Electromagnetic and Aeromechanical Analysis of Sabot Discard for Railgun Projectiles

Abstract

Experimental and engineering modeling techniques were applied to examine the role that electromagnetics plays during the sabot discard process in a solid-armature railgun. Current and magnetic eld diffusion in the conducting 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 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 eld was measured downrange from the launcher. Electrical parameters that characterize the magnetic elds were used to calculate the residual current owing in the armature after exit. An engineering model that includes both aeromechanicaland electromagnetic forces was developed to predict the sabot discard process. The model for the magnetic clamping force used the inferred current from the magnetic 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 are in close proximity. The analysis shows that the aerodynamic moment acting on the sabot is relatively small following launch, and small magnetic clamping forces could delay sabot separation. NomenclatureA pocket = horizontal area of sabot petal pocket a, b = downrange and transverse current loop dimensions, respectively C m a = slope of the pitching moment coef cient with angle of attack d = downrange distance from muzzle dl 0 = incremental arc-length vector along current loop F aero = sabot petal lifting force F EM = electromagnetic clamping force F pocket = sabot petal pocket force F x-react = sabot petal reaction force in horizontal direction F z-react = sabot petal reaction force in vertical direction H = magnetic eld vector H x = downrange component of magnetic eld H y = orthogonal component of magnetic eld I = local inferred loop current in armature I o = average inferred loop current at muzzle exit I t = projectile transverse moment of inertia l aero = moment arm of sabot petal lifting force l EM = moment arm of electromagnetic clamping force l pocket = moment arm of sabot petal pocket force M 1 = freestream Mach number m = projectile mass P t2= stagnation pressure behind normal shock wave P 2 = static pressure behind normal shock wave P 1 = freestream static pressure p = separation distance between current loops in adjacent sabot petals r = local position vector r 0 = position vector along current loop S = planform area of sabot petal

Effect of electromagnetic launch on armature/sabot discard

“…However, because the armature/sabot design was nearly identical to that used in [4], the time constants for the residual current decay may be assumed to be the same. Also, the initial magnitude of the residual current can be scaled by the ratio of the peak transport currents used in the two test series.…”

“…The electromagnetic restraining force can then be calculated by assuming that the residual current flows in two separate loops (one in each petal) and determining the Lorentz force acting on each current filament. Additional detail on the experiments as well as the analysis can be found in [4].…”

“…Also, the initial magnitude of the residual current can be scaled by the ratio of the peak transport currents used in the two test series. With these assumptions, the electromagnetic restraining force was calculated using the same analysis procedures discussed in [4]. The initial magnitude and time constant for the estimated EM restraining force history, shown in Fig.…”

“…The electromagnetic restraining force was determined experimentally in a separate study [4] using a nearly identical armature/sabot, albeit at a lower average peak current of about 450 kA and an average velocity of about 1097 m/s. The EM force was deduced from the measurements of the time rate of change in the axial component of the armature/sabot magnetic field (i.e.,…”

“…This is because in EM guns, launch dynamics and, hence, the resultant rates at muzzle exit are expected to be significantly different than those observed in conventional guns. Furthermore, recent studies [2]- [4] have suggested that the residual currents Publisher Item Identifier S 0018-9464(01)00236-9. and magnetic fields, present in the armature/sabot petals after muzzle exit, would delay sabot separation which may, in turn, amplify the adverse effects. Earlier modeling efforts [5]- [7] that focussed on the sabot discard problem met with mixed success.…”