Eisele was originally assigned to the prime crew of Apollo 1, a shakedown cruise of the Block I CSM. Apollo 2 was, at the time, scheduled as a second Block I CSM flight to carryout any further tests and checks NASA might have missed on Apollo 1. Apollo 3 would debut the Block II CSM, the advanced version that could dock with the lunar module on missions to the Moon. But in the course of training, Eisele twice dislocated his left shoulder in NASA’s hollowed out KC-135, the aircraft flown in parabolas to give astronauts brief periods of weightlessness. The long bone in his upper arm dislocated laterally, and on January 27, 1966, he entered the Methodist Hospital in Houston for surgery. He was expected to make a full recovery, but wouldn’t be fit to fly on Apollo 1. He switched places with Roger Chaffee, becoming the CMP for Apollo 2.