Operation DISTANT PLAIN is the nickname given for the US participation in a series of six non-nuclear experiments conducted in Alberta, Canada during CY 1966-67 in connection with the Tripartite Technical Cooperation Program (TTCP). This test series was designed to provide experimental data utilizing explosives and detonable gas filled balloons for application to the solution of nuclear weapon problems based on specific military requirements.
In the late 1950s and throughout the 1960s, an extensive series of experiments was undertaken at Suffield, Canada, to study the physical properties of blast waves produced by spherical and hemispherical charges of TNT, ranging in mass from 25 kg (60 lb) to 500 tonne. An alternative explosive was sought because of the relatively high cost of using TNT. One suggested alternative was to use gaseous explosions involving stoichiometric mixtures of propane or methane with oxygen. The resulting series of experiments was known in Canada as FE567 or as Operation DISTANT PLAIN in USA.
DNA was supporting efforts to determine the merits of detonable gases. In 1965, project SLEDGE (Simulating Large Explosive Detonable Gas Experiment) was initiated. Detonable gases had the promise of attractive features: 1) if the gas mixture was lighter than air,, then a balloon filled with the gases would permit conducting large scale, i.e., multi ton, blast tests at high altitudes to help determine the response of in flight missiles and aircraft in a realistic environment; 2) the microscale homogeneity of the gas mixture should be better than that of TNT, thus leading to better detonation properties and, hence, better blast fields without serious anomalies as compared to TNT shots; and 3) the low density of the detonable gas mixture would match the density of the surrounding air better than TNT, and thus, reduce the Taylor instabilities which were advanced as another source of anomalies for TNT charges.
The fact that detonable gases would produce peak pressures considerably lower in magnitude than those generated by TNT was of minimum concern. Most military targets of interest were usually exposed at less than the 600 psi maximum expected from detonable gases. In fact, this low peak pressure had an advantage: for surface bursts, with the gas charge resting on the ground, cratering with its deleterious ejecta of crater material would be reduced or even eliminated.
Analyses, small scale experiments with oxygen methane and oxygen propane mixtures, and a large test of a surface burst with 20 tons of oxygen propane in a 125 ft diameter hemispherical balloon (operation DISTANT PLAIN, Event 2a, 1966), showed that, indeed, the promises could be realized. On none of the tests were anomalies observed. The 20 ton surface test produced a slight surface depression under the charge with no crater ejecta, and. a 1000 1b mixture of oxygen and methane could fly and be detonated.
However, serious operational problems faced the gas balloon endeavor. The very construction of large balloons, 100 ft in diameter and more, was itself a challenge, which was only partially met; on one test with a 110 ft diameter balloon, the balloon material failed, .aborting the experiment. Additionally, the cost of fabrication for a large balloon and filling it with the detonable gases was estimated to be rather high, perhaps $500,000 for a 380 ft diameter balloon necessary to contain 500 tons of detonable gases. Handling and filling the balloon were difficult. Long filling times were required far large quantities of gases. The balloon was susceptible to wind damage during the filling procedure even though an airfilled ballonet was used to achieve some structural rigidity of the balloon while the slow oxygen and methane or propane filling was taking place.
The vast serious problem was the one concerned with safety. Static charges could build up on the balloon material through wind action even though attempts were made to cover the balloon material with a conductive coating. The hazard of discharging static charges ,through arcing in an atmosphere of detonable gases is, of course, well known. On 22 October 1966 premature static discharge ignited a 110 ft diameter balloon filled with oxygen and about half of its quota of methane.
The first experiment of this series involved a 20 tn (US short ton = 907.19 kg) sphere of TNT, which was successfully detonated at the top of a 25m tower. The second experiment was to have involved a spherical balloon, 33.5m diameter, filled with a mixture of methane and oxygen and tethered with its center 25m above ground. While it was being filled there was a structural failure of the balloon and a large volume of oxygen was spilled resulting in a fire that seriously injured one person.
A large hemispherical explosion of a stoichiometric mixture of propane and oxygen, carried out at Suffield, Canada, in 1966, and known in Canada as FE567/2a and in USA as Operation DISTANT PLAIN Event 2a. Experiment 2a used a hemispherical balloon, 38m diameter, filled with a stoichiometric mixture of propane and oxygen with a nominal total mass of 20 tn. The filling process was extremely difficult due to the wind conditions, and as a result the centre of the balloon was displaced approximately 3m (10 ft) from its planned position. This charge was successfully detonated on 22 July 1966. The diameter of the hemispherical bag was 125 ft. The gas mixture ratio was 3.5 moles of oxygen per mole of propane. The internal overpressure was 0.44 in. of water. The ambient atmospheric pressure was 13.7 psi. The external ground temperature was 118 F and the air temperature at a height of 6 ft was 74 F.
The blast waves produced by the detonation of different types of explosives are usually compared with the blast wave produced by an equivalent mass of TNT. There are a number of reasons for doing this. The main one is that more information is available about the physical properties of the blast waves produced by TNT, as a free air, surface-burst or height-ofburst explosion, than for any other explosive. For overpressures greater than 1 atm there is a strong dependence on the distance from the centre of the explosion, but at lower overpressures the equivalence factors have almost constant values of 0.55 for the propane-oxygen mixture and 1.95 for the propane alone.
Several techniques were applied to measure the blast wave generated by the gaseous explosion, but some were marred by technical failures and all by the displacement by the wind of the center of the gas bag by about 3m. A number of piezo-electric transducers were deployed, flush mounted with the ground surface, to measure hydrostatic pressuretime histories. Two high-speed photogrammetric methods were used. The refractive image of the primary shock front was photographed against a black and white striped background, and the displacement by the blast of a series of smoke tracers was photographed to provide data for a particle trajectory analysis. In this case, in addition to the uncertainty about the position of the charge center, problems were encountered with the accuracy of the timing marks on the films. None of these measurements provided sufficient information to make a reliable calculation of the TNT equivalence over a wide range of distances.
As a result of the difficulties and hazards experienced in carrying out these experiments, no further large scale gaseous explosions were attempted and an alternative solid explosive, ammonium nitrate-fuel oil (ANFO), was used to replace TNT in large scale blast experiments.
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