On February 21, 1967, tragedy struck the Apollo moon landing program. During a dress rehearsal or “plugs out” test for Apollo 1 – the first planned shakedown mission of the new spacecraft – an electrical wire sparked and started a fire in the capsule. In the pressurized, pure oxygen environment of the cabin, the flames quickly raged out of control, leading to the deaths of astronauts Gus Grissom, Ed White, and Roger Chaffee. The Apollo 1 disaster shook the US space program to its core, forcing a major reevaluation of safety procedures and dramatically showcasing the dangers of using pure oxygen atmospheres aboard spacecraft. But this lesson was very nearly learned a full two years earlier, in a now-forgotten incident that almost resulted in two astronauts suffering an equally fiery fate. This is the story of the near-disaster of Gemini 6A.

Following on the heels of Project Mercury, the United States’ first effort to put astronauts in space, Project Gemini was designed to test and perfect all the procedures and capabilities needed for the upcoming Apollo lunar program – including long-duration spaceflight, orbital rendezvous and docking, orbital maneuvering, and extra-vehicular activity or “spacewalking”. While the astronauts had had considerable input in the design of the earlier Mercury capsule, the Gemini spacecraft was a different beast entirely. Custom-tailored to the particular tastes of its fighter jock pilots, the cabin layout, controls, and instrument panels were all modelled after a high-performance fighter jet. Indeed, so influential was Mercury 7 astronaut and former Air Force test pilot Gus Grissom in Gemini’s design that the spacecraft became affectionately known as the “Gusmobile.” Another distinctly fighter-like feature of Gemini was its launch escape system or LES, designed to carry the crew to safety in case of an emergency during countdown or liftoff. The LES on the earlier Mercury spacecraft consisted of a solid fuel rocket motor mounted on a tower above the capsule, which in an emergency would ignite and launch the capsule – and its crew – safely away from the launch vehicle. Gemini, by contrast, was fitted with aircraft-like ejection seats – and it was these devices which almost caused the fiery deaths of the Gemini 6A crew.

The fourth manned Gemini mission, Gemini 6 was intended to test two of the most important capabilities required for the upcoming Apollo missions: orbital rendezvous and docking. NASA had recently chosen the Lunar Orbit Rendezvous or LOR method for landing on the moon, which involved the use of a lightweight spacecraft known as the Lunar Module or LEM which would undock from the Command-Service Module or CSM, land on the moon, take off again, and re-dock with the CSM in lunar orbit. Ensuring the astronauts could perform these procedures reliably and safely was vital to the success of the entire Apollo program. To this end, the Gemini 6 mission called for its crew to rendezvous and dock with an unmanned Agena target vehicle, which would be launched into orbit ahead of them.

Selected for the mission were Command Pilot Walter M. Schirra Jr. and Pilot Thomas P. Stafford. A member of the original Mercury Seven astronauts, Schirra had previously orbited the earth during the Sigma 7 mission on October 3, 1962, becoming the fifth American and ninth human in space. Stafford, by contrast, was a rookie, part of the second astronaut group  – AKA “the New Nine” or “Next Nine” – selected  by NASA in 1962. At 9:45 AM on October 25, 1965, Schirra and Stafford rode the elevator to the top of Cape Canaveral’s Pad LC-19 gantry and were strapped into their spacecraft. Fifteen minutes later, the Agena target vehicle, carried atop an Atlas booster, roared off the nearby pad LC-14. But while the Atlas first stage performed flawlessly, moments after separation a flaw in the second-stage engine caused the Agena to explode at an altitude of 240 kilometres. 50 minutes later, with the primary mission objective now a cloud of debris raining down over the Atlantic, Gemini 6 was scrubbed and Schirra and Stafford were removed from their spacecraft.

In the wake of this mishap, NASA planners scrambled to salvage the Gemini 6 mission. As a new Agena target vehicle could not be readied in time, NASA proposed an alternate plan: the new mission, designated Gemini 6A, would instead rendezvous with the Gemini 7 spacecraft, due to be launched in early December. Crewed by astronauts Jim Lovell and Frank Bormann, Gemini 7’s main objective was to test the feasibility of long-duration spaceflight by remaining in orbit for 14 days – the maximum time it would take to fly to the moon and back. Wanting to get the most bang for their buck out of the mission, NASA also proposed that Gemini 6A Pilot Tom Stafford switch places with Gemini 7 Pilot Jim Lovell by performing a spacewalk. Lovell, however, nixed this plan, as it would require him to spend 14 days wearing a bulky and uncomfortable EVA spacesuit instead of the lightweight orbital suit he and Borman had been issued.

Gemini 7 was successfully launched on December 4, 1965. Eight days later, Schirra and Stafford returned to Pad 19 for their second launch attempt. At first, all went exactly as planned. The preflight checks went smoothly, the countdown reached zero without incident, and the engines ignited right on schedule. But then, 1.5 seconds after ignition, the engines suddenly shut down and Gemini 6A’s Titan II booster settled back onto the pad. In that moment, Command Pilot Wally Schirra faced a critical decision. While all rockets are essentially giant bombs, the Titan II was particularly dangerous, for it was fuelled by a volatile combination of Hydrazine Hydrate and Nitrogen Tetroxide. These fuels are hypergolic, meaning they ignite on contact with one another. Thus, if a Titan II fell back onto the pad, the propellant tanks could split and the propellants mix, creating a giant fireball. Mission rules thus dictated that in the event of an engine failure on liftoff, Schirra was to immediately pull the ejection handle and launch himself and Stafford away from the exploding rocket.

But instead, Schirra hesitated. Though he had heard the engines ignite, he hadn’t felt the rocket lift off. Thus, when the engines shut down, he deduced that he and Stafford were in no danger and decided not to eject from the spacecraft. An hour later, pad crews rendered the malfunctioning rocket safe and the two astronauts climbed out of their spacecraft, their mission thwarted once again. But Schirra’s quick thinking and coolness under pressure likely saved their lives, as Tom Stafford explained in a 1997 interview:

“So it turns out what we would have seen, had we had to [eject], would have been two Roman candles going out, because we were 15 or 16 psi, pure oxygen, soaking in that for an hour and a half. Jesus, with that fire going off [from the ejection seat rockets], it would have burned the suits. Everything was soaked in oxygen. So thank God. That was another thing: NASA never tested it under the conditions that they would have had if they would have had to eject. They did have some tests at China Lake [California] where they had a simulated mock-up of Gemini capsule [on a rocket sled], but what they did is fill it full of nitrogen. They didn’t have it filled full of oxygen in the sled test they had.”

And even if the astronauts’ suits hadn’t ignited, the 20 g acceleration required to launch them 250 metres from the spacecraft – the minimum safe distance to escape the fireball – would likely have inflicted severe injuries. Yet had Schirra felt any motion at all, he would have pulled the handle, for, as he later stated: “It was death or the ejection seat.”

The engine shutdown was traced to a control umbilical cable which had released prematurely, and the Titan II rocket was quickly returned to flight readiness. On December 15, 1965, Schirra and Stafford climbed aboard the Gemini 6A for their third launch attempt. This time, everything went as planned, the Titan II lifting off at 1:37 PM. 4 hours later, Gemini 6A successfully intercepted Gemini 7 – the first orbital rendezvous by manned spacecraft in history. Over the next 5 hours the crews practiced orbital maneuvering and formation flying, coming within 30 centimetres of one another. Then, after a rest period, Gemini 6A broke formation and initiated reentry, splashing down in the Pacific Ocean and 3:28 PM on December 16 – 26 hours after liftoff. Meanwhile, Lovell and Borman carried on with their mission, reentering and splashing down two days later on December 18. With few other mission objectives to occupy them, the astronauts found the two week mission excruciatingly monotonous, while by the end of the flight the cramped conditions and rapidly-dwindling storage space for waste bags and other garbage made conditions inside the cabin almost unbearable. Nonetheless, the mission was a success, proving that astronauts could survive and function in space for long periods.

The next attempt to dock with an Agena target vehicle came on March 16, 1966 with the launch of Gemini 8, crewed by Command Pilot Neil Armstrong and Pilot David Scott. However, this mission also nearly ended in disaster when, shortly after docking with the Agena, a stuck thruster caused the spacecraft to start spinning out of control. Only quick action by Armstrong saved the crew from blacking out and being killed by the rapidly-mounting G-forces. The next mission, Gemini 9, was similarly plagued with problems. As on Gemini 6, the Agena target vehicle failed on launch, forcing the original mission to be scrubbed. However, NASA had planned for this contingency, and two weeks later launched the Augmented Target Docking Adaptor or ATDA, a simpler backup docking target based on the front half of the Gemini capsule. Two days later Gemini 9A launched into orbit. Unfortunately, when they caught up with the ATDA, they discovered that its streamlined launch fairing had failed to separate, making docking impossible. The first successful docking of a Gemini and Agena would not take place until July 19, 1966 during the Gemini 10 mission, crewed by Command Pilot John Young and Pilot Michael Collins. From here, NASA quickly made up for lost time. By the time Gemini 12 splashed down on November 15, 1966, Project Gemini had fulfilled all its objectives, and NASA was ready to move on to Project Apollo – and the moon. That the project was able to proceed so rapidly without a fatal mishap was largely due to the quick thinking and superb piloting skills of astronauts like Wally Schirra and Neil Armstrong.

However, Schirra’s potentially lifesaving decision during Gemini 6A only delayed the inevitable, with the tragic deaths of the Apollo 1 astronauts in February 1967 publicly exposing the dangers of using pure oxygen aboard spacecraft. It is often claimed that up to this point NASA was completely ignorant of these dangers, but this is not the case. Indeed, several near-fatal accidents involving fires in high-pressure oxygen test chambers had occurred prior to Apollo 1, including two at Brooks Air Force Base in San Antonio and the Navy Air Crew Equipment Laboratory in Philadelphia in  1962. On February 16, 1965, just 10 months before Gemini 6A, Navy Divers Fred Jackson and John Youmans were killed when a fire broke out in their decompression chamber at the Experimental Diving Unit in Washington, D.C. So why, then, did NASA persist in using pure-oxygen atmospheres aboard its spacecraft? There are several reasons. First, pure oxygen is breathable at much lower pressures than regular air, meaning the cabin pressure could be kept lower and the spacecraft structure lighter. Second, breathing a nitrogen-oxygen atmosphere introduces the risk of decompression sickness, also known as “the Bends”. And finally, maintaining a mixed atmosphere requires additional gas tanks, plumbing, and other equipment that all add weight and complexity to the spacecraft. NASA thus judged a pure oxygen atmosphere to be the lighter, simpler – and yes, safer – option. Following the Apollo 1, fire, however, engineers were forced to reevaluate this decision, and ultimately came up with a compromise. On all subsequent Apollo missions, a mixed nitrogen-oxygen atmosphere would be used during launch, but swapped out for pure oxygen once the spacecraft reached orbit. Most of the flammable material that had fuelled the Apollo 1 fire – such as velcro and nylon cloth – was also stripped from the spacecraft and replaced with fire-resistant alternatives. As for the Gemini’s potentially lethal ejection seats, these were replaced on Apollo with a conventional tower-mounted launch escape system. The only other time NASA used ejection seats on a spacecraft was during the first two Space Shuttle missions in 1981, after which they were deleted from the orbiter’s design. By this time, the cabin atmosphere had also been switched to a more earth-like oxygen-nitrogen mix. Thanks to these measures, NASA has not had another incident involving fire aboard a spacecraft – and hopefully never will again.

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