2. Description of the Prior Art Many thousands of heavyweight and medium-weight airdrops are conducted each month. Airdropping heavyweight and medium-weight runway improvement equipment onto damaged or unimproved runway sites is needed. Presently, the survival rate of the airdropped equipment is low, and the cost of strengthening the equipment for airdrop service is very high.

In an attempt to overcome these disadvantages, rocket deceleration systems have been developed. In these conventional systems, downward firing solid rocket motors are fixed to the bottom of the airdropped equipment. As the parachute payload approaches the ground, the solid rocket motors are fired creating intense heat, noise, and mass flow under the payload. The creation of these elements is dangerous, noisy, and expensive.

Accordingly, there exists a need for an active parachute riser system which activates as the payload approaches the ground. Additionally, a need exists for an active parachute riser system which increases the survival rate of airdropped equipment.

Furthermore, there exists a need for an active parachute riser system which contracts at controlled positions near or proximate to the ground.

SUMMARY The present invention is a parachute riser system for decreasing the velocity of a payload attached to a parachute. The parachute riser system comprises a pressure vessel mounted between the payload and the parachute. A piston is slidably mounted within the

pressure vessel with the piston secured to the payload. A gas generation mechanism generates a charge and moving the piston in a general direction toward the parachute.

In addition, the present invention includes a reusable deceleration device for decreasing the horizontal and vertical velocities of a payload. The payload is attached to a parachute and moves in a general direction toward the ground. The device comprises piston means for moving the payload in a general direction away from the ground and toward the parachute and activation means for sensing the position of the payload relative to the ground and activating the piston means upon the payload achieving a predetermined distance from the ground.

The present invention further includes a method for decreasing the velocity of a payload attached to a parachute. The method comprises moving the payload in a general direction away from the ground and toward the parachute, sensing the position of the payload relative to the ground, and activating the piston means upon the payload achieving a predetermined distance from the ground.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an elevational side view illustrating the active parachute riser system, constructed in accordance with the present invention, with the active parachute riser system disconnected from the payload ; FIG. 2 is an elevational side view illustrating the active parachute riser system, constructed in accordance with the present invention, with the active parachute riser system activated as the payload approaches the ground; FIG. 3a is another elevational side view illustrating the active parachute riser system, constructed in accordance with the present invention, with the active parachute riser system being in the extended position prior to payload reaching the ground; FIG. 3b is another elevational side view illustrating the active parachute riser system, constructed in accordance with the present invention, with the active parachute riser system being in the retracted position as the payload reaches and contacts the ground;

FIG. 4 is a perspective view illustrating the active parachute riser system, constructed in accordance with the present invention, with the active parachute riser system being in the extended position prior to payload reaching the ground; FIG. 5 is a cut-away view illustrating the active parachute riser system, constructed in accordance with the present invention, with the piston and the gas generation charges; FIGS. 6a and 6b are graphs illustrating prototype parachute/active riser experimental data: (a) digitized video data and (b) vertical velocity data; FIG. 7 is a graph illustrating instrumented prototype parachute test results with the approximate point in time when the active riser was triggered is given a t = 0 and force and altitude data were obtained from a load cell mounted on the top of the active riser and an encoder measuring the displacement of the same attachment point; and FIG. 8 is a perspective view illustrating three active risers attached through a frame above the payload so that cooperative riser actuation results in payload attitude, vertical velocity, and horizontal velocity correction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As illustrated in FIGS. 1-5, the present invention is an active parachute riser system, indicated generally at 10, for improving the survival rate of airdropped equipment. The active parachute riser system 10 of the present invention meets the pressing needs of many organizations for improved medium weight equipment airdrop capability. Presently, the survival rate of airdropped equipment is low, and the cost of strengthening the equipment for airdrop service is very high. The purpose of the present active parachute riser system 10 of the present invention is to cost-effectively reduce the impact severity and tipping tendency of airdropped equipment so as to increase survival rates and reduce the need for costly equipment modification.

As illustrated in FIGS. 1 and 2, the active parachute riser system 10 of the present invention includes one or more gas powered mechanical elements 12 placed between parachute risers 14 and a payload 16. The gas powered mechanical elements 12 contract at controlled positions near or proximate to the ground. The contraction of the active

parachute riser system 10 reduces the vertical and horizontal velocity of the payload 16 and corrects the payload orientation with respect to the ground.

FIGS. 3a, 3b, and 4 illustrate the operation of the active parachute riser system 10 of the present invention with one or more high pressure gas generator powered pressure vessel elements 12 connected between the parachute risers 14 and the payload 16.

Furthermore, FIGS. 3a, 3b, and 4 illustrate a comparison between the active parachute riser system 1 0 in the extended position and in the retracted position. A ground proximity system 18 fires gas generator charges 20 to pressurize the volume under the traveling piston 22 driving the piston 22 upwards. This results in reduced riser length and a deceleration of the parachute payload 16 near the ground thus reducing the impact velocity of the payload 16.

As previously stated, a ground proximity system 18 fires gas generator charges to pressurize the volume under the traveling piston 22 driving the piston upwards. This results in reduced riser length and a deceleration of the parachute payload 16 near the ground thus reducing the impact velocity of the payload 16.

FIG. 5 further illustrates the components of the active parachute riser system 10 of the present invention. The pressure vessel 12 houses a traveling piston 22 and gas generation charges 20. The pressure vessel 12 is preferably a low cost filament wound glass fiber composite structure although other pressure vessels 12 are within the scope of the present invention. The piston 22 and tensile rod 24 is preferably constructed from high strength steel although constructing the piston 22 and tensile rod 24 from other materials is within the scope of the present invention.

The gas generation charges 20 are preferably constructed from solid rocket fuel similar to (but significantly larger than) that of an automotive airbag charge. The ignition of the gas generation charges 20 pressurizes the volume below the piston 22, forcing it upward. It is therefore necessary that an adequate pressure seal exists between the outside of the piston 22 and the inside radius of the pressure vessel 12. Ground sensing can be accomplished by a number of methods depending on the drop terrain. The preferred ground sensor 18 would be a reusable pulse echo ultrasonic sensor.

As stated above, the active parachute riser system 10 of the present invention includes gas generator charges 20 to power the internal piston 22 for decelerating a parachute payload 16. The operation of the system 10 would be ground proximity controlled in such a way as to optimally reduce vertical and horizontal impact velocities and payload impact attitude. This would be accomplished by a simple, lightweight, reusable system 10 compatible with existing airdrop equipment.

Experiment A prototype active riser system has been designed and built. The prototype system consisted of a 2-meter diameter parachute and the active riser. The energy source for the riser contraction effort is a compressed spring, of known properties. The riser activation was accomplished remotely after the parachute had deployed and was descending at a constant rate. The tests were performed in a university athletic indoor sports arena, under controlled conditions. Sequential digital video records of the parachute's descent coordinates were digitized. Basic position and (inferred) velocity data are summarized in FIG. 6a and 6b.

In this exploratory experiment, the descent velocity of the payload was reduced by more than 50% by the dynamic spring retraction (from approximately 4 m/s to 1.5 m/s).

The deceleration was of the order of 1-g (the design objective for this prototype). The parachute was observed to maintain its deployment (i. e. , the parachute geometry remained essentially the same during the riser deployment) and was not significantly accelerated downward during this deceleration loading. The large"virtual mass"of the parachute canopy during this dynamic loading exhibited the expected high inertia behavior, where essentially all of the vertical displacement due to the contraction of the active riser was in the deceleration of the payload. This virtual mass effect may be represented in an aerodynamic drag equation as an added force term: FDRAG=FSTEADY+FVIRTUALMASS=CD1/2#V2Aprojected+##VIRTUALdV/dt , where CD is the steady drag coefficient, p is the air density, V is the descent velocity, Projected is the horizontally projected parachute area and VIRTUAL is the effective volume

of displaced air during the acceleration, dV/dt. Such a calculation has been performed, based on the instrumented drop test data shown in FIG. 7, where there is excellent agreement between the measured and predicted dynamic force that was applied to dynamically lifting the payload. This data was obtained from a force transducer located at the top of the active riser and displacement data from an encoder line attached at the same location.

FIG. 8 illustrates a conventional heavy parachute drop with several independent parachutes attached to the payload 16. Each of the parachutes attach to a single central attachment point. The terminal riser strap from each of the parachutes points in different, well-spaced directions.

We will modify the present conventional geometry so that each of the parachutes attach through active risers to a triangular attachment yolk located just above the payload.

A microprocessor based cooperative intelligent sensing and control system will simultaneously minimize ground attitude and velocity errors during payload touchdown.

This effort is very important as current heavy construction equipment drops often result in vehicle damage, excessive ground impact bounce, and frequent roll-overs.

Each of the three active risers will simultaneously influence payload touchdown attitude, vertical velocity, and horizontal velocity. A payload active sonic sensor package (preferably reusable) will measure realtime altitudes and vertical velocities of three widely spaced ground contact points. An onboard GPS sensor will measure horizontal velocity while an on-board digital compass will orient the payload and active riser attachment points to the horizontal drift measurement.

The features of the present invention are an advantage over existing systems where downward firing solid rocket motors are fixed to the bottom of heavy equipment such as tanks. With the conventional system, as the parachute payload approaches the ground, the solid rocket motors are fired creating intense heat, noise and mass flow under the payload. This makes the present conventional system dangerous, noisy, and expensive.

A significant advantage of the active parachute riser system of the present invention is that all combustion processes are performed inside the pressure vessel thereby strongly reducing noise and the risk of fire. The system is reusable in that a two- person crew will be able to pick up individual active risers and return them for the installation of new gas generation charges and re-rigging onto a payload. These features are an advantage over an existing system where downward firing solid rocket motors are fixed to the bottom of heavy equipment such as tanks. As the parachute payload approaches the ground the solid rocket motors are fired creating intense heat, noise and mass flow under the payload. This makes the present systems dangerous, noisy, and expensive.

In closing, adding intelligence to a parachute landing system will significantly increase the success rate of airdropping heavy equipment. It will also significantly expand the ground terrain-wind condition envelope available for successful airdrops. The cost of the system will be more than paid for by eliminating the current need for expensive equipment modification and by reducing equipment losses through landing accidents. Ultimately, the user will have quicker more reliable field repair of forward logistic airfields and will, therefore, expedite the flow of men and material.

The foregoing exemplary descriptions and the illustrative preferred embodiments of the present invention have been explained in the drawings and described in detail, with varying modifications and alternative embodiments being taught. While the invention has been so shown, described and illustrated, it should be understood by those skilled in the art that equivalent changes in form and detail may be made therein without departing from the true spirit and scope of the invention, and that the scope of the present invention is to be limited only to the claims except as precluded by the prior art. Moreover, the invention as disclosed herein, may be suitably practiced in the absence of the specific elements which are disclosed herein.