While Explorer 1 and the Explorer-series satellites which followed returned a wealth of new data, they were limited by the tiny 11 kilogram payload capability of the Juno I launch vehicle (see “Explorer 1: America’s First Satellite”). In order to orbit heavier payloads carrying a larger range of instrumentation, von Braun’s team at the Army Ballistic Laboratory (ABMA) in Alabama, which built the Juno I, developed the Juno II. This more powerful launch vehicle would be used to orbit the next generation of more capable Explorer satellites carrying a wider range of instruments to investigate the new frontier of space
The Launch Vehicle
While the Juno I upper stage cluster of solid rocket motors was retained in the new launch vehicle, the Juno II used a modified Jupiter IRBM instead of the much smaller Redstone as a first stage. As the first stage of the Juno II launch vehicle, Jupiter’s kerosene and LOX tanks retained their original 2.67 meter diameter but were lengthened by a total of 0.92 meters making the rocket 16.84 meters long. This prolonged the burn time of the 668 kilonewton thrust Rocketdyne S3D engine by 20 seconds to a total of 182 seconds.
Diagram of the Juno II launch vehicle. Click on image to enlarge. (NASA)
Mounted on top of the modified Jupiter first stage under an aerodynamic shroud was the “high speed assembly” consisting of an instrument compartment and a three-stage solid rocket cluster developed by JPL similar to that used on ABMA’s Juno I rocket. This cluster of rocket motors was a spin-stabilized tub of 11 scaled-down JPL Sergeant rockets with seven used for the second stage, three for the third and a single motor for the fourth stage. The spinning was required to provide stability to the assembly as well as even out the effects of performance variations in each stages comparatively primitive solid rocket motors. Modifications of this cluster from the version used on the Juno I and earlier Jupiter-C used for high speed reentry tests (the latter of which did not include the single motor fourth stage – see “America’s First Satellite… Almost”) included filling the third and fourth stage motors with a higher performance propellant and changing the original stainless steel casing of the fourth stage to a lighter weight titanium casing.
Diagram illustrating the upper stages of the Juno II launch vehicle with a second-generation Explorer satellite. Click on image to enlarge. (NASA)
After the Jupiter first stage burned out during a typical launch, the spinning high speed assembly would jettison its aerodynamic shroud and use a cold gas jet attitude control system to maintain orientation as it continued to coast towards apogee. At the prescribed time, the three stages would fire in quick succession to accelerate its payload to orbital velocity. Unlike with the earlier Explorer satellites launched by the Juno I, the fourth stage would separate from its payload instead of remaining attached. This allowed the satellite to spin stably in a fixed orientation as it orbited the Earth as well as aid in thermal control. The Juno II was designed to place 43 kilograms of payload in a 480-kilometer high Earth orbit or up to 7 kilograms of useful payload on a direct ascent escape trajectory. This new combination was first used to launch the Pioneer 3 and 4 lunar probes in December 1958 and March 1959, respectively (see “Vintage Micro: The Pioneer 4 Lunar Probe”).
The successful launch of Pioneer 4 towards the Moon using Juno II Round AM-14 in the early morning hours of March 3, 1959. (MSFC/NASA)
The S-1 Satellite Payload
The first of the new series of larger Explorer satellites was the 39.7-kilogram satellite (a “microsat” by today’s definition) designated S-1 by NASA which had assumed control of the Explorer program not long after the agency was established in October 1958. Built by JPL, the spin stabilized S-1 consisted of a pair of fiberglass cones joined at their bases. The interior included a central instrument column and a self in the middle for additional equipment with the weight distributed so that the satellite would spin stably in orbit. The S-1, with a diameter and height of 76 centimeters each, had an extensive scientific payload of miniaturized instruments to study the Earth, Sun and space environment. The instruments included:
Diagram of NASA S-1 payload. Click on image to enlarge. (NASA)
A pair of Geiger-Mueller counters to detect cosmic rays and charged particles trapped in the newly discovered Van Allen radiation belts. These were similar to those flown on earlier Explorer and Pioneer spacecraft allowing easy comparison of the results.
A pair of gas ionization chambers to measure the intensity of ultraviolet Lyman-α and X-ray emissions striking the atmosphere of the Earth.
A set of detectors to monitor the amount of light incident on the Earth, the amount of light reflected by the Earth as well as the amount of heat being radiated by the Earth in the infrared. This experiment would provide data on the heat balance of the Earth – a vital piece of data to understand weather.
An argon-filled ionization chamber to monitor the flux of heavy nuclei content of primary cosmic rays.
An optically-based micrometeorite penetration sensor to measure the flux of these high speed particles.
And finally, engineering measurements including temperature readings to assess the satellite’s thermal control system.
This wide ranging payload and its support systems were powered by a bank of nickel-cadmium and mercury batteries providing about 2.7 watts of electrical power. These batteries were recharged by 3,000 solar cells mounted on the satellite’s exterior. A set of these solar cells were instruments to provide data on their degradation in the space environment. This advanced payload was equipped with a timer to turn itself off after about a year in orbit.
The Launch & Afterwards
Juno II Round AM-16 was designated to launch NASA’s S-1 payload. Its high speed assembly, Cluster 14, arrived at Cape Canaveral, Florida on May 20, 1959. Cluster 14 was unpacked, inspected and modified before it started pre-launch and spin testing with S-1. The S-1 payload was returned to ABMA for additional environmental testing on June 16. Following it return on July 6, field operations at the Cape resumed.
The cluster was moved to Launch Complex 5 on July 9 for mating with its modified Jupiter booster. Launch operations formally began during the evening of July 15 for a 700-minute countdown with an additional 60 minutes of planned holds. Issues encountered during the final preparations of the S-1 payload delayed its delivery to LC-5 by about 1½ hours but most of this time was recovered requiring only a 20 minute hold at the T-220 minute mark when LOX loading would start.
Juno II Round AM-16 undergoing prelaunch checkout at LC-5. (NASA)
As the countdown proceeded, additional problems were encountered. An unscheduled 50-minute hold started at the T-100 minute mark to replace a faulty pitch gyro in the Jupiter. At T-47 minutes, another 13-minute hold was required to clear the pad area before the start of RF (radio frequency) testing. At T-50 seconds, a 4 minute hold was required to clear up a problem with the telephone line between the blockhouse and the Range Safety Office. With the countdown recycled to the T-2 minute mark, everything proceeded as planned until liftoff at 12:37:03.18 EST on July 16, 1959.
Juno II Round AM-16 with its gantry rolled back as final preparations were made for launch on July 16, 1959. (NASA)
Unfortunately, failure struck as soon as the Juno II left the pad. A power inverter used to convert the 24-volt DC current from the launch vehicle’s battery into AC current to power vital control functions failed at ignition due to a short between a pair of diodes. With power cut to the control system, the rocket immediately veered sharply to the west with the Range Safety Officer transmitting the destruct command just 5½ seconds after liftoff (see the video below).
The flaming debris of AM-16 crashed about 75 meters to the northwest of the launch table at LC-5 and just 90 meters to the southwest of the blockhouse where launch personnel resided. The observers inside saw at least one of the rocket motors from Cluster 14 burning from both ends after the missile was destroyed. Transmissions from S-1 were received for 7 seconds before its crash. All of the ordinance and all but a couple of kilograms of solid rocket motor propellant was consumed in the ensuing fire. Components of the instrument compartment, high speed assembly and the S-1 payload were later found spread across an area 30 meter across. Despite the shock and fire damage, some of the components of S-1 were still functional prompting consideration of their reuse. All the debris from the high speed assembly and payload were sent to JPL for analysis.
The debris of the high speed assembly after the launch failure of Juno II Round AM-16. (NASA)
With the first attempt to launch a satellite using the Juno II a failure, an investigation was immediately started to determine its causes and take corrective action. As other payloads were prepared for launch, JPL assembled a backup satellite designated S-1a which was essentially identical to its predecessor (see “Vintage Micro: The Second-Generation Explorer Satellites”). This time, Juno II Round AM-19A successfully placed what was now called Explorer 7 into a 557 by 1,069 kilometer orbit inclined 50.3° on October 15, 1959. NASA’s newest Explorer returned much new information on the spatial and temporal structure of the inner edge of the Van Allen radiation belts that complimented earlier data as well as that taken concurrently by later satellites. Explorer 7 returned continuous real-time data through February 1961 and then intermittently until August 24 when the satellite finally fell silent. Data returned by this flight showed a definite correlation between solar activity levels and the intensity of radiation in the Van Allen belts bringing us one step closer to understanding the new space environment.
A view of S-1a before its launch on October 15, 1959 to become Explorer 7. (NASA)
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Related Videos
Here is an excellent 1959 CBS News documentary, entitled Biography of a Missile, about the preparation and launch of NASA’s S-1 payload and its Juno II Round AM-16 launch vehicle:
Related Reading
“Vintage Micro: The Second-Generation Explorer Satellites”, Drew Ex Machina, September 3, 2015 [Post]
“Vintage Micro: The Pioneer 4 Lunar Probe”, Drew Ex Machina, August 2, 2014 [Post]
“Explorer 1: America’s First Satellite”, Drew Ex Machina, January 31, 2018 [Post]
General References
Josef Boehm, Hans J. Fichtner, and Otto A. Hoberg, “Explorer Satellites Launched by Juno 1 and Juno 2 Space Carrier Vehicles”, in Aeronautical Engineering and Science, Ernst Stuhlinger, Frederick I. Ordway III, Jerry C. McCall, and George C. Bucher (editors), pp. 218-239, McGraw-Hill, 1963
Ray V. Hembree, Charles A. Lundquist, and Arthur W. Thompson, “Scientific Results from Juno-Launched Spacecraft”, in Aeronautical Engineering and Science, Ernst Stuhlinger, Frederick I. Ordway III, Jerry C. McCall, and George C. Bucher (editors), pp. 281-297, McGraw-Hill, 1963
C.F. Mohl, Juno Final Report Volume III: Juno II Earth Satellites, JPL Technical Report No. 32-31, June 28, 1962