SAND85- 1180 • Unlimited Release • U C - 13 Printed August 1985

An Introduction to Deploy able Recovery Systems

Jan Meyer

Prepared by Sandia National Laboratories Albuquerque, New Mexico 87185 and Livermore, California 94550 for the United States Department of Energy under Contract DE-AC04-76DP00789

TL752 .M49 1985

Issued by Sandia National Laboratories, operated for the United States Department of Energy by Sandia Corporation. NOTICE: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Govern­ment nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or pro­cess disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government, any agency thereof or any of their contractors or subcontractors. The views and opinions expressed herein do not necessarily state or reflect those of the United States Government, any agency thereof or any of their contractors or subcontractors.

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, HALL LIBRARY

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SAND85-1180 Unlimited Release

Printed August 1985

An Introduction to Deployable Recovery Systems

Jan Meyer Parachute Systems Division

Sandia National Laboratories Albuquerque, NM 87185

Abstract This report provides an introduction to deployable recovery systems for persons with little or no background in parachutes but who are knowledgeable in aerodynamics. A historical review of parachute development is given along with a description of the basic components of most deployable recovery systems. Descriptions are given of the function of each component and of problems that occur if a component fails to perform adequately. Models are presented for deployable recovery systems. Possible directions for future work are suggested in the summary.

Distribution Category UC-13

3-4

Contents Introduction 7

Historical Review 7 Applications 14 Preview of Rest of Article 14

Deployable Recovery Systems 15 Deceleration Subsystem 15 Deployment Subsystem 17 Description of Deployment 19 Parachute Malfunctions 23

Development of Parachute Systems 25 Parachute Performance Modifications 26 Example of System Developmental Stages 27 Theoretical Models 28 Experimental Testing 29

Summary 30 References 31

Figures 1 Guide Surface Parachute 8 2 2-Foot Hyperflo Composite Construction for 1400°F Stagnation Temperature at Mach 4 9 3 21-Foot Diameter "Little Man" Piglet Parachute 9 4 Two Types of High Performance Round Parachutes With a "Pulled Down" Apex 10 5 Single and Double Keel Parawing Parachutes 12 6 Ram-Air Parachute 14 7 Basic Parachute Components 16 8 Static Line Deployment Initiator With a Pilot Chute Assist System 17 9 Pilot Chute Extraction and Bridle Line Extension 20

10 Bag Extraction From Payload Container 20 11 Suspension Line Unstowing 21 12 Canopy Extraction From Bag 21 13 Initial Phase of Canopy Inflation 22 14 Line Sail 25 15 Cluster of Four Parachutes 26 16 Drive Slots in the Form of Double L Modifications 27

LINDA HALL LIBRARY KANSAS CITY, MO 5-6

An Introduction to Deployable Recowery Systems

Introduction

Historical Review The first known written account of a parachute

concept is found in da Vinci's notebooks (cl495).' The sketch he drew consisted of a cloth material pulled tightly over a rigid pyramidal structure. Although da Vinci never made the device, he is given credit for the concept of lowering man to the earth safely using a maximum drag decelerator. Fauste Veranzio con­structed a device similar to da Vinci's drawing and jumped from a tower in Venice in 1617.1,2 Over a century would pass before further developments would be made by the famous balloonists, Joseph and Jacques Montgolfier.1"3 In 1783 they succeeded in lowering animals to the ground from rooftops or bal­loons. During the same year Sebastian Lenormand jumped from a tower using a 14-foot diameter para­chute. The first emergency use of a parachute was made by Jean Pierre Blanchard in 1785 after the hot-air balloon he was in exploded. Blanchard also worked on a foldable silk parachute, for until then all para­chutes were constructed with a rigid frame.3 In 1797, Andrew Garnerin made the first jump with a para­chute without a rigid frame. One of Garnerin's balloon jumps from 8000 feet, a very high altitude for the time, was observed by a French astronomer, Lalandes. As the parachute descended, severe oscillations were in­duced in the canopy. Lalandes suggested cutting a small hole near the apex of the canopy to inhibit the oscillations.1"3 This modification is now known as the vent and does indeed dramatically reduce canopy oscillations.

During the next century, parachute use was con­fined to carnivals and daredevil acts. Acrobats would perform stunts on a trapeze bar suspended from a decending parachute. The parachute was released from a hot-air balloon by attaching the top of the parachute to the equator of the balloon with a cord that broke after a person jumped from the basket. Public opinion became very unfavorable towards the use of parachutes when Robert Cocking fell to his

death in 1837.13 Cocking jumped an inverted cone-shaped parachute (point down) from 5000 ft. and distinguished himself by becoming parachuting's first fatality.

The next major contribution to parachute systems was the development of a harness by Captain Thomas Baldwin in 1887.1 The concept of folding or packing the parachute in a knapsak-like container was devel­oped by Paul Letteman and Kathchen Paulus in 1890. Kathchen Paulus also demonstrated an intentional breakaway. After a first parachute inflated, it was released and pulled open a second parachute.

The first jump from an airplane has been claimed by both Grant Morton and Captain Albert Berry in 1911 or 1912.1|2 Morton jumped with a silk parachute folded in his arms which he threw out as he left the plane. Captain Berry had a 36 ft. parachute packed into a metal case beneath the fuselage. The parachute had a trapeze bar for him to hold on to as he jumped and decended to the ground. Also in 1911, an Italian, Pino, invented the pilot chute or drogue chute.1 He attached a small parachute with a rigid frame to his helmet. The pilot chute would easily inflate, pull the helmet off and then pull the parachute out into the airstream.

The first freefall jump was made by Georgina "Tiny" Broadwick in 191413, but the military still did not believe that the human body could tolerate the experience of freefall for more than a few seconds before "blacking out." The skeptics were convinced in 1919 by Leslie Irvin and Floyd Smith. They demon­strated freefall jumps3 and developed the ripcord at the parachute testing and training center at Wright Field, established in 1918.

From World War I to the early 1930's, con­ventional round silk (now known as solid cloth) para­chutes remained unchanged in structure. They were primarily used by military air corps in Europe, Russia, and the United States.4 During the 1930's Germany's

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Luftwaffe established the essential ingredients for air supremacy. Kurt Student conceived and implemented a rapid deployment strike force by parachuting men, equipment, and weapons from gliders and aircraft, such as the Junker JU52/3m. Germany demonstrated the effectiveness of airborne troops delivered to the battle scene by parachutes during World War II.

The development of modern parachutes deployed at high speeds and high altitudes started in the 1930's. Knacke and Madelung6,6 developed the ribbon para­chute in Germany for decelerating heavy high speed payloads. After World War II Knacke invented the ring slot parachute which is used for moderate sub­sonic speeds. This parachute is used primarily for cargo delivery and aircraft deceleration. The ring slot

parachute is significantly cheaper to manufacture than the ribbon parachute. The ring sail parachute, developed by Ewing, is used to decelerate payloads at low to moderate subsonic deployments speeds. The ring sail parachute was used as the final stages of the Mercury, Gemini, and Apollo projects because of its slow inflation rate and stability. H. G. Heinrich in­vented the guide surface parachute (Figure 1) used as a pilot chute or for vehicle stabilization in the super­sonic (to Mach 3) deployment regime.7 The hyperflo parachute (Figure 2) developed by Sims,6 is used as a hypersonic pilot chute for the Mach 1 to Mach 5 speed domain. The specific construction details of these modern high speed parachutes may be found in the Recovery Systems Design Guide.6

Figure 1. Guide Surface Parachute

parachutes. Piglet parachutes are much smaller and much more reliable than solid cloth parachutes. The other three high performance rounds (Figure 4) are characterized by many modifications in the form of turning and drive slots. The top of the canopy, or apex, is pulled closer to the payload by means of additional rigging lines. These parachutes are also-known for their relatively high malfunction rate.

Figure 2. 2-Foot Hyperflo Composite Construction for 1400°F Stagnation Temperature at Mach 4

The development of sport parachutes beyond solid cloth parachutes began in the early 1960's. Solid cloth parachutes were modified with drive slots that provide greater stability and horizontal speed. A class of sport parachutes, known as high performance rounds, includes Piglets, Paracommanders, Sierras, and Papillon parachutes. Piglets (Figure 3) are similar to modified solid cloth parachutes, but are con­structed from less porous material than solid cloth

,. ^,;css»*

Figure 3. 21-Foot Diameter "Little Man" Piglet Parachute

9

(a)

Figure 4. Two Types of High Performance Round Parachutes With a "Pulled Down"

10

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f^J^***,!/ *«p"",jr

i . ' . 'S ' *" . ! • : '•'••'

Stf*

(b)

Figure 4. Concluded

During the early space projects, Rogallo devel­oped a single membrane flexible wing, known as the parawing8 (Figure 5). The large parawings designed for recovery of reentry vehicles did not have reliable opening characteristics at high speeds and were not used in the actual manned flights. There are several review articles916 describing the subsonic deployment and control of parawings. The parawing parachute, designed for maximum lift as opposed to maximum drag, was primarily used in sport parachuting during the 1970's.

By the late 1970's the parawing was replaced by the parafoil, invented in the middle 1960's by Domina Jalbert,1718 a kite maker. The parafoil or ram-air parachute (Figure 6) is a deformable airfoil that main­tains its profile by trapping air between two rectangu­larly shaped membranes, sewn together at the trailing edge and sides, but open at the leading edge. Several ribs are sewn to the inside of the upper and lower surfaces, maintaining an airfoil cross section in the spanwise direction. Stabilizers are added to prevent side slipping. Most personnel ram-air parachutes have

11

a nominal aspect ratio of two and a forward speed of 25 to 30 mph. Dynamic stalls may be performed with a ram-air so that landings are made with zero velocity. The ram-air may also be flown backwards by deflect­ing the trailing edge past the stall configuration. The parawing and parafoil are hybrids of maximum drag decelerators and rigid wing technology. The vastly

superior maneuverability of a ram-air parachute makes it one of the most promising decelerators. The deployment of ram-air parachutes at high speeds tends to degrade or destroy the parachute compo­nents. As suitable methods of reefing and staging of deployment are developed the ram-air parachute may have unlimited use.

(a)

Figure 5. (a) Single and (b) Double Keel Parawing Parachutes

12

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Figure 5. Concluded

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(b)

13

TRAILING EDGE

UPPER SU

LEADIN EDGE

RIB

Figure 6. Ram-Air Parachute

Applications Parachutes are used in a variety of ways. There

are also as many different types of parachutes as there are uses. Personnel uses include paratroop deploy­ment, sport skydiving, and military reconasissance missions. As a purely deceleration device, the para­chute is used as a braking system in car and motorcy­cle racing and also as landing brakes in a number of high performance aircraft. Parachutes also are used to stabilize aircraft and to control bomb trajectories. Low altitude delivery systems also use parachutes to slow the descent of needed supplies in military or crisis situations. Planetary exploration is also utilizing

parachutes to decelerate a data gathering probe enter­ing extraterrestrial atmospheres. The design and test stages of many weapons systems use parachutes to recover prototype vehicles.6

Preview of Rest of Article This article will continue to give a brief explana­

tion of the various components in deployable recovery systems. An explanation of the way in which para­chute systems function and how they do not function is also included so that the reader will become familiar with the processes involved in recovery. The round parachute may be an easy concept, but the parachute

14

may be severely damaged by the atmosphere during deployment. From an aerodynamic perspective, the analysis of a deploying parachute involves viscous, unsteady, turbulent, compressible wake-dominated flow over a continuously deforming geometry. The flow may also be supersonic. The porosity of the canopy further complicates the problem by allowing fluid to pass through the parachute material. After completing this article the reader should have an appreciation of the complexity of deployable recovery systems.

Deployable Recovery Systems

Deployable recovery systems consist of two major subsystems, the deployment and the recovery subsys­tems. The deployment subsystem consists of the com­ponents that are used to unpack the decelerator and allow it to inflate properly without damage. In most applications, the main decelerator cannot be deployed directly into the airstream, due to the high speed of the payload or unreliable inflation characteristics. The system must be staged using a deployment inia-tor, a device which provides for a lines first deploy­ment and a device which controls the rate of inflation. The decelerator subsystem includes a braking device and the mechanisms used to couple the payload to the decelerator. The decelerator may be a maximum drag decelerator, a soft-wing decelerator with gliding capa­bilities or an inflatable balloon decelerator. The em­phasis of this article is on a maximum drag decelera­tion device. These subsystems will be described in detail; also included is the description of the deploy­ment sequence.

Deceleration Subsystem The discussion of the deceleration subsystem in­

cludes the components specifically used to provide the required braking action for the payload, and the com­ponents used to suspend the payload and attach it to the canopy. These components are the canopy, sus­pension lines, risers, harness, and container.

Canopy Construction A conventional or round parachute is the most

widely used type of canopy. Parachutes are usually refered to by their diameters, construction or perfor­mance characteristics, such as 28 ft. flat, conical, bi-conical, hemispherical, parawing or parafoil. Even the "conventional" or round parachutes used today are not

so conventional, but they do have some basic features in common. As an introduction to these basic features, consider the most rudimentary of conventional para­chutes, the solid cloth, flat circular canopy.* When the canopy is laid out, it forms a flat circular surface. The circular surface is divided into an even number of pie-shaped sections, called gores. The seams defining the gores are called radials. A gore may be constructed out of a single piece of material or may contain several smaller sections called panels. If the gore consists of a single piece, the bias of the material is parallel or perpendicular to the central radial of the gore. This method of construction is the block or straight cut. It is an inefficient method because the strength of the canopy is at most equal to the strength of the individ­ual threads within the material. The strength of the weave does not contribute to the strength of the canopy unless the material is cut on the bias. In a bias construction the gores are divided into several panels. The bias runs at a 45-degree angle to the radial along the center of the gore. The bias is parallel among the panels within a gore, but alternate directions for adja­cent gores. The bias cut enhances the strength of the parachute because the strength of the weave is utilized.

A conical parachute may be thought of as a flat circular parachute with one or more gores removed. Parachutes may be constructed with several different sized conic sections, hence the names biconical, tri-conical, etc. Continuous ribbon parachutes are con­structed as hemispherical, biconical, etc. from 2-inch wide nylon webbing sewn circumferentially around the parachute with spacings varying from 0.5 to 2.0 inches. The strength of the nylon webbing typically changes over the canopy. Stronger webbing is required in the upper third of the canopy.

The solid cloth canopy was once constructed from silk. Nylon has replaced silk because of its strength, durability, and smaller bulk. Nylon canopies are nearly indestructable under good deployment condi­tions, but are very susceptible to ultraviolet light or exposure to salt water or chemicals.

*This is the parachute Spencer Tracy uses on the late, late show when he jumps out of a DC-3 to save stranded orphans on a mountainside in the midst of an earthquake.

15

A circular hole is cut at the center of the canopy. This vent reduces the oscillations of the payload-parachute system during steady-state descent. The top portion of the canopy is also called the apex. The canopy is further divided into two regions called the crown and skirt. The crown is the part of the inflated canopy above the maximum projected inflated diame­ter and the region below this diameter is the skirt. Figure 7 indicates the basic components of conven­tional round parachutes.

Figure 7. Basic Parachute Components

Suspension Lines The payload is attached to the parachute via

suspension lines, risers and harness. The suspension lines are most often continuous from one riser to a diametrically opposed riser. A line runs from one riser up to the skirt and then is sewn along a radial seam between gores. It then passes through the vent area and is sewn to the gore seam along the same diameter. The line continues from the skirt to an attachment on the opposite riser. Suspension lines are fabricated of nylon, Dacron, or more recently, Kevlar.

Risers The risers are the connection between the suspen­

sion lines and the harness supporting the payload. For a system with four risers, the suspension lines are divided into four groups. Each line group is then attached to the risers by a connector link or may be sewn directly to the risers. The connector link is a metal rectangle with a locking mechanism to prevent an inadvertent disconnection that allows the ends of the suspension lines to be readily slipped on or off. The other end of the risers is connected to the pay-load's harness. This connection may be permanent or it may be a quick release type.

The main canopy used by a sport parachutists has a quick release type connection so that it can be readily jettisoned if it malfunctions. The reserve canopy is permanently attached by sewing the risers directly to the main lift webbing of the harness. A quick release system is also used on payloads landing in the ocean so that the payload and parachute may be recovered separately. Parachutes are extremely cum­bersome when they become wet. A quick release sys­tem for half of the risers was used on Shuttle launch IV, but unfortunately the release system was activated during deployment and the boosters were not recov­ered. Sport parachuting has also experienced its share of premature activations and also the opposite prob­lem, a quick release that won't release. This situation increases the probability that a deploying reserve canopy may become entangled with the still connected and malfunctioned main canopy.

In a four riser system, each riser must withstand at least one fourth of any loading the payload experi­ences. A payload with some roll during peak loading will load the risers differentially. Risers must be de­signed to withstand many times the peak loads they may experience. This required safety factor makes the risers one of the bulkiest components, besides the canopy itself.

Harness The harness is the device used to keep the payload

attached to the parachute system. The risers are the actual interface between the harness and parachute. The risers may be attached to the harness in several ways, as discussed above. The harness is constructed from nylon webbing and is wrapped around the pay-load in a convenient and safe manner. The webbing which supports the majority of the load during de­ployment and steady state is called the main lift webbing. On personnel harnesses the webbing is a single loop running from the front of the right shoul­der, down the right side of the body, underneath the

16

right leg (forming a leg strap), diagonally across the back, over the left shoulder, down the left side of the body, underneath the left leg, and diagonally across the back to the right shoulder. Front and rear risers are attached to the main lift webbing at each shoulder. During deployment the leg straps and main lift web­bing over the shoulders experience and distribute the peak loads. The diagonals across the back do not experience much of the load. A chest strap is used to prevent a person from falling forward, out of the harness. Similar methods are used in other systems when the riser attachments cannot be bolted directly to the payload.

Container The container is the device used to store the

parachute canopy, suspension lines, and risers on the payload before activation of the system. The container must open readily for deployment, but care should be exercised to prevent premature activation.

Personnel parachute containers were originally constructed from a canvas material; usually a knap­sack was used. Today the container is constructed from man-made materials such as nylon, Dacron, or polyester. These materials are also lighter and more durable than canvas.

A metal casing is used as a container for sounding rockets, oglive cylinders, etc. The casing may be jetti­soned, in part or completely, during deployment and may be recovered by using a separate parachute sytem.

Deployment Subsystem The function of the deployment subsystem is to

ensure an orderly deployment of the decelerator. It consists of a deployment initiator, devices providing for a line's first deployment, and reefing devices that control the rate of inflation. The deployment subsys­tem is described in the order of deployment. This is the reverse order of the packing of the system.

Deployment Initiator The method used to initiate the deployment se­

quence greatly depends on the mission of the payload, the deployment speed, and the altitude of release. Initiation is successfully accomplished when the con­tainer is opened and the pilot chute is ejected into the airstream with a predetermined relative velocity with respect to the payload. The two crucial functions of the initiator are opening the container in which the parachute is packed and ejecting the pilot chute with the appropriate relative velocity. Static lines, velocity,

and/or altitude activated pyrotechnics or timed pyro­technics and manual methods are available to initiate deployment.

A static line is used in low altitude delivery sys­tems where the overall mission is to deliver the pay-load to the target area as quickly as possible. One end of the static line is rigidly attached to the aircraft delivering the payload and the other end acts as a fastener closing the container. As the payload falls away from the aircraft, the static line becomes taut and eventually is pulled away from the payload, open­ing the container (Figure 8). The pilot chute is pulled away from the payload by a pilot chute assist system, which is an extension from the static line that is temporarily attached to the pilot chute. The tempo­rary attachment, or breakcord, breaks when a suffi­cient load is applied to it. During the time it takes to supply this load, the pilot chute is out of the recircula­tion zone of the payload. In some static line systems, the line is directly and permanently attached to the bag. This eliminates the pilot chute from the system. In this type of system, the payload and recovery system separate completely from the aircraft after the deceleration device is extracted from the bag. The static line and bag remain attached to the aircraft. Static line systems are used in low altitude parachute delivery systems to deliver hardware, such as jeeps, howitzers, and tanks, for paratroopers and for orienta­tion jumps in sport parachuting.

Figure 8. Static Line Deployment Initiator With a Pilot Chute Assist System

17

Manual methods are available if personnel are using the deceleration system. A ripcord with a spring-loaded pilot chute consists of a cable attached between a rigid handle and metal pins which close the con­tainer. The ripcord handle is stowed in an elastic pocket on the main lift webbing, and the cable is routed through a housing, usually over the shoulder. As the ripcord handle is pulled away from the person's body, the pins are removed from the locking loops or locking cones. A compressed spring sewn inside of the pilot chute is now able to extend itself. As it does, it pushes the container flaps apart and the remaining potential energy of the compressed spring is converted to kinetic energy, resulting in a relative separation velocity between the pilot chute and payload.

There are predominately two other manual deployment initiators that use pilot chutes without springs, known as hand-deploy systems. In one method a pilot chute is stowed within the container with the parachute. As the deployment handle or pud is pulled, the container opens when a metal pin clears the closing loop and then the pilot chute is extracted from the container, away from the body. The pilot chute is vigorously thrown and released at arm's ex­tension. This method is known as the "pull out" pilot chute.

In the other hand-deploy method the pilot chute is stowed in a small pouch, external to the container with the packed parachute. The pouch may be located in a variety of places, but the most common is on the back of the right leg strap. The pilot chute is extracted from the pouch and thrown into the airstream at arm's extension. A curved metal pin is attached to the bridle line and is extracted from the closing loop, opening the container when the bridle line is fully extended. This method is known as the "throw out" pilot chute.

The two methods differ in potential malfunctions and the location of the extraction handle on the pilot chute. In the pull out method, the handle is fixed to the base of the pilot chute, allowing for pilot chute inflation prior to release, whereas in the throw out pilot chute method the deployment handle is attached to the apex of the pilot chute, completely inhibiting pilot chute inflation prior to release. The primary advantage of hand deploy methods is the ease of packing without a spring loaded pilot chute. There is no advantage from a safety standpoint. Sport para­chutists' secondary or reserve parachutes are still packed with a spring loaded pilot chute. Both hand deploy methods require a vigorous arm extension so that the pilot chute will not become trapped in the recirculation region of the person's wake. These man­ual methods may be backed up with pyrotechnic

activation devices. These methods are more com­monly used in military or space programs.

Pyrotechnic deployment initiators contain a small explosive which fires on the command of an altitude sensor, a velocity and altitude sensor, or a timing device. When the explosive fires it ejects the closing pins or bolts, permitting the container to be opened. The pilot chute may be ejected with a small booster or, as in aircraft emergency escape systems, the required separation velocity may be achieved by a spring loaded pilot chute.

Pilot Chute and Bridle Line The pilot chute is a small parachute also known as

a drogue chute. It is deployed in order to stabilize the payload if it is tumbling. If the payload is stable or only spinning, the inflated pilot chute provides the required force to extract a bag with the packed para­chute from the payload's container. The pilot chute provides a pull from above, so to speak, when the payload is very stable. Without help from the pilot chute, it would be difficult to consistently and reliably deploy a main canopy directly into the airstream, especially for stable payloads. A secondary function may be to provide a small amount of deceleration. The pilot chute also supplies tension in the suspension lines as they deploy in order to keep them straight.

The bridle line is a cord attached to the base of the pilot chute and the bag containing the packed para­chute. The bridle line may or may not be permanently attached to the main canopy. The function of the bridle line is to allow a sufficient distance between the pilot chute and the payload before extracting the main canopy from the payload. The length should allow the pilot chute to inflate downstream of the payload's recirculation zone. A bridle line that is too long may induce larger than necessary forces to open the con­tainer or pull the bag from the container, which could result in damage. A long bridle line will increase the overall deployment time, and a too short bridle line may inhibit inflation completely if the pilot chute becomes trapped in the recirculation zone. For an emergency escape system, this is a critical design factor.

Canopy Stowage The parachute canopy is stowed in a relatively

small volume before it is used. Most types of round parachutes are packed in a similar manner. The canopy must be packed so that it is free and clear of lines, pilot chute, or any other materials near the canopy. The first step in packing is to inspect the

18

canopy and lines for damage. Tension is put on the canopy by attaching the apex at one end of a packing table and the suspension lines at the other end of the packing table. The suspension lines are now checked for proper routing and are cleared of entanglements. The risers are also checked for twists ox loops through the lines. An easy technique is the 4-line check for a canopy with four risers. When the canopy is under tension the front gore rests on top of the rear gore. If these two gores are lifted slightly off the table, the four lines defining the gores are picked up so that each hand holds one line from the front gore and one line from the rear gore. These lines are the innermost lines on the connector links. Following these lines towards the risers clearly indicates if an entanglement, twist or loop exists. It is a positive indication that the lines are free and clear. Once the lines are straight and free of entanglements the canopy is ready to be folded prop­erly. The end result of flaking the canopy is that each gore is folded in half along its central radial; half of the gores are to the left of the grouped suspension lines and half of the gores are to the right. The canopy and suspension lines resemble a Christmas tree after flak­ing. The skirt hems of the gores to the right and left are now folded 45 degrees such that the skirt hems are parallel to the suspension lines. The canopy is now S-folded from the apex to the skirt into a bag or other similar device.

Suspension Line Stowage The suspension lines are stowed to provide for an

orderly extension of the lines, preventing the lines from entangling with each other and with the payload. The best systems stow the lines securely on the bag so that the lines leave the area near the payload with the bag before the lines unstow. The relative velocity between the payload and pilot chute-bag system pro­vides the necessary force to sequentially extract the lines from the stows. The last stow of lines (the suspension line closest to the skirt of the canopy) keeps the mouth of the canopy closed or the canopy in the bag, preventing inflation, until the lines are com­pletely deployed. The last stow is called the locking stow. The common technique for stowing the suspen­sion lines is to S-fold the lines back and forth across the bag. A stowing device is placed near the "fold" in the lines to hold them in place. For a comparison, the stowing device used in sport parachuting is a rubber band or thin bungee cord, whereas in most of Sandia's systems19 the stowing is done with "line ties" with a breaking strength ranging from 100 to 1500 lb.

Reefing Systems The function of a reefing system is to control and

slow down inflation. The reefing system consists of a device to restrict the amount of canopy inflation and a device to deactivate the restriction at an appropriate time. Recovery systems using large ribbon canopies have a circumferential band around the lowermost portion of the canopy, the skirt, which is a smaller than the fully inflated canopy. The effect is to tempo­rarily have a smaller canopy. Then a knife or reefing cutter is used to sever the band to allow the canopy to inflate fully.

Description of Deployment A deployment sequence consists of a series of

stages. Specific events occur during each stage, and frequently must occur sequentially to ensure that the entire system deploys properly. Each of these stages will be considered with respect to required events occuring, how the events are made to happen in a specific order, and what happens when the deploy­ment sequence leads to malfunctions, or a parachute that does not work properly.

The recovery system is assumed to contain a single conventional round parachute that performs as a maximum drag decelerator. The parachute is as­sumed to be packed in a bag for a "lines first" deploy­ment from a freely falling payload. There are no reefing systems, and the deployment is initiated by a pilot chute ejected into the airstream.

Pilot Chute Extraction and Bridle Line Extension

The first stage of deployment (Figure 9) is the pilot chute ejection from the payload and the exten­sion of the bridle line. The bridle line is a cord attached to the base of the pilot chute and the bag containing the packed parachute. The fully inflated pilot chute must provide stabilization of the payload or pull the bag from the payload, or both. The relative ejection velocity between the payload and pilot chute must be great enough to ensure that the pilot chute does not become trapped in the near field wake or recirculation zone of the payload. The pilot chute may be ejected in a transverse direction from the payload's trajectory, allowing the pilot chute to fully inflate before entering the payload's wake further down­stream. It may also be ejected directly down stream with a large enough velocity to emerge from the recir­culation zone. The bridle line must be long enough so that the pilot chute may inflate downstream of the recirculation zone.

19

PILOT CHUTE

BRIDAL LINE-

PAYLOAD'

Figure 9. Pilot Chute Extraction and Bridle Line Extension

Bag Extraction From Payload Container

The second stage of deployment (Figure 10) is the separation of the payload and the bag containing the packed parachute. In large weapon recovery systems, this stage is accomplished either by strategically plac­ing a mortar so that a metallic container containing the bag with the packed parachute is completely sheared, allowing separation, or by firing bolts which couple the payload and bag together. A relative sepa­ration velocity between the payload and metallic con­tainer with the bag is attained either by an additional mortar or by the pilot chute, which slows the bag appreciably since the payload has not slowed a signifi­cant amount, if at all. In slow speed systems, the drag from the pilot chute is adequate to extract the bag from the container.

A

DEPLOYMENT BAG

PILOT CHUTE

BRIDAL LINE

RISERS

STOWED SUSPENSION

LINES

PAYLOAD

Figure 10. Bag Extraction From Payload Container

Suspension Line Unstowing Early parachutists1,4 (cl940) discovered that the

opening forces were greatly reduced when the lines were allowed to extend fully before any air entered the canopy mouth. They devised methods of packing which lead to this type of staged deployment se­quence. The suspension lines are packed so that they will unstow before the canopy starts to inflate (Figure 11). This is known as a "lines-first" deployment se­quence. There are many devices available that are used to accomplish a lines first deployment. One of these devices is the bag, which is of concern here. Other devices include a diaper, a sleeve, or a line stow pouch on the canopy.

20

PILOT CHUTE

SUSPENSION LINES

•BRIDLE LINE

BAG

RISERS

PAYLOAD'

Figure 11. Suspension Line Unstowing

Canopy Extraction From Bag The next stage of deployment (Figure 12) is the

extraction of the canopy from the bag. The falling payload and the drag from the pilot chute must pro­vide the required forces to extract the canopy. If the pilot chute did not provide enough drag, the canopy would not be removed from the bag, resulting in total system failure.

/ f T \ \ - * — — PILOT CHUTE

CANOPY

BRIDLE LINE

BAG

SUSPENSION LINES

SERS

PAYLOAD

Figure 12. Canopy Extraction From Bag

21

Initial Phase of Canopy Inflation After the canopy is extracted from the bag and

line stretch occurs, air starts to accumulate in the upper one-third of the canopy (Figure 13). This part of the canopy must be stronger than the rest of the canopy due to the high pressures imposed during this first part of inflation. Air also flows through the vent at the apex of the canopy and through the porous canopy material. Turbulence generated by the pay-load may help or hinder the initial stage of inflation. If the wake generates a flow that tends to push the canopy closed, the inflation tends to take a longer time. This sort of problem was noticed in a GALILEO test drop.20 Although the parachute system eventually worked, it took a substantially longer time and dis­tance to inflate. This would compromise the effective­ness of the mission which is to study the Jovian atmosphere. The problem of long inflation times was avoided by redesigning the parachute system (i.e., lengthening the suspension lines) so that the para­chute would inflate in the wake further downstream. Turbulence may also help inflation, especially if a streamering condition exists. The turbulence may mo­mentarily reduce the external pressure on the canopy so that air may enter the mouth of the canopy. During this phase of inflation the pilot chute is deprived of air by the inflating main canopy. It starts to collapse and falls to the canopy surface.

Final Phase of Canopy Inflation The final phase of inflation may be considered as

a hemispherical ball of air expanding outward and downward from the apex towards the skirt of the canopy. The air fills the entire interior volume of the canopy, and the canopy and payload come to a com­mon velocity.

BAG

PILOT CHUTE

BRIDLE LINE

CANOPY

SUSPENSION LINES

PAYLOAD

Figure 13. Initial Phase of Canopy Inflation

22

Parachute Malfunctions Parachutes are designed to provide a specific de­

scent rate for the payload under particular conditions. Normally, the steady-state terminal velocity of the payload and inflated parachute system is the crucial design factor of a recovery system. The other compo­nents of the parachute system, for instance, the reef­ing devices, ensure that the recovery system is acti­vated, deployed, and inflated at the appropriate time. These components also prevent the payload and para­chute from experiencing damaging loads during de­ployment and inflation. Occasionally, a parachute, even though it is properly designed for the maximum descent rate of the payload, may not work. When a parachute does not perform properly, it is called a malfunction. Malfunctions may be grouped into two broad classes: high-speed and low-speed malfunc­tions. The names describe the performance of the malfunctioned recovery system. High-speed malfunc­tions mean that the payload descends at a rate that will severely damage or completely destroy the pay-load as it impacts the ground. Low-speed malfunc­tions imply that the performance of the parachute is degraded, but the payload will suffer only minor dam­age when it lands. Some low-speed malfunctions tend to degrade into high-speed malfunctions, such as a broken line on a ram-air canopy. This malfunction starts as a slow spiraling turn which rapidly progresses to a fast spiral which then deflates the canopy, result­ing in a streamering parachute. These malfunctions and several of the more common malfunctions are described in this section.

Total Malfunctions Malfunctions that completely inhibit deployment

are known as total malfunctions. These may be caused by a variety of reasons. The failure of a pyrotechnic device will lead to a disasterous system malfunction no matter how well the rest of the system might have had performed. In manual systems, total malfunctions occur when the ripcord or extraction handle cannot be pulled due to a bent pin or misrouting of the bride line or ripcord cable.

Pilot Chute Hesitations Pilot chute hesitations occur when the pilot chute

becomes trapped in the recirculating air in the wake. Pilot chute hesitations are caused by an insufficient ejection speed and/or incorrect ejection direction. They may also be caused by a bridle line which is too short to allow the pilot chute to fully inflate down­

stream of the recirculation zone. This problem inhib­its deployment completely unless pyrotechnic devices are used in subsequent stages. However, even in these cases the recovery system may not function properly if the stability or descent rate of the payload exceeds the deployment speed of the later stages. A pilot chute hesitation may lead to a propagation of malfunctions. The best cure for pilot chute hesitations, once they occur, is to change the attitude of the payload with roll so that the recirculation zone is sufficiently disturbed by the freestream, allowing the pilot chute to inflate and be carried downstream.

Preventing pilot chute hesitations is accom­plished by proper design and execution. The relative velocity of the pilot chute ejection and length of the bridle line in relation to the payload are the critical design factors.

Bag Lock Bag locks most often are traceable to a large

rotation or tumbling of the bag as it is extracted from the container. The tension in the bridle line and suspension lines retard the rotation of the bag as it pivots out of the container. If there is a lack of tension in these lines, the bag may rotate far enough to wrap the bridle line or suspension lines around the bag. Lack of tension in the bridle line is due to insufficient drag from the pilot chute because of under-inflation or poor design.

Bag locks may also occur if the line stows prevent the complete paying out of the suspension lines. The line stows closest to the skirt of the canopy usually lock the canopy within the bag. If the locking stows cannot be opened during deployment, then the canopy remains packed inside the bag, resulting in a high speed bag lock malfunction. Lines stows cannot be removed from the system because they provide an orderly deployment of the suspension lines and help distribute load forces over a longer time interval.

Broken Lines Broken suspension lines are a very common occur­

rence. The performance of conventional round para­chutes is not severely compromised when a few lines break. Broken lines become a high speed spinning malfunction if they occur on soft wing, high glide canopies such as the parawing and parafoil. Suspen­sion lines usually break when the loading of the lines is unequal. Whenever the payload has roll or the risers become entangled, one side of the suspension lines is loaded more than the other side, increasing the proba­bility of broken line.

23

Squldding ©r Streamering A canopy that does not inflate because the exter­

nal pressure effectively pinches off the mouth of the canopy is said to be a squidding or streamering canopy. This malfunction is common to all para­chutes, regardless of size, deployment regime, or ap­plication. Even though this is a most common occur­rence, it is one of the least understood phenomena. A streamering canopy can not be predicted a priori, and it cannot be consistently reproduced even under iden­tical test conditions and packing methods.

Inversions A common problem that occurs during the infla­

tion stage of deployment is that of an inverted canopy. An inverted canopy is one that partially or completely inflates inside-out. This happens when part of the canopy skirt is pushed under another part of the skirt and inflates. The majority of partial inversions work themselves out, leaving an inflated canopy with minor damage from friction burns. Inversions are caused by random motion, a strong cross wind, or an uneven skirt. The random motion generated in the turbulent wake of the payload may contribute to the probability of inversions. A strong cross wind may also increase the probability of an inversion. The most common cause of inversions is the fact that the skirt is uneven. This may be due to a non-zero roll angle of the payload, or an entangled riser which effectively short­ens the distance between the payload and skirt on one side of the canopy. The skirt may also become uneven if the suspension lines do not have the same length under maximum loading. Inversions most likely occur at line stretch because the mouth of the canopy has the smallest diameter at that time.

Line Twists and Line Sail Most of the problems that may occur during any

stage of deployment are due to instability of the payload. A spinning payload tends to turn with re­spect to the bag, producing line twists which hinder canopy inflation performance. Generally, line twists work their way out and the system functions properly. Systems that suffer significantly from line twists which may compromise the overall mission are para­trooper systems. If a paratrooper exiting from an aircraft at 800 ft. AGL has more than 7 line twists he will land with line twists. This increases the descent rate because the lines are effectively shortened, which decreases the inflated diameter of the canopy. The paratrooper also must dissipate the angular velocity produced by the unwinding lines. Landing with line twists is much more difficult and potentially danger­ous to the paratrooper. Removing line twists from a round canopy is accomplished by pulling the risers apart, a relatively simple task. Line twists in parawing and parafoil canopies are more difficult to remove. The jumper literally must kick his way out against the turning of the canopy.

Another phenomenon that occurs is high speed deployment systems is that the amount of line de­ployed is greater than the actual distance between the bag and payload (Figure 14). This is called line sail and is due to a strong relative cross wind. The wind may be due to the shock waves from the delivery aircraft, weather conditions, or insufficient drag from the pilot chute. Line sail depends on the pilot chute drag, number and strength of line ties and bag design.21

24

Figure 14. Line Sail

Development of Parachute Systems

Recovery of payloads has been accomplished by deploying parachutes into the atmosphere up to alti­tudes of 260,000 ft.,22 at speeds ranging from near zero to greater than the speed of sound. Payload weights may vary from less than 100 lb to 168,000 lb, the Space Shuttles' Solid Rocket Booster casing weight. Para­chute systems are also used on other planets to decel­erate data collecting probes.18,20 Originally, aircraft uses were only for emergency escape systems. As high performance aircraft were built, additional means were required for braking the aircraft during final approach and landing. Emergency escape systems have also been designed-for supersonic aircraft. The parachute system must have reliable deployment in the severe supersonic environment, but must not im­part fatal loads to the pilot. Rockets and prototype weapons that reach the upper atmosphere are also recovered by parachutes.23 The vast range of applica­tions of deceleration and recovery systems using para­chutes indicates a need for analytical design tools and experimental testing based upon the aerodynamics of parachute deployment and inflation, as well as the physical and dynamical configuration of the parachute.

Recovery systems function properly when the parachute both deploys and inflates. Deployment is the extraction of the parachute from its packed con­figuration within the payload to line stretch. Line stretch is used to denote the point when the parachute is fully extended in the airstream. Inflation is the accumulation of air within the parachute surface. The force of the air on the parachute provides the braking action on the payload. The maximum forces tolerable by the payload must not be exceeded during the deployment and inflation stages. These stages depend on the envelope of velocity, pressure, density, and temperature in which the recovery system is expected to perform. Physical characteristics, such as para­chute diameter at line stretch through full open, sus­pension line length and rigging, geometric porosity throughout the parachute surface, vents, and modifi­cations, also influence deployment and inflation.

Many studies have been conducted to determine the influence of various aspects of parachute systems on the rate of inflation and the forces imparted to the payload and parachute. A review of some of the major topics is presented in this chapter. This provides a background in the analytical and experimental contri­butions to the development of parachute deceleration and recovery systems. Included topics discuss specific parachute components which influence inflation or steady state characteristics of the parachute, exam­ples of developmental techniques for low altitude-high speed systems, theoretical and experimental studies of the deployment and inflation sequences.

25

Parachute Performance Modifications

Parachutes have been improved in many ways. Most changes in recovery systems appear to be very minor or subtle, but the resulting improved perfor­mance is significant. The vent, suggested by Lalandes,13 is typical of how such a simple modifica­tion can greatly improve the stability and overall performance of the parachute. This section describes some of these "simple" changes which dramatically effect the parachute's performance and reliability.

Apex Drogue Roberts24 shows that the tension in the suspension

lines increases when a pilot chute or drogue is perma­nently attached to the apex of the main parachute. This increases the inflation time and reduces the forces on the parachute and payload. The inflation rate is inhibited by the inward radial component of the additional tension in the suspension lines induced by the pilot chute. The longer inflation time reduces the maximum forces inparted to the payload and parachute. The weight and bulk of the system are reduced as the strength requirements decrease. Trade-offs are made between the smaller parachute and the increased deployment distance due to the slower inflation rate.

Line Ties Line ties have been used by early parachutists

since 1940.M The ties provide for an orderly deploy­ment of the suspension lines. They also help reduce possible entanglements between the suspension lines and payload. Experimental19 and theoretical25'26 work show that line ties can dramatically reduce opening forces.

Clusters Many heavy payloads are recovered by a group or

cluster of parachutes instead of a single large para­chute (Figure 15). The inflation time for several smaller parachutes deploying simultaneously is less than one large parachute. The altitude loss is signifi­cantly reduced during the deployment of the clustered parachutes.27 This is a critical design factor for low altitude delivery systems. If the clustered parachutes do not deploy in unison, the inflation time and forces may become greater than those of a single parachute. The clustered parachutes must have additional strength requirements. Clustered parachutes are also

less likely to suffer from a total system failure than is a single parachute. Clusters are cost effective because smaller parachutes are easier to manufacture, retrieve, and pack.

Figure 15. Cluster of Four Parachutes

The drag provided by the cluster of parachutes is less than the number of parachutes in the cluster times the drag of an individual parachute because of interference effects among the parachutes. The dy­namics of fully inflated parachute clusters is described by Wolf and Spahr.28 Their dynamical model includes five degrees of freedom, roll neglected, for each para­chute, six degrees of freedom for the payload, and an interference force inversely proportional to the square of the distance between the center of pressure of the parachutes. The solution to the dynamical equations is in the form of a computer generated movie and agrees with actual flight films.

26

Drive Slots Jorgensen and Cockrell29 developed an analytical

model showing that symmetrical drive slots (Figure 16) constrain the motion of a conventional parachute to the plane of symmetry with a positive angle of attack. Changes in the differential pressure distribu­tion over the parachute surface and the elimination of pressure forces over the drive slot areas make the descent much more stable for a modified parachute than for an unmodified parachute. The net velocity of a modified parachute is slightly larger than the un­modified parachute. The modified parachute per­forms better than the unmodified parachute because the drive slots give the parachute a larger horizontal velocity and smaller descent rate.

DOUBLE L MODIFICATIONS

Figure 16. Drive Slots in the Form of Double L Modifications

Flexibility From experimental wind tunnel tests Heinrich

and Hektner30 show that increasing the flexibility of a parachute decreases the inflation rate. This means a flexible parachute has a longer filling time than does a stiff parachute. As the inflation time increases, the loads decrease and are distributed over a longer time interval. The probability of squidding decreases as the flexibility increases.

Example of System Developmental Stages

The development of a system for delivery of or-danance at high speed and low altitude is an illustra­tive example of the design techniques widely used in the parachute industry. The first system, designed in 1972, consisted of a single ribbon parachute with one reefing stage that could withstand the high dynamic pressures of deployment at Mach 0.57 to 1.7.31 Sys­tems were constructed to deliver 5 to 45,000 lb pay-loads with parachute diameters ranging from 1 to 130 ft. Testing was done with rocket sleds and full flight drops. Further details of the experimental arrange­ment and measurement procedures are described by Maydew and Johnson.31 Results were repeatable for maximum snatch load, reefed stage opening shock, and second stage opening shock. The inflation time of the first stage decreases as Mach number increases and agrees with the empirical filling time theories.

The delivery system was dramatically improved by Rychnovsky32 by replacing the single ribbon para­chute by a small lifting ribbon parachute and a large parachute. The lifting ribbon parachute, strong enough to survive high dynamic pressures, lifts the payload to an altitude above the release altitude and slows the payload sufficiently to deploy the large, less strong parachute. Near the apogee of the trajectory, the ribbon parachute acts as a pilot chute for the larger parachute. The impact kinetic energy of the two parachute system was ten percent of the previous single parachute system. The payload had a near vertical landing. An analytical study of this system with nine degrees of freedom33 adequately predicts the system's velocity, axial acceleration, and dynamic pressure. The trajectory predictions are correct during the lifting process, but overpredict the range and maximum altitude of the system.

The lifting ribbon parachute was designed by wind tunnel testing. It had low porosity material over the leading edge area and slanted ribbons in the rear of the parachute to increase the porosity. A lift to drag ratio of 0.4 or greater was obtained, but was limited by the collapse of the parachute at high angles of attack. Roll control was attained by two reaction jets.

27

Further tests in 1977 were conducted in the NASA Ames 40 ft. X 80 ft. wind tunnel to study the ribbon parachute motion relative to the payload. Croll and Peterson34 used a 13 ft. constructed diameter ribbon parachute in conjunction with a 1.5 ft. diameter, 12 ft. long payload to measure roll, yaw damping, and forces imparted to the payload by the parachute as a func­tion of time. They found that high geometric porosity at the sides of the parachute increased the yaw damping.

Modifications to the lifting parachute were tested in a low speed wind tunnel, a whirl tower and free flight tests.35 Configurations tested had a pulled down apex, more slanted ribbons in the rear portion of the parachute, additional less-porous material at the lead­ing edge area, side vents, ram-air chambers at the leading edge "flow" deflectors, and removal of material at the leading edge. The promising modifications in the low speed wind tunnel tests were then tested on the whirl tower. Only the ram-air chambered para­chute was tested in free flight. The ram-air chambers separated from the parachute in the first flight test. The seams were re-enforced for subsequent tests.

The development of the low altitude high speed delivery system continued for more than 10 years and consisted of a systematic trial and error scheme for design improvements. Movies of the free flight test drops of the system are quite impressive and indicate that educated guessing may lead to excellent results.

Theoretical Models

Dynamic Stability The motion of a fully inflated parachute consists

of a steady-state vertical descent and of horizontal glide, pitch, and roll oscillations induced by payload relative motion or atmospheric turbulence. The theoretical analysis of the dynamic stability of fully inflated parachutes applies Newton's law to the trans-lational and rotational motions of the parachute-payload system. Two models36,37 assume that the pay-load and parachute are rigidly attached, that is, there is no relative motion between the parachute and pay-load. White and Wolf's36 model neglects rotation about the axis of symmetry, but it is able to predict the criterion for longitudinal and lateral stability. Tory and Ayres37 account for the symmetrical rotation and also for wind gusts. Wolf38 improves these models by accounting for the relative motion between the payload and parachute. The analysis of the nonrigid system shows that decreasing the riser length and parachute weight increases the stability of the system.

Inflation A model39 for the final stage of inflation assumes a

point mass payload with no aerodynamic forces, a common trajectory for the parachute and payload, inelastic suspension lines and canopy, and that the canopy shape is a function of the maximum radius of the parachute. The model accounts for the relative velocity between the parachute and payload. Wolf solves dimensionless forms of variable mass momen­tum equations for the parachute which are coupled to the momentum equations for the payload. The model adequately predicts the observed increase of inflation time and nondimensional forces with altitude. This is because the parachute's inertial becomes relatively larger than the surrounding atmosphere.

Roberts40 analyzes parachute inflation with con-formal transformations. The parachute's surface is represented by a parabola of revolution with a time varying focal length. In order to satisfy the tangency flow boundary condition on the surface, several sources and sinks are added to the representation along the stagnation streamline in the physical plane. The Kutta condition at the canopy skirt (leading edge of paraboloid) is satisfied by adding a starting vortex ring to the flow. Roberts transforms through four planes. This makes the problem a nonlinear second order differential equation that is solved numerically. Roberts method predicts trends found in parachute deployment adequately even though the method has severe limitations. Roberts' method is restricted to simple two-dimensional geometries due to the confor-mal transformation techniques involved, and the wake of the canopy is not modeled.

The theoretical model for parachute inflation by Klimas41"43 assumes a priori the shape of the canopy, the radial and axial velocity distributions, the axial deceleration, and the load. Klimas models the canopy surface by a spherical cap and several conical frus­tums. The ratio between the cap and frustums de­pends on the point in the inflation process. Each segment of the surface has a time dependent linear variation of a line vortex distribution. The application of a tangent flow boundary condition at one discrete point along a frustum yields the strengths of the linear distributions. The model predicts the pressure distri­butions of the inflation from a reefed condition to a fully inflated condition by integrating the pressure and matching it to a known load. The model has a good representation for the surface, but has no model for the wake of the inflating canopy.

28

Experimental Testing

Flow Visualization Klimas44 performed flowfield visualization wind

tunnel experiments to obtain velocity field data for the late stages of inflation. Small helium bubbles are introduced into the flow. These bubbles do not signifi­cantly change the oncoming flow and are readily visi­ble when illuminated with a high intensity light source. Photographs in two orthogonal directions are taken to determine the axial and radial velocity com­ponents. Velocity fields are obtained for the time at which the parachute first reaches its fully inflated steady state diameter and at a steady state condition. In a subsequent experiment45 at higher speeds, ve­locity profiles were obtained at earlier times of inflation.

The results indicate that the inflating parachute pertubations are localized to the fluid near the para­chute while the influence of the steady-state para­chute extends far from the body. The wake region behind the steady-state parachute is approximately 2.4 times as wide as the wake region behind the parachute when it first reaches its inflated diameter. Klimas also states that the steady-state motion indi­cates ring vortices parallel to the plane of the para­chute mouth and vortices in the wake region. The ring vortices are thought to be the starting vortices shed near the mouth of the parachute when the tension in the suspension lines restricts the parachute diameter. Due to the opaque nylon used to construct the para­chute, no data were obtained in the interior region of the parachute.

Stress Measurements Stresses imparted to the parachute material dur­

ing deployment determine strength requirements when the parachute is manufactured. An omega sen­sor, developed by H. G. Heinrich,46 measures stresses in elastic parachute materials. Braun and Doherr47

describe the use of the omega sensor in free flight tests of a block-constructed, flat, circular, solid cloth para­chute. Heinrich46 measures stresses in block- and bias-constructed, flat, circular, solid cloth parachutes. Heinrich and Saari48 measure stresses in block-constructed, flat, circular, solid cloth and ringslot parachutes. The stresses in the solid cloth parachute are largest at the vent region during inflation and in steady state. The radial stress is larger than the cir­cumferential stress at steady state. The ringslot para­chute has lower stresses during inflation than the solid cloth parachute. The ringslot parachute also has a

minimum circumferential stress at the middle of the canopy during inflation and at steady state. For both parachutes, the maximum force on the parachute does not occur at the same time as the maximum stresses.

Further measurements by Garrard and Konicke49

on bias-constructed, solid cloth, flat canopies indicate that the fill direction of the material experiences larger stresses than the warp direction.* This may be due to different elastic properties of the material in the warp and fill directions caused by the weaving and manufacturing processes. Konicke and Garrard60 also measure circumferential stress in block constructed flat circular ribbon parachutes. They observe a peak stress about half-way between the vent and skirt and a maximum stress at the skirt during inflation and at steady state.

Most parachute theoretical stress and tension model51"'2 require a priori knowledge of the detailed pressure distribution over the parachute surface in order to obtain results. Whenever possible, experi­mental pressure distribution data are used as input to theoretical stress calculations.51 Comparisons between experimental and theoretical stress calculations are given by Garrard and Muramoto.53 Some discepancies are due to pressure and stress measurements per­formed under different conditions. Theoretical work by Asfour,54 confirmed experimentally by Heinrich and Saari,48 indicates that the local velocity and aero­dynamic forces at the parachute surface are required to predict peak stresses in the parachute material.

Wind Tunnel Tests Experimental work on parachute inflation

conducted in wind tunnels is limited in its capability to accurately simulate actual drop tests. The small scale parachutes used in wind tunnel testing have a higher relative stiffness than full scale parachutes. This significantly changes the velocity flowfield and pressure distributions.30 It is also questionable if dy­namic simulation is applicable, and it is not clear what conditions must be met for parachute inflation. The most severe limitation is the changing relative wind during actual parachute inflation which cannot be adequately simulated in a wind tunnel.

*The fill and warp directions refer to the threads running perpendicular and parallel, respectively, to the edge of the cloth.

29

Summary Early parachute experimenters deployed para­

chutes from hot air balloons by attaching the apex of the canopy to the equator of the balloon with a breakable cord. The canopy suspension lines were fully extended as the payload dropped from the bal­loon's basket. The deployment sequence was simply the accumulation of air mass inside the canopy, known as inflation. The design of the parachute de­pended on the payload's weight and maximum allowa­ble descent rate. There was no real need to consider the forces during the inflation process in detail be­cause the forces were only slightly larger than the payload's weight.

In most of today's systems, recovery or decelera­tion occurs near the final stages in the overall mission of the payload. In order for the recovery system not to interfere with previous mission tasks, it is necessary to pack it within a relatively small volume on the pay-load, until its use is required. Even if there are no previous tasks to be performed by the payload, the recovery system must be packed to guard against entanglement of the deceleration device with the air­craft dropping the payload. Obviously, in order to recover the payload, the recovery system must be eventually unpacked or deployed into the airstream. The recovery of an undamaged payload is the result of successful packing and deployment of the decelera­tion device.

Todays' designers of recovery systems must consider the payload's weight, maximum allowable descent rate, and the payload's attitude, stability, velocity, altitude and forces the payload and decelera­tion system can tolerate during deployment. The ideal analytical design tool for deployable recovery systems would use only system specifications to determine an optimal parachute system. The major problem in de­veloping such a design tool is predicting the local velocity and differential pressure distribution over the deploying and inflating parachute surface.

Existing deployment and inflation models, such as the one by McVey and Wolf,55 calculate net forces imparted to the payload and parachute, but do not predict a detailed pressure distribution over the para­chute surface. Predicting detailed pressure distribu­

tions and local velocities at the parachute surface requires prediction of the unsteady, rotational fluid flowfield. Purvis66 models an arbitary, time-varying parachute surface and the interaction of the fluid dynamics and the motion of the payload. The irrota-tional flow over an unsteady geometry, viscous effects at the parachute surface and the wake and its effect on the canopy also need to be modeled in order to effec­tively determine the unsteady flowfield. The local pressure distribution during inflation is also re­quired48,64 since it affects the peak stresses in the parachute material. The wake must be simulated be­cause it effectively determines the drag of the para­chute and may severely compromise the performance of some systems.57

Future efforts in developing analytical models of parachute deployment should concentrate on the fluid dynamics. Analytical studies on parachute systems are following the steps of aircraft design. Early air­plane progress was based on the intuition of the backyard pilot; later stages used mathematical approaches with many simplifing assumptions. Re­cently, huge computer codes have been developed. These approaches are based on solving simplified versions of the Navier-Stokes' equation for flow over an aircraft. Obviously, the fluid dynamical approach has been very productive; consider the many different kinds of aircraft in use today.

New deployable recovery systems being developed today are at the limits of current design techniques. Full scale test drops are becoming so expensive that "design by testing methods" are no longer feasible. More and more of the parachute design, right down to the stitching patterns, must be done analytically. Re­cent approaches4043 have considered parachute de­ployment as a potential flow problem, and others56

have coupled the fluid dynamics to the trajectory equations of the payload-parachute system.

Future analytical parachute models may be com­parable to today's aircraft design models. The device that may force this to happen is the ram-air para­chute. The ram-air is a mixture of parachute technol­ogy and of rigid wing technology. The merging of these technologies has produced a working, deformable and deployable airfoil. It seems natural that the analytical tools used in the two technologies should also merge.

30

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BF. M. Rogallo, Flexible Wings, Astronautics and Aero­nautics, 8; 6, Aug 1968, pp 50-54.

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10W. C. Sleeman, Jr. and T G. Gainer, Status of Re­search of Parawing Lifting Decelerators, Journal of Air­craft, 6; 5, Sept/Oct 1969, pp 405-409.

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16P. M. Kenner, Structural Analysis of a Parawing During Deployment, Journal of Aircraft, 9; 2, Feb 1972, pp 168-177.

17C. F. Knapp and W. R. Barton, Controlled Recovery of Payloads at Large Glide Distances, Using a Parafoil, Jour­nal of Aircraft, 5; 2, Mar/Apr 1968, pp 112-118.

18J. D. Nicolaides, R. J. Speelman III, and G. C. L. Menard, A Review of Parafoil Applications, Journal of Aircraft, 7; 5, Sept/Oct 1970, pp 423-431.

19W. B. Pepper and D. C. Cronin, Experimental Re­search on Parachute Deployment Load Control by the Use of Line Ties, Journal of Aircraft, 6; 1, Jan 1969, pp 79-80.

2(,B. A. Smith, Deployment Problem Forces Galileo Test Reschedule, Aviation Week and Space Technology, Sept 20, 1982, pp 106-107.

21 J. W. Purvis, Prediction of Parachute Line Sail During Lines-First Deployment, Journal of Aircraft, 20; 11, Nov 1983, pp 940-945.

22M. N. Silbert, Deployment of a Spin Parachute in the Altitude Region of 260,000 ft., Journal of Spacecraft and Rockets, 20; 1, Jan/Feb 1983, pp 11-14.

23W. B. Pepper and F. M. Collins, Development of the ARIES Parachute System, Journal of Aircraft, 19; 7, July 1982, pp 600-601.

24B. W. Roberts, Use of an Apex Drogue as a Means for Controlling Parachute Inflation, Journal of Aircraft, 17; 5, May 1980, pp 376-378.

25K. E. French, Effects of Lines Ties on Parachute Deployment, Journal of Spacecraft, 17; 3, May/June 1980, pp 260-262.

2BE. K. Huckins III, Snatch Force During a Lines First Parachute Deployment, Journal of Spacecraft, 8; 3, March 1971, pp 298-299.

27K. H. Fu and R. Koch, Altitude Loss of Parachute Load Systems Using Clustered Parachutes, Journal of Air­craft, 14; 1, Jan 1977, pp 3-4.

28D. F. Wolf and H. R. Spahr, Parachute Cluster Dy­namic Analysis, Journal of Aircraft, 14; 4, April 1977, pp 321-322.

29D. S. Jorgensen and D. J. Cockrell, Effects of Drive Slots on Parachute Performance, Journal of Aircraft, 18; 6, June 1981, pp 501-503.

30H. G. Heinrich and T. R. Hektner, Flexibility as a Model Parachute Performance Parameter, Journal of Air­craft, 8; 9, Sept 1971, pp 704-709.

3,R. C. Maydew and D. W. Johnson, Supersonic and Transonic Deployment of Ribbon Parachutes at Low Alti­tudes, Journal of Aircraft, 9; 7, July 1972, pp 497-502.

32R. E. Rychnovsky, A Lifting Parachute for Very-Low Altitude, Very High-Speed Deliveries, Journal of Aircraft, 14; 2, Feb 1977, pp 184-187.

3;,P. R. Schatzle and W. H. Curry, Flight Simulation of a Vehicle With a Two-Stage Parachute System, Journal of Aircraft, 17; 8, Aug 1980, pp 545-546.

34R. H. Croll and C. W. Peterson, A Computer-Controlled Video Instrument for Wind Tunnel Testing of Lifting Parachutes, Journal of Aircraft, 16; 4, April 1979, pp 269-274.

35W. R. Bolton, I. T. Holt, and C. W. Peterson, Develop­ment of New Lifting-Parachute Designs with Increased Trim Angle, Journal of Aircraft, 19; 11, Nov 1982, pp 947-953.

3CF. M. White and D. F. Wolf, A Theory of Three-Dimensional Parachute Dynamic Stability, Journal of Air­craft, 5; 1, Jan/Feb 1968, pp 86-92.

37C. Tory and R. Ayres, Computer Model of a Fully Deployed Parachute, Journal of Aircraft, 14; 7, July 1977, pp 675-679.

38D. F. Wolf, Dynamic Stability of a Nonrigid Para­chute and Payload System, Journal of Aircraft, 8; 8, Aug 1971, pp 603-609.

31

39D. Wolf, A Simplified Dynamic Model of Parachute Inflation, Journal of Aircraft, 11; 1, Jan 1974, pp 28-33.

40B. W. Roberts, Aerodynamic Inflation of Shell Type Parachute Structures, Journal of Aircraft, 11; 7, July 1974, p390.

4IP. G. Klimas, Internal Parachute Flows, Journal of Aircraft, 9; 4, April 1972, pp 313-314.

42P. G. Klimas, Fluid Mass Associated with an Axisym-metric Parachute Canopy, Journal of Aircraft, 14; 6, June 1977, pp 577-580.

4 iP. G. Klimas, Inflating Parachute Canopy Differential Pressures, Journal of Aircraft, 16; 12, Dec 1979, pp 861-862.

44P. G. Klimas, Helium Bubble Survey of an Opening Parachute Flowfield, Journal of Aircraft, 10; 9, Nov 1973, pp 567-569.

45P. G. Klimas and D. F. Rogers, Helium Bubble Survey of a Parachute Opening Flowfield Using Computer Graphics Techniques, Journal of Aircraft, 14; 10, Oct 1977, p 952.

46H. G. Heinrich, Biaxial Stress Measurements on Cloth Samples and Bias-Constructed Parachute Models, Journal of Aircraft, 17; 7, July 1980, pp 487-492.

47G. Braun and K. F. Doherr, Experiments with Omega Sensors for Measuring Stress in the Flexible Material of Parachute Canopies, Journal of Aircraft, 17; 5, May 1980, pp 358-364.

48H. G. Heinrich and D. P. Saari, Parachute Canopy Stress Measurements at Steady State and During Inflation, Journal of Aircraft, 15; 8, Aug 1978, pp 534-539.

49W. L. Garrard and T. A. Konicke, Stress Measure­ments in Bias-Constructed Parachute Canopies During In­flation and at Steady State, Journal of Aircraft, 18; 10, Oct 1981, pp 881-886.

60T. A. Konicke and W. L. Garrard, Stress Measure­ments in a Ribbon Parachute Canopy, Journal of Aircraft, 19; 7, July 1982, pp 598-600.

61H. G. Heinrich and L. P. Jamison, Parachute Stress Analysis During Inflation and at Steady State, Journal of Aircraft, 3; 1, Jan/Feb 1966, pp 52-58.

62W. D. Sundberg, Finite Element Modeling of Para­chute Deployment and Inflation, AIAA paper No. 75-1380, Nov 1975.

5;iW. L. Garrard and K. K. Muramoto, Calculated and Experimental Stress Distributions in a Ribbon Parachute Canopy, Journal of Aircraft, 19; 12, Dec 1982, pp 1095-1097.

54K. J. Asfour, Analysis of Dynamic Stress in an Inflat­ing Parachute, Journal of Aircraft, 45, Sept/Oct 1967, pp 429-434.

66D. F. McVey and D. F. Wolf, Analysis of Deployment and Inflation of Large Ribbon Parachutes, Journal of Air­craft, 11; 2, Feb 1974, pp 96-103.

66J. W. Purvis, Theoretical Analysis of Parachute In-flaction Including Fluid Kinetics, Journal of Aircraft, 19; 4, April 1982, pp 290-296.

67H. R. Spahr and D. F. Wolf, Theoretical Analysis of Wake Induced Parachute Collapse, AIAA Paper No. 81-1922, Oct 1981.

32

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