42 IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 1, JANUARY 2003 Magnetic Blow-Off in Armature Transition John P. Barber and Ian R. McNab, Senior Member, IEEE Abstract—Magnetic blow-off forces play an important role in arcing and failure of electrical joints and contacts in electrical sys- tems under high current fault conditions. Magnetic blow-off forces arise from concentration of current in the contact interface and tend to blow the contacts apart, causing separation and arcing. Simple models have been developed to predict the magnitude of these forces and permit the design of joints and contacts that can successfully resist the forces. In this paper, the role of magnetic blow-off in railgun armature contacts is explored. It is shown that magnetic blow-off forces are important and may be the final step in a series of events that ultimately leads to transition. Models that predict the conditions under which blow-off will occur are devel- oped, and supporting experimental data is described. Index Terms—Armature, electromagnetic forces, railgun. I. INTRODUCTION E ARLY railgun research (e.g., Barber [1]) started with the use of metallic armatures that form an electrical connec- tion between the two rails in a simple railgun. The current ap- plied to the railgun breech is transferred through the sliding con- tacts at the armature-to-rail interfaces, and current flow through the armature results in the force that accelerates the launch package along the barrel. Early experiments showed that this technique operated satisfactorily at velocities up to about 1 km/s, but thereafter only intermittent contact could be main- tained between the rails. This caused the interface voltage be- tween the armature and rails to rise from the few volts typ- ical of a metallic contact to voltages of tens to hundreds of volts as arcing contacts caused plasmas to become the path for the current flow. The rise in contact voltage became known as “contact transition” to convey the concept of transition from metallic to arcing contact. Continued research into high-velocity railgun operation, therefore, took the path of using fuse-initiated plasmas to provide the entire path for the current flow. The ad- vantage of this approach is that the armature mass is very low (micrograms), but the disadvantages are that high contact volt- ages are created, and that it is generally necessary to preinject the projectile package into the railgun breech to prevent the ex- cessive ablation that results from a standing high-current arc. Manuscript received January 14, 2002. The research reported in this work was performed in connection with Contract DAAD17-01-D-0001 with the U.S. Army Research Laboratory. The views and conclusions contained in this docu- mentation are those of the authors and should not be interpreted as presenting the official policies or position, either expressed or implied, of the U.S. Army Research Laboratory or the U.S. Government unless so designated by other au- thorized documents. Citation of manufacturer’s or trade names does not consti- tute an official endorsement or approval of the use thereof. The U.S. Govern- ment is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon. J. P. Barber is with IAP Research Inc., Dayton, OH 45429 USA (e-mail: johnb@iap.com). I. R. McNab is with the Institute for Advanced Technology, The University of Texas, Austin, TX 78759 USA (e-mail: mcnab@iat.utexas.edu). Digital Object Identifier 10.1109/TMAG.2002.805855 Fig. 1. Pretransition eroded armature. In view of these disadvantages, there has been a renewed in- terest in recent years in trying to understand how the metal-on- metal sliding process may be made to work effectively at higher velocities. The objective is to find methods to guarantee transi- tion-free operation of railgun armatures up to velocities as high as 2500 m/s. Several mechanisms that can cause, or lead to, armature con- tact transition have been identified and investigated and exten- sively reported in prior Electromagnetic Launch (EML) Sym- posia. While all of these mechanisms are important and con- tribute to transition, they do not account for all the observations of transition. One transition mechanism that has not been ex- plored in railgun armatures is magnetic blow-off. Erosion of the armature edges (trailing, top and bottom, and occasionally leading) has been observed (an example is shown in Fig. 1), and this clearly leads to current concentrations in the contact zone. Current concentrations will generate magnetic forces that can cause unloading and may ultimately lead to the complete “blow-off” of the contact. In this paper, the basic mag- netic blow-off mechanism is described, the criteria for blow-off induced transition in railgun armatures is developed, and exper- imental data that supports this analysis is shown. II. MAGNETIC BLOW-OFF FORCES Magnetic blow-off of electrical contacts is a well-known and understood phenomena in electrical equipment design (see, e.g., Holm [2]). Magnetic repulsive forces can develop between con- tacts if there are current concentrations in the contact interface. The magnetic forces are always repulsive and tend to separate the contacts. If the magnetic blow-off forces exceed the external forces that are applied to keep the contacts together, contact sep- aration and transition to arcing will result. The development of the blow-off force for spot concentra- tions in the contact is described by Holm [2] and is repeated 0018-9464/03$17.00 © 2003 IEEE