PARATROOPERS may glide serenely through the air, but their landings can be
anything but graceful. Parachute landings are a major cause of injury and damage
to equipment, and countermeasures can be expensive鈥攖he US Army spends
millions of dollars each year making sensitive electronics shock-resistant for
airdrops.
Airdropping medical supplies, such as blood and plasma for transfusion, is an
even bigger problem: the jerk of a parachute opening is enough to cause
haemolysis, the bursting of red blood cells. Currently, Army medical teams have
to discard up to 25 per cent of airdropped blood, a potentially life-threatening
waste.
But Army engineers now believe they have solved these problems. All it takes
is a simple distance sensor鈥攐f the type used to help truck drivers reverse
safely鈥攃oupled to a pneumatic piston that decelerates the load just before
impact (see Diagram).
Using this relatively cheap and reusable design, Nicholas
Rosato and Joseph McGrath of the US Army鈥檚 Research Institute of Environmental
Medicine in Natick, Massachusetts, say they have made parachute landings as
gentle as stepping onto a pavement. They presented their initial results at the
22nd Army Science Conference in Baltimore, Maryland, last month.
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The retraction mechanism comprises two nested cylindrical chambers that sit
between the parachute and the load. A chamber inside the cylinder contains two
pulley blocks, through which the parachute cable is attached to a piston. When
the chute opens, the sudden deceleration on the cable pulls the lower pulley
block upwards, compressing the air above the piston and in the gap between the
two cylinders. The damping from the piston鈥檚 motion halves the impact of the
jerk.
During descent, the radar system鈥攚hich is fitted to the bottom of the
load鈥攎onitors the distance to the ground and relays this information to a
microcomputer. At a critical point just before impact, the microcomputer
triggers a solenoid valve, which releases the compressed air back into the inner
chamber. This drives the piston to the bottom of the cylinder again and draws
over 2 metres of cable back into the cylinder, pulling the load upwards relative
to the canopy. The process is timed to draw the load up at the same speed at
which the load is descending, momentarily stopping it a few centimetres above
the ground.
Rosato and McGrath have tested the system, dropping loads off a 250-metre
tower in Fort Benning, Georgia. While a standard payload received a shock of
almost 30 g on impact (equivalent to jumping off a wall 2.5 metres high
and landing without bending your knees), the retraction mechanism reduced the
deceleration to 2.7 g. 鈥淭hat鈥檚 about equivalent to the force on your
foot when you step up onto a kerb,鈥� McGrath says. The researchers believe a
similar mechanism could reduce injuries to paratroopers.