Opening Loads Analyses for Various Disk-Gap-Band Parachutes

18 Pages·2003·0.93 MB·English

by  Cruz, J. R.

AIAA 2003-2131
Opening Loads Analyses for Various
Disk-Gap-Band Parachutes
J. R. Cruz
NASA Langley Research Center
Hampton, VA
M. Kandis and A. Witkowski
Pioneer Aerospace
South Windsor, CT
17th AIAA Aerodynamic Decelerator Systems
Technology Conference and Seminar
19–22 May 2003
Monterey, California
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OPENING LOADS ANALYSES FOR VARIOUS DISK-GAP-BAND PARACHUTES
Juan R. Cruz*
NASA Langley Research Center
Hampton, Virginia
Mike Kandis† and Allen Witkowski‡
Pioneer Aerospace Corporation
South Windsor, Connecticut
.ABSTRACT C parachute steady-state drag coefficient for a fully
D
0
inflated parachute
Detailed opening loads data is presented for 18 tests
C opening load factor
x
of DGB parachutes of varying geometry with nominal
D band diameter
B
diameters ranging from 43.2 to 50.1 ft. All of the test
D disk diameter
D
parachutes were deployed from a mortar. Six of these
D constrained vent diameter
V,c
tests were conducted via drop testing with drop test
D nominal parachute diameter
vehicles weighing approximately 3,000 or 8,000 lb. 0
F force generated by the parachute during inflation
Twelve tests were conducted in the National Full-Scale p
g acceleration of gravity
Aerodynamics Complex 80- by 120-foot wind tunnel at
H band height
the NASA Ames Research Center. The purpose of B
H gap height
G
these tests was to structurally qualify the parachute for
k nondimensional constant used in the calculation
0
the Mars Exploration Rover mission. A key
of the parachute apparent mass
requirement of all tests was that peak parachute load
L suspension lines length
S
had to be reached at full inflation to more closely
m parachute apparent mass
a
simulate the load profile encountered during operation
m parachute physical mass
at Mars. Peak loads measured during the tests were in p
q dynamic pressure
the range from 12,889 to 30,027 lb. Of the two test
q effective dynamic pressure
methods, the wind tunnel tests yielded more accurate eff
q dynamic pressure in the wind tunnel far upstream
∞
and repeatable data. Application of an apparent mass
of the parachute canopy
model to the opening loads data yielded insights into
S projected parachute area
p
the nature of these loads. Although the apparent mass
S maximum projected parachute area
model could reconstruct specific tests with reasonable pmax
S parachute projected area ratio
accuracy, the use of this model for predictive analyses r
S nominal parachute area
was not accurate enough to set test conditions for either 0
t time
the drop or wind tunnel tests. A simpler empirical
t inflation time from start of inflation to full
model was found to be suitable for predicting opening inf
inflation
loads for the wind tunnel tests to a satisfactory level of
V velocity (airspeed)
accuracy. However, this simple empirical model is not
V effective velocity (airspeed)
applicable to the drop tests. eff
V airspeed in the wind tunnel far upstream of the
∞
parachute canopy
SYMBOLS AND ACRONYMS
λ geometric porosity based on the constrained vent
A wind tunnel cross sectional area at the test section g,c
area
AD disk area Γ volume of a hemisphere with diameter equal to D
AV,c constrained vent area ρ0 fluid density 0
C S parachute drag area
D
DGB Disk-Gap-Band

DTST Drop Test
*Aerospace Engineer, [email protected] DTV Drop Test Vehicle
†Analytical Engineer, [email protected] FI Full Inflation
‡Engineering Program Manager, SI Start of Inflation
[email protected], Member AIAA LS Line Stretch
This material is declared a work of the U.S. MER Mars Exploration Rover
Government and is not subject to copyright protection MF Mortar Firing
in the United States.
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American Institute of Aeronautics and Astronautics
MSL Mean Sea Level inflation, was key to the design and execution of both
NFAC National Full-Scale Aerodynamics Complex the drop and wind tunnel tests conducted in support of
PL Peak Load the MER structural qualification program. During the
WTMD Wind Tunnel Mortar Deployment design of the drop test vehicles (DTV), opening loads
analyses of various types (including those described
INTRODUCTION later in this paper) were used to determine the mass
required to achieve peak opening load at full inflation,
The structural qualification of the parachute for the as well as the time from DTV release to mortar fire
Mars Exploration Rover (MER) mission1,2 was needed to obtain the desired peak load. Those
conducted through a combination of full-scale low- predictions were hampered by lack of a suitable
altitude drop tests3,4 and wind tunnel tests in the analysis. Predicting the opening loads during the full-
National Full-Scale Aerodynamics Complex (NFAC) scale wind tunnel tests was somewhat simpler, since the
80- by 120-foot wind tunnel at the NASA Ames wind tunnel provides an almost constant and
Research Center.5 The parachute used by MER is of predictable dynamic pressure during parachute
the Disk-Gap-Band (DGB) design developed for the inflation.
Viking mission6 in 1976 and used on several other
missions including Mars Pathfinder7,8 in 1997. As with The objective of this paper is to present the opening
prior missions,9 the MER parachute will be deployed by loads data and retrospective analyses for the MER
a mortar.10 parachute drop and wind tunnel structural qualification
tests. This information can provide insights into
The primary objective of the tests discussed herein opening loads analyses at subsonic speeds that may be
was to show compliance with the MER parachute’s useful for future programs using similar DGB
structural qualification requirement. For the MER parachutes. Although results are presented for both the
mission, this requirement stated that the parachute had drop and wind tunnel tests, more emphasis is placed on
to withstand a load 25 percent greater than the the latter because the data obtained during the wind
calculated maximum opening load at Mars without tunnel test series was more accurate and thus easier to
sustaining damage that would affect parachute interpret.
performance. The maximum predicted opening load at
Mars, including various safety factors and allowances An understanding of, and the ability to model
for uncertainties, was calculated to be 19,360 lb. This unsteady aerodynamic forces was critical to the opening
resulted in a peak load qualification requirement of loads analyses. The apparent mass model used to
24,200 lb for the MER parachute. During Mars analyze the data is presented in the next section. This
operation, parachute inflation occurs under near-infinite model is based on the separation of opening loads into
mass conditions resulting in an almost constant two components: the first due to steady-state drag
dynamic pressure during the parachute deployment and forces and the second due to apparent mass effects.
inflation. As a consequence the peak opening load Modification of this analysis to account for wind tunnel
during Mars operation occurs when the parachute blockage effects, for use in analyzing the wind tunnel
reaches full inflation. Thus, an additional requirement test data, is also presented. This is followed by a
for the structural qualification tests was that the peak presentation of the data and analyses for the drop tests
opening load had to occur at full inflation. It should be and the wind tunnel tests. Comparisons of the results
noted that these tests were only approximations of the for these two test programs are also discussed. Finally,
conditions to be encountered at Mars. It was not an assessment of the suitability of the analytical model
feasible to reproduce all of the MER mission to predict opening loads is presented.
parameters of importance. All tests were conducted at
low altitudes on Earth and at subsonic speeds. On Mars ANALYTICAL FRAMEWORK
the parachute will be deployed at supersonic speeds in a
low-density atmosphere (approximately two orders of The equations used to analyze the data are a variant
magnitude less dense than Earth’s atmosphere at sea of those presented by McEwan,12 with the main
level). A discussion of the factors relevant to the differences appearing in how the apparent mass and
testing and qualification of parachutes at low altitudes steady state drag force are modeled. During a vertical
on Earth and subsonic speeds, when the intended drop, following mortar deployment of the parachute,
operation is in a low-density atmosphere at supersonic the force generated by the parachute on the DTV from
speeds, is given in reference 11.
The ability to predict the peak opening load, and the
point in time at which it occurs during parachute
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American Institute of Aeronautics and Astronautics
the start of inflation§ to full inflation, F , is assumed to Using test data from various sources, one can determine
p
be given by: all quantities in equation 6 except for k . Since the peak
0
opening load is of greatest interest, substituting the
d {( ) } appropriate values of the various quantities into
F =qC S+ m +m V −m g (1)
p D dt a p p equation 6 at peak opening load allows for the
where q is the dynamic pressure, C S is the parachute determination of k . Having identified a value for k , an
D 0 0
drag area, t is time, m is the apparent mass (due to the analytical reconstruction of a given test that yields the
a
fluid), m is the physical mass of the parachute, V is the same peak opening load as the test data can be
p
velocity (assumed to be equal to the airspeed), and g is generated. However, the reconstruction between the
the acceleration of gravity.¶ Performing the indicated start of inflation and full inflation will only be
differentiation yields: approximate due to the simplicity of the model and
various measurement uncertainties. Predictive analyses
dV dm for another test can be conducted with this value of k
F =qC S+(m +m ) + a V −m g (2) 0
p D a p dt dt p and the associated Sr vs time curve.
The apparent air mass is assumed to be described by: It is realized that equation 6 represents an
approximate model for a problem of greater physical
m =k ρΓ S32 (3) complexity. However, the expectation is that enough
a 0 0 r
flexibility is available in this approximate model to
where ρ is the fluid density and Γ is the volume of a yield predictions of engineering value when calibrated
0 with relevant experimental data.
hemisphere with a diameter equal to the nominal
diameter of the parachute, D : Γ =πD3 12. S is the
0 0 0 r ADJUSTMENTS FOR WIND TUNNEL TESTS
parachute projected area ratio, a nondimensional ratio,
S =S S , where S is the projected parachute area
r p p p When performing parachute tests in a wind tunnel,
max
at a given point in time during the inflation and S is the effect of wind tunnel blockage must be accounted
p
max
the maximum projected parachute area. The constant k for. This blockage increases the effective velocity and
0
is nondimensional. Performing the differentiation for dynamic pressure experienced by the canopy. In the
ma indicated in equation 2 results in: present investigation the effective velocity, Veff, and
effective dynamic pressure, q , are used in place of V
eff
dm dρ 3 dS and q for analyzing the data obtained from wind tunnel
a =k Γ S32 + k ρΓ S12 r (4)
dt 0 dt 0 r 2 0 0 r dt tests. The methods used to determine Veff and qeff are
discussed in this subsection.
The term C S is approximated as:
D Prior wind tunnel tests of the Space Shuttle Orbiter
C S=C S S (5) ribbon parachute13 conducted in the 80- by 120-foot test
D D0 0 r section of NFAC, found that the following blockage
equation yielded good results:
where C is the steady-state parachute drag coefficient
D
0
for a fully inflated parachute and S is the nominal F
0 q =q +1.85 p (7)
parachute area. Substituting equations 3, 4, and 5 into eff ∞ A
equation 2, and neglecting terms involving m and dρ/dt
p
since they are small as compared to the other terms over where q is the wind tunnel dynamic pressure far
∞
the time period of interest, yields the basic equation for upstream of the parachute canopy and A is the wind
the analysis of the test data and calculation of opening tunnel cross sectional area at the test section which was
loads: assumed to be 9,600 ft2 for the 80- by 120-foot wind
tunnel# (note that the notation here is somewhat
dV 3 dS
F =qC S S +k ρΓ S32 + k ρΓ S12 r V (6) different from that used in reference 13). This equation
p D0 0 r 0 0 r dt 2 0 0 r dt was based on work performed by Macha and
Buffington14 and derived under the assumption of
steady-state conditions. Tests performed by the MER
§ Start of inflation is used to refer to the point where the team5 validated its applicability to DGB parachutes
load is at a minimum following the snatch load.
¶ Force is used to denote aerodynamic forces; load is
used to denote both aerodynamic forces and forces of # The actual cross sectional area of the tunnel is slightly
other origins (e.g., mortar recoil, snatch). smaller due to an acoustic lining around the test section.
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American Institute of Aeronautics and Astronautics
under steady-state conditions. In the present TEST PARACHUTES
investigation the conditions are dynamic: as the
parachute inflates blockage increases over a time scale All parachutes used in the drop test series were
of several seconds. Given that the blockage arises due geometrically similar to that used for the Mars
to parachute inflation (with a corresponding increase in Pathfinder mission7,8 and thus were classified to be of
projected area), F in equation 7 was replaced by the MPF type. This parachute type is a modified
p
q C S S in the present investigation. Solving the version of that used by Viking.6 The changes from the
eff D 0 r
0
Viking type are mainly in the ratios of band height, gap
resulting equation for q yields:
eff height, and vent diameter to nominal diameter. The
q = q∞ (8) dduesriinggn tnhoem dinroapl dteiasmt seeterire, sD w0,a so f4 9th.5e fpt.a r aCcohnustetrsu cutsioedn
eff C S S
1−1.85 D0 0 r details for these parachutes are given in table 1. Minor
A
structural variations existed between these test
parachutes. All parachutes were pressure packed to a
V can then be determined from:
eff density of approximately 45 lb/ft3 in bags suitable for
mortar deployment.
2q
V = eff (9)
eff ρ TEST SETUP AND PROCEDURES
When analyzing or predicting the outcome of a wind DROP TEST VEHICLES AND INSTRUMENTATION
tunnel test, q and V in equation 6 were replaced by q
eff One of the drop test vehicles is shown in figure 2. It
and V , respectively. During the wind tunnel tests
eff consists of a cylindrical/conical steel nose (serving as
conducted under the present investigation, q was
∞
ballast and accounting for most of the DTV’s weight), a
almost constant from the start of inflation to full
cylindrical aluminum body, and large wire-braced fins
inflation. The changes in q and V were almost
eff eff fabricated from aluminum honeycomb sheet. The
entirely due to blockage effects as the parachute
cylindrical body served to house most of the
inflated.
instrumentation and the mortar. This DTV design
minimized the vehicle’s mass moments of inertia, thus
Two key points need to be emphasized regarding the
maximizing the effect of aerodynamic forces and
blockage correction. First, equation 7 was validated for
reducing the total angle of attack at mortar firing. The
steady-state test conditions. Its use in the present
fins were considered to be expendable and were
investigation for dynamic conditions is an extrapolation
replaced as necessary after each test. The steel nose
that has not been validated. Second, replacing F in
p and the cylindrical body suffered little to no damage
equation 7 by q C S S to yield equation 8 was done
eff D0 0 r after each test, and were re-used with minimal
to avoid unrealistic blockage effects. Since F at full
p refurbishing. Two versions of the DTV were used: one
inflation is significantly greater than the steady-state
with a nominal weight of 3,000 lb and a second with a
drag of the parachute just a fraction of a second earlier,
nominal weight of 8,000 lb. A more detailed
equation 7 yielded high values of the blockage at full
description of the design of the DTV is given in
inflation. These values were deemed to be unrealistic -
reference 4.
calculating blockage by equation 8 yielded more
plausible values without a high rate of change in
The instrumentation most relevant to this study were
blockage near full inflation. Given that the inflated
the accelerometers and digital video cameras. Two sets
parachute changed little in projected diameter from
of accelerometers were mounted to the DTV. A
peak load to a fraction of a second later,** the behavior
Crossbow VG400CB-200 gyroscope provided
predicted by equation 8 is more plausible.
accelerometer data with respect to three mutually
orthogonal axes. This gyroscope had an accelerometer
DROP TESTS range of ±10 g in each axis. Data from the Crossbow
VG400CB-200 was stored in an on-board digital data
Six tests were conducted during the drop test series.
logger using a 12-bit analog-to-digital converter. The
These tests were denoted by DTST (Drop Test) and a
uncertainty associated with this combination of
three digit number: DTST 037, DTST 038, DTST 040,
accelerometer and analog-to-digital converter was
DTST 049, DTST 050, and DTST 071.
±0.11 g. The second set of acceleration measurements
was provided by a Crossbow CXL10P3 accelerometer
** Due to their construction, MER DGB parachutes did device. Accelerations along three mutually orthogonal
not experience significant over-inflation. axes, with a range of ±10 g in each axis, were obtained
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American Institute of Aeronautics and Astronautics
from this device and recorded in an on-board digital logger recording was initiated upon release. At a pre-
data logger using a 8-bit analog-to-digital converter. set time following release the mortar was fired by an
The uncertainty associated with this combination of on-board timing sequencer, deploying the parachute.
accelerometer and analog-to-digital converter was The time from release to mortar firing was calculated
±0.26 g. Both the 12- and 8-bit data loggers recorded and set based the desired peak opening load for each
data at 200 Hz. For all but one of the drop tests (DTST test. Upon landing, the parachute was secured and
071), the data from the Crossbow VG400CB-200 stored removed from the DTV. All recording instruments
in the 12-bit data logger were used in the analyses were then turned off and removed for downloading of
presented in this paper because of its higher accuracy. data. The DTV was then reloaded on its launch cradle
For drop test DTST 071 the data from the CXL10P3 for transportation back to the hangar where it was
stored in the 8-bit data logger was used in the analyses refurbished as necessary and prepared for the next test.
presented in this paper (the data from the VG400CB-
200 stored in the 12-bit data logger for DTST 071 was RESULTS AND ANALYSES
corrupted due to landing damage). The accelerometer
data were used to calculate loads, DTV velocity and Six tests were conducted during the drop test series.
altitude (by integrating the accelerometer data), and The first two tests (DTST 037 and 038) were conducted
dynamic pressure (by combining the calculated velocity with a DTV of 3,000 lb nominal weight, whereas the
and altitude with the appropriate atmospheric sounding final four tests (DTST 040, 049, 050, and 071) were
data). In addition, accelerometer data were used to conducted with a DTV of 8,000 lb nominal weight. A
determine the timing of key events such as mortar summary of the drop test results is given in table 2.
firing. Note that the accelerometers were selected for
the determination of loads. They were adequate for this It became evident after the first two drop tests that it
purpose. However, the analyses presented in this paper was impossible to achieve the desired peak load at full
required additional information. Although a best effort inflation for these parachutes (a test requirement) with
was made to derive accurate velocities and dynamic the 3,000 lb DTV – the weight was insufficient to yield
pressures as described above, these quantities had this outcome. As can be seen from table 2, peak load
significant uncertainties. This should be kept in mind for both of these tests occurred at a time approximately
when considering the results of analyses that follow. 66 percent from the start of inflation to full inflation. A
Additional relevant data was obtained from two aft- second lower load peak occurred at full inflation.
facing digital video cameras mounted on the DTV Figure 3 shows the load, dynamic pressure, and
cylindrical body. These digital video cameras recorded parachute projected area ratio vs time for DTST 038
at 30 frames per second. Data from these video with key events identified. The non-dimensional
cameras were used to determine S . Digital video projected area ratio, S, is shown increasing from zero
r r
cameras operated by observers on the ground yielded to one during the inflation time (S is multiplied by 10
r
additional timing information of key test events. There in this and subsequent figures to make it visible in the
was additional instrumentation on the DTV (e.g., angle same scale as q). At mortar firing, the DTV was
of attack and sideslip vanes and transducers), but the accelerating and the dynamic pressure was increasing.
data generated was not directly related to the subject As the parachute inflated, the parachute drag force
matter of this paper (i.e., opening loads). began reversing the DTV acceleration, causing the
dynamic pressure to reach a peak value and then drop
TEST PROCEDURES rapidly. This resulted in a lower dynamic pressure at
full inflation than experienced earlier in the inflation
Drop testing was conducted at the Idaho Army process. Under these conditions, the combination of
National Guard Orchard Training Range south of Boise, dynamic pressure, parachute drag area, and unsteady
Idaho. The DTV was assembled in the hangar and aerodynamic forces resulted in the peak load occurring
secured to a launch cradle that held it vertically. The prior to full inflation. After examining the data from
instrumentation was then installed and checked. A DTST 037 and 038, it was decided to increase the
packed parachute was placed in the mortar mounted on weight of the DTV to 8,000 lb for subsequent tests. By
the cylindrical body of the DTV. The fully assembled doing this, the deceleration caused by the parachute
DTV, on its launch cradle, was then transported to the during the inflation process would be reduced, thereby
drop zone. The mortar gas generator was installed at limiting the decrease in the dynamic pressure, and
the test site. After starting all video cameras and yielding the desired peak load at full inflation.
connecting the lifting cable and release to the DTV, a
helicopter lifted the DTV to the desired release altitude. The four additional drop tests (DTST 040, 049, 050,
Once stabilized at the required altitude, the DTV was and 071) were conducted using the 8,000 lb DTV. All
released at the command of the test coordinator. Data four of these tests yielded peak loads at full inflation.
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American Institute of Aeronautics and Astronautics
Figure 4 shows load, dynamic pressure, and parachute equation 4) and is very sensitive to dS /dt at full
r
projected area ratio vs time for DTST 040. Although inflation. This implies that an accurate determination
the dynamic pressure decreased from its peak value, the of dS /dt at full inflation is critical. Unfortunately,
r
drop was not sufficient to cause the peak load to occur determining an exact value of this derivative at full
before full inflation. The observed plateau in the load inflation was difficult. The S vs time data obtained
r
profile is attributed to a hesitation in the parachute from the digital video camera had to be splined to
inflation as seen in the S curve. A plot of peak load vs obtain a smooth function S(t). However, this splining
r r
mortar firing time, as shown in figure 5 highlights the process is not unique; various equally valid smooth
key difficulty encountered in undertaking the drop test functions S(t) can be derived from the same digital
r
series: predicting the peak opening load. The video data. This non-uniqueness becomes even more
correlation between peak load and mortar firing time critical for dS /dt at full inflation since S increases
r r
was poor. For example, a comparison of the results rapidly at this point. An additional source of uncertainty
from DTST 040 and DTST 049 indicated that two tests in this analysis arose from the manner in which the
with significantly different mortar firing times; 3.10 and velocity and dynamic pressure were determined. These
2.32 seconds, yielded essentially the same peak load; quantities were derived by integration of the
29,731 and 29,518 lb (see table 2). During the drop test accelerometer data. Although this was considered to be
program, predicting the required mortar firing time to the best possible way of obtaining these quantities from
achieve a desired peak load was accomplished by the available data, concerns remained regarding
applying simple drag-area models and the apparent accuracy. Discrepancies in the values of V and q have
mass model to the available data. The accuracy of significant effects on the two main terms in the force
these predictions was poor – drop tests often resulted in calculation: qC S S and m˙ V. In addition, the total
D 0 r a
0
peak loads very different from those desired. This was
angle of attack at mortar firing varies from drop to drop
in sharp contrast to wind tunnel testing (described in the
as shown in table 2. These issues may partially account
next section) where load prediction by a simple
for the varying degrees of success observed in matching
empirical method yielded more accurate and consistent the load data for 0 ≤ t/t < 1, and the range of values of
inf
results. The non-dimensional inflation distances
k obtained from these four drop tests (0.425 to 0.685)
obtained from these four tests, as shown in table 2, were 0
as shown in table 2.
fairly consistent, varying from 7.93 to 9.12 nominal
diameters, with an average of 8.60.
WIND TUNNEL TESTS
Apparent mass model reconstructions and test data
Twelve tests were conducted during the wind
for the four drop tests conducted with the 8,000 lb DTV
tunnel test series. These tests were denoted WTMD 1
(i.e., DTST 040, 049, 050, and 071) are shown in
through WTMD 12, where WTMD stands for Wind
figures 6 through 9. Note that the results are plotted
Tunnel Mortar Deployed.
against non-dimensional time, t/t , where t is the
inf inf
inflation time from the start of inflation to full inflation
TEST PARACHUTES
(also peak load for these tests), and time t is zero at the
start of inflation. All four of the parachutes suffered
Construction details for each of the parachutes used in
structural damage at full inflation, therefore no data is
these tests are given in table 3. During this test series
presented for times after full inflation. In these figures
various parachute designs were investigated. Among
the experimental and analytical total loads are shown,
the parachute parameters varied were band height,
as well as components of the analytical load: qC S S ,
D 0 r constrained vent diameter, fabric material, and
0
m˙ V, and m V˙. By design, the experimental and construction technique. The objective in varying these
a a
parameters was to determine the parachute design that
analytical values of the peak load agree exactly, since k
0
would yield the best combination of drag, stability, and
was set to a value that would yield such agreement. For
0 ≤ t/t < 1, the agreement between the analytical total inflation reliability while meeting the structural design
inf
and packed volume requirements. After structural
load and the experimental load data is fair (DTST 040)
problems were encountered with the parachutes tested
to good (DTST 071). The analysis indicated that the
in drop tests DTST 040, 049, 050, and 071, the MER
qC S S term accounted for approximately 50 percent
D0 0 r parachute had to be re-designed. The wind tunnel tests
of the total load with the m˙ V term being of
a described here were part of the re-design and final
approximately equal magnitude. The m V˙ term is structural qualification. All parachutes were pressure-
a
negative and significantly smaller in absolute value packed to a density of approximately 45 lb/ft3. The
than either the qC S S or m˙ V terms. Note that the final parachute design, selected for the MER mission,
D 0 r a
m˙ V term depends0 on the time derivative of S (see was tested in WTMD 11 and 12. The parachute used
a r
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American Institute of Aeronautics and Astronautics
for WTMD 11 was an engineering prototype, while that Dynamics Tunnel15 indicated that the trim angle of
used for WTMD 12 was a qualification test article built attack for this type of DGB parachute was less than 10°.
concurrently with the flight lot. Thus, in the present investigation, the error induced by
not differentiating between the tangential and drag
TEST SETUP AND PROCEDURES forces (or their coefficients) was negligibly small and
was therefore ignored. The force reading from the load
The final structural qualification test series was cell during inflation was an accurate measurement of
conducted in the NFAC 80- by 120-foot wind tunnel F . Dynamic pressure was measured approximately
p
with full-scale parachutes. These tests used an existing 150 ft upstream of the parachute canopy, yielding q .
∞
facility strut and ball-and-socket joint that allowed the With the transducers mounted close to the pitot-static
placing of test-specific hardware near the centerline of tube, the lag in the q measurement was considered to
∞
the test section. The test-specific hardware consisted of be negligible. Both the load cell and dynamic pressure
a mounting system that held the mortar, load cell, and measurements were sampled at 200 Hz by the data
two aft-facing digital video cameras. This mounting acquisition system. The aft-facing digital video
system was counterbalanced and allowed to pivot on cameras recorded at 30 frames per second. Data from
the ball-and-socket joint on top of the strut. By these video cameras was used to determine S. A more
r
allowing the mounting system to pivot, side loads on thorough description of the wind tunnel test setup and
the load cell were reduced to a minimum, enhancing its operation is presented in reference 5.
measurement accuracy and reducing the possibility of
load cell failure due to side loads. In addition, attaching RESULTS AND ANALYSES
the two aft-facing digital video cameras directly to the
pivoting mounting system enabled them to track the Figure 12 shows a typical example of the data
parachute at all times. Figure 10 shows the mounting obtained during a wind tunnel test. From the load cell
system during final installation on top of the strut (note data, key points in the deployment and inflation
the two aft-facing digital video camera housings on sequence can be determined: mortar firing followed by
either side of the mortar barrel). During testing, the the peak mortar firing reaction load, line stretch
mortar was held at a 10° angle above horizontal by a set followed by the peak snatch load, the start of inflation,
of Kevlar™ break-ties. These break-ties held the and the peak load. The term “start of inflation” is used
mortar aligned during firing. By pointing the mortar up to refer to the point where the load is at a minimum
by 10°, the parachute pack traced a parabolic trajectory following the snatch load. This point provided a clearly
which placed the parachute directly downstream of the identifiable point to start the inflation analyses. From
mortar at the onset of the inflation. As the parachute figure 12 it can also be seen that q is essentially
∞
inflated, and the drag force increased to approximately constant during parachute inflation. However, the
1,000 lb, the break-ties released, allowing the mounting effective dynamic pressure, q , increases during
eff
system to pivot freely. The test procedure involved inflation to a value 18 percent greater than q at full
∞
increasing the wind tunnel airspeed until the desired inflation for this test. This is due to the wind tunnel
dynamic pressure was reached, starting the data blockage as discussed earlier; q is calculated from
eff
acquisition system, and firing the mortar which resulted equation 8. The curve for S indicates that the peak
r
in the deployment and inflation of the parachute. Once load occurred at full inflation to within the accuracy of
inflation was completed, the data acquisition system the time tag in the digital video camera from which it is
was turned off and the airspeed reduced. At airspeeds derived. The digital video data for S was smoothed by
r
below 17.5 knots, personnel were allowed in the tunnel a spline for plotting and use in the analyses that
test section to inspect the parachute and gather it as the followed. Note that the plateau observed in the drop
airspeed was reduced to zero. Figure 11 shows the test load curves (see figures 6 through 9) was also
strut, mounting system, and inflated parachute. present in the wind tunnel test results. Again, the
hesitation in the inflation, as shown by the S curve, is
r
The instrumentation most relevant to this study are the presumed reason for this plateau.
the load cell, dynamic pressure transducer, and aft-
facing digital video cameras. The load cell was A summary of the wind tunnel test results is
calibrated over a range from –16,000 to 35,000 lb and presented in table 4. Mortar performance and parachute
had an estimated accuracy of ±35 lb. Since the load extraction from its deployment bag are consistent
cell tracked the parachute (because it was mounted on a across tests, as can be seen from the times from mortar
pivoting mechanism), it measured the parachute firing (MF) to line stretch (LS), and from mortar firing
tangential force, not the drag force. However, earlier to the start of inflation (SI). The peak mortar firing
wind tunnel tests with sub-scale models performed at reaction load had a range from –9,091 to –11,150 lb,
the NASA Langley Research Center Transonic and the peak snatch load a range from 867 to 1,450 lb.
7
American Institute of Aeronautics and Astronautics
Note that these were transient loads, the 200 Hz In the present analysis a value of 727.5 ft2 was used for
( )
sampling rate limited the accuracy to which these peak C S . This is the value of C S for the
loads could be resolved. The non-dimensional inflation D0 0 normalized D0 0
parachute used in the final qualification test (WTMD
distance of the parachutes (i.e., last row of table 4)
12). The normalized peak load is plotted vs q in
varied greatly. During test WTMD 2 the parachute eff
figure 14. As can be seen, a good correlation can be
partially inflated, and assumed a stable state that was
established between the normalized peak load and q .
not fully inflated. While lowering the wind tunnel eff
A linear regression through the data yielded a fit with a
airspeed, the parachute ultimately achieved a fully
maximum residual of –1,207 lb, and a 95 percent
inflated shape at an airspeed significantly lower than
confidence interval for individual predictions (i.e.,
that at mortar firing. The data at peak load shown in
future tests) of approximately ±2,000 lb about the linear
table 4 for WTMD 2 was at this lower airspeed. In test
regression. From the standpoint of loads predictions for
WTMD 4 the parachute again assumed a stable state
similar wind tunnel tests, this figure yielded two
that was not fully inflated, but eventually inflated fully.
interesting conclusions. First, for tests with DGB
This was reflected in the non-dimensional distance
parachutes, the linear fit shown in figure 14 could be
required to inflate the parachute in this test: 39.09. No
used to estimate the peak load during similar wind
clear connection could be established between the
tunnel tests to a useful level of accuracy. Second, for
parachute geometry and the inflation distance across the
tests with other parachute types, a similar relationship
parachutes tested. For otherwise identical parachutes
between the normalized peak load and q might be
however, decreasing the vent area decreased the non- eff
generated from the results of a few tests. Such a
dimensional inflation distance (compare tests WTMD 7
correlation, if it is generated with data at lower peak
and 8 against WTMD 9 and 10). Blockage of the wind
loads, can help in determining the value of q required
tunnel test section increased q by 14 to 18 percent eff
eff to achieve higher peak loads with less risk of overload.
above q at peak load, depending on the parachute drag
∞
During the present investigation, the peak loads for the
coefficient and nominal area. Blockage can be assessed
first few tests were predicted using data from the drop
by the value of q /q . For all tests q was essentially
eff ∞ ∞ test program and analyses based on the apparent mass
constant during inflation.
model. After a few data points were acquired, an
empirical fit similar to the one shown in figure 14 was
The peak loads (excluding test WTMD 2) ranged
generated to predict the peak load of later tests. This
from 15,336 to 29,442 lb. In all cases, peak load
empirical fit was updated as additional data was
occurred at full inflation. An interesting parameter to
acquired.
study is C, the opening load factor, which is given by:
x
Data from all wind tunnel tests were analyzed using
Peak Load
C = (10) the apparent mass model discussed earlier. Results for
x q C S
eff D 0 three of these analyses are presented here. The three
0
tests chosen for presentation are WTMD 5, 11, and 12.
This is the ratio of the peak load generated by the Test WTMD 5 was chosen because the parachute used
parachute at full inflation (due to both unsteady and in this test was the same as that used during drop test
steady components) to the steady state force at an DTST 038 and thus provided a good comparison
equivalent dynamic pressure. As can be seen from between the drop tests and the wind tunnel tests. Tests
table 4, the values of Cx lie within the range from 1.57 WTMD 11 and 12 were chosen because they were the
to 1.86 across all parachutes tested. Figure 13 shows Cx final qualification tests for the MER parachute, using
plotted vs qeff for all tests except WTMD 2. As can be identical parachutes at the same test condition. The
seen, Cx is shown to be dependent on qeff. The Recovery parachute used in WTMD 11 was an engineering
System Design Guide17 cites a value of ~1.30 for DGB prototype; the parachute used in WTMD 12 was a
parachutes (p. 76). It is possible that this value was qualification unit fabricated concurrently with the flight
generated using data from the numerous high-altitude parachutes. Figure 12 shows the key parameters for test
supersonic flight tests of DGB parachutes in the 1960’s WTMD 5; figures 15 and 16 present the same data for
and 1970’s where apparent mass effects were smaller. tests WTMD 11 and 12. A comparison of figures 15
A more interesting way of looking at the peak load test and 16 indicates that the test conditions and results for
data is by normalizing it to a common parachute drag tests WTMD 11 and 12 are almost identical, showing
coefficient and nominal area through the equation: good test-to-test repeatability. Figures 17, 18, and 19
( ) show the experimental results and analytical
C S reconstructions (using the apparent mass model) for
Peak Load =Peak Load D0 0 normalized (11) tests WTMD 5, 11, and 12. Note that the results are
normalized C S
D0 0 plotted against non-dimensional time, t/tinf, where tinf is
8
American Institute of Aeronautics and Astronautics
the inflation time from the start of inflation to full as did wind tunnel test WTMD 5. The data used for
inflation (also peak load for the wind tunnel tests) and most of the following comparisons can be found in
the time t is zero at the start of inflation. For test table 2 for the drop tests and table 4 for the wind tunnel
WTMD5 (figure 17) no data is shown for t/t > 1 since test.
inf
the parachute was damaged at full inflation voiding
further data analyses. In these figures the experimental The non-dimensional inflation distance for the four
and analytical total loads are shown, in addition to the drop tests varied from 7.93 to 9.12 with an average of
components of the analytical load: q C S S , m˙ V , 8.60. The value for wind tunnel test WTMD5 was
eff D 0 r a eff
0
and m V˙ . After peak inflation the m˙ V and m V˙ 10.27 - above the range and average value for the drop
a eff a eff a eff tests. Calculated values of k , for the drop tests
terms are negligible, and the drag is mainly due to 0
considered here varied from 0.425 to 0.685 with an
q C S . As with the analysis of the drop test data, average of 0.534. The calculated value of k for wind
eff D0 0 0
the wind tunnel experimental and analytical values of tunnel test WTMD 5 was 0.420 - at the lower end of the
the peak load agree exactly since k was set to a value range and below the average value for the drop tests. In
0
that would yield such agreement. For 0 ≤ t/t < 1, the comparing values of the opening load factor, C , the
inf x
analytical total load prediction agreed well with the dynamic pressure at full inflation should be as close as
experimental data for test WTMD 5; the agreement was possible since, as shown in figure 13, C is dependent
x
only fair for tests WTMD 11 and 12, with a tendency on this parameter. Drop test DTST 071 was chosen for
for the analysis to overpredict the total load. The comparison against wind tunnel test WTMD5 because
analysis indicated that the q C S S term dominated these two tests had similar values of q and q at full
eff D 0 r eff
0 inflation (q = 15.1 psf for DTST 071; q = 14.5 for
the load calculation. However, the m˙ V provided a eff
a eff WTMD 5). The opening load factor for tests DTST
significant contribution to the total load at full inflation.
071 and WTMD 5 were calculated to be 1.85 and 1.75,
The m V˙ term was positive and accounted for only a respectively – within 6 percent of one another. The
a eff
small fraction of the total load; not more than a few observed discrepancies in these quantities (i.e., inflation
hundred pounds at full inflation. Again note that the distance, k , and C ) may be within the test-to-test
0 x
m˙ V term depends on the time derivative of S (see variation and/or the accuracy of the drop test
a eff r
measurement capability. However, these differences
equation 4) and is very sensitive to dS /dt at full
r
may also indicate problems with the apparent mass
inflation. The remarks made in the drop test section
model as currently implemented or reflect differences
regarding the accuracy of dS /dt and its effect on the
r
in the inflation of the parachutes due to the test method.
analysis results also apply to the wind tunnel test. This
The limited amount of tests that can be compared
issue may account for the varying degrees of success
observed in matching the load data for 0 ≤ t/t < 1 and directly (i.e., one wind tunnel test and four drop tests)
inf
makes it impossible to make definitive statements
the range of values of k obtained from the eleven valid
0
without further analysis.
wind tunnel tests (0.420 to 0.640). Note that concerns
regarding uncertainties in the velocity (airspeed) and
Figure 20 shows the non-dimensional inflation
dynamic pressure in the discussion of the drop test data
curves (i.e., S vs t/t ) for the four drop tests and wind
are not applicable to the wind tunnel test data. The r inf
tunnel test WTMD 5. Since the purpose was to
variation in k obtained from the wind tunnel test data
0
compare drop and wind tunnel tests, inflation curves for
analyses also reflects the differences in parachute
all drop tests are shown as dashed lines and the wind
geometries during the wind tunnel test series. For the
tunnel test is shown as a solid line. All non-
cases examined here in detail, k was 0.420 for test
0
dimensional curves are very similar and exhibit the
WTMD 5, 0.597 for test WTMD 11, and 0.589 for test
same hesitation in the inflation process at some point
WTMD 12.
between t/t = 0.6 and 0.9. The similarity in these
inf
curves indicates that the parachute inflation profile, as
COMPARISON OF DROP AND WIND TUNNEL TEST
given by S, does not differ significantly between drop
DATA AND ANALYSES r
and wind tunnel tests.
The parachute used in wind tunnel test WTMD 5
was of the same size and geometry as those used in the CONCLUDING REMARKS
drop test series. This provided the opportunity to make
Parachute opening load test results for various DGB
a direct comparison of the results of drop and wind
parachutes have been presented. These test results were
tunnel tests. The drop tests considered in this
obtained from both drop tests and wind tunnel tests. Of
discussion are DTST 040, 049, 050, and 071. These
these two test methods the wind tunnel tests yielded
parachute drop tests reached peak load at full inflation,
more accurate and repeatable data.
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American Institute of Aeronautics and Astronautics