Critical velocity for rails in hypervelocity launchers

This paper is related to the dynamic and strength analysis and optimized design of hypervelocity electromagnetic launchers. Projectile motion along the rails at critical velocity is associated with damaging resonant regimes. These regimes reveal increased displacements and stress that can lead to failure of a launcher. To calculate critical velocity and to visualize dynamic deformations of the launcher we have developed two alternative approaches utilizing analytical and finite element models. The first approach employs a closed form analytical solution for critical velocity that is based on the Bernoulli–Euler model of a beam resting on an elastic foundation and subjected to a moving load. The critical velocity is expressed as a function of geometric and material parameters of the rail and equivalent stiffness of the supporting structure. The stiffness of the supporting structure is found from a 2D finite element model. In the second approach, we employed beam finite element and 3D solid finite element models to visualize and measure the “natural” and “forced” bending waves traveling along the rails. These approaches helped to better understand the transient resonant dynamic processes and offered insight on how to alter the launching device materials and geometry to reduce the critical-velocity effects.     

Elastic waves and solid armature contact pressure in electromagnetic launchers

Electromagnetic launchers suffer a phenomenon referred to as armature transitioning: when the armature and rails suddenly lose contact with each other, damage can occur to the armature and the rails of the launcher. In this paper, we explore transient elastic waves as a possible explanation for the transitioning of solid armatures in electromagnetic launchers. We use a finite-element code to model the transient dynamics of a typical electromagnetic launcher guide rail. We found that dynamic rail deflections caused by the movement of the armature are similar in magnitude to those caused by the magnetic field, and that the contact pressure between the armature and the rails changes dramatically when the speed of the armature reaches the critical velocity of the rails.     

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.     

Development of a scramaccelerator based hypervelocity launcher

The Scramaccelerator, a novel type of supersonic-combustion, tube-based launcher has been developed that can accelerate projectiles to velocities of 3 to over 7 km/sec. Extremely flexible in application, the Scramaccelerator could launch impact specimens, wind tunnel specimens, projectiles, satellites, or spacecraft. This paper describes the technology demonstration of the concept by firing 120 gram projectiles into a 38 mm barrel at 2.8 to 3.2 km/sec at the Titan/CRT Impact Research Laboratory in Albuquerque. This technology promises an upward scalability beyond that of any conventional ballistic guns and electromagnetic launchers for high mass hypervelocity applications. It is the objective of this program to demonstrate the practical application of detonation physics to hypervelocity launchers. Critical test issues discussed include sabot seperation, venting requirements, Scramaccelerator tube requirements, and test performance. The current data indicate projectile accelerations were achieved in excess of 5,000 g's. Hence, these tests finally demonstrate that oblique detonation/supersonic combustion can be harnessed as a useful mechanism for hypervelocity propulsion. In addition, these tests demonstrate hypersonic propulsion at Mach numbers above 9, acceleration at greater than 3 kilometers per second, and system integration technology sufficient to accomplish this success. Scalability of the device allows for the hypervelocity launch of large masses.     

The use of hypervelocity launchers to explore previously inaccessible states of matter

The dynamic response of materials at very high pressure and temperature is important in a wide variety of scientific and programmatic applications. Thermodynamic regions of interest include the high-pressure response of condensed phases and the highly expanded states of materials. Since these regimes are normally studied with flat-plate impact techniques, it is necessary that projectile velocities achievable with highvelocity launchers be sufficient to access the desired thermodynamic regions. In this paper, we discuss the equation-of-state regions that would be accessible with a plate impact capability of 15 km/s and summarize the status of a hypervelocity launcher under development which will provide the required velocity capability.     

Historical overview of hypervelocity impact diagnostic technology

The ability to understand hypervelocity impact phenomena and to validate predictive models of material behavior is largely dependent on the diagnostic tools available to the experimenter. These tools range from simple, post-impact examination of targets to extremely sophisticated and complex techniques, which simultaneously measure a myriad of impact parameters on a time scale of nanoseconds to milliseconds. This wide range of available techniques represents the challenge and opportunity if hypervelocity impact experimentation.

There has been a continual challenge to develop diagnostic techniques with ever-increasing resolution, as higher velocities and pressures are achieved. Today's techniques provide the experimenter the means of measuring most hypervelocity impact parameters, including velocity, displacement, temperature, radiation, volumetric change, impluse, stress, and strain of the materials involved. However, the prospective quantum leap in impact velocities to be produced by electromagnetic and electrothermal launchers will require corresponding advances in diagnostic systems.

This paper examines the capabilities and limitations of the major diagnostic techniques for hypervelocity impact experimentation and traces their evolvement as useful laboratory tools.     

Hypervelocity impact on CFRP: Testing,material modelling,and numerical simulation

This paper describes the derivation and validation of a numerical material model that predicts the highly dynamic behaviour of CFRP (carbon fibre reinforced plastic) under hypervelocity impact. CFRP is widely used in satellites as face sheet material in CFRP-Al/HC sandwich structures (HC = honeycomb) that can be exposed to space debris. A review of CFRP-Al/HC structures typically used in space was performed. Based on this review, a representative structure in terms of materials and geometry was selected for study in the work described here. An experimental procedure for the characterisation of composite materials is documented by Riedel et al. [ADAMMO – advanced material damage models for numerical simulation codes. ESA CR(P) 4397, EMI report I 75/03, Freiburg; October 31, 2003.]. The test results from the CFRP of the current study allow for the derivation of an experimentally based orthotropic continuum material model data set that is capable of predicting the mechanical behaviour of CFRP under hypervelocity impact. Such a data set was not previously available. In the work by Riedel et al. [Hypervelocity impact damage prediction in composites: part II – experimental investigations and simulations. International Journal of Impact Engineering, 2006;33:670–80.] an orthotropic material data set was used for modelling HVI on AFRP (aramid fibre reinforced plastic), which shows relatively high deformability before failure. The enhancements of the modelling approaches in previous studies [Riedel W, Harwick W, White DM, Clegg RA. ADAMMO – advanced material damage models for numerical simulation codes. ESA CR(P) 4397, EMI report I 75/03, Freiburg; October 31, 2003. Hiermaier S, Riedel W, Hayhurst C, Clegg RA, Wentzel C. AMMHIS – advanced material models for hypervelocity impact simulations. Final report, EMI report E 43/98, ESA CR(P) 4305, Freiburg; July 30, 1999.] necessary to model brittle CFRP are specified. An experimental hypervelocity impact campaign was performed at two different two-stage light gas guns which encompassed both normal and oblique impacts for a range of impact velocities and projectile diameters. Validation of the numerical model is provided through comparison with the experimental results. For that purpose measurements of the visible damage of the face sheets and of the HC core are conducted. In addition, the numerically predicted damage within the CFRP is compared to the delamination areas found in ultrasonic scans.     

Stress analysis of the rails of a new high velocity armature design in an electromagnetic launcher

In an electromagnetic launcher, the magnetic field creates a dynamic force that moves the armature forward. In an electromagnetic launcher, the armature reaches a critical velocity during the launch which causes high amplitude stress and strain. In addition, high stress and strain damage the rails and reduces its life span. The purpose of this paper is to investigate the effect of armature velocity as well as the rails physical and geometrical properties on the dynamic response of the rails in an electromagnetic launcher. In this study the second moment of inertia of the rails cross-section, Young modulus, foundation stiffness and density of the rails are constant in location and time. In our formulation of governing non-linear differential equations, Maxwell equations and deflection equation are applied to the rails under dynamic loading. To solve the non-linear governing differential equations a finite difference method is utilized.     

Hypervelocity macroparticle accelerator experiments at CEM-UT

Several railgun experiments designed to accelerate projectile masses of 2 to 5 g to velocities greater than 6 km/s were performed. Two parallel rail-type accelerators with 12.7 mm square bores were used for the experiments. One gun is 2 m long, has molybdenum rails and alumina ceramic insulators. The other is 1 m long, has molybdenum rails and granite insulators. The greatest velocity achieved to date during the experiments was 5.1 km/s. During the test program, the following ideas to enhance launcher performance were tested: stiff-gun structures to reduce plasma leakage and rail movement, refactory bore materials to reduce ablation and frictional losses, and prefilling the gun bore with gases which will eliminate precursor arcs. After three experiments utilizing the 2 m long launcher, with peak currents ranging from 660 to 780 kA, a gun barrel comprised of 96% pure alumina ceramic insulators and 99.9% pure molybdenum rails has survived with minimal damage and no degradation of seals     

High velocity linear induction launchers

Electromagnetic launchers (EMLs) have received great attention in the last decades because of their potential application to a variety of energy, transportation, space, and defense systems. Particularly, they can serve as kinetic weapons, such as ground-based and naval artillery, space-based anti-missile guns, Earth-to-Orbit launcher, and mass transportation. The main advantage is that EMLs can accelerate projectiles to hyper velocities, i.e. velocities greater than those achievable with conventional cannons. The Linear Induction Launcher (LIL) is an air-cored electromagnetic coil launcher operating on the principle of the induction motor. Polyphase excitation of the coils constituting the barrel is designed to create an electromagnetic wave packet, which travels with increasing velocity from the breech to the muzzle. The projectile is a hollow conducting cylinder (sleeve) carrying the payload within it. Relative motion (slip) of the wave packet with respect to the projectile induces azimuthal currents in the sleeve that interacts with the exciting magnetic field to produce both propulsive and centering forces. This paper deals with the design of a high velocity linear induction launcher with muzzle velocity up to 6000 m/s. It addresses the design specifications of the launcher and utilizing a projectile weighing 1 kg. In the paper, the design specifications with simulation results for the phase voltages, the currents, the velocity, and the temperature rise of the sleeve are presented.     

Effect of a high energy electrical pulse on the dynamic stress-strain behavior of a copper alloy

Electromagnetic launchers (EML) dispatch projectiles at extreme velocities (Mach 7-8), using copper/copper alloy rails which are subjected to high rates of loading under a high energy electrical pulse. Results from Split Hopkinson Pressure Bar (SHPB) testing of a copper alloy at high strain rates in the order of 10³ s^− 1 with simultaneous application of a high energy electrical pulse show that the plastic deformation of the copper alloy is increased and a higher degree of work hardening is observed under these conditions.     

Development of a three-stage, light-gas gun at the University of Dayton Research Institute

An elusive goal of the hypervelocity impact community has been the evaluation of the ballistic response of space hardware to impact velocities ranging from 8 to 11 km/s using projectiles with known properties. The design, development, and use, during the 1960s, of a three-stage, light-gas gun at McGill University is reviewed. The developers of this gun claim that they were able to launch cylindrical, 12.7-mm-diameter Lexan disks with masses of 1.5 and 1.1 g to velocities of 9.6 and 10.5 km/s, respectively. This paper presents the results of an internally funded program at the University of Dayton Research Institute (UDRI) to duplicate the published performance of the McGill University launcher. A support structure and various components of a third stage which used an 8.1-mm-diameter launch tube were added to the UDRI 75/30-mm, two-stage, light-gas gun, making the arrangement of the components similar to the one used by McGill University. Work on the development of the UDRI three-stage, light-gas gun is a continuing effort, with the goal of successfully launching small diameter (3 mm or less) aluminum spheres to velocities in excess of 9 km/s. To date, the highest projectile velocity achieved with the UDRI three-stage, light-gas gun has been 8.65 km/s.     

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     

Hypervelocity research and the growing problem of space debris

Man-made orbital debris has increased in number so that it poses a potential barrier to the exploration of space. The ever-increasing number of objects in space has created an increasing hazard to all spacecraft, including manned shuttles, unmanned satellites, and manned space stations. Although international efforts are underway to reduce the proliferation of space debris, the number of objects continues to climb.

The majority of debris tracked by earth observation is classed either as ‘operational debris’ (spent boosters and satellites, discarded hardware from manned flight, etc.) or as “fragmentation debris” (debris created by explosions aboard boosters or satellites or by impacts between objects in orbit). While there is considerable information available about operational debris, statistics on fragmentation debris are more suspect, since it is difficult to predict with any accuracy the fragments resulting from an explosion or impact on a space structure.

As realization of the importance of the problem grows, the hypervelocity launcher and impact communities are becoming increasingly involved. This paper defines the major problems to be solved and outlines the requirements for launchers, diagnostics, and modeling. A bew U.S. space program to model and the fragmentation of satellites impacted by space debris described. The results of tests against actual satellites are discribed in terms of their importance to the modeling effort.     

A hypervelocity projectile launcher for well perforation

Current oil well perforation techniques use low- to medium-velocity gun launchers for completing wells in soft rock. Shaped-charge jets are normally used in harder, more competent rock. A launcher for a hypervelocity projectile to be used in well perforation applications has been designed. This launcher will provide an alternative technique to be used when the conventional devices do not yield the maximum well perforation. It is an adaptation of the axial cavity in a high explosive (HE) annulus design, with the axial cavity being filled with a low density foam material. Two configurations were tested; both had an HE annulus filled with organic foam, one had a projectile. Comparison of the two shots was made. A time sequence of Image Intensifier Camera photographs and sequential, orthogonal flash x-ray radiographs provided information on the propagation of the foam fragments, the first shock wave disturbance, the projectile motion and deformation, and the direct shock wave transmission from the main HE charge. Perforation tests of both device configurations (with and without the pellet) into steel-jacketed sandstone cylinders were made. Static radiographs of the cavities in the sandstone showed similar cavities, however, the perforation of the steel cap was larger in response to the pellet. DYNA2D calculations were made to assist in the interpretation of the experimental records. The preliminary results show promise that a useful perforating tool can be developed. Plans for an extended experimental program are outlined.     

Predicting bore deformations and launcher stresses in railguns

The structural responses of launchers are important because they affect the projectile performance and the operating limits of the railgun system. Structural analysis makes it possible to make better decisions in launcher design. Example analyses of the Los Alamos HIMASS and Lethality Test System launchers are presented in this paper. Also, a discussion of the benefits and limitations of these analyses is included.     

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.     

Thunderbolt [railguns]

The Thunderbolt program facility will provide 60 MJ of energy at 21 kV from capacitor banks for powering breech-fed or distributed energy railgun electromagnetic launchers. After discharge from the capacitor banks, the energy pulse is shaped with room-temperature solenoidal inductors, with switching being provided by D and E-sized ignitrons. Fast recharging of the capacitor banks could permit repetitive operation. A screened room provides protection for the control and data acquisition equipment, while a single-point ground scheme protects against ground loops. A substantial foundation provides support for the catch tank, while high-vacuum equipment will permit operation at high velocities with the round-bore railgun. Preinjection of the test projectiles to velocities of about 1 km/s is provided by a helium gas gun to minimize damage to the breech section of the railgun. In addition to providing details on Thunderbolt, operating experience and results obtained with a subscale launcher are described     

EMET technology for rail launchers

The EMET concept is a marriage of electrothermal plasma jet technology with rail accelerators using plasma armatures. By injecting a structured plasma immediately behind a moving projectile prior to the current pulse, the plasma armature properties can be highly controlled. Parameters of interest are the armature mass and length, molecular weight, specific heat ratio gamma, and temperature. Proper control of these parameters leads to control of problems facing rail launchers such as wall ablation and viscous wall drag. In support of EMET, a Material Test Facility (MTF) has been developed for performing basic physics and materials research on hypervelocity launchers, by making direct measurements of the plasma pressure and jet velocity in a 1 cm bore. These measurements are then compared with theoretical models for various types of plasmas, in order to understand and eliminate barrel ablation. The paper discusses measurement techniques used on MTF, and the approaches being taken to develop EMET in the laboratory.