File:Stafford Air & Space Museum, Weatherford, OK, US (12).jpg

English: A pressurized nitrogen gas bottle for the Apollo Command Module's gas-driven side hatch counterbalance system. The side hatch of the Apollo command module, through which the astronauts entered and exited the spacecraft, was designed to open quickly in an emergency. (During the Apollo 1 ground test fire that killed the first Apollo flight crew, the inability to quickly open the hatch had been a key cause of their deaths, and the hatch was extensively redesigned in response.) Although the hatch weighed 350 pounds (160 kilograms), it was designed to swing open rapidly once the astronauts or ground crew released its latches. The hatch was pushed open by a mechanical counterbalance system driven by nitrogen gas pressurized to 5,000 pounds per square inch (34,000 kPa). Two gas bottles were provided for the system. One of these was opened to charge the system just before the astronauts boarded (which meant ground crew had to push against the system's pressure to close the hatch). The system could be recharged from the second bottle if needed for spacewalks or after landing. This system replaced the explosive bolts that had been used in Project Mercury; after the splashdown of Mercury-Redstone 4, these bolts had been triggered accidentally, blowing the Mercury capsule's hatch prematurely and causing it to sink before it could be recovered. Displayed at the Stafford Air & Space Museum, Weatherford, Oklahoma, U.S.