feasibility studies and proceed with Faget's design, focused on the lunar landing mission. The contract to build Apollo was awarded to North American Aviation. The Apollo Command/Service Module (CSM) was originally designed to take three men directly to the surface of the Moon, atop a large landing stage with legs. The Command Module sized out at in diameter, by long. The Service Module was long, with a total vehicle length of including the engine bell. The hypergolic propellant service propulsion engine was sized at to lift the CSM off the lunar surface and send it back to Earth. This required a single-launch vehicle much larger than the Saturn V, or else multiple Saturn V launches to assemble it in Earth orbit before sending it to the Moon. Early on, it was decided to use the lunar orbit rendezvous method, using a smaller Lunar Excursion Module (LEM) to ferry two of the men between lunar orbit and the surface. The reduction in mass allowed the lunar mission to be launched with a single Saturn V. Since significant development work had started on the design, it was decided to continue with the existing design as Block I, while a Block II version capable of rendezvous with the LEM would be developed in parallel. Besides addition of a docking tunnel and probe, Block II would employ equipment improvements based on lessons learned from the Block I design. Block I would be used for uncrewed test flights and a limited number of Earth orbit crewed flights. Though the service propulsion engine was now bigger than required, its design was not changed since significant development was already in progress; however, the propellant tanks were downsized slightly to reflect the modified fuel requirement. Based on astronaut preference, the Block II CM would replace the two-piece plug door hatch cover, chosen to avoid an accidental hatch opening such as had happened on Gus Grissom's Mercury-Redstone 4 flight, with a one-piece, outward-opening hatch to make egress easier at the end of the mission. The Mercury-Gemini practice of using a prelaunch atmosphere of pure oxygen proved to be disastrous in combination with the plug-door hatch design. While participating in a pre-launch test on the pad on January 27, 1967, in preparation for the first crewed launch in February, the entire crew of Apollo 1—Grissom, Edward H. White, and Roger Chaffee—were killed in a fire that swept through the cabin.