Simulation of hollow shaped charge jet impacts onto aluminium whipple bumpers at 11 km/s

The computational technique of Smoothed Particle Hydrodynamics (as implemented in the hydrocodes AUTODYN-2D and AUTODYN-3D) has been used to simulate the impact of hollow shaped charge jet projectiles onto stuffed Whipple bumper shielding. Due to limited availability of material models, the interim Nextel/Kevlar-Epoxy bumper was modelled as an equivalent thickness of aluminium. Stuffed Whipple bumper shields are used for meteoroid and debris impact protection of the European module of the International Space Station (the Columbus APM). A total of 56 simulations were carried out to investigate the impact processes occurring for shaped charge jet impact. Sensitivity studies were carried out on the influence of projectile shape, pitch, yaw and strength at 11 km/s to determine the range of debris cloud morphologies. The debris cloud structure was shown to be highly dispersed, and no projectile remnant was observed at the centre of the cloud. The mass of an aluminium sphere producing equivalent damage to a shaped charge jet projectile was in the range 1.5 to 1.75 times greater than the mass of the shaped charge jet projectile. Upon loading by the dispersed debris cloud, the interim bumper failed by spallation, producing fragments moving at 2 km/s or less. The fragments distorted the rear wall (pressure wall) of the shield but did not perforate it. The experimental data show rear wall deformation but to a lesser degree. Perforation of the rear wall, observed for one test, was not reproduced by the simulation. Nextel/Kevlar-epoxy material models are required to reproduce correctly the interim bumper failure under debris cloud loading.     

Performance of Whipple shields at impact velocities above 9 km/s

The results of 18 impact tests performed on Whipple shields were compared to the predicted ballistic limits of the shields in the region where the impact velocity of the threatening particle was high enough to produce melting and incipient vaporization of the particle. Ballistic limit equations developed at NASA Johnson Space Center were used to determine nominal failure thresholds for two configurations of all-aluminum Whipple shields. In the tests, 2017-T4 aluminum spheres with diameters ranging from 1.40 to 6.35 mm were used to impact the shields at impact velocities ranging from 6.94 to 9.89 km/s. Two different aluminum alloys were used for the rear walls of a simple Whipple shield. The results of 13 tests using these simple Whipple shields showed they offered better-than-predicted capability as impact velocity increased and that the strength of the rear wall material appeared to have a smaller-than-predicted effect on the shield performance. The results of five tests using three configurations of a scaled Space Station shield - a plain shield at 0 degrees, two shields with multilayer insulation in the space between the bumper and the rear wall (also at 0 degrees), and two tests with the plain shield at 45 degrees obliquity - showed that these shields met their predicted capabilities.     

Whipple shield ballistic limit at impact velocities higher than 7 km/s

The Whipple bumper shield was the first system developed to protect space structures against Meteoroids and Orbital Debris (M/OD), and it is still extensively adopted. In particular, Whipple shields are used to protect several elements of the International Space Station, although the most exposed areas to the M/OD environment are shielded by innovative low weigh and high resistance systems.

Hydrocode simulations were used to predict the ballistic limit of a typical aluminium Whipple shield configuration for space applications in the impact velocity range not accessible by the available experimental techniques. The simulations were carried out using the AUTODYN-2D and the PAMSHOCK-3D codes, allowing to couple the gridless Smoothed Particles Hydrodynamics with the Lagrange grid-based techniques. The global damage of the structure after the impact was determined with particular attention to the back wall penetration, and the results obtained with the two hydrocodes were compared with those given by semi-empirical damage equations.

A few hypervelocity Light Gas Gun impact experiments, performed on the same shield configuration at velocities up to 7.2 km/s, were previously simulated in order to assess the capability and limitations of the two hydrocodes in reproducing the experimental results available in the lower velocity regime. The influence of material models on the numerical predictions is discussed.     

Whipple shield performance in the shatter regime

A series of hypervelocity impact tests have been performed on aluminum alloy Whipple shields to investigate failure mechanisms and performance limits in the shatter regime. Test results demonstrated a more rapid increase in performance than predicted by the latest iteration of the JSC Whipple shield ballistic limit equation (BLE) following the onset of projectile fragmentation. This increase in performance was found to level out between 4.0 and 5.0 km/s, with a subsequent decrease in performance for velocities up to 5.6 km/s. For a detached spall failure criterion, the failure limit was found to continually decrease up to a velocity of 7.0 km/s, substantially varying from the BLE, while for perforation-based failure an increase in performance was observed. An existing phenomenological ballistic limit curve was found to provide a more accurate reproduction of shield behavior that the BLE, prompting an investigation of appropriate models to replace linear interpolation in shatter regime. A largest fragment relationship was shown to provide accurate predictions up to 4.3 km/s, which was extended to the incipient melt limit (5.6 km/s) based on an assumption of no additional fragmentation. Alternate models, including a shock enhancement approach and debris cloud cratering model are discussed as feasible alternatives to the proposed curve in the shatter regime, due to conflicting assumptions and difficulties in extrapolating the current approach to oblique impact. These alternate models require further investigation.     

Hypervelocity testing of advanced shielding concepts for spacecraft against impacts to 10 km/s

Experiments have been performed on NASA state-of-the-art hypervelocity impact shields using the Sandia Hypervelocity Launcher (HVL) to obtain test velocities greater than those achievable using conventional two stage light-gas sun technology. The objective of the tests was to provide the first experimental data on the advanced shielding concepts for evaluation of the analytical equations (shield performance predictors) at velocities previously unattainable in the laboratory, and for comparison to single Whipple Bumper Shileds (WBS) under similar loading conditions. The results indicate that significantly more mass is required on the back sheet of the WBS to stop an approximately flat-plate particle impacting at 7 km/sec and at 10 km/sec that the analytical equations (derived from spherical particle impact data) predicted. The Multi-Shock Shield (MSS) consists of four ceramic fabric bumpers, and is lighter in terms of areal density by up to 33%, but is as effective as the heavier WBS under similar impact conditions at about 10 km/s. The Mesh Double Bumper shield (MDB) consists of an aluminum wire mesh bumper, followed by a sheet of solid aluminum and a layer of Kevlar^® fabric. It provides a weight savings in terms of areal density of up to 35% compared to the WBS for impacts of around 10 km/s.     

Analysis of the UDRI tests on Nextel multi-shock shields

This paper analyzes the results of further development of the Nextel ceramic cloth, multiple-bumper or multi-shock shield, first presented at the 1989 HVIS and published as Cour-Palais and Crews (1990). The supporting hypervelocity impact testing was done by the University of Dayton Research Institute, Dayton, Ohio, in their Impact Physics Laboratory, using 0.953cm aluminum spheres and equal-mass (l/d=0.16) aluminum discs. The projectiles were launched at 6.6 to 6.9km/s by a 50/20mm, two-stage light gas gun, normal to the targets. The objective of this development project was to investigate light-weight, flexible, multiple-bumper shields for possible use as protection for some elements of Space Station Freedom. The analysis discusses the performance of shields consisting of different combinations of Nextel ceramic cloth bumpers and aluminum rear sheets. Several Nextel fiber strengths and weaves were investigated as bumpers and a baseline, light-weight shield that met the failure criteria was established using the spherical aluminum projectiles. This same target was then tested against the aluminum discs to investigate the effect of projectile shape. The multi-shock phenomena was also investigated during this project using the UDRI multiple, orthogonal x-ray system to observe the first three or four sequential impacts of the projectile fragments. Some of these are reproduced in the paper, together with views of the associated rear sheet damage. Similarities between the shock effects of the Nextel and thin aluminum bumpers are shown, and the aluminum multiple-bumper shield results are used to further understand the multi-shock process. Finally, the paper modifies the equation constants given by Cour-Palais and Crews (1990), adds constants for the l/d=0.16 disc, and provides evidence that they scale with momentum to 10km/s.     

Hypervelocity impact tests and simulations of single whipple bumper shield concepts at 10km/s

A series of experiments has been performed to evaluate the effectiveness of a Whipple bumper shield to orbital space debris at impact velocities of 10 km/s. Upon impact by a 19 mm (0.87 mm thick, L/D 0.5) flier plate, the thin aluminum bumper shield disintegrates into a debris cloud. The debris cloud front propagates axially at velocities of 14 km/s and expands radially at a velocity of 7 km/s. Subsequent loading by the debris on a 3.2 mm thick aluminum substructure placed 114 mm from the bumper penetrates the substructure completely. However, when the diameter of the flier plate is reduced to 12.7 mm, the substructure, although damaged is not perforated. Numerical simulations performed using the multi-dimensional hydrodynamics code CTH also predict complete perforation of the substructure by the subsequent debris cloud for the larger flier plate. The numerical simulation for a 12.7 mm flier plate, however, shows a strong dependence on assumed impact geometry, i. e., a spherical projectile impact geometry does not result in perforation of the substructure by the debris cloud, while the flat plate impact geometry results in perforation.     

A multi-shock concept for spacecraft shielding

The results of an advanced spacecraft shielding program conducted at the NASA Johnson Space Center Hypervelocity Impact Research Laboratory (HIRL) are presented. The results include two new aspects of shielding design: the geometrical configuration and the type of material used for the shield. The geometrical configuration of the shield will be the prime focus of this paper due to its application over a large range of materials. The uniqueness of this concept is in the utilization of a multi-shock (MS) shielding technique where ultra-thin (t_s) spaced (ΔS), shield elements are used to repeatedly shock the impacting projectile (diameter d_p) to a high enough energy state to cause melting and vaporization at velocities which normally would not produce these results. Although the concept of multi-sheet shields has been proposed and tested many times (Christiansen, 1987; Gehring, 1970; Rajendra and Elfer, 1989; Richardson, 1970), the t_s/d_p ratio has always been large enough that the shield material has provided a large percentage of the debris plume mass which the back sheet must withstand. This concept does not produce the same results. The low t_s/d_p adds very little shield material to the debris plume allowing a substantial decrease in the thickness (strength) of the backsheet and the proper spacing between sheets prevents the debris plume from destroying successive sheets prior to the particulates reaching the sheet. The present concept, using aluminum as an analog for comparison to a dual sheet (aluminum) “Whipple shield” results in a 30% reduction in weight.

The use of other materials with this concept can result in even greater weight savings. The concept was tested at normal impact, oblique impact, and low velocity impact (2.7 km/s) and performed as well as an equivalent dual sheet shield. The scaling characteristics of the new cincept were tested and verified for impacting projectiles of mass 45 milligrams and 1.27 grams at velocities of 6.7 km/s. The new concept provides a shield which can be tailored to meet many design requirements, produce minimal secondary debris particles, provide a means for designing an augmentable shielding system, and most important reduce the weight of debris shielding.     

A material point method model and ballistic limit equation for hyper velocity impact of multi-layer fabric coated aluminum plate

A multi-layer fabric coated aluminum plate is usually used in the hard upper torso of space suit to protect astronauts from getting hurt by space dust. In this paper, the protective performance of the multi-layer fabric coated aluminum plate is investigated. To establish its ballistic limit equation, thirteen hyper velocity impact tests with different impact velocities (maximum velocity is 6.19 km/s) and projectile diameters have been conducted. To provide data for impact velocity higher than 6.2 km/s which is hard to be obtained by tests due to the limitations of test equipment capacity, a material point method (MPM) model is established for the multi-layer fabric coated aluminum plate and validated/corrected using the test results. The numerical results obtained using the corrected MPM model for impact velocity higher than 6.2 km/s are used together with the test results to develop the ballistic limit equation. The corrected MPM model and the ballistic limit equation developed for the multi-layer fabric coated aluminum plate provide an effective tool for the space suit design.     

Penetration mechanics and performance of segmented rods against metal targets

Terminal ballistic experiments confirm theoretical predictions that a segmented rod will penetrate a semi-infinite metal target deeper than a continuous rod of the same material and having equal mass, diameter and velocity. For copper segmented rods impacting aluminum targets and tantalum segmented rods impacting 4340 (BHN 300) steel, penetration depths of at least 50 percent greater than that for a corresponding continuous rod are measured at impact velocities of 4 to 5 km/s. Spacing between segments of only about 2.5 segment diameters or more are required to achieve these results. Reducing the Li/D of the segments to less than 1 improves the penetration efficiency of a segmented rod. For segmented rods with segment L_i/D < 1, experiments suggest that penetration may increase with impact velocity rate greater than V^2/3.     

Ballistic limit for shielded plates subjected to hypervelocity impact

A new method of determining ballistic limits for hypervelocity impact is proposed. This method applies to cases of impulsive impact on a plate protected by a multi-shock shield, which corresponds to projectile velocities in the range above 6 km s⁻¹. It is shown, by using experimental and analytical results, that the plate will not fail, provided the impact parameters are bounded by certain critical values. A simple equation that relates the critical values is established. Curves are presented for the critical projectile radius versus the projectile velocity, and for the critical plate thickness versus the velocity. These curves are in good agreement with curves that have been generated empirically.     

5A06铝合金单层板超高速撞击弹道极限分析

日益增长的空间碎片对在轨航天器的安全运行构成了严重威胁,毫米级空间碎片的防护已成为航天器结构设计必须考虑的问题之一.航天器的蒙皮是抵御空间碎片超高速撞击的最基本防护结构.采用数值仿真并结合试验验证的方法,对5 mm厚5A06铝合金单层板承受2A12铝合金球形弹丸正撞击下的弹道极限进行了研究.研究表明,在验证实验速度范围内,数值仿真结果与实验结果吻合良好;使用数值仿真对实验速度以上的区间进行拓展研究,获得了其弹道极限曲线和弹道极限方程;数值仿真和实验结果与已有经验方程对比表明,经验方程与具体材料的弹道极限有较大偏差,因此,应具体问题具体分析.     

Advanced shield design for space station freedom

Due to the predicted increase in the severity of the orbital debris environment in low-Earth orbit, the baseline meteoroid/debris protection system for Space Station Freedom (S.S. Freedom) must be augmented on orbit. In response to this need, an advanced shield design effort is underway at NASA's Marshall Space Flight Center (MSFC). The results to date of this program are presented.

A series of 18 hypervelocity impact tests were conducted at MSFC's Space Debris Simulation Facility. These tests consisted of launching aluminum projectiles at velocities up to 7 km/s to evaluate various design solution. Parameters investigated include shield material and geometric configuration (thickness, spacing, orietation, and arrangement) in relation to the baseline aluminum “Whipple” bumper.

The results of the hypervelocity impact tests are presented. Comparison with protection offered by the baseline protection system is made. Evaluation of protection offered by candidate augmented systems and hydrocode simulations is performed. An assessment of the often-overlooked structural design onsiderations such as launch loads, on-orbit loads, extravehicular activity requirements, maintainability, etc., is presented. These analyses lead to identification of a candidate system to augmented the baseline meteoroid/debris protection system for the habitable modules of S.S. Freedom.     

Predicting orbital debris shape and orientation effects on spacecraft shield ballistic limits based on characteristic length

We propose the use of “characteristic length,” based on radar cross section, as a metric for comparing the performance of orbital debris impactors of differing shapes, and the use of NASA's standard breakup model (SBM) “flake” shape as the representative particle for predicting orbital debris penetration effects. We also propose the use of a 26-view methodology for examining non-spherical particles such as cylinders, rectangular prisms, octahedrons, etc., with the intent to describe their potential impact orientations while minimizing the number of hydrocode runs needed to develop orientation-dependent ballistic limit curves. Using this methodology and the smooth particle hydrodynamic code (SPHC), we predict the ballistic limit for SBM-based particles against a typical spacecraft dual-wall shield at normal obliquity and velocities of 7, 8, and 12 km/s. Finally, we compare these results with ballistic limits produced by spherical impactors of the same characteristic length as the SBM-based particles.     

Critical response of shielded plates subjected to hypervelocity impact

A ballistic limit equation for hypervelocity impact on thin plates is derived analytically. This equation applies to cases of impulsive impact on a plate that is protected by a multi-shock shield, and is valid in the range of velocity above 6 km/s. Experimental tests were conducted at the NASA Johnson Space Center on square aluminum plates. Comparing the center deflections of these plates with the theoritical deflections of a rigid-plastic plate subjected to a blast load, one determines the dynamic yield strength of the plate material. The analysis is based on a theory for the expansion of the fragmented projectile and on a simple failure criterion. Curves are presented for the critical projectile radius verus the projectile velocity, and for the critical plate thickness versus the velocity. These curves are in good agreement with curves that have been generated empirically.     

板间距对铝板多冲击结构高速撞击防护性能影响的试验研究

为了研究板间距变化对铝板多冲击结构高速撞击损伤与防护特性的影响,采用二级轻气炮发射铝球弹丸对具有不同板间距的双层、三层、四层和五层铝板结构进行了高速撞击试验,弹丸直径分别为3.97 mm、5 mm和6.35 mm,撞击速度为1.72~4.88 km/s,撞击角度为0°。结果表明:在铝球弹丸的弹道段撞击速度区间,板间距变化对铝板多冲击结构的高速撞击防护性能无显著影响;在铝球弹丸的破碎段撞击速度区间,对于相同的总防护间距,具有不同板间距的铝板多冲击结构的高速撞击防护性能存在明显差异;基于该试验数据定义的三层、四层和五层铝板结构的板间距因子,可为具有高效抗高速撞击能力的铝板多冲击结构的板间距设计提供依据。     

Numerical simulation of hypervelocity impact on CFRP/Al HC SP spacecraft structures causing penetration and fragment ejection

A representative carbon fiber reinforced plastic/aluminum honeycomb sandwich panel (CFRP/Al HC SP) spacecraft structure has been modeled in the hydrocode AUTODYN using the state-of-the-art ADAMMO material model [Riedel W, Harwick W, White D, Clegg R. Advanced material damage models for numerical simulation codes. ESA CR(P) 4397, 2003] to study the performance of the structure during impact events that cause perforation and fragment ejection. A new procedure combining a series of existing theoretical methods has been developed and applied to derive a full set of coarse material data. The data set has been implemented in AUTODYN, and the results of the numerical simulation have been compared to experimental impact test data. For impact tests performed near the structural ballistic limit, quantitatively accurate results were obtained over a range of impact velocities and angles. A further increase in the projectile size resulted in significant destruction of the sandwich panel front face-sheet and diversion from the experimental damage measurements. Inspection of the numerical model has shown non-localized propagation of inter-laminar delaminations, possibly caused by an under-prediction of the laminate dynamic inter-laminar tensile strength. The effects of the delamination propagation occur over an extended time scale and were not found to affect the state and trends of the fragment cloud ejected into the satellite interior. Accordingly, experimental trends of fragment cloud dispersion have been qualitatively reproduced.     

The effect of phase changes on target response

This paper presents the results of two impact studies with lead projectiles and lead targets. Impact velocity varied between 2.65 and 8.3 km/s, a range of velocities that induces a range of response in lead from fragmentation to vaporization. The first study considers the response of a lead target to impact by a 1.5 mm tungsten carbide sphere. Target response measurements included crater parameters and target momentum. Normalized target momentum, i.e. the ratio of target to projectile momentum, was observed to increase non-linearly with impact velocity, obtaining a value of 7.1 at 8.3 km/s. The second study compares the response of a shielded aluminum target to impact by either a lead or molybdenum projectile (the shield material was the same as the projectile). Test variables included shield to target spacing, shield thickness, target orientation and impact velocity. The four test variables affected the two test conditions differently, with the most similar results observed for tests with thick shields, minimal spacing and low impact velocity.     

Projectile density, impact angle and energy effects on hypervelocity impact damage to carbon fibre/peek composites

This paper explores the effects of projectile density, impact angle and energy on the damage produced by hypervelocity impacts on carbon fibre/PEEK composites. Tests were performed using the light gas gun facilities at the University of Kent at Canterbury, UK, and the NASA Johnson Space Center two-stage light gas gun facilities at Rice University in Houston, Texas. Various density spherical projectiles impacted AS4/PEEK composite laminates at velocities ranging from 2.71 to 7.14 km/s. In addition, a series of tests with constant size aluminum projectiles (1.5 mm in diameter) impacting composite targets at velocities of 3, 4, 5 and 6 km/s was undertaken at incident angles of 0, 30 and 45 degrees. Similar tests were also performed with 2 mm aluminum projectiles impacting at a velocity of approximately 6 km/s. The damage to the composite was shown to be independent of projectile density; however, debris cloud damage patterns varied with particle density. It was also found that the entry crater diameters were more dependent upon the impact velocity and the projectile diameter than the impact angle. The extent of the primary damage on the witness plates for the normal incidence impacts was shown to increase with impact velocity, hence energy. A series of tests exploring the shielding effect on the witness plate showed that a stand-off layer of Nextel fabric was very effective at breaking up the impacting debris cloud, with the level of protection increasing with a non-zero stand-off distance.     

Penetration of semi-infinite AD995 alumina targets by tungsten long rod penetrators from 1.5 to 3.5 km/s

The performance of confined AD995 Alumina against L/D 20 tungsten long rod penetrators was characterized through reverse ballistic testing. The semi-infinite ceramic target was cylindrical with a diameter approximately 30 times the rod diameter. The target configuration included a titanium confinement tube and a thick, aluminum coverplate. The impact conditions ranged from 1.5 to 3.5 km/s with three or four tests performed at each of six nominal impact velocities. Multiple radiographs obtained during the penetration process allowed measurement of the penetration velocity into the ceramic and the consumption velocity, or erosion rate, of the penetrator. The final depth of penetration was also measured.

Primary penetration approaches 75% of the hydrodynamic limit. Secondary penetration is very small, even at 3.5 km/s. The effective R_t value decreased from 90 kbar to 70 kbar with increasing impact velocity over the range of velocities tested.

In tests in which the ratio of target diameter to penetrator diameter was reduced to 15, R_t, dropped by 30% to 50%. When a steel coverplate was used, total interface defeat occurred at 1.5 km/s.