the astronauts' pressure suits was not changed. Because of the rapid drop in cabin (and suit) pressures during ascent, decompression sickness was likely unless the nitrogen had been purged from the astronauts' tissues before launch. They would still breathe pure oxygen, starting several hours before launch, until they removed their helmets on orbit. Avoiding the "bends" was considered worth the residual risk of an oxygen-accelerated fire within a suit. Nylon used in the Block I suits was replaced in the Block II suits with Beta cloth, a non-flammable, highly melt-resistant fabric woven from fiberglass and coated with Teflon. Block II had already been planned to use a completely redesigned hatch which opened outward, and could be opened in less than five seconds. Concerns of accidental opening were addressed by using a cartridge of pressurized nitrogen to drive the release mechanism in an emergency, instead of the explosive bolts used on Project Mercury. Flammable materials in the cabin were replaced with self-extinguishing versions. Plumbing and wiring were covered with protective insulation. Aluminum tubing was replaced with stainless steel tubing that used brazed joints when possible. Thorough protocols were implemented for documenting spacecraft construction and maintenance. New mission naming scheme The astronauts' widows asked that Apollo 1 be reserved for the flight their husbands never made, and on April 24, 1967, Mueller, as Associate Administrator for Manned Space Flight, announced this change officially: AS-204 would be recorded as Apollo1, "first manned Apollo Saturn flight – failed on ground test". Even though three uncrewed Apollo missions (AS-201, AS-202, and AS-203) had previously occurred, only AS-201 and AS-202 carried spacecraft. Therefore, the next mission, the first uncrewed Saturn V test flight (AS-501) would be designated Apollo4, with all subsequent flights numbered sequentially in the order flown. The first three flights would not be renumbered, and the names Apollo2 and Apollo3 would officially go unused. Mueller considered AS-201 and AS-202, the first and second flights of the Apollo Block I CSM, as Apollo2 and3 respectively. The crewed flight hiatus allowed work to catch up on the Saturn V and lunar module, which were encountering their own delays. Apollo4 flew in November 1967. Apollo1's (AS-204) Saturn IB rocket was taken down from Launch Complex 34, later reassembled at Launch complex 37B and used to launch Apollo5, an uncrewed Earth orbital test flight of the first lunar module, LM-1, in January 1968. A second uncrewed Saturn V