RAPID - THE DESIGN OF A LOW ALTITUDE PARACHUTE

Elsa J. Hennings* Recovery Systems Division

Aerosystems Department Naval Weapons Center, China Lake, CA 93555-6001

As aircraft operational speeds become greater, protective ejectable capsules may begin to replace open ejection seats. These capsules increase the recovery weight, requiring larger parachutes. Because larger parachutes take longer to inflate, low altitude recovery at both high and low speeds becomes increasingly difficult.

The Regulated Area Progressive Inflation Decelerator (RAPID) parachute was designed to address this problem. Using a unique reefing system, the parachute was designed to open very quickly at low speeds and to open at a rate controlled by the force on the parachute at high speeds, optimizing the inflation process.

In fiscal yea. 1990, the Naval Weapons Center, China Lake, Calif. (NWC), funded a program to determine the feasibility of this new parachute design. Because of limited funding, only a few drop tests could be performed. These tests compared per- formances of the 28-foot flat circular parachute with a RAPID prototype having an equivalent drag area. The results showed that at low speeds, the RAPID parachute consistently opened faster than the 28-foot flat, opening almost twice as fast during the lowest speed test. However, because the force-controlled disreefing mechanism was not available during this phase of the test program, the high speed performance was not evaluated. As a result of this testing, the RAPID parachute may be a strong candidate for use in the U.S. Navy's Advanced Technology Crew Station Program.

Low altitude ejections from military aircraft present a difficult problem in recovering the crewmember without injury. There is very limited time to inflate the parachute before ground impact.

Parachute opening speed is proportional to deployment speed. This results in two major deploy-

*~echanical Engineer, Member. AZAA.

ment problems at the velocity extremes of the opera- tional envelope: (1) excessive opening forces at high speeds due to fast inflation, and (2) premature ground impact at low speeds due to slow inflation.

. Parachute drag force is proportional to the square of the deployment velocity multiplied by the inflated area. Thus at high speeds when the parachute inflates very quickly, the drag forces can become excessive. Therefore in the high speed regime, the major problem is to slow down inflation to avoid excessive drag forces.

b w Speed Case. The opposite is true in the low speed regime, where the airflow is often too slow to fully inflate the parachute before ground impact. To increase the amount of time available for the parachute to inflate, the capsule must be propelled faster and higher during ejection which results in a

, weight penalty for the additional propellant. Thus in the low speed case, the major problem is to speed up inflation.

M i n e UD Inflatim

One method currently used on some ejection seat parachutes to speed up inflation at low speeds is the pulldown vent line (PDVL). In this method a line attached to the parachute's vent is pulled down prior to deployment When the parachute is deployed at low speed. the pulled-down vent forces the air entering the parachute to open the skirt quickly. allowing for a faster inflation. When the parachute is deployed at high speed, the PDVL simply breaks. allowing for a n o d inflation. However, the PDVL changes the opening shape of the parachute, which causes high forces to occur at a point halfway between the skirt and vent. Parachutes are normally weaker at this point than they are at the vent, which is designed for such loading.l

Another way of speeding up inflation, which is also used on ejection seat parachutes, is with a spreading gun. This gun explosively throws outward metal slugs that are attached around the perimeter of

This paper is declared a work of the U.S. Government and is not subject to copyright protection in the Unittd Stales.

the parachute skirt, which causes thc skirt to open very quickly. But at high speeds this fast opening is undesirable because it causes high opening forces.

Because smaller parachutes inflate faster, a third way of speeding up inflation is by using a cluster of smaller parachutes instead of one large one. However, this increases the weight and complexity of the system tremendously. Also, at high speeds, the cluster parachutes' inflation must be delayed to keep from opening too quickly and causing excessive forces.

The method of inflation delay currently used for high speed deployments consists of a reefing line that prevents the parachute skirt from fully opening until a timed pyrotechnic line cutter severs the reefing line. This method is commonly used with single parachutes and with the individual parachutes in a cluster. However, the failure of a single cutter (or premature or late firing of one) can result in system failure. Single point diseefing for cluster systems is experimental at the present time.

Low altitude recovery requires that the parachute be inflated quickly enough at low speeds to main an acceptable descent rate before ground impact but not inflated so quickly at high speeds that it would cause injury from high parachute opening forces. At high altitude, there is ample time to inflate the parachute at low speeds or, at high speeds, to gradually reduce the velocity before main parachute deployment. At low altitude, inflation speed must be controlled depending on deployment velocity.

As the state of the art advances in aircraft capabilities, so must it advance in emergency recovery. Thus, the RAPID parachute was designed to provide fast inflation at low speeds, and gradual inflation at high speeds to reduce the high opening forces. This is achieved by the use of a unique deployment method coupled with a slotted canopy and a controllable disreefing system.

The intent of this program was to determine the feasibility of the new design by comparing the performance of the 28-foot flat circular ejection seat parachute to a comparably sized RAPID prototype. If this design proves feasible, this type of parachute would increase the safe recovery envelope to more

closely resemble the operational envelope of the airaaft

Ln arder to have a single deployment system that allows rapid inflation at low speeds and slower inflation at high speeds (which is convary to the way parachutes naturally open), a unique deploy- mendreefing system was devised. This system involves packing the parachute with the skirt pulled up to a concentric band located one-third of the way down the gore from the vent This is accomplished by attaching internal lines that run from each skirt-radial joint through rings at each concentric band-radial joint and in to a single reefing webbing (see Figure 1). When this reefing webbing is pulled completely down so that the skirt is drawn up to the concentric band (see Figure 2). the drag area of the parachute is very small. The smaller the drag area, the faster the parachute inflates; therefore the parachute opens very quickly when deployed in this configuration.

The reefing webbing is designed to be attached to a device that will let it pay out, allowing the parachute to inflate until a preset maximum drag force is reached. When that force is reached, the webbing is locked in place, which stops the inflation process. When the velocity decays, the drag force will drop below the preset maximum force, so more webbing will be let out, allowing additional inflation. This sequence continues until the parachute is fully inflated. In the low speed case, the maximum force would not be reached. so the reefing webbing would pay out completely upon parachute deployment.

The slurt, which initially opens to the diameter of the concentric band (not completely shut as in normal deployments), drops straight down and is inflated very quickly. At low speeds, this system allows for a very fast opening, while at high speeds, the reefing webbing controls the inflation, keeping the opening force from exceeding human tolerance. Unlike the PDVL, this deployment method causes the opening loads to be highest at the vent, which is designed to withstand such forces.

In addition to the controllable reefing webbing, slots at each radial seam running from the skirt to the concentric band act to automatically reduce excessive forces by opening when the pressure inside the parachute is high (during the inflation process), and closing when the pressure drops (during steady state descent) (see Figure 3). This results in the maximum drag area (slots closed) when it is needed most, i.e., when the parachute is close to the ground.

SUSPENSION LINE

REEFING WEBBING c

SECTION A-A

FIGURE 1. RAPID Parachute in Diseefed Condition.

FIGURE 2. Reefed Condition With Skirt Drawn up to the Concentric Band.

SLOTS OPEN AT HIOH SPEED /l

- FIGURE 3. Effect of Velocity on Slots.

To test this theory and determine the feasibility of the design, a limited test program was conducted comparing the RAPID parachute to the 28-foot flat circular parachute. Testing was performed in three phases: phase 1 was scale model basic configuration testing, phase 2 was f d scale deployment testing, and phase 3 was actual comparison testing. Each of these phases and the test results are discussed below.

To determine the optimum configuration for the full size prototype, several 5-foot-diameter prototypes were constructed with different combinations of slot length and position, cloth air permeability, and top and skirt cone angles. These prototypes were then tested to determine approximate reefed to diseefed drag area ratios.

To determine the reefed to disreefed drag area ratios, each prototype was tow tested in nonturbulent air (the attachment point was 8 feet out from the side of the towing vehicle) at a constant velocity (around 20 miles per hour, depending on the particular prototype), first in the reefed mode and then in the disreefed mode. A force gauge was attached to the parachute and force measurements recorded for both the reefed and disreefed modes. Because the velocity was held constant between modes, the measured force was directly proportional to the drag area, and thus the drag area ratio could be determined. A ratio of 1:9 was the goal as this represents a ratio used with other conventional reefing techniques. A prototype was developed and tested to have a ratio of 1:8.3, so this design was chosen for full scale testing.

The full scale prototype was designed to provide the same drag area as the 28-foot flat circular parachute so that, in the third phase of testing, the two could be directly compared to determine the relative strengths and weaknesses in the RAPID design. It was also important to keep the parachute diameters as similar as possible since opening time is proportional to parachute size. Because the slots in the RAPID design vent air and thus reduce the drag, drag could be increased in the RAPID parachute by using a cloth with a lower air permeability, which has a higher drag coefficient. This would result in the parachute sizes being similar. Because cloth air permeability did not seem to affect the steady state shape of the 5-foot prototypes, 0 to 3 cubic feet per minute (cfm) cloth was chosen for the full scale

prototype to keep the drag and size as similar as possible to the %foot flat circular.

A drop test was performed with two 5-foot models, both flat circular, one with slots and one without, to determine what effect the slots had on the drag area The test results indicated a 26% loss of drag area for the design with slots. Using a drag coefficient of 1.2 for 0- to 3cfm cloth (this value was arrived at by extrapolating data from 30- and lOOcfm clothL*), and taking into account a 26% loss in drag due to the slots, the diameter of the RAPID prototype necessary to approximate the drag of the 28-foot flat circular parachute was calculated to be 26.5 feet

The full scale prototype was constructed with slots at each of the 26 radial seams reaching from the skirt band to a concentric band located 8.75 feet up from the skin. The suspension line length was 1.5 times the diameter of the parachute for maximum slot closure. The internal lines, which run from the skirt through rings at the concentric band and attach to a center reefing webbing, were 13.25 feet long, half the parachute diameter. The reefing webbing was 26 feet long. A 300-pound Kevlar tether line attached the end of the reefing webbing to the test vehicle to prevent the reefing webbing from tangling in suspension lines after release.

Phase 2~EdUcale De~lo~ment Testing

Test. Prior to the comparison testing phase, "checkout" tests were performed to ensure proper deployment and make modifications to the design where necessary. The first such test of the full scale prototype was performed to determine the baseline characteristics of the design, including deployment shape, opening time, full open shape, rate of descent, drag area, oscillation and rotation tendencies. For this test, the parachute was attached to a 250-pound test vehicle and static line deployed at 3000 feet above ground level (AGL) and 110 knots indicated air speed (KIAS).

The parachute opened from line stretch to first full open in 1.6 seconds. The reefing webbing, which was set to release at line stretch. released prematurely. The Kevlar tether line broke, causing the reefing webbing to whip into the canopy at full open, breaking an internal line and causing minor damage.

** Unpublished test results of the Missile Recovery System (MRS) program from David Hass, Naval Weapons Center, 1985.

Because the reefing webbing released before line stretch, the canopy skirt was no longer pulled up to the concentric band to provide for a fast, controlled opening, such as was seen on the later tests. Instead, the opening resembled that of a standard flat circular parachute, where the skirt flaps about, causing a delay in inflation. The slots, however, opened at inflation and closed during steady state descent, as predicted. The amount of oscillation and rotation during descent was unacceptable. It was felt that this movement was due to the tops of the slots being too close to the vent. Because of excessive instability, the rate of descent and drag area were not obtained.

The test results confirmed that the packing pr0cxdure.s were c o m t and that the slots opened and closed as predicted. However, slot length and position, as well as reefing webbing tether line strength, needed to be modifid.

Test. For the second test, the slots were shortened in an effort to reduce the oscillation and rotation problems. The reefing webbing tether line strength was increased to 550 pounds. Also, because an experimental force measuring device was available for this test, the reefed versus disreefed riser loads were to be measured, as was the reefed rate of descent to indicate the reefed drag area.

This test was performed exactly as the first test, except that a loop of 2000-pound Kevlar line was used to attach the reefing webbing to the test vehicle in its fully reefed position, until two 7-second cutters severed the line and released the webbing. Because of the RAPID parachute's geometry, half the opening force should be transferred through the suspension lines and half through the reefing webbing. The Kevlar line, in this looped configuration, should have withstood about 4000 pounds; but upon line stretch. the entire force of the reefed parachute (calculated to be about 2500 pounds) was inadvertently transferred through the reefing webbing and thus through the loop, which snapped the Kevlar loop. The low breaking strength was caused by the line being knotted instead of fingertrapped, which normally would have been suffkient since only half the force was expected. The reason alJ the farce was transferred to the Kevlar loop and no force was transferred through the suspension lines was that the Kevlar loop was 4.5 inches too short. When the Kevlar loop broke, releasing the reefing webbing, the tether line broke again and, as before, the reefing webbing whipped up into the canopy, ripping a gore from the concentric band to the skirt

The canopy opened extremely quickly, taking only 0.4 second to go from line stretch to reefed open and 0.3 second from reefed open to first Full open, for a total of 0.7 second from line stretch to first full open. This is more than twice as fast an opening as the previous test. which was 1.6 seconds from line stretch to first full open. This decrease in opening time was due to the skirt being pulled up to the concentric band at inflation, as intended, allowing for a fast, controlled opening. Also, the slots were smaller and allowed less air to escape during the inflation process. Unfortunately, because the reefing webbing separated from the test vehicle during this fast inflation, the canopy completely inverted. There was very little oscillation and rotation noted. probably because of the blown gore venting air.

The experimental force measuring device indicated a peak force of 2500 pounds, but did not return to the 250-pound suspended force during steady state descent, as it should have. Therefore these force data were considered suspect. Also, because a gore was blown, the rate of descent and drag area were not obtained.

The test results showed that the opening process (when the skirt is pulled up to the concentric band) is very fast, as predicted. Because it is easier to slow down a fast opening than to speed up a slow opening, the RAPID design shows some promise as a capsule recovery parachute. However, this test also showed the results of making the reefing webbing too short. Also, the reefing webbing tether strength needed to be increased.

Test 3. A third test was performed to ensure correct reefing function and determine the reefed and dis~eefed drag areas prior to the comparison testing. This was a repeat of the second test with the following changes:

1. Suspension lines were shortened to 26.5 feet (line length to parachute diameter ratio changed from 1.5 to 1.0) to eliminate possible deployment complications due to line sail.

2. Reefing webbing was lengthened to 28 feet long with a 3-ring attachment at the 14-foot mark for attachment in the reefed mode, and attached at the end to act as its own tether in the disreefed mode (see Figure 4). Because the reefing webbing was 6500- pound Kevlar, there should be adequate strength to prevent tether failure at diseefing.

Two 7-second cutters were to cut a line holding the 3-ring release in the closed position, allowing the parachute to be in the reefed mode f a 7 seconds. The 7 seconds allows enough time for the laser space positioning instruments to determine rate of descent, from which reefed drag a m can then be calculated. The reefing webbing in the reefed mode was 2 feet longer than it was for the last test, allowing the skirt to be 1 foot down from the concentric band while in the reefed mode (see Figure 4). The geometry of the suspension linehnternal line routing causes the skirt to descend from the concentric band one half the amount of increase in the reefing webbing length. This increase in the reefhg webbing length prevents the skirt rings from being pulled up against the concentric band rings (due to line stretch or a miscalculation in reefing webbing length), which would transfer the entire opening load into the center reefing webbing, as was seen on the last test.

The 253-pound total weight test vehicle was dropped at 3000 feet AGL and at 110 KIAS. To ensure a low speed deployment, an 8-foot, 20degree conical, W f m cloth drogue parachute was ridden for 4 seconds prior to main deployment The velocity at main deployment was about 60 feet per second (35.5 KIAS).

This time, the deployment of the RAPID parachute went perfectly. After the 4-second drogue ride, the RAPID parachute was deployed cleanly, taking 0.35 second to go from line smtch to reefed open. Seven seconds from line stretch it disreefed, taking 0.5 second to reach first full open. Once it was fully open, it started oscillating at an approximate half angle of 35 degrees and continued oscillating to ground impact. There was virmally no rotation.

The opening characteristics of the RAPID parachute were as predicted. Because the skirt was dropped 1 foot from the concentric band in the reefed mode, the bottom two-thirds of the parachute filled with air, resembling an annular parachute. This allowed the parachute to be very stable in the reefed mode but also increased the reefed drag area. Upon diseefing, the slots opened briefly and uniformly to vent high pressure air. The lack of canopy rotation may have been caused by the high angle of oscillation.

1 FOOT SEPARATlON

I REEFING WEBBING I

I I I

34UNGAlTACHMPrr 1 I I I I

I I I I I I i

L - - - - - - - - - - - - - A FIGURE 4. Reefing Webbing Attachment.

The rate of descent was measured by radar and corrected to standard day. sea level conditions. The corrected rate of descent and calculated drag areas for reefed and diseefed modes are l i d in Table 1.

Table 1. Test 3 Results.

which can measure rate of descent over a short time interval such as this, were not usable and radar, which requires more time in the reefed condition, had to be used.

Mode Reefed Disreefed

The drag area in the disreefed mode, which was supposed to have been around 490 square feet to be similar to the 28-foot flat? came out high, possibly due to the shortening of th? slots, which would tend to restrict the amount of air lost out the sides. In addition, the reefed drag area to disreefed drag area ratio, which was predicted to be 1:8.3. came out to be 1:2.6, which was probably due to the skin being dropped 1 foot from the concenmc band instead of pulled up against it. For the next test, this separation distance was shortened to 9 inches.

Because the deployment and opening characteristics looked good and reefed and diseefed drag areas were obtained. it was now time to begin the third phase of testing, the comparison to the 28- foot flat parachute. Prior to this test phase, however, the RAPID parachute vent was enlarged to reduce the oscillation problem and to lower the drag area somewhat, which would also make the RAPID drag area closer to the 28-foot flat drag area. (Enlarging the vent allows more air to escape out the top so that the parachute does not have to oscillate, spilling air out of the boaom.) The vent diameter was increased from 24 to 32 inches.

+These values may not be valid. The laser data,

Rate of descent 3 1 .O ftls* 19.35 ftls

Phase 3: Com~arison TeSring

Drag area 221.5 ft2* 568.5 ft2

Test. The two theoretical advantages of the RAPID parachute are its fast opening at low speed and its force-regulated opening at high speed. The purpose of the first test in this phase was to compare the low speed opening times of the RAPID parachute and the 28-foot flat parachute.

For the low speed lest, both parachutes were packed in identical deployment bags designed for lines-first deployment, the same as the RAPID had

been tested with previously. As before, the RAPID skirt was pulled up to the concentric band prior to packing, with a p i e . of 80-pound break tape holding the reefing webbing in the reefed position to ensure that the parachute reached line stretch prior to disreefing. Both parachutes were auached to test vehicles with total weights (including the parachute) around 250 pounds. The test vehicles were dropped one by one from a helicopter flying at 40 KIAS, the slowest the helicopter could fly carrying these drop vehicles, and at 1500 feet AGL This low altitude was chosen to facilitate good, close up ground video coverage. Timed video was used to determine opening time, and laser, radar, and weather data were recorded to & m i n e rate of descent. The rate of descent data were corrected to sea level, standard day conditions and drag area was calculated using these corrected rate of descent data Test results are shown in Table 2.

Table 2. Test 4 Results.

Measurement Total weight, lb Opening time, s

(Iine str. to first full open)

Rate of descent, ft/s (C-tea)*

Drag area, ft2 Oscillation, deg from

vertical Rotation from full open

to impact *Because of the low

GElzl release altitude, less than

1 minute of steady state descent dam was recorded. Therefore, these values may only be approximate.

The low speed opening time of the RAPID parachute was almost twice as fast as the standard 28- foot flat. The video tape showed that the RAPID parachute skirt opened immediately to the diameter of the concenmc band, which allowed a much greater volume of air into the canopy upon initial deployment than that allowed by the 28-foot flat parachute, whose skirt fully opened only after the parachute had inflated.

Results of this test showed that the drag areas of the RAPID and 28-foot flat were now similar. In addition, the test verified the first of the two theoretical advantages of the RAPID design over conventional parachutes: fast inflation time at low speeds. The second advantage, control of inflation forces at high speeds through the use of a continuous

dsreefing system, could not be fully analyzed during the remainder of the program because the reefing webbing force-regulated controller has yet to be developed. However a discontinuous disreefing system, using a timed line cutter, would give some information as to the function of the RAPID at higher speeds. Thus additional tests were conducted at higher speeds to analyze the force versus time functions of the RAPID parachute compared to the 28-foot flat.

M. This test evaluated the force versus time profile for the RAPID parachute at a moderate velocity to ensure structural stability prior to testing at higher speeds. Because the continuous disreefing controller mechanism was not available, an alternate single stage diseefing system using timed cutters was used

Because forces were needed for this test, both the RAPID and 28-foot flat parachutes were attached to instrumented torso dummies, with each parachute packed in the same deployment bag as was used for the previous test. Each deployment bag was then put into a 4-flap pack attached to a torso dummy.

To compare the fast opening RAPID parachute to the slower opening 28-foot flat, the RAPID parachute was reefed long enough so that both parachutes would go from line stretch to full open in the same amount of time. For this test, the RAPID parachute reefing webbing was pulled down further than in test 4, leaving a 6-inch separation between the skin and concentric band in the reefed mode compared to the 9-inch separation for test 4. This was done to reduce the drag caused by the lower two-thirds of the parachute inflating somewhat in the reefed mode. The reefing was performed as it was on test 3, with a 3- ring attachment holding the reefing webbing in the reefed position. Two 1.2-second cutters, armed at the beginning of line deployment (pack opening), were used to release the 3-ring attachment and allow the RAPID parachute to disreef.

Each deployment bag was static line deployed from a C-8 aircraft at 3000 feet AGL (5268 feet MSL) and at 100 KIAS. Radar and laser data, as well as weather data, were recorded for rate of descent and drag area calculations. The test results are listed in Table 3.

Table 3. Test 5 Results.

Measurement Total weight, lb Peak reefed force, III

Reefing webbing Suspension lines Total

Peak diseefed farce, Ib Reefing webbing Suspension lines Total

Opening time, s Line stretch to reefed open Diseef to first full open Pack open to first full open Pack open to ven descent

Corrected rate of descent, ftls Calculated drag area, ft2 Oscillation, deg from ven Rotation, revolutions

28-foot flat 25 1

- - -

- -

1582

- - 1.60 3.39

19.8 54 1.2 30 0

RAPID 255

1288 1708 29%

1180 l a 1 2380*

0.48 0.30 1.45 2.45

21.9 448 .O 35 0 A

*Reefing webbing and suspension line peak forces in the disreefed mode came at different timu. Therefore they do not dd up to the total peak disreefed force.

Figures 5 through 8 show the force versus time plots of reefkg webbing, suspension lines, and total for the W I D parachute and the total for the 28-foot flat parachute.

FIGURE 5. RAPID: Reefing Webbing Force Versus Time, Test 5.

FIGURE 6. RAPID: Suspension Line Force Versus Time. Test 5.

FIGURE 7. RAPID: Total Force Versus Time, Test 5.

FIGURE 8. 28-Foot Flat: Total Force Versus Time, Test 5.

The large difference in peak forces between the two parachutes was probably due to several factors. First, the opening characteristics of the 28-foot flat are historically unrepeatable, even in a very controlled deployment such as this test. Thus, on one test, such as this one, the skirt will hesitate before opening, causing a slow, low force deployment. On a different

test, such as the next one proved to be, the skirt will open quickly, causing a fast, high force deployment. Second, because the RAPID opened faster than the 28-foot flat on this test, one would expect the forces to be higher. The RAPID parachute's high reefed force was probably due to the fact that the RAPID parachute skirt was not pulled completely up to the concentric band. This gap of 6 inches allowed air to inflate the skirt somewhat, causing additional drag force. Also, the RAPID parachute was manufactured from 0- to 3cfm cloth, which traps more air than lOOcfm cloth which was used for the =-foot flat. This aids in fast opening times but causes higher opening forces, also. The force versus time curve of the RAPID parachute would probably smooth out if the material in the top was changed to 100-cfm cloth and if the gradual diseefing mechanism was used.

In addition to measuring pack open to first full open time, pack open to vertical orientation was also measured. This was done to further compare the opening characteristics of the two parachutes, as time needed to achieve vertical orientation is an important parameter in low altitude recovery. There was almost a full second difference in the pack open to vertical times, with the 28-foot flat taking 38% longer. More importantly, there was probably a large difference in altitude loss during opening, with the RAPID parachute using much less altitude to achieve vertical orientation.

Test results showed high reefed and disreefed forces for the RAPID parachute compared to the 28-foot flat parachute. Results also showed that the RAPID parachute, even when reefed, opened much faster than the 28-foot flat However, the RAPID parachute oscillated once again, for unknown reasons.

Test 6. The next comparison test gathered additional force versus time data for the RAPID and the 28-foot flat parachutes at the same 100 KIAS airspeed as test 5. In addition, the altitude loss during opening was measured to further analyze the opening characteristics of the two parachutes. Cameras were mounted on the test dummies to get a close-up view of the inflation.

For this test, the parachutes were packed in the same deployment bags and 4-flap packs as were used on test 5. However, the C-8 aircraft was unavailable, so the helicopter was used. Since the helicopter could not fly 100 KIAS carrying both dummies externally, the dummies were allowed to freefall for 7 seconds to attain the desired 100 KIAS deployment speed. A

small pilot chute attached to each torso dummy to prevent tumbling was released 7 seconds after deployment. As this small parachute released, it deployed the main parachute.

The RAPID parachute was deployed with the skirt 6 inches from the concentric band. as it was in test 5; but this time the RAPID parachute was not reefed. Both torso dummies were dropped at 5000 feet AGL (7268 feet MSL). Radar, laser, Askania, ground and onboard video, and weather data were recorded. The test results are listed in Table 4.

Table 4. Test 6 Results.

Measurement Total weight, lb Pack open speed, KIAS Peak opening force, Ib

Reefing webbing Suspension lines Total

Opening time, s Pack open to first full

open Opening distance, ft

Pack open to first full open

Corrected rate of descent, ft/s Calculated drag am, ft2 Oscillation, deg from vert Rotation, rprn

*Reefmg webbing and came at different times. The]

2&foot flat 265 1 16.0

- 3303.0 3303.0

1.55

276

25.6 341.6 28 0.42

;pension line peak forces re they Q not add up to

RAPID 269 114.6

1427.3 2124.6 3194.3*

1.42

255

17.1 77 1.6

12 1.5

the total peak force.

Both parachutes opened cleanly. The opening force of the unreefed RAPID parachute was more constant than in test 5 for the reefed RAPID. Also, the peak opening force for the RAPID was a little lower than for the %foot flat. The probable reason for this difference between the RAPID and the 28-foot flat is the unrepeatability of the 28-foot flat, as was discussed earlier. Also, the slots on the RAPID parachute opened during the latter part of inflation, resulting in a lower force, while the 28-foot flat parachute's peak force occurred during the latler part of inflation. Again, the high initial force on the RAPID parachute was probably caused by the 0- to 3cfm cloth and partial inflation of the skin. due to the initial 6-inch opening between the skirt band and concentric band

Figures 9 through 12 show the force versus time plots of reefing webbing, suspension lines, and total for the RAPID parachute, and the total for the =foot flaf parachute.

FIGURE 9. RAPID: Reefing Webbing Force Versus Time, Test 6.

FIGURE 10. RAPID: Suspension Line Force Versus T i e , Test 6.

I I II- . . . .

,= . . . . ' . . . I . . . I . . . . . , . . . .

$I ,om I FIGURE 1 1. RAPID: Total Force Versus Time, Test 6.

FIGURE 12. 28-Foot Flat Total Force Versus Time. Test 6.

Once again, the RAPID parachute opened faster and in a shorter distance than the 28-foot flat. The RAPID parachute oscillated very little compared to the 28-foot flat (which oscillated up to 38 degrees at times). These large oscillations were probably what caused the rate of descent for the 28-foot flat to be higher than normal. The RAPID parachute rotated at 1.5 revolutions per minute (rpm), compared to a rate of 0.42 rpm for the 28-foot flat.

Test results showed that the RAPID parachute opened faster, in less distance, and with a lower peak force than the 28-foot flat. It also showed that even with a very controlled deployment. the 28-foot flat opened much differently during this test than it did for test 5. The RAPID parachute, on the other hand, showed a good consistency in its opening patterns between tests 5 and 6, with the peak opening forces being almost the same. This is probably due to the unique deployment method that controls the all- important skirt opening dynamics. Thus, while the 28-foot flat skirt sometimes opens quickly but sometimes flaps awhile first, the RAPID skirt always opens in the same way. This indicates that the opening repeatability will be much better for the RAPID parachute than for the standard 28-foot flat. This test also showed that, compared to the 28-foot flat parachute, the RAPID parachute's oscillation and rotation were not a problem.

To determine the minimum drag area of the parachute, and thus what the reefed force would be at higher speeds, a reefed to the ground drop test was needed

portion would be required. A reefed drag area of 60 square feet was pmhcted from the preliminary 5-foot model tests (a ratio of reefed drag area to full open drag area of 19.3). Because none of the tests to date could be used to &tennine the reefed drag area, a final drop test was conducted.

For this test, the RAPID parachute was reefed completely, with the skirt pulled up to the concenmc band. The 3-ring release mechanism was tied shut and taped to ensure that the parachute would not disreef. The parachute was packed in its deployment bag in the same manner as in previous tests. It was then attached to a beer mug test vehicle. with a total weight of 248 pounds.

The test vehicle was dropped from a helicopter at 3000 feet AGL (5268 feet MSL) and at 40 K I M . Radar, laser, and weather data were recorded. The test results are listed in Table 5.

Table 5. Test 7 Results.

Opening time, line stretch to reefed open, s

CaTeaed reefed rate of descent, ftk

Calculated reefed drag area, ft2

Average full open drag area, ft2 (averaged from tests 4.5, and 6)

Predicted full open to reefed open drag area ratio

Actual full open to reefed open drag area ratio

During the descent, the skirt. which was pulled completely up to the concenmc band, did not appear to inflate. During previous tests the skirt was not pulled completely up and thus inflated somewhat, generating additional drag. Therefore, the reefed drag area from this test represents the minimum reefed drag area for the RAPID design in the present c ~ ~ g u r a t i o n using 0- to 3cfm cloth. The actual full open to reefed open drag area ratio was lower than expected; thenfore a change to lOOcfm cloth, in the top area at least, might reduce the reefed drag area somewhat.

Test 2. If the minimum drag area of the reefed RAPID parachute was too high, a change in material from 0- to 3cfm cloth to lOOcfm cloth for the reef&

References

This concludes the fiscal year 1990 feasibility 1. US. Air Force. Recovery Systems Design Guide, testing of the RAPID parachute. In summary, the test by E. Ewing and others. December 1978. P. 248. results indicated that the RAPID design consistently (USAF Report no. AFFDL-TR-78-151.) opened faster than the standard 28-foot flat parachute- almost twice as fast at low speeds. The two opening 2. Naval Weapons Center. Decelerator Systems force profiles of the RAPID parachute were very Engineering, by T. W . Knacke. July 1985. Pp. 5-71, similar to each other, indicating good opening Table 5-7. (NWC TP 6575.) repeatability. The two opening force profiles of the 28-foot flat were not similar to each other. When the RAPID parachute vent was enlarged, oscillation and rotation did not seem to be a problem.

The full open to reefed open drag area ratio was lower than expected due to the large reefed drag area. This large reefed drag area could cause excessive forces at high speeds. However, because no high speed tests have been conducted, this was not evaluated

The RAPID parachute's controllable diseefing system was not evaluated either because the force- regulated controlling mechanism was not available. Hopefully, this mechanism will be made available ior use in further testing in fiscal years 1991 and 1992.

The RAPID parachute design proves feasible when a fast opening parachute is required. The constant opening force claim cannot yet be justified, as the controlling mechanism was not available. Proposed proof-of-concept testing for fiscal years 1991 and 1992 will include development and fabrication of the controlling mechanism; low, moderate, and high speed testing, along with some minor modifications to the parachute, such as increased cloth air permeability in the top portion; and a change from a flat profile to a conical profile to help eliminate the post-inflation collapse. Based on the testing done to date, the RAPID parachute may be a strong candidate for use in the Navy's Advanced Technology Crew Station program.