On a gray January afternoon in Houston, Walt Cunningham leaned into his Eames Lounge Chair and clasped his hands behind his head, the better to try and bend his thoughts back across five decades. Floor-to-ceiling windows let in a dull light that outlined Cunningham; it was a gloomy backdrop that mirrored the Apollo astronaut’s melancholy mood.

As a backup crew member for the initial Apollo mission, Cunningham recalled clambering into the first Apollo capsule on Jan. 26, 1967 for some pre-flight work. All had gone well, and no one thought the next day’s test, when the capsule would rely on its own internal power for the first time, would prove fatal. “We always expected that we’d lose at least one mission before we landed on the Moon, because of how far we were reaching out,” he said. “But we didn’t expect it to be on the ground.”

The Apollo astronauts, most of them confident or even cocky test pilots, were accustomed to risk. In those early days as NASA invented spaceflight on the fly, all of the vehicles had flaws. The early astronauts trusted that they could handle any situation that came their way. “Rightly or wrongly, we thought we were going to be good enough to compensate for whatever it was,” Cunningham explained.

But, infamously, a fire in the pure oxygen environment of the Apollo 1 spacecraft could not be compensated for—even by arguably NASA's best test pilot, Gus Grissom, veteran astronaut Ed White, and rookie Roger Chaffee. Just seconds after a spark ignited inside the capsule, a conflagration burned hotter than 1,000 degrees Fahrenheit. As materials inside the spacecraft were incinerated, they gave off toxic fumes. Opening the spacecraft’s cumbersome hatch, under the best of circumstances, required a minimum of 90 seconds.

The aftermath of the grisly accident found America questioning its previously infallible space program, which since the late 1950s had chased, caught, and surpassed the Soviet Union in the race to the Moon. Ultimately, however, the Apollo 1 fire probably saved NASA’s lunar ambitions. The fire forced a hard reset of a space program that had been rushing headlong toward the Moon, but had lost its way due to overconfidence. A better Apollo capsule was born from the accident.

“I’m always asked if I was nervous about that flight,” said Cunningham, the lunar module pilot on Apollo 7, which marked NASA’s return to flight after the accident. “We never really had any doubts. Everybody was working to get us there. Thousands of people. We were sitting at the end of the arrow so we got the glory, but those other people had the same attitude we did—if this mission fails it won’t be because of me.”

Oxygen and wires

To understand the whirlwind in which NASA existed in the mid-1960s, consider that Gordon Cooper flew the final Mercury mission in May 1963. Then, in March of 1965, the first Gemini crewed mission launched, beginning a series of tests in low Earth orbit that would prove the technologies needed to go to the Moon. Several months before that Gemini flight, however, technicians cut the first metal on Apollo flight hardware, known as Spacecraft 012. Amazingly, this was NASA’s third new spacecraft in just five years. In the 51 years since, NASA has flown just one other crewed spacecraft: the Space Shuttle.

During 1966, NASA flew five Gemini missions. Yet even amid this hectic schedule, engineers with the agency confidently conducted final design reviews on the Apollo capsule. In August, the agency took delivery of Spacecraft 012 at Kennedy Space Center. Its maiden launch was planned for February 1967.

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McDonnell Aircraft had won NASA contracts to construct the Mercury and Gemini capsules, but the Apollo award went to North American Aviation. This well regarded company had earned plaudits for the X-15, a hypersonic, rocket-powered aircraft that had flown as high as 107.8km in 1963, crossing the threshold into outer space. Nevertheless, although North American had much experience with airplanes, it had little with actual spacecraft.

In the early days of the capsule’s design, engineers at NASA made another critical—and ultimately fatal—decision. Knowing full well the premium on weight in a spacecraft launched from Earth to the Moon in a single stack, managers looked for every opportunity to trim mass from the capsule and three-stage rocket. A pure oxygen atmosphere was one option, as it required a lighter environmental control system to produce than a more complex mix of nitrogen and oxygen.

The risk, of course, is that pure oxygen under high pressure requires but a spark to ignite and rapidly burn. However, because a pure oxygen atmosphere had worked without incident for the Mercury and Gemini capsules, NASA decided it would be safe to press ahead with the same for the larger Apollo capsule.

The new spacecraft was more complex, with considerably more electronic equipment inside. The initial version of the spacecraft contained more than 600 switches, indicators, and computers, all of which had to be connected and powered by an extensive amount of wiring. In his biography Forever Young, A Life of Adventure in Air and Space, the Gemini, Apollo, and Space Shuttle veteran John Young recounted the unfortunate differences in wiring between the Gemini and Apollo spacecraft.

To save on labor costs, North American had used machines to bundle the miles of wires that snaked through the Apollo spacecraft, the arrangement of which seemed arbitrary to Young. Some wires even appeared frayed. As he surveyed the capsule that Grissom, White, and Chaffee were to fly in, Young saw a seemingly endless amount of short circuits. “I knew it when I saw it, and I saw it in spades in the command module,” he wrote.

In the hurly burly world of NASA at the time, however, such a problem hardly stood out with the imperfect vehicle. Spacecraft 012 had shipped from California to Florida with more than 100 “significant” engineering orders still not completed, according to the accident report. For the astronauts, if they complained too loudly, there was also the threat of getting pulled off the mission.

“While Grissom complained long and loud behind the scenes about many of the problems with his spacecraft, he likely tolerated the bad wiring because he, NASA, and its contractors were at that time firmly in the grip of a deadly malady called ‘Go Fever,’” wrote George Leopold in his recently published book on Grissom’s life, Calculated Risk. “Grissom and his crew were gambling that the growing list of problems with the spacecraft would somehow be fixed in time for the February launch.”