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Int..L Impact Engng, Vol. 17, pp. 903 914, 1995 Copyright (C 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0734-743X/95 $9.50+0.00
Pergamon
E X P E R I M E N T A L IMPACTS ABOVE 10 KM/S
James D. Walker, Donald J. Grosch, and Scott A. Mullin Southwest Research Institute San Antonio, Texas 78228
S u m m a r y - - I n the proceedings of the last symposium, recent work on a technique for launching small projectiles to hypervelocities above 10 km/s using an inhibited shaped charge was presented [1]. In the interim, experiments have been conducted using the inhibited shaped charge to launch aluminum, nickel, and molybdenum projectiles. This paper presents the results of the impact tests, as well as discusses the shaped charge design modifications for the nickel and molybdenum launchers. Radiographs are presented of the impacting projectiles, as are post test photographs of various targets. The data are unique in that they represent low D'D projectile impacts into both monolithic blocks and spaced plates at velocities above 10 km/s. The aluminum projectiles are being launched at 11.25_+0.20 km/s, the molybdenum projectiles at 11.72_+0.10 km/s, and the nickel projectiles at 10.81_+0.10 km/s. INTRODUCTION For satellites and spacecraft in low earth orbit, there are two possible sources of accidental impact. The first is impacts with meteoroids. The second, and of growing concern, is impacts from material left in orbit on previous flights, usually referred to as "orbital debris." From orbital mechanics, accidental impacts between man made objects can occur at velocities up to 15 km/s. These impact velocities are high enough that questions of melt and vaporization need to be addressed. As research is still necessary to validate, verify, and incorporate complex equation of state and constitutive models into hydrodynamic codes, it is still necessary to experimentally test the designs of shields intended to protect personnel and equipment. There are three approaches to hypervelocity impact testing for this problem: light gas guns (up to 9 km/s), shaped charge techniques (up to 12 km/s), and dissimilar material scaling (up to a scaled velocity of 18 km/s). Each technique has its advantages and drawbacks. A particular advantage of the inhibited shaped charge launcher (ISCL) is the ability to launch one-gram aluminum fragments above 11 km/s. This provides the ability to test complex shield designs at 11 km/s, which is close to the impact velocity that would be seen in a collision between objects in polar and equatorial orbits. This paper describes developments in the aluminum shaped charge launcher, extensions of the technique to molybdenum and nickel projectiles, and test results of several space station shield designs. The protection of space station components and on-board personnel from micrometeoroid and orbital debris impact is the motivation of the tests. T H E INHIBITED SHAPED CHARGE A shaped charge is a cavity within an explosive which is lined with metal. When detonated, this device generates a long, plastically-deforming jet of material that travels at high speeds [2]. Shaped charges have great penetrating capabilities, and have been used for many years in anti-armor warheads and oil well perforators. The long jet produced by standard shaped charges is not similar to the accidental threat to spacecraft. It is believed that most debris in orbit has a length to diameter ratio (L/D) of 1 to 2. Thus, the shaped charge was modified to isolate the high-speed jet tip from the rest of the jet. This was done by placing an inhibitor within the cavity of the charge [3-6,1]. The inhibitor allows the jet tip to form but prevents the remainder of the jet from forming or travelling down range. The isolated jet tip is the space debris simulant. In order to utilize the explosive launcher in an environment that simulates the conditions in space, an evacuated hypervelocity launcher facility was designed and fabricated at Southwest Research Institute (SwRI) for NASA-JSC [7]. 903
J.D. WALKER et al.
904
Design Modifications A number of modifications have occurred to the testing arrangement described in [1]. Detonation Configuration. To reduce the translational scatter of the impact point, the previous shaped charge initiation system was replaced by one using a precision initiation coupler (PIC). The PIC reduces the tolerances required on the positioning of the detonator by providing a large surface for the detonator to initiate. In cross section, the PIC is hour-glass shaped, varying from a large detonator-surface diameter to a small disk and the.n increasing to an even larger diameter on the high-explosive side (Fig. I). This geometry provides a point initiation of the explosive, the point being the center of the hour-glass configuration. Due to the delicacy of the PIC, a reduced-energy detonator (RP-80 Explosive Bridgewire instead of an RP-83) was used to prevent the detonator from overwhelming the PIC and preventing it from working as intended. Additional efforts were aimed at ensuring that the PIC detonation point was aligned with the conical liner and not with the explosive cylinder, as the explosive cylinder may not be concentric with the centerline of the liner due to uneven curing of the explosive. A vacuum-mandrel was fabricated that fit inside the liner and allowed the charge to be machined about the centerline of the liner. Using this technique, a 45* chamfer was cut on the rear end of the explosive. This 45 ° chamfer mated with a 45 ° chamfer on an aluminum PIC holder. This arrangement insured that the detonation train was positioned along the centerline of the liner. A marked improvement in the repeatability of the shotline was seen after this procedure was initiated. After a flight of 220 cm, scatter with the previous booster arrangement had been roughly +/- 6 cm, while scatter with the new detonation system has been roughly +/- 3 cm. Aluminum Liners. Aluminum is thought to comprise most man-made orbital debris, and is therefore the most common liner material used with the inhibited shaped charge launcher. The 1100-alloy aluminum shaped charge liners are now fabricated using a three step process. The initial step is to forge a round bar of the aluminum to a solid cone. This is completed with two knock-down forging operations. The second step is a forging operation, requiring about six passes, in which the cone is hollowed-out. The final fabrication step is to machine the roughed-out cone to final dimensions. About 0.10 cm of material is removed from both the inside and the outside of the cone during this step. The liners have a 30 ° included angle and a rounded apex with inner radius of 0.254 cm (Fig. 2).
RP-80 DetonatorWell ~ \
Pic \
PICH°lder ~ /
/- PBXN-5 / Explosive
/ 1/
l%%?~o?~.%o_~.o.o.~.<>i \
Octol Explosive 45 Degree Mating Surface Figure 1. Precision Initiation Coupler (PIC) and Assembly
Figure 2. 30* Liner Design
New Projectile Materials: Molybdenum and Nickel Liners In the past two years, the inhibited shaped charge launcher has been used to launch projectiles of materials with a density greater than that of aluminum , s 2.7 g/cm 3 [8]. The experimental work focused on two materials, molybdenum and nickel, with densities of 10.2 g/cm 3 and 8.9 g]cm 3, respectively. There is an upper bound on the launch velocities achievable by a shaped charge for the jet tip to be in one piece [9]. An attempt to drive the velocity of the jet greater than this velocity results in a radial expansion and break-up of the jet, referred to as incoherence, which defeats the purpose of launching a single projectile. For most materials, the requirement is that the collapse velocity of the charge is less than the low pressure longitudinal sound speed in the collapse point reference frame [10]. In the laboratory frame, the empirical engineering rule ~,,,p < 2.41c0
(1)
is used to determine the maximum velocity, where co is the low pressure bulk sound speed of the material [2]. This engineering requirement produces roughly the same results as the underlying collapse point frame
Experimental impacts above 10 km/s
905
requirement for some geometries of interest. According to Eqn. (1), the maximum velocity for an intact projectile would be roughly 12.3 km/s for molybdenum and 11.1 km/s for nickel. To design the shaped charge, an analytic shaped charge model was used. The model was based on the Pugh-Eickelberger-Rostocker [11] theory of collapse, modified to account for liner acceleration [12]. The model uses information about the geometry of the charge, the liner material, and the explosive to calculate details of the resulting shaped charge jet. The liner is divided into elements. Based on the calculated jet velocities and masses for each liner element, a velocity and a mass for the jet tip are calculated. Similarly, an average collapse velocity for the jet tip material can be calculated. The collapse velocity is the velocity at which the liner material, driven by the explosive pressures, converges on the axis of the liner, as observed in the collapse point reference frame. In general, higher velocities are achieved for thinner liners, as there is less mass Molybdenum Liner, 1 mm fhick, Oc|ol 1.6 , , , = , to be driven by the explosive. However, a thinner liner also means there is less material .~ 1.4 going into the jet, leading to a smaller projectile. It was found that for molybdenum E 1.2 and nickel to exceed 10 km/s with the Octol t~ ntal DataPoint explosive, the liner could not be much more -~ ~.o than 1 mm thick. To provide a flavor of what is involved in the design process, Fig. 3 was E 0.8 produced with the analytical shaped charge '~. model. The figure is for a shaped charge with ....... ~ ~ ! ~ ...... a through thickness of 1 mm on the liner, for ,~ 0.6 various liner angles. In order for the jet to be coherent, it is necessary that the collapse 0.4 velocity (the lower solid curve) be less than the maximum coherent collapse velocity (dotted 0.2 curve). The figure shows that a velocity of 11.8 km/s should be possible with I I I i I I I I 0.0 20 40 60 80 molybdenum for a 20 ° total liner angle, and that the jet tip should still be coherent. Also on the Liner Angle (deg) plot is the experimentally determined value of Figure 3. Molybdenum Liner Tip Velocity, the jet tip velocity from our tests Collapse Velocity, (ll.7:H).I kin/s), and the model is seen to and Tip Mass Versus Liner Angle accurately predict the experiments. ,
The 20 °, 1 mm through thickness molybdenum liners were fabricated with the same technique as the aluminum liners. The billet material for these liners was formed from pure (99.95%) molybdenum powder through powder metallurgy compression and sintering. The nickel liners were machined from a bar of commercially pure nickel alloy Ni 200 (in the "as received" cold drawn form, o,tt = 6.7 kbar). E X P E R I M E N T A L RESULTS To simulate the environment in orbit, the inhibited shaped launcher is used in conjunction with a vacuum chamber. The vacuum chamber has a variety of view ports and access ports to allow for extensive diagnostic equipment. To date, diagnostics have been primarily restricted to flash X-rays, with as many as eight X-ray heads used per test. During most tests, the heads are arranged to observe the projectile before impact. With this arrangement, the projectile geometry is observed and measurements of the relative position of the projectile versus the X-ray times allows its velocity to be calculated. In other tests, the flash X-rays are positioned to allow for the examination of the debris pattern behind the target plate. The flash X-rays are triggered at various delay times from the initial signal to the detonator. As the jet tip velocities are very repeatable, the delay times are calculated based on the position of each X-ray head along the shotline. Kodak direct exposure film (DEF) is used. This film does not require the use of intensifier screens. A 0.32 cm thick lexan sheet is placed over each film holder to protect the film from particles resulting from the impact test. A piece of felt is placed between the film holder and the lexan cover to reduce bruising of the film.
Aluminum Projectile The aluminum projectile produced by the inhibited shaped charge launcher is a hollow cylinder with an approximate L/D of 2 (see Fig. 4). Often, the leading and trailing edges of the projectile appear jagged or even slightly turned back. It is felt that these slight irregularities have little effect on the results of the impact tests. The cavity along the axis of symmetry is probably due to the jet being on the verge of
J . D . WALKER et al.
906
incoherence [10]. Overall, the aluminum projectiles produced are consistent and the inhibited shaped charge launcher provides a repeatable means of launching hypervelocity projectiles. Table 1 contains a complete statistical summary of the projectile geometry.
Table 1. Statistical Summary of Aluminum Projectiles
Mass (g)
L/D
Length (mm)
Outer Diameter (mm)
Velocity (km/s)
Impact Angle
Average
1.15
1.86
13.00
7.11
11.25
37.4 °
Std. Dev.
0.21
0.47
2.53
0.60
0.097
23.9 °
Maximum
1.56
2.92
18.16
7.95
11.42
75.0 °
Minimum
0.78
1.32
9.50
5.87
11.03
2.0 °
Figure 4. Orthogonal Radiographs of Two Aluminum Projectiles
1 Jet Temperature. Questions regarding the temperature of the projectile are occasionally raised. The concern is that the large deformations during the formation of the shaped charge jet leads to substantial heating, perhaps even enough to approach the melt temperature of the jet material. Conventional wisdom in the shaped charge community is that the jets produced by both aluminum and copper shaped charges are solid [13]. There have been a number of experimental efforts to directly address this question. The most extensive work has been performed for copper. For 42 ° copper lined charges, vol Holle and Trimble [14] performed two color infrared radiometry with a response time of less than one microsecond. For Comp-B charges they found an average jet temperature of 430°C and for Octol charges they found an average jet temperature of 540°C. Boih these values are well below the 1085°C melt temperature of copper. This indicates that copper jets are not molten. Experimental work has been performed with aluminum jets to address the temperature issue. Jamet [15] performed X-ray diffraction of aluminum jets in flight. These jets were produced by a 40 ° variable liner thickness with a TNT/RDX 35/65 charge, and had a jet tip velocity of 8.5 km/s. Distinct crystal structure demonstrated that the aluminum jet material was solid in flight. We performed shaped charge collapse calculations with CALE [ 16]. These calculations give an upper bound on the temperature rise of jet material of 450°C, which is below the aluminum melt temperature of 661 °C. The temperature rise is mostly due to dissipative plastic work during the formation of the jet, as large plastic strains occur. Also, flash X-rays of the projectile in flight (e.g., Fig. 4) reveal sharp edges, further evidence of the solidity of the material in the projectile. Thus, though the projectile material is at an elevated temperature, all available evidence argues that the material in the jet is solid.
Experimental impacts above 10 km/s
907
C o m a Characterization. It became apparent during testing that a cloud of fine particles travels just ahead of and in-line with the ISCL projectile. This cloud will be referred to as the c o m a of the jet tip. In addition, a number of small particles typically travel adjacent to the projectile. Since a single projectile impact on the target is of interest, these particles are undesirable. However, as it appeared to be impossible to remove the coma, an effort was made to characterize it, with a view to determining whether it significantly influenced test results. As background, in some early tests, the jet tip passed through thin sheets of mylar in order to reduce the projectile/_,/D. One to eight 0.025 m m thick (1 mil) mylar sheets were placed directly in the shotline of the projectile. Use of the mylar sheets shortened the projectile without breaking it up, and was believed to have also removed some of the coma material. However, it was determined that the mylar, launched by the impacts with the jet tip, could deform the initial target plate. During a test in which mylar was used, a 0.127 cm thick, aluminum 6061-T6 plate with a 7.62 cm diameter center hole and six 6.35 cm long radial cuts to allow the plate to petal-back (similar to Fig. 7, where eight radial cuts were used) was placed in the shotline. The plate was placed 28 cm after the mylar shortening station and 28 cm in front of the target. The projectile travelled through the center hole without interference. After the test, the pre-formed petals had permanent rearward deflections ranging from 1.27 cm to 5 cm. A large amount of (what appeared to be) burned mylar was detected on the surface of the plate. It was felt that the deposition of mylar onto the aluminum plate caused the petals to deflect. To determine if this was the case, an aluminum plate, prepared the same way was placed in the shot line of a second test in which no mylar was used. During this test, although a small amount of aluminum was deposited on the surface of the plate, the petals did not deflect. An additional observation of this test was that the aluminum deposits lacked a directional quality, unlike the radial streak patterns travelling away from the impact point seen on the front of the impacted target plates (Fig. 5).
Figure 5. Aluminum Splash Pattern Seen on a Typical I S C L Test
Figure 6. Coma Characterization Sheet
Although the petals did not deflect during this second test, the fact that mylar could petal the test plate led to concern about the effect of the coma on the target. Wenzel and Gehring [5] suggested that such material only affects the first target sheet impacted by an inhibited shaped charge projectile: in secondary sheets "the mode of failure appears to be the same as that observed in targets recovered from light-gas gun tests." If this were the case, the coma around the inhibited shaped charge projectile would not effect secondary plates. As most spacecraft shield designs contain multiple plates, the coma would then not affect the "fail/no fail" performance of the shield. To determine whether or not the shield performance would be affected, a quantitative estimate of how much material was travelling with the projectile was needed. Therefore, a coma characterization test was performed. In this test, five thin aluminum sheets were placed 7.62 cm apart. Each plate contained a centerline hole, with diameters 7.62, 6.35, 5.08, 3.81, and 2.54 cm, decreasing in the direction of flight. The sheets had eight 6.35 cm radial cuts, as shown in Figs. 6 and 7. These sheets were placed 3.8 cm behind a steel plate with a 7.62 cm diameter hole at the shotline.
908
J.D. WALKER
et al. ×e
Figure 8. Geometry of the Coma Characterization Petal
Figure 7. Example Coma Characterization Test Plate
To determine the impulse versus deflection for the petals, the work of Parkes [17-19] on the deformation of a cantilever beam carrying a mass at its tip subjected to a short pulse loading was extended to the irregular triangular shape of the petals in our test. Figure 8 describes the geometry of the section, which is idealized as a trapezoid bending about a given x(t). The width of the trapezoid for a given x is = 2(x +xc)tan(0/2), where x = -xc is the intersection with the x axis of the trapezoid edge extension, and 0/2 is the angle of intersection. For eight radial cuts, 0 = 45*. h is the thickness of the plate, and is the vertical deflection of the x = 0 end of the plate.. A linear velocity distribution in the normal direction is assumed in the plate, of the form
z(x)
y(t)
v(x, t) = (1
-x/-x)~t
(2)
dy/dt
(so = v(0, t)). The particle hitting the edge of the plate is assumed to have mass m with initial velocity v(0, 0). The momentum balance for the plate is
dt~,Joz(')phz(x)v(x,t)dx)=O
d (m v(O, t)) + d ( f
(3)
This leads to
+ x~ -x-2 dEy xc x dydx= {m 2phtan(O/2)(--~+~3}-ff-t~t2+2phtan(O/2)(~+-~)-~-~ O
(4)
Taking the first moment about the tip of the plate gives
(5) or
1 ~ +XcX -2l-~t2 d2y +(-~--+ 3-~ 2x~x _)-~-~-~ dyd-x} =Mu(-x) ~phtan(O/2){ C-~
(6)
where the plastic bending moment is given by
Mu =1 Yh2z(x)
(7)
Experimental impacts above 10 km/s In this equation, Y is the flow stress of the material. The initial conditions are that y(0)=0, dy/dt(O) equals the impact velocity, andS(0) = 0. Initially, this system is singular (because d'x/dt(O) = +oo), and so was solved numerically by an implicit midpoint scheme. When ~ equals the length of the cut in the plate, all further bending is assumed to occur around that point, and the first moment is taken around that point (the momentum balance no longer holds, as the rest of the plate is viewed as fixed in space). The moment equation can then be explicitly integrated, as M v is constant, and the closed form solution is similar to those found in Refs. [17-19]. These equations were used to determine the mass impacting each plate, based on the measured deflection of each petal. Figure 6 shows the final coma test sheet with hole diameter of 2.54 cm. Aluminum material from both the coma and previous sheet impacts is seen deposited on the surface of the sheet (it is assumed that the momentum transferred to the petals all originated with coma material). Figure 9 shows the deflection versus normalized impact mass with the impact velocity of 11.2 km/s for the 0.030, 0.102, and 0.127 cm thick sheets. The mass values were normalized by dividing the calculated mass on each petal by the presented area of that petal (the curves for the two different hole sizes for the thicker sheets nearly overlay). Figure 10 shows the deflections and the masses to produce those deflections for each of the forty petals. The masses are small relative to the mass of the projectile. The largest mass on a petal was 3.61 mg, or 0.3% of the average projectile mass (1150 mg). The total mass hitting the five plates was 73 mg, or 6.3% of the projectile mass. These low values imply that damage to secondary plates due to the coma material is unlikely, and that the coma material only affects the first target plate, and not subsequent ones. An average density versus distance from the centerline is plotted in Fig. 11. Semi-Infinite Target Tests. Several aluminum projectile tests have been performed against semi-infinite blocks of aluminum. One type of semi-infinite target was a block of 6061-T6 aluminum measuring 15.25 cm in diameter and 10.16 cm thick. A second semi-infinite target was a pure aluminum ingot, measuring 15.25cm in diameter and 12.70 cm thick. The results of the tests into these targets is given in Table 4.
909
•
0.127cm Thlck Pk~te
i
p
i
i
r
i
i
i
i
0.5
I
1,5
2
2,5
3
3.5
4
4.5
Mass (mg) / Area (cm2)
Figure 9. Deflection versus Impact Mass
Figure 10. Deflections and Masses for Each Petal
3.5 3
R E
2.s
1.5
0.5 q 1,5
2
J 2.5
3
3.5
Radius (cm)
Figure 11. Coma Density Distribution
J.D. WALKER et al.
910
Table 4. Aluminum Projectiles into Semi-Infinite Aluminum Targets
Test No.
Projectile Impact Crater Crater Projectile Mass Velocity Depth Diameter Length (grams) (km/s) (cm) (cm) (cm)
Target Material
Projectile Diameter (cm)
Impact Angle
3513-17B A1 6061-T6
0.31
11.00
2.69
2.72
1.14
0.38
0°
4416-12
A1 6061-T6
0.74
11.17
2.93
4.45
1.39
0.52
18*
5993-1
Pure A1
1.15
11.28
4.88
6.15
1.72
0.59
7*
To compare with similar experiments conducted at lower velocity, Figs. 12 and 13 are plots of normalized penetration (P/L) into various types of semi-infinite aluminum blocks by/_/D = 2 and 3 aluminum projectiles (and one nickel projectile, discussed later). Figure 12 includes data from the literature up to impact velocities of 6 km/s [20], and four data points obtained with the inhibited shaped charge launcher above 10.0 km/s. Figure 13 displays the data from Fig. 12 using impact velocity normalized by the projectile density, p, and the target ultimate strength, a,, and the depth normalized by a, the square root of the projectile density divided by the target density. On Fig. 13, the normalization process is seen to group the two aluminum shaped charged tests that impacted A1 6061-T6 in line with the lower velocity data. However, the aluminum shaped charge impact into the soft aluminum target indicates that the cratering data curve may continue to slope upwards for large values of normalized impact velocity. 4.0
.•
.= . in
•
-
.
l
.
.•
.
,
.
-.
.m .
4,0 LID
Atlov
•
3
Ni pmj., 6061 *T6
0
3
II0C~0
•
3
2024-T3
3
6061-T6
2
t 100-0
~llov
3
Ni p m ) ,
6061 -T6
3.0
"~- 2.0
O
3
1100-0
•
3
2024-T3
•
3
6061T6
A
2
1100-0
3.0
O
[
O
O
•
~d2. 0
o go °~3 o •
0~0
O m
~o 0
1.0
1.O
,,'0 I I
l
.
:.o
~
,
.
4.0
.
6.0
,
•
v
s.o
D
,
10.0
.
i
10.0
12.0
i
20.0
(p V 2
Velocity (km/s) Figure 12. Normalized Penetration versus Impact Velocity
I
,
3O.O '
'
1
40 i 0 Q 1/2
501.0
60.0
,)
Figure 13. Normalized Penetration versus Scaled Impact Velocity
Whipple Bumper Shield Tests. The primary use of the inhibited shaped charge launcher has been the impact of thin, spaced plates. Two groups of such tests will be discussed: the first is a collection of various spaced plate targets with the results summarized in Table 5; the second group is several space-station shielding designs. Both groups were impacted with aluminum projectiles launched from the inhibited shaped charge launcher for NASA-JSC. The second group of tests was performed against two types of targets, an all-aluminum Whipple and a stuffed Whipple, both at 45 ° obliquity to the shot line (Fig. 15). Both targets were 2/3rds the size of the true shield design (2/3rds scale). The configuration of these shield targets is given in Table 6. 2.00 0
Sfuffed
1.75
Whipple
(no failure)
1:3
AII-AI Whipple
•
AII-AI
1.50
(no failure)
1.25
o
Whipple
(failed)
8 O
1 .00
O
0.75 O
ffl
0.50
0.25 0.00
I 0.0
I
0.5
i
I
1 .0
I
I
1 .5
i
l"
2.0
i
I
I
2.5
L/D
Figure 14. Whipple Shield Tests
3.0
Both the all-aluminum Whipple target and the stuffed Whipple were impacted four times. In all but one of the all-aluminum tests, the rear-wall of the target failed (was perforated). None of the stuffed Whipple shields that were tested had a rear-wall failure. However, due to the variation in the projectiles, it is not possible to draw a conclusion regarding which shield design is better. As can be seen in Fig. 14, the stuffed Whipple was impacted by smaller projectiles than the all-aluminum Whipple (some of these projectiles were shortened by mylar sheets).
Experimental impacts above 10 km/s
911
Table 5. Aluminum Projectiles into Spaced Aluminum Plate Targets Projectile Mass (grams)
Impact Velocity (km/s)
Target Description (Dimensions in cm)
Rear-Wall Failure (Yes or No)
4416-13 Task 2
1.02
11.16
0.317 A1 6061-T6 30.48 Gap 0.635 A1 6061-T6
N
4416-15 Task 2
1.05
11.30"
5098-5
0.62
11.30"
4416-5 Task 3
0.66
11.50
4416-7 Task 3
1.00
11.50
4416-6 Task 3
1.27
5098-6
Test No.
If Failure, Rear Wall Hole Dia. (cm)
Y
No Hole, Only Spallation Damage
Y
4.45 to 6.98 (elliptical)
Y
4.45 to 6.03 (elliptical)
"
Y
3.81 to 6.03 (elliptical)
11.32
"
Y
3.81
0.78
11.30*
0.127 AI 6061-T6 10.0 Gap 0.317 AI 6061-T6 10.0 Gap 0.317 AI 6061-T6
N
5993-7
0.66
11.29
4416-11 Task 3
0.70
11.50
0.229 AI 6061-T6 30.48 Gap 0.483 AI 6061-T6 (at 45°)
N
4416-10 Task 3
0.84
11.37
0.254 AI 6061-T6 30.48 Gap 0.635 AI 6061-T6 (at 45 °)
N
0.127 AI 6061-T6 20.0 Gap 0.317 A1 6061-T6
10.0305 AI 6061-T6 5x/ 7.62 Gap 0.317 A! 6061-T6
N
Velocity was not measured: this value is the average velocity from the other tests.
Table 6. Whipple Target Descriptions Target
Description (Dimensions in cm)
All-Aluminum Whipple
0.127 Thick AI 6061-T6 2.54 Gap 14 Layers of multi-layer insulation (MLI) 2.54 Gap 0.3175 Thick AI 2219-T87 7.62 Gap 0.1016 Thick AI 2024-T3 (Witness Plate)
Stuffed Whipple
0.127 Thick AI 6061-T6 2.794 Gap 14 Layers of multi-layer insulation (MLI) 4 Layers of Nextel (AF62) 4 Layers of Kevlar (710) 3.048 Gap 0.3175 Thick A! 2219-T87 7.62 Gap 0.1016 Thick AI 2024-T3 (Witness Plate)
J.D. WALKER et al.
912
Figure 15. Before and After Views of a Stuffed Whipple Shield
Molybdenum Projectiles For hypervelocity lethality testing, the inhibited shaped charge was modified to launch higher density materials, in particular, molybdenum and nickel. For these materials, however, only four tests each have been performed. These tests were design tests for the inhibited shaped charge, and the geometry of the inhibitor was undergoing modification. Thus, the launching of molybdenum and nickel projectiles has not undergone the charge design refinement required for a consistent projectile as has the aluminum launcher. The molybdenum tests used a 20 ° (total angle) liner. The inhibitor was made of OHFC copper.
Experimental impacts above 10 km/s
913
For the initial molybdenum test, a block of aluminum 606 l-T6 was used as the target. The block was 15.24 cm in diameter and 10.16 cm thick. The 2.50 gram projectile produced during this test hit very near the edge of the witness block, spalling offa 5.84 cm section of the side of the block2. Measurements were made of the impact crater assuming that, had the edge of the material not spalled off, it would have been symmetric. With this agsumption, the impact crater was approximately 5.5 cm in diameter and 4.3 cm deep. The crater was not smooth along its interior surface due to the irregular shape of the projectile. A second test was performed into a block of aluminum 606 l-T6. The projectile formed was rod-like, with a mass of approximately 4 grams. This projectile produced a crater of 3.7 cm deep and 5.3 cm wide. The semi-infinite impact data with molybdenum projectiles is given in Table 7. The third test with the molybdenum projectile was launched against a triple layer multi-shock target. The configuration for this target is shown in Table 8. All three layers of this target were perforated by the 2.07 gram, 11.72 km/s projectile produced during this test. The jagged hole in the third plate was approximately 3.81 cm in diameter. Even though this third plate was failed, minimal damage occurred to the witness plate (only a small number of shallow craters with diameters of 0.25 cm). The final molybdenum projectile was launched into a classifted target.
Table 7. Molybdenum and Nickel Projectiles into Semi-Infinite Aluminum 6061.T6 Targets Projectile Impact Crater Crater Mass Velocity Depth Diameter (grams) (km/s) (cm) (cm)
Projectile Length (cm)
Projectile Diameter (cm)
5.5
3.04
0.32
0°
5.3
3.34
0.42
32*
3.2
5.1
2.01
0.31
90 °
4.0
6.8
2.06
0.37
56 °
Test No.
Projectile Material
5133-6
Molybdenum
2.50
11.68
4.3
5133-7
Molybdenum
4.00
11.81
3.7
5133-3
Nickel
1.11
10.85
5133-4
Nickel
1.94
10.75
Impact Angle
Table 8. Multi-Shock Target Used for the Molybdenum and the Nickel Projectile Spaced Plate Impact Tests Target Arrangement (Dimensions in cm) 0.127 A1606 l-T6 10.16Gap 0.3175 A1606 l-T6 10.16Gap 0.3175AI6061-T6 15.24 Gap 15.24 Diameter by 10.16 Thick A1606 l-T6 (Witness Block)
Nickel Projectiles A total of four inhibited shaped charge tests have been performed using nickel liners. These tests used a 30 ° (total angle) liner. These tests were performed in the design of the inhibited shaped charge. For the first nickel test, a block of aluminum 606 l-T6 was used as a target. This target was 15.24 cm in diameter and 10.16 cm thick. The 1.11 gram nickel projectile produced during this test created a crater in the aluminum block with an approximate diameter of 5.1 cm and a depth of 3.2 cm. The surface of the impact crater was jagged due to the irregular shape of the projectile. A second test with the nickel projectile used the same target as the first. In this case, a 1.94 gram projectile was created that produced a 6.8 cm diameter by 4.0 cm deep crater in the block of 6061-T6. This crater measurement is an approximation, since some of the edge of the aluminum block was spalled offby the impact. The "effective" length of this projectile, computed by multiplying length by the cosine of the impact angle, provides aL/D ratio of 3.12 for this test. As such, it should compare to the data plotted on Fig. 13. It can be seen on Fig. 13, that the nickel data point lies below the aluminum data point. Such limited data precludes a firm conclusion about high speed cratering. The next nickel projectile test also resulted in an edge-impact of an aluminum 606 l-T6 block. In this test, a 1.82 gram projectile travelling at 10.86 km/s impacted near the edge of the aluminum block and again spalled off some of the target material. This spall, and the fact that the projectile broke up, precluded a data point from this test. The semi-infinite impact data with nickel projectiles is given in Table 7.
2 The molybdenum and nickel tests were performed before the new detonation configuration was introduced.
914
J.D. WALKERet al.
The final test using nickel liners was launched against the three plate multi-shock target described in Table 8. All three plates were perforated by the 1.60 gram nickel projectile travelling at 10.76 km/s. The jagged, petalled-hole in the third plate was approximately 8.26 m in diameter. The witness block had minimal damage (again, only a few shallow craters with surface diameters less than 0.25 cm). CONCLUSIONS The inhibited shaped charge launcher has now been in use for a number of years and is performing routine tests on spacecraft shield designs. The aluminum launcher produces well characterized projectiles, travelling at 11.25:£-0.20 km/s, and the impact data generated agree well with trends seen in lower velocity light gas gun testing and scaled approaches. The inhibited shaped charge launching technique has also been successfully extended to molybdenum and nickel, launching projectiles at 11.72_+0.10 km/s and 10.81:k-0.10 km/s, respectively. ACKNOWLEDGEMENTS We thank Jeanne Crews of NASA-JSC for primary funding support and encouragement for the aluminum work. We also thank Tom Tsai at the Defense Nuclear Agency who funded the molybdenum and nickel work. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
J.D. Walker, D. J. Grosch, and S. A. Mullin, A Hypervelocity Fragment Launcher Based on an Inhibited Shaped Charge. Int. J. Impact Engng. 14, 763-774 (1993). W.P. Waiters and J. A. Zukas, Fundamentals of Shaped Charges. New York, Wiley-Interscience (1989). S. Kronman and A. Merendino, Inhibited Jet Charge. Proceedings of the Sixth Symposium on Hypervelocity Impact, Cleveland, Ohio (1963). S. Merendino, J. M. Regan and S. Kronman, A Method of Obtaining a Massive Hypervelocity Pellet From a Shaped Charge Jet. Ballistic Research Laboratories Memorandum Report No. 1508, Aberdeen Proving Ground, Maryland (1963). A.B. Wenzel and J. W. Gehring, Hypervelocity Impact Studies Against Apollo-Type Structures Up to 16.5 km/s. General Motors Defense Research Laboratories Final Report TR65-56, Santa Barbara, California (1965). A.B. Wenzel, A Review of Explosive Accelerators for Hypervelocity Impact. Int. J. Impact Engng. 5, 681-692 (1987). D. Grosch, J. Walker, and S. Mullin, Design of an Evacuated Hypervelocity Launch Facility. Southwest Research Institute Final Report 06-4416, San Antonio, Texas (1993). S.A. Mullin, J. D. Walker, and D. J. Grosch, Application of the Inhibited Shaped Charge Hypervelocity Launcher to Lethality Testing. Southwest Research Institute Final Report 06-5133, San Antonio, Texas (1993). P.Y. Chanteret, Studies of Maximum Velocities for Coherent Shaped Charge Jets. 13th International Ballistics Symposium, Stockholm, Sweden (1992). J.D. Walker, Incoherence of Shaped Charge Jets. 14th International Ballistics Symposium, Quebec City, Canada (1993). E.M. Pugh, R. J. Eichelberger, and N. Rostoker, Theory of Jet Formation by Charges with Lined Conical Cavities, J. Appl. Phys. 23(5), 532-536 (1952). P.C. Chou, J. Carleone, E. Hirsch, W. J. Flis, and R. D. Ciccarelli, Improved Formulas for Velocity, Acceleration, and Projection Angle of Explosively Driven Liners. 6th International Ballistics Symposium, Orlando, Florida (1981). L. Zernow and L. Lowry, Does Momentary Core Melting Occur in Copper Shaped Charge Jets? 12th International Ballistics Symposium, San Antonio, Texas (1990). W . G . yon Holle and J. J. Trimble, Shaped Charge Temperature Measurement. Proceeding of the Sixth Symposium on Detonation. Coronado, California, 691-699 (1976). F. Jamet, Investigation of Shaped Charge Jets Using Flash X-Ray Diffraction. 8th International Ballistics Symposium, Orlando, Florida (1984). R. Tipton, CALE Users Manual, Version 901101. Lawrence Livermore National Laboratory, Livermore, California (1990). E.W. Parkes, The permanent deformation of a cantilever struck transversely at its tip. Proc. Royal Soc. Series A 228(1175), 462-476 (1955). P.S. Symonds and W. T. Fleming, Jr., Parkes revisited: on rigid-plastic and elastic-plastic dynamic structural analysis. Int. J. Impact Engng. 2(1), 1-36 (1984)o J. Lubliner, Plasticity Theory. Macmillan Publishing Company, New York, New York (1990). C.E. Anderson, Jr., D. L. Littlefield, N. W. Blaylock, S. J. Bless, R. Subramanian, The Penetration Performance of Short L/D Projectiles. High-Pressure Science and Technology--1993 (edited by S. C. Schmidt, J. W. Shaner, G. A. Samara and R. Ross), 1809-1812, AIP Press, New York (1994).
Pergamon
E X P E R I M E N T A L IMPACTS ABOVE 10 KM/S
James D. Walker, Donald J. Grosch, and Scott A. Mullin Southwest Research Institute San Antonio, Texas 78228
S u m m a r y - - I n the proceedings of the last symposium, recent work on a technique for launching small projectiles to hypervelocities above 10 km/s using an inhibited shaped charge was presented [1]. In the interim, experiments have been conducted using the inhibited shaped charge to launch aluminum, nickel, and molybdenum projectiles. This paper presents the results of the impact tests, as well as discusses the shaped charge design modifications for the nickel and molybdenum launchers. Radiographs are presented of the impacting projectiles, as are post test photographs of various targets. The data are unique in that they represent low D'D projectile impacts into both monolithic blocks and spaced plates at velocities above 10 km/s. The aluminum projectiles are being launched at 11.25_+0.20 km/s, the molybdenum projectiles at 11.72_+0.10 km/s, and the nickel projectiles at 10.81_+0.10 km/s. INTRODUCTION For satellites and spacecraft in low earth orbit, there are two possible sources of accidental impact. The first is impacts with meteoroids. The second, and of growing concern, is impacts from material left in orbit on previous flights, usually referred to as "orbital debris." From orbital mechanics, accidental impacts between man made objects can occur at velocities up to 15 km/s. These impact velocities are high enough that questions of melt and vaporization need to be addressed. As research is still necessary to validate, verify, and incorporate complex equation of state and constitutive models into hydrodynamic codes, it is still necessary to experimentally test the designs of shields intended to protect personnel and equipment. There are three approaches to hypervelocity impact testing for this problem: light gas guns (up to 9 km/s), shaped charge techniques (up to 12 km/s), and dissimilar material scaling (up to a scaled velocity of 18 km/s). Each technique has its advantages and drawbacks. A particular advantage of the inhibited shaped charge launcher (ISCL) is the ability to launch one-gram aluminum fragments above 11 km/s. This provides the ability to test complex shield designs at 11 km/s, which is close to the impact velocity that would be seen in a collision between objects in polar and equatorial orbits. This paper describes developments in the aluminum shaped charge launcher, extensions of the technique to molybdenum and nickel projectiles, and test results of several space station shield designs. The protection of space station components and on-board personnel from micrometeoroid and orbital debris impact is the motivation of the tests. T H E INHIBITED SHAPED CHARGE A shaped charge is a cavity within an explosive which is lined with metal. When detonated, this device generates a long, plastically-deforming jet of material that travels at high speeds [2]. Shaped charges have great penetrating capabilities, and have been used for many years in anti-armor warheads and oil well perforators. The long jet produced by standard shaped charges is not similar to the accidental threat to spacecraft. It is believed that most debris in orbit has a length to diameter ratio (L/D) of 1 to 2. Thus, the shaped charge was modified to isolate the high-speed jet tip from the rest of the jet. This was done by placing an inhibitor within the cavity of the charge [3-6,1]. The inhibitor allows the jet tip to form but prevents the remainder of the jet from forming or travelling down range. The isolated jet tip is the space debris simulant. In order to utilize the explosive launcher in an environment that simulates the conditions in space, an evacuated hypervelocity launcher facility was designed and fabricated at Southwest Research Institute (SwRI) for NASA-JSC [7]. 903
J.D. WALKER et al.
904
Design Modifications A number of modifications have occurred to the testing arrangement described in [1]. Detonation Configuration. To reduce the translational scatter of the impact point, the previous shaped charge initiation system was replaced by one using a precision initiation coupler (PIC). The PIC reduces the tolerances required on the positioning of the detonator by providing a large surface for the detonator to initiate. In cross section, the PIC is hour-glass shaped, varying from a large detonator-surface diameter to a small disk and the.n increasing to an even larger diameter on the high-explosive side (Fig. I). This geometry provides a point initiation of the explosive, the point being the center of the hour-glass configuration. Due to the delicacy of the PIC, a reduced-energy detonator (RP-80 Explosive Bridgewire instead of an RP-83) was used to prevent the detonator from overwhelming the PIC and preventing it from working as intended. Additional efforts were aimed at ensuring that the PIC detonation point was aligned with the conical liner and not with the explosive cylinder, as the explosive cylinder may not be concentric with the centerline of the liner due to uneven curing of the explosive. A vacuum-mandrel was fabricated that fit inside the liner and allowed the charge to be machined about the centerline of the liner. Using this technique, a 45* chamfer was cut on the rear end of the explosive. This 45 ° chamfer mated with a 45 ° chamfer on an aluminum PIC holder. This arrangement insured that the detonation train was positioned along the centerline of the liner. A marked improvement in the repeatability of the shotline was seen after this procedure was initiated. After a flight of 220 cm, scatter with the previous booster arrangement had been roughly +/- 6 cm, while scatter with the new detonation system has been roughly +/- 3 cm. Aluminum Liners. Aluminum is thought to comprise most man-made orbital debris, and is therefore the most common liner material used with the inhibited shaped charge launcher. The 1100-alloy aluminum shaped charge liners are now fabricated using a three step process. The initial step is to forge a round bar of the aluminum to a solid cone. This is completed with two knock-down forging operations. The second step is a forging operation, requiring about six passes, in which the cone is hollowed-out. The final fabrication step is to machine the roughed-out cone to final dimensions. About 0.10 cm of material is removed from both the inside and the outside of the cone during this step. The liners have a 30 ° included angle and a rounded apex with inner radius of 0.254 cm (Fig. 2).
RP-80 DetonatorWell ~ \
Pic \
PICH°lder ~ /
/- PBXN-5 / Explosive
/ 1/
l%%?~o?~.%o_~.o.o.~.<>i \
Octol Explosive 45 Degree Mating Surface Figure 1. Precision Initiation Coupler (PIC) and Assembly
Figure 2. 30* Liner Design
New Projectile Materials: Molybdenum and Nickel Liners In the past two years, the inhibited shaped charge launcher has been used to launch projectiles of materials with a density greater than that of aluminum , s 2.7 g/cm 3 [8]. The experimental work focused on two materials, molybdenum and nickel, with densities of 10.2 g/cm 3 and 8.9 g]cm 3, respectively. There is an upper bound on the launch velocities achievable by a shaped charge for the jet tip to be in one piece [9]. An attempt to drive the velocity of the jet greater than this velocity results in a radial expansion and break-up of the jet, referred to as incoherence, which defeats the purpose of launching a single projectile. For most materials, the requirement is that the collapse velocity of the charge is less than the low pressure longitudinal sound speed in the collapse point reference frame [10]. In the laboratory frame, the empirical engineering rule ~,,,p < 2.41c0
(1)
is used to determine the maximum velocity, where co is the low pressure bulk sound speed of the material [2]. This engineering requirement produces roughly the same results as the underlying collapse point frame
Experimental impacts above 10 km/s
905
requirement for some geometries of interest. According to Eqn. (1), the maximum velocity for an intact projectile would be roughly 12.3 km/s for molybdenum and 11.1 km/s for nickel. To design the shaped charge, an analytic shaped charge model was used. The model was based on the Pugh-Eickelberger-Rostocker [11] theory of collapse, modified to account for liner acceleration [12]. The model uses information about the geometry of the charge, the liner material, and the explosive to calculate details of the resulting shaped charge jet. The liner is divided into elements. Based on the calculated jet velocities and masses for each liner element, a velocity and a mass for the jet tip are calculated. Similarly, an average collapse velocity for the jet tip material can be calculated. The collapse velocity is the velocity at which the liner material, driven by the explosive pressures, converges on the axis of the liner, as observed in the collapse point reference frame. In general, higher velocities are achieved for thinner liners, as there is less mass Molybdenum Liner, 1 mm fhick, Oc|ol 1.6 , , , = , to be driven by the explosive. However, a thinner liner also means there is less material .~ 1.4 going into the jet, leading to a smaller projectile. It was found that for molybdenum E 1.2 and nickel to exceed 10 km/s with the Octol t~ ntal DataPoint explosive, the liner could not be much more -~ ~.o than 1 mm thick. To provide a flavor of what is involved in the design process, Fig. 3 was E 0.8 produced with the analytical shaped charge '~. model. The figure is for a shaped charge with ....... ~ ~ ! ~ ...... a through thickness of 1 mm on the liner, for ,~ 0.6 various liner angles. In order for the jet to be coherent, it is necessary that the collapse 0.4 velocity (the lower solid curve) be less than the maximum coherent collapse velocity (dotted 0.2 curve). The figure shows that a velocity of 11.8 km/s should be possible with I I I i I I I I 0.0 20 40 60 80 molybdenum for a 20 ° total liner angle, and that the jet tip should still be coherent. Also on the Liner Angle (deg) plot is the experimentally determined value of Figure 3. Molybdenum Liner Tip Velocity, the jet tip velocity from our tests Collapse Velocity, (ll.7:H).I kin/s), and the model is seen to and Tip Mass Versus Liner Angle accurately predict the experiments. ,
The 20 °, 1 mm through thickness molybdenum liners were fabricated with the same technique as the aluminum liners. The billet material for these liners was formed from pure (99.95%) molybdenum powder through powder metallurgy compression and sintering. The nickel liners were machined from a bar of commercially pure nickel alloy Ni 200 (in the "as received" cold drawn form, o,tt = 6.7 kbar). E X P E R I M E N T A L RESULTS To simulate the environment in orbit, the inhibited shaped launcher is used in conjunction with a vacuum chamber. The vacuum chamber has a variety of view ports and access ports to allow for extensive diagnostic equipment. To date, diagnostics have been primarily restricted to flash X-rays, with as many as eight X-ray heads used per test. During most tests, the heads are arranged to observe the projectile before impact. With this arrangement, the projectile geometry is observed and measurements of the relative position of the projectile versus the X-ray times allows its velocity to be calculated. In other tests, the flash X-rays are positioned to allow for the examination of the debris pattern behind the target plate. The flash X-rays are triggered at various delay times from the initial signal to the detonator. As the jet tip velocities are very repeatable, the delay times are calculated based on the position of each X-ray head along the shotline. Kodak direct exposure film (DEF) is used. This film does not require the use of intensifier screens. A 0.32 cm thick lexan sheet is placed over each film holder to protect the film from particles resulting from the impact test. A piece of felt is placed between the film holder and the lexan cover to reduce bruising of the film.
Aluminum Projectile The aluminum projectile produced by the inhibited shaped charge launcher is a hollow cylinder with an approximate L/D of 2 (see Fig. 4). Often, the leading and trailing edges of the projectile appear jagged or even slightly turned back. It is felt that these slight irregularities have little effect on the results of the impact tests. The cavity along the axis of symmetry is probably due to the jet being on the verge of
J . D . WALKER et al.
906
incoherence [10]. Overall, the aluminum projectiles produced are consistent and the inhibited shaped charge launcher provides a repeatable means of launching hypervelocity projectiles. Table 1 contains a complete statistical summary of the projectile geometry.
Table 1. Statistical Summary of Aluminum Projectiles
Mass (g)
L/D
Length (mm)
Outer Diameter (mm)
Velocity (km/s)
Impact Angle
Average
1.15
1.86
13.00
7.11
11.25
37.4 °
Std. Dev.
0.21
0.47
2.53
0.60
0.097
23.9 °
Maximum
1.56
2.92
18.16
7.95
11.42
75.0 °
Minimum
0.78
1.32
9.50
5.87
11.03
2.0 °
Figure 4. Orthogonal Radiographs of Two Aluminum Projectiles
1 Jet Temperature. Questions regarding the temperature of the projectile are occasionally raised. The concern is that the large deformations during the formation of the shaped charge jet leads to substantial heating, perhaps even enough to approach the melt temperature of the jet material. Conventional wisdom in the shaped charge community is that the jets produced by both aluminum and copper shaped charges are solid [13]. There have been a number of experimental efforts to directly address this question. The most extensive work has been performed for copper. For 42 ° copper lined charges, vol Holle and Trimble [14] performed two color infrared radiometry with a response time of less than one microsecond. For Comp-B charges they found an average jet temperature of 430°C and for Octol charges they found an average jet temperature of 540°C. Boih these values are well below the 1085°C melt temperature of copper. This indicates that copper jets are not molten. Experimental work has been performed with aluminum jets to address the temperature issue. Jamet [15] performed X-ray diffraction of aluminum jets in flight. These jets were produced by a 40 ° variable liner thickness with a TNT/RDX 35/65 charge, and had a jet tip velocity of 8.5 km/s. Distinct crystal structure demonstrated that the aluminum jet material was solid in flight. We performed shaped charge collapse calculations with CALE [ 16]. These calculations give an upper bound on the temperature rise of jet material of 450°C, which is below the aluminum melt temperature of 661 °C. The temperature rise is mostly due to dissipative plastic work during the formation of the jet, as large plastic strains occur. Also, flash X-rays of the projectile in flight (e.g., Fig. 4) reveal sharp edges, further evidence of the solidity of the material in the projectile. Thus, though the projectile material is at an elevated temperature, all available evidence argues that the material in the jet is solid.
Experimental impacts above 10 km/s
907
C o m a Characterization. It became apparent during testing that a cloud of fine particles travels just ahead of and in-line with the ISCL projectile. This cloud will be referred to as the c o m a of the jet tip. In addition, a number of small particles typically travel adjacent to the projectile. Since a single projectile impact on the target is of interest, these particles are undesirable. However, as it appeared to be impossible to remove the coma, an effort was made to characterize it, with a view to determining whether it significantly influenced test results. As background, in some early tests, the jet tip passed through thin sheets of mylar in order to reduce the projectile/_,/D. One to eight 0.025 m m thick (1 mil) mylar sheets were placed directly in the shotline of the projectile. Use of the mylar sheets shortened the projectile without breaking it up, and was believed to have also removed some of the coma material. However, it was determined that the mylar, launched by the impacts with the jet tip, could deform the initial target plate. During a test in which mylar was used, a 0.127 cm thick, aluminum 6061-T6 plate with a 7.62 cm diameter center hole and six 6.35 cm long radial cuts to allow the plate to petal-back (similar to Fig. 7, where eight radial cuts were used) was placed in the shotline. The plate was placed 28 cm after the mylar shortening station and 28 cm in front of the target. The projectile travelled through the center hole without interference. After the test, the pre-formed petals had permanent rearward deflections ranging from 1.27 cm to 5 cm. A large amount of (what appeared to be) burned mylar was detected on the surface of the plate. It was felt that the deposition of mylar onto the aluminum plate caused the petals to deflect. To determine if this was the case, an aluminum plate, prepared the same way was placed in the shot line of a second test in which no mylar was used. During this test, although a small amount of aluminum was deposited on the surface of the plate, the petals did not deflect. An additional observation of this test was that the aluminum deposits lacked a directional quality, unlike the radial streak patterns travelling away from the impact point seen on the front of the impacted target plates (Fig. 5).
Figure 5. Aluminum Splash Pattern Seen on a Typical I S C L Test
Figure 6. Coma Characterization Sheet
Although the petals did not deflect during this second test, the fact that mylar could petal the test plate led to concern about the effect of the coma on the target. Wenzel and Gehring [5] suggested that such material only affects the first target sheet impacted by an inhibited shaped charge projectile: in secondary sheets "the mode of failure appears to be the same as that observed in targets recovered from light-gas gun tests." If this were the case, the coma around the inhibited shaped charge projectile would not effect secondary plates. As most spacecraft shield designs contain multiple plates, the coma would then not affect the "fail/no fail" performance of the shield. To determine whether or not the shield performance would be affected, a quantitative estimate of how much material was travelling with the projectile was needed. Therefore, a coma characterization test was performed. In this test, five thin aluminum sheets were placed 7.62 cm apart. Each plate contained a centerline hole, with diameters 7.62, 6.35, 5.08, 3.81, and 2.54 cm, decreasing in the direction of flight. The sheets had eight 6.35 cm radial cuts, as shown in Figs. 6 and 7. These sheets were placed 3.8 cm behind a steel plate with a 7.62 cm diameter hole at the shotline.
908
J.D. WALKER
et al. ×e
Figure 8. Geometry of the Coma Characterization Petal
Figure 7. Example Coma Characterization Test Plate
To determine the impulse versus deflection for the petals, the work of Parkes [17-19] on the deformation of a cantilever beam carrying a mass at its tip subjected to a short pulse loading was extended to the irregular triangular shape of the petals in our test. Figure 8 describes the geometry of the section, which is idealized as a trapezoid bending about a given x(t). The width of the trapezoid for a given x is = 2(x +xc)tan(0/2), where x = -xc is the intersection with the x axis of the trapezoid edge extension, and 0/2 is the angle of intersection. For eight radial cuts, 0 = 45*. h is the thickness of the plate, and is the vertical deflection of the x = 0 end of the plate.. A linear velocity distribution in the normal direction is assumed in the plate, of the form
z(x)
y(t)
v(x, t) = (1
-x/-x)~t
(2)
dy/dt
(so = v(0, t)). The particle hitting the edge of the plate is assumed to have mass m with initial velocity v(0, 0). The momentum balance for the plate is
dt~,Joz(')phz(x)v(x,t)dx)=O
d (m v(O, t)) + d ( f
(3)
This leads to
+ x~ -x-2 dEy xc x dydx= {m 2phtan(O/2)(--~+~3}-ff-t~t2+2phtan(O/2)(~+-~)-~-~ O
(4)
Taking the first moment about the tip of the plate gives
(5) or
1 ~ +XcX -2l-~t2 d2y +(-~--+ 3-~ 2x~x _)-~-~-~ dyd-x} =Mu(-x) ~phtan(O/2){ C-~
(6)
where the plastic bending moment is given by
Mu =1 Yh2z(x)
(7)
Experimental impacts above 10 km/s In this equation, Y is the flow stress of the material. The initial conditions are that y(0)=0, dy/dt(O) equals the impact velocity, andS(0) = 0. Initially, this system is singular (because d'x/dt(O) = +oo), and so was solved numerically by an implicit midpoint scheme. When ~ equals the length of the cut in the plate, all further bending is assumed to occur around that point, and the first moment is taken around that point (the momentum balance no longer holds, as the rest of the plate is viewed as fixed in space). The moment equation can then be explicitly integrated, as M v is constant, and the closed form solution is similar to those found in Refs. [17-19]. These equations were used to determine the mass impacting each plate, based on the measured deflection of each petal. Figure 6 shows the final coma test sheet with hole diameter of 2.54 cm. Aluminum material from both the coma and previous sheet impacts is seen deposited on the surface of the sheet (it is assumed that the momentum transferred to the petals all originated with coma material). Figure 9 shows the deflection versus normalized impact mass with the impact velocity of 11.2 km/s for the 0.030, 0.102, and 0.127 cm thick sheets. The mass values were normalized by dividing the calculated mass on each petal by the presented area of that petal (the curves for the two different hole sizes for the thicker sheets nearly overlay). Figure 10 shows the deflections and the masses to produce those deflections for each of the forty petals. The masses are small relative to the mass of the projectile. The largest mass on a petal was 3.61 mg, or 0.3% of the average projectile mass (1150 mg). The total mass hitting the five plates was 73 mg, or 6.3% of the projectile mass. These low values imply that damage to secondary plates due to the coma material is unlikely, and that the coma material only affects the first target plate, and not subsequent ones. An average density versus distance from the centerline is plotted in Fig. 11. Semi-Infinite Target Tests. Several aluminum projectile tests have been performed against semi-infinite blocks of aluminum. One type of semi-infinite target was a block of 6061-T6 aluminum measuring 15.25 cm in diameter and 10.16 cm thick. A second semi-infinite target was a pure aluminum ingot, measuring 15.25cm in diameter and 12.70 cm thick. The results of the tests into these targets is given in Table 4.
909
•
0.127cm Thlck Pk~te
i
p
i
i
r
i
i
i
i
0.5
I
1,5
2
2,5
3
3.5
4
4.5
Mass (mg) / Area (cm2)
Figure 9. Deflection versus Impact Mass
Figure 10. Deflections and Masses for Each Petal
3.5 3
R E
2.s
1.5
0.5 q 1,5
2
J 2.5
3
3.5
Radius (cm)
Figure 11. Coma Density Distribution
J.D. WALKER et al.
910
Table 4. Aluminum Projectiles into Semi-Infinite Aluminum Targets
Test No.
Projectile Impact Crater Crater Projectile Mass Velocity Depth Diameter Length (grams) (km/s) (cm) (cm) (cm)
Target Material
Projectile Diameter (cm)
Impact Angle
3513-17B A1 6061-T6
0.31
11.00
2.69
2.72
1.14
0.38
0°
4416-12
A1 6061-T6
0.74
11.17
2.93
4.45
1.39
0.52
18*
5993-1
Pure A1
1.15
11.28
4.88
6.15
1.72
0.59
7*
To compare with similar experiments conducted at lower velocity, Figs. 12 and 13 are plots of normalized penetration (P/L) into various types of semi-infinite aluminum blocks by/_/D = 2 and 3 aluminum projectiles (and one nickel projectile, discussed later). Figure 12 includes data from the literature up to impact velocities of 6 km/s [20], and four data points obtained with the inhibited shaped charge launcher above 10.0 km/s. Figure 13 displays the data from Fig. 12 using impact velocity normalized by the projectile density, p, and the target ultimate strength, a,, and the depth normalized by a, the square root of the projectile density divided by the target density. On Fig. 13, the normalization process is seen to group the two aluminum shaped charged tests that impacted A1 6061-T6 in line with the lower velocity data. However, the aluminum shaped charge impact into the soft aluminum target indicates that the cratering data curve may continue to slope upwards for large values of normalized impact velocity. 4.0
.•
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.
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.
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.
,
.
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2024-T3
3
6061-T6
2
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~llov
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6061 -T6
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"~- 2.0
O
3
1100-0
•
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,
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,
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.
i
10.0
12.0
i
20.0
(p V 2
Velocity (km/s) Figure 12. Normalized Penetration versus Impact Velocity
I
,
3O.O '
'
1
40 i 0 Q 1/2
501.0
60.0
,)
Figure 13. Normalized Penetration versus Scaled Impact Velocity
Whipple Bumper Shield Tests. The primary use of the inhibited shaped charge launcher has been the impact of thin, spaced plates. Two groups of such tests will be discussed: the first is a collection of various spaced plate targets with the results summarized in Table 5; the second group is several space-station shielding designs. Both groups were impacted with aluminum projectiles launched from the inhibited shaped charge launcher for NASA-JSC. The second group of tests was performed against two types of targets, an all-aluminum Whipple and a stuffed Whipple, both at 45 ° obliquity to the shot line (Fig. 15). Both targets were 2/3rds the size of the true shield design (2/3rds scale). The configuration of these shield targets is given in Table 6. 2.00 0
Sfuffed
1.75
Whipple
(no failure)
1:3
AII-AI Whipple
•
AII-AI
1.50
(no failure)
1.25
o
Whipple
(failed)
8 O
1 .00
O
0.75 O
ffl
0.50
0.25 0.00
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I
0.5
i
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1 .5
i
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2.0
i
I
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2.5
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Figure 14. Whipple Shield Tests
3.0
Both the all-aluminum Whipple target and the stuffed Whipple were impacted four times. In all but one of the all-aluminum tests, the rear-wall of the target failed (was perforated). None of the stuffed Whipple shields that were tested had a rear-wall failure. However, due to the variation in the projectiles, it is not possible to draw a conclusion regarding which shield design is better. As can be seen in Fig. 14, the stuffed Whipple was impacted by smaller projectiles than the all-aluminum Whipple (some of these projectiles were shortened by mylar sheets).
Experimental impacts above 10 km/s
911
Table 5. Aluminum Projectiles into Spaced Aluminum Plate Targets Projectile Mass (grams)
Impact Velocity (km/s)
Target Description (Dimensions in cm)
Rear-Wall Failure (Yes or No)
4416-13 Task 2
1.02
11.16
0.317 A1 6061-T6 30.48 Gap 0.635 A1 6061-T6
N
4416-15 Task 2
1.05
11.30"
5098-5
0.62
11.30"
4416-5 Task 3
0.66
11.50
4416-7 Task 3
1.00
11.50
4416-6 Task 3
1.27
5098-6
Test No.
If Failure, Rear Wall Hole Dia. (cm)
Y
No Hole, Only Spallation Damage
Y
4.45 to 6.98 (elliptical)
Y
4.45 to 6.03 (elliptical)
"
Y
3.81 to 6.03 (elliptical)
11.32
"
Y
3.81
0.78
11.30*
0.127 AI 6061-T6 10.0 Gap 0.317 AI 6061-T6 10.0 Gap 0.317 AI 6061-T6
N
5993-7
0.66
11.29
4416-11 Task 3
0.70
11.50
0.229 AI 6061-T6 30.48 Gap 0.483 AI 6061-T6 (at 45°)
N
4416-10 Task 3
0.84
11.37
0.254 AI 6061-T6 30.48 Gap 0.635 AI 6061-T6 (at 45 °)
N
0.127 AI 6061-T6 20.0 Gap 0.317 A1 6061-T6
10.0305 AI 6061-T6 5x/ 7.62 Gap 0.317 A! 6061-T6
N
Velocity was not measured: this value is the average velocity from the other tests.
Table 6. Whipple Target Descriptions Target
Description (Dimensions in cm)
All-Aluminum Whipple
0.127 Thick AI 6061-T6 2.54 Gap 14 Layers of multi-layer insulation (MLI) 2.54 Gap 0.3175 Thick AI 2219-T87 7.62 Gap 0.1016 Thick AI 2024-T3 (Witness Plate)
Stuffed Whipple
0.127 Thick AI 6061-T6 2.794 Gap 14 Layers of multi-layer insulation (MLI) 4 Layers of Nextel (AF62) 4 Layers of Kevlar (710) 3.048 Gap 0.3175 Thick A! 2219-T87 7.62 Gap 0.1016 Thick AI 2024-T3 (Witness Plate)
J.D. WALKER et al.
912
Figure 15. Before and After Views of a Stuffed Whipple Shield
Molybdenum Projectiles For hypervelocity lethality testing, the inhibited shaped charge was modified to launch higher density materials, in particular, molybdenum and nickel. For these materials, however, only four tests each have been performed. These tests were design tests for the inhibited shaped charge, and the geometry of the inhibitor was undergoing modification. Thus, the launching of molybdenum and nickel projectiles has not undergone the charge design refinement required for a consistent projectile as has the aluminum launcher. The molybdenum tests used a 20 ° (total angle) liner. The inhibitor was made of OHFC copper.
Experimental impacts above 10 km/s
913
For the initial molybdenum test, a block of aluminum 606 l-T6 was used as the target. The block was 15.24 cm in diameter and 10.16 cm thick. The 2.50 gram projectile produced during this test hit very near the edge of the witness block, spalling offa 5.84 cm section of the side of the block2. Measurements were made of the impact crater assuming that, had the edge of the material not spalled off, it would have been symmetric. With this agsumption, the impact crater was approximately 5.5 cm in diameter and 4.3 cm deep. The crater was not smooth along its interior surface due to the irregular shape of the projectile. A second test was performed into a block of aluminum 606 l-T6. The projectile formed was rod-like, with a mass of approximately 4 grams. This projectile produced a crater of 3.7 cm deep and 5.3 cm wide. The semi-infinite impact data with molybdenum projectiles is given in Table 7. The third test with the molybdenum projectile was launched against a triple layer multi-shock target. The configuration for this target is shown in Table 8. All three layers of this target were perforated by the 2.07 gram, 11.72 km/s projectile produced during this test. The jagged hole in the third plate was approximately 3.81 cm in diameter. Even though this third plate was failed, minimal damage occurred to the witness plate (only a small number of shallow craters with diameters of 0.25 cm). The final molybdenum projectile was launched into a classifted target.
Table 7. Molybdenum and Nickel Projectiles into Semi-Infinite Aluminum 6061.T6 Targets Projectile Impact Crater Crater Mass Velocity Depth Diameter (grams) (km/s) (cm) (cm)
Projectile Length (cm)
Projectile Diameter (cm)
5.5
3.04
0.32
0°
5.3
3.34
0.42
32*
3.2
5.1
2.01
0.31
90 °
4.0
6.8
2.06
0.37
56 °
Test No.
Projectile Material
5133-6
Molybdenum
2.50
11.68
4.3
5133-7
Molybdenum
4.00
11.81
3.7
5133-3
Nickel
1.11
10.85
5133-4
Nickel
1.94
10.75
Impact Angle
Table 8. Multi-Shock Target Used for the Molybdenum and the Nickel Projectile Spaced Plate Impact Tests Target Arrangement (Dimensions in cm) 0.127 A1606 l-T6 10.16Gap 0.3175 A1606 l-T6 10.16Gap 0.3175AI6061-T6 15.24 Gap 15.24 Diameter by 10.16 Thick A1606 l-T6 (Witness Block)
Nickel Projectiles A total of four inhibited shaped charge tests have been performed using nickel liners. These tests used a 30 ° (total angle) liner. These tests were performed in the design of the inhibited shaped charge. For the first nickel test, a block of aluminum 606 l-T6 was used as a target. This target was 15.24 cm in diameter and 10.16 cm thick. The 1.11 gram nickel projectile produced during this test created a crater in the aluminum block with an approximate diameter of 5.1 cm and a depth of 3.2 cm. The surface of the impact crater was jagged due to the irregular shape of the projectile. A second test with the nickel projectile used the same target as the first. In this case, a 1.94 gram projectile was created that produced a 6.8 cm diameter by 4.0 cm deep crater in the block of 6061-T6. This crater measurement is an approximation, since some of the edge of the aluminum block was spalled offby the impact. The "effective" length of this projectile, computed by multiplying length by the cosine of the impact angle, provides aL/D ratio of 3.12 for this test. As such, it should compare to the data plotted on Fig. 13. It can be seen on Fig. 13, that the nickel data point lies below the aluminum data point. Such limited data precludes a firm conclusion about high speed cratering. The next nickel projectile test also resulted in an edge-impact of an aluminum 606 l-T6 block. In this test, a 1.82 gram projectile travelling at 10.86 km/s impacted near the edge of the aluminum block and again spalled off some of the target material. This spall, and the fact that the projectile broke up, precluded a data point from this test. The semi-infinite impact data with nickel projectiles is given in Table 7.
2 The molybdenum and nickel tests were performed before the new detonation configuration was introduced.
914
J.D. WALKERet al.
The final test using nickel liners was launched against the three plate multi-shock target described in Table 8. All three plates were perforated by the 1.60 gram nickel projectile travelling at 10.76 km/s. The jagged, petalled-hole in the third plate was approximately 8.26 m in diameter. The witness block had minimal damage (again, only a few shallow craters with surface diameters less than 0.25 cm). CONCLUSIONS The inhibited shaped charge launcher has now been in use for a number of years and is performing routine tests on spacecraft shield designs. The aluminum launcher produces well characterized projectiles, travelling at 11.25:£-0.20 km/s, and the impact data generated agree well with trends seen in lower velocity light gas gun testing and scaled approaches. The inhibited shaped charge launching technique has also been successfully extended to molybdenum and nickel, launching projectiles at 11.72_+0.10 km/s and 10.81:k-0.10 km/s, respectively. ACKNOWLEDGEMENTS We thank Jeanne Crews of NASA-JSC for primary funding support and encouragement for the aluminum work. We also thank Tom Tsai at the Defense Nuclear Agency who funded the molybdenum and nickel work. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
J.D. Walker, D. J. Grosch, and S. A. Mullin, A Hypervelocity Fragment Launcher Based on an Inhibited Shaped Charge. Int. J. Impact Engng. 14, 763-774 (1993). W.P. Waiters and J. A. Zukas, Fundamentals of Shaped Charges. New York, Wiley-Interscience (1989). S. Kronman and A. Merendino, Inhibited Jet Charge. Proceedings of the Sixth Symposium on Hypervelocity Impact, Cleveland, Ohio (1963). S. Merendino, J. M. Regan and S. Kronman, A Method of Obtaining a Massive Hypervelocity Pellet From a Shaped Charge Jet. Ballistic Research Laboratories Memorandum Report No. 1508, Aberdeen Proving Ground, Maryland (1963). A.B. Wenzel and J. W. Gehring, Hypervelocity Impact Studies Against Apollo-Type Structures Up to 16.5 km/s. General Motors Defense Research Laboratories Final Report TR65-56, Santa Barbara, California (1965). A.B. Wenzel, A Review of Explosive Accelerators for Hypervelocity Impact. Int. J. Impact Engng. 5, 681-692 (1987). D. Grosch, J. Walker, and S. Mullin, Design of an Evacuated Hypervelocity Launch Facility. Southwest Research Institute Final Report 06-4416, San Antonio, Texas (1993). S.A. Mullin, J. D. Walker, and D. J. Grosch, Application of the Inhibited Shaped Charge Hypervelocity Launcher to Lethality Testing. Southwest Research Institute Final Report 06-5133, San Antonio, Texas (1993). P.Y. Chanteret, Studies of Maximum Velocities for Coherent Shaped Charge Jets. 13th International Ballistics Symposium, Stockholm, Sweden (1992). J.D. Walker, Incoherence of Shaped Charge Jets. 14th International Ballistics Symposium, Quebec City, Canada (1993). E.M. Pugh, R. J. Eichelberger, and N. Rostoker, Theory of Jet Formation by Charges with Lined Conical Cavities, J. Appl. Phys. 23(5), 532-536 (1952). P.C. Chou, J. Carleone, E. Hirsch, W. J. Flis, and R. D. Ciccarelli, Improved Formulas for Velocity, Acceleration, and Projection Angle of Explosively Driven Liners. 6th International Ballistics Symposium, Orlando, Florida (1981). L. Zernow and L. Lowry, Does Momentary Core Melting Occur in Copper Shaped Charge Jets? 12th International Ballistics Symposium, San Antonio, Texas (1990). W . G . yon Holle and J. J. Trimble, Shaped Charge Temperature Measurement. Proceeding of the Sixth Symposium on Detonation. Coronado, California, 691-699 (1976). F. Jamet, Investigation of Shaped Charge Jets Using Flash X-Ray Diffraction. 8th International Ballistics Symposium, Orlando, Florida (1984). R. Tipton, CALE Users Manual, Version 901101. Lawrence Livermore National Laboratory, Livermore, California (1990). E.W. Parkes, The permanent deformation of a cantilever struck transversely at its tip. Proc. Royal Soc. Series A 228(1175), 462-476 (1955). P.S. Symonds and W. T. Fleming, Jr., Parkes revisited: on rigid-plastic and elastic-plastic dynamic structural analysis. Int. J. Impact Engng. 2(1), 1-36 (1984)o J. Lubliner, Plasticity Theory. Macmillan Publishing Company, New York, New York (1990). C.E. Anderson, Jr., D. L. Littlefield, N. W. Blaylock, S. J. Bless, R. Subramanian, The Penetration Performance of Short L/D Projectiles. High-Pressure Science and Technology--1993 (edited by S. C. Schmidt, J. W. Shaner, G. A. Samara and R. Ross), 1809-1812, AIP Press, New York (1994).