 Every time I discuss the Apollo 1 fire, somebody asks the question of why NASA sent a spacecraft up into orbit with a pure oxygen environment. We know there are three things you need to make a fire. Heat, oxygen, and fuel. So why give ample oxygen? Well, I'm gonna walk you through the decision-making process. In the later half of 1961, not long after President Kennedy promised America a moon landing by the end of the decade and that crew's safe return, NASA started putting out requests for proposals for various contractors hoping to build the Apollo spacecraft. Now, no one at the time really knew what that spacecraft was going to look like or even what it was going to do. Initially, the idea was that this mothership would also be the landing vehicle. So what NASA did was put everything it knew in the official request for proposals as constraints on the designers. Now, there were a number of contractors that submitted bids back to NASA hoping to win the bid to actually build the spacecraft and have the glory of landing their machine on the moon. Among them was North American aviation. North American aviation was already pretty well known in the aviation industry as the builder of the P-51 Mustang, the B-25, and of course the X-15, which took pilots to the fringes of space beginning in the late 1950s and into the 1960s. North American, like the other contractors hoping to build Apollo, didn't really know what was going to be involved in going to the moon. This was a very early concept study that NASA hoped would eventually lead to a flight article. So, North American figured the best bet was to just give the space agency exactly what it asked for. And it did. It basically took all of NASA's constraints and put them into a spacecraft that NASA said it wanted. It wanted a truncated cone because that's what the Mercury spacecraft was, was building on experience. Five companies ended up submitting bids to build the Apollo spacecraft. And NASA ranked all of them. And in that list, North American was second. But NASA couldn't ignore North American's background in early aviation and spaceflight because mainly of the X-15. So it gave North American the benefit of the doubt that it had the best experience to transfer from aviation into spacecraft. So that's how North American ended up winning the bid to build the Apollo spacecraft. Within the constraints in this initial call for proposal, NASA had stipulated that the cabin environment for the crew be a dual gas environment, oxygen and nitrogen, similar to the air we breathe on Earth. This was known to be good for humans and also not super flammable. So that's what North American gave NASA back, a dual gas environment. A year later, however, in November of 1962, NASA changed its mind. Weight was becoming the most precious commodity on Apollo and shaving off pounds at the top of the stack was ideal. Playing the numbers game and looking to lose those last few stubborn pounds, NASA zeroed in on the crew cabin as a place to save weight. The space agency realized that the dual gas environment was very heavy. Not only did you need two different kinds of tanks to hold the two gases, the hardware and plumbing to feed both of those gases into the crew cabin added more hardware and therefore more weight. Not to mention it was a very difficult system for North American to develop. See, you can't just open up the tanks and have the environment being flushed with oxygen and nitrogen. You need to keep it in the certain right balance. The air we breathe on Earth is about 78% nitrogen and 21% oxygen. The rest is trace gases. So the environment for the crew cabin would have to be balanced very precisely, which meant that North American would not only have to figure out how to feed the gases into the cabin in that right arrangement, it would have to develop a gauge that would be able to tell how the cabin environment was shifting as the astronauts did things like breathe and be able to react instantly to any changes to keep the environment stable. All of that was not only complicated, it added a lot of weight. So NASA said, well, we're doing pure oxygen in Mercury. Why not just do pure oxygen in Apollo? Humans can survive with pure oxygen in space because it's not super concentrated. In a spacecraft on Apollo, the gas was only pressurized to be at five pounds per square inch. So even though it's pure oxygen and that can be dangerous, as long as you purge your body of nitrogen beforehand, it was actually okay for the crew to live in that environment for two weeks. But North American wasn't in favor of the change. From the contractor's perspective, the simplicity of a single gas system didn't offset the danger it posed to the crew. North American engineers knew, as did NASA engineers, that a flame in the presence of pure oxygen has the potential to become an explosion very fast. North American did not want to build a spacecraft that risked exploding with a crew on board. However, NASA countered that the gas would be at such a low pressure, a flame wouldn't become an explosion. It would burn, but not so fast that the crew couldn't respond to deal with it in time. It ultimately fell to NASA to make the final decision. It was, after all, a NASA program, not a North American aviation program. The official switch from a duel to a single gas environment came via a formal contract change notice signed by Robert Gilruth, director of the Man's Spacecraft Center in Houston, on August 28, 1962. What's really amazing in the story of the single gas environment for Apollo leading up to the Apollo 1 fire is that NASA had more than enough evidence that this was maybe going to be a problem. In 1961, cosmonaut Valentin Bondarenko was killed after leaving an isolation chamber test in which the environment was 68% oxygen. He took off his bio-med sensors threw him into a corner and it inadvertently landed on a heating coil, and it erupted into flame and he was killed. In September of 1962, a fire broke out in a simulated spacecraft cabin at Brooks Air Force Base where the cabin was pressurized to 5 psi with pure oxygen. Luckily, the two subjects were protected by pressure suits but were treated for smoke inhalation. In November of 1962, four men inside the U.S. Navy's Air Crew Equipment Laboratory sat for 17 days in an environment pressurized with 5 psi of 100% oxygen. When an exposed wire arced and started to fire, it spread rapidly before they were rescued. Two Navy divers were killed in February of 1965 during a test of the Navy's Experimental Diving Unit, which was pressurized to 55.6 psi to mimic depth conditions at 92 feet. This was actually a multi-gas environment with 28% oxygen, 36% nitrogen, and 36% helium. But somehow the carbon dioxide scrubbers designed to remove the toxic gas from the air caught fire. The pressure inside the chamber rose, making it impossible for technicians to open the door and save the men. And even closer to home, on April 28, 1966, a fire broke out during an unmanned test of the Apollo Environmental Control System. No one was injured, but spacecraft hardware was destroyed. This was the first time NASA really considered flammability issues on Apollo in light of the pure oxygen environment. However, because the heater involved in this incident was a commercial heater and not something that would be going to the moon, no one really paid much attention. It wasn't likely that this would be repeated in space. However, NASA did look at the flammable material inside the spacecraft, but only the Block 2, the version of the Apollo spacecraft that could support a lunar mission because this one alone could dock with a lunar module. NASA decided to go through the entire cabin and make sure nothing flammable was too close to anything that could be an ignition point. However, that was only for the Block 2. Apollo 1 was an Earth Orbital mission without a lunar module, and one of the only missions designed to fly a Block 1 spacecraft. The flammability survey that fixed the Block 2 issues did not fix the Block 1 issues. NASA didn't change the cabin environment after the Apollo 1 fire, but it did change the environment launch. The real problem with the Apollo 1 fire was that the oxygen was under pressure. To mimic the pressure differential of a spacecraft going to the moon at 5 psi against the vacuum space, the spacecraft on the launch pad had to be pressurized to about 16.8 psi. It was that amount of oxygen under that high pressure that was really the problem. So NASA decided on a dual gas environment on the launch pad when it was at high pressure. Then that was slowly bled out during the launch and replaced with pure oxygen. That way, when the crew took off their helmets, they were breathing pure oxygen from a separate system at the time of launch. They were able to move seamlessly into the cabin environment. I hope that answers your questions about why NASA knowingly sent Apollo to the moon with a pure oxygen environment. In space at a low psi, pretty safe. On the launch pad under high pressure, not so safe. If you have other things you'd like to know about the Apollo era, any questions about flight, space, all of those things in that time, leave me all of your questions and comments in the comment section below. And if this is your first time visiting vintage space, welcome and please consider subscribing so you never miss an episode. 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