The Saturn I was the United States' first heavy-lift dedicated space launcher, a rocket designed specifically to launch large payloads into low Earth orbit. Most of the rocket's power came from a clustered lower stage consisting of tanks taken from older rocket designs and strapped together to make a single large booster, leading critics to jokingly refer to it as "Cluster's Last Stand". However, its design proved sound and very flexible. Its major successes were launching the Pegasus satellites and flight verification of the Apollo Command and Service Module aerodynamics in the launch phase. Originally intended as a near-universal military booster during the 1960s, it served only for a brief period and only with NASA; ten Saturn Is were flown before it was replaced by the derivative Saturn IB, which featured a more powerful upper stage and improved instrumentation.