Jerald V. Parker

Experiments to measure gouging threshold velocity for various metals against copper

Hypervelocity gouging is a form of damage that can occur to surfaces in sliding contact at high relative velocity. Gouges, which are in the form of teardrop-shaped craters, have been observed on rocket sled tracks, in light gas gun barrels, and in the bore of railguns. One aspect of gouging that has not been adequately explained is the existence of a minimum velocity (or threshold velocity) for a given material pair below which gouging does not occur. This paper reports a series of experiments to test the hypothesis that the onset of gouging is governed by the hardness of the harder material and by the density and sound speed of both materials. In the tests, samples of various metals were accelerated to 2.2 km/s while in direct sliding contact with CD110 copper rails. The samples were carried in a Lexan polycarbonate forebody, modified to apply normal loads of 40-80 MPa to the sample/rail interface. The portion of the armature directly in line with the samples was cut away to avoid contaminating the gouge track. Visual inspection of the resulting gouges was used to establish a gouging threshold for each metal. The tests were conducted in the 40 mm square bore electromagnetic launcher at the Institute for Advanced Technology (IAT). Metals tested include AISI 1015 steel, silver, molybdenum, pure copper, tungsten, nickel, magnesium, and 7075 aluminum alloy. The results of the experiments show the existence of a straight line fit between hardness of the harder material and the shock pressure for a normal collision at the gouging threshold velocity.

Investigation of Damage to Solid-Armature Railguns at Startup

This paper describes work investigating a rail damage mechanism observed in solid-armature railguns at the Institute for Advanced Technology, The University of Texas at Austin. The damage occurs in the starting section of rails and is not associated with high-speed phenomena such as hypervelocity gouging or transition to arcing contact. The damage, which we call grooving, is localized to the region of the rails adjacent to the insulators. In this paper, we describe grooving damage observed in multiple tests using copper rails. In one series, in which we tested up to 20 shots on one pair of copper rails, we obtained grooves on the order of a millimeter deep and several millimeters wide. We present evidence that grooving is caused by liquid erosion and is not a result of plasma heating or mechanical deformation

Demonstration of a hypervelocity mass-efficient integrated launch package

Tests were conducted whereby a kinetic energy penetrator was launched from an electromagnetic launcher. The penetrator was fabricated from a tungsten alloy. An armature, which supported the penetrator during acceleration and conducted the rail current, was fabricated from 7075-T6 aluminum. The transition of the armature contacts from a low voltage to a high voltage was identified as the mechanism that caused the aft section of the subprojectile to fail. Transition occurred fairly consistently at an average velocity of 2050 m/s. Removal of armature material at the rod-armature interface increased the survivability of the subprojectile and was demonstrated with a launch package mass of 308.5 gm, yielding a useful mass fraction of 49%. Structural integrity of the subprojectile at a launch velocity of 2350 m/s was verified with an orthogonal flash X-ray image at the muzzle.

A Plasma Railgun Experiment Addressing Launch-to-Space Issues

The Institute for Advanced Technology has recently been awarded a grant by the U.S. Air Force Office of Scientific Research to undertake research in the area of hypervelocity, plasma-driven electromagnetic launch. This paper describes our technical approach to achieving 7 km/s at accelerations that are lower than typical for plasma launchers. Our approach to managing plasma restrike and secondary arc formation is to reduce the emission of ablated materials into the gun bore through the use of augmentation and ceramic sidewalls. In addition, preinjection to 1 km/s is used to provide an initial velocity for the launch package while introducing minimal dense cold gas, since such gas can also contribute to restrike. This paper describes the hardware that is being put in place for this experiment

Development of a Plasma Railgun for Affordable and Rapid Access to Space

The Institute for Advanced Technology (IAT) of The University of Texas at Austin (UT) is developing a plasma-driven railgun to launch low-mass projectiles of roughly 5–10 g to a velocity in excess of 7 km/s. Accomplishing this goal requires overcoming the problem of bore ablation, which has been linked to an observed velocity ceiling of about 6 km/s in plasma armature launchers. Bore ablation is a direct consequence of the intense heat radiated by plasma armatures. Controlling bore ablation requires a coordinated approach that includes: 1. using magnetic augmentation to reduce power dissipation in the plasma, 2. using high-purity alumina insulators to raise the ablation resistance of the bore, 3. using pre-acceleration to prevent ablation of the bore materials at low velocity, and 4. using a synchronously driven, distributed power supply to electrically isolate stages.

Developments in making space access rapid and affordable using a plasma railgun

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.

Development of a Plasma-Driven Railgun to Reach 7 km/s

Research in the area of plasma-armature railguns is currently underway at the Institute for Advanced Technology (IAT) as part of an Air Force multidisciplinary university research initiative (MURI). The program is aimed at investigating the possible use of an electromagnetic launcher for the rapid and affordable launch of microsatellites (~1 to 10 kg) into low earth orbit. In the experiment, the IAT is developing a plasma-driven railgun to launch low-mass projectiles of roughly 5-7 g to a velocity in excess of 7 km/s. This goal requires overcoming the problem of bore ablation, which has been linked to an observed velocity ceiling of about 6 km/s in plasma-armature launchers. Bore ablation is a direct consequence of the intense heat radiated by plasma armatures. This paper describes the consequences of excessive bore ablation, the rationale for the IAT experiment, and results obtained from the hardware that has been designed and tested so far.