Complex sentence: Boilerplate spacecraft are most commonly used to test manned spacecraft; for example, in the early 1960s, NASA performed many tests using boilerplate Apollo spacecraft atop Saturn I rockets, and Mercury spacecraft atop Atlas rockets (for example Big Joe 1).
Split into simpler sentences: Boilerplate spacecraft are most commonly used to test manned spacecraft; for example, in the early 1960s, NASA performed many tests of boilerplates. Such boilerplates were made for Apollo spacecraft atop Saturn I rockets, and Mercury spacecraft atop Atlas rockets (for example Big Joe 1).