1American Institute of Aeronautics and Astronautics

Substitution of Reefing Loops for Reefing Rings in PressurePacking Applications

Elsa J. Hennings1

Naval Air Warfare Center Weapons DivisionChina Lake, California, 93555

High density pressure packing a parachute containing metal parts is not trivial. Muchtime must be spent ensuring that the metal parts do not contact each other, the packingfixture, or pinch the canopy fabric between them. One of the metal parts most susceptible todamage are the reefing rings, which, if nicked or broken, can easily sever the reefing line,causing catastrophic failure of the parachute. Many devices have been used over the years toprevent this problem but they all take time to implement and are not foolproof. With theexplosion of new materials available to the parachute industry over the last decade, however,some very different solutions to this problem are now feasible. This paper describes onesuch solution.

NomenclatureCdSo = full open drag areaCdSr = reefed drag areaDo = nominal diameterKEAS = Knots Equivalent Air Speedn = fill factorV = disreef velocity

I. Introduction

A recovery system is currently being developed for a United States Navy program, which requires a 39 ft.nominal diameter parachute be packed into a very small, oddly shaped compartment previously containing a 30 ft.nominal diameter chute. This requirement necessitated pressure packing of the parachute to a fairly high density(approximately 45 lbs/ft3). During the packing process, the reefing rings frequently incurred damage from contactwith each other or with the reefing line cutters, which was unacceptable. Since pack volume was already criticallysmall, a new reefing line attachment method was needed.

II. Design Methodology

In 2003, the Naval Air Warfare Center Weapons Division was tasked with the development of a collapsingmechanism for the separation drogue parachute used for NASA’s X-37 Approach and Landing Test Vehicle(ALTV). The mechanism must allow for full canopy inflation while the parachute is attached to the ALTV, butmust collapse it after release to prevent it from drifting off the range1. The tested design involved three elastomercords in a Teflon fabric sleeve attached at the skirt of the parachute in the same manner as a reefing line, whichwould stretch out under load to allow for full parachute inflation and retract the skirt after parachute release.Reefing rings could not be used to attach this “reverse reefing line” due to the large size of the elastomer bundle.Instead, Teflon-covered Kevlar loops were fabricated which were sewn to the radials and performed quite well (seeFigure 1). Based on the success of this attachment method it was decided to modify it for use as a reefing ringsubstitute for the new 39 ft. parachute.

1 Recovery Systems Design EngineerHuman Systems Department, Code 466100DMember AIAA

19th AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar21 - 24 May 2007, Williamsburg, VA

AIAA 2007-2536

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

2American Institute of Aeronautics and Astronautics

Figure 1. Teflon-covered Kevlar loop used for elastomer in X-37 Program

III. Reefing Loop Design

The important functions of the reefing line attachment method are to securely attach the reefing line to theparachute skirt, while also providing a smooth, non-restricting, tangle-resistant guide for the reefing line to feedthrough after it is cut. The Teflon-covered Kevlar worked quite well as a secure restraint method, but would it allowa reefing line to slide through as quickly and easily as a metal ring does after the line is severed?

The optimum reefing loop design should be very smooth, small and relatively stiff to reduce the possibility ofreefing line entanglement as the reefing line is sliding through it. The attachment method must not only withstandthe reefing line force, but must also allow the loop to widen its opening when the line is under tension so as toprevent binding of the line. In addition, the reefing line should be smooth and relatively stiff to resist entanglementas it feeds through the loops. For this reason, Spectra was chosen as the reefing line material.

The first prototype reefing loop consisted of a length of ½ inch wide Kevlar tape routed through a length oftubular Teflon cloth. The reefing loops were attached to the line attachment loops on an existing 5 foot diameter flatcircular parachute model in a tent-fashion as shown in Figure 2. This parachute model was then tested at low speedto verify that the attachment method would work and that the reefing line would slide through the loops easily upondisreef.

Figure 2. Reefing Loop Attachment on Model Chute Figure 3. Reefing Loop with Reefing Line on Full-Scale Parachute

Teflon-covered Kevlar loop

3American Institute of Aeronautics and Astronautics

IV. Results of Model Test

For this model test, the parachute was towed using a side boom on a truck. With the parachute attachment point4 feet from the side of the truck, the truck was accelerated to 30 miles per hour at which point the parachute wastossed into the airstream in a reefed configuration (the parachute was reefed to 2%, which was the reefing ratio ofthe full-scale parachute). The parachute was towed in this fashion for approximately 30 seconds and videotaped.Post-test inspection of the loops and video showed no damage to the loops and a good symmetrical spacing of thereefing loops under load. For the second test, the truck was accelerated to 30 miles per hour again and the parachutetossed into the airstream, this time also firing a nine second reefing line cutter. The parachute disreefed at nineseconds and the truck began to slow down about 10 seconds after disreef. This test was videotaped as well. Post-test inspection of the loops and video again showed no damage to the loops, a symmetrical reefing, and a smooth,fast disreef. Based on this quick test, we were satisfied that the technique had merit and decided to modify one ofthe full-scale 39 foot parachutes in this manner (see Figure 3) and test it in a flight-representative drop test.

V. Full-Scale Implementation

For the full-scale parachute, the reefing loops were fabricated and attached in the same manner as for the model.In the standard reefing ring configuration, there is a ring attached on either side of each of the two line cutters, whichwere ostensibly added to prevent reefing lines from contacting the line cutter edges before the advent of Tefloninserts in line cutters. Since this parachute had been previously tested using reefing rings, and since the diameter ofthe reefing rings is much larger than the line cutter standoff from the radial, the reefing line never touched theserings but was retained solely by the line cutter. For that reason, no reefing loops were used at the line cutterattachment points since the cutter itself appears to perform quite well in attaching the reefing line to the skirt in thisapplication.

The criteria selected to evaluate the reefing loops during the full-scale test were as follows:

A. pressure packing integrityB. shape of reefed skirt compared to reefing ring systemC. shape of disreefing skirt compared to reefing ring systemD. disreefing fill factor compared to reefing ring systemE. structural integrity during reefing and disreefing

VI. Results of Full-Scale Test

As expected, the parachute packed much easier with the reefing loops compared to reefing rings. The fabricloops could be pressed closed on top of the reefing line, which gave us a small amount of extra pack volume, but thebiggest benefit was not having to spend the time carefully positioning the rings prior to the last canopy press.Following the packing process, the pack was carefully opened and the reefing loops examined, with noabnormalities noted.

For this first test of the new reefing loop design, it was decided to deploy the parachute at the low end of theairspeed required for this parachute system, which was 140 KEAS. For this test series, a 460 lb. test vehicle is liftednose down to an altitude of 10,000 MSL by an HH-1 helicopter, at which point it is released. It is allowed to freefallto the required velocity, at which point an onboard timer fires a line cutter allowing a Hot Shot pilot chute to deploythe test pilot chute from the rear of the vehicle. The test pilot chute then withdraws the main chute deployment bagand the canopy deploys lines first. For this test, everything went as planned, with the timer releasing the Hot Shotpilot chute at 8.9 seconds from release from the helicopter. The vehicle velocity was 148 KEAS at Hot Shot pilotrelease, and the main parachute deployed cleanly. The onboard camera showed the reefed skirt took on a somewhatrectangular shape, which was similar to the skirt shape using reefing rings. During the disreef, the mouth evolvedinto a three-lobed shape and then opened fully, which was also similar to the skirt evolution during disreef usingrings. The disreefing fill time was determined to be approximately 1.9 seconds, at a disreef velocity of 78.3 KEAS(149.4 ft/sec). Based on a nominal diameter (Do) of 39 ft, a reefed drag area (CdSr) of 33 square feet and a full-opendrag area (CdSo) of 1124.5 square feet (these values were determined from test data), the disreef fill factor (n) iscalculated to be:

4American Institute of Aeronautics and Astronautics

V

)/CdSCdS-(CdSDntimefillDisreef

oroo⋅= (ref. 2, page 5-44)

149.4

33)/1124.5-(1124.539n1.9

⋅=

n = 7.4

This compares well with a disreef fill factor of 7 for a fully extended skirt parachute using reefing rings (ref. 2, table5-6). Upon post-test inspection, neither the reefing loops nor the attachment stitching were damaged, indicatingsufficient structural integrity for this application.

The results of this first test were very encouraging. Not only did the parachute pack much easier with reefingloops, but the substitution of loops for rings did not appear to affect the opening characteristics significantly. It wasdecided to make this design change permanent and continue the test program. The following eight drop testsshowed mostly similar results to the first (see Table 1), so the design change was considered a success for this typeof parachute system and deployment method.

Table 1: Drop Test Results

Test date

line stretchvelocity(KEAS)

disreefvelocity

(KEAS) (ft/sec)

MeasuredCdSr

(sq. ft.)

disreeftime(sec.)

MeasuredCdSo

(sq. ft.)

FillFactor

n8 Feb (1) 163.9 78.3 149.4 33 1.90 1124.5 7.3927 Apr (1) 230.6 103.1 190.8 20 2.30 1124.5 11.35*27 Apr (2) 336.6 (no test)6 July (1) 225.4 93.0 173.2 28 1.27 1405.6 5.706 July (2) 264.3 102.0 186.3 25 2.67 1290.9 12.88*9 Aug (1) 219.2 90.9 171.0 29 1.59 1163.0 7.069 Aug (2) 291.3 90.4 164.9 30 1.74 1076.2 7.4628 Sept (1) 134.0 80.4 155.0 30 1.65 1149.9 6.6428 Sept (2) 296.2 91.9 167.3 30 1.54 1203.4 6.69

* Some skirt infolding noted at disreef, delaying inflation

The average disreef fill factor for the six nominal test cases (the two tests with skirt infolding were not included)was 6.82, with a standard deviation of 0.65. This average fill factor compares well to the value listed in ref. 2.

VII. Conclusions

Based on the results of the drop test series using reefing loops as a substitution for reefing rings, this design isvalid for this type parachute and deployment method. Since the testing of this concept was confined to a singleparachute in a limited application, additional evaluation of the concept should be performed for different types andsizes of parachutes in a multitude of applications to determine its validity across the spectrum of parachute usage,however.

References

1Hennings, E., Wolf, D., Jensen, S., and Granica, D., “Design and Testing of a Collapsible Drogue Parachute for the X-37Vehicle,” AIAA 18th Aerodynamic Decelerator Systems Technology Conference, Paper no. AIAA-2005-1660, May 2005

2Knacke, T. W., “Parachute Recovery Systems Design Manual,” Naval Weapons Center Technical Publication 6575, March1991