Hypersonic kinetic penetrator projectile
Kinetic weapons rely on velocity and mass to cause damage, while the conventional weapons typically use stored chemical energy (i.e., explosives) to achieve their effect. Kinetic energy weapons have the advantage of being mechanically simpler, as well as cheaper, than conventional weapons with the disadvantage that they must be traveling at great speeds to achieve the same destructive capability. When the weapon system is launched, it is imbued with kinetic energy by the boosters which transfers to potential energy as its altitude increases. After that, it is a relatively simple matter of converting the potential energy of the system back into kinetic energy using gravity.
Such a “Hypervelocity Rod Bundles” weapon was mentioned in the November 2003 “US Air Force Transformation Flight Plan”. This plan discussed hypervelocity rod bundles that could strike ground targets anywhere in the world from space. This theoretical weapon system in many circles has come to be known as the “Rods from God”. The concept was devised by Jerry Pournelle while he was in operations research in the 1950s at Noeing. He developed the notion of "orbital telephone poles" made of tungsten.
A variety of hardened, buried targets is included in the Air Force worldwide target list. The GBU-24 was adequate against hardened aircraft shelters during Desert Storm. However, a new GBU-28 penetrator was quickly fielded to attack deeply buried command and control (C2) targets. More effective penetrator weapons are needed for global power projection. The hypersonic speed increases the penetration depth by a factor of 2 compared to current conventional weapons, but increases the targets at risk by only a few percent.
Measurement of roll angle is generally required for projectile guidance in either gun-launched or rocket-launched ordinance; however, launch acceleration forces experienced by the projectile may damage conventional inertial measurement guidance systems. Various methods of measuring roll angle using solid-state electronics capable of withstanding launch acceleration have been previously demonstrated. A guidance system for use in both gun-launched munitions and rocket-launched missiles suitably configured to deliver kinetic energy projectiles would be desirable. This would result in greater mission flexibility and reduced inventory for guided kinetic penetrator projectiles.
Increased deployment of gun-launched and rocket-launched ordinance has resulted in their application to a wider variety of targets, which in turn has resulted in the production of different types of munitions and rockets adapted to carry kinetic penetrator rods. The science and measurement of penetration of materials by long-rod penetrators has been studied for many years. It has major applications in the areas of armor penetration and deep earth penetrators. The interaction physics is different in these two applications, and it depends strongly on the velocity of the penetrator. The different types of munitions and rockets required to defeat a variety of targets generally increases the need for producing and maintaining a large inventory of munitions and missiles. Because they are unguided, gun-launched munitions that include kinetic penetrator rods are generally effective at relatively short ranges. Accurate guidance may extend the effective range of these gun-launched munitions. Rocket-launched missiles, such as direct fire missiles, may include kinetic penetrator rods and have a range in excess of gun-launched munitions.
A steel penetrator with a mass of 300 kg (660 lb) moving at 1.2 km/sec (4,000 ft/sec) has a kinetic energy of 216 MJ, or an energy equivalent of approximately 50 kg (100 lb) of high explosive. An explosion of this magnitude produces a hole about 1 m deep in loose soil. The large penetration depths produced by a rod penetrator result from the fact that a large force per unit area or pressure is produced at impact, and this pressure persists for a long period compared to the detonation of high explosives.
A steel penetrator begins losing its strength at about 4,000 ft/sec (1,200 m/sec). At 6,000 ft/sec, one would expect that the penetrator would have very little mechanical strength left and that the penetration would approach that of a fluid jet, much like a shaped-charge penetrator. A second observation is that granite looks mostly like steel in this velocity range, and the above observations apply also to a granite target. In concrete, the onset of the hydrodynamic regime appears to occur even at the lowest velocities. In the low-velocity regime, the interaction is like a solid-steel rod penetrating a target of putty. In the higher-velocity regime, the penetration interaction is similar to the impact of putty on putty, which is very complicated. At higher velocities, the interaction can be treated using the simple conservation laws of fluid dynamics.
It has been demonstrated that a hardened long-rod penetrator delivering in excess of 5 megajoules of energy at hypersonic velocity to the armor of a tank can penetrate the armor and destroy a tank. This generally involves boosting the rod to hypersonic velocity using a gun-launched munition or a rocket motor. A 250 lb hypersonic penetrator can acquire the same penetration depth and impact as a 5,000 lb gravity bomb. By one calculation, a 4,000-lb Conventional Penetrator can reach a depth of 150 feet, while a 2,500-lb Hypersonic KE Penetrator can reach a depth of 300 feet.
Hypersonic penetrators have a maximum effective delivery speed of about 5,000 ft/sec (at approximately Mach 5) and have the potential to destroy some deeply buried targets (DBTs). The maximum penetration depth is a function of the mass of the device and the velocity with which the device strikes the ground. Maximum penetration depths for granite are less than 100 ft. While there are many critical targets within the penetration depth capability of hypersonic penetrators, many of the most critical targets are more deeply buried than the penetration depth limit for these devices.
Comparison of the weights of a large gravity bomb and an equivalent hypersonic penetrator indicate that the penetrator can be reduced from 5,000 to 250 lb to get a comparable penetration depth. For the GBU-28, the impact velocity is 1,300 ft/sec. In practice, the velocity scaling is more favorable, and the scaled velocity for the lighter-weight penetrator is only about 3,000 ft/sec. This could be an important advantage for use with hypersonic strike aircraft or UAVs. A booster motor on a hypersonic penetrator could provide precision control on the point and angle of impact of the penetrator. These are important parameters in maximizing the penetration depth of the weapon.
Experiments by Orbital Sciences Corporation in the late 1990s indicated that they could deliver a 300-kg (660-lb) penetrator with an impact velocity of 4,000 ft/sec into an earth granite target. The measured depth of penetration was 45 ft. The penetrator that was recovered after the experiment indicated little erosion and loss of mechanical strength, although the penetrator did appear to be slightly bent. The length of the penetrator was 5 ft and the diameter was 9 inches, which gives L/D = 6.6. The areal mass M/A = 10.4 psi. In another experiment, a 4-ft-long, 256-lb steel penetrator impacted granite at a velocity of 3,300 ft/sec. The penetration depth was 31 ft in granite. The diameter of the penetrator was 6 inches, which gives L/D = 8 and an M/A = 9 psi. The penetrator in these tests was a solid body with no interior space for a warhead. In a practical device, such space would be required, which would weaken the mechanical strength of the penetrator and reduce the penetration depth.
A study in 2000 by the Air Force Science Advisory Board concluded "The operational benefit for these hypersonic penetration weapons for DBTs is not judged to be applicable to the hypersonic vehicle program, given the effective limit of these weapons to around Mach 5. Enhanced effects may be produced at much higher penetration speeds (35,000 to 40,000 ft/sec) where impact angle is not a factor."
High-speed missiles could provide critical capability against hardened buried targets. However, an important point for consideration is that at supersonic speeds up to Mach four a steel penetrator retains its strength, but at hypersonic speeds the penetrators should have a much harder casing such as tungsten (which might increase the cost of the weapon). Similarly, the current material used in missile fuselage loses its strength at hypersonic speeds necessitating use of tungsten-based nose caps, structures based on nickel alloys and other special carbon-based material to withstand the enormous temperatures. The final point for consideration is the impact of hypersonics on the current range of navigation and terminal guidance systems also requires further study.
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