In 1967, during a routine countdown simulation on the Apollo 1 spacecraft, an electrical fire erupted inside the cabin. Under ideal conditions, the three-part hatch could be opened inwards within 60 to 90 seconds, but the fire spread quickly within the pure oxygen environment, and the atmospheric pressure difference was too great. The astronauts -- Gus Grissom, Edward H. White II and Roger B. Chaffee --were unable to exit in time.
Following the Apollo 1 tragedy, engineers were tasked with designing a new hatch that could be opened in three seconds and allow the crew to egress in under half a minute.
The new hatch design integrated the three layers into one, and equipped the perimeter of the door with 15 latches, actuated by five strokes of a ratcheting handle. It also included a plunger mechanism, a gas powered piston to push the hatch open and attenuate travel, a manually operated pressure dump valve, and a screw jack attachment for emergency closure.
This impressive feat of engineering was unprecedented. It is estimated around 150 new tools were designed and built just to work on it. One account refers to the unified hatch as "the most carefully engineered and manufactured door ever built."