is a reminder of the engineer who smuggled it on board because he thought it might be needed. He was right.

The Line That Says It All

Jack Garman smuggled a slide rule onto Apollo 13 because he thought the astronauts might need it—and when the oxygen tank exploded, when the computers failed, when the astronauts were 200,000 miles from Earth with no way to calculate the trajectory that would bring them home, they took out the slide rule, did the math by hand, fired the engine at the right time for the right number of seconds, and came home—because a young engineer had been told that the astronauts would not need a slide rule, and he did not believe it.