Each vehicle consisted of a modified Redstone ballistic missile with two solid-propellant upper stages. The tanks of the Redstone were lengthened by 8 ft to provide additional propellant. The instrument compartment was also smaller and lighter than the Redstone's. The second and third stages were clustered in a "tub" atop the vehicle. The second stage was an outer ring of eleven scaled-down Sergeant rocket engines; the third stage was a cluster of three scaled-down Sergeant rockets grouped within. These were held in position by bulkheads and rings and surrounded by a cylindrical outer shell. The webbed base plate of the shell rested on a ball bearing shaft mounted on the first-stage instrument section. Two electric motors spun in the tub at a rate varying from 450 to 750 rpm to compensate for thrust imbalance when the clustered motors fired. The rate of spin was varied by a programmer so that it did not couple with the changing resonance frequency of the first stage during flight. The upper-stage tub was visibly spun-up before launch. During first-stage flight, the vehicle was guided by a gyro-controlled autopilot controlling both air-vanes and jet vanes on the first stage by means of servos. Following a vertical launch from a simple steel table, the vehicle was programmed so that it was traveling at an angle of 40 degrees from the horizontal at burnout of the first stage, which occurred 157 seconds after launch. At first-stage burnout, explosive bolts fired and springs separated the instrument section from the first-stage tankage. The instrument section and the spinning tub were slowly tipped to a horizontal position by means of four air jets located at the base of the instrument section. When the apex of the vertical flight occurred after a coasting flight of about 247 seconds, a radio signal from the ground ignited the eleven-rocket cluster of the second stage, separating the tub from the instrument section. The third stage then fired to raise the apogee. Through this system, designed by Wernher von Braun in 1956 for his proposed Project Orbiter, the Jupiter-C obviated the need for a guidance system in the upper stages.
The Juno I was a satellite launch vehicle based on the Jupiter-C, but with the addition of a fourth stage, atop the "tub" of the third stage. The Juno name derived from Von Braun wishing to make the satellite launch appear as peaceable as the Vanguard rocket, which was not a weapon, but was developed from a weather study rocket, the Viking. Since the Juno I was the same height as the Jupiter-C, with the added fourth stage being hidden inside the shell, this vehicle which successfully launched the first orbital satellite of the United States is often incorrectly referred to as a Jupiter-C.
The Jupiter-C was part of the IRBM project, and the sequence of manufacture of the rockets was considered a military secret. So the designation painted on the sides of the rocket was not a serial number in clear text, but employed a simple transformation cypher that the staff would be sure not to forget. The key was taken from the name of the design and test base: Huntsville, Alabama, giving HUNTSVILE, with duplicated letters dropped: H was used for 1, U for 2,..., E for 9 and X for 0. For example, the Jupiter-C modified to launch Explorer 1 had "UE" painted on the side, indicating it was S/N 29.
General characteristics
Weight as configured for Explorer 1 launch, loaded/empty
