<p>Fourth stage and payload bay of an engineering back-up for the Jupiter-C rocket designated "UE".  On display in the Smithsonian Air and Space Museum in Washington, D.C.
</p><p>The Jupiter rockets were linear descendants of the V-2 -- a ballistic missile developed by Nazi Germany during World War II.  In 1954, two programs were under way to develop an American intermediate range intercontinental ballistic missile (IRBM). One was Project Vanguard, run by the U.S. Navy.  The other was Project Orbiter, run by the Navy and the U.S. Army.  By 1955, it was clear that Project Vanguard had won -- but the Jupiter rockets served a useful purpose for a time, and have a unique role in U.S. history.
</p><p>The Redstone rocket (first launched in 1953) was derived from the V-2, and was the first short-range, surface-to-surface ballistic missile developed by the United States.  Although other missiles had been used (the Corporal, the Sergeant), this was a BALLISTIC missile because it actually left the atmosphere before coming back down to hit its target.
</p><p>The Jupiter-A was a rocket derived from the Redstone, and used only as a test vehicle for components that would later become the Jupiter medium-range ballistic missile (MRBM).  Built in 1954 and deployed in 1956, the Jupiter MRBMs (by now woefully obsolete) would be pulled from service in 1961 as part of the agreement pulling nuclear missiles out of Cuba following the Cuban Missile Crisis.
</p><p>The Jupiter-C was a sounding rocket.  Sounding rockets were not intended to reach orbit.  Rather, they were designed to reach high into the atmosphere (between 30 and 950 miles) to gather data and test instruments.  Jupiter-Cs were launched from Cape Canaveral in Florida.
</p><p>Like the Jupiter-A, the Jupiter-C was a test rocket.  Its purpose was to test the re-entry capabilities of the nuclear bombs it carried.  These bombs were wrapped in ablative material, and testing the ablative material was the reason for the Jupiter-C.
</p><p>The Jupiter-C's first stage was a Redstone that was lengthened by eight feet accommodate more fuel.  The motor was a Rocketdyne A-7 engine that burned liquid oxygen and "Hydyne" (a rocket fuel developed by Rocketdyne that used the highly toxic and unstable dimethylhydrazine and diethylenetriamine).
</p><p>The second and third stages looked like a single stage, but they were not.  The second stage was a ring of 11 rocket engines taken from the Sergeant short-range solid-fuel missile.  These were scaled down by the Jet Propulsion Laboratory.  They surrounded the fuel tank, and were held in place by steel rings.  A webbed bulkhead fore and aft isolated the stage from the other stages.  A cylinder of aluminum protected the engines.
</p><p>The third stage was exactly like the second stage, but consisted of just three scaled-down Sergeant rocket engines.
</p><p>The fourth stage was a single scaled-down Sergeant rocket.
</p><p>The Jupiter-C had a variety of guidance systems.  The first stage was topped by an instrument package that contained  gyroscopic flight controls.  Maneuverable fins and vanes in the rocket exhaust controlled the flight. The rocket was programmed to travel at a 40-degree angle when the first stage burned out at 157 seconds into the flight.  The rocket coasted upward for another 100 seconds.  At this point, electric motors began spinng the axle-mounted second/third/fourth stages.  The rate of spin varied from 450 to 750 rpm, and was controlled from the ground (so that the motors would compensate only for the second/third/fourth stage attitude and not the first stage -- which was still firing).  This stabilized them, so that a guidance system was not needed.
</p><p>The instrument package and second/third/fourth stages separated from the first stage using explosive bolts and springs.  A radio signal from the ground ignited the second stage, whose exhaust blew away the instrument package.
</p><p>The third and fourth stages also used electric motors to spin up.
</p><p>It was clear right away that the Jupiter-C was so powerful, it could launch a satellite into orbit.  But it needed a fourth stage to do so.
</p><p>Werner von Braun, the German scientist who ran the American space program, worried that the Jupiter-C looked too military and would be perceived as an offensive weapon.  So when the fourth stage was added, von Braun re-designated the rocket as a "Juno I."  (NASA considers the Jupiter-C and the Juno I the same rocket, although Wikipedia and other sources like to think they are not.)
</p><p>There were just three launches of the Jupiter-C in the three-stage configuration, but six launches of it in the four-stage configuration.  
</p><p>It was in the four-stage configuration that the Jupiter-C made world history.  On January 31, 1958, the Jupiter-C put Explorer I -- America's first satellite -- into orbit.  Explorer I discovered the Van Allen radiation belts.  On March 26, 1958, a Jupiter-C put Explorer III into orbit, and on July 26, 1958, a Jupiter-C put Explorer IV into orbit.
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Remember, the Jupiter program was a military one.  Rockets needed to be numbered, but the military did not want the Russians seeing how many rockets had been built or flown.  The base where the Jupiter-Cs were built was Huntsville, Alabama.  A simple cypher was used to name the rockets:  H=1, U=2, N=3, T=4, etc.  The Jupiter-C that launched Explorer 1 had "UE" painted on the side, indicating it was rocket number 29 (U=2, E=9).
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/23165290@N00">Tim Evanson</a> from Cleveland Heights, Ohio, USA
CC BY-SA 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/timevanson/7259408162/">Payload of the Jupiter-C - Smithsonian Air and Space Museum - 2012-05-15</a>