Defenses Against Hypersonic Glide Vehicles

Hypersonic reentry of an HGV
Image Credit: Defense Advanced Research Projects Agency (DARPA)

In my last post I wrote about the unique threat against U.S. national security posed by the Hypersonic Glide Vehicle (HGV) programs of China and Russia. The reasons HGVs pose such a threat are they go hypersonic (speeds greater than five times the speed of sound, Mach 5) in mid-flight, much sooner than a conventional Inter-Continental Ballistic Missile (ICBM); and they have some capability to maneuver to evade Anti-Ballistic Missile (ABM) missiles. In my first post on HGVs I discussed the development programs of China and Russia, as well as some attempts by the U.S. to catch up. China’s program appears to have been particularly successful, and would appear to pose a near-term threat — possibly within a decade. HGVs would greatly enhance possible Chinese designs in the East China Sea and in the South China Sea.In this post I will look at possible U.S. defenses against HGV weapons, particularly the THAAD system and rail guns. In a following post I will also take a look at high power laser defense systems, as well as a look at U.S. political reactions to the problem.

Problems Shooting Down An HGV

The problem with an HGV warhead homing on a target is that it is both fast and shifty, making it hard for ABM countermeasures of any kind to hit it. Some feeling for the problems can be gained from looking at the graphical representation below of a flight profile for the American Hypersonic Technology Vehicle 2 program.

The flight profile displayed would be typical for most HGVs, with the possible exception of the very end where the illustration shows an “emergency controlled descent”. All of the HTV-2 flight tests were failures because the skins of the vehicles could not take the heat of reentry at Mach 20. (The Chinese and Russian HGVs travel at a relatively more sedate Mach 10, about 7,680 and 7000 mph, respectively.) As a result when the HTV-2’s flight grew erratic, the onboard computer ordered an emergency plunge into the ocean. An actual HGV attack on a target might have a trajectory at the end different from a vertical plunge. During its reentry into the atmosphere, an HGV can use a reaction control system and flight control surfaces to change the attitude of the HGV and its trajectory, allowing turns and altitude changes to evade ABM countermeasures. This is especially true when the weapon is far from its target. When it gets very close to the target, even small changes in trajectory might cause the HGV to miss its target, and traveling at hypersonic speeds, the range from target at which HGV maneuvering becomes impractical must be in the hundreds of miles. For example, traveling at 7000 mph an HGV would cover 100 miles in 51.4 seconds!

However, to hit such a target with ordinary ABM missiles, the defense must also fire on the incoming HGV when it is at a relatively long range. Otherwise the reaction time available would be too short for the ABM system to respond. Just for the sake of argument, let us suppose it takes 60 seconds for the ABM system to respond to a threat and fire its missile. In that 60 seconds, a 7000 mph HGV will travel 117 miles. Therefore, the defense would have to engage the HGV at ranges of much greater than 117 miles. The faster the ABM missile is, the closer the HGV could come to the target before it must be fired on. The faster the incoming HGV is, the more distant the HGV range at which the ABM battery has a last chance to shoot at it. Let us assume for simplicity that the ABM battery is co-located with the target, and that: R_M is the minimum safe distance at which the already fired ABM missile must hit the HGV, R_H is the distance of the HGV from the target when the ABM missile is fired, V_H is the speed of the HGV, and V_M is the speed of the ABM missile. Then it is very easy to show (derivation is left as an exercise for the student) that

R_H = (1 + V_H/V_M)R_M

Note that as the speed of the HGV increases relative to the ABM missile speed, the larger the HGV distance at which it must be fired on in order to hit  it at the minimum safe distance. The larger R_H is, the more chance and time the HGV will have to maneuver to evade the ABM missile.

THAAD

The system the U.S. Army uses for missile defense for an Army theater is called the Terminal High Altitude Area Defense (formerly called the Theater High Altitude Area Defense), or THAAD. It was originally designed to shoot down short-range and medium-range missiles, as well as Intermediate Range Ballistic Missiles (IRBMs) in the terminal phase of their flights.

However, because the radar system guiding the missile was found to have a greater capability than originally thought, the system also has a capability to engage ICBMs. The THAAD missiles have an operational range greater than 200 km (124 miles) and a speed of Mach 8.24 or about 6,260 mph. Clearly, the THAAD missiles are somewhat slower than both the Chinese and Russian HGVs.

Lockheed has proposed to upgrade the system to make it more capable of countering HGVs. Primarily, the changes would be to the missile to increase its range and speed, but there are limits as to how much speed the kinetic kill warhead could be given and still keep the entire system both mobile and economically practicable. The basic problem with both rockets and ordinary artillery is that the thrust to the projectiles is limited by the exhaust speed of the gasses propelling the artillery shell or rocket.  This limitation is removed by the next system at which we will look: rail guns.

Rail Guns

Rail guns can be thought of as something like artillery, except propellent gasses are not used to accelerate the projectile. Instead a magnetic force acting on a current, often called a Lorentz force, is used. The basic idea of how it works is shown in the figure below. The basic idea of the rail gun is illustrated

in the right-most figure, which is labeled “The principle behind the Lorentz force.” In that picture current flows from the power supply on the left down one of the conducting rails to a conducting armature, which is allowed to slide freely along the parallel rails. The current is then shunted across the armature to the second rail, and travels to the left on the second rail back to the power supply. If you were to take the thumb of your right hand and point it in the direction of the current, curling your fingers around the rail, the curled fingers would point in direction of the magnetic field lines produced by the current, which are approximately circular around the conducting rail. This is called the “right hand rule,” and the magnetic field lines produced are illustrated as the blue circles surrounding the rails. Note that in the figure, this produces magnetic fields that all point upwards in-between the rails, and downwards outside the rails.

The currents are then shunted across the armature, and the right hand rule again gives us circles centered on the armature, with the field lines again pointing upwards inside the current loop, therefore adding to the magnet field inside the current loop. The current crosses the armature to the second rail and races back down the rail to the power supply, completing the electrical circuit. Applying the right hand rule for a third time again gives us circular field lines surrounding the rail, with the fields again pointing upwards inside the current loop formed by the rails, the armature, and the power supply. Therefore the fields from all parts of the current loop add to each other constructively inside the current loop. Maxwell’s electromagnetic field equations tell us the amplitude of the produced field is directly proportional to the amplitude of the current.

In addition, a magnetic field produces a force on the current, the direction of which can be determined by yet another right hand rule: If you point the fingers of your right hand in the direction of the current and then curl them in the direction of the magnetic field, the thumb will point in the direction of the force, as illustrated in the figure. The rails and the power supply are clamped down so that they are not allowed to move. The armature however is allowed to slide precisely in the direction the force on it is pointing, causing it to accelerate in the parallel direction of the rails. If we attach some kind of projectile to the armature, or even better make the armature part of the projectile, such that the projectile is released when the armature reaches the end of the rails, the projectile is accelerated together with the armature to be released into free-space at the end of the rail gun. Because the magnetic field is directly proportional to the current magnitude, and the Lorentz force is directly proportional to both the magnetic field and current being accelerated, the acceleration of the projectile down the rail gun is directly proportional to the square of the current: double the current and you get four times the acceleration.

The result is a gun whose muzzle velocity is not limited by the speed of expansion of a burning gas, but by the amplitude of the current that can be forced through the rails and the sliding armature. Although there are problems to be answered by materials science, the biggest problem is obtaining power supplies large enough to produce the needed currents.

To persuade you that the rail gun is something that really exists and is not something out of science fiction, watch the video produced by CBS News below. In it the Chief of the Office of Naval Research, Rear Admiral Matthew Klunder, notes that the cost of a rail gun projectile is about 1/100th the cost of more conventional missiles used against aerial threats, costing many millions of dollars each. Each projectile of the type he picks up costs approximately $25,000. Why does it cost that much? Klunder tells us it has an electronic guidance system that allows it to change direction slightly to match changes in trajectory of the target. If it were to be a mere slug of metal it could cost orders of magnitude less. If the target is another ship, relatively slow moving compared to an HGV, the Navy might indeed use projectiles that are just solid metal. Also, Klunder notes that a Navy warship could carry many hundreds (probably thousands) of such warheads due to their very small size and weight.

One of the leading choices for basing rail guns are the new U.S. Navy Zumwalt class destroyers, of which there will be, unfortunately, only three. The first, the USS Zumwalt (DDG-1000) is undergoing sea trials, and the other two are yet to be built. Costs, limited budgets, and what was thought to be a changing threat environment cut down the final authorization of these ships from 32 to three.

The advantages of placing a rail gun on a Zumwalt destroyer are several. First, as a stealthy ship, the Zumwalt enhances the survivability of the rail gun system. Second, the Zumwalt could provide fleet defense against ICBMs and HGVs. Third, because of its mobility, it could move close to an enemy coast to engage ordinary ICBMs or HGV bearing missiles in their boost phase.

Another choice for basing rail gun systems would be to post them close to high-value targets to act as terminal phase point defense.

In the next post, I will examine the defensive possibilities for high power laser weapons, and the political reactions to the HGV problem.

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