Patent Publication Number: US-7707938-B2

Title: Apparatus and method for avalanche control

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention generally relates to ammunition and explosives. More specifically, the invention relates to blasting and terrain clearance. Another specific aspect of the invention relates to fuel air explosive. The invention is a device and method for artificially triggering an avalanche, such as a snow avalanche. 
   2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
   Avalanche control is a safety practice by which known snow slide areas are artificially triggered to release a snow slide or avalanche. Such artificial release reduces the possibility that an avalanche will occur at another time or in greater volume. Avalanche control is especially important in ski areas, where an unexpected avalanche might threaten human safety. Avalanche control also is important near mountain highways, where an unexpected avalanche might damage or close a road, in addition to threatening vehicles and human safety. Consequently, managing avalanches offers the advantage of causing the avalanche to take place at a planned, favorable time, such as when people and vehicles are not present and when any necessary machinery and equipment is available. 
   A basic method of releasing an avalanche is by use of an explosive to dislodge the snow. Various types of explosive charges are in use. Placing or throwing a charge of TNT is effective. However, such a method involves the difficulties and dangers of storing and directly handling explosives. 
   A method of avalanche control in common use in the United States is to fire an artillery shell at the snow slide area. The exploding shell can trigger an avalanche. If the avalanche fails to release, additional artillery shells can be fired. This method has the advantage of being simple, effective, and flexible. The artillery gun can be fired from a safe distance and repositioned as necessary to fire at additional snow slide areas. However, storing functional artillery guns and shells can be a public danger, since such equipment can be misappropriated and misused. For this reason, artillery guns and shells are being retired from use in avalanche control. 
   In 1975, the Washington University, Seattle, Dept. of Civil Engineering, reported a study titled “Alternate Methods of Avalanche Control,” published by the U.S. Department of Commerce. This study investigated several methods of releasing snow slides by mechanical action or explosive force applied from below the snow. 
   According to the first reported method, inflatable air bags were placed in known snow slide zones prior to snowfall. After snowfall, the air bags were inflated in an attempt to trigger the release of an avalanche. This method had many problems, including the inability to adjust bag placement once the snow layer was in place, various types of bag failures, and danger that the bags would slide with the avalanche. 
   In the second reported method, explosive gas was detonated in various types of chambers installed before snowfall. One device was a steel cylinder that constituted an explosion chamber. The chamber carried a movable lid mounted on guide rods and held by return springs. After snow had buried the chamber, explosive gas injected into the chamber was detonated to explosively raise the lid from the cylinder. The rising lid and escaping gas from the explosion disrupted the covering snow. In a second variation of an explosion chamber, a length of metal pipe was perforated with holes. After being buried by snow, the pipe was filled with explosive gas, which was detonated. The explosion passed through the perforations to directly disrupt the snow. In a third explosion chamber, a large truck tire and rim were used without a split ring so that part of the rim could move. The explosive gas was introduced into the tire and detonated. According to the study, these gas exploder systems were not uniformly reliable for triggering avalanches. 
   Other types of gas exploders deliver a detonation and shock wave above the snowfield. U.S. Pat. No. 5,107,765 to Schippers teaches the use of a gas gun in the form of an explosion chamber configured as a tank with a discharge port pointed in the direction of the slope where the avalanche is to be triggered. The gas gun must be solidly anchored to the mountainside. Anchoring such a gas gun to a sufficiently solid foundation is difficult, particularly where native rock is crumbling. Further, the vibrations from explosions in the gas gun tend to fracture the foundation, damaging the mountain environment and requiring periodic remounting of the gas gun. During winter conditions, likely it is impossible to rebuild a foundation for the gas gun, resulting in loss of avalanche control at the site for the remainder of the season. 
   U.S. Pat. No. 6,279,481 to Schippers discloses a device characterized as an explosive hammer for dislodging cornices. A structure likened to the hammerhead is an explosion chamber configured as a hollow cylinder with closed top and open bottom. A structure likened to the hammer handle is an arm connected to the hollow cylinder and anchored to a mountainside by a pivot. A foundation secures the end of the arm to the mountainside. After a cornice forms over the cylinder, the explosion chamber is filled with explosive gas, which is detonated. Upon detonation, the gas raises the explosion chamber from the ground. The gas can escape from the open bottom of the explosion chamber. The discharging gas and the rise and fall movement of the explosion chamber combine to dislodge the cornice. However, over time the stress from the confined explosion and the hammering motion of the cylinder are likely to degenerate local rocks or the foundation. 
   Explosion chambers present additional limitations. For safety, combustible gases and oxidizing gases are not mixed prior to the approximate time of the intended explosion. The two types of gas are fed into the explosion chamber, where a degree of mixing takes place. However, short term mixing tends to be imperfect, which can reduce explosive yield when the mix is detonated. The explosion chamber may further limit explosive yield by preventing access to ambient oxygen, which otherwise might supplement the imperfect mixing within the chamber. 
   U.S. Pat. No. 6,324,982 to Eybert-Berard et al teaches a one-time-use explosion chamber similar to a butyl rubber weather balloon to hold explosive gas above the snowfield. Detonating the gas in the balloon destroys the balloon and generates an aerial pressure wave for releasing an avalanche. Such a frangible explosion chamber offers an advantage, as the balloon suffers no problems with degenerating foundations. However, handling and positioning a large balloon in the typically windy conditions of a mountaintop cause this solution to be impractical. Also, mountain locations are notable for having many hungry rodents, which often eat plastic, rubber, and almost any other material they can gnaw. The balloons are to be preinstalled in containers on a mountain and connected to a gas supply tube system. Quite likely the balloons, any nonmetallic portions of the containers, and any nonmetallic gas supply lines would present a food source to the rodents and quickly become non-operational. 
   It would be desirable to create an avalanche control system that is conveniently used in the often-difficult conditions on a mountaintop, that enjoys the relative safety and convenience of using explosive gases, but that does not tend to fracture its foundation or the rock structures of the mountain. 
   It would be desirable to create an avalanche control system that employs a combustible gas mixed with an oxidizing gas, in which ambient oxygen is readily available to the mixture to ensure good explosive yield. 
   To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the method and apparatus of this invention may comprise the following. 
   BRIEF SUMMARY OF THE INVENTION 
   Against the described background, it is therefore a general object of the invention to provide an avalanche control system that employs explosive gas ignited in open air. 
   According to the invention, an avalanche control apparatus employs an open air gas explosion to trigger an avalanche. The apparatus includes suitable sources of at least two gases that can be combined to form a detonable mixture. These gases may comprise a combustible gas and an oxidizing gas. Each gas is supplied at a pressure above atmospheric pressure so that the pressure differential from atmosphere drives the gases to mix and deploy as a detonable cloud, in open air, and in communication with an igniter at a known location, in a predictable time interval. A controller initiates the mixing and deployment of the gas cloud. Within the predictable time interval, the controller fires the igniter to detonate the explosive gas mixture in open air. 
   The apparatus provides a means for mixing the two gases to form the explosive gas mixture. A suitable means for mixing the two gases includes a means for directing the gases into mixing contact in a mixing area and for directing the mixed gases toward a point of use. The mixing means is adapted to receive the two gases under greater than atmospheric pressure, to mix the two gases to form an explosive gas mixture, and to further mix the explosive gas mixture with ambient air. 
   The igniter is juxtaposed to the mixing area and is in communication with the gas mixture at the point of use. The controller synchronizes the operation of the igniter with the projected location of the mixed gases to fire at a suitable time to ignite the explosive gas mixture in open air. 
   The controller contains a microprocessor that executes programmed instructions to regulate the release of component gases for mixing and the subsequent firing of the igniter. The microprocessor sequences the mixing of component gases and detonation of the mixture in open air. 
   According to a method of the invention, an explosive batch of mixed gases is detonated in open air in proximity to an avalanche start zone to trigger release of an avalanche. According to the method, at least two component gases are provided, initially at greater than atmospheric pressure. The two gases are chosen such that, when mixed together, they form an ignitable, explosive gas mixture. The predetermined quantity of each of the two gases is mixed to form an ignitable, explosive gas mixture. At a first preselected time, the predetermined quantity of each gas is delivered at greater than atmospheric pressure into a mixing area in a manner such that the two gases mix with the one another to form a batch of mixed gas. The batch of mixed gas is then located in open air, in communication with an igniter. At a second preselected time subsequent to and coordinated with the first preselected time, and while the mixed gas is in communication with the igniter, the igniter fires to ignite the batch, thereby producing an explosion in open air. The explosion generates an airborne shockwave suitable for triggering release of an avalanche from the avalanche start zone. 
   The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention, and together with the description, serve to explain the principles of the invention. In the drawings: 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a side elevational view of the exploder, showing a representative mounting location. 
       FIG. 2  is an isometric assembly view of the exploder and tank cage. 
       FIG. 3  is a rear assembly view of the exploder and mounting bullet base. 
       FIG. 4  is an enlarged detail isometric view of a mixer. 
       FIG. 5  is a schematic diagram of components of the exploder and connections thereof, with functional connections between an electronic control module and other components shown by broken lines. 
       FIG. 6  is a flow chart showing control software and method steps in operating the exploder. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention is an apparatus and method for mitigating avalanche danger in known avalanche slide paths. The invention, which will be generally called an “exploder,” provides a substantially unconfined batch, charge, cloud, or body of explosive gas that is detonable to produce an airborne shockwave for triggering the release of an avalanche. Associated equipment for storing, delivering, mixing, and detonating the gas body can be installed near the area of the known avalanche slide path. The substantially unconfined character of the detonable gas body reduces the degree of shock and vibration delivered to the associated equipment, as compared to prior art equipment that employs an explosion chamber that houses the exploding gas charge. Thus, the method and apparatus of the invention provides a suitable explosion to trigger an avalanche while the equipment delivering the gas charge experiences greatly reduced shock due to the explosion being in open air, protecting both the equipment and the structure of the mountainside terrain where the equipment is located. 
   As a general guideline, triggering an avalanche requires a force equivalent to the explosion of at least seven pounds or three kilograms of TNT. The explosion of one gram of TNT supplies one thousand calories. Using this information, it is possible to calculate the volume of various explosives that would be equivalent to three kilograms of TNT. Thus, it is readily possible to determine an effective quantity of various explosives. U.S. Pat. No. 6,324,982 issued Dec. 4, 2001 to Eybert-Berard et al provides an enabling sample calculation for a mixture of hydrogen and air, thus guiding similar determinations for other gas mixtures. The teachings of U.S. Pat. No. 6,324,982 are hereby incorporated by reference for this purpose. 
     FIGS. 1-5  illustrate the structure of the exploder  100 . The components include a means for supplying component gases in a suitable ratio and quantity to form a detonable gas body that, when detonated at an avalanche start zone, is sufficient to trigger release of an avalanche. The components further include a means for receiving and mixing the metered gases, a means for locating or discharging mixed gases to open air as a detonable gas body or gas cloud, and a means for igniting the gas cloud in open air. A support structure carries the aforesaid components in operable relationship. An electronic controller times and sequences the operation of the exploder. The controller actuates the release of the component gases to the mixer for mixing. At a subsequent time, the controller fires the igniting means after allowing a suitable time interval for the mixed gas to discharge from the mixer to open air in igniting proximity to the igniting means, thereby detonating the mixed gas body in open air. 
   In use, the exploder  100  includes a means for mounting the support structure in an area useful for artificially triggering release of an avalanche. In theory, the support structure might be portable. However, the frequent reality of operating in mountainous areas is that equipment should be pre-positioned before the winter, since moving equipment into close access to avalanche start zones during winter can be difficult. 
   The components of the exploder  100  are best located near an initiation zone, known as the “start zone” near the top of an avalanche slide path. Such zones are readily identified or known by experts, such as professional avalanche control personnel. Terrain analysis also is helpful to identify potential slide zones and start zones. Of course, empirical observation over long periods of time also allows identification of avalanche paths and start zones. 
   Various studies have attempted to characterize the physical qualities of start zones and slide paths in order to enable prediction of their location. The most important characteristic appears to be slope angle. The start zone often has a slope angle between thirty and forty-five degrees, while the avalanche path may have a more gentle slope angle between twenty and thirty-five degrees. Some reports suggest a slope angle as low as twenty-five degrees is suitable for a start zone. Steeper slopes, such as forty to sixty degrees, might be suitable for start zones, although such angles can be too steep for sufficient snow accumulation. Another relevant characteristic is the aspect of the slope relative to prevailing wind direction. Snow slabs on slopes downwind or parallel to prevailing wind direction appear to be more prone to avalanches. The shape of the terrain favors an avalanche on convex slopes, steep planar slopes, and along ridges with cornices. Start zones tend to be in open, non-forested areas, especially rocky surfaces above tree line. These physical characteristics enable prediction of a start zone and slide path by inspection of terrain. 
   Avalanche slide paths not only can be identified by inspection, but also by empirical observation. Over the course of years such slide paths are empirically identified by the natural and repeated occurrence of avalanche slides. Thus, in areas where long-term records are available, such as at ski areas and along mountain roadways, avalanche slide paths and start zones are identified by observation. 
   As suggested by  FIG. 1 , an appropriate mounting location for exploder  100  typically will be on a mountainside  102 , near avalanche start zone  103  at the top of a known slide path, often along a ridge. Various components of the exploder are best mounted on an elevated support structure such as a mounting mast  106 , suitable to support the components above the level of expected accumulated snow  104 . A pilot shaft  108 ,  FIG. 3 , carried on a bolt-down mounting flange  110  serves as a means for attaching the mast  106  to the mountainside  102 . The pilot shaft  108  may be fixed to the ground, while the mast and its attachments can be removed and replaced as necessary, such as by helicopter transport. Any of various types of supplemental fasteners such as bolts may secure the base flange to a suitable pad or foundation. Optionally, the mast  106  and pilot shaft  108  may be keyed together to support the mast in a single rotational position. For example, the pilot shaft and mast may engage by telescoping structures configured such that the mast cannot rotate on the pilot shaft. 
   The means for supplying component gases may include a dispenser that meters the gases in a desired ratio. An accumulator tank for each gas is suitable to measure, contain, and dispense each gas. An accumulator tank contains and dispenses a quantity of one gas suitable for one operational cycle of the exploder. The mast  106  carries two or more accumulator tanks. One of the gaseous components can be characterized as a combustible agent, and another of the components can be characterized as an oxidizing agent. For safety reasons, the two gases are stored and handled separately until mixed immediately prior to detonation. More than two gases may be employed, as desired. Impurities in the component gases are acceptable and expected, although they should be substantially harmless to the eventual mixing and detonation of the gases. 
   Propane gas is a preferred combustible agent because of its broad acceptance, wide availability, safety of storage and use, and standardized handling equipment. Other combustible gases are suitable and may be selected for use in the explosive mixture. Examples of other suitable combustible gases include acetylene, hydrogen, and butane. Combustible gas may be obtained from a source that is in any state, such as either liquid or gas. 
   When propane is selected as the combustible gas, the relative volumes of the accumulator tanks and available gas pressures are selected to produce approximately a 5:1 ratio of oxygen to propane in an end mixture. Propane may be used as a representative example of a combustible gas. Further examples may use the properties of propane when describing the structure and operation of the exploder  100 . The naming of a specific gas should be understood to be by way of example and not limitation. Structural details of the exploder  100  often will be designed for handling propane and may be unsuitable for handling a different combustible gas. Therefore, for use with any other combustible gas than propane, the exploder  100  may require suitable modification for properly handling the chosen gas. 
   A first accumulator tank  112  is sized to contain an oxidizing gas in a first preselected quantity. A second accumulator tank  114  is sized to contain combustible gas in a second preselected quantity. The comparative volumes of tanks  112  and  114  are selected with consideration for available or chosen gas pressures under expected temperatures at point of use, to produce a suitable ratio of gases in the end mixture. As noted above, a suitable ratio for oxygen and propane is 5:1. Each accumulator tank holds a quantity of gas suitable to be combined in a mixture for a single operation of the exploder  100 . 
   A sample calculation yields sample volumes of the gases and for the accumulator tanks for a single operational cycle of the exploder. The calculation shows an explosive yield for those volumes: 
   According to Boyles law, V1×(P1/P2)=V2 where P1 and P2 are values expressed as p.s.i.a. If the oxygen tank volume is 2.76 cu.ft. with an operating pressure of 85 p.s.i.g., the volume of oxygen is 18.73 cu.ft. at standard temperature and pressure. If the propane tank volume is 3.47 cu.ft. at 19 p.s.i.g., the volume of propane is 7 cu.ft. at standard temperature and pressure. Commercial propane really is LP-gas or LPG (liquefied petroleum gases) and varies in heat of combustion yields due to the differing percentages of the constituent gases. These may include propane, butane, isobutane, propylene, butylenes and other light hydrocarbons occurring in the paraffin series between ethane and pentane. 
   Explosive yield for propane can be calculated using a value for heat of combustion of 2522 BTU/cu.ft. and the following conversion factors: 1 btu=1055.87 Joules; 1000 calories=1 g TNT; 1 calorie=4.1868 Joules; and 1 lb=453.59 g. Therefore, 7.048 cu.ft. of propane yields a theoretical equivalent to 9.883 lbs of TNT. 
   The theoretical ratio of propane to oxygen is approximately 5 volumes of oxygen to 1 volume of propane or 16.6 percent propane by volume. The theoretical ratio of propane to air is 4.02 percent propane by volume. Since the ratio of propane to oxygen in the preferred embodiment is 27 percent, then during testing, either atmospheric oxygen is making up the difference or a less than ideal combustion is occurring. The former seems to be the case due to the observed effectiveness of the detonation and apparent lack of excess residual propane. 
   Optionally, in order to operate the exploder repeatedly, each accumulator tank is associated with another source or larger reservoir supplying the appropriate gas, of suitable size to recharge the accumulator tank, repeatedly. Such an arrangement is best shown in the schematic view of  FIG. 5 . The oxygen accumulator tank  112  is fed from a reservoir of oxygen contained in an oxygen supply tank  116 , which may be a commercial oxygen tank available from many sources. Such commercial tanks carry a mechanical shutoff valve and are configured for attachment to a standard oxygen regulator. An oxygen pressure regulator  118  is connected to oxygen supply tank  116  to control an allowed rate of oxygen flow from tank  116 . An oxygen supply solenoid valve  120  is connected in-line with the regulator. Valve  120  is electrically opened to allow oxygen from supply tank  116  to charge accumulator tank  112 . The solenoid valve  120  is spring loaded to closed position so that oxygen is not released accidentally, without the presence of an active electrical signal. The oxygen accumulator tank  112  should be suitably structured to receive and hold oxygen gas at a pressure from at least about fifty p.s.i.g. to about one hundred p.s.i.g. 
   The propane accumulator tank  114  is similarly fed from a larger reservoir of propane such as in a propane supply tank  124 . This tank also is a standard commercial propane tank equipped with a mechanical shutoff valve and configured for attachment to a standard propane regulator. A propane regulator  126  is attached in-line with the regulator and controls an allowed rate of flow from propane supply tank  124  to accumulator tank  114 . A propane supply solenoid valve  128  electrically opens to allow gas from supply tank  124  to charge accumulator tank  114 . The solenoid valve  128  is spring loaded to closed position so that propane is not released accidentally. The propane accumulator tank should be suitable to receive and hold gas at a pressure of at least about nineteen p.s.i.g., which is typical of available propane gas pressures at cold, mountaintop temperatures. 
   An electronic control module  122  is functionally connected to the solenoid valve  120  in a suitable manner, such as by signal wire  121 , to supply an electrical signal to selectively open the solenoid valve. The electronic control module  122  is functionally connected to the solenoid valve  128 , such as by signal wire  129 , to supply an electrical signal to selectively open valve  128 . The control module causes these valves to close by terminating the signal. The described electric valves permit automated control over charging the accumulator tanks from the larger supply tanks. 
   The control module  122  may be a programmed general-purpose computer, controller, or other microprocessor-controlled device capable of carrying out programmed instructions and issuing electrical signals to control external electrical equipment. The electronic control module is preferred to be self-contained so that it can operate in remote areas. It is provided with a power supply to operate the module and supply necessary electrical signals and actuations. Suitable known interfaces are used, as allow a general-purpose computer or industrial control module to monitor and control chosen electrical equipment such as switches and valves. Suitable power sources include a battery or a solar panel, or, optionally, power from a utility grid, as may be available. 
   An optional pressure responsive means may regulate or monitor the charge in each of the accumulator tanks. As one option, the pressure responsive means is a transducer in each accumulator tank  112 ,  114 ; in the gas lines  138 ,  140 ; or in valves  120 ,  128 ,  158 ,  160  communicating with each of the accumulator tanks. As another option, a pressure-actuated switch will shut the gas valves  120 ,  128  at suitable preselected pressures, such as nineteen p.s.i.g. for a propane accumulator tank and eighty-five to one hundred p.s.i.g. for an oxygen accumulator tank. Each accumulator tank  112 ,  114  or valves  120 ,  128 ,  158 ,  160  optionally may incorporate a pressure switch  119 ,  127  that closes upon detecting a suitable pressure for the chosen component gas within the tank. Thus, an electronic monitoring system of control module  122  can determine, by reading the open or closed condition of such pressure switches  119 ,  127 , when each accumulator tank  112 ,  114  contains a suitable charge of a chosen gas. 
   Alternatively, the pressure regulators  118 ,  126  can be used in place of a pressure switch. A pressure regulator can be preset for a selected maximum pressure of gas to monitor and control the filling process. For example, if a propane pressure regulator is set at nineteen p.s.i.g., the pressure regulator will limit the accumulator charge to the set pressure. A standard pressure regulator may lack facility to communicate with an electronic monitoring system of control module  122 . Therefore, when a standard pressure regulator is used in place of a monitorable pressure switch, the process of filling an accumulator tank can be carried out over a surplus time period for the full charge to be delivered to the accumulator tank. A period of one minute typically will prove to be adequate for this purpose. 
   Nitrogen supply tank  130  is an optional but recommended component. Tank  130  contains a supply of nitrogen gas. Tank  130  may be a commercial nitrogen tank equipped with a mechanical shut-off valve and configured for attachment to a nitrogen regulator. A nitrogen pressure regulator  132  controls the rate of allowable nitrogen flow from tank  130 . Optionally, a nitrogen supply solenoid valve  134  is connected in-line with the regulator to allow controller  122  to electrically release nitrogen gas. Released nitrogen flows through nitrogen supply line  136  to the accumulator valves, where it is a source of pneumatic power for operating the accumulator valves. Nitrogen gas can serve as a source of power for other purposes, as may be required. 
   The gas supply tanks  116 ,  124  are connected to the accumulator tanks by respective feed lines  138 ,  140 . The gas supply tanks may be located remotely from the mast  106 , at a safe distance from the operation of the exploder and at a convenient location for periodically replacing exhausted tanks or for servicing a plurality of grouped exploder installations. As best shown in  FIGS. 1-3 , an alternative and preferred location for the gas supply tanks  116 ,  124 , and  130  is mounted on mast  106 . A housing or partially open cage  142  is mounted on one side of the mast  106 , such as on the uphill or backside of the mast so that the mast helps to shield the cage from operational explosions of the exploder  100 . The walls of cage  142  are formed with an open structure, such as a partially open mesh, so that any leaked gases cannot accumulate inside the cage. However, as best shown in  FIG. 2 , the front or mounting wall  144  of the cage may be a substantially imperforate or solid shield wall that further protects the contents of the cage from the force of operational explosions. Thus, the shield wall approximately faces the area where the gas mixture will be detonated. 
   Desirably, the gas supply tanks are adequate to provide a sufficient reservoir of respective gases for the needs of an entire winter or other period of service. Nevertheless, the cage can be serviced to replace spent gas supply tanks  116 ,  124 , and  130  as necessary. For this purpose, the cage includes an openable or removable rear door  146  to allow removal and reinstallation of supply tanks. A combination of factors makes on-site servicing difficult during winter conditions. These factors include the often-remote location where the exploder may be installed, the deep snow that may surround the exploder, and the difficulty of transporting and installing bulky or heavy supplies to the exploder during heavy snow conditions. To overcome these factors, the cage is structured as a removable unit from the mast  106 . 
   As best shown in  FIG. 2 , the cage  142  mounts on mast  106  by a gravity connection so that the housing can be removed through vertical lifting that disengages the connection. A suitable gravity connection is a hook and receptor system. A plurality of laterally separated hooks  148  on the cage  142  engages a laterally elongated receptor  150  on the mast to provide a stable mounting with fixed relative orientation. The cage is further stabilized on the mast  106  by a standoff  152 . One or more lift eyes, such as the laterally spaced pair of lift eyes  154  at the top of cage  142 , provide a means for lifting the cage and its contained tanks from mast  106  to enable replacement of the tanks. Another cage holding fresh tanks can be delivered to replace a removed cage having one or more spent tanks. A helicopter is a suitable vehicle for lifting or lowering such large or heavy objects as the cage at remote locations. Thus, the described mounting system between the mast  106  and cage  142  is adapted to allow the cage to be lifted from or lowered to the mast. Even during heavy snow conditions, ground maintenance workers should be able to reach the location by snow vehicle, skis, or other means in order to aid the replacement by disconnecting or reconnecting gas feed lines to the like and hooking or unhooking a helicopter winch line with eyes  154 . 
   As thus far described, the replaceable cage  142  carries pressurized reservoir tanks of propane, oxygen, and nitrogen. Suitable regulators and electrically controlled valves  120 ,  128  enable and control the flow of oxygen and propane to gas quantity measuring devices such as the accumulator tanks  112 ,  114 . Under control of a computer or other electronic control module  122 , electrically controlled valves  120 ,  128  can be selectively opened to feed gases under pressure into the accumulator tanks, after which the valves  120 ,  128  close under a default force system such as a spring. The accumulator tanks  112  and  114  each are sized to hold a separate charge of gas in quantity sufficient to form an explosive mixture when mixed together, suitable for a single operation of the exploder  100 . 
   According to one embodiment, the accumulator tanks  112 ,  114  selectively supply measured amounts of the combustible gas and the oxidizing gas at above-ambient pressures to a means for mixing gas, such as gas mixer  156  of  FIGS. 1-5 . A first accumulator valve  158  controls release of gas flow exiting the first accumulator tank  112 ; and a second accumulator valve  160  controls release of gas flow exiting the second accumulator tank  114 . 
   As may be required, suitable oxygen conduit  162  and a propane conduit  164  connect oxygen accumulator tank  112  and propane accumulator tank  114  to the mixer  156  under control of the valves  158 ,  160 . The accumulator tanks are oriented with discharge openings at their bottoms so that any residual gas will not pool or stagnate in these tanks after valves  158 ,  160  have released the gas charges. Thus, valves  158 ,  160  and conduits  162 ,  164  connect to the discharge openings at the tank bottoms and provide a downwardly open drainage pathway for any residual gases. 
   The best use of the electronic controller  122  employs a benchmark event to initiate timing to predict the movement of the gases from the supply until discharge to open air. Opening of one or more of the accumulator valves  158 ,  160  establishes such a reliable benchmark event. This benchmark time initiates a precalculated interval during which the component gases are released to the gas mixer  156 , and this release timing can be either simultaneous or sequential according to such factors as relative lengths of conduits  162 ,  164  to ensure the gases reach the mixer at a suitable time to be mixed. When an accumulator valve  158 ,  160  is open, any gas within the associated accumulator tank at greater than atmospheric pressure will flow through the accumulator valve and conduit to mixer  156 . Such flow is known to occur rapidly, at near sonic speed. 
   A variety of commercially produced valves, such as electric solenoid valves or pneumatic ball valve assemblies can be used as the accumulator valves. Preferred accumulator valve assemblies  158  and  160  are commercially available ball valves that are spring-loaded to open position and closed by pneumatic pressure. Many such valves require about 50 ms to fully open. For preferred operation of the exploder  100 , the ball valve or other type valve used on the accumulators should close on command and open rapidly, within 100 ms, when commanded to open. Nitrogen gas from tank  130  may supply the pneumatic energy to close the pneumatic valves. Because the accumulator valves are mechanically biased open unless receiving gas pressure to close them, the accumulator tanks normally are vented to atmosphere. In default operation, due to loss of nitrogen pressure or loss of electricity, the valves are vented to atmosphere. During normal operation, an electrical signal from controller  122  triggers the valve assemblies  158 ,  160  to close, and the absence or termination of the electrical signal causes the valves to open. 
   The accumulator valve assemblies  158 ,  160  and the conduits  162 ,  164  should be large enough to allow the detonable gas mixture to form a contiguous cloud to the exploder  100  within the precalculated time interval, which is before ambient wind can dissipate the cloud. The full-accumulated charge of gases from the accumulator tanks  112 ,  114  should be able to discharge promptly, without undue friction from the valves or conduit. For this purpose, the diameter of the valve orifice in valve assemblies  158 ,  160 , when open, is the same size or larger than the inner diameter of conduits  162 ,  164 . For preferred operation of the exploder  100 , a suitable nominal inner diameter of the conduit is two inches, which two-inch schedule  40  steel pipe offers. A smaller size pipe or valve orifice could overly restrict the flow of gases. Such undue restriction could slow the escape of gases to a sufficient degree that the full-accumulated charge of gases does not escape from the accumulator tanks quickly enough to form a contiguous cloud of explosive mix before ambient wind could dissipate the cloud. Therefore, the preferred nominal size of the accumulator valves and gas conduit is two inches or more. 
   For safety related reasons, a suitable accumulator valve assembly  158  on the oxygen accumulator tank  112  must not trap oxygen in any part of the valve mechanism. Trapped oxygen might form an explosive mixture with combustible dirt, oil, or grease. A suitable valve choice is a stainless steel ball valve that has been cleaned for oxygen and is rated for oxygen use. Similarly, all oxygen accumulator tanks, conduits, and fittings must be oil, grease and dirt free and specially cleaned for oxygen use. 
   As an example of a commercially available stainless steel ball valve that is suitable, DuraValve, Inc., 2331 Eastern Ave., Elk Grove, Ill. 60007, produces model DM240, which is a two-piece valve with full port and threaded ends. Electrical commands actuate this ball valve, and pneumatic pressure operates the valve. It is an industry-standard type of pneumatically actuated ball valve assembly. 
   As an example of how an industry standard valve may function within the exploder  100 , a control line  159  runs from the electronic control module  122  to the ball valve assembly  158 . The pressure regulated line  136  from the nitrogen tank  120  runs to the ball valve assembly  158 . When the ball valve is to be closed, the control module  122  may transmit a suitable electrical signal to the ball valve assembly  158 . 
   The electric signal from the control module  122  activates a small solenoid that is a component of the ball valve assembly. The solenoid pulls open a small, spring loaded poppet valve, which allows nitrogen from line  136  to push a lightly spring loaded pilot spool valve to an open position. The spool valve allows a large flow of nitrogen from the tank line  136  to enter a one hundred millimeter diameter piston actuator in the ball valve assembly. The piston accumulator causes a rack and pinion arrangement to rotate a splined shaft by 90 degrees. The splined shaft is coupled to the stem shaft of the two-inch ball valve and rotates the ball to closed position. 
   The ball valve is normally open until actuated. Control module  122  can cease sending the electrical signal to the ball valve assembly  158  in order to command the valve to open. Without the signal from the control module, power is removed from the small solenoid on the valve assembly and the small poppet valve closes under spring force. In turn, the lightly spring loaded pilot spool valve is returned to the closed position. This simultaneously shuts off the flow of nitrogen from the tank line  136  to the one hundred millimeter piston and vents remaining pressurized nitrogen in the one hundred millimeter cylinder to the atmosphere. The valve is returned to the normally open position by means of a set of return springs that are built into the one hundred millimeter piston housing and oppose the one hundred millimeter diameter piston travel. The piston had previously compressed the return springs upon actuation. 
   Accumulator valve  160  on the propane accumulator tank may operate in the same industry standard manner described above. A control line  161  runs from the control module  122  to the ball valve assembly  160  and provides a signal to close valve  160 . Because of their speed and reliability, industry standard pneumatic valves are preferred. If another type of valve is used, such as an electric solenoid valve, the alternative valve should similarly close on command, open within the previously specified 100 ms limit, and open as the default valve position. 
   The gas mixer  156  receives flow of pressurized gases from accumulator tanks  112  and  114  when accumulator valves  158 ,  160  are open. The flow is rapid, occurring at near sonic speeds at the valves  158 ,  160  and through the conduits  162 ,  164 . With reference to  FIG. 4 , the mixer provides gas input ports  166 ,  168  that respectively connect to conduits  162 ,  164  and are nominally of the same inner diameter, i.e. two inches. The input ports receive and direct the oxygen and propane gases into a mixing chamber  170 , where the oxidizing gas and combustible gas are combined to form a detonable gas mixture. The mixing chamber  170  may be a cylindrical chamber of larger diameter than the input ports  166 ,  168  or conduits  162 ,  164 . For example, the mixing chamber may have an inside diameter of about eight inches and a height of about six inches, which provides room for the two gas streams to mix. 
   In mixer  156 , the inlet ports are positioned to direct the gas streams from conduits  162 ,  164  through ports  166 ,  168  along intersecting or impinging pathways. Thus, for example, the ports are arranged at ninety degrees on the cylindrical sidewall of the mixing chamber  170 . Streams of gas entering the mixing chamber through ports  166 ,  168  are turbulent and actively mix with one another. 
   The mixer  156  is open to atmosphere so that the mixed gas can discharge from it. The discharging mixed gas is directed to a known or substantially predictable open-air proximity to the mixer. Reference to a known or substantially predictable open-air proximity is time dependent. Typically, the time is several hundred milliseconds from release of the component gases from their source, i.e, from the accumulator tanks. A divergent nozzle  172  at an axial end of the mixing chamber  170  discharges and directs the detonable gas mixture to the open-air proximity from the mixing chamber. A lip  174  defines a mixing chamber exhaust port. The opposite axial end of the mixing chamber  170  is closed. The nozzle  172  to direct the gas cloud out of the mixer and into selected open-air proximity to the exploder. 
   The volume of mixed gas is substantially larger than the volume of the mixer. For example, the mixed gas may occupy about twenty-five cubic feet at standard temperature and pressure, while the mixer typically has a volume of less than one-half cubic foot. Thus, mixed gas in a single batch is rapidly and progressively discharged to open atmosphere through divergent nozzle  172 . 
   The gas mixture expands to ambient pressure as its passes through nozzle  172  and into open air beyond. With such expansion, gas velocity decreases in open air. In the open air, the mixed gas undergoes additional mixing, including mixing with ambient air. Such additional mixing supplies supplemental oxygen from atmosphere to the mixture, which helps to ensure that sufficient oxygen will be available within the mixture to produce a powerful and efficient explosion. 
   The body of mixed gas exits the mixer at nozzle lip  174  and enters open-air proximity to the exploder. A means for igniting the mixed gas, such as a spark ignition unit  176 , is located in the open-air proximity of the mixed gas cloud to the exploder. More specifically, the igniter  176  is located where the trailing portion of the gas cloud will be located at a time when the igniter is actuated, which is within a time interval measured from the benchmark release of gases from the accumulator tanks. The igniter may be juxtaposed to the mixer nozzle  172 , such as by attachment to the outlet lip  174  or other predetermined position. When the igniter  176  is proximate to the exhaust port defined by lip  174 , the igniter will be near the trailing edge of the discharging cloud. Thus, the igniter can detonate the cloud from its trailing edge, ensuring that the explosion takes place in open air. The igniter selectively provides a spark to ignite and detonate the gas cloud  178 , schematically illustrated in  FIG. 5 . 
   Thus, the mixed gas cloud is discharged from the mixer in open-air proximity to the igniter, which is in a predetermined position. The shield wall  144  of cage  142  approximately faces this predetermined position near the mixer  156 . The igniter should be proximate to the discharge port from the mixer or to the final portion of the mixing area for the component gases. When the mixer partially confines the component gases, as is the case with mixer  156 , the discharge port at lip  174  provides a suitable predetermined position for carrying the igniter. Igniting the trailing portion of the mixture cloud is preferred because the detonation will propagate from the exploder toward the area of open air. Also, the location of the trailing portion as it exits the confinement of the mixer is reliable. 
   The electronic control module  122  is functionally connected to the igniter  176 . For example, a wire  179  connects module  122  to the igniter for selectively firing the igniter. The electronic control module  122  coordinates the time for ignition from an earlier reference point, which is the benchmark opening of the accumulator valves  158 ,  160 . The time interval necessary for movement of the gases from benchmark time until the mixed cloud has exited the mixer to open air is determinable. With suitable coordination, the mixed gas cloud reliably resides in the area of open atmosphere in communication with the predetermined position of the igniter. 
   The time for igniting the gas mixture is best determined by projecting the location of the gases with respect to time. The control module  122  may coordinate the release of each component gas so that the two gases arrive at the mixer substantially at the same time. As best shown in  FIG. 2 , the accumulator valves  158 ,  160  are located such that conduits  160 ,  162  are of approximately equal length between the valve and the mixer. The equal lengths and diameters should produce approximately equal resistance to gas flow. Equal resistance to flow over the length of the conduits conveniently permits the valves  158 ,  160  to be opened simultaneously. In the illustrated embodiment of  FIGS. 1-3 , the conduits  160 ,  162  may be about three feet in length, and the time delay from opening the accumulator valves until firing the igniter can be about 200 milliseconds. Conduits lengths could be extended to about thirty or forty feet from the accumulator valves to the mixer without significant flow restriction. With such longer conduit lengths, the suitable firing delay might be increased. 
   The described mixer  156  relies upon mixing between intersecting high-speed gas streams, aided by impingement of the gas streams on one or more walls of the mixing chamber  170 . Other known types of gas mixers could be used. These include static mixers, entrainment mixers, and venturi mixers. The partially closed mixing chamber  170  of mixer  156  may be regarded as an optional convenience for supporting the gas conduits and igniter. 
   Mixing can be achieved in open air by aiming gas conduits  162 ,  164  to direct their discharging gas streams along intersecting vectors. Thus, the gas conduits  162 ,  164  may be supported in the configuration shown in  FIG. 2 , where the gas streams will intersect at ninety degrees. In the absence of a physical mixing chamber  170 , the gas streams would mix in open air. Thus, the physical mixing chamber  170  is optional. An impingement plate such as one or more wall of chamber  170  can be used to assist open air mixing and to redirect the movement of the mixed gas cloud as required. 
   An intersection angle of less than ninety degrees between component gas streams could produce mixing while simultaneously directing the mixed gas cloud in a selected direction, such as toward an avalanche start zone. As noted, the ambient air itself serves as a barrier, thus causing turbulent mixing. Thus, mixing in open air is optionally possible. During and after mixing, or as the gas cloud enters open air, the speed of the gas cloud slows substantially due to encountering ambient air. 
   Within a partially confined mixing chamber  170  or substantially any other mixing arrangement, it is advisable that the gas streams not directly oppose one another, so that the higher pressure gas stream does not reverse flow of the lower pressure gas stream within a gas supply conduit. For example, oxygen conduit  162  should not direct oxygen, typically at a relatively higher pressure, into the lower pressure propane conduit  164 . It would be undesirable to mix oxygen and propane within the propane conduit  162 . Therefore, an impingement angle of ninety degrees or less is preferred between the discharging gas streams from the accumulator tanks. 
   Advantageously, the exploder  100  provides means for directing or delivering the mixed cloud to a position removed from the exploder. The exit port  174  or nozzle  172  can direct the gas cloud. As illustrated, the conduits  162 ,  164  carry the component gases away from mast  106  to gas mixer  156  that can be located remotely from the mast  106 . The conduits  162 ,  164  may be either rigid or flexible. If rigid, the conduits may be connected to valves  158 ,  160  by pivot joints serving as a means for variably positioning the mixer. For example, the conduits can be pivoted to various angles of elevation to accommodate the level of snow pack. The discharge location from mixer, such as the discharge nozzle  172 , should remain the low point in the gas pathway to avoid pooling. With suitably large diameter to prevent flow restriction, the conduits may be of selected increased length, including variably extendable lengths, to deliver the gases to a mixer at a more remote point for detonation. 
   If conduits  162 ,  164  are flexible hoses of variably selected lengths, the mixer  156  can be moved over the range permitted by hose length. Thus, a single mixer  156  can be placed to suit conditions at one or more avalanche zones. If a first detonation fails to trigger a snow slide, the mixer  156  can be repositioned so that a subsequent detonation can be applied from a different location in the slide initiation zone. Flexible hoses  162 ,  164  also permit the mixer to be disposed at variably selected heights, such as from a nearby tree or selectively placed pole. Any flexible hoses should be managed to avoid traps or low points where gases can pool. 
   As further illustrated, the conduits may direct the component gases to a selected elevation, such as downwardly from the accumulator tanks. Where the mixer  156  is employed, the orientation of the mixer and its discharge nozzle may direct and deliver the mixed gas cloud to a pre-selected location remote from mast  106 . The mixer  156  and, optionally, additional portions of the exploder  100  may be disposed at a small angle to the mast  106 . For example, the discharge opening of the mixer may be aimed downhill from the mast and toward a slide initiation zone of a known avalanche area. Such an angle is desirable for directing the detonable gas mixture toward an intended target area while, at the same time, preventing the gas mixture from engulfing the exploder  100 . 
   The detonable gas mixture is discharged into airspace above the snow mantle  104 . After a timed delay from the initiation of component gas discharge from the accumulator tanks, the electronic control module  122  causes the igniter  176  to become energized and detonate the mixed gas cloud. The time delay allows turbulence within the gas cloud to complete the mixing process. An appropriate time delay should produce a thorough mix while detonating the mixture before it can be dissipated or removed by wind. 
   Detonation produces a supersonic shockwave with a high overpressure front, which fractures the snow mantle beneath the explosion. A vacuum or low-pressure wave follows passage of the high-pressure shockwave. The vacuum lifts the snow layer  104 , thereby initiating release of a snow slide. 
   The electronic control module  122  provides a microprocessor and suitable memory to carry out programmed instructions to control the timing and operation of the accumulator valves  158 ,  160 ; the gas supply solenoid valves  120  and  128 ; and the igniter  176 . A remote device such as a handheld radio  180  forms a radio link to the control module  122 . Through the radio link, avalanche control personnel may initiate execution of the programmed instructions and operate the exploder  100  from a safe distance. The programmed instructions close valves  158  and  160  by sending an appropriate signal to those valves. The control module then opens valves  120  and  128  for a suitable time to allow the gases to flow under pressure from the oxygen and propane supply tanks to the accumulator tanks  112 ,  114 . After the suitable time interval, valves  120 ,  128  are allowed to close. Then, the control module  122  opens valves  158 ,  160 , such as by ceasing to send a “close” signal so that, by default mode, the valves open. The accumulator tanks discharge their contents through the open valves  158 ,  160  to the mixer  156 . The control module  122  optionally can open valves  158 ,  160  either simultaneously or according to a programmed sequence in order to control the relative time when the component gases will reach the mixer. 
   The programmed instructions also provide suitable time for the explosive gas mixture to discharge from the mixer to open air, although this time period is limited so that the gas cloud is not dissipated or removed by wind from open-air proximity to the igniter. The control module actuates the igniter  176  at a suitable time to initiate an open-air detonation of cloud  178  of the explosive mixture. A suitable time delay from the opening of accumulator valves  158 ,  160  until actuating the igniter allows a batch of explosive gas  178  to substantially discharge from the mixer. The igniter  176  can be actuated at a suitable time when a trailing portion of the explosive gas cloud is proximate to the predetermined position of the igniter. 
   The following Wind Speed Chart shows the possible displacement from the mixer of the mixed gas cloud at wind velocities ranging from 5 m.p.h. through 75 m.p.h. The igniter typically is fired in about 200 milliseconds from the opening of the accumulator valves. This timing is suitable to ignite the gas cloud at any of the shown wind speeds. 
   
     
       
         
             
          
             
                 
             
             
               Wind Speed Chart 
             
          
         
         
             
             
             
             
             
          
             
                 
               M.p.h. 
               Ft/sec 
               In/sec 
               In/ms 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               75 
               110.03 
               1320.30 
               1.32 
             
             
                 
               70 
               102.69 
               1232.28 
               1.23 
             
             
                 
               65 
               95.36 
               1144.26 
               1.14 
             
             
                 
               60 
               88.02 
               1056.24 
               1.06 
             
             
                 
               55 
               80.69 
               968.22 
               0.97 
             
             
                 
               50 
               73.35 
               880.20 
               0.88 
             
             
                 
               45 
               66.02 
               792.18 
               0.79 
             
             
                 
               40 
               58.68 
               704.16 
               0.70 
             
             
                 
               35 
               51.35 
               616.14 
               0.62 
             
             
                 
               30 
               44.01 
               528.12 
               0.53 
             
             
                 
               25 
               36.88 
               440.10 
               0.44 
             
             
                 
               20 
               29.34 
               352.08 
               0.35 
             
             
                 
               15 
               22.01 
               264.06 
               0.26 
             
             
                 
               10 
               14.67 
               176.04 
               0.18 
             
             
                 
               5 
               7.34 
               88.02 
               0.09 
             
             
                 
                 
             
          
         
       
     
   
   The residual velocity of the ignited gas cloud can carry the igniting explosive cloud  178  clear of the exploder  100  to open air. It is desirable to eliminate the explosion from the confines of the exploder  100  to reduce the shock borne by the exploder. The mounting of the exploder  100  primarily experiences an external shockwave from the open-air explosion, which the foundation and surrounding rock structures readily can tolerate. 
   A method of operation relies upon the electronic control module  122 , which is controlled by a microprocessor to execute programmed instructions for controlling the timing and operation of the valves and igniter. The remote radio  180  and the control module  122  communicate by a radio link. Together, they cooperate to execute a programmed routine according to software shown in flow chart form at  FIG. 6 . Module  122  runs the programmed instruction routine, with a starting point at block  182 . A watchdog timer can operate to restart the microprocessor after any defined period when no code is being executed, such as after a power interruption. The microprocessor will then automatically boot up the software and start executing code at block  182 . 
   The initial instruction produces a test loop that checks for reception of a valid radio code. At block  184 , control module  122  monitors the radio link to a radio  180  for a start code, repeatedly checking at block  186  for a detected code and looping back to block  184  to check again if a code was not detected. When a start code is detected, processing advances to block  188  the module reports its status or other information stored in memory to the radio link. The status report may consist of error codes showing previously detected information held in memory. Examples are low battery power, low oxygen pressure, low propane pressure, or inoperative solar panel where a solar panel is used. For security, at block  190  the control module also requests encryption codes via the radio link. The radio link is monitored at block  190  for receipt of valid encryption codes, looping back to request these codes again if valid codes are not received. When valid encryption codes are received, processing advances to block  192 . 
   In preparation for a detonation, at block  192  the control module closes the accumulator valves  158 ,  160  from the propane and oxygen accumulator tanks. At block  194 , the gas supply valves  120 ,  128  are opened to allow gas from supply tanks  116 ,  124  to charge the accumulator tanks. 
   A pressure check at block  196  can confirm that an adequate charge is present in the accumulator tanks, but the processing in block  196 ,  198 , and  202  is optional. The detonation process can continue at blocks  200  and  204 . An attempted detonation without a pressure check is without significant adverse consequence. For example, if an accumulator tank is not adequately charged, the detonation may be small or may fail, in either case providing equivalent notice to an operator that the exploder requires service. The use of a pressure check at blocks  196 ,  198 , and  202  is preferred so that operating personnel can isolate the reason why a detonation did not occur. 
   As one option, processing advances from block  194  to blocks  200  and  204 , allowing a prior preselected time period, such as one minute, for the accumulator tanks to charge. Where the fixed charging time option is employed, gas supply valves  120 ,  128  are closed after the preselected period has elapsed. 
   As another option for charging the accumulator tanks, the control module monitors pressure switches at accumulator tanks  112 ,  114 . At block  196  the control module reads whether the oxygen pressure switch  119  and the propane pressure switch  127  have closed, indicating that adequate charging pressure is present. For example, the pressure switch  127  in the propane tank may close at 19 p.s.i.g., providing a closed reading if this pre-selected minimum pressure is achieved. At block  198 , the module checks for adequate propane charge by checking whether the propane pressure switch  127  is closed and loops back to block  196  if the switch remains open. If a closed pressure switch  127  is detected, the processor closes the propane supply valve  128  at block  200 . Similarly and in parallel, the oxygen pressure switch  119  is checked at block  202 , providing a closed reading when a suitable oxygen pressure, such as 85 p.s.i.g., is present. If the oxygen pressure switch  119  is not closed, processing returns to block  196 . If the pressure switch  119  is closed, processing advances to block  204 , where the processor closes the oxygen supply valve  120 . 
   When the gas supply valves have been closed at blocks  200  and  204 , processing advances to block  206  where the module  122  sends a “ready to fire” code to the radio link. At this point at block  208 , the control module awaits receipt of a “fire” code from the radio link and, at block  210 , checks whether such a “fire” code is received. If this code is not received, the program loops back to block  208  to further await receipt of the code. Block  208  also measures elapsed time since the “ready to fire” code was sent. If the elapsed time period is longer than a pre-established limit such as five minutes without receipt of the “fire” code, the control module purges the accumulator tanks and advances to block  220  to end the program cycle. 
   When ready to proceed, an operator supplies a “fire” code via the radio link. At block  210  the program detects receipt of the “fire” code. Processing advances to block  212 , where the module  122  opens the accumulator tank valves  158 ,  160  to discharge the component gases to the mixer. As previously noted, these valves open in less than 100 ms. The gas charges then flow rapidly to the mixer and out the mixer nozzle as a mixed gas cloud. At block  214 , the control module pauses for a suitable time delay interval such as 200 milliseconds for the component gases to reach the mixer, mix, and emerge to open air as the mixed gas cloud. The time delay is measured from block  212  and is sufficient that at least a portion of the gas cloud is positioned in communication with the igniter. 
   After the time delay has elapsed, at block  216  the control module energizes the igniter to spark for a suitable period, such as 0.5 seconds, to ignite the gas cloud for detonation. At block  218  the control module reports a “firing completed” code to the radio link so that the operator is aware the firing cycle has been completed. Finally, at block  220  the program cycle ends. 
   In a further embodiment, the handheld radio  180  is capable of communication with a plurality of exploders  106  in an array. By suitable programming, the radio link can fire a plurality of the exploders in a preselected sequence, which may include simultaneous firing. Thus, it becomes possible to generate complimentary shock waves in the area of the array. Shock waves can be generated in a reinforcing sequence, such that the second and subsequent detonations are cumulatively reinforced by the first and previous detonations. 
   The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly all suitable modifications and equivalents may be regarded as falling within the scope of the invention as defined by the claims that follow.