At about 6:31 p.m. EST on 27 January 1967, a spark of unknown origin — almost certainly in the wiring harness — ignited a fire inside the Apollo Command Module during a pre-launch test at Pad 34, Cape Kennedy. Astronauts Gus Grissom, Ed White, and Roger Chaffee died of smoke inhalation; the cabin reached about 1,000°C within two minutes — the mission (originally AS-204) was renamed Apollo 1 in their honor in April 1967.
The cabin was pressurized with pure oxygen at 16.7 psi for the test, the standard procedure for Block I Command Module ground tests. Pure oxygen at that pressure makes almost any organic material readily ignitable. A spark of unknown origin, almost certainly in the wiring harness, ignited a fire at 6:30:54 p.m. The fire spread in seconds; the cabin reached about 1,000°C within two minutes. The hatch opened inward and required the cabin pressure to be vented, taking 60–90 seconds; the crew had no chance.
The NASA Apollo 204 Review Board, chaired by Floyd L. Thompson, identified the contributing factors: the pure-oxygen atmosphere, the hatch design, flammable cabin materials (nylon netting, polyurethane foam, Velcro), and inadequately protected wiring. The redesigned Block II Command Module used a 60/40 oxygen–nitrogen atmosphere on the pad, an outward-opening hatch operable in 5 seconds, fire-resistant materials, and a fail-safe electrical system. The redesign cost 20 months and several hundred million dollars; it also produced a spacecraft that did not fail in 12 subsequent crewed missions.
For the rest of the Apollo article and the space race overview.