By the winter of 1967, President John F. Kennedy’s goal of “landing a man on the moon and returning him safely to Earth” by the end of the decade appeared to be in doubt. A three-month delay in the delivery of a newly designed spacecraft had pushed back the Apollo program’s first manned mission to February 1967, and repeated testing failures plagued the most complex flying machine ever engineered.

The three men set to blast off on Apollo 1—rookie astronaut Roger Chaffee and veterans Virgil “Gus” Grissom and Ed White—had issues with the new craft as well. They voiced their concerns about the quantity of flammable nylon and Velcro in the command module with Joseph Shea, manager of the Apollo Spacecraft Program Office, before presenting him with a gag version of their crew portrait in which their heads were bowed and hands were clasped in prayer. “It isn’t that we don’t trust you, Joe, but this time we’ve decided to go over your head,” read the inscription.

In spite of the incredible danger inherent in space travel, the National Aeronautics and Space Administration (NASA) had launched 16 manned space flights in its Mercury and Gemini programs without a single casualty. “Success had become almost routine for us,” NASA flight director Gene Kranz wrote in his book Failure Is Not an Option. “The country had gotten complacent.” Perhaps NASA had gotten complacent as well. In spite of orders from Shea, the flammable materials were never removed from the Apollo 1 command module.

With 25 days left before the scheduled launch, the crew of Apollo 1 climbed out of a NASA van into sparkling Florida sunshine on January 27, 1967, and ascended the tower of launch pad 34 for a routine simulated launch test. Clad in their spacesuits and carrying their portable air conditioning packs like office workers toting briefcases, the astronauts crossed the 218-foot-high catwalk with vistas of the blue Atlantic waters washing up on the white beaches of Cape Canaveral before climbing inside their command module perched atop a massive booster rocket.

The “plugs-out” test, during which the module was disconnected from the launch pad’s electrical systems and operating under its own power, was classified as non-hazardous since the rocket was unfueled. To make the countdown rehearsal as realistic as possible, the launch pad team sealed the hatch after the astronauts were strapped into their seats inside the cabin pressurized with pure oxygen.

Throughout the afternoon, minor glitches and communication issues between the spacecraft and Mission Control in Houston caused repeated delays. Hours behind schedule, early evening darkness settled around the launch pad as the simulated countdown reached a hold with 10 minutes left as attempts continued to resolve the radio issues. “How are we gonna get to the moon if we can’t talk between two or three buildings?” quipped a frustrated Grissom at 6:30 p.m.

Less than a minute later, the engineers watching the capsule cabin on a closed-circuit television screen were startled by a flash. A spark that likely came from faulty electrical wiring behind a panel door below Grissom’s feet suddenly ignited in the capsule. Fed by the cabin’s pure oxygen, the spark took only seconds to morph into an inferno that tore through the flammable nylon netting and Velcro surrounding the astronauts.

“Hey! We’ve got a fire in the cockpit!” yelled one of the astronauts. Horrified engineers watched on their screens as smoke filled the cabin while White desperately attempted to release the cumbersome hatch. “We have a bad fire! We’re burning up!” came another screaming transmission from the cockpit.

Then silence.

Pad safety workers grabbed extinguishers and rushed to the capsule, but the dense smoke reduced visibility to nearly zero. Even rescuers wearing smoke masks were overcome by toxic fumes, and the tremendous heat burned through their gloves.

It took more than five minutes for the pad crew to open the complicated latch system on the hatch. By that point, it was far too late. The astronauts had virtually no time to unstrap themselves from their seats, let alone escape the flash fire. Burning at hotter than 1,000 degrees Fahrenheit, the blaze melted the astronauts’ space suits and oxygen tubes. The crew likely lost consciousness and died from asphyxiation from inhaling toxic gases. The process of removing the men from the charred capsule couldn’t begin until six hours after the fire, and it took 90 minutes to extricate their bodies, which were fused to the nylon of the cabin interior.

“We did not do our job! We were rolling the dice, hoping that things would come together by launch day, when in our hearts we knew it would take a miracle,” an emotional Kranz told his flight control team three days after the tragedy. “We were too ‘gung-ho’ about the schedule, and we blocked out all of the problems we saw each day in our work. Every element of the program was in trouble, and so were we.”

Just weeks before his death, Grissom had told a reporter, “If we die we want people to accept it. We hope that if anything happens to us it will not delay the program. The conquest of space is worth the risk of life.” NASA indeed pressed ahead with the Apollo program, but more than 20 months elapsed before American astronauts returned to the skies. During that time, NASA made thousands of changes to the Apollo spacecraft, including redesigning the hatch, altering the cabin atmosphere to include nitrogen and replacing flammable materials from the interior.

“It was perhaps the defining moment in our race to get to the moon,” Kranz wrote of the fire aboard Apollo 1. “The ultimate success of Apollo was made possible by the sacrifices of Grissom, White and Chaffee. The accident profoundly affected everyone in the program. There was an unspoken promise on everyone’s part to the three astronauts that their deaths would not be in vain.” Out of the ashes of the Apollo 1 tragedy came crucial safety and performance improvements that allowed NASA to fulfill Kennedy’s pledge in July 1969 by landing Neil Armstrong and Edwin “Buzz” Aldrin on the moon and returning them safely to Earth. Before departing the lunar surface, the Apollo 11 astronauts left behind a reminder of their fallen colleagues—a commemorative medallion bearing the names of Grissom, White and Chaffee.