Misty Castle: High-Explosive Nuclear Effects Simulations at White Sands Missile Range

Misty Castle III: Minor Scale (1985)

Date: 27 June 1985
Location: Permanent High-Explosive Test Site (PHETS), White Sands Missile Range
Explosive Charge: 4,744 tons, ANFO

Objectives

Primary Objective: Provide an airblast and ground shock environment for DoD sponsored experiments designed to determine the response of tactical and strategic weapon systems, communications equipment, vehicles, and a variety of structures to this environment.

Secondary Objective I: Provide a simulated precursor environment for several other experiments.
Secondary Objective II: Provide a thermal environment (in addition to airblast) for several experiments.

By the Numbers

Event Summary

Planning for the Minor Scale test event, the third test in the Misty Castle series, began in December 1983, when the Defense Nuclear Agency (DNA) began sending invitations for experiment proposals to research and testing organizations both in the United States and foreign partner countries. This test would be the largest high-explosives nuclear simulation test to date, with a planned amount of 4,880 tons of ANFO poured into a dome-shaped container on the Permanent High Explosives Test Site (PHETS) at White Sands Missile Range. A technical reviewing committee was assembled by the DNA to screen and select submitted experiments. This phase was complete by March 1984.

Minor Scale included over 50 experiment groups, about a quarter of the total number of experiments, that were classified or included classified materials. Due to operational security requirements, there were fewer people outside of the participating agencies who knew about the test. Like similar tests before it, the DNA also brought on board foreign agencies from Canada, West Germany, France, Norway, Sweden, and the United Kingdom. The following is a summarized list of the majority of the tests, which measured:

• Blast and thermal effects on above surface and partially buried shelters, blast shelters, and industrial buildings;
• Blast and thermal effects on various radomes and antennas;
• Blast and thermal effects on anthropomorphic dummies;
• Blast and thermal effects on military communication and protective equipment;
• Blast and thermal effects on various armored vehicles
• Blast effects on the propagation of electromagnetic signals, such as radar and communications traffic, in airborne systems;
• Phenomenology diagnostics and;
• Remote pressure sensing in the air and on the ground.

Like the Direct Course event, project officers decided to include thermal radiation sources (TRS) as a part of the test to simulate a combined airblast and thermal environment. The TRS units consisted of a linear array of four nozzles, each of which produces a flame two meters wide and six meters high. The TRS units use a combination of liquid oxygen and aluminum powder to create a heat source that produces approximately 50 megawatts of radiant heat. In many photos and videos of Minor Scale, the TRS units can be seen as several flashes of light and columns of smoke that appear a few seconds before the actual explosion.

In order to hold the planned 4,880 tons of ANFO, project directors and engineers designed a fiberglass hemisphere with a radius of 44 feet. Personnel then lined the dome’s inside floor with a layer of polyethylene sheet to prevent the ANFO from absorbing moisture from the ground. Construction of the dome began in March 1985 and was completed in early April.

Beginning on 17 June 1985, the construction team began to fill the dome with ANFO. Due to the sheer volume of ANFO required, the project team had to build a mixing plant, which added diesel oil to the ammonium nitrate, near the site. Fuel oil delivered to the mixing plant in trucks was discharged into an auger carrying the ammonium nitrate from the hopper to elevators. The mixed product was then poured onto trucks and transported to the dome at Ground Zero. Even with a dedicated mixing plant, the team could only work with 100 tons of ammonium nitrate and 100 tons of diesel fuel oil at a time.

On 21 June, four days after the start of filling the dome, inspectors discovered small cracks, less than 1/8 inches, running up the side of the fiberglass from the ground level up to 10 feet in height. Two days later, technicians working around the dome heard a loud crack as a 2-inch breach opened up on one of the sides. Later that night, the fissure opened again, this time to a width of 6 inches. Soon, another crack on the other side of the dome opened up. Loose ANFO began pouring out of both openings, with the first crack spilling approximately 120 tons of ANFO while the second crack spilled approximately 100 tons.

With the test scheduled only three days away and the spillage mostly contained and stabilized, program managers decided to leave the 220 tons of spilled ANFO in place and stop the filling process immediately. Minor Scale had been planned for a charge of 4,880 tons of ANFO. However, by the date of the test, only 4,744 tons, or around 97.2%, of ANFO was in place.

The date of execution for Minor Scale was originally planned for 25 June 1985 at 1000 hours. However, high winds on the range meant that one of the tests, which used helium bags to simulate precursor effects, would not be able to run. To give time for the wind to die down, the Test Group Director placed the countdown on hold and rescheduled the event for 1000 hours on 27 June. On event day, several hold criteria necessitated the delay of zero time until noon. The actual detonation took place at 1220.

The post-event reentry plan called for four phases. The top priority was the safety sweep of the remains of ground zero. Personnel conducting the sweep were primarily looking for unexploded ordnance, unactivated thermal radiation source units, high pressure helium bags, and residual radiation sources. Once the site had been declared safe, the next priority was dismantling classified experiments and retrieving data collectors and film. Once classified operations were complete, the testbed was declared open for all other experimenters and support personnel.

As intended, the testbed around ground zero was largely destroyed. The explosion created a crater that was 345 feet in diameter and 80 feet in depth. The depth was about twice as deep as predicted by theoretical simulations. The 4,744 tons of ANFO generated an explosion equivalent to 4 kilotons of TNT. The airblast was calculated to be the equivalent of an 8-kiloton nuclear weapon. Measurements from the Air Force Weapons Laboratory estimated the speed of the blast as between 16,830-17,300 feet per second (5,130-5,270 meters per second).

Post-test results were largely positive, although a few experiments experienced equipment failures in one way or another. Initially, there were no overtly noticeable signs that the two leaks in the container affected the blast effects. However, high-speed cameras in the west radial showed blast anomalies caused by the ANFO spillage. The results of the unclassified projects were presented at the Minor Scale Symposium held from 24-28 February 1986.

Although the explosion at a warehouse storing ammonium nitrate in the Port of Beirut, Lebanon in 2020 has been referred to as the largest man-made, non-nuclear explosion in history in professional research publications, Minor Scale resulted in an approximate yield (roughly 4 kilotons) four to eight times larger than the explosion in Lebanon (best estimates and the reasonable upper limits are 0.5 and 1.12 kilotons of TNT, respectively). Prior to Minor Scale, the largest known man-made, non-nuclear explosion was the 1917 Halifax Explosion in Nova Scotia, Canada, estimated to have the yield of approximately 2.9 kilotons of TNT.