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Hypervelocity Rod Bundles (HRB)

"Hypervelocity rod bundles" are a weapon concept that involves firing tungsten rods from satellites at targets on the ground. The rods would be launched at speeds of over 10,000 feet per second, penetrating deep into the ground without explosives. The destructive force of these weapons is directed almost entirely in the path of the weapon’s travel; for this reason, suitable targets include missile silos [particulary], ships, hardened aircraft shelters, command bunkers and tall buildings. Against softer area targets such as military factories, they would like mainly seve to punch narrow but deep holes into the factory floor.

The idea for "Rods from God" originated in the late 1950s with science-fiction writer Jerry Pournelle. He called the weapon Thor, and as he explained in an interview, “People periodically rediscover it.”

Data derived from Project Thor served as the foundation for subsequent developments related to the Rods from God concept. Project Thor, established in 1947 and conducted at Aberdeen Proving Ground with involvement from Johns Hopkins University, aimed “to correlate the mass and velocity of shell fragments with the probability of damage to an aircraft component.” The project spawned a number of technical reports and led to the development of several equations, which were likely used in the 1980’s to study penetration mechanics of much larger projectiles.

The U.S. Air Force included "hypervelocity rod bundles" in its November 2003 Transformation Flight Plan as a future weapon system goal. "Currently, striking conventional targets across the globe from the United States requires employing long-range bombers, which takes many hours and enables mobile targets to hide before the strike force arrives. The key to achieving DoD’s current transformational objective of denying sanctuary to adversaries is: 13. Rapid and precise attack of any target on the globe with persistent effects A non-nuclear, prompt, global attack capability will provide the United States with a range of options for deterrence and flexible response when rapid response is absolutely critical, risks associated with other options are too high, or when no other courses of action are available. Such rapid global attack would likely be used against extremely high value targets such as hardened command and control facilities, terrorists, fixed and mobile integrated air defense system elements, theater ballistic missile launchers, and CBRNE production, storage, and delivery.

"This capability would be a key enabler of the Global Response CONOPS’ mission of holding terrorist-related targets at risk everywhere. It would also allow the United States to project power almost immediately in areas with no forward-deployed forces or easy access. Indeed, the traditional US method of deploying air and ground forces at or through ports and airfields will grow more problematic as national and commercial satellite services, missiles, and CBRNE technology rapidly evolve. This capability would also buy valuable time should additional forces need to be deployed to the theater."

Such a "tungsten telephone pole" would have an energy roughly equal to its own mass in TNT, and it would go straight through the target, probably depositing most of this energy in the ground below. As a precision "bunker buster", it would need precision guidance, and entry at that speed would make it difficult to implement, considering the plasma sheath will prevent using forward-looking sensors. The best of the best inertial guidance of Peacekeeper is ~100 m accurate, good enough for an H-bomb but not much good for tungsten rods. In practice, air drag detracts from the advantage gravity gives, and deorbiting such an object, not to mention orbiting it in first place, are both rather expensive. In combat the windows for taking action can stay open briefly.

Because Rods from God involved the use of long rods dropped from space, research on long rod penetration mechanics was important to its development. One primary report in this focus area, produced by the U.S. Army Armament Research and Development Command’s Ballistic Research Laboratory in 1983, studied the effect of long tungsten rods penetrating steel. It found that during penetration, the rod creates a cavity while simultaneously eroding away. A key component of this process comes from the eroding rod model.

The eroding rod model presents a sound theory of penetration for long rods” as evidenced by its predictions. The eroding rod model demonstrates that penetration is proportional to the length of the penetrator, all things being equal. This model also indicates that for fixed Lo/D and a fixed amount of kinetic energy, there is an optimum striking velocity for maximized penetration. As target strength increases, the curve moves to the right, and as penetrator strength increases, the curve moves left.

Hypervelocity Rod Bundles are similar to other proposed kinetic-energy weapons designed for use against terrestrial targets. These weapons, frequently referred to as “eroding rods,” seek to destroy targets by converting the KE [kinetic energy] associated with the weapon’s high velocity (5 to 11 km/s [kilometers/second]) into work and heat. For example, a two-meter rod weighing 25 kilograms and penetrating a depth of six to eight meters is similar to detonating 25 kilograms of explosive in a hole slightly larger in diameter than the rod.

But the 2003 Air Force report noted that a 6.1 by 0.3 meter (20 ft × 1 ft) tungsten cylinder impacting at Mach 10 (11,200 ft/s; 3,400 m/s) has kinetic energy equivalent to approximately 11.5 tons of TNT (48 GJ). The mass of such a cylinder is itself greater than 9 short tons (8.2 t), so the practical applications of such a system were limited to situations where its other characteristics provide a clear and decisive advantage.

Due to their extremely high speed and lack of vulnerable points, defense against the solid metal rods would be very difficult inside the atmosphere. The best approach might be finding and attacking them in space before the penetrators reenter. For a constellation of eroding-rod space-based weapons,trade-offs between total “delta” velocity (energy needed to deorbit), impact velocity (destructive power), area coverage, and reentry angle suitable for accuracy seem to yield an optimum orbit altitude of around eight thousand kilometers and a response time of 1.5 to two hours. Such a deployment would add to global strike capabilities, with responsiveness better than that of current manned aircraft, and some unique munitions effects.

However, enemy reaction must be considered. Such global strike weapons would confer on hostile nations much greater increases in combat power, in proportional terms, than they would for the United States. A RAND study concluded that because of their extremely high velocity, these kinetic-energy weapons are very difficult to defend against during their brief transit through the atmosphere and might therefore be particularly interesting against heavily defended targets. These weapons may be of only limited interest to the United States, which has other means of global power projection. However, they may be a very good fit for another country, such as one seeking global power projection without duplicating the American terrestrial investment, or one seeking to deny access to U.S. power projection forces. For example, instead of playing catch-up against highly evolved air and submarine defenses, a country might prefer these space weapons to bypass the defense entirely.

The RAND Corporation outlined, in one of their many publications on policy and strategy, entitled Space Weapons Earth Wars, four types of space-based weapons. These include directed-energy, kinetic-energy vs. missiles, kinetic-energy vs. surface targets, and conventional space-based weapons. Directed energy weapons include electronic jamming, laser cutting torches, and a variety of similar weapons. None of the directed energy weapons currently employed on a practical scale are powerful enough to accomplish these kinetic missions.

The latter three depend on the transfer of potential energy to destroy the target. Kinetic weapons rely on velocity and mass to cause damage, while the conventional Rods from God [are] kinetic energy weapons capable of deliving destruction against certain hard targets like missile silos on the scale of nuclear weapons due to their enormous mass. These are weapons that RAND refers to 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.

This works well with space-based systems because the high altitude means the projectile has a high potential energy, which translates directly to kinetic energy as the Earth's gravity accelerates it towards a chosen target. The basic physics involved is set forth in the following equation: Potential Energy = gravitational acceleration x projectile mass x altitude. 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.

Kinetic Energy Weapons were being considered for the force application missions of BMD and orbital bombardment of very hard, highvalue, terrestrial targets. Orbital bombardment seeks to destroy targets by converting the KE associated with the weapon’s high velocity (5 to 11 km/s) into work and heat. Such projectiles could have a number of configurations, including long thin rods, ultrahard penetrating warheads, or warheads that fragment shortly before impact.

As with most weapons, trade-offs must be made when designing weapons for orbital bombardment. To attain velocities in the range of 10 to 11 km/s, satellites must be in orbits with an altitude of more than 40,000 km, but these high-altitude orbits sacrifice responsiveness to achieve high-impact velocities. For instance, a weapon in a 40,000 km orbit would need about five hours to reach the earth’s surface and would have an impact velocity of about 10 km/s. The actual time required to hit a specific target would probably be longer since it is unlikely that the weapon would be in the proper position to initiate an immediate attack.

Lower orbits could yield shorter response times; for instance a satellite placed in a 500-mile (926 km) orbit could strike in less than 12 minutes if the orbital geometry was ideal. The trade-off is that a weapon in LEO would impact at less than 5 km/s. One design for such a KE projectile is a thin, heavy, metallic rod one to two meters in length. Such a weapon could be used against hard targets that are not too deeply buried. Depending on what they are made of, the rods can penetrate two to three times their length into a target. As long as the rod impacts at a velocity in excess of 3 km/s, the depth it penetrates depends exclusively on the composition of the target and the rod, with only slight differences among specific hard target materials.53 The mechanism used for penetration is progressive erosion of the tip of the rod coupled with progressive erosion of the substance being penetrated. The pressure generated at the tip of the rod causes both the rod and the target to liquefy in the vicinity of the tip. As the rod penetrates the target, its progress is similar to that of a high-pressure jet of water penetrating earth.

The results of hitting a target with one of these rods is similar to boring a hole, placing in the hole an amount of explosive comparable in weight to that of the rod, and detonating it. For example, a two-meter rod weighing 50 pounds and penetrating to a depth of six to eight meters is similar to detonating 50 pounds of explosive in a hole slightly larger in diameter than the rod. As long as the rod penetrates to the interior of the target, the results are devastating. A drawback of this type of weapon is that very deep targets would necessitate rods too massive to be practical.

A concept that could solve some of the technical problems associated with orbital bombardment deals with the problem of high reentry velocities by using a maneuverable reentry vehicle. Deployed from an orbiting satellite, the weapon would slow from orbital speeds to speeds low enough to dispense conventional munitions. As it slowed, the weapon would be capable of aerodynamically maneuvering thousands of kilometers to either side of the orbital track without needing additional propellant. A weapon of this type has been proposed by the Armament Product Group at Eglin Air Force Base, Florida, and is called the common aero vehicle (CAV). Air-launched suborbital missiles or ICBMs, as well as orbital platforms could deliver the CAV. If an orbital system were pursued, it would be possible to station large numbers of CAVs in LEO and de-orbit them when needed. With guidance, navigation, and aerodynamic controls within the atmosphere, the CAV would dispense its submunitions at the appropriate geographic location.

Weapons that do not need explosive charges generate two benefits. The first is that they are inert at ground level. A tungsten rod 56 | Air & Space Power JournalSuborbital Strike! travelling at zero miles per hour is able to injure somebody only if he or she trips over it. Therefore, one can use cluster munitions without the political backlash they generate from unexploded munitions left behind. In a war zone, if an ammunition ship or ammo bunker filled with these weapons is hit, there will be no subsequent detonations that lead to further damage to the ship convoy or base.

Second, without the need for explosives, the weapon itself can be made smaller, allowing the vehicle to carry more of them. Typically, the mass of a conventional munition is about half explosive filler and half steel casing - heavy penetrator cases typically cntain a mush smaller proportion of explosive. Equivalent mass hyoperveclocity solid rods wold produce more energy on imact that explosive filled projectiles, a exact increment depending on impact velocity.

A recurring argument against this idea is noted on the Popular Science website: "Launching heavy... rods into space will require substantially cheaper rocket technology than we have todaY". While this is a legitimate concern for the powerful weapon system mentioned by the Air Force, the force of a nuclear weapon is far greater than the kinetic energy required to destroy targets in a conflict, conventional or non-conventional. Although the cost of a space-based weapon system would be great, it would not be so prohibitively high as to prevent its implementation due to the far smaller weight of any useable weapon.

The greatest challenge with this system is the problem of atmospheric reentry. The smallest and cheapest system would destroy ballistic missiles outside of the atmosphere where reentry would not be an issue. This limits the window of opportunity to the brief time when the target is above sixty kilometers. In order to engage below that altitude, larger projectiles with atmospheric reentry capabilities would need to be built, greatly increasing the cost of the system. A reentry vehicle is required for such weapons. kinetic or conventional, intended for use against ground or aerial targets. The limitations of such kinetic weapons include the fact that, in order to maintain velocity, their maneuverability and target window is severely limited. In addition, because they derive their power from the pull of gravity, reentry angles must be steep, giving the weapon system a very narrow scope of targets at any given time.

Because of this, a useful space-based system would require deployment of a larger number of satellites to be in position to strike targets nywhere around the globe in a reasonable amount of time. According to RAND's study, six platforms in high orbit would only provide targeting opportunities every two to three hours.