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Why Hypervelocity Weapons Are So Powerful

Despite the intuitive appeal of kinetic energy, some skeptics of hypervelocity weapons have remained, well, skeptical. An article in the March 2005 IEEE Spectrum reported that “The physics of high-velocity impact limits the penetration depth; basically, too much energy at impact causes the projectile to distribute its energy laterally, rather than vertically. Tests done since the 1960s by Sandia National Laboratories… confirm that maximum penetration is achieved at a velocity of about 1 to 1.5 km/s.”

Contrary to this, over 900 hypervelocity tests of tungsten rods against armor steel conducted at the Institute for Advanced Technology of the University of Texas at Austin show that as impact velocity rises from 1.5 km/s to 2.5 km/s, penetration depth increases by about 50% (see Figure 1 below).


Figure 1: Penetration of tungsten longrod into rolled homogeneous armor steel vs. impact velocity. P/L is penetration per length of projectile.

Sandia’s Michael J. Padilla says the discrepancy hinges upon the purposes of the tests. The Sandia tests were designed to learn how fast their “Hard Target defeat penetrators” composed of “cylindrical bodies… surrounding cavities for the high-explosive fill material and electronic or mechanical fuzes” could impact yet still manage to detonate properly. The Sandia experiments had nothing to do with purely kinetic weapons. (The nature of the intended explosive is left to the imagination of the reader.)

Two other factors drive penetration depth:  length of penetrator and ratio of densities between it and its target:

 
where P/L is penetration per length of projectile,  rp is projectile density and rt is target density. Effects of other geometrical variables such as hollow rods and segmented penetrators have been experimentally explored but remain classified.

Therefore these long, thin, dense projectiles must be able to take high g forces, leave the barrel without damaging it, and have an aerodynamic shape that keeps them from losing too much velocity on the way to the target. One consequence is that they typically are launched inside a sabot which protects the projectile while it travels through the barrel. Figure 2 shows an “integrated launch package” that does all this for a hypervelocity launch at 8 MJ muzzle energy.


Figure 2: “Integrated launch package” including sabot for launch at 8 MJ muzzle energy.

TIGER (tongue-in-groove extending rod) is a technique to increase the length to mass ratio of a projectile (see Figure 3). The tongue, which is extended an instant before impact, doubles impactor length.

Figure 3: TIGER (tongue-in-groove extending rod).

 

“Star-Crossed,” by Bruce M. DeBlois, Richard L. Garwin, R. Scott Kemp and Jeremy C. Marwell, IEEE Spectrum, March 2005, Pg. 46.

“Hypervelocity Electromagnetic Kinetic Energy Weapons,” by Harry Fair, in press.

“Design of an 8-MJ Integrated Launch Package,” by S. Satapathy, I.R. McNab, M. Erengil, and W.S. Lawhorn, IEEE Transactions on Magnetics, Vol. 41, No. 1, Jan. 2005, pg. 426.

 

 
       © 2013 Carolyn Meinel