AIDJEX Bulletin #40 - Polar Science Center - University of Washington

hours. The twenty-cell battery pack has a voltage
of 14.4 volts and a capacity of 110 amp hours.
A single additional cell is used to provide
switch and regulator bias. Expected life from
the above pack is 7.5 months.
The antenna is a modified canted turnstile
antenna which provides right hand circular polarization
and good hemispherical coverage. It consists
of four quarter wave vertical stubs canted
45O tangentially around a half wave diameter
circle. Each of the stubs is fed 90' out of
phase with the preceding one, progressing around
the circle.
Although the original ADRAMS was designed for
tracking only, there was soon a requirement to
add a data capability. Several units were modified
to incorporate a precision barometer and an
internal temperature sensor. Both sensor outputs
were converted to 10-bit digital words and interfaced
to the digital encoder. The temperature
sensor range is +30° to -5OOC with better than
. l0C resolution. The barometer pressure range
is 950 to 1050 millibars with .1 millibar resolution.
Since the buoy had to be water tight,
a special teflon membrane was used for the
pressure port which allowed barometric pressure
changes to enter the buoy but kept moisture out.
5. Mechanical Design
The major mechanical tasks associated with
the development of ADRAMS included the following:
development of a subsystem for paradropping
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:
1) the inner gimbal which contains the
antenna and electronics; 2) the sphere which
houses the gimbal; 3) the impact pad which
-ahsorbs the shock of the landing and 4) the
parachute and its releasing mechanism. These
are shown in Figures 1 and 2.
Gimbal Subsystem
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. Tlie
gimbal apprcach 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 4 teflon bearings which are free to
slide over the inside surface of the sphere.
These bearings are spring-mounted onto the gimbal
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 -5OOC.
Buoy Hull
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 thinwalled
apex (about 118") thereby reducing the
strength of the sphere. Polycarbonate plastics
were then investigated. These plastics have
about 16 times the impact strength of ABS and
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 s-urface. 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
interferring with the gimbal, Twelve 318" x 2"
nylon bolts are used as fasteners for joining
the hemispheres.
The flange Ltself is square-shaped instead of
round to reduce any tendency to roll in a wind.
Shock Absorbing Pad
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 dnd
crushes to 30X of its original depth so that the
proper area nf material can be determined. To
provide a broad support area, the impact pad was
designed as a 15" diameter cylinder with the
required 16 in2 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 feetlsec.
Parachute-Release Subsystem
The parachute is designed for a d:op rate of
18 to 20 feet/sec with an 80 pound payload. The
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