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.     

Crater morphology at impact velocities between 8 and 17 km/s

The crater morphology of impacts in the velocity range between 8 and 17 km/s has been investigated Glass projectiles with diameters between 20 ωm and 200 ωm were impacted on targets of gold, tungsten, iron and aluminum. Data have been compiled for the dependence of the crater diameter D_c and the crater depth T_c on the projectile velocity.     

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.     

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.     

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.     

Ultra-high velocity impacts: cratering studies of microscopic impacts from 3 km/s to 30 km/s

Cratering experiments performed under carefully controlled conditions at impact velocities ranging from 3 km/s to 30 km/s into a wide variety of target materials are presented. These impact experiments use the 6 MV vertical Van de Graaff accelerator of the Ion Beam Facility at the Los Alamos National Laboratory to electrostatically accelerate highly charged iron micro-spheres. The sub-micron spheres, from a random size distribution, are shocklessly accelerated along an 8 m flight path. Ultra-sensitive charge detectors monitor the passage of the projectiles at a rate of up to 100 projectiles/second. An online computer records and displays in real time the charge, velocity and mass of the projectiles and provides cross correlation between the events observed by the several in-flight charge detectors and impact detectors. Real-time logic controls an electrostatic kicker which deflects projectiles of selected charge and velocity onto the target. Thus each experiment consists of an ensemble of 10 to 40 impacts onto a single target within a narrow window of the projectile parameter space, providing excellent statistical resolution of each data point.

The target materials used include single crystal copper and single crystal aluminum, gold, and quartz as well as pyrolytic graphic and anoxy used in composite materials of interest to space applications. We also conducted impact experiments onto thin Mylar and nickel foils. This paper presents these experiments and summarizes the cratering characterization performed to date. Emphasis is placed on cratering results in several materials over a range of impact velocities.     

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.     

Experimental impacts above 10 km/s

In the proceedings of the last symposium, recent work on a technique for launching small projectiles to hypervelocities above 10 km/s using an inhibited shaped charge was presented [1]. In the interim, experiments have been conducted using the inhibited shaped charge to launch aluminum, nickel, and molybdenum projectiles. This paper presents the results of the impact tests, as well as discusses the shaped charge design modifications for the nickel and molybdenum launchers. Radiographs are presented of the impacting projectiles, as are post test photographs of various targets. The data are unique in that they represent low L/D projectile impacts into both monolithic blocks and spaced plates at velocities above 10 km/s. The aluminum projectiles are being launched at 11.25±0.20 km/s, the molybdenum projectiles at 11.72±0.10 km/s, and the nickel projectiles at 10.81±0.10 km/s.     

Response of space structures to orbital debris particle impact

All long-duration space and aerospace and transportation systems, such as the Space Station Freedom and the Space Shuttle, are susceptible to impacts by pieces of orbital debris. These impacts occur at high speeds and can damage the flight-critical systems of such spacecraft. Therefore, the design of a structure that will be exposed to a hazardous orbital debris environment must address the possibility of such hypervelocity impacts and their effect on the integrity of the entire structural system. A technique is developed for analyzing the response of dual-wall structures to oblique Hypervelocity projectile impact. Ballistic limit curves that predict the potential of an impacting projectiles to perform the main wall of a dual-wall strucutral system are obtained using the techniques and are compated against experimentally derived curves. Comparisons are performed for a variety of impact velocities, trajectory obliquities and projectile masses. It is shown that the results obtained using the technique developmed herein compare very well with experimetanl results.     

An impact technique to accelerate flier plates to velocities over 12 km/s

Very high pressure and acceleration is necessary to launch flier plates to hypervelocities. In addition, the high pressure loading must be uniform, structured, and shockless, i.e., time-dependent to prevent the flier plate from either fracturing or melting. In this paper, a novel technique is described which allows the use of 100 GPa megabar loading pressures and 10⁹-g acceleration to launch intact flier plates to velocities of 12.2 km/s. The technique has been used to launch nominally 1-mm thick aluminum, magnesium, and titanium alloy plates to velocities over 10 km/s, and 0.5-mm thick aluminum and titanium alloy plates to velocities of 12.2 km/s.     

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.     

Measured charge generation by small mass impact at velocities between 1 and 45 km/s

The electrical charge that is generated by the impact of a small mass at velocities between 1 and 45 km/s was investigated using the Electrostatic Dust Accelerator of the Max-Planck-Institut in Heidelberg (MPI) and the Plasma Accelerator of the Lehrstuhl fu¨r Raumfahrttechnik (LRT) of the Technische Universita¨t Mu¨nchen (TUM). Glass beads were accelerated, and the targets were of different materials i.e., (Au, W, Fe, Al). The mass/velocity range of the accelerated small masses was: MPI: 10⁻¹⁵g−10g/ 1km/s−10g−5g/ 2km/s±_total of both polarities can be described by the empirical formula: Q^±_total =Cmv^β[Coulomb], C being a function of the density ratio of target/projectile, is approximately unity and β between 2.92 and 3.77. The charge detector is described and the results that were obtained in test series at both facilities are discussed in relation to the empirical formula.     

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.     

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.