Juno II

History

The Juno II rocket flew ten missions carrying satellites, with six ending in failure. Its payloads included Pioneer 3, Pioneer 4, Explorer 7, Explorer 8, and Explorer 11, all launched from Cape Canaveral’s Launch Complex 5 and Launch Complex 26B. The first flight, carrying Pioneer 3 on December 6, 1958, ended early when the first stage cut off prematurely. This kept the upper stages from gaining enough speed to reach the Moon.

Pioneer 3 remained trapped in Earth orbit, but it sent back data for about 40 hours before burning up in the atmosphere. Investigators traced the problem to a faulty propellant depletion circuit, though they could not pinpoint the exact defect. The circuit was later redesigned.

Pioneer 4 launched successfully on March 3, 1959. It became the only first-generation American lunar probe to meet all its mission objectives, and the only successful U.S. Moon probe until 1964. After this flight, NASA turned its lunar ambitions toward the larger Atlas-Able rocket and decided to use the Juno II for missions in Earth orbit. Removing the fourth stage nearly doubled how much weight the rocket could carry.

On July 16, 1959, the attempt to launch Explorer S1 went badly wrong. The Juno II lost control right after liftoff, tumbling into a cartwheel before the range safety officer triggered its destruction. The nearly full booster crashed just a few hundred feet from the launch pad, and crews in the blockhouse watched in shock as the upper stage motors burned on the ground.

The cause was quickly found: a short circuit between two diodes in a power inverter cut power to the guidance system at liftoff. This made the engine gimbal to its full stop, flipping the rocket onto its side before the destruct command came. To prevent this from happening again, engineers applied better coatings to the booster’s circuit boards.

Development

The Juno II's upper stages were solid-fueled rockets based on the MGM-29 Sergeant. The second stage used eleven of these motors, the third used three, and the fourth used one—the same arrangement found on the smaller Juno I. When the fourth stage was omitted for low Earth orbit missions, the vehicle could carry an extra nine kilograms of payload. Development moved quickly because the rocket was assembled entirely from existing parts. Work started in early 1958, and the first launch happened by the end of that year.

Chrysler managed the overall contract, Rocketdyne built the first stage engine, and the Jet Propulsion Laboratory handled the upper stage propulsion. The first three Juno IIs were actually converted Jupiter missiles, but all later boosters were built as Juno IIs from scratch. Compared to the Jupiter, the Juno II had stretched propellant tanks, giving the first stage about 20 seconds more burn time. Its structure was reinforced to handle the extra weight of the upper stages, and the inertial guidance system was swapped for a radio ground guidance package, which was moved up into the upper stages.