with pure oxygen, and also to seal the plug door hatch cover. During the launch, the pressure would have been gradually reduced to the in-flight level of , providing sufficient oxygen for the astronauts to breathe while reducing the fire risk. The Apollo1 crew had successfully tested this procedure with their spacecraft in the Operations and Checkout Building altitude (vacuum) chamber on October 18 and 19, 1966, and the backup crew of Schirra, Eisele and Cunningham had repeated it on December 30. The investigation board noted that, during these tests, the command module had been fully pressurized with pure oxygen four times, for a total of six hours and fifteen minutes, two and a half hours longer than it had been during the plugs-out test. Flammable materials in the cabin The review board cited "many types and classes of combustible material" close to ignition sources. The NASA crew systems department had installed of Velcro throughout the spacecraft, almost like carpeting. This Velcro was found to be flammable in a high-pressure 100% oxygen environment. Astronaut Buzz Aldrin states in his book Men From Earth that the flammable material had been removed per the crew's August 19 complaints and Joseph Shea's order, but was replaced before the August 26 delivery to Cape Kennedy. Hatch design The inner hatch cover used a plug door design, sealed by higher pressure inside the cabin than outside. The normal pressure level used for launch ( above ambient) created sufficient force to prevent removing the cover until the excess pressure was vented. Emergency procedure called for Grissom to open the cabin vent valve first, allowing White to remove the cover, but Grissom was prevented from doing this because the valve was located to the left, behind the initial wall of flames. Also, while the system could easily vent the normal pressure, its flow capacity was utterly incapable of handling the rapid increase to caused by the intense heat of the fire. North American had originally suggested the hatch open outward and use explosive bolts to blow the hatch in case of emergency, as had been done in Project Mercury. NASA did not agree, arguing the hatch could accidentally open, as it had on Grissom's Liberty Bell 7 flight, so the Manned Spacecraft Center designers rejected the explosive design in favor of a mechanically operated one for the Gemini and Apollo programs. Before the fire, the Apollo astronauts