The Past, Present, and Future of Mid-Air Retrieval

Dean S. Jorgensen*, Roy A. Haggard† and Glen J. Brown‡

Vertigo, Inc., Lake Elsinore, California 92531

Aircraft mid-air retrieval (MAR) of high-value payloads descending on round parachutes dates back more than 40 years to the mid-1960s. However, since 1989 no US military units have flown MAR missions. In 2004, NASA certified the Genesis Sample Return Capsule MAR system – featuring a parafoil instead of a round canopy – for recovery of payloads up to 425 lb. Since 2002, industry has been actively evaluating new methods of helicopter MAR that would permit the retrieval of 80% or more of the aircraft’s external payload capacity. Studies and subscale feasibility demonstrations have led investigators to the conclusion that payloads up to 22,000 lb can be retrieved in mid-air without any technological breakthroughs.

I. The Legacy of Mid-Air Retrieval – “The Past” EVERAL payload mid-air “pickup” systems were developed that held multiple engagement hooks below the aft with poles and a network of connecting lines (Fig. 1 and

2). The hooks were connected to a special constant-tension winch that controlled the peak force after engagement by paying out line at a pre-set tension, generally about 1.25 times the weight of the payload. A number of different parachute configurations have been used with some success. Larger, heavier payload systems used separate main and engagement parachutes so that the main could be released and the smaller engagement parachute wound up on the winch. The heaviest drones retrieved in this way weighed 3,000 lb and some capsules that heavy were also retrieved. Studies were done that concluded that up to 5,000 lb could be retrieved with existing winches, but operations at this weight were never attempted.

aircrS

Figure 1. CH-3 Sea King Configured for MAR.

Reproduced in Table 1 are data from the Air Force’s “Recovery Systems Design Guide”1 that summarize the various types of parachutes used for mid-air retrieval through 1978. The annular system listed as in development was later successfully used in the development testing of the Air-Launched Cruise Missile (ALCM) through 1989. Since the end of that program, no US military units have flown MAR missions.

Figure 2. C-119J MAR of Discoverer XIV.

There were difficulties with using round parachutes for MAR, particularly from the pilot’s perspective. The small, sometimes unstable, target parachute must be hit accurately just as the aircraft flies through the wake of the parachute. High relative velocity then causes flight transients upon engagement. The most advanced legacy MAR systems were developed for recovery of air-launched cruise missiles. Table 2 summarizes the characteristic of those systems.

* Director of Business Development, Associate Fellow AIAA. † CEO, Associate Fellow AIAA. ‡ President, Associate Fellow AIAA.

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18th AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar AIAA 2005-1673

Copyright © 2005 by Vertigo, Inc. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

The high pack density of both systems certainly stands out. These densities were accomplished with great effort, multiple heat cycles and X-ray inspection to assure that hardware items were not damaged in the process. In the case of the AGM-86, an additional 36 lb of structure was necessary to contain the pressure-packed parachutes.

Table 1. Comparison of Mid-Air Retrieval Parachutes Systems (from Ref. 1).

Table 2. Air-Launched Cruise Missile MAR Parachute Systems.

Payload Type AGM-86 (Boeing ALCM)

AGM-109 (Air-launched version of Tomahawk)

Recovery Weight 2,000 lb 1,400 - 2,500 lb

Main Parachute 71.6 ft Annular 71.8 ft Triconical

Parachute System Weight 86.05 lb* 83.9 lb

Pack Density 51 lb/ft3 43 lb/ft3

*Total recovery system weight = 135.5 lb

II. Current Mid-Air Retrieval Systems – “The Present”

A. Mid-Range Unmanned Air Vehicle High-glide ram-air-inflated parafoils offer advantages over round parachutes in many respects. The forward

flight characteristic of the parafoil is generally better for the pilot, allowing him to set up a stabilized approach and also providing a consistent “sight picture,” or visual cueing, necessary for a visual approach and engagement.

Vertigo introduced a parafoil-based MAR system for Teledyne Ryan Aeronautical and the US Navy in 1990 for the Mid-Range Unmanned Air Vehicle (UAV) program2 (Fig. 3). Flight recovery demonstrations with this 1,700 lb, Mach 0.9 UAV showed the advantages of the parafoil-based system over the earlier non-gliding systems then in use (e.g., ALCM). While totally successful, this system was also quite complex, using a total of seven parachutes to decelerate, recover, and finally stabilize the UAV in tow under the UH-60 Black Hawk helicopter.

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B. Genesis Sample Return Capsule Genesis was selected in October 1997 as a flight in NASA’s

Discovery program of competitively selected, low-cost solar system exploration missions with highly focused science goals. NASA’s Jet Propulsion Laboratory, Pasadena, California, manages Genesis and Lockheed Martin Astronautics, Denver, Colorado, built the spacecraft.

The Genesis spacecraft was launched on August 8, 2001, from Cape Canaveral, Florida, and was placed into orbit around L1, a point between Earth and the sun where the gravity of both bodies is balanced. Once in the L1 orbit, Genesis deployed its collector arrays and began collecting particles of the solar wind that imbedded themselves in specially designed high purity wafers. In April 2004, the sample collectors were re-stowed for a September 2004 return to Earth.

Lockheed Martin proposed parafoil MAR for the Genesis Sample Return Capsule (SRC) to protect the payload from landing impact loads, and to minimize the delivery time to the receiving facility where a clean gas purge could be applied to the sample canister. Vertigo was contracted in 1997 to adapt parafoil MAR technology to suit the Genesis capsule, and to perform a proof-of-concept demonstration. The MAR system for the 450 lb SRC would be far simpler than the Mid-Range UAV (MRUAV) MAR system, and parafoil-based MAR promised unprecedented engagement reliability with reduced training requirements. The Genesis main parafoil (Fig. 4) is comparable in size to the engagement parafoil of the MRUAV system. High-strength cordage was added to the main parafoil canopy to create a reinforced load network for the parafoil, which doubled as the engagement parachute. The system required only one other parachute, a supersonic

drogue to stabilize the capsule through the transonic regime and to deploy the main parachute. This simplicity was not without cost, however. Since the main parachute was not cut away, it remained a lightly loaded airfoil after capture, and would “fly” if towed at or above its normal flight speed. Wind gusts could make this flight mode unstable, leading to an oscillation between flight and pendular motion. Towing characteristics were investigated at various airspeeds, and in one test case, following a totally successful engagement and capture, a gust induced the parafoil to inflate and fly to such an extent that the towline unloaded momentarily, causing the hook to disengage. This led to the subsequent development of two post-capture security measures, only one of which was eventually adopted. Vertigo developed a pyrotechnically operated latching hook, which was successfully

qualified and adopted. Parafoil containment sleeves were also developed and demonstrated to allow tow speeds up to 60 kt, but the program team chose instead to have the recovery crew land and remove the parafoil prior to long-line tow of a clean payload.

Figure 3. Mid-Range UAV.

Figure 4. Engagement of Genesis Parafoil.

III. Third Generation MAR – “The Future”

C. The Concept of Third Generation MAR Vertigo has recently developed a new method of helicopter mid-air retrieval3 that permits the retrieval of 80% or

more of the aircraft’s external payload capacity. The technical approach relies on near-zero relative velocity, no winch, and the use of a parafoil instead of a round parachute. This method is referred to as “third generation MAR,”

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where round parachute MAR is first generation and parafoil MAR with a winch is second generation. Third generation MAR (3GMAR) is divided into three distinct events: the intercept, the engagement, and the pickup. 1. Intercept

In discussing target intercept, we assume the retrieval helicopter to be loitering at the edge of a safety radius. This would be several tens of miles for atmospheric reentry payloads. As the descending payload approaches, range radar will start providing a continuous stream of position information to the MAR helicopter including vectors to intercept and test vehicle altitude. Confirmation of drogue and main parafoil deployment will be provided. In the case of a parachute system malfunction, the MAR helicopter will be vectored away from the test vehicle.

The MAR helicopter will fly directly toward the payload under parafoil until visual contact is achieved. The retrieval helicopter then maneuvers into a right echelon formation with the parafoil/payload (Step 1 in Fig. 5), maintaining an altitude about 50 feet above the parafoil and about 50 feet to the left side of the parafoil centerline. This completes the intercept phase.

Figure 5. Third Generation Mid-Air Retrieval.

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4

3

2

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2. Engagement Upon completion of the intercept phase, the MAR helicopter then slides laterally to the right just past the parafoil

centerline stopping this motion once the trailing engagement line is resting against the capture hook suspension cable (Fig. 5 Step 2). The helicopter then climbs gently until the capture hook assembly has the parafoil engagement line in the mouth of one or more of its latching arms (Fig. 5 Step 3). Sensing the presence of the parafoil engagement line, it automatically latches onto the line, trapping it. This completes the engagement line capture maneuver. 3. Pickup

The parafoil is equipped with a “slider” reefing system, a reinforced rectangular piece of fabric with a large grommet installed in each corner. The slider is an aid to controlled parafoil inflation. Prior to canopy rigging, the parafoil suspension lines are divided into four groups, corresponding to each of the four quadrants of the parafoil canopy lower surface. Each line group is then passed through a slider grommet. Finally, the lines are rigged to the canopy. Prior to packing the parafoil, the slider is positioned directly beneath the parafoil canopy lower surface so that when the parafoil is deployed, the canopy is partially reefed and the lines are restricted from spreading to their fully deployed geometry. In this way, peak opening loads on the canopy are dramatically reduced. As the parafoil inflates, the lateral forces acting on the canopy force the slider to translate smoothly down the suspension lines until it hits a set of stops attached to the canopy risers.

The 3GMAR system engagement line runs from the parafoil slider up to the bottom surface of the parafoil canopy, and through a Teflon-lined channel to the top surface of the canopy. It then extends about 60 feet aft to a small drogue that keeps it tensioned. During the pickup maneuver, the helicopter then slowly climbs and accelerates past the parafoil (Step 4). This pulls the slider up and progressively deflates the parafoil until the helicopter carries the full weight of the payload (Step 5). The deflating parafoil provides excellent energy absorption during the pickup.

D. Helicopter Equipment A major benefit of the 3GMAR concept is that no modifications are required for the helicopter airframe. Prior

concepts required a reel and a hook-on-boom to be mounted to the airframe. Parts of those modifications are external to the airframe and so require FAA involvement to grant an Experimental category classification, or the

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expensive and time-consuming procedure of recertifying the airframe as airworthy with the modifications. 3GMAR helicopter equipment is entirely suspended from the standard helicopter cargo hook and so requires no FAA involvement. This is possible because 3GMAR engages a trailing engagement line instead of the parafoil itself. This in turn allows for engagements with very low helicopter/parafoil relative speeds and the use of a towline and grappling hook instead of a hook-on-boom. The low speed engagements greatly reduce the transient engagement forces, eliminating the need for a heavy-duty reel. The reel may be eliminated altogether or a small over-force-protection reel may be suspended from the helicopter cargo hook, depending on the results of future analyses and testing. A prototype grappling hook was fabricated for use in flight tests to demonstrate the 3GMAR concept. The hook is shown in Fig. 6. The hook arms, in the open position, guide the engagement line to the bottom of the maw of the hook. The hook arms then close, trapping the line, but leaving a slot with enough space to allow the line to slide through the maw. When a stop on the engagement line pilot chute reaches the hook, it cannot pass through the hook slot. At this point the helicopter can begin climbing and transferring payload weight from the parafoil to the helicopter.

Figure 6. 3GMAR Grappling Hook.

The hook is actuated by a servo in the box below the arms. The servo is commanded by radio control by a spotter on the helicopter. In the future, the hook will be revised to incorporate a sensor in the maw to detect the engagement line and then capture the line automatically.

The ultimate hook design will likely take one of two forms. A hook could be opened and closed for training exercises where a payload is captured, elevated to a higher altitude, released, and then reengaged multiple times for cost effective training. The operational hook will likely close only, in order to preclude the possibility of an inadvertent opening.

E. 3GMAR Proof-of-Principle Demonstration In January 2003, Lockheed Martin Space Systems Company contracted with Vertigo to demonstrate the 3GMAR

concept. Tests were performed at Skylark Airport in Lake Elsinore, California. An Aerospatiale Astar 350 helicopter was configured with its doors removed, an anemometer installed on a skid, support hardware in the cockpit, and rear

Figure 7. 3GMAR Demonstration.

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jump seats stowed. The 3GMAR hook cable was attached to the sling hook with a ⅝-inch rapide link. For a secondary attachment to the helicopter, the rapide link was attached with Spectra webbing to a hard point in the aircraft. The secondary attachment could then be manually cut should release of the tow cable become necessary.

The demonstration test configuration was a 260-lb paratrooper (all-up weight) jumping from a De Havilland DHC-6 Twin Otter and descending on a 140-ft2 parafoil, as shown in the photo sequence in Fig. 7. With the helicopter loitering near the drop zone, the helicopter crew first made visual contact as the parafoil descended. Proper main parafoil and trailing drogue deployment were confirmed and the helicopter was maneuvered into a left echelon (helicopter behind and to the left) formation with the test jumper, maintaining an altitude about 40 feet above the parafoil and about 50 feet to the left side of the parafoil centerline. The test jumper maintained a straight flight line.

The helicopter slipped laterally to the right past the parafoil centerline, stopping this motion once the trailing engagement line was resting against the capture hook suspension cable. In this flight configuration, the helicopter was laterally located between the half and full canopy semi-span. The helicopter then climbed gently until the capture hook assembly had the parafoil engagement line in the maw of one or more of its latching arms between the deployment bag and the pilot chute. Upon a visual confirmation of the engagement, the hook was activated by radio control, trapping the line. The helicopter then ascended vertically, centering over the parafoil. This pulled the slider up, progressively deflating the parafoil until the helicopter carried the full weight of the payload. The jumper’s total velocity prior to engagement was estimated to be around 30 knots.

The photo sequence in Fig. 7 shows Steps 1 (intercept), 3 (engagement), and 5 (pickup) as defined in Fig. 5. In the left and center pictures the slider is clearly visible immediately above the jumper’s head. 1. First Test

In the first demonstration test, the jumper exited the Otter at 12,000 feet AGL while the helicopter loitered at 9,000 feet AGL. The helicopter crew confirmed the open canopy and proceeded to the intercept. Three practice engagement approaches were flown with the hook in the closed position. The fourth engagement maneuver was flown with the hook in the open position and resulted in the mid air retrieval of the test jumper, Scott Smith. The capture and pickup maneuver and parafoil deflation went as planned.

The parafoil showed no tendency to reinflate. The pilot, Cliff Fleming of South Coast Helicopter, noticed no moments transferred to the helicopter during the pickup maneuver. In contrast, during Genesis MAR, the nose-down pitching moment transferred to the helicopter is very noticeable, the main cause being the post-capture reinflation of the parafoil. This is completely eliminated using the 3GMAR method. After capture, the helicopter pilot ferried the test jumper at airspeeds up to approximately 45 mph. 2. Second Test

Prior to the second test, the decision was taken to shorten the 3GMAR hook cable by 25 feet in order to make the engagement maneuver easier to fly. As in the first test, the jumper exited the Otter at 12,000 feet AGL while the helicopter loitered at 9,000 feet AGL. Upon completion of the intercept maneuver, the second MAR was accomplished with less than 1,000 feet of altitude loss from the point of parafoil opening.

F. Payload Tracking During the 3GMAR demonstration flight tests, target payload tracking was visual. Operationally, the payload

typically requires acquisition tens of thousands of feet above the altitude at which the engagement maneuver is initiated. 1. Genesis SRC Tracking

Genesis reentry was targeted to the Utah Test and Training Range (UTTR), located on the Great Salt Lake Desert in western Utah. UTTR is a Major Range and Test Facility Base operated by the Department of Defense. It was created in 1979 for cruise missile testing by combining pre-existing operations of the Hill AFB, Wendover, and Dugway ranges. During the course of the cruise missile testing, it was the site of the mid-air retrievals in Table 2. The nature of UTTR’s mission has required it to become one of the most extensively instrumented test sites in the world. Primary Genesis tracking was assigned to the two High Accuracy Digital Instrumentation Radars (HADIRs) that blanket the range airspace from opposite sides. The SRC contained no transponder, making it dependent on skin tracking. Numerous optical trackers are networked with the radars, and Forward Looking Infrared (FLIR) is available at many sites. For a reentry body experiencing aerodynamic heating, FLIR was expected to make the first contact and feed data to the radars to speed acquisition. In practice, the SRC was spotted on IR at 300,000 ft altitude, before peak heating occurred.

Backup tracking was provided in the form of an onboard GPS receiver feeding position data to the ground via a digital communications network linked to the mission control center. Due to the need to protect the antenna during

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entry heating, this system was not active until main parachute deployment, so it would provide less time for rendezvous and intercept than the nominal tracking. 2. Alternate Payload Tracking Method

As the payload descends it may pass under the horizon of ground-based radar sites. For retrieval over open water, ship-based radars may be available, but an alternative is Automatic Dependent Surveillance (ADS) for which two protocols have been implemented, ADS-B and ADS-C*. ADS is a method for using aircraft-provided GPS data, transmitted over a Mode S data link and shared over a network, to derive tracking information and displays of tracking data on multiple targets. More information is needed to understand the costs of full implementation of a stand-alone tracking system, but implementation on the payload may be as simple as installing a Mode S transponder.

The lists below summarize the differences between ADS-C and ADS-B. It appears that ADS B is more suitable for stand-alone (non-ATC) systems and would support air-to-air operation.

ADS-Contract • Requires a point-to-point data link service • Data link connections must be initiated before ADS-C operation can begin • Transmitted reports are acknowledged • The recipient can specify which information he needs • Compatible with existing communication infrastructures (Inmarsat, SITA, ARINC, etc.) • Existing standards allow only for air/ground operation

ADS-Broadcast • Requires a broadcast data link service • Establishment of data link connections is not required • Transmitted reports are not acknowledged • Air-to-air operation is allowed • Incompatible with ATN and ACARS systems

G. Parafoil MAR Reliability To date, Vertigo has performed over 34 MAR training missions and 22 end-to-end parafoil MAR missions

(culminating in pick-up), all successfully. Seven Genesis retrievals were beyond visual range, demonstrating the ground controlled intercept. As a component of helicopter pilot training, dozens of “bare pole” strikes – actual parafoil intercepts flown without an engagement hook – have been flown. Table 3 summarizes the history of actual pick-ups.

This reflects the inherent reliability of both second generation and third generation parafoil MAR. However, what is not reflected in the numbers is the ease with which helicopter pilots adapt to the maneuver. There are no active MAR pilots – fixed wing or helicopter – from the 1960s, ’70s or ’80s. Today’s helicopter MAR pilots have been recruited primarily from the logging industry where they have developed relevant long-line towing experience, and from the movie industry where helicopter stunt pilots routinely demonstrate extremely high levels of flying skill. What stands out unequivocally is that all of these pilots, after only one day of training, were able to successfully fly a MAR mission on the second day and achieve a successful engagement on their first pass.

Table 3. Summary of All Parafoil Mid-Air Retrievals to Date.

Program

Payload Weight (lb)

Year

Mid-Air Retrievals Attempts / Engagements

Teledyne Ryan Mid-Range UAV 1,700 1990 3 / 3

Genesis SRC (Phase B) 500 1998 2 / 2

Genesis SRC (Phase C/D) 500 1999 3 / 3

3GMAR Dem/Val 260 2003 2 / 2

Genesis SRC (Phase E) 500 2004 12 / 12

* Technical information on ADS-B and ADS-C was acquired from two sources, an April 4, 2002, Raytheon news release, http://www.raytheon.com/newsroom/briefs/040402b.html, and a Eurocontrol Experimental Centre ADS Studies and Trials Project report, http://www.eurocontrol.fr/projects/ads/eng/protocols_adsc.htm.

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H. Applications The most recent mission for mid-air retrieval was the Genesis Sample Return Capsule. And whereas the retrieval

mission had to be scrubbed in real time due to the failure of the spacecraft to initiate the parachute deployment sequence, the mid-air retrieval development and qualification program was an unqualified success.4 NASA, the US Department of Defense, and major aerospace companies have expressed new and renewed interest in parafoil MAR as a reliable and economical way to retrieve high-value airborne assets. 1. Scientific Payload MAR

In early 2003, NASA Wallops Flight Facility contracted with Vertigo to evaluate the feasibility of performing mid-air retrievals of two types of scientific payloads – a 1,000-lb sounding rocket payload, and a 600-lb sphere-cone reentry vehicle (RV) that was to be used in the Next Generation Launch Technology (NGLT) program.5 Investigators considered traditional, fixed-wing MAR and third generation helicopter MAR. The study concluded that 3GMAR was feasible and practical in both cases and NASA was on track to pursue a MAR development program. However, the January 2004 presidential initiative for human and robotic exploration of the Moon and Mars led to a reprioritization of federal funds, resulting in the termination of the NGLT program.

As part of this study, historical data and information on C-130 MAR installations were accessed with the assistance of Mr. Robert Veazey of ESCO Engineered Arresting Systems Corporation, who has first-hand knowledge of these legacy systems. The study concluded that re-creating old MAR equipment was theoretically possible but was not practical. Creating a modern version of this equipment was feasible, if relatively expensive. However, at least in the case of the NGLT RV, this approach proved unfeasible because the round parachute system that would be needed for this MAR method would not fit in the volume available in the test vehicle. The helicopter parafoil MAR approach was found to be feasible with respect to parachute system packing volume. However, the difficulty with helicopters becomes their more limited range, compared to fixed-wing aircraft, for most of the missions defined by Wallops Flight Facility. One viable solution was sea-based operations, which becomes feasible with Navy support, ocean barges, or NASA-operated ships.

NASA defined two trajectories of interest for the 9-ft long NGLT RV. For the “proof-of-concept mission,” the ground-track distance to the nearest population (Bermuda) was 460 nm. For the “experiment mission,” the ground-track distance to the nearest population (Azores) was 290 nm. In both cases, the hazard area is defined as a circle of 30 nm radius around the predicted impact point, outside of which the retrieval aircraft shall remain until main parachute deployment.

For the case of sounding rocket payload retrieval, NASA would launch the missions from Wallops Flight Facility in Virginia over the Atlantic Ocean, with possible intercept distances from Wallops of 70, 115, 175, and 230 nm. In all cases, the hazard area is defined as a circle of 35 nm radius around the predicted impact point. Both day and nighttime operations were envisioned.

For both payloads, a 3GMAR parachute recovery system will incorporate three parachutes: a small transonic pilot parachute for extracting a stabilization-and-deceleration drogue parachute that in turn extracts the main parafoil. The drogue parachute will serve the function of stabilizing the payload and decelerating it from transonic velocity at high altitude to a velocity and altitude suitable for parafoil deployment. Experience on a range of high performance parafoil development programs, including the X-38 Crew Return Vehicle6 and the GPADS-Light cargo delivery system7 indicates that the drogue should be sized to reduce free-stream dynamic pressure at the time of parafoil deployment to around 25 psf to minimize peak deployment loads on the parafoil. Experience also dictates that the parafoil should generally be deployed at altitudes below 30,000 ft MSL, where atmospheric density is sufficiently high to ensure reliable and consistent pressurization of the parafoil canopy.

Parafoil size is governed primarily by three system design parameters: available pack volume; resultant total system velocity, Vtot; and descent or sink rate, Vv.

The MAR engagement speed should be at least 1.2 times the helicopter translation speed to assure good helicopter towing performance. Small-to-medium helicopter translation speeds are in the 20-knot range. With a 1.2 factor, the minimum engagement speed should be 1.2 × 20 = 24 knots = 40.5 ft/s. The sink rate of the payload on parafoil should be less than the maximum helicopter sink rate, which for small-to-medium helicopters is around 2,000 ft/min or 33 ft/s.

Based on experience with the parafoil MAR of the Teledyne Ryan Mid-Range UAV and NASA Genesis spacecraft, the optimum altitude for engagement is around 10,000 ft AGL. This allows sufficient margin for multiple passes at the target (although no parafoil MAR pilot has ever failed to engage the target on his first pass).

Given the system performance specification and all of the design constraints, Vertigo was able to establish parachute recovery system (PRS) design concepts for both payloads. A notional parafoil will have the design and performance characteristics defined in Table 4.

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Table 4. Candidate Scientific Payload Parachute Recovery Systems

NGLT RV Sounding Rocket Parafoil Canopy Area 242 ft2 827 ft2

Wing Loading 2.53 lb/ft2 1.25 lb/ft2

Glide Ratio 2.0:1 2.3:1

Vtot @ 1,000 ft MSL 64.8 ft/s 46.2 ft/s

Vv @ 10,000 ft MSL 33.3 ft/s 20.9 ft/s

Parafoil Weight 10.3 lb 30.2 lb

Total PRS Weight 11.5 lb 32.0 lb

PRS Pack Volume 0.32 ft3 1.08 ft3

PRS Pack Density 35.9 lb/ft3 29.6 lb/ft3

For safety, the recovery helicopter is required to remain outside a hazard area around the predicted ballistic

impact point until main parachute deployment. The parafoil will be deployed at 25,000 ft and will fly off on a random heading at an initial horizontal airspeed of 50.5 kt for the case of the NGLT RV and 37.0 kt for the case of the sounding rocket payload. Given the fact that the flight heading of the parafoil is unpredictable, it is just as likely that the parafoil will fly away from the loitering helicopter as it will toward it. For this reason, two helicopters loitering on opposite sides of the safety circle will greatly improve mission efficacy. In addition, when operating at sea, a second helicopter will provide a real-time on-site search-and-rescue capability. 2. Atlas V Booster ATS

In late 2002, Lockheed Martin Space Systems Company and Vertigo undertook a study to extend the concept of MAR to very heavy high-value payloads.8 Work on the NASA X-38 program6 had already proven that objects in the 20,000 to 25,000-lb range could be successfully recovered by parafoil. Preliminary Lockheed studies on delivering heavy, impact-sensitive payloads from helicopter-borne carriage to awaiting ground suspension systems – thereby eliminating nearly all impact loads and forestalling contamination – had also been performed. Given the parallel advances in parafoil MAR, it could then be contemplated that mid-air retrieval of 20,000 to 25,000-lb payloads could be achieved.

The booster on the Atlas V launch vehicle has a separated weight of between 45,000 and 63,000 lb. However, more than half of this weight is the relatively low value tank elements and unburned propellants. These tank elements are not only bulky but are relatively delicate and contain potentially hazardous materials (LO2, kerosene, and destruct explosives).

The single highest value object on the Atlas launch vehicle is the booster engine. When combined with the adjacent Aft Transition Structure (ATS) and fire shield, this segment weighs approximately 22,000 lb – well within demonstrated parafoil recovery capability. In addition, this structure is relatively compact, strongly built and (relatively) readily severed from the remainder of the booster. It is certainly the strongest element of the entire launch vehicle and as such would provide optimal protection from pre-parachute deployment heating and loads. It also presents the best primary structure from which to suspend the recovered element. The configuration is such that the ATS will also provide a near-perfect location for the stowage of the recovery parachute and for direct access to load-bearing primary structure without compromising already space-critical external areas.

Helicopter MAR operations in the past have been limited to approximately 25% of the helicopter’s rated external load capacity. There are several reasons higher fractions have not been demonstrated:

• Legacy (i.e., there was no push from the user community to increase the mass fraction) • High relative velocity at engagement causes flight transients that are more easily handled with

excess lift • Existing winches were only rated for use up to 3,000 lb (the All American Engineering Model

90 winch could be extended up to 5,000 lb for helicopter use) The technical approach for retrieving 80% or more of the helicopter’s external payload capacity relies on the

differences attributable to 3GMAR: • Near-zero relative velocity • No winch • Parafoil instead of round parachute

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Retrieval mission trajectory options include starting the ATS separation and recovery sequence as early as apogee, which is at greater than 750,000 ft and Mach 5+. Another option is to delay separation until after atmospheric deceleration to subsonic airspeeds. If high altitude separation were to prove necessary, supersonic deceleration components could be added, but the description of the subsonic components in this study still applies. The MAR feasibility study was therefore confined to atmospheric ATS separation at 62,000 ft MSL and Mach 0.93, at which point the recovery system initiation occurs.

As illustrated in Fig. 8, separation of the ATS from the vehicle core triggers the deployment of a small, transonic pilot parachute that, in turn, extracts a large, round drogue parachute. After deceleration to a relatively low dynamic pressure, the drogue is released from the ATS and deploys the main parafoil. The baseline parafoil is assumed to not have active

steering, but rather to turn in a slow, gentle, constant turn when in steady-state glide. Therefore, no account has been taken of u

1) t = 0 min

space

2

143 statute miles 7.5 min

3 4

5

6

25,000 ft ASL 13.5 min

5,000 ft ASL 22.5 min

7 8

1200 nautical miles

Figure 8. Atlas V ATS Mid-Air Retrieval Scenario.

sing parafoil glide to reduce the helicopter intercept distance and time. The sequence of events is as follows: 1. Liftoff 2. First stage separation 3. ATS separation at approximately 60,000 ft MSL, triggering the release of the pilot chute. Alternately, not

shown, separation may occur in vacuum. 4. The pilot chute begins to extract the drogue. 5. The pilot strips the drogue deployment bag from the drogue canopy. 6. Full inflation is achieved after two intermediate disreefing stages. The ATS is decelerated to subsonic speed. 7. The drogue is released and the main parafoil canopy is deployed. Nine minutes later, the helicopter engages

the trailing load line. 8. The helicopter climbs, sliding a parafoil canopy slider up the suspension lines, collapsing the main parafoil

(although completely releasing the parafoil when the helicopter begins to support the ATS weight is also a viable scenario).

There are at least three helicopters in operation today with the capability to tow an external load of 22,000 lb. These include the Boeing CH-47 Chinook, the Sikorsky CH-53E Super Stallion, the Russian-built Mi-26 Halo, and related variants. The Mi-26 is oversized for the Atlas V MAR mission. The CH-53E is the best match. It has sufficient excess payload capacity to accommodate reasonable crew, fuel, and payload transient load requirements. It also has mid-air refueling capability and has been used in maritime applications.

Results of this study indicate that the concept is feasible, with the lowest risk actually associated with the design and implementation of the parachute system and parafoil engagement device. Helicopter positioning prior to rocket liftoff is a major area for follow-on study and cost optimization. Again, MAR engagement speed should be at least 1.2 times the helicopter translation speed to assure good helicopter towing performance. The MAR engagement speed should be below the helicopter’s maximum demonstrated heavy tow speed.

Large helicopter translation speeds are in the 25-knot range (±5 kt). With a 1.2 factor, the minimum engagement speed should be 1.2 × (25 + 5) = 36 kt. For the range of canopy areas considered – 5,000 to 9,500 ft2 – the slowest engagement speed is 41 kt. Therefore, all canopies in the size range of interest have ample speed margin above helicopter translation speed. Large helicopter tow speeds are well above 100 kt and calculations indicate a maximum parafoil speed of 76 kt for the fastest case considered. Therefore, all canopies in the size range of interest have

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ample speed margin below helicopter maximum tow speed. The canopy/payload glide speeds are well within the helicopter’s operational speed range. Therefore, capture speed is not a driver in selecting the canopy area.

Operationally, the helicopter will be loitering at the edge of the predicted elliptical footprint. As the descending booster section is approaching, range radar will start providing a continuous stream of position information to the MAR helicopters including vectors to intercept and booster altitude. Confirmation of drogue and main parafoil deployment and tank separation from the ATS will be provided. Range radar will also track the severed tank section and provide safe separation vectors as required. The MAR helicopter will fly directly toward the ATS under parafoil until visual contact is achieved. The helicopter then maneuvers into a right echelon formation with the parafoil/ATS, maintaining an altitude about 50 feet above the parafoil and about 50 feet to the left side of the parafoil centerline. This completes the intercept phase. As described earlier, upon completion of the intercept phase, the 3GMAR helicopter then slides laterally to the right just past the parafoil centerline stopping this motion once the trailing engagement line is resting against the capture hook suspension cable. The helicopter then climbs gently until the capture hook assembly has the parafoil engagement line in the mouth of one or more of its latching arms. Sensing the presence of the parafoil engagement line, it automatically latches onto the line trapping it. This completes the engagement line capture maneuver.

The very large parafoils demonstrated on the X-38 program – 5,500 to 7,500 ft2 – use a method of multi-stage spanwise disreefing for load management6 rather than the more conventional slider reefing. However, slider reefing systems are now being used successfully on parafoils with canopy areas up to 3,000 ft2 (Ref. 9). Plans are underway to extend their use to parafoils of the size necessary to recover the Atlas V ATS. To adapt current parafoil designs that incorporate the spanwise reefing system to very heavy 3GMAR, parafoil system rigging will require modification. The engagement line will be routed from the ATS risers up past the front of the parafoil, off the trailing edge, and back to the engagement drogue. During the mission pickup maneuver, which is flown exactly as described above, the engagement line strips off the top of the parafoil and is pulled forward of the leading edge into its release window. When the required tension load is achieved, the main parafoil is released. This design modification incorporates lessons learned from early MAR of round parachute systems with those used on the MAR system of the Teledyne Ryan Mid-Range UAV. Releasing the parafoil is the most conservative approach to this mission permitting, as it will, higher helicopter tow speeds (resulting in shorter ferry times and reduced fuel consumption) and reduced handling burden during payload transfer to a ground receiving unit and transport cradle. 3. ATREP and ATPAD

There are other applications for MAR that are now conceivable as a result of the demonstration of the 3GMAR concept and the ongoing development of slider-reefed large parafoils. For example, it is now feasible to transfer supplies from a cargo aircraft to a ship using the same helicopters currently used for Vertical Replenishment (VERTREP). This feasibility is the result of those advances in MAR techniques that provide improved safety and increased payload capacity of MAR operations. These advances also eliminate the special equipment previously necessary to configure a helicopter for MAR.

Dubbed Aerial-Transfer Replenishment, the ATREP operation starts with a cargo aircraft approaching the ship’s location at 15,000 ft MSL or higher and the retrieval helicopter airborne at 10,000 ft MSL. The cargo unit is extracted and first deploys a stabilizing drogue followed the parafoil. The retrieval helicopter engages and picks up the cargo, tows it to the ship, and places it on deck. The parafoil is recovered for future use.

A number of helicopters are suited to ATREP using 3GMAR. For light and medium-weight loads, these include the Bell TH-57 Sea Ranger, the Sikorsky SH-60 Seahawk, the Kaman SH-2G Super Seasprite, the Sikorsky MH-53E Sea Dragon, and the Boeing CH-46 Sea Knight. These helicopters are all currently in use for utility and shipboard delivery missions. As shown during the Atlas V ATS MAR study program, the Sikorsky CH-53E Super Stallion is ideally suited to heavyweight ATREP using 3GMAR. It has sufficient excess payload capacity to accommodate reasonable crew, fuel, and payload transient load requirements. It also has mid-air refueling capability and is used in maritime applications.

In common parlance, precision airdrop has become synonymous with the electromechanical autopilot flight of a controllable cargo parachute delivery system such as the round-parachute-based Affordable Guided Airdrop System (AGAS), and the parafoil-based Sherpa. In fact, ATREP is also a precision aerial delivery system capable of transferring supplies from a cargo aircraft to the ground with extreme accuracy using unmodified medium and heavy-lift helicopters. Aerial Transfer Precision Aerial Delivery (ATPAD) is similar to ATREP operation. In this case, the retrieved cargo unit can be flown to any desired location and placed on the ground or directly into a truck for ground transport. If the cargo is a vehicle, it can be driven away immediately, since there is no need for impact attenuation materials (e.g., honeycomb). While not an autonomous delivery system, ATPAD does meet the Army’s desire for precision airdrop of payloads weighing 10,000 lb and up (Joint Precision Airdrop System – Light) as well as soft landing improvements (Rapid Rigging and Derigging Airdrop Capability).

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All Army and US Marine Corps helicopters equipped with external load capability are suitable for ATPAD. For the Army these include the UH-60 and CH-47 for medium and heavy loads, respectively. For the Marine Corps the CH-46 and the CH-53 series are all suitable. Again, based on Atlas V ATS recovery studies, the CH-53E may be capable of ATPAD payloads of over 22,000 pounds.

IV. Conclusion The history of mid-air retrieval as a method of recovering high-value payloads dates back over 40 years.

Originally a relatively complicated and moderately risky method for hooking round parachutes, MAR has evolved to become a highly reliable recovery method, due largely to advances in parafoil technology. Gliding parafoils offer pilots a better visual approach. Helicopters are particularly well suited for setting up a stabilized approach to the parafoil. The engagement maneuver becomes easier still, with the advent of the third generation method of mid-air retrieval. Instead of hooking the parafoil, itself, the pilot can set up on a trailing engagement line, ease the retrieval hook onto the capture line, and gradually collapse the canopy, greatly reducing snatch loads on the payload. The 3GMAR method will conceivably permit the retrieval of 80% or more of the aircraft’s external payload capacity. Given the scalability of 3GMAR, the current state of the art of very large parafoils, and the load carrying capacity of the world’s largest helicopters, it is conceivable that 3GMAR can be used to safely, reliably, and economically recover high-value payloads of up to 22,000 lb.

References 1Ewing, E.G., Bixby, H.W., and Knacke, T.W., “Recovery System Design Guide,” AFFDL-TR-78-151, Wright-Patterson Air

Force Base, OH, Dec. 1978. 2Hintzke, S., and Haggard, R., “Demonstration of a Parafoil Based Mid-Air Retrieval System for an Unmanned Air Vehicle,”

CP912, AIAA 11th Aerodynamic Decelerator Systems Technology Conference, AIAA, Reston, VA, 1991, pp. 55-64. 3Haggard, R.A., “Parafoil Mid-Air Retrieval,” U.S. Patent No. 6,824,102, 30 November 2004. 4Fogelman, J.L., Norton, B, and Haggard, R., “Genesis Phase E MAR System Upgrades Report,” Doc. No. 3506 21 0137,

Vertigo, Inc., Lake Elsinore, CA, November 2004. 5“Next Generation Launch Technologies Reentry Vehicle Mid-Air Retrieval (NGLT RV MAR) Concept Study,” Doc. No.

2005-11-0001, Vertigo, Inc., Lake Elsinore, CA, June 2003. 6Smith, J.J., Bennett, T, and Jorgensen, D.S, “Recent Development of the NASA X-38 Parafoil Landing System,” SAFE

Association 37th Annual Symposium Proceedings [CD-ROM], SAFE Association, Creswell, OR, 1999. 7Jorgensen, D.S., and Patel, S., “Qualification of the Guided Parafoil Air Delivery System – Light (GPADS-Light),” CP976,

AIAA 14th Aerodynamic Decelerator Systems Technology Conference, AIAA, Reston, VA, 1997, pp. 234-243.8“Atlas V Booster ATS Parafoil Mid-Air Retrieval System Concept Study, Final Report,” Doc. No. 3505-11-0001, Vertigo,

Inc., Lake Elsinore, CA, Jan. 2003. 9Berland, J.C., and George, S., “Development of a Low Cost 10,000 lb Capacity Ram-Air Parachute,” 2005 AIAA Meeting

Papers on Disc [CD-ROM], Vol. 10 (to be published), AIAA, Reston, VA, 2005.

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