LV_P- 985

September 2, 1971

" LANGLEY WORKING PAPER

THE EFFECT OFSUSPENSION-LINE LENGTH

ON VIKING PARACHUTE INFLATION LOADS

Prepared by

Theodore A. Talay

and __ _, L__Charles H. _nitlock

•Approveclby .....

/q_Son D. Davis, Jr., Chief.pace Technology Division

distribution by /_'/. _/_" 2(_ Clifford H_elson

F Director for Spade

LANGLEY RESEARCH CENTER

NATIONAL AERONAUTICS AND SPACE ADMIN_TRATION

• THE EFFECT OF SUSPENSION-LINE LENGTH

. ON VIKING PARACHUTE INFLATION LOADS .

By Theodore A. Talay, Lamont R. Poole,and Charles H. Whitlock

SUMMARY

Analytical calculations have considered the effect on maximum load of

increasing the suspension-line length on the Viking parachute. Results indicate

that unfurling time is increased to 1.85 seconds from 1..45seconds, and that

maximum loads are increased approximately 5 percent with an uncertainty of

-4 percent tO *3 percent.

INTRODUCTION

Wind-tunnel tests have shown that the effect of the bl_nt Viking entry

vehicle is to.reduce the drag forces produced by the parachute decelerator.

As a result, the suspension lines on the Viking'parachute were recently

lengthened to move the canopy farther rearward in the wake of the entry vehicle.

The result: is increased drag produced by the parachute to meet mission design

requirements. The effect of the increase in suspension-line length on the

opening loa_ is unknown. It is the purpose of this•document to define the

effects of suspension line length on this design parameter. •

Specifically, the effects of suspension,line length change will be

examined in the Min _,s atmosphere which is the worst-case load condition

on which the parachute is designed. First, trajectory effects will be defined

in that the differences in the length of time for parachute unfurling will be

examined. Trajectory conditions at the end of unfurling will provide the

initial conditions for the inflation process. Next, the effect of suspension-

1

line length on the opening loads felt both at the entry vehicle and on the

canopy will be analyzed. The relative effects of uncertainties in opening

load magnitude, suspension system damping, and wind-tunnel test results will

be considered.

BES units are used throughout this document to simplify application of

the r_sults to the Viking Project.

SYSTEM DE_.CRIPTION

The total deployment process for the Viking decelerator consists of an

unfurling phase and an inflation phase as shown in figure 1. Unfurling is

the phase in which the parachute is stl'ung out from the deployment bag in a

lines-first manner following mortar fi:'e. The canopy inflation phase is

assumed to begin at the end of the un_rling process. Overall dimensions for "

the Viking decelerator system were taken from reference 1 and are shown in

figure 2. The distribution of weight along the parachute strung-out length is

shown in figure 3. Suspension line and bridle material force-elongation

characteristics are presented in figure 4. These static data were obtained using

unsterilized material in tests at the Langley Research Center. Manual pull

tests were used to define characterist_.cs for loads under 50 pounds. These

results were then combined with Instron test data for loads above 50 pounds.

Damping characteristics of suspension-:_ystem material are presently undefined.

For purposes of this'analysis, an average value of damping will be assumed.

Final results will then be compared for several values of damping to examine

sensitivity. A constant damping coefficient value of 25 pound-seconds is

assumed for each suspension line. Thi3 coefficient is the ratio of the force

caused by damping to the strain rate {in percent per second). The bridle

2

material is assumed to have zero damping, and each leg is considered to have

" four layers ofmaterial for computationof force-elongation characteristics

for the total bridle assembly.

Entry vehicle and decelerator mass and aerodynamic properties assumed for

this analysis are given in the table.

SIMULATION AND RESULTS

Program Inputs

The unfurling sequence is simulated using the analytical model described

in reference 2. Entry vehicle and decelerator physical properties aze used as

program inputs. Initial conditions to begin the unfurling computations were

taken from reference 3 for theMin Hp, s atmosphere and were adjusted for

" different atmospheric properties from reference 4. The adjuste_ initial

conditions for mortar fire are Mach number equal 2.2, dynamic pressure equal

10.55 pounds per square foot, and flight path angle equal -10.81 degrees. For

the X/D = 6.03 configuration, a mortar velocity of 90 feet per second is used

for consistency with reference 3. A mortar velocity of 98 feet per second is

used for the X/D = 8.69 configuration based on current mortar.system design.

An increased ejection velocity is required to insure completion of the

unfurling process when the length of the system is increased.'

The inflation sequence is simulated using a currently unpublished

analytical model. The model considers nonlinear elastic properties with

suspension-system geometry corrections as a function of instantaneous canopy

- diameter. As usual in most models of elastic systems, the weight of the

canopy is considered attached to a massless spring, the suspension-system.

The model also considers the effect of entry vehicle-decelerator •relatiVe

3

velocity in the computation of dynamic pressureacting on the parachute

canopy, Geometry changes during inflation are prescribed by the canopy

projected-diameter profile shown in figure 5. This curve was constructed

using data from the Planetary Entry Parachute Project. The CDS history

shown in figure 6 is estimated for the X/D = 6.03 configuration. A CDS

history relative to the figure 6 history will be defined for the X/D = 8.69

configuration during this study. Final results will be compared for several

different magnitudes of the figure 6 C[,Shistory to examine the effects of

uncertaintyin opening load magnitude.

Program Results

Unfurlin_ Sequence.- The effect of longer suspension lines is to increase

the unfurling tim_ between the events of mortar fire and bag strip. For the

Viking system, however, the difference is not great because the mortar velocity

is increased from 90 to 98 feet per second. Figure 7 shows the unfurled length

as a function of time £or both th_ X/D = 6.03 and 8.69 configurations. These

curves show that unfurling time is increased to 1.83 seconds from 1.45 seconds

by lengthening the suspension lines. The effect of longer unfurling time is

to reduce the Mach number and dynamic pressure at bag strip by 0;02 and O.12

pounds per square foot, respectively. Thus, the inflation process begins at

slightly less severe conditions for the X/D = 8.69 configuration.

Inflation Sequence.- Initial inflation calculations were completed using

the figure 6 CDS history for both the 6.03- and 8.69-1ength configurations.

Inflation load histories for both the canopy and entry vehicle are presented

in figure 8. From these curves, it should be noted that the maximum loads

experienced by the canopy are 5.8 percent higher than those of the vehicle

4

for the X/D = 6.03 configuration and 6.3 percent higher for the X/D = 8.69

system given the same CDS curve. Thus, increased suspension line length

attenuates the loads transmitted to the entry vehicle by only a small amount.

It should be observed in figure 8 that the maximum vehicle load for the

X/D = 8.69 system is approximately 4 percent lower than the load on the

X/D = 6.03 system. Approximately i percent of this amount is because inflation.

begins at a slightly lower dynamic pressure for the long configuratio n. The

remaining reduction is caused by the fact that longer suspension line lengths

cause increased stretch velocities durirg inflation. The increase in relative

velocity between the vehicle and decelerator causes reduced dynamic pressures

and loads at the canopy. This in turn means lower loads at the vehicle for

increased suspension line length if the CDS history of the canopy is unchanged.

For the Viking system, the CDS history of the canopy is not identical for

both parachute • configurations. The X/D = 8.69 system has the parachute

farther rearward in the wake to produce more drag. For this reason, it is

expected that the CDS values for that configuration should be larger than the

figure 6 curve. An appropriate CDS history for the 8.69 configuration can be

derived using preliminary wind-tunnel d_ta from recently completed tests at

AEDC. The indirect procedure used to derive canopy drag area from vehicle-

location measurements is described in the following paragraph.

Parachute inflation loads were measured for X/D values 6.0 and 8.5 at

' Mach number 2.2 in the AEDC-PWT facility. Preliminary results indicate that

the nondimensional opening load parameter measured at the vehicle is 6.2 percent

higher for the 8.5 configuration than the 6.0 system. Thus if all other factors

were equal, the canopy CDS curve should be adjusted until the vehicle forces

are larger for the long system than the short system by 6.2 percent.

5

Unfortunately all other factors are not equal because of trajectory differences.

Unfurling calculations indicate that the longer system begins inflation at a

dynamic pressure 1.2 percent lower than the short system. Thus to equal the

6.2 percent load difference measured in the wind tunnel, the X/D = 8.69 canopy

drag area curve should be adjusted until the vehicle loads for the long

system are 4.8 percent larger than those of the X/D = 6.0 configuration.

Using an iteration procedure, an approximate adjustment was made and the CDS

history shown on figure 9 was derived for the X/D = 8.69 configuration. The

important point to note is that a i0.0 percent increase in canopy CDS is required

to achieve 4.7 percent higher vehicle loads for the long system over the short

system.

An increase of i0 percent in CDS does not mean canopy loads are increased

by an equal amount when suspension lines are lengthened. As CDS is increased,

differences in the trajectory profiles and the,dynamic pressure reduction Caused

by stretch velocity are amplified. The actual loads felt in the system are

given below:

Configuration Vehicle Load Canopy Load(lbs) (lbs)

X/D = 8.69 13 296 14 077

X/D = 6.03 12 706 13 419

Ratio 1.047 1.049

These results indicate that when both trajectory and elastic differences are

taken into account, actual loads increase only approximately 5 percent both

at the canopy and vehicle when suspension lines are lengthened.

EFFECTS OF UNCERTAINTrES

The previous results are based on an assumed average value of damping

and an estimated canopy CDS history for the X/D = 6.03 configuration (figure 6).

The effects of uncertainties in these parameters are shown in figures iO, II,

and 12. Figure I0 shows the ratios of loads between the two configuration

for values of the average damping coefficient between 0 and 50 pound-seconds.

The uncertainty in damping coefficient causes an uncertainty of -3 percent to

+i percent about the nominal 5 percent value. Figure ii shows the effect of

damping on the ratio of loads between the canopy and the vehicle, These curves

indicate that damping has a significant effect on the distribution of loads

between the canopy and vehicle. It also significantly influences the magnitude

of loads experienced by the canopy.

The effect of uncertainty in the baseline CDS history assumed for the

X/D = 6.03 configuration is shown in figure 12. Values are shown as a function

of maximum vehicle loading to increase the usefulness of results. The figure 6

CDS curve was adjusted by use of a multiplier tO higher valnes to simulate

unknown Mach number or wake effects. As baseline CDS is adjusted up the loads

on the X/D = 6.03 vehicle are also increased. Thus the abscissa of figure 12

represents the effect of uncertainty in the baseline CDS history. The CDS

history for the X/D = 8.69 system was adjusted such that it was always

i0 percent above the X/D = 6.03 history to remain consistant with wind-tunnel

results. The curves in figure 12 indicate that the uncertainty in baseline

CDS causes an uncertainty of -2 to +0 percent in the nominal S percent value.

Figure 13 shows the actual vehicle loads for the long configuration as a

function of those expected in the short system when the damping coefficient is

equal 25 pound-seconds.

7

The analysis of this document is highly dependent on opening load data

obtained from the Mach 2.2 parachute tests at AEDC. The relative CDS histories

between the two parachute configurationsarebased on the fact that the opening

load parameter measured at the vehicle for the long parachute was 6.2 percent

higher than that for the short configuration. Examination of the analog plots

from the tunnel tests indicate that the uncertainty about the 6.2 percent value

is -i percent to +3 percent. This uncertainty will feed almost directly into

the results of this document.

An uncertainty level considering the combined effect of all parameters

may be estimated using the root-sum-square technique. Based on the above

uncertainty levels for suspension material damping, baseline CDS history, and

wind-tunnel results, the combined uncertainty level for the 5 percent load

increase as a result of lengthening the Viking parachute's Suspension lines

is -4 percent to +3 percent.

CONCLUSIONS

Analytical calculations have considered the effect on maximum load of

increasing the suspension-line length on the Viking parachute. Complex

interactions between the trajectory environment and decelerator elastic

characteristics have been simulated for the Min Hp, s atmosphere. Based on the

results of this study, the following conclusions are made:

i. Decelerator unfurling time is increased to 1.83 seconds from

1.45 seconds and the Math number and dynamic pressure at bag strip are reduced

0.02 and O.12 pounds per square foot, respectively.

2. Maximum loads at both the canopy and vehicle are increased approxi-

mately 5 percent with an uncertainty of -4 percent to +3 percent.

It should be noted that design changes in the Viking system can invalidate

the results of this study. The calculations are particularly sensitive to

significant changes in deployment conditions, mortar ejection velocities, entry

vehicle ballistic coefficient, and decelerator physical properties.

REFERENCES

i. Goodyear Aerospace Corp. Drawings No. 3064130-102 and 3064110-104.

2. Peele, Lament R.; and Huckins, Earle K. III: An Approximate Techniquefor Analyzing the E_fects of Suspension-Line Elasticity During the ParachuteUnfurling Process. Prospective NASA TN.

3. Moog, R,D.: Viking Dynamics Data Book, VER-19. Martin Marietta Corp.TR-37200S2, Revision A, April 1971.

4. Mars Engineering Model. Viking 75 Project Document M75-125-I,December 1970.

i.

9

TABLE

Mass and Aerodynamic Properties

Weight-Earth pounds

Entry vehicle 1888

Parachute X/D = 6.03 X/D = 8.69Canopy 46.15 46.15Suspension lines 23.31 30.73Bridle 6.00 6.00

Total 75.-4"6 82.88

Entry Vehicle Drag Coefficient

Mach CD

O 1.081.3 1.O81.6 i.iii

1.0 1.3221.4 1.5522.0 1.5973.0 1.6124.0 1.6176.0 1.622

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