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.     

Ballistic limit for shielded plates subjected to hypervelocity impact

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

Measuring hypervelocity impact velocity from micrometeoroid crater geometry

Guided by half-space computer simulations showing hypervelocity impact crater formation for an iron particle impacting an aluminum target and characteristic crater geometry changes with impact velocity over the range 8–40 km s⁻¹, we examined normal surface crater views and cross-sectional views through craters (>0.5 mm diameter) from samples retrieved from the NASA LDEF satellite and examined in the scanning electron microscope (SEM). While geometrical features suggested in the computer simulations were indeed observed for micrometeoroid craters in 6061-T6 aluminum targets and 303 stainless steel targets, there was no consistent estimate for impact velocities in any of the experimental samples, and velocity estimates based on measuring ratios of ejecta width/crater diameter and ejecta height/crater depth as well as ejecta height/crater diameter varied from 8 to 42 km s⁻¹; over the same range simulated. These results point to the need to create reference data from actual hypervelocity impact experiments in the laboratory, and systematic observation of residual crater geometries in the SEM. These experiments also demonstrate the uncertainty in assuming a fixed impact velocity for all impact craters in space materials as well as an apparent futility in attempting to correlate impacting particle velocity with post-mortem characteristics of a given crater.     

Survey of diagnostic tools used in hypervelocity impact studies

This paper surveys a variety of diagnostic tools that are available for use in studies of hypervelocity impact. Emphasis is on time-resolved methods for measuring pressure or particle velocity histories of stress waves induced under these conditions, and on the new developments of the past decade.     

Influence of the critical velocity on deformation of launcher components

This paper is related to the dynamics of hypervelocity electromagnetic launchers. A projectile accelerating along launcher rails may cross a range of critical velocities and induce structural resonance. As a result, the rails and other components exhibit increased displacements and stress that may affect launcher performance and lead to premature launcher failure. This work is a continuation of our previous studies of the critical velocity and resulting transient resonance that was performed for a notional hypervelocity launcher [Nechitailo NV, Lewis KB. Critical velocity for rails in hypervelocity launchers. In: Proceedings of the 2005 hypervelocity impact symposium. International Journal of Impact Engineering Dec. 2006; 33: 485–495; Lewis KB, Nechitailo NV. Transient resonance in hypervelocity launchers at critical velocities [Selected papers from the 13th Electromagnetic Launch Technology (EML) Symposium, Potsdam, Germany, May 22–25, 2006]. IEEE Transactions on Magnetics Jan. 2007; 43 (No. 1, Part II): 157–162 [1,2]]. Analytical models including Bernoulli–Euler model of a beam resting on an elastic foundation and the Timoshenko and Flügge tube models as well as finite element tools helped to better understand the transient resonant regimes in launcher components and offered insight on how to alter the launching device materials and geometry to reduce the critical-velocity effects. Analysis showed that the various components of a launcher can have different critical velocities and there is a possibility of enhanced group resonance in the assemblies. The resonance in the launcher assembly can be reduced by controlling the bending stiffness of the individual components. Finite element models were used to illustrate the influence of variations in materials of launcher components on the resulting critical velocities, intensity of the group resonance, and resulting maximum displacements and stress.     

Application of a 100-kV electric gun for hypervelocity impact studies

The Lawrence Livermore National Laboratory 100-kV electric gun has been used to launch flat-plate projectiles for use in studies of spall and hypervelocity impact penetration of thin plates. Impactors were 0.3-mm thick Kapton with dimensions and velocities ranging from 100 mm² at 4 km/s to 10 mm² at 18 km/s. A Fabry-Perot laser velocimeter, an electronic streak camera, and a flash x ray were used as diagnostics of the flyer-plate impact on the selected specimen. Experiments generally included the recovery of the remnant specimen and fragments for detailed examination, permitting a study of incipient spall, onset of melting, and fraction fragmented. Experiments to be described include spall measurements on simple and composite target walls at normal and oblique incidence and “reverse ballistics” impacts of the thin-plate impactor on a stationary penetrator (e.g., Kapton impactors at 15 km/s incident on rods of steel, aluminum, and lead) for calibration of hypervelocity impact codes.     

Characterization of prompt flash signatures using high-speed broadband diode detectors

Impact flash is a brief, intense flash of light released when a target is impacted by a hypervelocity particle. It is caused by emissions from a jet of shocked material which is thrown from the impact site. Impact flash phenomenology has been known for decades, and is now being considered for applications where remote diagnostics are required to observe and diagnose impacts on satellites and space craft where micrometeoroid and orbital debris impacts are common. Additionally, this phenomena and remote diagnostics are under consideration for missile defense applications. Currently, optical signatures created from hypervelocity impact can be utilized as the basis for detectors (spectrometers, pyrometers), which characterize the material composition and temperature. More recent interest has focused on study of hypervelocity impact generated debris and the physics of the associated rapidly expanding and cooling multiphase debris cloud. To establish this capability technically in the laboratory, we have conducted a series of experiments on a two-stage light gas gun at impact velocities ranging from 6 to 19 km/s, which is representative for light emissions resulting from hypervelocity impacts in space. At these high impact velocities jetting is no longer the dominant mechanism for observed impact flash signatures. The focus of this work is to develop fast, inexpensive photo-diodes for use as a reliable prompt flash, and late time radiating debris cloud diagnostic to: (a) characterize material behavior in the shocked and expanding state when feasible; (b) ascertain scaling of luminosity with impact velocity; (c) determine the temperature of the impact flash resulting from radiating emissions when multiple silicon diodes are used in conjunction with narrow band pass filtering at specific wavelengths as a pyrometer. The results of these experiments are discussed in detail using both a metallic target, such as aluminum, and an organic material such as Composition-B explosive.     

Simulations of hypervelocity impact damage and fracture of aluminum targets using a constitutive-microdamage material model

The material damage and fracture of Aluminum 1100 target plates that experience hypervelocity impact by glass projectiles traveling at 6 km/s are simulated using a proposed constitutive-microdamage material model. The model is best suited for polycrystalline metals that are subject to hypervelocity impact at the lower range of velocities. Simulations are performed for three projectile diameter-target thickness ratios that produce a wide range of damage features. The predicted damage is compared with that of the corresponding test laboratory specimens, illustrating the capability of the constitutive-microdamage model.     

Micrometeorite impact simulation at EMI — a review

Activities at EMI in the field of hypervelocity impact techniques are reported. Optimization experiments have been carried out with a light gas gun in order to achieve projectile velocities up to 10 km/s. Different methods for measuring the projectile velocities have been developed and adapted according to respective velocity and mass ranges of projectiles. Experimental efforts have been undertaken to accelerate also microgram particles in light gas guns. Masses as small as 37 μg can be accelerated as individual particles. As examples, several contributions to recent space projects are described.     

Simulation of orbital debris impact on the Space Shuttle wing leading edge

A series of three dimensional hypervelocity impact simulations has been performed to study the effects of orbital debris impact on the Space Shuttle wing leading edge. The simulations employed an improved hybrid particle-finite element method and an orthotropic elastic-plastic material model recently developed for reinforced carbon–carbon. The simulation results are consistent with the available experimental data, and suggest the use of momentum scaling to estimate damage effects for impact conditions outside the range of current light gas gun technology. Projectile shape and orientation effects appear to be modest for flat plate projectiles at impact velocities above the ballistic limit.     

Searching for momentum enhancement in hypervelocity impacts

In conjunction with the Los Alamos National Laboratory hypervelocity microparticle impact (HMI) team effort to produce higher impact velocities and to understand the physics of crater formation and momentum transfer, we have implemented a low noise microphone as a momentum detector on both the 6 MV Van de Graaff and 85 KV “test stand” particle accelerators. Calculations are presented showing that the impulse response of a circular membrane. When used as a momentum impulse detector, the microphone theoretically may detect impulses as small as 8.8 × 10⁻¹⁵ N s. Sensitivity of the microphone in this application is limited by the noise threshold of the electronic amplifiers and the ambient microphonic vibration of the system. Calculations lead us to anticipate detection of particles over the full range of the Van de Graaff acceleration capability and up to 7 km/s on the test stand. We present momentum enhancement data in the velocity range between 10 km/s and 20 km/s. Preliminary work is presented on momentum impulse calibration of the microphone using laser-pulse photon momentum as an impulse source.     

A review of explosive accelerators for hypervelocity impact

A technical overview of experimental methods using high explosive techniques for conducting hypervelocity impact studies is presented. The explosive techniques use the explosive detonation fronts as means of accelerating the projectile, or as means of compressing a light gas which is then used to launch the projectile.

The explosive launchers are in six subdivisions: high explosive pellet accelerators, flyer plate accelerators, shaped charges, explosive-formed projectiles, fragment and microparticle accelerators, and explosive gas guns. Each one of the subdivisions presents the various techniques, their advantages and disadvantages, the range of mass and velocity capable of being accelerated, and whether the technique can be scaled for larger or smaller masses.     

An engineering fragmentation model for the impact of spherical projectiles on thin metallic plates

In this paper, an engineering fragmentation model is presented for the case of hypervelocity impact of a spherical projectile on a thin bumper plate at normal incidence. The range of impact velocities covered is the solid fragmentation regime up to the limits of complete melting of projectile and target material. The model was developed for an axisymmetric fragment cloud by consideration of the conservation laws for mass, momentum, and energy, as well as making a few assumptions on the morphology of the cloud. The fragment cloud is modeled discretely, i.e. each particle of the fragment cloud is considered separately in the analytical calculation. The model consists of mainly analytical relationships and a few empirical fit functions, where no analytical formulation was available. The model distinguishes between fragments originating from the projectile and fragments originating from the bumper plate. The projectile fragments are split into the central fragment and spall fragments. An exponential distribution function is assumed for the mass distribution of the projectile's spall fragments. The fragments from the bumper are assumed to have a uniform mass. All fragments are assumed to be of spherical shape. The fragmentation model was applied and calibrated during experiments, in which Al spheres impact on thin Al plates. The calibration experiments, performed using a two-stage light gas gun, were in the range of impact velocities between 4.8 and 6.7 km/s. In this velocity range, the model was calibrated against residual velocities measured and fragment mass distribution, which was indirectly determined by measuring the crater depth distributions in rear walls.     

高速粒子撞击作用下LF6合金多层靶结构防护能力研究

针对总厚度为４ｍｍ的ＬＦ６合金双层靶和总厚度为２ｍｍ的三层靶进行了直径为２ｍｍ,速度分别为５．８和７．２ｋｍ／ｓ的ＧＣｒ１５粒子 撞击试验,并对双层靶进行了不同前靶厚度和靶间距的撞击试验,试验结果表明：与同样碰撞条件下半无限体靶上产生的破坏情况相比,多层靶被击穿的总厚度远淖于半无限体靶上形成的弹坑深度,采用多层靶结构可显著提高材料的抗高速粒子撞击能力,并大大降低航天器抗高速粒子撞击的防护结构的重量     

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.     

Debris fragment characterization in oblique hypervelocity impacts

A case history in debris characterization is presented for oblique impacts of chunky tungsten projectiles against thin plates. The integrated approach of scaled experiments and hydrocode simulations led to a semi-analytic model of behind the plate debris fragment distributions. This debris distribution model agreed quite well with the experimental fragment distributions derived from witness plate measurements. The 1/4 scale test program included three projectile masses, two target geometries (single and dual plates), a velocity range of 4–7 km/s and a strike angle range of 15–55 degrees. Close correlation of measured and predicted fragment distributions encouraged the extension of the model to higher velocities not currently obtainable in the laboratory.

The paper also includes discussions of critical features of debris in oblique hypervelocity impact, the scalability of fragment data, and the utilization of the derived fragment models in semi-analytic damage assessment codes.     

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.     

Hypervelocity impact of small masses on large surfaces of piezoelectric ceramics

The hypervelocity impact of small masses on large surface piezoceramics was investigated to study the impact behavior of hypervelocity projectiles. From a linear elastic model obtained at lower velocities, solutions were found for the hypervelocity case which determine both the size and the momentum of impacting projectiles from the rising slope of the charge signal generated by the impact. The results lead to the development of a new generation of impact detectors for small masses at hypervelocities which consists only of a plate of piezoceramic material.     

Development and optimization of a multibumper design model for spacecraft protective structures

The development and optimization of a design model for multibumper spacecraft protective structures to defeat orbital space debris is presented. The Marshall Space Flight Center (MSFC) Materials and Processes (M&P) Laboratory Hypervelocity Impact Database is first filtered to experiments comprising metallic configurations without multilayered insulation present and for projectile velocities exceeding 2.5 km/sec. This filtering results in 337 single, double, and triple bumper hypervelocity impact experiments. Regression variables of interest include projectile diameter, density, velocity, and impact angle, bumper standoff distances, bumper densities and thicknesses, wall density and thickness, and number of bumpers. The dependent regression variable is the total number of plate penetrations, beginning with the wall and continuing through the witness plates. A unique intrinsically linear regression form, which accounts for the number of bumpers employed and invokes a posynomial (polynomial with positive coefficients, positive valued independent variables, and real valued exponents) form, is chosen based on a comparison of various regression forms using correlation coefficient and F-statistic as measures of effectiveness. The least squares regression is performed followed by an ANOVA, tests of the correlation and F value, and graphical examination of residuals. Regression results indicate that statistically significant least squares is possible using the chosen form on the MSFC M&P database with small residual effects. Generic nonlinear regression forms are also investigated.

The resulting regression model is next used in the formulation of a nonlinear optimization program. This program is devised to minimize the protective structures areal density subject to a limitation on total standoff distance between the first bumper and the wall. The decision variables of interest are the optimal values of the areal densities of the bumpers and wall, as well as the optimal individual standoff distances. The problem is solved using the dual transformation of geometric programming. The optimal independent variables and minimum system areal density are solved for analytically in terms of the systemic parameters. A sensitivity analysis to these parameters is then performed. Additionally, the optimal number of bumpers is evaluated in this sensitivity study. The most significant results from a hypervelocity impact standpoint are that additional hypervelocity impact tests and analyses should be performed to support understanding of multiple bumper, large particle diameter, large separation, large particle mass density, various particle impact angles, and spallation phenomenologies. Additionally, more emphasis should be placed on understanding the transition regions between particle shatter, melt, and vaporization, while less emphasis should be placed on small velocity differences within these regions. Major protective structures design results indicate that for Space Station Freedom impact scenarios of interest, and within the limitations of the regressed hypervelocity impact database, at most four metallic bumpers are optimal. In particular, a transition region from optimal number of bumpers of 2 to 3 (and 3 to 4) has been identified for particle diameters in the 0.25–0.5 cm (and 1 to 1.25 cm) range. An interesting transition region from 3 to 4 optimal number of bumpers has been discovered for standoff distances between 10 and 15 cm. Furthermore, the optimal protective structures design sensitivity to impact angle is very low. Finally, the results of this investigation indicate that this combination of regression form and resulting optimization approach is useful in identifying protective structures design trends for spacecraft subject to hypervelocity impact environments.     

An analytical model of hole size in finite plates for both normal and oblique hypervelocity impact for all target thicknesses up to the ballistic limit

This paper presents, for the first time, a single comprehensive analytical model for the hole size produced by hypervelocity impact into finite plates. This model is based on experimental data for 2017 aluminum spheres impacting 2014, 2024 and 6061 aluminum plates.

The significance of this model is that it spans the entire range of target thickness from very thin to very thick, which makes it possible to determine when the impact conditions are those of thin target behavior (where the hole size increases with increasing target thickness and debris formation and damage is important) and when the impact conditions are those of thick target behavior (where the hole size decreased with increasing target thickness and the debris formation is significantly decreased). The model makes it clear that the target thickness that divides the thin target regime from the thick target regime is a function of velocity. This means that an impact configuration which exhibits thick target behavior at common experimental velocities could actually exhibit thin target behavior at velocities in the tens of kilometers per second such as that of meteroid impacts. This hole size model also includes the effects of oblique impact and computes both the major and the minor diameters of the hole.

This paper also raises, for the first time, the possibility that the commonly accepted models for crater diameter (and by implication those for penetration depth as well), which are taken to be a power function of velocity, might be wrong. Only a linear dependence on velocity for the crater diameter is consistent with the linear velocity dependence of this and all other accepted models of hole diameter in finite plates. If this is correct, it would raise questions about the validity of using any target damage computer models, that are based on the old crater modeling equations, to extrapolate to higher velocities.