Mesh double-bumper shield: A low-weight alternative for spacecraft meteoroid and orbital debris protection

A number of new, innovative, low-weight shielding concepts have resulted from a decade of research at the NASA Johnson Space Center (JSC) Hypervelocity Impact Test Facility (HIT-F). One such concept, the mesh double-bumper (MDB) shield is a highly efficient method to provide protection from meteoroid and orbital debris impacts. Hypervelocity impact (HVI) testing of the MDB shield at the HIT-F and other facilities have demonstrated weight savings of approximately 30% to 50% at light gas gun velocities compared with conventional dual-sheet aluminum Whipple shields at normal impact angles. Even larger weight savings, approximately 70%, have been achieved at 45 degree oblique angles. The MDB shield was developed to demonstrate that a Whipple shield could be “augmented” or modified to substantially improve protection by adding a mesh a short distance in front of the Whipple bumper and inserting a layer of high strength fabric between the second bumper and rear wall. From the test results, formulas have been developed that allow the design engineer to size MDB shield elements for spacecraft applications.     

Crater distribution on the rear wall of AL-Whipple shield by hypervelocity impacts of AL-spheres

All spacecraft in low orbit are subject to hypervelocity impact by meteoroids and space debris, which can in turn lead to significant damage and catastrophic failure. In order to simulate and study the hypervelocity impact of space debris on spacecraft through hypervelocity impact on AL-Whipple shield, a two-stage light gas gun was used to launch 2017-T4 aluminum alloy sphere projectiles. The projectile diameters ranged from 2.51 mm to 5.97 mm and impact velocities ranged from 0.69 km/s to 6.98 km/s. The modes of crater distribution on the rear wall of AL-Whipple shield by hypervelocity impact of AL-spheres in different impact velocity ranges were obtained. The characteristics of the crater distribution on the rear wall were analyzed. The forecast equations for crater distribution on the rear wall of AL-Whipple shield by normal hypervelocity impact were derived. The results show that the crater distribution on the rear wall is a circular area. As projectile diameter, impact velocity and shielding spacing increased, the area of crater distribution increased. The critical fragmentation velocity of impact projectile is an important factor affecting the characteristics of the crater distributions on the rear wall.     

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.     

Three dimensional hypervelocity impact simulation for orbital debris shield design

This paper assesses a Whipple shield impact simulation method which is both accurate and computationally efficient. The paper documents the simulation methodology and results of Whipple shield simulations at an oblique impact angle of 30°. These results are compared with HVI experiments to demonstrate the accuracy of the simulation technique. In addition, simulations of Whipple shields in the velocity regime above 8km/s were completed and the results compared to published ballistic limit equations to demonstrate the reliability of these equations. Finally, the paper documents computational efficiency of the simulation technique.     

Design and performance equations for advanced meteoroid and debris shields

This paper provides equations defining the performance capability of various types of meteoroid and debris shielding systems. These equations have been developed at the NASA Johnson Space Center (JSC) Hypervelocity Impact Test Facility (HIT-F). Equations are included that are applicable for aluminum Whipple shields, Nextel® Multi-Shock (MS) shields, hybrid Nextel®/Aluminum MS shields, and Mesh Double-Bumper (MDB) shields. The MS and MDB shields are advanced shields with demonstrated weight and performance advantages over conventional Whipple shields.     

Shielding against space debris. A comparison between different shields: The effect of materials on their performances

The space environment requires the Space Station to be shielded against orbital debris. A technological programme undertaken by the European Space Agency has led to a preliminary definition of the shield configuration for the European Attached Pressurized Module. The envisaged shield is a modified Whipple shield. A second bumper is located midway between the first bumper and the backwall.

The work described has been initiated to quantify experimentally the merits of different shields compatible with the APM system requirements. For this technological investigation, two requirements had to be satisfied. The spacing between the front bumper and the backwall had to be limited to 120 mm. The backwall thickness could not be reduced to technological limits as it has structural functions as well. In addition, the long life requirements of the Space Station precludes the use of unproved materials for the external parts of the shield.

Different materials have been tried as second bumper. The effect of the first bumper thickness on the projectile fragmentation has been explored as well. Shields based on Aluminium, Kevlar and Glare have been investigated. Kevlar 29 fabrics impregnated with epoxy resin were used for this work. Glare is a material developed to improve the fatigue strength of metal structures. It is primarily intended for aircraft skin applications. Glare consists of a 60 percent fibre volume adhesive prepreg with high-strength unidirectional or cross-ply R-glass fibres. A variety of lay-up sequences is available ranging from 2/1 (two layers of aluminium alloy sheet bonded by one layer of prepeg) to any number of layers. The 2/1 layers version of the Glare material has been used for this work.

The tests results indicate the performances of materials can change significantly with the impact conditions. Glare shows the best performances in the low velocity regime while Kevlar is very promising in the high velocity regime. It is concluded the use of Kevlar can improve substantially the performances of the APM shield.     

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.     

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.     

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。     

Ballistic limit evaluation of advanced shielding using numerical simulations

The advanced shielding concept employed for the Columbus module of the International Space Station consists of an aluminum bumper and an intermediate shield of Nextel and Kevlar-epoxy. Until recently, the lack of adequate material models for the Nextel cloth and Kevlar-epoxy has precluded the practical usage of hydrocodes in evaluating the response of these shields to hypervelocity impact threats. Recently hydrocode material models for these materials have been proposed [1,2] and the further development and completion of this model development is reported in this paper. The resulting models, now implemented in AUTODYN-2D and AUTODYN-3D, enables the coupling of orthotropic constitutive behavior with a non-linear (shock) equation of state. The model has been compared with light gas gun tests for aluminum spheres on the advanced shield at impact velocities between 3.0 and 6.5km/s [3]. Reasonable correspondence has been obtained at these impact velocities and thus the models have been used to perform preliminary assessment of predicted ballistic limits at velocities from 7 to 11km/s. The predicted ballistic limits are compared with ballistic limit curves derived on the basis that damage is proportional to projectile momentum     

Impact damage on sandwich panels and multi-layer insulation

Most spacecraft rely intensively on sandwich construction for external structures with multi layer thermal insulation where appropriate. Experience gained in ESA with various spacecraft (ROSETTA, METOP, ATV,…) covers a substantial range of materials and configurations. In this work, the applicability of simple damage equations (e.g. those presently used for single or Whipple shield ballistic limits) to more complex configurations (e.g. sandwich plates with and without MLI) is analyzed. The different sandwich configurations which were submitted to testing are reviewed, impact test results are presented and compared with impact reference data on single plates and Whipple shields. It has been found that sandwich panels have a better tolerance to hypervelocity impacts than monolithic structures. MLI placed in front of the sandwich panels contributes significantly to the overall protection performance in the range of the projectile diameters tested. The complexity of the sandwich structure is responsible for a considerable scatter in the test results. The predictors for Whipple shields applied to sandwich panels with and without MLI can only be considered on a case by case basis for risk assessment analysis.     

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.     

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.     

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.     

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.     

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.     

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.     

泡沫铝防护屏的Whipple防护结构弹道极限数值模拟研究

为研究泡沫铝板作为防护屏的Whipple防护结构抵御空间碎片超高速撞击的特性,模仿泡沫金属的生产原理建立了泡沫金属细观结构几何模型,结合自编的光滑质点流体动力学程序进行了超高速撞击数值仿真,通过与实验对比验证了模型的有效性.分别对相对密度为23.2%的理想均匀和非均匀开孔泡沫铝板作为防护屏的Whipple防护结构进行了数值仿真,得到了它们的弹道极限曲线,并与实心铝板作为防护屏的Whipple防护结构进行了对比分析.结果表明,相同面密度的泡沫铝板相对于实心铝板能够在更低的速度上将弹丸粉碎、液化及气化.泡沫铝板作为防护屏,在总体上拥有更好的防护性能;相同面密度的理想均匀泡沫铝板的防护性能总体上优于非均匀泡沫铝板.     

铝双层板结构撞击损伤的板间距效应实验研究

为了研究空间碎片对航天器防护结构的超高速撞击损伤特性,采用二级轻气炮发射球形弹丸,对铝双层板结构进行了超高速撞击实验研究.弹丸直径为3.97 mm,撞击速度分别为(2.58±0.08)km/s、(3.54±0.25)km/s和(4.35±0.11)km/s,板间距为10~100 mm.实验得到了铝双层板结构在不同撞击速度区间的后板损伤模式.结果表明,弹丸撞击速度一定时,后板弹坑分布随前后板间距的不同而不同.前板背面返溅影响区和后板弹坑分布区随板间距的增大而增大,各弹坑分布区扩散角随板间距的增大而减小.