Sabot Discard Research Articles

A Priori Information Based Time-Resolved 3D Analysis of the Trajectory and Spatial Orientation of Fast-Moving Objects Using High-Speed Flash X-ray Imaging.

This paper shows that the X-ray analysis method known from the medical field, using a priori information, can provide a lot more information than the common analysis for high-speed experiments. Via spatial registration of known 3D shapes with the help of 2D X-ray images, it is possible to derive the spatial position and orientation of the examined parts. The method was demonstrated on the example of the sabot discard of a subcaliber projectile. The velocity of the examined object amounts up to 1600 m/s. As a priori information, the geometry of the experimental setup and the shape of the projectile and sabot parts were used. The setup includes four different positions or points in time to examine the behavior over time. It was possible to place the parts within a spatial accuracy of 0.85 mm (standard deviation), respectively 1.7 mm for 95% of the errors within this range. The error is mainly influenced by the accuracy of the experimental setup and the tagging of the feature points on the X-ray images.

Sabot Discard Characteristics under Different Spin Rates of the Rifled Barrel Launching APFSDS

The sabot discard asymmetry caused by spinning affects the exterior ballistic characteristics and shooting accuracy of a gun with the rifled barrel. To gain a deeper understanding of the complex sabot discard performance for the armor-piercing, fin-stabilized discarding sabot (APFSDS), a numerical investigation is performed to assess the effects of the spin rate on the sabot discard characteristics. We obtain the calculation boundary by the interior ballistics and the firing conditions and carry out a numerical simulation under different spin rates using computational fluid dynamics (CFD) and a dynamic mesh technique. We analyze four aspects of sabot discard characteristics, namely, sabot separation, rod surface pressure, rod aerodynamic parameters, and discarding quantization parameters. Computational results show that the sabot separation nearly presents perfect symmetry at 0 rad/s, and when the initial rate of the sabot increases, there is more obvious separation asymmetry, and it contributes to the relative position variation among the sabots and the rod. The distinction of rod surface pressure indicates that the choked flow is the strongest flow source, and the spin rate has almost no effect on the pressure of the rod front part. When the monitoring point moves towards the fins, the pressure distribution and intensity change more dramatically. The initial spin rate and separation asymmetry produce a variation in the surface pressure, which further influences the rod aerodynamic characteristics. The discarding quantization parameters exhibit a certain variation rule with its spin rate. 2,000 rad/s has a significant influence on the rod aerodynamic coefficients during the weak coupling phase. When the spin rate is in the range of 0–900 rad/s, the discarding characteristics remain the same. However, when the spin rate exceeds 900 rad/s, the separation time and aerodynamic impulse have a quadratic polynomial relationship with the rate. Additionally, a spin rate of 1,000 rad/s is the optimal value for a rifled barrel gun.

Behavior of a Railgun Launch Package at the Muzzle and During Sabot Discard

Projectiles launched with ISL’s PEGASUS railgun achieve muzzle velocities well beyond 2000 m/s. The design of the corresponding 1.3-kg launch package of caliber $40\times 40$ mm2 was presented at the 18th EML-Symposium in 2016. Meanwhile, the phase after muzzle exit has been studied, and in particular, the sabot separation in the air flow has been investigated. To that end, a numerical model was used which had been previously validated by means of shock tunnel experiments. Besides the subprojectile, the launch-package essentially consists of the armature, on which the Lorentz force is acting, and further, of two sabot parts with integrated bore riders. The numerical model describes the launch-package flight in moving coordinates in the shot axis direction, i.e., the package is considered to be initially fixed in space, surrounded by a constant velocity air flow. The launch package components are able to move and to deform in the three dimensions thereafter. It is further possible to introduce lateral perturbation, e.g., linear and angular motion due to the muzzle disturbance. Although the airflow is modeled with a Eulerian code neglecting viscous effects, the computation results agree very well with the experiments. Hereby, it is demonstrated that the numerical model can effectively be used as a tool for launch package design optimization. An example is given for a fin-stabilized projectile which shall be launched with the new 6-m-long NGL60 railgun having a bore cross section of $60\times 60$ mm2.

Numerical investigations on the sabots discard process of an APFSDS at different angles of attack

Sabot asymmetric discard after the projectile being launched from the muzzle at various angles of attack and 4Ma is investigated. This is implemented by the coupling computational fluid dynamics (CFD) and six degrees-of-freedom exterior ballistic code through the unstructured dynamic mesh and user defined function. The flow field characteristics during sabot discard process and the trajectory parameters of all three sabots have been obtained. In addition, the aerodynamic coefficients of the projectile are also obtained. The numerical results show that the asymmetric discard of sabot is more obvious along with the increasing angle of attack. Moreover, the aerodynamic forces of projectile have a larger change and the pressure distribution of its surface is more asymmetric and complex. This means the aerodynamic interference at a non-zero angle of attack contributes more significantly to shooting dispersion and flight stability than that at zero angle of attack and the influence increases as the angle of attack increases.

Simulation of Sabot Discard for Electromagnetic Launch Integrated Projectile

The process of the sabot discard of an electromagnetic integrated launched projectile (EMILP) has a great effect on the operational effectiveness of integrated launched projectile (ILP). Aiming at the characteristics of high-velocity discard, strong muzzle magnetic field of the EMILP, a coupled model of magnetic field and flow field for simulating the EMILP sabot discard has been established by using the harmonic analysis method, time–frequency analysis method and fitting interpolation method to gain the time-varying functions of electromagnetic force and torque, which serve as the external force and moment of six degrees of freedom (SDOF) movement, and meanwhile, by using the dynamic mesh technique to couple the fluid flow control equations and the SDOF movement equations. Taking for example, the EMILP named IAT-HVP designed by the Institute of Advanced Technology (IAT), University of Texas, Austin, TX, USA, this paper makes a simulation analysis of the sabot discard of ILP at a different angle of attack, and makes a comparison between it and test data. Simulation results show that the model proposed in this paper is high precision and effective.

Numerical investigation of the three-dimensional dynamic process of sabot discard

The sabot discard process of an armor-piercing, fin-stabilized discarding sabot (APFSDS) is crucial for the flight stability of the projectile. In this paper, the sabot discard behavior after projectile ejection from the muzzle is investigated at Mach number 4.0 and angle of attack of 0°. 3D compressible equations implemented with a dynamic unstructured tetrahedral mesh are numerically solved with a commercial computational fluid dynamics (CFD) code (FLUENT 12.0). Six-degrees-of-freedom (6DOF) rigid-body motion equations is solved with the CFD results through a user-defined function to update the sabot trajectory at every time step. A combination of springbased smoothing and local re-meshing is employed to regenerate the meshes around the sabot and describe its movement at each time step. Computational results show three different separation processes during the sabot discard process. Furthermore, the aerodynamic forces of APFSDS are calculated, and the trajectories of the three sabots are illustrated through the numerical solution of 6DOF equations. The results of the present study agree well with typical experimental results and provide detailed parameters that are important for analyzing the stability of the projectile. The present computations confirm that the numerical solution of the governing equations of aerodynamics and 6DOF rigid-body equations are a feasible method to study the sabot discard processes of APFSDS.

Computational Fluid Dynamics and Experimental Validations of the Direct Coupling Between Interior, Intermediate and Exterior Ballistics Using the Euler Equations

For several years we have been working on the development of a computational fluid dynamics ballistics code called FREIN. This code is the result of a strong cooperation between Nexter Munitions and the University of Poitiers. In the last years, efforts have been carried out to improve the 3D modeling. In a fully unsteady way, the interior, intermediate and exterior ballistics were modeled as well as the weapon system environment. The complex phenomena encountered are investigated by an adapted numerical simulation approach using the Euler equations for two immiscible gases. The method involves moving bodies with respect to fixed Cartesian meshes and the aerodynamic forces are used to compute the trajectories. In this paper, theoretical developments and computations have been applied mainly to the simulation of the firing of an advanced 120 mm lightweight tank demonstrator. In comparison with firing experiments, first computation validation results concerning interior ballistics, muzzle brake flow, sabot discard and blast wave propagation and reflection are presented and are very satisfactory.

Effect of electromagnetic launch on armature/sabot discard

This paper presents the validation of a sabot discard model, developed at the Institute for Advanced Technology (IAT), for predicting armature/sabot separation, and quantifies the effect of electromagnetic launch on sabot discard as well as on the sub-projectile. The experimental data used to validate the model were obtained at the Army Research Laboratory (ARL) Transonic Experimental Facility at Aberdeen Proving Ground, MD. Five shots were conducted at a nominal velocity of 1350 m/s and armature/sabot petal trajectories were reduced from X-ray images taken at six downrange stations. The residual electromagnetic force on the launch package after muzzle exit was also determined from experimental data. The sabot discard model uses analytical expressions to estimate surface pressure distributions, which are then integrated, in a time-marching algorithm, for linear and angular accelerations, and subsequently for sabot petal trajectories. The most significant attribute of this model is that it does not rely on any empirical data for modeling and is therefore expected to be widely applicable. The results from this study have shown that the comparison between predicted and measured armature/sabot petal trajectories is excellent. Furthermore, the model has demonstrated quantitatively the delayed armature/sabot discard caused by the residual electromagnetic force and the additional deceleration of the sub-projectile due to the delayed mechanical disengagement.

Electromagnetic and Aeromechanical Analysis of Sabot Discard for Railgun Projectiles

Experimentaland engineering modelingtechniqueswereapplied toexaminetherolethatelectromagneticsplays duringthesabotdiscardprocessina solid-armaturerailgun.Currentandmagnetice elddiffusionintheconducting armature and sabot produce residual forces that tend to clamp the sabot petals during in-bore acceleration and after launch. The effect of this force on sabot discard was investigated. The experimental portion of this work used the measured downrange magnetic e elds to quantify the effect on subprojectile performance. The time rate of change of launcher and switch currents and launcher voltages were measured. In addition, the time rate of change of the armature magnetic e eld was measured downrange from the launcher. Electrical parameters that characterize the magnetic e elds were used to calculate the residual current e owing in the armature after exit. An engineeringmodelthatincludesbothaeromechanicalandelectromagneticforceswasdevelopedtopredictthesabot discard process. The model for the magnetic clamping force used the inferred current from the magnetic e elds measured downrange. The results from the sabot discard analysis are in good agreement with the measured sabot trajectory data, particularly when the sabot petals arein close proximity. The analysisshows thattheaerodynamic moment acting on the sabot is relatively small following launch, and small magnetic clamping forces could delay sabot separation.

Effect of railgun electrodynamics on armature discard

An experiment was conducted to examine the role that electromagnetics plays during the sabot discard process. This investigation is less rigorous than an accuracy test but uses a number of performance indicators, including measured downrange magnetic fields, to quantify the effect on subprojectile performance. The test program consisted of two sets of shots with different armature current profiles. A muzzle switch, which closed prior to exit, was employed to tailor the current and armature acceleration profiles. The exit velocity was 1100 m/s for both profiles so as to maintain the same aerodynamic pressure loads during armature discard. The time rate of change of launcher and switch currents and launcher voltages were measured. In addition, the time rate of change of the armature magnetic field was measured downrange from the launcher. The residual magnetic field in the armature after exit produced no observable effect on the sabot discard process.

Computational modeling of multibody aerodynamic interference

The research effort has focused on the development and application of a versatile overset grid numerical technique to solve multibody aerodynamic problems. The developed numerical capability has been used successfully to determine the aerodynamics and provide critical information on the design and development of two multibody systems: sabot discard from the kinetic-energy (KE) projectile and brilliant anti-armor (BAT) submunition dispersal from the Army Tactical Missile System (TACMS). The complexity and uniqueness of these types of multibody problems result from the aerodynamic interference of the individual components, which include three-dimensional (3-D) shock–shock interactions, shock–boundary layer interactions, and highly viscous-dominated separated-flow regions. The overset grid technique, which is ideally suited to this problem, is used to determine the 3-D interacting flow field of the multibody system and the associated aerodynamic forces and moments at different positions and orientations without the need for costly regridding. The complex physics and fluid dynamics structure of the 3-D aerodynamic interference for these multibody problems has been uniquely identified.

Pressure Distribution on Sabots in Hypervelocity Flight

Experimental and computational investigations of thepressure distribution on sabotsin hypervelocity e ight are performed to investigate the accuracy of a computational model previously developed for predicting sabot discard trajectories. The pressure distribution on the sabot front scoop and underside are measured experimentally and compared to results predicted by the modie ed numerical code. The results of this study indicate that although, overall, the modie ed code does a reasonable job of predicting sabot trajectories, further ree nement to the pressure modeling in the code may be needed. Specie cally, ree nement to the sabot front scoop pressure model is indicated. Additionally, a simple analytical model developed to help verify the trends predicted by the code is shown to produce accurate results, especially considering the simplicity of the model.

Large calibre armature firings at Green Farm electric gun test facility

The DNA/Maxwell Laboratories SSG 90 mm Rail Gun has been used to fire a number of solid armature projectiles in conjunction with the United Kingdom, (UK) Defence Research Agency, (DRA). These projectiles have been fully tactical, discarding sabots rounds fired at short range into RHA steel targets. This paper gives details of those shots, the diagnostics used to capture the data and some of the design methodology behind the test. Visual recordings have been assisted in the analysis of the results, showing good sabot discard and flight characteristics. Velocities of over 2000 m/s have been achieved with fully discarding tactical packages and typical accelerating lengths. The tests have shown for a limited sample size, that EM projectiles can be launched as accurately as ones from a conventional powder gun, and that the differing launch conditions do not pose a major problem to the shot designers.

Hypervelocity launch dynamics

The launch dynamics from a generic hypervelocity cannon are considered over the velocity range from 1500 through 3500 m/s. Both fin and flare stabilized projectiles are examined for the influence of in-bore dynamics, muzzle blast, sabot discard, and free flight aerodynamics upon their trajectory. Computations are performed using codes developed and validated for conventional cannon with ordnance muzzle velocities (approximately 1700 m/s). While the extension into the hypervelocity regime is not supported by experimental data, this initial study is intended to aid in defining potential problem areas.

Influence of muzzle brakes upon the trajectory of fin-stabilized projectiles

Abstract : The dispersion of groups of fin-stabilized projectiles fired from high velocity cannon was measured for cases with and without a muzzle brake in place. The data clearly show degradation in precision due to the presence of the device. Diagnostic experiments and analyses were performed which indicate that the difficulties arise mainly due to gasdynamic loads within the muzzle device influencing both the projectile motion and sabot discard. Muzzle device design modifications are suggested to solve the problems. (Author)

Aerodynamics of asymmetric sabot discard

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Disturbance to the Launch of Fin-Stabilized Projectiles

The sabot discard from a gun launched, fin-stabilized projectile is examined from the time of shot ejection until the round enters undisturbed flight. Trajectory perturbations due to muzzle blast, fin shadowing, mechanical contact, and aerodynamic interaction are evaluated and a relative ranking is determined. Analysis shows that muzzle blast and fin shadowing result in negligible changes to the launch angular momentum of the projectile; however, mechanical contact with sabot components markedly alters both the projectile momentum and the subsequent trajectory of the sabot sections. The resultant geometric asymmetry in the pattern of these discarding sections is shown to cause strong aerodynamic interaction between the sabot flowfields and the projectile. For the case investigated, aerodynamic interaction was determined to be the major source of postseparation trajectory perturbation.

Aerodynamic Interference during Sabot Discard

Abstract : An experimental program has been conducted to investigate disturbances to the trajectories of gun-launched, fin-stabilized projectiles during the sabot discard process. Correlated near muzzle and down range measurements of sabot component and projectile motion are acquired using both orthogonal, flash X-rays and a ballistics range. Apparent anomalous yawing motion of projectiles correlates with asymmetries in the sabot discard geometry. The perturbations to the flow around the projectile due to these asymmetries are calculated and shown to be of sufficient magnitude to produce significant alterations in the desired trajectory. For the rounds tested, the impulse due to aerodynamic interaction during sabot discard is of the same magnitude as the transverse impulse imparted to the projectile during in-bore acceleration.