the effects of microgravity and rapid acceleration. The astronauts spent time in simulators, aboard cargo planes that simulated weightlessness, underwater to practice extravehicular activities (EVAs), and on visits to manufacturing plants to check on the progress of the hardware. Project Gemini Every astronaut was required to have a specialty, and Chaffee's specialty was communications. He focused on the Deep Space Instrumentation Facility (DSIF), which the astronauts needed for navigation in space. At the Manned Spacecraft Center in Houston, Chaffee served as capsule communicator (CAPCOM) in March 1965 for Gemini 3. Later that year, he was CAPCOM, along with Virgil "Gus" Grissom and Eugene Cernan, for the Gemini 4 mission, in which Ed White performed the first spacewalk by an American. As CAPCOM, Chaffee relayed information between the crew members and the Director of Flight Operations, Chris Kraft. He never got a seat on a Gemini mission, but was assigned to work on flight control, communications, instrumentation, and attitude and translation control systems in the Apollo program. During this time, along with Grissom, he also flew chase planes at an altitude of between to take motion pictures of the launch of an uncrewed Saturn 1B rocket. Apollo program Chaffee received his first spaceflight assignment in January 1966, when he was selected for the first crewed Apollo-Saturn flight, AS-204. At the time, he was the youngest American astronaut to be selected for a mission. Joining Command Pilot Grissom and Senior Pilot White, he replaced the injured Donn F. Eisele in the third-ranked pilot position. Eisele required surgery for a dislocated shoulder, which he sustained aboard the KC-135 weightlessness training aircraft. He was reassigned to a second Apollo crew, commanded by Wally Schirra. The crew announcement was made public on March 21, 1966. The two-week flight of Apollo1 was to test the spacecraft systems and the control and ground tracking facilities. While Chaffee had monitored the manufacture of the Gemini spacecraft, he had not witnessed the building of the Apollo spacecraft. Three days after being selected for the Apollo1 crew, he flew to the North American Aviation Plant in Downey, California, to see it. Later in April, the crew traveled to Chapel Hill, North Carolina, to study stars that were programmed into their flight computer. In October, the six crewmembers planned to test the spacecraft in sea level and altitude conditions. The failure of an oxygen regulator prevented them from performing the vacuum