Juno I

History

The Juno I was developed as part of the Explorer Project, with the original goal of placing an artificial satellite into orbit. After the Soviet Union's launch of Sputnik 1 on October 4, 1957, and the failure of the Vanguard 1 launch attempt, the program received funding to match Soviet space achievements.

The first launch occurred in early 1958, with the successful launch of Explorer 1 on February 1, 1958, at 03:47:56 GMT, after Sputnik 1. Explorer 1 confirmed the existence of the Van Allen radiation belt. Five more Juno I launch attempts followed, with two successes and three failures; the final attempt on October 23, 1958, ended in failure.

Launch vehicle

The Juno I launch vehicle used a Jupiter-C first stage, which itself was derived from the Redstone missile, and added three solid-fuel upper stages built from Sergeant missile components to generate the extra thrust needed to reach orbit. Its fourth stage sat atop the third stage’s tub and ignited after the third stage burned out, accelerating the payload and fourth stage to 8 km/s under 25 to 51 g. While still on the launch pad, the tub and fourth stage were spun up to create gyroscopic stability, eliminating the need for thrust vectoring, vernier thrusters, or a reaction control system.

After first-stage burnout, the booster’s guidance package, still attached to the tub, separated to control attitude until the second stage fired. Wernher von Braun designed this multistage arrangement in 1956 for Project Orbiter, and it avoided the complexity of an upper-stage guidance system, though it meant the payload could not reach a highly accurate orbit. The four-stage Juno I and the three-stage Jupiter-C were both 21.2 meters tall, with the Juno I’s extra fourth stage housed inside the third stage’s nose cone.

Launch history

The Army Ballistic Missile Agency launched the Juno I six times during 1958, each flight aimed at putting a satellite into low Earth orbit. The rocket's first mission, on February 1, 1958, successfully placed Explorer 1, America's first satellite, into space. After that triumph, the Juno I flew five more times before being phased out for the Juno II. Only two of those later launches—Explorer 3 and Explorer 4—succeeded, meaning the vehicle achieved a 50 percent overall success rate.

The Juno II, which used a PGM-19 Jupiter first stage instead of a Redstone, replaced it in 1959. The serial numbers painted on the rocket's side came from a simple code based on the name of the design and test base, Huntsville, Alabama. With duplicate letters removed, the word HUNTSVILE gave H for 1, U for 2, N for 3, T for 4, S for 5, V for 6, I for 7, L for 8, E for 9, and X for 0. The "UE" on the Juno I that launched Explorer 1 thus stood for serial number 29.

The American public felt relief and satisfaction that the country had finally orbited a satellite, following earlier failures with the Vanguard and Viking programs.