Copper Rails Research Articles

Limits to the velocity of solid armatures in railguns

Velocity limits of a solid armature in a railgun are the result of temperature and internal forces which surpass the yield strength of the material. A two-dimensional finite-element model for the magnetic and temperature fields in a railgun is presented for several rail and armatures designs. Copper rails and a molybdenum armature are identified as candidates for future solid armature testing. All simulations are performed for a 1/2-inch-square-bore railgun. >

Wear of conductors in railguns: metallurgical aspects

The wear debris generated by an aluminum alloy armature sliding on copper rails was studied experimentally. The experimental parameters were set to maximize armature velocity. Preliminary rail and armature wear observations were documented as part of a database for solid armature development. A variety of debris collection and characterization approaches were used. Both in-bore debris and muzzle-exit debris were collected for size and chemical analysis. Debris constituents were copper-rich, and the compound Cu/sub 9/Al/sub 4/ was observed in the splat-quenched muzzle debris. The approach and method are detailed to indicate the potential of wear studies in defining the activity at the sliding interface. Some implications of rail surface roughening for zero wear measurements are also discussed, and the adoption of wear mechanism maps is proposed. >

Erosion measurement techniques for plasma-driven railgun barrels

Plasma-driven railguns are now in operation at several locations throughout the world. All share common problems in barrel erosion arising from the fact that the bore surface must contain a high temperature plasma armature which transmits the acceleration force to a projectile. The plasma temperature at the core of the armature is estimated to be 30 000 K or higher. Such conditions are erosive to most materials even when the exposure time is 100 μs or less. We have adapted two accelerator based techniques to aid in the study of this erosion. The first technique involves the collection and analysis of material ablated and left behind by the plasma. This analysis is based on the unfolding of the Rutherford backscattered (RBS) spectra of 1 MeV deuterons incident on residue collected from a railgun bore. The second technique is an erosion measurement involving thin layer activation (TLA) of surfaces. In this process, the copper rail surface is activated by 2.4 MeV protons creating a relatively thin (3 m) layer sparsely seeded with a long lived zinc isotope. Monitoring the decay of the activated sample before and after a firing can detect surface wear of about 0. 1 m. Results from the RBS and TLA experiments on the BRL plasma driven railgun are described.

Electromagnetic railgun extended-life bore material tests results

During the last five years the LTV Aerospace and Defense Company (LTVAD) has built and operated a Distributed Energy Source (DES) railgun laboratory. As part of the efforts to improve the efficiency of this gun design, research in the area of rail materials erosion phenomena has been one of the major concerns. Studies have been centered around the basic synergies associated with material's erosion and railgun design; such as, the conductors, the insulators, the bore, the projectile and the gas environment. A data base, consisting of well over 200 shots, has been generated on the erosion rates of copper rails and various insulating sidewall materials at current densities ranging from 200 - 475 kAmps/cm. During these tests the insulating support structures suffered from considerably more erosion than the copper rails and will represent the largest deterent to repetitive operation of a railgun system. Consequently, research in the increased erosion resistance of insulating materials is an area of focus at LTVAD. This work was sponsored by the Army Ballistic Missile Defense Advanced Technology Center.

Railgun conductor heating from multiple current pulses

A numerical technique for solving current- and thermal-diffusion problems in railgun conductors has been used to study joule heating in rails that are subject to multiple current pulses. Copper rails that are 25 mm high by 12.5 mm wide with a 20-mm-square bore and a current pulse with 1-MA peak current and 1-ms pulse width at half maximum were assumed. This combination of parameters is sufficient to accelerate an 80-g projectile to 2-3 km/s with each current pulse. Three parameters were varied in the analysis: the repetition rate or current pulse frequency (3.3 to 100 Hz), the coolant heat-transfer coefficient (5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> and 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .K), and the coolant channel distribution in the rail. Detailed results are used to illustrate the acceptability or unacceptability of particular combinations of these parameters for operation at steady state. An uncooled rail was not acceptable for steady-state operation. Repetition rates of about 30 Hz were acceptable with the higher coolant heat-transfer coefficient and the best coolant-channel distribution; this included cooling the rail exterior surface.

Rail damage and armature parameters for different railgun rail materials

Rails made from different metals, and copper rails with different surface coatings were tested in a small calibre railgun. Electrical parameters and velocities were recorded and the rail surfaces were examined to assess damage after each firing. It is shown that some materials resist arc damage better than others and that damage mechanisms vary depending on material or coating. Some insight into railgun electrical parameters for the different materials is given.

The structural response of a rail accelerator

The transient response of a 0.4 by 0.6 cm rectangular bore rail accelerator was analyzed using a three-dimensional finite-element code. Results are presented for the case of a 210-kA current input and 5-cm arc length. Typically, the copper rail deflected to a peak value of 0.08 mm in compression and then oscillated at an amplitude of 0.02 mm. Simultaneously the insulating side wall of glass fabric base, epoxy resin laminate (G-10) was compressed to a peak value of 0.13 mm and rebounded to a steady state in extension. Projectile pinch or blow-by due to the rail (or side-wall) extension or compression, respectively, can be identified by examining the time history of the rail (or side-wall) displacement. For the case presented, the effect of blow-by was most significant at the side wall characterized by mm-size displacement in compression. Dynamic stress calculations indicate that the G-10 supporting material behind the rail is subjected to over 21 MPa - at which the G-10 could fail if the laminate was not carefully oriented. Results for a polycarbonate resin (Lexan) side wall show much larger displacements and stresses than for G-10. Therefore the tradeoff between the transparency of Lexan and the mechanical strength of G-10 for side-wall material is obvious. Displacement calculations from the modal method are smaller than the results from the direct integration method by almost an order of nagnitude, because the high-frequency effect is neglected. However, this effect is significant and dominating for a highly-impulsive, wave-propagation problem such as the rail accelerator structure.

Rail arcs: Interaction of magnetic and fluid dynamic forces

The motion of arcs driven along rail electrodes is predicted by balancing the driving magnetic force and the resisting fluid dynamic force. High current (2 to 14 kA) arcs in ambient pressure air are driven along enclosed copper rails by the self-induced magnetic field. Arc velocities and pressures along the travel length are obtained with steady flow demonstrates that the arc velocity can change abruptly. Future arc motion studies must define parameters such as travel length, channel inlet and exit conditions. The application of wave analysis can similarly predict the arc motion existing in circuit breakers and advantageously affect design changes of electrode and flow configurations accordingly.