Apollo 10 Astronauts Thomas P. Stafford and Eugene A. Cernan in the Lunar Module Mission Simulator. (Credit: NASA) Fire, as we know, needs three things: a source of heat, fuel and oxygen. Apollo lunar missions had all three in spades. There was plenty of electricity running through the spacecraft, lots of material that could be fuel and a 100 percent oxygen atmosphere under pressure. So why exactly did NASA design a spacecraft that was an explosion waiting to happen? (This is a question I get *a lot* so I hope this gives a full answer!) [embed]https://www.youtube.com/watch?v=FvA7N_j_8os[/embed] Not long after President Kennedy famously challenged America to a manned lunar landing by the end of the 1960s, NASA started figuring out how it was going to complete this daring mission, and one of the first things it needed was a spacecraft. As it had done with the Mercury spacecraft, the space agency released a Call For Proposals to industry partners inviting them to bid on the contract to build that spacecraft. Of course, it wasn’t an open call. The RFP, which was sent out on July 28, 1961, included certain design constraints. The shape for Apollo, for example, had to be some kind of truncated cone like the Mercury spacecraft. As for the environment, NASA asked for an oxygen-nitrogen mixed gas atmosphere for the crew to mimic the air we breathe on Earth. Five bids for the Apollo spacecraft came back to NASA on October 11, 1961. General Dynamics/Astronautics teamed up with the Avco Corporation. General Electric Company, the Douglas Aircraft Company, the Grumman Aircraft Engineering Corporation and Space Technologies Laboratories Inc. submitted a joint bid. McDonnell Aircraft (the contractor behind the Mercury spacecraft) partnered with the Lockheed Aircraft Corporation, the Hughes Aircraft Company, and the Chance Vought Corporation for another joint bid. Only the Martin Company and North American Aviation submitted bids alone. After two months of deliberation, NASA revealed its scores. Martin was the winner with an overall score of 6.9 out of 10. The General Dynamics team and North American tied for second place, each with 6.6 points. The General Electric and McDonnell teams tied for third place with a score of 6.4. But NASA couldn't ignore NAA’s experience. The company had a sterling legacy for building exceptional aircraft such as the P-51 Mustang, the B-25 bomber, and the X-15. The eventually pushed NASA to favor North American. The winning bid was announced on November 28, 1961. A year of development later, there were still sticking points between NASA and NAA regarding Apollo’s design, one of which was the environment. NASA changed its mind on the dual-gas system because weight was increasingly becoming an issue. The spacecraft was getting heavy, too heavy to launch to the Moon with the Saturn V, and shaving pounds off the top of the stack was ideal: every pound removed at the top translates to tens of additional pounds of thrust at the moment of launch. And so NASA zeroed in on the crew cabin environment as a place to save weight. The tanks to hold both oxygen and nitrogen as well as the associated hardware and plumbing to deliver them into the crew cabin was be heavy. A single gas system significantly reduced the mass. But that wasn’t all. A dual gas system isn’t as simple as just pumping oxygen and nitrogen into the air. Balancing the gases demanded North American invent some way of measuring the mixture continually, adjusting the mix of gases constantly with every change. If that system failed, the crew might lose consciousness before realizing there was a problem. A pure oxygen system wouldn’t just be lighter, it would be far simpler; all the crew would need was a simple pressure sensor to ensure the cabin was adequately pressurized. This rationale was enough to have NASA change the Apollo crew cabin from a mixed gas to a pure oxygen environment. North American disagreed saying that the simplicity of a single gas system didn’t offset the danger it posed to the crew. NAA engineers knew that in a pure oxygen environment, a single spark could turn into a raging fire with explosive consequences, and they made their concerns known to NASA. The space agency countered that the risks of a fire in the crew cabin was minimal because it was a low-pressure environment. The spacecraft would be pressurized to just 5 pounds per square inch with pure oxygen. At such a low density, a fire wouldn't explode, it could be managed by a fast-acting crew. Not to mention the Mercury missions were flying with pure oxygen without any problems. Why complicate something that didn't need to be changed? The space agency ultimately had final say. The official switch from a dual to single gas environment came via a formal contract change notice signed by Robert Gilruth, Director of the Manned Spacecraft Center in Houston, on August 28, 1962. As a portent of the potential danger, a fire broke out during an unmanned test of the Apollo environmental control system on April 28, 1966. No one was injured, but spacecraft hardware was destroyed, and because the bulk of the hardware damage was attributed to the commercial grade strip heater inside the cabin that wouldn’t be present on any mission, the incident was dismissed on account of the non-flight material involved. What it did do was prompt NASA to have North American revisit the amount and placement of flammable materials in the cabin as well as survey the vehicles' layout to ensure no potentially combustible materials were in contact with or close proximity to any electrical systems. The space agency also had North American identify and eliminate potential fire hazards stemming from fluid leaks, overheating lamps, or large areas of exposed fabric and foam. But the subsequent changes were only made in the Block II Apollo command module, the one that could dock with the lunar module and so would fly on lunar missions. What this didn't do was affect the earlier Block I version, the one slated to take the Apollo 1 crew into orbit. It was deemed too late to make any changes to that Block I spacecraft since it was already well on its way to the launch pad. It was pressurized oxygen on the launch pad that was the problem. To mimic the pressure differential of 5psi inside the cabin against the vacuum of space, the spacecraft had to be pressurized with 16psi at sea level. That played a big part in the Apollo 1 fire. It was only after the Apollo 1 fire that NASA changed the cabin environment for launch; it was too late to change the cabin for the full mission. When the spacecraft was on the launch pad, it was an oxygen-nitrogen mix. Those gases were bled out and replaced with pure oxygen for the remained for the mission. There was even more evidence that pure oxygen under pressure was dangerous; I talked about some previous oxygen fires that *should* have been red flags in an old blog post for Scientific American.
Sources, beyond the linked article: “The Apollo Spacecraft, Volume IV January 21, 1966 — July 13, 1974.” Ivan D. Ertel and Roland W. Newkirk with Courtney G. Brooks; Mike Gray. Angle of Attack: Harrison Storms and the Race to the Moon; Chariots for Apollo, available online here. This is also part of a book section I wrote ages ago that never got published.