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The following information has been excerpted from Brown, W. P. and E. G. Kerut. 1978. Air droppable RAMS (ADRAMS) buoys. AIDJEX Bulletin. 40:21-29.
The major mechanical tasks associated with the development of ADRAMS included the following: development of a subsystem for para-dropping ADRAMS and releasing the parachute after ice impact; packaging the system to survive ice impact; making provision for maintaining the antenna in a vertical position regardless of orientation of the overall package; and selecting materials compatible with the impact loads and sub-zero temperatures.
The mechanical design consists of four subsystems:
In order to maintain the antenna in a vertical position, the entire electronics assembly was designed as a pendulous self-leveling gimbal revolving inside a 22" diameter sphere. The gimbal approach was found to be much simpler and more reliable than attempting to deploy automatically a fixed antenna in an upright position on the rough surface of the ice pack. The gimbal insures that the antenna will be vertical not only upon landing but also during any future disturbances due to wind, ice surface changes due to movement, melting or polar bear interference.
The weight of the entire gimbal including the electronics, batteries and antenna is 50 pounds and rests on four Teflon bearings which are free to slide over the inside surface of the sphere. These bearings are spring-mounted, allowing them to retract on impact. The deceleration forces are then picked up by a polyurethane pad which distributes the impact load evenly over the bottom of the sphere.
The Teflon bearings have a low coefficient of friction but to further enhance the self-leveling capability a lubricant was used on the inside of the polycarbonate sphere. Several oils and greases were tested for both lubrication and freezing properties. A silicone base grease was selected which has excellent lubricant qualities to temperatures below -50 degrees C.
A number of materials and fabrication approaches were considered for the outer sphere. The antenna could not tolerate metals within its pattern, so the sphere had to be non-metallic. It also had to be capable of withstanding the impact loads. Vacuum formed acrylic and ABS plastic hemispheres were tried, but the manufacturing technique produced a very thin-walled apex (about 1/8") thereby reducing the strength of the sphere. Polycarbonate plastics were then investigated. These plastics have 40 times the impact strength of acrylics. In addition, they can be "rate-formed" to provide a uniform wall thickness. Rate-forming involves heating powdered resin inside a complete spherical mold which is heated and rotated about all axes. The result is a sphere with a smooth inside surface. The sphere is cut in half and fiberglass flanges are installed.
A Teflon gasket material is used to seal the two hemispheres. Caulking was rejected because of the possibility of leaks into the sphere interfering with the gimbal. Twelve 3/8" X 2" nylon bolts are used as fasteners for joining the hemispheres.
The flange itself is square-shaped instead of round to reduce any tendency to roll in a wind.
The impact pad was designed to absorb the shock of the landing and to house the parachute release switches. The shock absorbers are cubes made of polystyrene foam that crush at a constant force level. For a rate of descent of 20 feet/sec and a desired deceleration of 20 g's, a deceleration distance of 3.7 inches is required. The polystyrene used yields at 35 psi and crushes to 30 percent of its original depth so that the proper area of material can be determined. To provide a broad support area, the impact pad was designed as a 15 inch diameter cylinder with the required 16 inches-squared of polystyrene distributed around the perimeter in 2" X 2" blocks. To provide lateral stability, the cylinder was divided into 3 layers of 2" cubes with each layer separated by a wood disk. This provided for a rigid structure whose crush force would produce a 20 g deceleration in a 60 pound load impacting at 20 feet/sec.
The parachute is designed for a drop rate of 18 to 20 feet/sec with an 80 pound payload. The chute supplied by Paranetics Inc. is constructed in a "paraform" shape rather than the typical hemispherical design. The paraform is a modified cross which provides a highly stable, low-oscillation descent thus minimizing impact side loads.
The chute must be released immediately after initial impact to prevent wind dragging. To accomplish the release, an explosive cutter is mounted on the chute's main shroud, actuated upon landing by the impact switches and a four-volt battery. Each of the four switches located in the impact pad consists of a large "needle" and a metallic foil embedded in a 2" polystyrene cube. Upon impact the needle penetrates the foil and closes the circuit.
Several drop tests were conducted both statically from a platform and by airplane. The static drops were without parachute at a height of 6'2" to achieve a velocity of 20 feet/sec, which is equivalent to the actual rate of descent of the parachute. These first drops were primarily to test the impact pad and cutter switches, but valuable structural design information was also gained. Four local test air drops were then conducted to observe the overall mechanical operation of the buoy.
Deployment is extremely simple. It can be handled by any aircraft having an opening of 25" X 40". The buoys are fully operating when they are placed aboard the aircraft. They weigh only 90 pounds including the parachute and therefore deployment can be handled by one man. The aircraft should be above 300 feet and the speed should be below 100 knots. For accurate placement of the buoy in a specified target area, a low altitude of 400 to 500 feet is preferred. The deployment sequence is as follows:
The design of the chute is such that the buoy descends within a few degrees of vertical and lands on its crash pad. The foam crash pad compresses, absorbing the landing shock. At least one of the switches built into the crash pad is actuated by the compression and in turn actuates a guillotine cutter which separates the chute from the buoy. The buoy falls over on its side after impact and the flanges bite into the snow crust thus stabilizing the housing against being blown by the wind. The electronics package rotates on its Teflon bearings to assume its proper orientation.
