YUCCA - Balloon Nuclear Detonation
Yucca was the code name of the first and unique high altitude test of a nuclear device ever carried by a stratospheric balloon.
The main goal of the US Department of Defense and Los Alamos Scientific Laboratory was to study the effects of the detonation of a "W-25" air defense warhead at high altitude as part of the developmental work of an Anti Ballistic Missile. This test, along with two other exoatmospherics detonations performed using rocket-borne warheads of megaton scale (code named "Teak" and "Orange") were included under Operation Newsreel, as part of Operation Hardtack I.
As occured in most nuclear test, it was neccesary to setup a series of monitoring stations and instrumented vehicles to retrieve the data at the moment of the explosion. The measurements of the Electromagnetic Pulse generated by the blast was performed by the Army Signal Research and Development Laboratory from two stations located at Wotho and Kusale, approximately 100 miles and 460 miles from Bikini respectively.
Two RB-36 USAF bombers were modified to act as observing platforms, including the modification of the fuselage for the placement of cameras, spectrographic equipment, and other optical instruments, as well the addition of special navigation aids. Also, to obtain basic data in the infrared region a P2V patrol plane of the US NAVY was instrumented with infrared detectors and recorders.
Sandia Corporation was in charge of the development of the weapon, while a different project was started for the balloon development, under the Air Force Cambridge Research Center. This included development work on the balloon that carried the YUCCA device and work on launching and tracking techniques.
Balloon flight-testing for YUCCA began in 1956 at various locations in the United States like Vernalis Naval Air Station in California, Holloman AFB in New Mexico y Goodfellow AFB in Texas, for testing different componentes and to train launch crews. Next year, attempts were made by the Air Force Cambridge Research Center (AFCRC) to launch large balloons from the airfield at Enewetak Island, but due to the high surface winds there, these launches were generally not satisfactory.
As a result another scheme was devised: to launch the balloon from the deck of an aircraft carrier, which could move at the same speed and in the same direction as the surface wind to create a "zero-wind" condition. The practicality of this scheme was demonstrated with five successful launches carried out from the USS Boxer, the ship designated for the task, off San Diego in September 1957.
Eleven more launches were made from Boxer in the waters near Eniwetok from 9 March to 18 April 1958 just before HARDTACK I beginning. The purpose of these flights was to familiarize the Air Force launch crew and USS Boxer crew with launch procedures. These flights were also used to check instrumentation and telemetry systems.
In all, a total of 76 balloons were launched in preparation for the event.
On the morning of April 28, 1958, the USS Boxer departed from Bikini to the launch point. After the bomb was assembled and armed, the balloon was released at 11:25 hours. During ascent the nuclear device was separated from the balloon a distance of 568 feet by a hydraulic load-lowering device, and the measurement instrumentation was additionally deployed along a nylon line at specific intervals totaling 3,000 feet below it.
At 12:50 the balloon reached floating altitude of 85.500 ft and finally at 14:40 the nuclear device was detonated by radio command in a point located 85 miles NE of Eniwetak. The device produced a yield of 1.7 kilotons, a small amount for the standards of the times.
The fact that an Electro Magnetic Pulse (EMP) is produced by a nuclear explosion was known since the very first days of nuclear weapons testing, but the magnitude of the EMP and the significance of its effects were not realized for some time. According to the data published in the final report of HARDTACK which was declassified several years later, YUCCA provided the first indication in United States testing that high-altitude EMP could be more than 1000 times as intense as low-altitude EMP.
The data obtained was so different from the expected results that it was apparently dismissed as an anomaly, and was generally ignored until after the 1962 tests of Operation Fishbowl had proven the earlier theory of high-altitude EMP to have been dramatically incorrect.
EMP History - an excelent article on YUCCA EMP effects published in Futurescience, LLC website