- Email: [email protected]
Sub-millimeter Debris Impact Damage of Unmanned Spacecraft Structure Panel
Available online at www.sciencedirect.com
Procedia Engineering 58 (2013) 517 – 525
The 12th Hypervelocity Impact Symposium
Sub-millimeter Debris Impact Damage of Unmanned Spacecraft Structure Panel Masumi Higashidea,*, Naomi Onosea, Sunao Hasegawab b
a Japan Aerospace Exploration Agency, 7-44-1 Jindaiji Higashimachi, Chofu, Tokyo, Japan Institute of Space and Aeronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Japan
Abstract This study investigated the damage caused by sub-millimeter debris impact on the structures of unmanned spacecraft owing to the increase in the amount of sub-millimeter debris in the low earth orbit. Panel structures are mainly made of honeycomb sandwich panels, and the chassis of electronic devices are mounted on the panel. Because damage to the electronic device strongly influences the mission success of a spacecraft, determining the damage to the chassis wall behind the structure panel is important in debris protection design. Hypervelocity impact experiments were performed on the sets of an aluminum honeycomb sandwich panel and an aluminum alloy plate. The aluminum alloy plate was installed behind the honeycomb sandwich panel, without a standoff distance. In the experiments, submillimeter steel spheres (diameters of 0.15-1.0 mm) were accelerated up to 6 km/s by a two-stage light gas gun. Impact angles were varied from 0° to 30°. The depths of craters on the aluminum alloy plate were measured after the impact. A crater depth equation that applies when impact angles are less than 16.1° was calculated empirically. © 2012 The Published by Published Elsevier Ltd. SelectionLtd. and/or peer-review under responsibility of the Hypervelocity Impact Society. © 2013 Authors. by Elsevier Selection and peer-review under responsibility of the Hypervelocity Impact Society Keywords: Space Debris; Honeycomb Sandwich Structure; Sub-millimeter Debris ; Crater Depth Equation
Nomenclature dp p vp vn Cw
projectile diameter (mm) maximum crater depth (mm) impact velocity (km/s) normal component of impact velocity (km/s) sound speed in witness plate (km/s) impact angle (°)
1. Introduction Space debris impact is one of critical risks for spacecraft. Figure 1 shows spatial density of debris in below 2000 km in altitude calculated by the debris environment model, MASTER2009. In the decade, the amount of debris less than 1 mm in diameter increased to approximately 1.5 times. Consequently, the probability of their impact with spacecraft has also increased. Unmanned spacecrafts are vulnerable to sub-millimeter debris impact because they are made of light and thin materials. Therefore, risk assessment for debris impact should be carried out before launch to protect mission-critical components of the spacecraft. Many important components are installed on the interior surfaces of structure panels. Therefore, determining the debris damage limit of structure panels is necessary. The purpose of this study was to
* Corresponding author. Tel.: +81-50-3362-3191; fax: +81-422-40-3236. E-mail address: [email protected].
1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the Hypervelocity Impact Society doi:10.1016/j.proeng.2013.05.059
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Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
investigate the sub-millimeter debris impact damage to the structure panels of unmanned spacecraft in low earth orbit. Structure panels are typically made of a sandwich of aluminum or carbon fiber reinforced plastic face sheets and an aluminum honeycomb core. Hypervelocity impact studies on such sandwich structures have been conducted since the 1960s, and these have yielded some ballistic limit equations for honeycomb sandwich panels [1-3]. However, these results are not applicable to structure panels because those panels had thicker face sheets than those of a typical unmanned spacecraft. R. Putzar et al. [4] performed hypervelocity experiments on the structure panels of a typical unmanned spacecraft and investigated the debris impact damage to electronic devices. They found that even projectiles that are less than 1.5 mm can cause damage to the electronic devices. In the present study, damage produced by the impact of projectiles less than 1 mm to the structure panel was investigated. Sandwich panels made of thin face sheets (0.25 mm thick) were used as experimental specimens. Electronic devices in an unmanned spacecraft are usually mounted in an aluminum chassis installed on the interior surface of a structure panel as shown in Fig. 2. If debris perforates the structure panel but is stopped by the chassis wall, the debris impact will not affect the equipment and will not affect the probability of mission success. Thus, the perforation threshold of the chassis wall behind the structure panel is equal to the ballistic limit of the unmanned spacecraft structure. This study investigated the damage to a chassis wall installed behind a structure panel. The depths of craters in the chassis wall were measured, and the crater depth equations were calculated empirically. 2. Experimental Conditions To simulate a structure panel and chassis wall, a honeycomb sandwich panel was fixed to a 5 mm thick A2024 aluminum alloy plate, without a standoff distance, as shown in Fig. 3. The characteristics of the used honeycomb sandwich panel are listed in Table 1. The surface impacted by a projectile was defined as the front face sheet, and its opposite surface was defined as the back face sheet in this study. Hypervelocity impact experiments were performed with a two-stage light gas
cumulative spatial density of debris [1/km3]
10 1999
8
2009 6 4 2 0 1.0E 06
1.0E 05 1.0E 04 debris diameter [m]
1.0E 03
Fig. 1. Spatial density of debris in below 2000 km in altitude calculated by MASTER2009.
structure panel
electronic devices
Fig. 2. Schematic of electronic devices in unmanned spacecraft.
equipment chassis
Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
honeycomb sandwich panel
aluminum alloy plate
impact direction
Fig. 3. Target setup.
Table 1. Tested honeycomb sandwich panel. Face Sheet
Honeycomb Core
Material
A2024
Thickness
0.25 mm
Material
A5056
Core thickness
25.4 mm
Cell size
6.35 mm
Foil thickness
18 m
projectile
honeycomb sandwich panel
aluminum alloy plate
Fig. 4. Definition of impact angle.
gun at the Institute of Space and Aeronautical Science, JAXA. Steel spheres were launched at 6 km/s. Alumina is the dominant material in sub-millimeter debris in low earth orbit, and the average impact velocity on a spacecraft is approximately 10 km/s [5-6]. However, advanced techniques are required to accelerate small solid projectiles up to such speeds, so this study used a higher-density projectile material to simulate the impact pressure caused by alumina projectiles at 10 km/s. When a steel sphere impacts at 6 km/s, the impact pressure is almost equal to that produced by an alumina sphere at 9 km/s. The projectile diameters were 0.15-1.0 mm. Projectiles less than 0.5 mm were launched using a scattershot method: multiple projectiles were placed into a sabot and impacted on a target at almost the same velocity. Using this method, 10-20 projectiles could impact a target in a single shot. The projectile impact angles were varied to 0°, 15°, and
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30°. Figure 4 defines the impact angle. After the impact experiments, the depths of craters in the aluminum alloy plate resulting from the projectile impacts were measured using optical and laser microscopes. 3. Experimental Results 3.1. Results of Normal Impact Experiments Results of normal impact experiments are summarized in Table 2. Under all conditions, the projectiles perforated the front face sheet of the honeycomb sandwich panel. The diameters of the perforated holes were 2.0dp to 4.8dp. The honeycomb sandwich panels impacted by projectiles of dp = 0.15 mm are shown in Fig. 5. The perforation of the back face sheet only occurred when two or three projectiles impacted a single honeycomb cell; when only one projectile impacted a cell, there was no perforation hole on the back face sheet. Consequently, 0.15 mm projectiles did not perforate the honeycomb sandwich panel. In the experiments using projectiles over 0.3 mm in diameter, damage produced by impact of the fragment cloud was observed on the back face sheet and surface of the aluminum alloy plate, as shown in Fig. 6. The projectile was considered to have been changed into the fragment cloud by the impact on the front face sheet. The honeycomb core acted similarly to the standoff of a double-wall bumper shield and dispersed the impact energy of the projectiles. After the impact experiments, the honeycomb sandwich panels were examined by X-ray radiography. The results are shown in Fig. 7. Under the impact points, the honeycomb cells were deformed. As shown on the left side of Fig. 7, the honeycomb cells adjacent to the impact points were not deformed by the impacts of the 0.3 and 0.5 mm projectiles. The fragment cloud produced by the impact on the front face sheet may have been captured in the foils of the honeycomb cell under the impact point. On the other hand, the impacts of 0.8 and 1.0 mm projectiles ruptured the honeycomb cells adjacent to the impacted cell, as shown in the right side of Fig. 7. However, perforated holes on the back face sheets were only generated in the area under the impacted cells. The foils of cells adjacent to those impacted were damaged, but the fragments did not perforate the back face sheet under the adjacent cell. This result shows that, in only the impacted cell, the fragments kept sufficient energy to perforate the face sheet and that the honeycomb foils acted as bumpers for the fragment cloud. Table 2. Normal impact experiments.
ID
Projectile Diameter (mm)
Number of Projectiles
Impact Velocity (km/s)
Result
AL-HCSP-23
0.15
3
5.96
Projectiles stopped on the back face sheet.
AL-HCSP-24
0.15
37
5.78
Projectiles stopped on the back face sheet.
AL-HCSP-11
0.3
9
5.88
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-16
0.3
12
5.92
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-20
0.5
3
5.86
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-22
0.5
4
6.02
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-26
0.8
1
5.84
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-27
0.8
1
5.71
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-28
1.0
1
5.86
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-29
1.0
1
5.94
Projectiles perforated the panel and produced craters on the witness plate
3.2. Results of Oblique Impact Experiments The impact angles were adjusted to 15° and 30°. Results of oblique impact experiments are summarized in Table 3. When dp = 0.3 mm and the impact angle was 30°, the back face sheet was not perforated. The results of 0.5 mm projectile impacts are shown in Fig. 8. The damage to the aluminum alloy plates decreased when the impact angle was increased. At dp = 0.3, 0.8, and 1.0 mm, damage to the back face sheets and aluminum alloy plate also became less at larger impact angles.
Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
10mm
521
10mm
Fig. 5. Honeycomb sandwich panel impacted by dp = 0.15 mm projectiles at 5.78 km/s: (left) front face sheet, (right) back face sheet
10mm
10mm
Fig. 6. Honeycomb sandwich panel impacted by dp = 0.3 mm projectiles at 5.92 km/s: (left) front face sheet, (right) back face sheet
5mm
5mm
Fig. 7. X-ray radiographs of honeycomb sandwich panels: (left) dp = 0.5 mm, vp = 5.86 km/s, (right) dp = 0.8 mm, vp = 5.71 km/s.
The back face sheets shown in Fig. 9 are the results of 1 mm projectile impacts at an impact angle of 15°. Both experiments were performed under almost the same experimental conditions, but different shapes of perforated holes were generated. The honeycomb sandwich panels were examined by X-ray radiography, as shown in Fig. 10. When the projectile impacted the center of a honeycomb cell, as shown on the left side of Fig. 10, the back side of only the impacted cell was damaged. Foils constituting cells adjacent to the impacted cell were slightly deformed, but there was no significant damage to the back side of the adjacent cells. On the back face sheet, only one perforated hole was observed. Next, the panel shown on the right side of Fig. 9 was examined in the same manner as that shown on the right side of Fig. 10. The projectile impacted the foil of a honeycomb cell, and back wall damage was produced under several cells in contact with the impact point. The projectile seems to have been broken by the impact on the foil, and some large fragments then damaged on the back face sheet. To investigate the dispersing region of the fragment cloud, the number of damaged foils was counted. In this study, one foil is defined as a side of a honeycomb cell hexagon. In the panel where the impact point was at the center of a honeycomb cell, 37 foils were damaged. On the other hand, the panel where the impact was on a foil had 55 damaged foils. Several large fragments were generated, so the damaged area of the honeycomb core was expanded when the projectile impact was on the foil.
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Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525 Table 3.Oblique impact experiments. Projectile Diameter (mm)
ID AL-HCSP-37
0.3
Number of Projectiles 1
Impact Angle (°)
Result
6.39
15
Projectiles perforated the panel and produced craters on the witness plate Projectiles perforated the panel and produced craters on the witness plate
Impact Velocity (km/s)
AL-HCSP-41
0.3
2
6.03
15
AL-HCSP-39
0.5
6
6.33
15
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-43
0.5
6
6.33
15
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-45
1.0
1
6.56
15
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-47
1.0
1
6.65
15
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-38
0.3
6
6.35
30
Projectiles stopped on the back face sheet.
AL-HCSP-42
0.3
10
6.08
30
Projectiles stopped on the back face sheet.
AL-HCSP-40
0.5
3
6.26
30
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-44
0.5
5
6.26
30
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-46
1.0
1
6.66
30
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-48
1.0
1
6.62
30
Projectiles perforated the panel and produced craters on the witness plate
10mm
10mm
10mm
Fig. 8. Aluminum alloy plates behind honeycomb sandwich panels impacted by dp = 0.5 mm projectiles: (left) 6.33 km/s, (right) = 30°, vp = 6.26 km/s.
10mm
= 0°, vp = 6.02 km/s, (center)
= 15°, vp =
10mm
Fig. 9. Back face sheets of honeycomb sandwich panels impacted by dp = 1 mm projectiles with
= 15°: (left) vp = 6.56 km/s, (right) vp = 6.65 km/s.
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10mm
10mm
Fig. 10. X-ray radiographs of honeycomb sandwich panels impacted by dp = 1 mm projectiles with
= 15°: (left) vp = 6.56 km/s, (right) vp = 6.65 km/s.
max. crater depth, p [mm]
2.0
1.5
1.0
0.5
0.0 0.0
0.2
0.4 0.6 0.8 projectile diameter, dp [mm]
1.0
1.2
Fig. 11. Depth of craters on aluminum plates, produced by normal impacts of projectiles.
4. Crater Depth Equations 4.1. Equation for Normal Impacts The depths of craters on the aluminum alloy plate were measured using optical and laser microscopes. The maximum crater depth was obtained for each impact point. The relationship between dp and crater depth p is shown in Fig. 11 the depths of craters resulting from multiple impacts on a single honeycomb cell are omitted. Because the projectiles of dp = 0.15 mm did not perforate the honeycomb sandwich panel, p = 0. The impact energy was assumed to be proportional to the crater volume; the crater depth is thus proportional to the projectile diameter because the impact velocity, projectile density, and target density were almost constant. Thus, the following empirical equation was obtained from Fig. 11:
p 2.18d p
0.454
(1)
The standard deviation of the regression line in Fig. 11 was 0.084 mm. The upper limit of Eq. (1) expresses the severest damage on a chassis wall caused by a debris impact because an aluminum alloy was used to simulate the chassis of equipment mounted in a spacecraft. 4.2. Application to Oblique Impacts When a projectile impacts obliquely on a double-wall structure, a ballistic limit curve can be applied to the oblique impact results by using the normal component of an impact velocity on a target surface as the impact velocity [7]. A honeycomb sandwich panel is a double-wall structure consisting of front and back face sheets. Therefore, the effect of
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Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
impact velocities was added to Eq. (1). The assumption that the impact velocity is constant was excluded from the method given in the previous section. Accordingly, the relationship between crater depths, projectile diameters, and impact angles is expressed below.
p
(2)
d p vn2 3
where, vn is a normal component of an impact velocity on a target surface vpcos . The experimental results for each impact angle are plotted in Fig. 12. The differences in the horizontal positions for each impact angle express differences in the projectile diameters because the impact velocities were nearly constant in this study. By focusing on the deepest crater data produced by each projectile diameter, the results of = 0 and 15° seem to have the proportional relationship predicted by Eq. (2). From these results, the following empirical equation was calculated.
p 2.30d p v n C w
23
0.588
(3)
The dashed lines in Fig. 12 show the standard deviation of the upper limit of Eq. (3). Although the crater depths produced at = 15° and 30° can be expressed by Eq. (3), the results of = 30° were much less than the upper limit shown by Eq. (3). The reason for this was considered to be the projectile paths, as shown in Fig. 13. The honeycomb sandwich panel used in this study had a 25.4 mm core height and 6.35 mm cell size. At = 15°, some paths do not pass through foils of the honeycomb core. If a projectile does not pass through the foils, the honeycomb sandwich panel is equivalent to a doublewall structure. Consequently, crater depths can be estimated by using vn as in previous studies. At = 30°, however, projectiles always impact on the foils, as shown in Fig. 12. The kinetic energy of the fragment cloud generated by the impact on the front face sheet seems to have been decreased by impact on the foil. Although the foil thickness was lower than 1/10 of the projectile diameter, the foil is thought to be able to protect against the fragment cloud. When the impact angle is less than 16.1°, some paths do not pass through the foils in the honeycomb sandwich panel used in this study. If 16.1°, the upper limit of the maximum crater depth is considered lower than Eq. (3).
3.0
max. crater depth, p [mm]
0deg 15deg 2.0
30deg
1.0
0.0 0.0
0.5
1.0 dp(v n/Cw)2/3
Fig. 12. Crater depths on aluminum plates, produced by normal and oblique impacts of projectiles.
1.5
Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
honeycomb core foils projectile path 15°
30°
Fig. 13. Cross section of honeycomb core.
5. Summary Because mission-critical components of unmanned spacecrafts are installed on the interior surfaces of structure panels, this study investigated the debris impact damage to an equipment chassis wall mounted on a structure panel. Hypervelocity impact experiments were performed on a simulated set of a structure panel and chassis wall. In normal impact experiments, projectiles of 0.15 mm in diameter did not perforate the back face sheet of a honeycomb sandwich panel simulating the structure panel. In oblique impact experiments, the panel was not perforated by 0.3 mm diameter projectiles at a 30° impact angle. If the panels were perforated, craters were produced on the aluminum alloy plates simulating the chassis wall. The depths of the craters were measured, and the crater depth equations were calculated empirically with the assumption that impact energy is proportional to the crater volume and a honeycomb sandwich structure is equivalent to a double wall. The equation is considered to be applicable when the impact angle is less than 16.1°. An X-ray radiography examination of the panels after the impact experiments showed that the internal honeycomb foils of the panel act as bumpers for fragment clouds produced by impacts on the front face sheet. To obtain a crater depth equation that can be applied to large impact angles, the effect of the protection capability of the honeycomb foil should be considered. Acknowledgements The work was supported by the Space Plasma Laboratory, ISAS, JAXA. The authors acknowledge the valuable advice given by members of JAXA Spacecraft Design Standard WG3. The authors also thank Ms. Noriko Kanai from Mitsubishi Electric Co. for her great support. References [1] Sennett, R.E., Lathrop, B.L., 1968, Effects of Hypervelocity Impact on Honeycomb Structures, Journal of Spacecraft 5, pp. 1496-1497. [2] Jex, D.W., Miller, A.M., MacKay, C.A., 1970, The Characteristics of Penetration for a Double-Sheet Structure with Honeycomb, NASA TM X-53974. [3] Turner, R.J., Taylor, E.A., McDonnell, J.A.M., Stokes, H. Marriott, P., Wilkinson, J., Catling, D.J., Vignjevic, R., Berthoud, L., Lambert, M., 2001, Cost Effective Honeycomb and Multi-Layer Insulation Debris Shields for Unmanned Spacecraft, International Journal of Impact Engineering 26, pp. 785-796. [4] Putzar, R., Schäfer, F., Stokes ,H., Chant, R., Lambert, M., 2005, “Vulnerability of Spacecraft Electronics Boxes to Hypervelocity Impacts,” Proceedings of the 56th International Astronautical Congress. IAC-05-B6.4.02. [5] Wegener, P., Krag, H., Rex, D., Bendisch, J., Klinkrad, H., 1999, “The Orbital Distribution and Dynamics of Solid Rocket Motor Particle Clouds for an Implementation into the Master Debris Model,” Advances in Space Research 23(1), pp. 161-164. [6] Johnson, N.L., 2005, “Orbital Debris Sesearch in the U.S.,” Proceedings of the Fourth European Conference on Space Debris, ESA SP-587, pp. 5-15. [7] Christiansen, E.L., 1993, Design and Performance Equations for Advanced Meteoroid and Debris Shields, International Journal of Impact Engineering 14, pp. 145-156.
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Procedia Engineering 58 (2013) 517 – 525
The 12th Hypervelocity Impact Symposium
Sub-millimeter Debris Impact Damage of Unmanned Spacecraft Structure Panel Masumi Higashidea,*, Naomi Onosea, Sunao Hasegawab b
a Japan Aerospace Exploration Agency, 7-44-1 Jindaiji Higashimachi, Chofu, Tokyo, Japan Institute of Space and Aeronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Japan
Abstract This study investigated the damage caused by sub-millimeter debris impact on the structures of unmanned spacecraft owing to the increase in the amount of sub-millimeter debris in the low earth orbit. Panel structures are mainly made of honeycomb sandwich panels, and the chassis of electronic devices are mounted on the panel. Because damage to the electronic device strongly influences the mission success of a spacecraft, determining the damage to the chassis wall behind the structure panel is important in debris protection design. Hypervelocity impact experiments were performed on the sets of an aluminum honeycomb sandwich panel and an aluminum alloy plate. The aluminum alloy plate was installed behind the honeycomb sandwich panel, without a standoff distance. In the experiments, submillimeter steel spheres (diameters of 0.15-1.0 mm) were accelerated up to 6 km/s by a two-stage light gas gun. Impact angles were varied from 0° to 30°. The depths of craters on the aluminum alloy plate were measured after the impact. A crater depth equation that applies when impact angles are less than 16.1° was calculated empirically. © 2012 The Published by Published Elsevier Ltd. SelectionLtd. and/or peer-review under responsibility of the Hypervelocity Impact Society. © 2013 Authors. by Elsevier Selection and peer-review under responsibility of the Hypervelocity Impact Society Keywords: Space Debris; Honeycomb Sandwich Structure; Sub-millimeter Debris ; Crater Depth Equation
Nomenclature dp p vp vn Cw
projectile diameter (mm) maximum crater depth (mm) impact velocity (km/s) normal component of impact velocity (km/s) sound speed in witness plate (km/s) impact angle (°)
1. Introduction Space debris impact is one of critical risks for spacecraft. Figure 1 shows spatial density of debris in below 2000 km in altitude calculated by the debris environment model, MASTER2009. In the decade, the amount of debris less than 1 mm in diameter increased to approximately 1.5 times. Consequently, the probability of their impact with spacecraft has also increased. Unmanned spacecrafts are vulnerable to sub-millimeter debris impact because they are made of light and thin materials. Therefore, risk assessment for debris impact should be carried out before launch to protect mission-critical components of the spacecraft. Many important components are installed on the interior surfaces of structure panels. Therefore, determining the debris damage limit of structure panels is necessary. The purpose of this study was to
* Corresponding author. Tel.: +81-50-3362-3191; fax: +81-422-40-3236. E-mail address: [email protected].
1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the Hypervelocity Impact Society doi:10.1016/j.proeng.2013.05.059
518
Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
investigate the sub-millimeter debris impact damage to the structure panels of unmanned spacecraft in low earth orbit. Structure panels are typically made of a sandwich of aluminum or carbon fiber reinforced plastic face sheets and an aluminum honeycomb core. Hypervelocity impact studies on such sandwich structures have been conducted since the 1960s, and these have yielded some ballistic limit equations for honeycomb sandwich panels [1-3]. However, these results are not applicable to structure panels because those panels had thicker face sheets than those of a typical unmanned spacecraft. R. Putzar et al. [4] performed hypervelocity experiments on the structure panels of a typical unmanned spacecraft and investigated the debris impact damage to electronic devices. They found that even projectiles that are less than 1.5 mm can cause damage to the electronic devices. In the present study, damage produced by the impact of projectiles less than 1 mm to the structure panel was investigated. Sandwich panels made of thin face sheets (0.25 mm thick) were used as experimental specimens. Electronic devices in an unmanned spacecraft are usually mounted in an aluminum chassis installed on the interior surface of a structure panel as shown in Fig. 2. If debris perforates the structure panel but is stopped by the chassis wall, the debris impact will not affect the equipment and will not affect the probability of mission success. Thus, the perforation threshold of the chassis wall behind the structure panel is equal to the ballistic limit of the unmanned spacecraft structure. This study investigated the damage to a chassis wall installed behind a structure panel. The depths of craters in the chassis wall were measured, and the crater depth equations were calculated empirically. 2. Experimental Conditions To simulate a structure panel and chassis wall, a honeycomb sandwich panel was fixed to a 5 mm thick A2024 aluminum alloy plate, without a standoff distance, as shown in Fig. 3. The characteristics of the used honeycomb sandwich panel are listed in Table 1. The surface impacted by a projectile was defined as the front face sheet, and its opposite surface was defined as the back face sheet in this study. Hypervelocity impact experiments were performed with a two-stage light gas
cumulative spatial density of debris [1/km3]
10 1999
8
2009 6 4 2 0 1.0E 06
1.0E 05 1.0E 04 debris diameter [m]
1.0E 03
Fig. 1. Spatial density of debris in below 2000 km in altitude calculated by MASTER2009.
structure panel
electronic devices
Fig. 2. Schematic of electronic devices in unmanned spacecraft.
equipment chassis
Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
honeycomb sandwich panel
aluminum alloy plate
impact direction
Fig. 3. Target setup.
Table 1. Tested honeycomb sandwich panel. Face Sheet
Honeycomb Core
Material
A2024
Thickness
0.25 mm
Material
A5056
Core thickness
25.4 mm
Cell size
6.35 mm
Foil thickness
18 m
projectile
honeycomb sandwich panel
aluminum alloy plate
Fig. 4. Definition of impact angle.
gun at the Institute of Space and Aeronautical Science, JAXA. Steel spheres were launched at 6 km/s. Alumina is the dominant material in sub-millimeter debris in low earth orbit, and the average impact velocity on a spacecraft is approximately 10 km/s [5-6]. However, advanced techniques are required to accelerate small solid projectiles up to such speeds, so this study used a higher-density projectile material to simulate the impact pressure caused by alumina projectiles at 10 km/s. When a steel sphere impacts at 6 km/s, the impact pressure is almost equal to that produced by an alumina sphere at 9 km/s. The projectile diameters were 0.15-1.0 mm. Projectiles less than 0.5 mm were launched using a scattershot method: multiple projectiles were placed into a sabot and impacted on a target at almost the same velocity. Using this method, 10-20 projectiles could impact a target in a single shot. The projectile impact angles were varied to 0°, 15°, and
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30°. Figure 4 defines the impact angle. After the impact experiments, the depths of craters in the aluminum alloy plate resulting from the projectile impacts were measured using optical and laser microscopes. 3. Experimental Results 3.1. Results of Normal Impact Experiments Results of normal impact experiments are summarized in Table 2. Under all conditions, the projectiles perforated the front face sheet of the honeycomb sandwich panel. The diameters of the perforated holes were 2.0dp to 4.8dp. The honeycomb sandwich panels impacted by projectiles of dp = 0.15 mm are shown in Fig. 5. The perforation of the back face sheet only occurred when two or three projectiles impacted a single honeycomb cell; when only one projectile impacted a cell, there was no perforation hole on the back face sheet. Consequently, 0.15 mm projectiles did not perforate the honeycomb sandwich panel. In the experiments using projectiles over 0.3 mm in diameter, damage produced by impact of the fragment cloud was observed on the back face sheet and surface of the aluminum alloy plate, as shown in Fig. 6. The projectile was considered to have been changed into the fragment cloud by the impact on the front face sheet. The honeycomb core acted similarly to the standoff of a double-wall bumper shield and dispersed the impact energy of the projectiles. After the impact experiments, the honeycomb sandwich panels were examined by X-ray radiography. The results are shown in Fig. 7. Under the impact points, the honeycomb cells were deformed. As shown on the left side of Fig. 7, the honeycomb cells adjacent to the impact points were not deformed by the impacts of the 0.3 and 0.5 mm projectiles. The fragment cloud produced by the impact on the front face sheet may have been captured in the foils of the honeycomb cell under the impact point. On the other hand, the impacts of 0.8 and 1.0 mm projectiles ruptured the honeycomb cells adjacent to the impacted cell, as shown in the right side of Fig. 7. However, perforated holes on the back face sheets were only generated in the area under the impacted cells. The foils of cells adjacent to those impacted were damaged, but the fragments did not perforate the back face sheet under the adjacent cell. This result shows that, in only the impacted cell, the fragments kept sufficient energy to perforate the face sheet and that the honeycomb foils acted as bumpers for the fragment cloud. Table 2. Normal impact experiments.
ID
Projectile Diameter (mm)
Number of Projectiles
Impact Velocity (km/s)
Result
AL-HCSP-23
0.15
3
5.96
Projectiles stopped on the back face sheet.
AL-HCSP-24
0.15
37
5.78
Projectiles stopped on the back face sheet.
AL-HCSP-11
0.3
9
5.88
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-16
0.3
12
5.92
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-20
0.5
3
5.86
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-22
0.5
4
6.02
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-26
0.8
1
5.84
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-27
0.8
1
5.71
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-28
1.0
1
5.86
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-29
1.0
1
5.94
Projectiles perforated the panel and produced craters on the witness plate
3.2. Results of Oblique Impact Experiments The impact angles were adjusted to 15° and 30°. Results of oblique impact experiments are summarized in Table 3. When dp = 0.3 mm and the impact angle was 30°, the back face sheet was not perforated. The results of 0.5 mm projectile impacts are shown in Fig. 8. The damage to the aluminum alloy plates decreased when the impact angle was increased. At dp = 0.3, 0.8, and 1.0 mm, damage to the back face sheets and aluminum alloy plate also became less at larger impact angles.
Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
10mm
521
10mm
Fig. 5. Honeycomb sandwich panel impacted by dp = 0.15 mm projectiles at 5.78 km/s: (left) front face sheet, (right) back face sheet
10mm
10mm
Fig. 6. Honeycomb sandwich panel impacted by dp = 0.3 mm projectiles at 5.92 km/s: (left) front face sheet, (right) back face sheet
5mm
5mm
Fig. 7. X-ray radiographs of honeycomb sandwich panels: (left) dp = 0.5 mm, vp = 5.86 km/s, (right) dp = 0.8 mm, vp = 5.71 km/s.
The back face sheets shown in Fig. 9 are the results of 1 mm projectile impacts at an impact angle of 15°. Both experiments were performed under almost the same experimental conditions, but different shapes of perforated holes were generated. The honeycomb sandwich panels were examined by X-ray radiography, as shown in Fig. 10. When the projectile impacted the center of a honeycomb cell, as shown on the left side of Fig. 10, the back side of only the impacted cell was damaged. Foils constituting cells adjacent to the impacted cell were slightly deformed, but there was no significant damage to the back side of the adjacent cells. On the back face sheet, only one perforated hole was observed. Next, the panel shown on the right side of Fig. 9 was examined in the same manner as that shown on the right side of Fig. 10. The projectile impacted the foil of a honeycomb cell, and back wall damage was produced under several cells in contact with the impact point. The projectile seems to have been broken by the impact on the foil, and some large fragments then damaged on the back face sheet. To investigate the dispersing region of the fragment cloud, the number of damaged foils was counted. In this study, one foil is defined as a side of a honeycomb cell hexagon. In the panel where the impact point was at the center of a honeycomb cell, 37 foils were damaged. On the other hand, the panel where the impact was on a foil had 55 damaged foils. Several large fragments were generated, so the damaged area of the honeycomb core was expanded when the projectile impact was on the foil.
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Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525 Table 3.Oblique impact experiments. Projectile Diameter (mm)
ID AL-HCSP-37
0.3
Number of Projectiles 1
Impact Angle (°)
Result
6.39
15
Projectiles perforated the panel and produced craters on the witness plate Projectiles perforated the panel and produced craters on the witness plate
Impact Velocity (km/s)
AL-HCSP-41
0.3
2
6.03
15
AL-HCSP-39
0.5
6
6.33
15
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-43
0.5
6
6.33
15
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-45
1.0
1
6.56
15
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-47
1.0
1
6.65
15
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-38
0.3
6
6.35
30
Projectiles stopped on the back face sheet.
AL-HCSP-42
0.3
10
6.08
30
Projectiles stopped on the back face sheet.
AL-HCSP-40
0.5
3
6.26
30
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-44
0.5
5
6.26
30
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-46
1.0
1
6.66
30
Projectiles perforated the panel and produced craters on the witness plate
AL-HCSP-48
1.0
1
6.62
30
Projectiles perforated the panel and produced craters on the witness plate
10mm
10mm
10mm
Fig. 8. Aluminum alloy plates behind honeycomb sandwich panels impacted by dp = 0.5 mm projectiles: (left) 6.33 km/s, (right) = 30°, vp = 6.26 km/s.
10mm
= 0°, vp = 6.02 km/s, (center)
= 15°, vp =
10mm
Fig. 9. Back face sheets of honeycomb sandwich panels impacted by dp = 1 mm projectiles with
= 15°: (left) vp = 6.56 km/s, (right) vp = 6.65 km/s.
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Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
10mm
10mm
Fig. 10. X-ray radiographs of honeycomb sandwich panels impacted by dp = 1 mm projectiles with
= 15°: (left) vp = 6.56 km/s, (right) vp = 6.65 km/s.
max. crater depth, p [mm]
2.0
1.5
1.0
0.5
0.0 0.0
0.2
0.4 0.6 0.8 projectile diameter, dp [mm]
1.0
1.2
Fig. 11. Depth of craters on aluminum plates, produced by normal impacts of projectiles.
4. Crater Depth Equations 4.1. Equation for Normal Impacts The depths of craters on the aluminum alloy plate were measured using optical and laser microscopes. The maximum crater depth was obtained for each impact point. The relationship between dp and crater depth p is shown in Fig. 11 the depths of craters resulting from multiple impacts on a single honeycomb cell are omitted. Because the projectiles of dp = 0.15 mm did not perforate the honeycomb sandwich panel, p = 0. The impact energy was assumed to be proportional to the crater volume; the crater depth is thus proportional to the projectile diameter because the impact velocity, projectile density, and target density were almost constant. Thus, the following empirical equation was obtained from Fig. 11:
p 2.18d p
0.454
(1)
The standard deviation of the regression line in Fig. 11 was 0.084 mm. The upper limit of Eq. (1) expresses the severest damage on a chassis wall caused by a debris impact because an aluminum alloy was used to simulate the chassis of equipment mounted in a spacecraft. 4.2. Application to Oblique Impacts When a projectile impacts obliquely on a double-wall structure, a ballistic limit curve can be applied to the oblique impact results by using the normal component of an impact velocity on a target surface as the impact velocity [7]. A honeycomb sandwich panel is a double-wall structure consisting of front and back face sheets. Therefore, the effect of
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Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
impact velocities was added to Eq. (1). The assumption that the impact velocity is constant was excluded from the method given in the previous section. Accordingly, the relationship between crater depths, projectile diameters, and impact angles is expressed below.
p
(2)
d p vn2 3
where, vn is a normal component of an impact velocity on a target surface vpcos . The experimental results for each impact angle are plotted in Fig. 12. The differences in the horizontal positions for each impact angle express differences in the projectile diameters because the impact velocities were nearly constant in this study. By focusing on the deepest crater data produced by each projectile diameter, the results of = 0 and 15° seem to have the proportional relationship predicted by Eq. (2). From these results, the following empirical equation was calculated.
p 2.30d p v n C w
23
0.588
(3)
The dashed lines in Fig. 12 show the standard deviation of the upper limit of Eq. (3). Although the crater depths produced at = 15° and 30° can be expressed by Eq. (3), the results of = 30° were much less than the upper limit shown by Eq. (3). The reason for this was considered to be the projectile paths, as shown in Fig. 13. The honeycomb sandwich panel used in this study had a 25.4 mm core height and 6.35 mm cell size. At = 15°, some paths do not pass through foils of the honeycomb core. If a projectile does not pass through the foils, the honeycomb sandwich panel is equivalent to a doublewall structure. Consequently, crater depths can be estimated by using vn as in previous studies. At = 30°, however, projectiles always impact on the foils, as shown in Fig. 12. The kinetic energy of the fragment cloud generated by the impact on the front face sheet seems to have been decreased by impact on the foil. Although the foil thickness was lower than 1/10 of the projectile diameter, the foil is thought to be able to protect against the fragment cloud. When the impact angle is less than 16.1°, some paths do not pass through the foils in the honeycomb sandwich panel used in this study. If 16.1°, the upper limit of the maximum crater depth is considered lower than Eq. (3).
3.0
max. crater depth, p [mm]
0deg 15deg 2.0
30deg
1.0
0.0 0.0
0.5
1.0 dp(v n/Cw)2/3
Fig. 12. Crater depths on aluminum plates, produced by normal and oblique impacts of projectiles.
1.5
Masumi Higashide et al. / Procedia Engineering 58 (2013) 517 – 525
honeycomb core foils projectile path 15°
30°
Fig. 13. Cross section of honeycomb core.
5. Summary Because mission-critical components of unmanned spacecrafts are installed on the interior surfaces of structure panels, this study investigated the debris impact damage to an equipment chassis wall mounted on a structure panel. Hypervelocity impact experiments were performed on a simulated set of a structure panel and chassis wall. In normal impact experiments, projectiles of 0.15 mm in diameter did not perforate the back face sheet of a honeycomb sandwich panel simulating the structure panel. In oblique impact experiments, the panel was not perforated by 0.3 mm diameter projectiles at a 30° impact angle. If the panels were perforated, craters were produced on the aluminum alloy plates simulating the chassis wall. The depths of the craters were measured, and the crater depth equations were calculated empirically with the assumption that impact energy is proportional to the crater volume and a honeycomb sandwich structure is equivalent to a double wall. The equation is considered to be applicable when the impact angle is less than 16.1°. An X-ray radiography examination of the panels after the impact experiments showed that the internal honeycomb foils of the panel act as bumpers for fragment clouds produced by impacts on the front face sheet. To obtain a crater depth equation that can be applied to large impact angles, the effect of the protection capability of the honeycomb foil should be considered. Acknowledgements The work was supported by the Space Plasma Laboratory, ISAS, JAXA. The authors acknowledge the valuable advice given by members of JAXA Spacecraft Design Standard WG3. The authors also thank Ms. Noriko Kanai from Mitsubishi Electric Co. for her great support. References [1] Sennett, R.E., Lathrop, B.L., 1968, Effects of Hypervelocity Impact on Honeycomb Structures, Journal of Spacecraft 5, pp. 1496-1497. [2] Jex, D.W., Miller, A.M., MacKay, C.A., 1970, The Characteristics of Penetration for a Double-Sheet Structure with Honeycomb, NASA TM X-53974. [3] Turner, R.J., Taylor, E.A., McDonnell, J.A.M., Stokes, H. Marriott, P., Wilkinson, J., Catling, D.J., Vignjevic, R., Berthoud, L., Lambert, M., 2001, Cost Effective Honeycomb and Multi-Layer Insulation Debris Shields for Unmanned Spacecraft, International Journal of Impact Engineering 26, pp. 785-796. [4] Putzar, R., Schäfer, F., Stokes ,H., Chant, R., Lambert, M., 2005, “Vulnerability of Spacecraft Electronics Boxes to Hypervelocity Impacts,” Proceedings of the 56th International Astronautical Congress. IAC-05-B6.4.02. [5] Wegener, P., Krag, H., Rex, D., Bendisch, J., Klinkrad, H., 1999, “The Orbital Distribution and Dynamics of Solid Rocket Motor Particle Clouds for an Implementation into the Master Debris Model,” Advances in Space Research 23(1), pp. 161-164. [6] Johnson, N.L., 2005, “Orbital Debris Sesearch in the U.S.,” Proceedings of the Fourth European Conference on Space Debris, ESA SP-587, pp. 5-15. [7] Christiansen, E.L., 1993, Design and Performance Equations for Advanced Meteoroid and Debris Shields, International Journal of Impact Engineering 14, pp. 145-156.
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