Blast provided mountains of data, and actual volcanoes

By Stephen Murray Special to the News on July 18, 2024.

@MedicineHatNews

Operation Snowball was the largest controlled detonation of conventional explosives at the time that 500-tons of TNT was set off, July 17, 1960, at Defence Research Station Suffield.

The huge force and fireball created by 30,000 stacked bricks of explosives took but a moment to subside, but the results of the test – devised and staged over two years by British, American and Canadian scientists – would be studied for years.

The test yielded valuable information about the response of military and civilian targets, as well as air blast and cratering phenomena.

Scientists also wanted to study how material, or ‘ejecta’ was thrown by the shockwaves and at the heart of the explosion itself. The shape of the charge – a hemisphere, or dome shape – was also devised to keep most of the explosive material compacted tightly to avoid outer portions being pushed away from the ignition point. Grey columns pictured extending out from the central black explosion represent ejecta, or ejected material.

Air blast results

One important result can be explained with the aid of the diagram below which shows a typical pressure-time profile at any point along a radial from the charge. The blast wave is characterized by a number of important parameters. The “time of arrival” (tA) is the elapsed time from initiation of charge detonation to the time the shock wave arrives at a given observation point. Upon arrival of the shock, the incident or “side-on” pressure increases almost instantaneously to a peak value (PSO) before decaying relatively slowly, eventually dropping back down to ambient pressure (P0). The time which elapses between shock arrival and the return of the pressure to ambient pressure is referred to as the positive phase duration (t0). The area under the pressure-time curve, up to that point, is known as the positive specific impulse (iS). The damage imparted to a particular target depends on both PSO and iS.

Following the passage of the blast wave, a region of sub-atmospheric pressure exists for a finite duration as the flow of air reverses direction. This is referred to as the “negative phase” of the event and is not generally as important from a blast damage perspective. One of the important findings of the trial was that the data for tA, t0, PSO and iS as a function of range were in agreement with the data from the earlier 5-ton, 20-ton, and 100-ton shots at Suffield when plotted using “Sachs scaling law.”

Craters reveal planetary phenomenon

The crater measured 240 feet in diameter in a shape that was expected except for a central uplift, a feature similar to many craters on the moon.

Long open cracks existed in the ground surface outside the crater rim.

At first, it was not appreciated that there were several sets cracks, some radial to the crater, and others circumferential.

There were certainly two, and perhaps up to four, circumferential cracks. These intersecting fissures were not immediately recognized because they were hidden by a light layer of ejecta. The radial cracking had not been observed on other major Suffield craters.

Several wetted areas were observed outside the crater immediately following the detonation However, the flow of water did not start until around 10 minutes later. Between 15 and 30 minutes later, an “explosive” eruption of water occurred, both at locations external to the crater and in the central uplift.

Canadian researcher Gareth Jones referred to the seven flooded areas as “pseudo-volcanic” flood provinces, and these were generally associated with one or another of the major circumferential and radial cracks.

Within these provinces, smooth sand cones created by the release of sand-laden water were present.

The resulting sand deposits were no more than inches thick, except in some of the main cones which could have been a foot high. A vent somewhat more than a foot in diameter was opened in the central uplift, and from that vent a gusher of sand-laden water rose to a height of some three or four feet. It continued for many hours. In fact, the flow continued to be quite rapid for some three days, and then gradually slowed to an effective end within a week.

These sand structures were dubbed pseudo-volcanic, both due to the appearance of individual cones and the similarity between the effects in Snowball and the distribution of small, presumably volcanic, cones associated with lunar craters. If the Snowball results were scaled up to a planetary impact crater 100 km in diameter, the corresponding size of the volcanoes would be the better part of a kilometre.

Complete analysis of the crater would take two years.

Canadian team behind the blast

John Dewey and Trevor Groves, two scientists recruited earlier to the Canadian Blast Program, left Suffield Experimental Station shortly after SNOWBALL to take up university teaching positions in Victoria and Calgary, respectively. Jones left about five years later to track icebergs in Labrador.

Another key member of the Canadian team, John Muirhead, spent the remainder of his career at Suffield, passing away only recently.

Dewey is still alive and publishing. The author owes him a debt of gratitude for the wealth of information he provided while the author was writing a book that is to be published in 2025.

(Stephen Murray is a retired defence research scientist specializing in air blast dynamics. He is based in Medicine Hat.)

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