PMA -- Pneumatic Muscle Soft Landing Actuator
Parachute retraction soft landing
is a subject of current study at the Mobility Directorate, SSCOM NRDEC where
both analysis and actual demonstrations have shown the potential of this
concept.
Soft landing of wheeled vehicles, such as the HMMWV, has high potential payoff
with the elimination of paper honeycomb and the ability to immediately drive
the vehicle.
Newly developed flexible composite technology makes compact and highly
efficient long-stroke actuators for parachute retraction possible.
These actuators are slender flexible tubes with means for mechanical attachment
on each end and a pneumatic fitting at one end. Prior to pressurization,
the actuator is flexible and can be packed for deployment with the parachute.
When pressure is applied through the end fitting, the actuator contracts
strongly, providing a force useful for a variety of applications.
The name "Pneumatic Muscle Actuator" (PMA) is descriptive of this
device.
One of the most striking characteristics of the PMA is that the initial
tension provided is 25 or more times the force of a pneumatic cylinder of
the same diameter. Other characteristics include extremely light weight
and the ability to be packed into a small volume prior to inflation.
These attributes are highly suggestive of recovery systems applications.
SYSTEM DESCRIPTION
The soft landing airdrop system consists of the payload, the parachute
and the retraction device. The payload and the parachute are assumed
to be completely standard, inventory items. For the airdrop of a HMMWV
we assume a payload weight of approximately 10,000 lb. and the use of two
G-11 parachutes. (We could also consider reducing parachute size or
number, descending at a higher velocity, and relying on retraction to reduce
impact velocity to an acceptable level). However, for the purpose
of this paper, we will consider only reduction of impact velocity while
using current airdrop practices.
The retraction device consists of a braided fiber tube,
a gas barrier liner, one end closure with a mechanical load attachment, and
one end with a mechanical load attachment, a pyrotechnic gas generator and
a ground proximity sensing initiator. Each of these elements is described
below:
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Braided fiber tube. The pneumatic muscle relies on a
low initial fiber bias angle for its high contraction force and effective
long stroke. The current prototype is braided at a 15 degree initial
bias angle, and this is assumed for the analyses of this paper. The
fiber used must have high strength-to-weight (tenacity), high tensile
stiffness (modulus), high flex fatigue tolerance and high abrasion
resistance. Vectran HS with T97 sizing is the only fiber that meets
all of these requirements at this time.
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Gas barrier liner. An elastomeric film material is
formed into a tube inside the braided tube and is used to contain the
pressurizing gas. The working elongation is slightly over 200%.
Urethane films are suitable, having break elongation of well over 300%,
and have been proven so in the current prototypes when used with cold
gas. We anticipate the need for protecting the liner from direct
hot gas impingement when a pyrotechnic generator is used. One
solution is a partial length tube of Teflon through which the hot gas
flows, but which is not asked to expand with the liner.
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End with load attachment. We have demonstrated mechanical
ends with high joint efficiency. We are also able to form a soft end,
using only the parent tube fibers to form an eye, for the end that does
not include a gas passage.
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Gas generator. Using a gas generator attached directly
to one end fitting eliminates flow restrictions of the plumbing otherwise
necessary. While stored gas and cold gas pyrotechnic generators are
both suitable and available, a hot gas generator requires the minimum weight
and volume. Hot gas is suitable because the retraction occurs in less
than one second, making the pressure drop due to heat loss over time
acceptable.
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Proximity sensing initiator. An integrated sensor without
connectors, wires or external power requirements is preferred. The
accuracy must be ±1 foot or better for minimum impact velocity.
Millimeter wave sensors are now in production at low cost as warhead fuses,
and can be adapted easily to this application. They are configured to
light the gas generator by direct ignition. Either a thermal battery
or a lithium battery can be used for long storage life. Alliant
Techsystems is a leader in this area and is working with Vertigo to provide
an integrated proximity initiator for the PMA retractor.
The question of false triggering on trees, bushes and small local
features is an important one because it can cause an early retraction and
higher impact velocity. One of the easiest solutions is to use multiple
sensors looking at a number of spots on the ground and to trigger using a
simple algorithm that eliminates a single anomalous close object. The
low cost of the sensors makes multiple sensors cost effective for a high value
payload.
The trigger is best placed at the top of the PMA, which gives the sensors
the steepest look-down angles that miss the payload. The sketch left
shows a configuration with multiple sensors mounted at the top end of the
PMA retractor.
Performance Estimate
The graphs at right show performance estimates for a 14 ft long, 6.0 inch
diameter PMA retracting a 10,000 lb payload. A gas generator burn
of 6.0 lb/sec for 0.20 seconds produces a peak acceleration of 8.8 g and
a DV of 18.8 ft/sec.
With an initial descend rate of 26.7 ft/sec (under two G-11 parachutes),
descent rate is reduced to less than 10 ft/sec for a period of 0.21 seconds.
If the trigger initiates at between 3.7 and 1.9 (2.8 ± 0.9) feet above
the ground then the impact will be less than 10 ft/sec.
Estimated weight of this device is less than 15 lb, most of the weight
being in the load attach fittings, and the gas generator case. The
PMA retractor assembly will pack into and deploy from a bag approximately
6 inches in diameter and 24 inches long.
This is an example of expected performance, but no attempt has been made
to optimize the design for any particular performance goal. The PMA
concept does allow for tailoring for a wide range of applications.
Validation Testing
NRDEC sponsored prototype fabrication and testing have validated PMA performance.
The graph to the right shows the force generated in a 1.05 inch diameter,
34 inch long PMA.
Actual force generated is less than the theoretical prediction. This
is to be expected from end effects, liner elasticity and fiber friction.
The ratio of actual to predicted force is also plotted. Over the operating
range, the efficiency of the muscle is in the range 85% to 65%, with the
highest efficiency during initial contraction.
In a straight tension test to failure at zero pressure, the prototype PMA
failed at 4600 lbf, which is approximately 44% of the theoretical strength
based on yarn tenacity. This is three times the planned peak generated
force. Parachute opening loads may be higher in some applications, necessitating
a heavier braid, or a parallel primary riser.
A dynamic test was also conducted, using a 300 lb torso dummy and
a 1.05 inch diameter, 14 ft long PMA. The test setup is shown in the
photo below.
The torso dummy was lifted off the ground by rapid inflation from
a tank starting at pressures from 25 to 150 psi.
The flow into the PMA was slowed by pipe losses compared to an integrated
gas generator, but the contraction was still strong and rapid.
In the 150 psi test, the dummy was thrown nearly to the top
of the PMA. PMA tension loads were measured and are plotted in the
first graph on the right.
The second graph shows peak acceleration as a function of tank pressure.
The label at each data point shows initial and final tank pressure.
A test of the heavy lift version of the PMA was also conducted.
The 9 inch diameter muscle pulled an 11,250 payload off the ground. A video
clip of the test is available by selecting the link on the below.
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Dynamic PMA Test
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