The major changes that occurred on SA-5 were that for the first time the Saturn I would fly with two stages - the S-I first stage and the S-IV second stage. The second stage featured six engines burning liquid hydrogen. Although this engine design (RL10) was meant to be tested several years earlier in the Centaur upper stage, in the end the first Centaur was launched only two months before SA-5. This rocket stage was delivered to the Cape by a modified B-377 aircraft, the "Aero Spacelines Pregnant Guppy".