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Int. J. Impact Engng, Vol. 20, pp. 165-172, 1997 ©1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0734-743X/97 $17.00+0.00
Pergamon
P R O J E C T I L E SHAPE EFFECTS ON SHIELDING P E R F O R M A N C E AT 7 KM/S AND 11 KM/S E R I C L. C H R I S T I A N S E N 1 AND J U S T I N H. K E R R l tNASA Johnson Space Center, Mail Code SN3, Houston, TX 77058 Ph. 713-483-5311, FAX 713-483-5276 Summary--NASA has developed enhanced performance shields to improve the protection of spacecraft from orbital debris and meteoroid impacts. One of these enhanced shields includes a blanket of Nextela~ ceramic fabric and Kevlarru high strength fabric that is positioned midway between an aluminum bumper and the spacecraft pressure wall. As part of the evaluation of this new shielding technology, impact data above 10 km/sec have been obtained by NASA Johnson Space Center (JSC) from the Sandia National Laboratories HVL ("hypervelocity launcher") and the Southwest Research Institute inhibited shaped charge launcher (ISCL). The HVL launches flyer-plates in the velocity range of 10 to 15 km/s while the ISCL launches hollow cylinders at -11.5 km/s. The >10 km/s experiments are complemented by hydrocode analysis and light-gas gun testing at the JSC Hypervelocity Impact Test Facility (HIT-F) to assess the effects of projectile shape on shield performance° Results from the testing and analysis indicate that nonspherical shapes are more penetrating than equal mass spheres (by factors of 1.2 to -2). Impact data also demonstrated the NextelT~/KevlarTM shield provides superior protection performance compared to an all-aluminum shield alternative. NOMENCLATURE d D 19 P t 0 V
projectile diameter (cm) hole or crater diameter (cm) density (g/cm 3) penetration depth (cm) thickness (cm) impact angle from surface normal (deg) projectile velocity (km/s)
Subscripts:
c h n p s t
crater/cavity hole normal component (of velocity vector) projectile surface spall target
INTRODUCTION The performance of hypervelocity impact shields is provided by "ballistic limit" equations which define the threshold particle size causing failure of the shield as a function of velocity, impact angle, particle density, and particle shape. As a practical matter, most ballistic limit equations are constructed from databases and analyses using spherical projectiles only [1]. The NASA Johnson Space Center (JSC) Hypervelocity Impact Test Facility (HIT-F) is currently conducting studies of the effects of projectile shape on shielding performance [2]. This is necessary as the orbital debris environmental threat which N A S A shields against consists of non-spherical particles. Figures 1 and 2 illustrate recent orbital debris particles recovered from a hypervelocity impact crater found on the exterior o f Shuttle OV-102 (Columbia) payload bay door after the STS-73 mission. These orbital debris particles were the largest orbital debris particles ever recovered after an impact on a spacecraft. The exterior of the payload bay door is - 8 r a m thick Nomex felt which contributed to the recovery of large pieces of the impactor. Two particles had at least one dimension over l m m in length. The largest particle was 1.5mm x l m m x l m m and was determined by Scanning Electron Microscope (SEM) X-Ray analysis to contain large amounts of lead and tin, with minor amounts of copper and silver. Using conventions established in studies o f other returned spacecraft materials, it has been determined that this particle represents a piece of lead/tin solder. Attached to the solder (Figure 1) is a piece of fiberous solid material which is identified as a piece of circuit board like material. The likely source o f this orbital debris impact has been identified as a piece of electric circuit board [3,4]. The second large piece of debris was another piece of circuit board type material (-1.2mm long x 0.5mm wide x 0.2mm thick). When the circuit board piece and solder are connected, the resulting orbital debris particle was approximately - 2 . 7 m m long x i m m wide x 0.2 to l m m thick (aspect ratio=3:l:l). This non-spherical shape is probably typical of orbital debris. Determining 165
166
E.L. CHRISTIANSENand J.H. KERR
the effects of non-spherical debris shapes on shielding performance is necessary to assess the true risk of meteoroid/orbital debris impacts on spacecraft.
Fig. 1. Scanning Electron Microscope view of a large (approximately 1.5mm x lmm x 1mm) particle extracted from a 17mm diameter impact into flexible reusable surface insulation (FRSI) on exterior of Shuttle payload bay door after STS-73.
Fig. 2. Another > l m m long particle extracted from 17mm diameter impact after STS-73
STUFFED WHIPPLE SHIELD
Several low-weight, enhanced performance shields have been developed by the NASA HIT-F to improve spacecraft protection from meteoroid/orbital debris (M/OD) impact over that offered by conventional 2-sheet Whipple shields [1,2,5]. For instance, the baseline shielding for high M/OD flux areas of the space station modules will incorporate a blanket between the outer aluminum bumper and inner pressure wall that combines two materials: Nextel T M ceramic fabric and Kevlar T M high strength fabric. These shields are referred to as "stuffed Whipple" shields. Figure 3 shows a typical example although many different stuffed Whipple shield configurations have been tested at the JSC HIT-F [5]. A major advantage of the ceramic cloth is that it generates higher shock pressures and greater disruption of an impacting particle than an equivalent weight aluminum bumper. High-strength Kevlar TM cloth follows the ceramic cloth. Because it has a much higher strength to weight ratio than aluminum, Kevlar T M is more effective at slowing any remaining projectile fragments and decreasing the expansion rate of the debris cloud before it subsequently impacts the rearwall of the shield. Other high-strength fabrics such as SpectraT M have also demonstrated good performance in JSC HIT-F testing. Extensive testing of Nextel I ' M /Kevlar T M stuffed Whipple and other multi-wall shields have been performed using light gas guns (LGG) to launch projectiles (spheres typically, but also cylinders, disks, etc.) up to velocities o f - 8 km/s [6]. Spacecraft in LEO are exposed to orbital debris with relative impact velocities ranging from <1 km/s to -14.5 km/s, with an average velocity o f - 1 0 km/s. To extend the impact database to velocities typical of approximately half of the orbital debris population, NASA JSC sponsored a multi-year effort to obtain test data at >I0 km/s to assess the performance of advanced spacecraft shields. This paper will describe results of >10 km/s tests on advanced shielding concepts using two launcher systems: the Sandia National Laboratories (SNL) flyer plate hypervelocity launcher (HVL) [7] and the Southwest Research Institute (SwRI) inhibited shaped-charge launcher (ISCL) [8]. The projectiles from both of these launchers are non-spherical which influence shielding performance. A combined test and hydrocode approach has been employed to assess the relative contributions of projectile shape and velocity. The JSC HIT-F is conducting light-gas gun tests with non-spherical shapes as well as hydrocode simulations to determine the effects of projectile shape in the HVL/ISCL tests and to use this data to extend the ballistic limit curves for spherical projectiles to velocities above 10 km/s. This assessment contributes to fully utilize the HVL and ISCL data in justifying changes to shielding "ballistic limit" equations used in spacecraft meteoroid/orbital debris risk assessments (Christiansen et al. 1993).
Projectile shape effects on shielding performance O
167
1.27cm AI 7 km/sec, 0 deg
g/cm^ 2 0.2cm 0.551 A16061-T6
MLI 0.035 I 6 Nextel AF62 0.60 6 Kevtar710 0.192
0.48cm AI 2219T87 1.361 Total A.D.
2.74
Fig. 3. Stuffed Whipple Shield (Full-Scale) Sensitivity of Target Mounting Conditions on HVI Test Results In assessing shield response with projectiles of different shapes, or in comparing different shield performance with the same projectiles, it is necessary to recognize the importance of target mounting conditions. The size and way a target is mounted can influence the results of hypervelocity impact testing as explained below. To avoid an "apples to oranges" comparison, the SW and all-aluminum 3-wall shield options discussed in this paper were tested with the same size targets and mounting techniques. Target Size. It has been recognize by the HVI community for some time that target size and edge effects have an important influence on the resulting damage to targets, especially brittle targets such as glass. A much greater degree of cracking and fracturing can be expected in HVI tests on a small glass sample compared to the same test on a larger panel. The size of test samples will also influence the results of shield tests, especially for the rearwall of a shield. For shielding rearwalls subjected to impulsive loading, as the size of the rearwall decreases, the greater the probability that the resulting plate deflection exceeds the allowable burst failure point (unless the size is reduced to such a point that the debris cloud "bypasses" some of the rearwall) [11]. Rearwall Mounting Conditions. More shielding perforations, larger holes and greater cracking of a rearwall can also be expected when the rearwall is rigidly mounted to a frame. Bulging of the rearwall subjected to impulsive loading is much reduced if it the mounting of the rearwall is loosely held in place [6], and failure is therefore not as likely to occur. The ballistic limits will be higher for a non-rigidly mounted rearwall. Nextel/Kevlar Blanket Mounting Conditions. Tests conducted at the HIT-F on 3 different techniques to hold the Nextel/Kevlar second bumper in the SW shield indicate that a loosely held Nextel/Kevlar blanket will reduce the critical particle size causing failure of the shield (i.e., the shield is more likely to fail) and increases hole and crack sizes in the rearwall. The 3 mounting techniques tested include: (1) loosely held Nextel/Kevlar with the blanket held in position by 4 threaded rods mounted at the comers of the shield, (2) rigidly held Nextel/Kevlar sandwiched in a frame and bolted -8 places around the frames, and (3) held with 4 aluminum "staves" that are sewn into the Nextel/Kevlar blanket in a square pattern with long threaded rods holding the staves in position. The "stave" mounting technique is being planned for Space Station SW support. HVI test data indicates that there is no difference in rearwall damage results between the rigidly held Nextel/Kevlar and use of staves. The loosely held Nextel/Kevlar failed from 5% to 10% smaller projectile diameters. 10-15 KM/S F L Y E R PLATE TEST DATA Table 1 provides data from five SNL flyer plate tests on stuffed Whipple and allaluminum shields (Fig.4) Undeformed dimensions of the flyer plate used in these experiments
E. L. CHRIST1ANSENand J.H. KERR
168
are tabulated in Table 1. The flyer is a thin aluminum or titanium disk in these experiments. It is subject to both deformation and tilting. In some cases, the flyer remains relatively flat in a slightly bowed (i.e., cupped) shape while in others the deformation is more significant and the plate folds into a more compact shape. Orthogonal flash x-rays are used to evaluate shape and integrity of the projectile prior to impact. A measure of the extent of flyer deformation and tilting is given in Table 1 by the ratio of maximum projectile length (along line of flight) to diameter (or width) normal to the line of flight. For instance, the flyer in test SWBS-2 is folded nearly in-half along one axis (-180 ° bend) and is bent -120 ° along an orthogonal axis. The resulting shape has a maximum length to diameter o f - 2 which from hydrocode and experimental results is more penetrating to multi-wall shields than a flat plate impact [9,10]. 67%-Scale MOD-2 Test Configuration e
67%-Scale AII-AI Test Configuration
d=Proj.Diameter(crn) V=Velocity(kin/s) ~=impactangle tom norma deg 0.127cm A16061-T6
Li=JL t~ 05
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Q
~
~
d=Proj.Diameter(cm) V=Velocity(km/s) o=mpactange tom norma dog j "
""
MLI
MLI 0.025 4 Nexte AF62 0.40 4Kevlar710 0.128
t - - - - - I I]
-, 1
g/cm^2 0.127cm 0.345 A16061-T6
I
~
0.025
0,20era 0.568 A12024T3
0.905 Total A.D.
1.803
Total A.D.
1.843
Fig. 4. Test ConfiguratJ on, 67%-scale shields: (b) All-Aluminum (a) Stuffed Whipple The SNL test results indicate the stuffed Whipple shield provided superior protection compared to the all-aluminum shield alternative (compare SWBS-2 and SWBS-3 in Table 1). The all-aluminum shield suffered a large perforation whereas the stuffed Whipple did not. The flyer plate tests also demonstrate that the stuffed Whipple shield is more effective at oblique impact angles than for impacts normal to the shield (compare rearwall damage of SWBS-5 and SWBS-6). This characteristic is particularly advantageous for spacecraft shielding since most orbital debris and meteoroid impacts will be oblique. However, it is a property not shared by some shields such as the Whipple shield, which can be more vulnerable to oblique impact than normal impacts in the 4-11 km/s velocity range [2]. Table 1. HVL Flyer Plate Tests Flyer Flyer Dia. x Flyer Max SNL Shield Angle Vel. thk. Mass Length/ No. Type* (deg) (kin/s) (mm) (g) Width SWBS-2 SW-67% 0 9,9 18.9xl.0 0.75 1.7 SWBS-5 SW-67% 0 10,0 17.4x0.9 0.57 0.4 SWBS-6 SW-50% 45 10.1 17.3x0.9 0.56 0.5 SWBS-4 SW-67% 0 14.7 5.92xl.0 0.12 6.1 SWBS-3 AII-AI 0 10,2 18.9xl.0 0.75 1.0 *note: SW=Stuffed Whipple, AII-Al=all-aluminum 67%-scale shield Impact
Impact
Flyer Integrity V. bowed, trailing fragments Slightly bowed Slightly bowed Flat, tilted on-edge Bowed, tilted
Rear Wall Damage No Perf (large bulge) No Perf (large bulge) No Perf(small bulge) Perforated (pinhole) Perforated (large)
11-12 KM/S INHIBITED S H A P E D - C H A R G E TEST DATA
An inhibited shaped charge capable of launching aluminum projectiles in excess of 11 km/s has been developed at SwRI over a number of years [6]. NASA sponsored a number of ISCL tests on the stuffed Whipple and all-aluminum shield configurations given in Fig.4. The ISCL launches an aluminum projectile at velocities that are consistently between 11 and 11.5 krn/s. The projectile is in the shape of a hollow cylinder (or thick-walled pipe) with length to outside diameter ratio (L/D) of from 1 to 3. Projectile mass typically ranges from 0.8g to 1.5g. Projectile mass is determined by SwRI from digitized X-ray images of the projectile. Two sets of
Projectile shape effects on shielding performance
169
orthogonal flash x-ray images capture the size, shape, and orientation of the projectile prior to impact. General observations from the ISCL tests are as follows: (1) The stuffed Whipple shield defeated a 0.85g projectile at 0° (normal impact angle) while the all-aluminum shield was perforated. (2) In 45 ° impacts, the stuffed Whipple provided successful protection from 0.6g to 1.5g ISCL projectiles (5 tests), while the all-aluminum shield failed in 3 tests from 0.9g to 1.0g projectiles. (3) The stuffed Whipple provided better protection from 45 ° oblique impacts than normal impacts. LIGHT-GAS GUN (LGG) SHIELD TESTING The effect of projectile shape must be understood to complete the assessment of the ISCL and HVL data. The JSC HIT-F is conducting research on shape effects on shield performance in two areas: LGG testing and hydrocode investigations. The HIT-F has launched hollow aluminum cylinders at 7 km/s similar in size and mass to the ISCL projectiles. This data and LGG data for spherical projectiles is used to determine the ballistic limit mass ratio (BLMR) which is the ratio of the mass of a sphere on the ballistic limit of a shield to the mass of a nonspherical projectile at the ballistic limit. The BLMR is expected to vary with projectile shape and impact velocity. A BLMR above 1 indicates the particular non-spherical shape is more penetrating than a solid spherical projectile. Table 2 lists BLMR's derived from the JSC HIT-F tests at 7 km/s and hydrocode simulations at 11 km/s. In general, non-spherical projectiles are more penetrating than equivalent mass solid spheres. This activity is still in progress. Eventually, enough confidence will be attained to apply the BLMR's to the ballistic limit mass found by the HVL/ISCL tests. This will allow the ballistic limit equations to be raised accordingly in the V> 10km/s region. Table 2. BLMR's (Ballistic Limit Mass Ratios) for 3-wall shields from JSC HIT-F tests and hydrocodes Shield Type All-aluminum All-aluminum Stuffed Whipple
Proj. Shape Hollow Cylinder Hollow Cylinder Flat Plate Hollow Cylinder
Velocity (km/s) 7 11 11 7
BLMR 1.3 1.9 1.7 1.2
Method Test Hydrocode Hydrocode Test
SW SHIELD BALLISTIC LIMITS Recent HVI test results on the SW and all-aluminum 3-wall shield are compared with the predicted perforation threshold ballistic limits for these shields in Figure 5 for normal (0 °) and Figure 6 for 45 ° angle impacts by aluminum projectiles. The SW ballistic limit equations that were used to generate the predicted BL shown in Figures 5-6 are given below for the specific shielding configuration illustrated in Figure 1. High-Velocity: when V _> 6 . 5 / ( c o s 0 ) 1/3, dc = 2.584 V 1/3 (cos0) -°5
(1)
Intermediate-Velocity: when 2.7/(cos0) °5 < V < 6 . 5 / ( c o s 0 ) 1/3, d c = 1.385 (cos0) "vls [(V - 2.7/(cos0)°s)/(6.5/(cos0) I/3 - 2.7/(cos0)°5)] + 0.636 (cos0) -4/3 [[(6.5/(cos0) ~/3- V ) / ( 6 . 5 / ( c o s 0 ) 1/3 - 2.7/(cos0)°5)]
(2)
Low-Velocity: when V < 2.7/(cos0) °5, dc = 1.233 V "2/3(cos0) -5/3
(3)
170
E.L. CHRISTIANSENand J.H. KERR
A general set of equations that can also be used to predict SW ballistic limits are given in Eqns.4-6. However, caution should be exercised when using these equations for designs far from the SW design given in Fig.3. The best method to calibrate the BL equations is to perform HVI tests on the particular shield configuration of interest. High-Velocity: when V _>6.5/(cos0) t/3, dc = 0.6 (tw low)1/3 pp-I/3 V-i/3 (c0s0)-o.5 Intermediate-Velocity: when 2.7/(cos0) °5 < V
<
$2/3 (~/40)1/6
(4)
6.5/(cos0) 1/3,
dc= 0.321 (t w low)1/3 190"1/3 (cosO) "7/18 S 2/3 (0"/40) 1/6 [(W - 2.7/(cos0)°5)/ (6.5/(C0S0)
I/3 -
2.7/(cos0)°5)] + 1.031 901/2 [tw (t~/40) °5 + 0.37 mb] (cosO)4/3 [[(6.5/(C0S0) j/3
- V)/(6.5/(cosO) 1/3 - 2.7/(cos0)°5)]
(5)
Low-Velocity: when V _<2.7/(cos0) °5, d~ = 2 V 2/3 (cosO) 5/3 [t~ (or/40) °5 + 0.37 rob] / [(cosO)5/3 9p°5 V 2/3]
(6)
Figs.5-6 show that the HVI test data indicates that the current ballistic limit equations for the Stuffed Whipple shields are in some cases conservative (i.e., overpredict damage) at velocities >10 km/s. More near-term experimental and hydrocode work is planned at the JSC Hypervelocity Impact Test Facility to improve the assessment of the relative effects of nonspherical projectile shapes and projectile velocity on the results obtained from the HVL and inhibited shaped charge. The SW BL equations will also be modified (i.e., increased) to reflect the HVL and ISCL data. On the other hand, the BL equations for the 3-wall all-aluminum shield are nonconservative in some cases (-7 km/s normal impacts) and will also need to be modified after all HVI data and analyses are completed. Balliatic Limit~ at 0 °
O
2
4
6 Imput
8
I0
12
14
16
V-Sooi~p, (kin/8)
Fig. 5. Ballistic limits for SW and all-aluminum shields for normal impacts.
Projectile shape effects on shielding performance
171
Ball/stie Limits at 45* S
_~ 11
0
, , , 1 + ,
0
I-
FallureExpected AboveL.urves
S
~
l
4
,
,
.
]
6
'
' • ~r.,m,m
'
+
l
'
'
8
Impact V l d o d t y
+
l
'
'
"
10
I '!
I + ' ' g ' ' "
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14
16
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Fig. 6. Ballistic limits for SW and all-aluminum shields for 45 ° impacts. CONCLUDING REMARKS HVI data from 3 different launchers and hydrocode assessments show that the Nextel/Kevlar intermediate layer in the stuffed Whipple (SW) Shield provides better protection than a aluminum second bumper for impact velocities that range up to 12 km/s. Non-spherical shapes are generally more penetrating to 3-wall shields (SW or all-aluminum) than equivalent mass spherical particles. From the work completed to data, the spherical mass that can be stopped by 3-wall shields is from 1.2 to 2 times greater than a cylinder or flat plate impactor. Preliminary evidence from tests and analysis indicates that this ratio is even higher for 2-wall Whipple shields; i.e., that spherical projectiles from 2-4 times greater mass can be stopped in impacts from 7-12 km/s compared to a cylinder of L/D = -2. This indicates that multiple wall shields are more effective than single-bumper shields in protecting spacecraft from non-spherical impacts. Optimizing shielding protection for spacecraft should involve increasing the number of bumpers exterior of the rearwall to 2 or more. Of course, other parameters such as overall shielding spacing should also be optimized to provide the most effective protection for a given shielding weight (as a rule of thumb, spacing should be increased to at least 30x the average particle size that it is desired the shielding protect against). The HIT-F is continuing to determine the effects of projectile shape through light-gas gun tests and sponsoring >10 km/s speed testing at SwRI, SNL and other locations. REFERENCES
1. J.L. Crews and E.L. Christiansen, NASA Johnson Space Center Hypervelocity Impact Test Facility (HIT-F), AIAA Paper No.92-1640, 1992. 2. E.L. Christiansen, Design and Performance Equations for Advanced Meteoroid and Debris Shields, Int. J. Impact Engng, 14, 145-156 (1993). 3. R.P. Bernhard and E.L. Christiansen, STS-73 Meteoroid/Orbital Debris Impact Damage Assessment Report, Part 1, NASA Johnson Space Center, JSC-27323 (1995). 4. R.P. Bernhard, E.L. Christiansen, F. Horz, and D.E. Kessler, Orbital Debris as Detected on Exposed Spacecraft, Int. J. Impact Engng, Proceedings of the 1996 HVIS (1996). 5. E.L. Christiansen, J.L. Crews, J.E. Williamsen, J.H. Robinson, and A.M. Nolen, Enhanced Meteoroid and Orbital Debris Shielding, Int. J. Impact Engng, 17, Proceedings of the 1994 Hypervelocity Impact Symposium (1995). 6. B.G. Cour-Palais, A.J. Piekutowski, K.V. Dahl, and K.L. Poormon, Analysis of the UDRI Tests on Nextel Multi-Shock Shields, Int. J. Impact Engng, 14, 193-204 (1993). 7. M.B. Boslough, J.A. Ang, L.C. Chhabildas, W.D. Reinhart, C.A. Hall, B.G. Cour-Palais, E.L. Christiansen, and J.L. Crews, Hypervelocity Testing of Advanced Shielding Concepts for Spacecraft Against Impacts to 10 kin/s, Int. J. Impact Engng, 14, 95-106 (1993). 8. J.D. Walker, D.J. Grosch, and S.A. Mullin, Experimental Impacts above 10 km/s, Int. J. Impact Engng, 17, 903-914 (1995).
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E. L. CHRISTIANSENand J.H. KERR
9. E.L. Christiansen, J.L. Crews, J.H. Kerr, and L.C. Chhabildas, "Hypervelocity Impact Testing above 10 km/s of Advanced Orbital Debris Shields," Shock Compression of Condensed Matter - 1995, AlP Conference Proceedings 370, Part 2, pp.1183-1186, Seattle, WA, August 13-18, 1995. 10. J.H. Kerr, E.L. Christiansen and J.L. Crews, "Hydrocode Modelling of Advanced Debris Shield Designs," Shock Compression of Condensed Matter - 1995, AIP Conference Proceeedings 370, Part 2, pp.1167-1170, Seattle WA, August 13-18, 1995. 11. Y.C. Angel and J.P. Smith, Critical Response of Shielded Plates Subjected to Hypervelocity Impact, Int. J. lmpact Engng, 14, pp.25-35 (1993).
Pergamon
P R O J E C T I L E SHAPE EFFECTS ON SHIELDING P E R F O R M A N C E AT 7 KM/S AND 11 KM/S E R I C L. C H R I S T I A N S E N 1 AND J U S T I N H. K E R R l tNASA Johnson Space Center, Mail Code SN3, Houston, TX 77058 Ph. 713-483-5311, FAX 713-483-5276 Summary--NASA has developed enhanced performance shields to improve the protection of spacecraft from orbital debris and meteoroid impacts. One of these enhanced shields includes a blanket of Nextela~ ceramic fabric and Kevlarru high strength fabric that is positioned midway between an aluminum bumper and the spacecraft pressure wall. As part of the evaluation of this new shielding technology, impact data above 10 km/sec have been obtained by NASA Johnson Space Center (JSC) from the Sandia National Laboratories HVL ("hypervelocity launcher") and the Southwest Research Institute inhibited shaped charge launcher (ISCL). The HVL launches flyer-plates in the velocity range of 10 to 15 km/s while the ISCL launches hollow cylinders at -11.5 km/s. The >10 km/s experiments are complemented by hydrocode analysis and light-gas gun testing at the JSC Hypervelocity Impact Test Facility (HIT-F) to assess the effects of projectile shape on shield performance° Results from the testing and analysis indicate that nonspherical shapes are more penetrating than equal mass spheres (by factors of 1.2 to -2). Impact data also demonstrated the NextelT~/KevlarTM shield provides superior protection performance compared to an all-aluminum shield alternative. NOMENCLATURE d D 19 P t 0 V
projectile diameter (cm) hole or crater diameter (cm) density (g/cm 3) penetration depth (cm) thickness (cm) impact angle from surface normal (deg) projectile velocity (km/s)
Subscripts:
c h n p s t
crater/cavity hole normal component (of velocity vector) projectile surface spall target
INTRODUCTION The performance of hypervelocity impact shields is provided by "ballistic limit" equations which define the threshold particle size causing failure of the shield as a function of velocity, impact angle, particle density, and particle shape. As a practical matter, most ballistic limit equations are constructed from databases and analyses using spherical projectiles only [1]. The NASA Johnson Space Center (JSC) Hypervelocity Impact Test Facility (HIT-F) is currently conducting studies of the effects of projectile shape on shielding performance [2]. This is necessary as the orbital debris environmental threat which N A S A shields against consists of non-spherical particles. Figures 1 and 2 illustrate recent orbital debris particles recovered from a hypervelocity impact crater found on the exterior o f Shuttle OV-102 (Columbia) payload bay door after the STS-73 mission. These orbital debris particles were the largest orbital debris particles ever recovered after an impact on a spacecraft. The exterior of the payload bay door is - 8 r a m thick Nomex felt which contributed to the recovery of large pieces of the impactor. Two particles had at least one dimension over l m m in length. The largest particle was 1.5mm x l m m x l m m and was determined by Scanning Electron Microscope (SEM) X-Ray analysis to contain large amounts of lead and tin, with minor amounts of copper and silver. Using conventions established in studies o f other returned spacecraft materials, it has been determined that this particle represents a piece of lead/tin solder. Attached to the solder (Figure 1) is a piece of fiberous solid material which is identified as a piece of circuit board like material. The likely source o f this orbital debris impact has been identified as a piece of electric circuit board [3,4]. The second large piece of debris was another piece of circuit board type material (-1.2mm long x 0.5mm wide x 0.2mm thick). When the circuit board piece and solder are connected, the resulting orbital debris particle was approximately - 2 . 7 m m long x i m m wide x 0.2 to l m m thick (aspect ratio=3:l:l). This non-spherical shape is probably typical of orbital debris. Determining 165
166
E.L. CHRISTIANSENand J.H. KERR
the effects of non-spherical debris shapes on shielding performance is necessary to assess the true risk of meteoroid/orbital debris impacts on spacecraft.
Fig. 1. Scanning Electron Microscope view of a large (approximately 1.5mm x lmm x 1mm) particle extracted from a 17mm diameter impact into flexible reusable surface insulation (FRSI) on exterior of Shuttle payload bay door after STS-73.
Fig. 2. Another > l m m long particle extracted from 17mm diameter impact after STS-73
STUFFED WHIPPLE SHIELD
Several low-weight, enhanced performance shields have been developed by the NASA HIT-F to improve spacecraft protection from meteoroid/orbital debris (M/OD) impact over that offered by conventional 2-sheet Whipple shields [1,2,5]. For instance, the baseline shielding for high M/OD flux areas of the space station modules will incorporate a blanket between the outer aluminum bumper and inner pressure wall that combines two materials: Nextel T M ceramic fabric and Kevlar T M high strength fabric. These shields are referred to as "stuffed Whipple" shields. Figure 3 shows a typical example although many different stuffed Whipple shield configurations have been tested at the JSC HIT-F [5]. A major advantage of the ceramic cloth is that it generates higher shock pressures and greater disruption of an impacting particle than an equivalent weight aluminum bumper. High-strength Kevlar TM cloth follows the ceramic cloth. Because it has a much higher strength to weight ratio than aluminum, Kevlar T M is more effective at slowing any remaining projectile fragments and decreasing the expansion rate of the debris cloud before it subsequently impacts the rearwall of the shield. Other high-strength fabrics such as SpectraT M have also demonstrated good performance in JSC HIT-F testing. Extensive testing of Nextel I ' M /Kevlar T M stuffed Whipple and other multi-wall shields have been performed using light gas guns (LGG) to launch projectiles (spheres typically, but also cylinders, disks, etc.) up to velocities o f - 8 km/s [6]. Spacecraft in LEO are exposed to orbital debris with relative impact velocities ranging from <1 km/s to -14.5 km/s, with an average velocity o f - 1 0 km/s. To extend the impact database to velocities typical of approximately half of the orbital debris population, NASA JSC sponsored a multi-year effort to obtain test data at >I0 km/s to assess the performance of advanced spacecraft shields. This paper will describe results of >10 km/s tests on advanced shielding concepts using two launcher systems: the Sandia National Laboratories (SNL) flyer plate hypervelocity launcher (HVL) [7] and the Southwest Research Institute (SwRI) inhibited shaped-charge launcher (ISCL) [8]. The projectiles from both of these launchers are non-spherical which influence shielding performance. A combined test and hydrocode approach has been employed to assess the relative contributions of projectile shape and velocity. The JSC HIT-F is conducting light-gas gun tests with non-spherical shapes as well as hydrocode simulations to determine the effects of projectile shape in the HVL/ISCL tests and to use this data to extend the ballistic limit curves for spherical projectiles to velocities above 10 km/s. This assessment contributes to fully utilize the HVL and ISCL data in justifying changes to shielding "ballistic limit" equations used in spacecraft meteoroid/orbital debris risk assessments (Christiansen et al. 1993).
Projectile shape effects on shielding performance O
167
1.27cm AI 7 km/sec, 0 deg
g/cm^ 2 0.2cm 0.551 A16061-T6
MLI 0.035 I 6 Nextel AF62 0.60 6 Kevtar710 0.192
0.48cm AI 2219T87 1.361 Total A.D.
2.74
Fig. 3. Stuffed Whipple Shield (Full-Scale) Sensitivity of Target Mounting Conditions on HVI Test Results In assessing shield response with projectiles of different shapes, or in comparing different shield performance with the same projectiles, it is necessary to recognize the importance of target mounting conditions. The size and way a target is mounted can influence the results of hypervelocity impact testing as explained below. To avoid an "apples to oranges" comparison, the SW and all-aluminum 3-wall shield options discussed in this paper were tested with the same size targets and mounting techniques. Target Size. It has been recognize by the HVI community for some time that target size and edge effects have an important influence on the resulting damage to targets, especially brittle targets such as glass. A much greater degree of cracking and fracturing can be expected in HVI tests on a small glass sample compared to the same test on a larger panel. The size of test samples will also influence the results of shield tests, especially for the rearwall of a shield. For shielding rearwalls subjected to impulsive loading, as the size of the rearwall decreases, the greater the probability that the resulting plate deflection exceeds the allowable burst failure point (unless the size is reduced to such a point that the debris cloud "bypasses" some of the rearwall) [11]. Rearwall Mounting Conditions. More shielding perforations, larger holes and greater cracking of a rearwall can also be expected when the rearwall is rigidly mounted to a frame. Bulging of the rearwall subjected to impulsive loading is much reduced if it the mounting of the rearwall is loosely held in place [6], and failure is therefore not as likely to occur. The ballistic limits will be higher for a non-rigidly mounted rearwall. Nextel/Kevlar Blanket Mounting Conditions. Tests conducted at the HIT-F on 3 different techniques to hold the Nextel/Kevlar second bumper in the SW shield indicate that a loosely held Nextel/Kevlar blanket will reduce the critical particle size causing failure of the shield (i.e., the shield is more likely to fail) and increases hole and crack sizes in the rearwall. The 3 mounting techniques tested include: (1) loosely held Nextel/Kevlar with the blanket held in position by 4 threaded rods mounted at the comers of the shield, (2) rigidly held Nextel/Kevlar sandwiched in a frame and bolted -8 places around the frames, and (3) held with 4 aluminum "staves" that are sewn into the Nextel/Kevlar blanket in a square pattern with long threaded rods holding the staves in position. The "stave" mounting technique is being planned for Space Station SW support. HVI test data indicates that there is no difference in rearwall damage results between the rigidly held Nextel/Kevlar and use of staves. The loosely held Nextel/Kevlar failed from 5% to 10% smaller projectile diameters. 10-15 KM/S F L Y E R PLATE TEST DATA Table 1 provides data from five SNL flyer plate tests on stuffed Whipple and allaluminum shields (Fig.4) Undeformed dimensions of the flyer plate used in these experiments
E. L. CHRIST1ANSENand J.H. KERR
168
are tabulated in Table 1. The flyer is a thin aluminum or titanium disk in these experiments. It is subject to both deformation and tilting. In some cases, the flyer remains relatively flat in a slightly bowed (i.e., cupped) shape while in others the deformation is more significant and the plate folds into a more compact shape. Orthogonal flash x-rays are used to evaluate shape and integrity of the projectile prior to impact. A measure of the extent of flyer deformation and tilting is given in Table 1 by the ratio of maximum projectile length (along line of flight) to diameter (or width) normal to the line of flight. For instance, the flyer in test SWBS-2 is folded nearly in-half along one axis (-180 ° bend) and is bent -120 ° along an orthogonal axis. The resulting shape has a maximum length to diameter o f - 2 which from hydrocode and experimental results is more penetrating to multi-wall shields than a flat plate impact [9,10]. 67%-Scale MOD-2 Test Configuration e
67%-Scale AII-AI Test Configuration
d=Proj.Diameter(crn) V=Velocity(kin/s) ~=impactangle tom norma deg 0.127cm A16061-T6
Li=JL t~ 05
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Q
~
~
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t - - - - - I I]
-, 1
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I
~
0.025
0,20era 0.568 A12024T3
0.905 Total A.D.
1.803
Total A.D.
1.843
Fig. 4. Test ConfiguratJ on, 67%-scale shields: (b) All-Aluminum (a) Stuffed Whipple The SNL test results indicate the stuffed Whipple shield provided superior protection compared to the all-aluminum shield alternative (compare SWBS-2 and SWBS-3 in Table 1). The all-aluminum shield suffered a large perforation whereas the stuffed Whipple did not. The flyer plate tests also demonstrate that the stuffed Whipple shield is more effective at oblique impact angles than for impacts normal to the shield (compare rearwall damage of SWBS-5 and SWBS-6). This characteristic is particularly advantageous for spacecraft shielding since most orbital debris and meteoroid impacts will be oblique. However, it is a property not shared by some shields such as the Whipple shield, which can be more vulnerable to oblique impact than normal impacts in the 4-11 km/s velocity range [2]. Table 1. HVL Flyer Plate Tests Flyer Flyer Dia. x Flyer Max SNL Shield Angle Vel. thk. Mass Length/ No. Type* (deg) (kin/s) (mm) (g) Width SWBS-2 SW-67% 0 9,9 18.9xl.0 0.75 1.7 SWBS-5 SW-67% 0 10,0 17.4x0.9 0.57 0.4 SWBS-6 SW-50% 45 10.1 17.3x0.9 0.56 0.5 SWBS-4 SW-67% 0 14.7 5.92xl.0 0.12 6.1 SWBS-3 AII-AI 0 10,2 18.9xl.0 0.75 1.0 *note: SW=Stuffed Whipple, AII-Al=all-aluminum 67%-scale shield Impact
Impact
Flyer Integrity V. bowed, trailing fragments Slightly bowed Slightly bowed Flat, tilted on-edge Bowed, tilted
Rear Wall Damage No Perf (large bulge) No Perf (large bulge) No Perf(small bulge) Perforated (pinhole) Perforated (large)
11-12 KM/S INHIBITED S H A P E D - C H A R G E TEST DATA
An inhibited shaped charge capable of launching aluminum projectiles in excess of 11 km/s has been developed at SwRI over a number of years [6]. NASA sponsored a number of ISCL tests on the stuffed Whipple and all-aluminum shield configurations given in Fig.4. The ISCL launches an aluminum projectile at velocities that are consistently between 11 and 11.5 krn/s. The projectile is in the shape of a hollow cylinder (or thick-walled pipe) with length to outside diameter ratio (L/D) of from 1 to 3. Projectile mass typically ranges from 0.8g to 1.5g. Projectile mass is determined by SwRI from digitized X-ray images of the projectile. Two sets of
Projectile shape effects on shielding performance
169
orthogonal flash x-ray images capture the size, shape, and orientation of the projectile prior to impact. General observations from the ISCL tests are as follows: (1) The stuffed Whipple shield defeated a 0.85g projectile at 0° (normal impact angle) while the all-aluminum shield was perforated. (2) In 45 ° impacts, the stuffed Whipple provided successful protection from 0.6g to 1.5g ISCL projectiles (5 tests), while the all-aluminum shield failed in 3 tests from 0.9g to 1.0g projectiles. (3) The stuffed Whipple provided better protection from 45 ° oblique impacts than normal impacts. LIGHT-GAS GUN (LGG) SHIELD TESTING The effect of projectile shape must be understood to complete the assessment of the ISCL and HVL data. The JSC HIT-F is conducting research on shape effects on shield performance in two areas: LGG testing and hydrocode investigations. The HIT-F has launched hollow aluminum cylinders at 7 km/s similar in size and mass to the ISCL projectiles. This data and LGG data for spherical projectiles is used to determine the ballistic limit mass ratio (BLMR) which is the ratio of the mass of a sphere on the ballistic limit of a shield to the mass of a nonspherical projectile at the ballistic limit. The BLMR is expected to vary with projectile shape and impact velocity. A BLMR above 1 indicates the particular non-spherical shape is more penetrating than a solid spherical projectile. Table 2 lists BLMR's derived from the JSC HIT-F tests at 7 km/s and hydrocode simulations at 11 km/s. In general, non-spherical projectiles are more penetrating than equivalent mass solid spheres. This activity is still in progress. Eventually, enough confidence will be attained to apply the BLMR's to the ballistic limit mass found by the HVL/ISCL tests. This will allow the ballistic limit equations to be raised accordingly in the V> 10km/s region. Table 2. BLMR's (Ballistic Limit Mass Ratios) for 3-wall shields from JSC HIT-F tests and hydrocodes Shield Type All-aluminum All-aluminum Stuffed Whipple
Proj. Shape Hollow Cylinder Hollow Cylinder Flat Plate Hollow Cylinder
Velocity (km/s) 7 11 11 7
BLMR 1.3 1.9 1.7 1.2
Method Test Hydrocode Hydrocode Test
SW SHIELD BALLISTIC LIMITS Recent HVI test results on the SW and all-aluminum 3-wall shield are compared with the predicted perforation threshold ballistic limits for these shields in Figure 5 for normal (0 °) and Figure 6 for 45 ° angle impacts by aluminum projectiles. The SW ballistic limit equations that were used to generate the predicted BL shown in Figures 5-6 are given below for the specific shielding configuration illustrated in Figure 1. High-Velocity: when V _> 6 . 5 / ( c o s 0 ) 1/3, dc = 2.584 V 1/3 (cos0) -°5
(1)
Intermediate-Velocity: when 2.7/(cos0) °5 < V < 6 . 5 / ( c o s 0 ) 1/3, d c = 1.385 (cos0) "vls [(V - 2.7/(cos0)°s)/(6.5/(cos0) I/3 - 2.7/(cos0)°5)] + 0.636 (cos0) -4/3 [[(6.5/(cos0) ~/3- V ) / ( 6 . 5 / ( c o s 0 ) 1/3 - 2.7/(cos0)°5)]
(2)
Low-Velocity: when V < 2.7/(cos0) °5, dc = 1.233 V "2/3(cos0) -5/3
(3)
170
E.L. CHRISTIANSENand J.H. KERR
A general set of equations that can also be used to predict SW ballistic limits are given in Eqns.4-6. However, caution should be exercised when using these equations for designs far from the SW design given in Fig.3. The best method to calibrate the BL equations is to perform HVI tests on the particular shield configuration of interest. High-Velocity: when V _>6.5/(cos0) t/3, dc = 0.6 (tw low)1/3 pp-I/3 V-i/3 (c0s0)-o.5 Intermediate-Velocity: when 2.7/(cos0) °5 < V
<
$2/3 (~/40)1/6
(4)
6.5/(cos0) 1/3,
dc= 0.321 (t w low)1/3 190"1/3 (cosO) "7/18 S 2/3 (0"/40) 1/6 [(W - 2.7/(cos0)°5)/ (6.5/(C0S0)
I/3 -
2.7/(cos0)°5)] + 1.031 901/2 [tw (t~/40) °5 + 0.37 mb] (cosO)4/3 [[(6.5/(C0S0) j/3
- V)/(6.5/(cosO) 1/3 - 2.7/(cos0)°5)]
(5)
Low-Velocity: when V _<2.7/(cos0) °5, d~ = 2 V 2/3 (cosO) 5/3 [t~ (or/40) °5 + 0.37 rob] / [(cosO)5/3 9p°5 V 2/3]
(6)
Figs.5-6 show that the HVI test data indicates that the current ballistic limit equations for the Stuffed Whipple shields are in some cases conservative (i.e., overpredict damage) at velocities >10 km/s. More near-term experimental and hydrocode work is planned at the JSC Hypervelocity Impact Test Facility to improve the assessment of the relative effects of nonspherical projectile shapes and projectile velocity on the results obtained from the HVL and inhibited shaped charge. The SW BL equations will also be modified (i.e., increased) to reflect the HVL and ISCL data. On the other hand, the BL equations for the 3-wall all-aluminum shield are nonconservative in some cases (-7 km/s normal impacts) and will also need to be modified after all HVI data and analyses are completed. Balliatic Limit~ at 0 °
O
2
4
6 Imput
8
I0
12
14
16
V-Sooi~p, (kin/8)
Fig. 5. Ballistic limits for SW and all-aluminum shields for normal impacts.
Projectile shape effects on shielding performance
171
Ball/stie Limits at 45* S
_~ 11
0
, , , 1 + ,
0
I-
FallureExpected AboveL.urves
S
~
l
4
,
,
.
]
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'
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'
+
l
'
'
8
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+
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'
'
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Fig. 6. Ballistic limits for SW and all-aluminum shields for 45 ° impacts. CONCLUDING REMARKS HVI data from 3 different launchers and hydrocode assessments show that the Nextel/Kevlar intermediate layer in the stuffed Whipple (SW) Shield provides better protection than a aluminum second bumper for impact velocities that range up to 12 km/s. Non-spherical shapes are generally more penetrating to 3-wall shields (SW or all-aluminum) than equivalent mass spherical particles. From the work completed to data, the spherical mass that can be stopped by 3-wall shields is from 1.2 to 2 times greater than a cylinder or flat plate impactor. Preliminary evidence from tests and analysis indicates that this ratio is even higher for 2-wall Whipple shields; i.e., that spherical projectiles from 2-4 times greater mass can be stopped in impacts from 7-12 km/s compared to a cylinder of L/D = -2. This indicates that multiple wall shields are more effective than single-bumper shields in protecting spacecraft from non-spherical impacts. Optimizing shielding protection for spacecraft should involve increasing the number of bumpers exterior of the rearwall to 2 or more. Of course, other parameters such as overall shielding spacing should also be optimized to provide the most effective protection for a given shielding weight (as a rule of thumb, spacing should be increased to at least 30x the average particle size that it is desired the shielding protect against). The HIT-F is continuing to determine the effects of projectile shape through light-gas gun tests and sponsoring >10 km/s speed testing at SwRI, SNL and other locations. REFERENCES
1. J.L. Crews and E.L. Christiansen, NASA Johnson Space Center Hypervelocity Impact Test Facility (HIT-F), AIAA Paper No.92-1640, 1992. 2. E.L. Christiansen, Design and Performance Equations for Advanced Meteoroid and Debris Shields, Int. J. Impact Engng, 14, 145-156 (1993). 3. R.P. Bernhard and E.L. Christiansen, STS-73 Meteoroid/Orbital Debris Impact Damage Assessment Report, Part 1, NASA Johnson Space Center, JSC-27323 (1995). 4. R.P. Bernhard, E.L. Christiansen, F. Horz, and D.E. Kessler, Orbital Debris as Detected on Exposed Spacecraft, Int. J. Impact Engng, Proceedings of the 1996 HVIS (1996). 5. E.L. Christiansen, J.L. Crews, J.E. Williamsen, J.H. Robinson, and A.M. Nolen, Enhanced Meteoroid and Orbital Debris Shielding, Int. J. Impact Engng, 17, Proceedings of the 1994 Hypervelocity Impact Symposium (1995). 6. B.G. Cour-Palais, A.J. Piekutowski, K.V. Dahl, and K.L. Poormon, Analysis of the UDRI Tests on Nextel Multi-Shock Shields, Int. J. Impact Engng, 14, 193-204 (1993). 7. M.B. Boslough, J.A. Ang, L.C. Chhabildas, W.D. Reinhart, C.A. Hall, B.G. Cour-Palais, E.L. Christiansen, and J.L. Crews, Hypervelocity Testing of Advanced Shielding Concepts for Spacecraft Against Impacts to 10 kin/s, Int. J. Impact Engng, 14, 95-106 (1993). 8. J.D. Walker, D.J. Grosch, and S.A. Mullin, Experimental Impacts above 10 km/s, Int. J. Impact Engng, 17, 903-914 (1995).
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9. E.L. Christiansen, J.L. Crews, J.H. Kerr, and L.C. Chhabildas, "Hypervelocity Impact Testing above 10 km/s of Advanced Orbital Debris Shields," Shock Compression of Condensed Matter - 1995, AlP Conference Proceedings 370, Part 2, pp.1183-1186, Seattle, WA, August 13-18, 1995. 10. J.H. Kerr, E.L. Christiansen and J.L. Crews, "Hydrocode Modelling of Advanced Debris Shield Designs," Shock Compression of Condensed Matter - 1995, AIP Conference Proceeedings 370, Part 2, pp.1167-1170, Seattle WA, August 13-18, 1995. 11. Y.C. Angel and J.P. Smith, Critical Response of Shielded Plates Subjected to Hypervelocity Impact, Int. J. lmpact Engng, 14, pp.25-35 (1993).