Design limitations on ultra-high velocity projectile launchers

The maximum velocity attainable in a gasdynamic gun is limited by the maximum sound speed in the driver gas. For a conventional 2-stage light gas gun, the limit is 10 km/s. Higher velocities are possible, but probably not without the destruction of the gun barrel. As long as this occurs on a time scale longer than the residence time of the projectile, a useful system may still result. Using a newly developed computer code called IGUN, we have evaluated the performance of several multistage designs capable of achieving ultra-high projectile velocities. The main problem is in maintaining the integrity of the projectile. Our calculations indicate that 20 km/s should be achievable without fracturing the projectile. If it is only required to retain near original areal density, velocities in excess of 30 km/s appear feasible.     

Continuous measurements of in-bore projectile velocity

The application of velocity interferometry to the continuous measurement of in-bore projectile velocity in a small-bore three-stage railgun is described. These measurements are useful for determining projectile acceleration and for evaluating gun performance. The launcher used in these studies consists of a two-stage light gas gun used to inject projectiles into a railgun for additional acceleration. Results obtained for projectile velocities to 7.4 km/s with the two-stage injector are reported, and potential improvements for railgun applications are discussed     

Particle launch to 19 km/s for micro-meteoroid simulation using enhanced three-stage light gas gun hypervelocity launcher techniques

Particle launch experiments were performed to study application of the enhanced hypervelocity launcher (EHVL), i.e. the third-stage addition to the two-stage gun, for launching micron to millimeter sized particulates at velocities unobtainable with a standard two-stage light gas gun launch. Three types of particles or fliers were tested along with several barrel designs. For micron scale particles fine-grain polycrystalline ceramics were impacted and fractured, launching particulate clouds at velocities of 15 km/s. Multiple titanium particles 400 μm diameter embedded in plastic were “shotgun” launched to velocities of 10 km/s. Flier plates of 3 mm diameter by 1 mm thick Ti6Al4V were launched to 19 km/s. All experiments used a second-stage projectile with graded density facing impacting a flier in an impact generated acceleration reservoir. This paper describes the modification and adaptation of the Sandia EHVL to provide micrometeoroid simulation capabilities.     

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.     

Analysis of the second stage of the STAR 28 mm two-stage light gas gun

An engineering model of the second stage operation of the 28 mm two-stage light gas gun at the Shock Technology and Applied Research (STAR) facility of Sandia National Laboratories has been developed. It describes the piston motion and loading of the accelerated reservoir (AR) and the coupled projectile motion. The model was developed to better assess the safety of the AR and to determine the significance of various parameters to the gun performance. Validity of the model is supported by calculated results that compare closely with projectile velocity data over a range of piston masses, piston velocities, and projectile masses and correlate satisfactorily with AR diameter growth data in a typical high-performance test. For this test, bounds on the factor of safety (FS) for the piston velocity are determined to be 1.3 < FS < 1.5. Results indicate that the projectile velocity can be increased and the AR loading reduced by changes in the piston density and break diaphragm opening pressure.     

Hypervelocity impact on honeycomb target structures: Experiments and modeling

This paper reviews some experimental and modeling works carried out at the CEG on hypervelocity ballistic properties of honeycomb structures representative of satellites structural bodies. Honeycomb structural panels were considered for HVI experiments using CEG's Persephone two-stage light gas gun in order to provide reference data to assess the Ouranos simulation software that was then used intensively during the second phase of the study. A ballistic limit equation (BLE) derived from Christiansen's equation was fitted for projectile velocities ranging from 2 to 10 km/s under normal and oblique impacts. In case of perforation, when the projectile diameter is greater than the critical perforation diameter, particle clouds orientation and amplitude in the plane of impact have been characterised and modeled analytically for inclusion within a global vulnerability software.     

Ballistic limit evaluation of advanced shielding using numerical simulations

The advanced shielding concept employed for the Columbus module of the International Space Station consists of an aluminum bumper and an intermediate shield of Nextel and Kevlar-epoxy. Until recently, the lack of adequate material models for the Nextel cloth and Kevlar-epoxy has precluded the practical usage of hydrocodes in evaluating the response of these shields to hypervelocity impact threats. Recently hydrocode material models for these materials have been proposed [1,2] and the further development and completion of this model development is reported in this paper. The resulting models, now implemented in AUTODYN-2D and AUTODYN-3D, enables the coupling of orthotropic constitutive behavior with a non-linear (shock) equation of state. The model has been compared with light gas gun tests for aluminum spheres on the advanced shield at impact velocities between 3.0 and 6.5km/s [3]. Reasonable correspondence has been obtained at these impact velocities and thus the models have been used to perform preliminary assessment of predicted ballistic limits at velocities from 7 to 11km/s. The predicted ballistic limits are compared with ballistic limit curves derived on the basis that damage is proportional to projectile momentum     

蜂窝铝的侵彻实验研究与有限元模拟

目的 研究弹体侵彻蜂窝铝的力学行为。方法 在实验中, 通过轻气炮加速的尼龙弹体冲击蜂窝铝靶体。利用Ansys/LS-DYNA建立了弹体侵彻蜂窝铝的壳单元有限元模型。结果 实验给出了冲击速度为140和167 m/s时弹体的加速度曲线。运用壳单元模型对蜂窝铝的侵彻进行模拟, 得到了弹体的加速度曲线以及蜂窝铝在侵彻过程中的变形图。结论 将模拟结果与实验结果进行了对比, 发现模拟结果与实验结果符合较好, 证明了壳单元有限元模型模拟蜂窝铝侵彻行为的可靠性。     

超高速撞击压力容器碎片云与气体介质相互作用建模研究

针对超高速撞击压力容器碎片云与气体介质相互作用问题,首先,建立了碎片云的初始模型;然后,应用气固两相流理论对碎片云与气体介质的相互作用进行分析,建立了计算模型,并通过与实验结果的比较,验证了计算模型的有效性。在此基础上,考察了弹丸直径、撞击速度及气体介质压力对碎片云与气体介质相互作用的影响。研究结果表明:碎片云的尖端速度及径向扩展速度均随着弹丸直径的增加而增加,气体冲击波强度随着弹丸直径的增加而增强;随着撞击速度及气体介质压力的增加,碎片云尖端减速运动的加速度及径向减速运动的加速度均增大,气体冲击波强度随着撞击速度及气体介质压力的增加而增强,并且衰减速度增大。     

Velocity Fluctuations in a Particle-Laden Turbulent Flow over a Backward-Facing Step

Dilute gas-particle turbulent flow over a backward-facing step is numerically simulated. Large Eddy Simulation (LES) is used for the continuous phase and a Lagrangian trajectory method is adopted for the particle phase. Four typical locations in the flow field are chosen to investigate the two-phase velocity fluctuations. Time-series velocities of the gas phase with particles of different sizes are obtained. Velocity of the small particles is found to be similar to that of the gas phase, while high frequency noise exists in the velocity of the large particles. While the mean and rms velocities of the gas phase and small particles are correlated, the rms velocities of large particles have no correlation with the gas phase. The frequency spectrum of the velocity of the gas phase and the small particle phase show the -5/3 decay for higher wave number, as expected in a turbulent flow. However, there is a "rising tail' in the high frequency end of the spectrum for larger particles. It is shown that large particles behave differently in the flow field, while small particles behave similarly and dominated by the local gas phase flow.     

Projectile density, impact angle and energy effects on hypervelocity impact damage to carbon fibre/peek composites

This paper explores the effects of projectile density, impact angle and energy on the damage produced by hypervelocity impacts on carbon fibre/PEEK composites. Tests were performed using the light gas gun facilities at the University of Kent at Canterbury, UK, and the NASA Johnson Space Center two-stage light gas gun facilities at Rice University in Houston, Texas. Various density spherical projectiles impacted AS4/PEEK composite laminates at velocities ranging from 2.71 to 7.14 km/s. In addition, a series of tests with constant size aluminum projectiles (1.5 mm in diameter) impacting composite targets at velocities of 3, 4, 5 and 6 km/s was undertaken at incident angles of 0, 30 and 45 degrees. Similar tests were also performed with 2 mm aluminum projectiles impacting at a velocity of approximately 6 km/s. The damage to the composite was shown to be independent of projectile density; however, debris cloud damage patterns varied with particle density. It was also found that the entry crater diameters were more dependent upon the impact velocity and the projectile diameter than the impact angle. The extent of the primary damage on the witness plates for the normal incidence impacts was shown to increase with impact velocity, hence energy. A series of tests exploring the shielding effect on the witness plate showed that a stand-off layer of Nextel fabric was very effective at breaking up the impacting debris cloud, with the level of protection increasing with a non-zero stand-off distance.     

On the hydrodynamic approximation for long-rod penetration

Steady-state hydrodynamic theory, or variations thereof, has been applied to long-rod penetration since the 1940s. It is generally believed that projectile strength is of little consequence at high velocities, and that hydrodynamic theory is applicable to long-rod penetration when penetration pressures are much greater than the target flow stress. Substantiating this belief is the observation that at approximately 2.5 km/s, for tungsten alloy projectiles into armor steel, normalized penetration (P/L) nominally saturates to the classical hydrodynamic limit of the square root of the ratio of the projectile to target densities. Experimental data herein, however, show penetration velocities and instantaneous penetration efficiencies fall below that expected from hydrodynamic theory, even at impact velocities as high as 4.0 km/s. Numerical simulations, using appropriate strength values, are in excellent agreement with the experimental data. Parametric studies demonstrate that both projectile and target strength have a measurable effect even at such high impact velocities.     

Current scaling of projectile velocities with and without asnowplow effect in electromagnetic launchers

Scaling laws are studied, with and without a snowplow (SP) effect, for macroparticle projectile velocities in electromagnetic launchers (EMLs) as a function of the peak EML current. Analytical formulae to predict the instantaneous position and velocity have been derived for both phases of rising and constant EML currents. The SP effect altered the scaling law of the short-pulse muzzle velocity (ν_xw), but not of the long-pulse muzzle velocity (ν_xm). The absolute values of both ν_xw and ν_xm are, however, seriously reduced in the SP regime, depending on the newly introduced parameter, ∈, which is a mass ratio of the armature/projectile system to the gas in the barrel. The critical ∈ s with which the SP effect alters the muzzle velocity and time by more than 10% are presented. Numerical examples are given for possible applications of EMLs on calibrated impact damage testers and impact thermonuclear fusion, where short (1 m) and long (1 km) barrels are to be used for projectiles of several grams to achieve the maximum velocities of 1 and 130 km/s, respectively     

Hypervelocity acceleration and impact experiments with the LLNL electric guns

In the electric gun, the explosion of an electrically heated metal foil and the accompanying magnetic forces are used to accelerate a thin flyer plate to velocities as high as 18 km/s. Here we report preliminary results of a study to extend this capability to the acceleration of projectiles or particles ranging in size from micrometeorites to chunky projectiles with a mass as high as 0.5 g. We also have started code calculations of projectile impacts on thick aluminum witness plates for comparison with observations of experimentally produced cratering.     

Deformation characteristics of low carbon steel subjected to dynamic impact loading

The effects of impact loading on changes in microstructure have been studied in low carbon steel. Low to moderate shock loading tests have been carried out on steel specimens using a single stage gas gun with projectile velocities ranging from 200 to 500 m/s. Stress history at the back face of the target specimen and projectile velocity prior to impact were recorded via manganin stress gauges and velocity lasers, respectively. A Johnson-Cook constitutive material model was employed to numerically simulate the material behavior of low carbon steel during impact and obtain the particle velocity at the impact surface as well the pressure distribution across the specimens as a function of impact duration. An analytical approach was used to determine the twin volume fraction as a function of blast loading. The amount of twinning within the α-ferrite phase was measured in post-impact specimens. A comparison between experimental and numerical stress histories, and analytical and experimental twin volume fraction were used to optimize the material and deformation models and establish a correlation between impact pressure and deformation response of the steel under examination. Strain rate controlled tensile tests were carried out on post-impact specimens. Results of these tests are discussed in relation to the effects of impact loading on the yield and ultimate tensile strength as well as the hardening and strain energy characteristics.     

Hypervelocity impact testing using an electromagnetic railgun launcher

An electromagnetic (EM) railgun launcher facility has been developed to routinely conduct hypervelocity impact tests. Two types of completely reusable EM launchers have launched sabot/impactor packages between 2 and 43 grams to velocities between 1.5 and 8.5 km/s. The highly reliable railguns have conducted over 250 projectile launchings and have established a projectile/launcer data base covering interior as well as exterior ballistic considerations. A conventional type instrumented ballistics range is compatible with the EM launcher and can be used to conduct anti-armor and lethality experiments at hypervelocities.     

The development of a projectile velocity measuring system for the RARDE No. 2 Hypervelocity Launcher

This paper outlines the principle of a projectile velocity measuring system fitted to the RARDE No. 2 Hypervelocity Launcher. The system is capable of measuring the velocity of small particles, (1g), travelling at velocities in excess of 6000m/s.     

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.     

High velocity penetration of homogeneous, segmented and telescopic projectiles into alumina targets

Segmented and telescopic projectiles are designed to make efficient use of the higher impact velocities achievable with new acceleration techniques. This concept has been found to work against steel armour. In this study, we compare the penetration capability into an alumina target for these unconventional projectiles with that of a homogeneous projectile. The influence of segment separation distance and core-to-tube diameter ratio were simulated for the impact velocities 2.5, 3.0 and 3.5 km/s. The simulated final penetrations are compared to test results for one type of each of the homogeneous, segmented and telescopic projectiles at 2.5 and 3.0 km/s. Both simulations and tests show that the unconventional projectiles have better penetration capability than a homogeneous projectile with the same initial geometry.