Apollo 4, (also known as Apollo-Saturn 501 and AS-501), was the first test flight for the Saturn V, the launch vehicle which was ultimately used by the U.S. Apollo program to send the first astronauts to the Moon. Apollo 4 flew without a crew, and was an "all-up test", meaning all rocket stages and spacecraft would be 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.