Whipple Shield Research Articles

Hypervelocity impact damage behavior of B4C/Al composite for MMOD shielding application

In order to improve the survivability of in-orbit spacecraft against the hypervelocity impact (HVI) of space debris, the lightweight B4C/Al composite is fabricated by the pressureless presintering and Al infiltration in vacuum and applied as the bumper of Whipple shield configuration. The HVI tests ranging from 3 km/s to 6.5 km/s are carried out to assess the shield performance of B4C/Al bumper. The fitting line of the ballistic limit for the B4C/Al bumper is obtained, and its critical projectile size can be improved by about 20% in comparison to the conventional Al alloy Whipple shield configuration. Based on the damage features of bumper and rear wall, the B4C ceramic phase of B4C/Al composite is the key reason to break the projectile into smaller fragments or liquid droplets of debris cloud. The Al plastic phase with three-dimensional network enhances the toughness of B4C/Al composite bumper, which plays an important role in avoiding structural failure caused by shock waves. Hence, the impact kinetic energy of debris cloud is obviously dispersed, and the subsequent impact damages on real wall are alleviated accordingly. The primary investigation provided theoretical and technical instructions for the design and application of B4C/Al composite in MMOD shield configuration.

Shielding performance of debris shield with separated rear wall

Generally, changing the properties of bumper or the stuffed wall in a debris shield is the traditional method to improve shielding performance. A new way to improve shielding performance by separating the rear wall of debris shield was explored in this paper. Both hypervelocity impact tests and simulations were performed to evaluate the performance of the shield with a separated rear wall. The projectiles were aluminum spheres with a diameter of 6.0 mm at the impact velocity of about 8.3 km/s, and a diameter of 4.0 mm at the impact velocity of about 4.7 km/s. The targets were the separated rear wall shield (SRW) and Whipple shield with the same areal density. The results show that the shielding performance of a SRW shield is improved and better than that of a Whipple shield with the same areal density, because the separated surface in SRW shield can attenuate the propagation of the impact energy wave, decrease the free surface velocity on the back of the rear wall, and then reduce the damage on the rear wall.

Hypervelocity impact of PrintCast 316L/A356 composites

Shielding elements used to protect against micrometeoroids and orbital debris (MMOD) (e.g., standard and stuffed Whipple shields, multi-shock shields) typically incorporate thin bumper sheets that intercept and vaporize incident MMOD traveling at speeds in excess of several km/s. In some applications, however, space limitations prevent the use of large stand-offs, and components must instead be protected by a single monolithic shielding element. Electronics, for example, are often only protected by their housing. With such applications in mind, we describe a class of spatially efficient composite shielding elements fabricated using a hybrid additive manufacturing approach termed PrintCasting. The PrintCast process consists of two steps: First selective laser melting is used to fabricate a lattice preform in the shape of the final component. Next this preform is infiltrated with a liquid metal that has a melting point lower than that of the lattice. The resulting solidified part is a periodic interpenetrating composite in which each constituent forms a continuous network. Using a combination of hypervelocity impact experiments and shock transmission calculations, we demonstrate that these interpenetrating composite shielding elements mitigate spallation and other failure modes through multiple internal shock reflections at the buried heterophase interfaces.

Microstructural material characterization of hypervelocity-impact-induced pitting damage

In a hypervelocity impact (HVI) between the micrometeoroids/orbital debris (MMOD) and multi-layered shielding mechanisms of spacecraft, the debris cloud, formed by shattered materials of the outer bumper layer and projectile, commits multitudinous pitting craters and cracks that are disorderedly scattered in the rear wall layer. Material degradation due to the pitting damage is a precursor of structural fragmentation and system failure of the space assets. In this study, microscopic material degradation of the rear wall of a typical dual-layered Whipple shield, initiated and intensified by the debris cloud-engendered pitting damage, is characterized using metallographic analysis including optical microscope (OM), laser scanning microscope (LSM), scanning electron microscope (SEM) and X-ray diffraction (XRD). Results have revealed that (1) the degree of material degradation shows difference in the central cratered area, the ring cratered area, and the spray area, respectively; (2) the dynamic recrystallization gives rise to the formation of fine grains adjacent to pitting craters; (3) the extents of recrystallization and dislocation depend on the strain rate levels during HVI; and (4) the temperature elevation, caused by the heat transformed from the adiabatic plastic deformation energy and shock heating, warrants the recrystallization. Two types of damage, namely micro-voids and micro-cracks, are identified beneath the pitting damage area; under the extremely high compressive strain rate induced by HVI, micro-voids are initiated by the nucleation of grains or deteriorate from existing material defects, and these micro-voids further expand at the grain boundaries and within the grains to form micro-cracks under a tensile-type wave converted from the HVI-induced shock wave.

A stuffed Whipple shield for the Chinese space station

Nine successful hypervelocity impact tests between 3.1 and 7.9 km/s were performed on shield samples designated to be used onboard the future Chinese space station. Two types of shields were impact-tested: a three-layer shield consisting of a bumper, stuffing and the rear wall, and a two-layer shield. The two-layer shield configuration corresponds to the three-layer shield configuration with the bumper plate removed. The three-layer shield configuration investigated is comparable to both the ESA Columbus module debris shield and the NASA stuffed Whipple shield. A new ballistic limit equation (BLE), adopted from the NASA Nextel/Kevlar stuffed Whipple shield equation, was derived from the available experimental data for the three-layer shield. For the two-layer shield, there is not sufficient test data available for derivation of a BLE. The new BLE and a second BLE from CAST are analyzed and discussed. Overall, all BLE investigated are conservative. Considering the anticipated orbit of the Chinese space station, the CAST BLE is more conservative than the new BLE. The individual components of the shield are discussed shortly. Ballistic limit curves are given and compared against all available experimental data.

CST: A new semi-empirical tool for simulating spacecraft collisions in orbit

This paper provides a general description of a new Collision Simulation Tool (CST) to model the consequences of orbital impacts involving large debris and satellites, with the aim to predict fragments distributions in case of sub-catastrophic and catastrophic impacts. The new tool is being developed in the framework of the ESA contract “Numerical simulations for spacecraft catastrophic disruption analysis”, carried out by the Center of Studies and Activities for Space CISAS “G. Colombo” of the University of Padova (prime contractor) and etamax space GmbH.The CST makes possible to model a large variety of collision scenarios involving complex systems, such as entire satellites with many design details included, and provides statistically accurate results with a computational effort orders of magnitude lower than hydrocodes. To this end, the simulation approach is based on a hybrid modelling strategy, in which every colliding object is described as a gross net of Macroscopic Elements (ME) representing spacecraft elementary building blocks. On one hand, individual fragmentation of Macroscopic Elements is addressed through the use of semi-empirical breakup models applied, at element level, only to those spacecraft parts which are involved in the collision. On the other hand, structural distortion, fracture, and separation of satellite broken parts are modelled with a discrete-element approach through the simulation of momentum transfer to Macroscopic Elements through the net, taking into account energy dissipation inside elements and across links.This paper provides a description of the CST simulation methodology and framework; preliminary results are also shown with respect to the tool validation process. Validation is done by comparing the tool predictions with empirical data from ground-based impact tests on simple targets (simple plates and Whipple Shields) as well as sub-scale spacecraft models. In the first case, the tool capability of predicting fragments distributions from plates and ballistic limit equations of shields is verified. In the second case, fragments distributions calculated by the software are compared with those derived from published experiments on a micro-satellite.

A debris cloud model for hypervelocity impact of the spherical projectile on reactive material bumper composed of polytetrafluoroethylene and aluminum

In this paper, a debris cloud model is presented for the case of hypervelocity impact of the spherical projectile on the thin PTFE (polytetrafluoroethylene)/Al (aluminum) reactive material bumper at normal incidence. The experiments of projectiles with hypervelocity impact on Whipple shields with PTFE/Al and Al2024 as bumper respectively were carried out by using two-stage light-gas gun, and the movement process of debris cloud was recorded through laser shadow photography system. According to the numerical simulation results and the laser shadow photographs of debris cloud obtained from the experiment of hypervelocity impact, two parts named as internal and external debris cloud are considered in the theoretical model. Several assumptions are made, and the theoretical model of debris cloud is developed by the conservation laws of mass, momentum and energy. Four characteristic velocities of the debris cloud are first calculated, and thus the whole evolution process of the debris cloud is plotted according to those velocities. By comparing the size of the debris cloud shown in the experiment, it can be concluded that the theoretical model agrees well with the experimental results. Moreover, the characteristic velocities of projectile fragments and bumper fragments in the debris cloud are calculated, and the mass areal density and momentum areal density distribution of the debris cloud on the rear wall are also obtained. Additionally, the effects of the detonation reaction of the reactive material bumper on the projectile during the impact are analyzed in contrast with the results on the Al2024 bumper. It is indicated that the detonation reaction of PTFE/Al can reduce the velocity of center of mass of debris cloud and increase the expansion velocity of debris cloud. PTFE/Al bumper can reduce damage caused by debris cloud to rear wall.

Hypervelocity impact damage prediction of stuffed Whipple shield based on Adaboost

Hypervelocity impact damage prediction of stuffed Whipple shield based on Adaboost

Comparison of shielding performance of Al/Mg impedance-graded-material-enhanced and aluminum Whipple shields

The gradient material bumper shows promise to improve shielding performance of Whipple shields. The purpose of this study is to investigate the main factors in performance improvement. Comparison experiments have been performed on Al/Mg impedance-graded-material-enhanced and conventional aluminum Whipple shields using a two-stage light-gas gun at impact velocities of 3.5 and 6.5 km/s. They show that the shielding capability of Al/Mg shields is greater than that of conventional aluminum shields with the same bumper areal density. Hypervelocity impact characteristics, including debris clouds and damage patterns on the rear wall, have been studied, and consistent results have been achieved. To explore the reasons for the superior shielding capability of Al/Mg shields, some theoretical analysis and calculations were carried out. The results show that the projectiles receive equal shock pressures and temperature rise when impacting on Al/Mg and aluminum bumpers, which because the bumpers possessing the same acoustic-impedance surface materials. However, the Al/Mg bumper can breakup the projectile into smaller parts and spread the projectile fragments over a greater area, because the particular shock wave transfer is affected by shock impedance mismatch. In addition, the shock heating effect for the Al/Mg bumper plays an important role in contributing to kinetic energy attenuation and bumper material fracture, thereby causing a sharp increase in protective capability for spacecraft meteoroids/debris shielding. Finally, the ballistic limit curve of Al/Mg shields was preliminarily obtained.

Preliminary study on shielding performance of debris shield with the rear wall combining light materials and an aluminum plate

Several kinds of shields such as Whipple shield, stuffed shield and multiple-wall shield were developed to defend from orbital debris impacting on spacecraft. The shielding performance of these shields was improved notably by changing the properties of bumper or stuffed wall. This work focuses on a new way to improve shielding performance by using a combination of light material attached to a rear aluminum wall instead of a single aluminum wall with the same areal density. Hypervelocity impact tests were performed to evaluate the new shielding concepts using aluminum spherical projectiles with 5.0 mm diameter at impact velocity ranging from 4.62 km/s to 4.90 km/s. The targets were three kinds of shields (aerogel/fiberglass composite, wood plate, or polyurethane foam plate attached to the rear aluminum walls in the shields individually), and Whipple shield with the same areal density. Test results showed that the shielding performances of these three kinds of shields are better than that of Whipple shield, based on the reduced damage on the rear wall. The advantage of combined wall using light materials attached to an aluminum plate to replace a single aluminum plate in traditional shield was verified preliminarily, and a feasible way was put forward to improve the shielding performance of debris shield in this paper.

Study of the shielding performance of a Whipple shield enhanced by Ti-Al-nylon impedance-graded materials

An improved meteoroid/debris shielding structure for spacecraft is presented, using a bumper constructed from impedance-graded materials. The hypervelocity impact performances of a shield enhanced by Ti-Al-nylon impedance-graded materials and an aluminum Whipple shield are investigated experimentally, using a two-stage light gas gun at velocities of 3.50 and 6.50 km/s. The impact characteristics, including debris clouds, penetration holes in the bumper, and damage patterns on the rear wall are studied. The results show that the shielding capability of a Ti-Al-nylon shield is greater than that of an aluminum Whipple shield where the bumper has the same areal density. A theoretical analysis and numerical simulation are performed to explore why Ti-Al-nylon shields achieve a better shielding performance. The results suggest that a Ti-Al-nylon bumper can generate higher shock pressures and induce a greater temperature increase, which is more effective for fragmenting a projectile. Furthermore, wave propagation in the projectile and bumper is discussed. It is found that the shockwave propagation is affected by the shock impedance mismatch in impedance-graded material bumpers, so that the shock heating effect and the expansion angle of the debris cloud are increased. Materials with lower melting and vaporization temperatures are helpful for improving the performance of a Ti-Al-nylon shield, thereby achieving a sharp increase in the protective capability for spacecraft meteoroid/debris shielding.

Three-dimensional characterization of debris clouds under hypervelocity impact with pulsed digital inline holography.

The increasing space debris poses a great threat to in-orbit spacecraft and satellites, because its hypervelocity impact can bring about fatal mechanical and electrical damage to them. This work applies pulsed digital inline holography (DIH) to measure three-dimensional (3D) positions and shapes of the debris clouds generated by the hypervelocity impact in the Whipple shield. Detailed operation procedures of synchronizing the pulse DIH system with the impact event and removing the strong plasma radiation are presented, ensuring the successful capture of the transient state of ultrafast ejecta. Experiments on a 2.25mm aluminum sphere impacting a 0.5mm thickness aluminum target plate with a velocity of 3.6 km/s are carried out at the Hypervelocity Impact Research Center of the China Aerodynamics Research and Development Center, and results show that the holographic fringes are clearly recorded and the debris fragments are reconstructed and located accurately, agreeing well with the results measured by laser shadowgraph. This work demonstrates the powerful capability and great potential of DIH in the diagnostics of hypervelocity impact.

Hypervelocity impact testing on stochastic and structured open porosity cast Al-Si cellular structures for space applications

A hypervelocity (6.7–7.0 km/s) impact testing campaign was conducted using Ø 2 mm, 95% Al projectiles onto A357 aluminium alloy stochastic foams with 4–5 mm typical pore dimensions, or alternatively diamond cubic periodic structures of cast AlSi12 with a 6 mm lattice parameter, in order to assess their performance as shielding material against orbital impacts. Either 0.15 mm Al foil or multi-layer insulation (MLI) was used as a front bumper material. It was found that the periodic structures failed to retain the impact debris for any incident angle of impact between 0° and 12°, at least in part due to ricochets and/or spalled material finding its way through open straight channels within the periodic structure. A porous material architecture traversed by no open, straight path is thus required for proper impact protection. Stochastic foams satisfy this criterion and indeed were found to stop the debris. Depending on bumper configuration, stochastic foams gave comparable performance to that of simple Whipple shield designs at 1.3–1.9 × the areal weight. We suggest that a finer pore structure with respect to the projectile diameter should yield a higher impact absorption per areal weight. As an auxiliary result, it was found that MLI as a front bumper was less efficient in fragmenting the projectile compared to Al foil of similar areal weight. In conclusion, open porosity stochastic foams are a promising material as sandwich panel cores for space applications, as they may reduce the need for a dedicated shield, so long as the small debris produced by the impact can be isolated from the satellite systems.

Study on launching technique of a 98 kJ electric gun for hypervelocity impact experiments

An electric gun with the maximum stored energy of 98 kJ is established for hypervelocity impact (HVI) ballistic tests on Whipple shields of space debris. The status of a launched flyer, being crucial of HVI ballistic tests, and the influence of the plasma following the flyer has been experimentally evaluated. By improving the fabrication technique of bridge-foil laminates, a flyer plate is driven by evenly distributed foil expansion and plasma expansion. A 10 mm-diameter and 0.2 mm-thick mylar flyer is launched up to ∼12 km/s and a 18 mm-diameter and 1.0 mm-thick mylar plate up to ∼4.5 km/s. Both the flyers maintain their completeness. The circuit of the electric gun is also reformed to eliminate the formation of plasma shock waves with the amplitude as high as 20 GPa. Such an electric gun has been applied not only for HVI ballistic tests on Whipple shields, but also as a versatile tool for dynamic high-pressure experiments.

Orthotropic node-separation finite element method for composite laminate in hypervelocity impact simulation

This paper reports a finite element modeling approach to simulate the hypervelocity impact (HVI) response of composite laminate. Node–separation finite element (NSFE) method based on scalar–element–fracture technique for isotropic material in HVI simulation has been presented in the previous study. To extend NSFE to composite materials, an orthotropic node-separation finite element (ONSFE) method is developed. This approach employs an orthotropic continuum material model and a corresponding orthotropic–element–fracture technique to represent the HVI behavior/damage of composite laminate. A series of HVI simulations are conducted and the developed ONSFE method is validated by comparing with the experimental data. The simulation results show that ONSFE can successfully capture the HVI phenomena of composite laminate, such as the orthotropic property, nonlinear shock response, perforation, fiber breakage and delamination. Finally, a HVI event of Whipple shield is simulated and the computational capability of ONSFE for predicting the damage state of the composite bumper is further evaluated.

Hypervelocity impact on flexible curable composites and pure fabric layer bumpers for inflatable space structures

Demands for large space structures recently increased in accordance with the growth of space applications for micro-gravity research and space travel. To satisfy growing demand, inflatable structures have been developed. For hypervelocity impact protection from micrometeoroid and orbital debris with enough structural flexibility to be applied to an inflatable space structure, a Multi-shock shield was developed and adopted. However, the multi-layered ceramic fabric bumper of the Multi-shock shield is inferior to the bumper of the conventional stuffed Whipple shield with respect to its hypervelocity impact protection performance. In this study, the applicability of directly curable carbon, Zylon and Twaron composites as a front bumper of the hypervelocity impact shield was examined through 22 experiments involving a pure fabric layer bumper. Through the experiments, it was found that directly curable composites can be more effectively used as a bumper of hypervelocity shield than pure fabric layers.

Experimental and Numerical Study of Submillimeter-Sized Hypervelocity Impacts on Honeycomb Sandwich Structures

This paper deals with hypervelocity impacts of submillimer-sized debris on honeycomb sandwich panels. These debris, which are mostly present within the low Earth orbit, indeed represent a real threat for spacecrafts and satellites. In fact, for debris large enough to be tracked, pre-determined debris avoidance manoeuvre is usually conducted to prevent any damage. Submillimer-sized debris, however, are too small to be identified and therefore spatial structures must be protected against such threat. Honeycomb structural panels and whipple shields have been used as primary shielding against orbital debris impact. The protection capability is usually estimated using Ballistic Limit Equations (BLE). These data have been built from experimental tests on whipple shield protection and transposed to honeycomb sandwich panels.In the case of Whipple shield, the debris cloud generated at the impact on the bumper sheet expands until reaching the rear wall. BLE for Whipple shields only depends on materials properties, protection geometry, angle of incidence and impact velocity. For honeycomb sandwich panels, the debris cloud is partially channelled within honeycomb cells, thus limiting its radial expansion. The channelling effect is thus a function of the honeycomb cell geometry. The honeycomb BLE presented by the Centre d’Etudes de Gramat (CEG) in 2008 has been introduced in order to take into consideration such effect.The present study proposes to extend the results of the CEG. The main approach is to consider the relative dimensions between the projectile diameter and the honeycomb geometry in order to evaluate the perforation risks of submillimer-sized hypervelocity impacts. The impact process on honeycomb sandwich panel has first been modelled using commercial hydrocode LS-Dyna using hybrid Lagrange and Smooth Particle Hydrodynamics (SPH) solvers. The numerical model has been validated through several hypervelocity impacts experiments carried out at Thiot Ingenierie Shock Physics Laboratory at velocities up to 9.3 km/s. This model has then been used to define a ballistic curve which defines the critical projectile diameter of a specific sandwich panel subjected to submillimer-sized debris impact. The results are finally compared to the ones obtained by the CEG leading to an updated estimation of the protection capability of honeycomb sandwich panels.

Shape Effect Analysis of Aluminum Projectile Impact on Whipple Shields

This report presents a numerical analysis study on the shape effects of prolate and oblate ellipsoid projectiles on the performance of spacecraft shields conducted in support of the Orbital Debris Program Office at the NASA Johnson Space Center with the Hypervelocity Impact Technology (HVIT) group [1]. The study served as an independent analysis of the experimental results obtained by the Ernst-Mach Institute, who conducted hypervelocity impact (HVI) tests using shaped-projectiles against a dual-wall Whipple shield to develop ballistic limit equations (BLE) with shape effects. The HVI have been simulated via the three-dimensional, nonlinear-structural-dynamics, simulation tool, CTH, which has been developed by Sandia National Laboratories to treat shock-wave propagation and large-deformation phenomena. The results from the numerical simulations for the sphere and prolate ellipsoid projectiles indicate numerical simulations are a viable tool for conducting HVI analyses, however, significant differences between the CTH models and published EMI test data for the oblate projectiles point to issues in modeling the impact physics of oblate ellipsoid geometries [2]. Consequently, further analysis should be performed to solve the problem in modeling these oblate projectile impacts. The unmodified BLE for spheres and modified BLE for prolate ellipsoids reasonably predict failure of a Whipple shield at low impact velocities, but based on the obtained data may need to be adjusted for velocities between 4 and 8 km/s.

Experimental study of hypervelocity impacts on space shields above 8 km/s

This paper deals with hypervelocity impacts of millimeter size space debris on space structures at velocity regime between 6 and 15 km/s. Millimeter size debris present a real threat to space structures since they cannot be tracked nor detected by radars. At the opposite, for larger debris, 5-10 cm size, avoidance manoeuvre can be conducted remotely from earth to shift the space structure from the threat trajectory. A lot of works (tests and simulations) have been done for debris smaller than 5 cm size. For debris smaller than 1 cm, the maximum impact velocity reachable with available facilities is mainly up to 8-8.5 km/s. As a result, the ballistic limit equation (BLE) for single plate, whipple shield and honeycomb structures are only based on analytical and numerical investigations for higher threat velocity.There is no efficient facility available to generate hypervelocity impacts in the 8-12 km/s regime for millimeter size projectile. To fulfil this need, the French space agency CNES granted a R&T contract to Thiot Ingenierie company for performing hypervelocity tests between 8 and 12 km/s. The contract specified Thiot Ingénierie for performing the analysis and the development of a special hypervelocity device that will be able to propel 1 to 3 mm aluminium sphere at this high velocity regime. With this innovative launcher, impacts of 1 mm sphere diameter at a speed of 10km/s and above is now reachable.The present study presents hypervelocity impacts on different types of targets: single plate, whipple shield and honeycomb structure have been tested. Impact velocities ranging from 7 to 10 km/s have been reached. Specific diagnostic tools (very high speed camera, X-rays flash photography …) are used to analyse the tests. Results, in terms of ejecta projection, post-impact damage are presented. The effects of ejecta tunnelling in the case of honeycomb shield is highlighted.

Weight-Efficiency of Conventional Shielding Systems in Protecting Unmanned Spacecraft from Orbital Debris

Unmanned spacecraft typically require protection only from much smaller orbital debris as compared to manned missions. This paper presents quantification and comparison of the weight efficiency of conventional shielding concepts, which were originally developed for manned spacecraft, when designed to protect a robotic satellite against small-size (1 mm) orbital debris impacts. The shielding systems under comparison comprise two categories: “single-purpose orbital debris shields,” represented by the Whipple shield and the stuffed Whipple shield; and “multipurpose structural panels,” represented by honeycomb-core and foam-core sandwich panels. First-order estimates of the shields’ parameters are obtained using the well-known ballistic limit equations. These estimates are then used as starting points for further optimization of the shields conducted by means of hydrocode simulations. The simulations employ a combination of the ANSYS Autodyn finite element and smooth particle hydrodynamics solvers. The results obtained indicate that, in the single-purpose orbital debris shields category, the simpler Whipple shield concept provides better performance than the stuffed Whipple configuration; whereas for the multipurpose structural panels, the foam-core sandwich panel can have less than half the weight as compared to a honeycomb-core panel with a similar ballistic performance.