DEVELOPMENTS IN MAKING SPACE ACCESS RAPID AND AFFORDABLE USING A PLASMA RAILGUN ∗ D. Wetz ξ , I. McNab, F. Stefani, D. Motes, and J. Parker Institute for Advanced Technology The University of Texas at Austin 3925 W. Braker Ln., Suite 400 Austin, TX 78759 ∗ Work supported by the U.S. Air Force Office of Scientific Research (AFOSR) under contract number DOA-8910. ξ email: david_wetz@iat.utexas.edu Abstract For the last four years, the Institute for Advanced Technology has been working on the development of a plasma driven electromagnetic launcher (EML), for economic access to space [1]. The research is focused on overcoming setbacks experienced in the early developmental days of plasma-driven EMLs, which prevented researchers from obtaining muzzle velocities in excess of 6 km /s [2]. The possibility of achieving muzzle velocities in excess of 7 km/s with an EML make its use attractive and cost-efficient means for launching small (~ 10 kg) microsatellites into low Earth orbit. For that reason, the research being performed is funded as part of a multidisciplinary university research initiative (MURI) by the United States Air Force Office of Scientific Research (AFOSR). In the summer of 2007, a muzzle velocity of 5.2 km/s was achieved with no evidence of restrike arcs or bore ablation, the effects of which are believed to limit the velocity of plasma railguns to no more than 6 km/s. Since then, a series of modifications have been made to the railgun bore to improve its performance and lifetime. Some of those modifications, and the experimental results obtained as a result, are discussed here. I. INTRODUCTION In the early days of electromagnetic launch (EML) research, experimentalists were under the assumption that they could use plasma-armature railguns to accelerate payloads up to muzzle velocities in excess of 10 km/s. During the initial studies, velocities of only 4–5 km/s were demonstrated for medium-bore (25–50 mm) railguns operating at typical accelerations of 400–600 kG [3],[4], and only 6–7 km/s velocities were obtained in smaller bore guns with accelerations of 1 MG or greater [5][6]. It was unclear for many years why velocities in excess of 5 km/s, at modest accelerations, could not be achieved until the 1980s, at which point several causes for the velocity ceilings were discovered and understood. The velocity ceiling for plasma armature railguns was found to be a direct consequence of ablation of the bore insulators, which causes the bore to fill up with a hot, dense, neutral gas [7]. This gas does not affect the performance of the railgun until, at high velocities, the voltage across the railgun breech increases to the point where conditions for high-voltage breakdown are met. When this occurs, additional plasma armatures, known as restrike or secondary arcs, are formed well behind the main armature. Ideally, the restrike arcs are not pushing against anything and should rapidly merge with the main armature, which is pushing the payload. However, in practice, the secondary armatures are retarded by viscous drag as they push the ablation products in the bore. This drag prevents the restrike arcs from catching up to the main armature—causing current, and thereby acceleration force, to be lost in the restrike arc—preventing any future acceleration of the payload. Several suggestions for overcoming the bore ablation problem were made [7] and implemented [8]. However, as soon as positive effects were starting to be observed, funding shifted to solid-armature tactical railguns, and plasma-armature railgun programs were discontinued. The Institute for Advanced Technology (IAT) at the University of Texas at Austin has resumed the research into plasma-driven railguns as part of a multidisciplinary university research initiative (MURI) sponsored the United States Air Force Office of Scientific Research (AFOSR). For its part, the IAT has set up a proof-of- principle experiment aimed at launching 5 g polycarbonate projectiles to muzzle velocities in excess of 7 km/s using a 7 m long laboratory-style electric launcher. The IAT’s technique for eliminating bore ablation involves a three-pronged approach that includes using magnetic augmentation to reduce power dissipation in the plasma, using high-purity alumina insulators to raise the ablation parameter of the bore, and using pre- acceleration to prevent ablation of the bore materials at low velocity [1]. Figure 1 contains plots of the ablation parameter, calculated using equation (1), as a function of time for a simple and augmented railgun. It is shown that without the use of augmentation as well as pre-injection, it is impossible to eliminate insulator ablation, even when insulators with a high ablation threshold like alumina are used. 742 U.S. Government work not protected by U.S. copyright