David J. Gardner

Modelling of microparticle hypervelocity oblique impacts on thick targets

Abstract For hypervelocity impacts the effects of impact angle obliquity on final crater morphology have been studied using the hydrocode AUTODYN-3D. The target material and velocity ranges have been chosen so that they correspond with typical space debris impacts on orbital spacecraft situations, such as NASAs LDEF and ESAs Eureca. The hydrocode calculations were first validated against oblique impact experiments for 1 mm aluminum spheres on aluminum at 6.5 km/s and for the normal impact of 1 μm iron spheres on copper at 16 km/s. The main study was for 1 pin aluminum and iron spheres impacting aluminum at velocities of 6.5, 10 and 16 km/s and impact angles from 0° to 85°. Results are presented which show the calculated relationships between the impact angle, and the main crater length, width and depth. A study of projectile size effects has also been performed by carrying out calculations for several different projectile sizes ranging from 1 mm to 1 μm The calculations used strain-rate dependent material models and the results suggest either that departure from exact size scaling cannot be explained by strain rate effects, or that the Johnson-Cook model used is not appropriate for the higher strain rates experienced in the case of smaller size projectiles.The calculations demonstrate that Lagrangian techniques, when coupled with an erosion algorithm, can be used for hypervelocity impact calculations; however care must be taken to use an appropriate erosion criterion.

Hole growth characterisation for hypervelocity impacts in thin targets

Abstract Interpretation of the flux data of hypervelocity impact craters and perforations from recovered metallic satellite surfaces (such as from NASAs LDEF and ESAs Eureca) is primarily directed towards the derivation of particle diameters. This requires careful calibration, especially in the transition region near to marginal perforation for targets of finite thickness. This calibration is usually in the form of a “hole growth equation”. Although several thin foil formulae exist and many thick target formulae, the only established equation which attempts to interpret near marginal impacts does not seek to interpret impacts where the perforation is smaller than the foil thickness. In this work intuitive trends are used to select an appropriate mathematical form for a new equation, after which hypervelocity impact data on thin foils are used for defining the parameters.

Near earth environment

Planet Earth provides an interface to the interplanetary environment; its atmosphere forms a protective shield against direct impacts and erosion and is a medium in which to observe the approach of meteoroids and even to capture intact smaller meteoroids. The Earth’s gravitational well enhances the flux of interplanetary dust and modifies its velocity distribution. We consider the effect of the Earth on the dynamical properties on the interplanetary dust population, the relative contribution of sporadic meteoroids and annual streams, the efficiency of the atmosphere in capturing and fragmenting meteoroids and the effect of space debris on in situ experimental results. We review the range of modelling tools necessary to interpret the complex interaction of these populations with spacecraft, with particular emphasis on the improved calibration of impact detectors and the application of software models. Analysis of the available data from 30 years of in situ impact experiments, and more recent recovered samples reveals evidence of the relative contributions from space debris and various astrophysical sources. While temporally and spatially averaged fluxes are well represented by existing isotropic interplanetary models for meteoroids responsible for penetrating experimental foils (of thickness F max) greater than approximately 30 μm, at smaller sizes a high degree of anisotropy is apparent in resolved data. An Earth apex component is observed for particles larger than a few microns in size whereas at smaller sizes, β-meteoroids from the solar direction dominate. Space debris forms an increasingly significant proportion of the LEO population at F max < 30 μm in addition to its dominance in the centimetre size range and above.

Micro-particle impact flux on the timeband capture cell experiment of the Eureca spacecraft

Measurements of hypervelocity impact fluxes (in both thick and thin targets) detected by the University of Kent at Canterburys Timeband Capture Cell Experiment (TiCCE) (flown on ESAs Eureca spacecraft) are presented. The foil perforations are used to derive the ballistic limit values, or the maximum thickness of Al perforated, for the impacting particles. This data is then combined with the thick target data to derive a unified ballistic limit flux. A significant enhancement in the observed large particle flux compared with LDEF is found, possibly due to the pointing history of Eureca compared to the Earths orbital direction. Comparisons are also made to predictions from ESABASE modelling. Preliminary results of a study of perforation morphology are also presented, providing insight into particle shape, density and directionality.

Hypervelocity impact on spacecraft carbon fibre reinforced plastic/aluminium honeycomb

Samples of a spacecraft primary external wall structure, as used in a low earth orbit remote sensing platform, have been tested to determine the response to the hypervelocity impact and ballistic limit (for mm-sized impactors) of the 47 mm thick structure at 5 km/s. A strong dependence of the ballistic limit on projectile density was identified. This programme was carried out using the two-stage light gas gun at the University of Kent at Canterbury. The equivalent diameters of the front and rear holes for each impact were analysed as a function of the impactor parameters. Damage equations derived by other experimenters were compared to the experimental results. X-ray non-destructive testing was used to determine the level of internal honeycomb damage for a sample. The dependence of the witness plate damage (placed behind the target to capture any ejecta from the rear surface) on the impactor parameters was recorded. It was found that the use of ‘equivalent thicknesses’ of aluminium may not be appropriate as a general conversion factor for carbon fibre reinforced plastic (CFRP) facesheets. A simple damage equation is presented, based on the total hole size as a function of the impact energy. The ballistic limit cannot be defined solely in terms of impact energy and shows an additional dependence with projectile density. The amount and type of ejecta produced is a strong function of density and a less strong function of projectile diameter, and its production cannot be linked with the rear hole diameter.

Meteoroids and space debris hypervelocity impact penetrations in LDEF MAP foils compared with hydrocode simulations

Abstract The continued analyses of penetrating impacts on MAP foils of Aluminium and Brass have produced data for several LDEF faces, i.e., Space, West, and East. These data have immediate bearing on the interpretation and design of devices to detect the penetration of a thin metallic film by a dust grain which have been tested both in the laboratory and in space. A crucial component of the analysis has been the theoretical calculation utilizing CTH, a Sandia National Laboratory Hydrodynamic computer code /1/ to assess the parameters of the hypervelocity penetration event. In particular theoretical hydrodynamic calculations have been conducted to simulate the hypervelocity impact event where various cosmic dust grain candidates, e.g., density = 0.998, 2.700, 7.870 (gm/cm 3 ), and velocities, i.e., 7 – 16 km/s, have been utilized to reproduce the events. Theoretical analyses of hypervelocity impact events will be reported which span an extensive matrix of values for velocity, density and size. Through a comparison between LDEF MAP foil measurements and CTH hydrocode calculations these analyses will provide an interpretation of the most critical parameters measured for space returned materials, i.e., for thin films, the diameter of the penetration hole, D b , and for semi-infinite targets, the depth-to-diameter ratio of craters, D c /T c . An immediate consequence of a comparison of CTH calculations with space exposed materials will be an enhancement of the coherent model developed by UKC-USS researchers to describe penetration dynamics associated with LDEF MAP foils.

Application of hydrocode modelling to the study of hypervelocity impact crater morphology

In order to obtain a better understanding and model of the natural and artificial particulate environment from measurements of impact damage features on returned spacecraft materials, it is necessary to be able to determine how the size and shape of an impact feature are related to the parameters of the impacting particle. The AUTODYN-3D hydrocode has been used to study the effects of projectile density, velocity and impact angle on the the depth, diameter and ellipticity of the impact craters. The results are used to determine the distributions of crater depth to crater diameter ratios and of crater ellipticities to be expected on an aluminium surface exposed to an isotropic distribution of incident particles of given densities and velocities. Comparison of these calculated distributions with those observed for craters on aluminium clamps on various faces of the Long Duration Exposure Facility shows that particles with a wide range of densities, including significant proportions both greater and smaller than that of aluminium, were responsible for these craters.