Apollo 4 was an "all-up" test, meaning all rocket stages and spacecraft were fully functional on the initial flight, a first for NASA. It was the first time the S-IC first stage and S-II second stage flew. It also demonstrated the S-IVB third stage's first in-flight restart. The mission used a Block I Command Service Module (CSM) modified to test several key Block II revisions, including its heat shield at simulated lunar-return velocity and angle.