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.     

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.     

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.     

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.     

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.     

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.     

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.     

Integral model for the description of the debris cloud structure and impact

The purpose of the present paper is to introduce a new integral model capable to describe the evolution of the debris clouds originated after normal-impacts of orbital debris over a Whipple shield. This work had been developed at Alenia Spazio in the context of a degree thesis. Several numerical SPH simulations of debris impacts on a Whipple shield configuration were performed to determine the ballistic limit and to compare it with semi-empirical damage equations. In the present paper, the numerical simulations were used to investigate the typical behaviour of experimental debris clouds ([6] and [9]) and to support the development of the integral model.

With respect to previous papers ([1], [2], [3], [10]) in which a spherical shell-wise debris cloud was considered, here we try to introduce more realistic assumptions. We approximate the cloud's shape also introducing ejecta veil effects, which produce a multiplication of the deposited momentum upon the underneath wall. In the present model, the most peculiar hypothesis is a cinematic self-similar behaviour that is, whatever the shape is, the debris cloud evolves keeping unchanged its shape. Then, the material is opportunely distributed inside a volume and the choice of that distribution is described taking into account the results of the numerical simulations. Knowing the spatial material distribution and treating the cloud as a fluid, we can estimate the load time history and the drag-unitary force induced by the cloud impacting upon the rear wall. Of course, such a method uncouples the dynamic response of the rear wall from the evolution of the debris cloud. The balances of mass, momentum and energy allow three global and unknown parameters to be determined. The one-dimensional theory of impact ([10]) is used to take into account the conversion of part of the initial kinetic energy into internal thermal energy. No integration of differential equations is performed since complex propagation phenomena are taken into account through the effects they globally produce. The model still presents some free parameters related to the integral formulation. These parameters cannot be calculated through any balance condition, but they must be imposed to get a good, global reproduction of the debris cloud. The choice of these parameters is still the weak aspect of the method, and it depends on the consideration of the results obtained with more sophisticated tools, as, for instance, SPH simulations. The spatially defined load time history obtained with the debris cloud integral model can be used for further analysis on the back up plate.     

The shape effect of non-spherical projectiles in hypervelocity impacts

This paper provides qualitative and quantitative analyses of regular non-spherical projectile hypervelocity impacts on basic Whipple shields using test data obtained by light-gas guns, flat plate accelerators and shaped charge launchers. Surrogate cadmium and zinc test results are used to extend light-gas gun data beyond 8 km/s. Advanced Whipple shield derivatives are shown to be necessary to protect against non-spherical projectiles.     

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.     

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.     

Modified Cour-Palais/Christiansen damage equations for double-wall structures

Ballistic limit equations (BLEs) are used for the damage prediction of spacecraft in a meteoroid and space debris environment. For double-wall configurations, the Cour-Palais/Christiansen equations have been modified to yield a general approach including the influence of the shield thickness. The ratio of shield thickness to particle diameter is considered as additional parameter in the equations. These equations result in the single-wall equation when the shield thickness approaches zero. The modifications can also be applied to other BLEs. Impact tests have been performed in order to validate the modified equations. In this paper, the test results are compared to the modified BLEs. Especially in the hypervelocity region, the new equations are more suitable for configurations with very thin shields than the original ones.     

Numerical simulation of non-perforating impacts on shielded gas-filled pressure vessels

In order to calibrate the output of hydrocode simulations of hypervelocity impacts on shielded gas-filled pressure vessels, Light Gas Gun impact experiments were performed. In a first step, tests were performed on so-called equivalent Whipple shield (EWS) configurations having basically the same set-up as the shielded pressure vessels (i.e. bumper thickness and - material, stand-off and backwall plate thickness and -material). Purpose was the determination of the impact conditions that lead to penetration into the backwall plate but not perforation of it or leakage through the impacted area. In a second step, impact tests on the corresponding shielded pressure vessels were performed with the same test conditions as the EWS. The purpose of the tests was the investigation whether leakage occurs when the vessel's front wall is not perforated, but just cratered. The test conditions lead to no leakage in all tests. The most important measured damage parameter was the crater depth of the deepest crater in the vessel's front wall/the backwall plate of the EWS, respectively. Hydrocode simulations were then performed to assess the capability of the numerical tool to correctly predict the damage on the impacted vessel surface. Normal impacts of aluminium spheres against shielded vessels were simulated using AUTODYN-2D, including and evaluating the effect of the static stress induced in the vessel walls by the inner pressure. Particular attention was focused on the exact determination of the maximum crater depth caused by the debris cloud impact on the vessel wall/the backwall plate of the EWS, respectively. Bumper and projectile were represented by SPH particles, the vessel shell was represented by a Lagrange grid. The results showed a very good agreement with the measured crater depths of the experiments.     

Ballistic resistance of double-layered armor plates

In this paper, the ballistic resistance of double-layered steel shields against projectile impact at the sub-ordnance velocity is evaluated using finite element simulations. Four types of projectiles of different weight and nose shapes are considered, while armor shields consist of two layers of different materials. In a previous study of the same authors, it was shown that a double-layered shield of the same metal was able to improve the ballistic limit by 7.0–25.0% under impact by a flat-nose projectile, compared to a monolithic plate of the same weight. Under impact by a conical-nose projectile, a double-layered shield is almost as capable as a monolithic plate. The present paper extends the analysis to double-layered shields with various metallic material combinations. The study reveals that the best configuration is the upper layer of high ductility and low strength material and the lower layer of low ductility and high strength material. This configuration results in some 25% gain in the ballistic limit under moderate detrimental impact. This research helps clarify the long standing issue of the ballistic resistance of the multi-layered armor configuration.     

Radiative and conductive heat transfer in a 4-K helium phase separator with minimum heat loss

Improvements in simulation and practice for the heat load of a helium phase separator are discussed. The separator cryostat (volume 100 L, cooling capacity 1.5 W at 4.2 K) re-condenses and stores liquid helium. An additional radiation shield was designed to minimize the radiative heat transfer and to decrease the heat conduction. The experimental results indicate that the heat load of the separator was decreased from 5.56 W to 1.555 W, a gain about 4 W of heat load, which is an improvement by 72%. Liquid helium (50 L) was stored in the separator stably for more than 90 h. Software implementing the finite-element method (FEM) was used to predict the temperature distribution of pipe fittings and the separator heat load with or without the additional radiation shield. The results of these simulations show that the temperatures of the pipe fittings were significantly decreased in the separator with additional radiation shields. For the heat load, the trends of simulation and experimental results were similar. This work provides a simple and effective method to minimize the radiation heat load of a separator. In this paper, we discuss in detail the improvements of the model, the experimental setup and the results of comparisons between experiments and simulations.     

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.     

Analytic 3-D Read Head Model With Finite Shields

This paper presents an analytic model of a read head with shields of finite length and width, suitable for replay at 1 Tb/in $^{2}$. The model determines the shield potential by iterative solution of the equations governing the Fourier coefficients, given an initial guess. The model is used to demonstrate the effect of varying the length and width of the shields. The paper gives results for the shield potential, equipotentials, vertical head field, and spectral response function. A head with shields that are the same length as the semi-shield—shield gap offers an almost constant magnitude response over a broad range of along-track frequencies, and the use of sufficiently wide side shields is advantageous.      

A novel simulation approach for electromagnetic gun barrels

An approach to modeling a railgun barrel for performance simulations is discussed that is simple and readily implemented and yet is more representative of actual barrel performance due to electric/thermal effects. This novel, but simple, methodology differs from most of the dynamics equations. Instead, the barrel is discretized into small "cell" sections whose performance parameters vary linearly from one cell to the next. Each cell's electrical/thermal parameters that impact the barrel performance are updated and their effects on the barrel summed in entirety each iteration step; in effect, a distance iteration with a different time increment, which is a funtion of acceleratoin and velocity for each step. This technique allows variations due to materials, resistance, inductance and barrel topology. The simulation also permits a simplified pseudo-two-dimensional analysis to approximate a complicated temporal and spatial heat flow of a rapid-fire scenario. Even though this technique approximates a complex solution, the integrated system simulation executes quickly and has been operated on a personal computer (PC). The numerous advantages and flexibility of this simulation approach are discussed and correlated with other simulations and test data.     

一种跨音速伺服气动弹性分析方法

基于CFD技术,运用系统辨识方法,建立了基于模态坐标的跨音速气动力降阶模型(ROM)。耦合气动状态方程、结构状态方程和伺服状态方程,建立了一个适合跨音速伺服气动弹性分析的数学模型。算例首先通过对比基于ROM技术的分析结果和直接仿真结果,以证明该模型的正确性和精度。在保证精度的同时,其计算效率比直接耦合CFD技术的仿真方法高1个～2个数量级。算例还研究了传感器安放位置和结构陷幅滤波器对该导弹伺服气动弹性特性的影响,结果显示结构陷幅滤波器的引入可以显著地降低开环气动弹性系统和控制系统的耦合。