April 30, 2016

Hypersonic boost glide weapon analysis, flight profiles, detection and early warning

From a purely technical perspective, hypersonic boost-glide weapons would offer certain unique attributes to military planners. Their speed is unmatched by any other kinetic weapon, except for ballistic missiles. And, compared to ballistic missiles, boost-glide weapons have potentially longer ranges, can generally transport a heavier payload over a given range, are capable of midcourse maneuvering, and fly at lower altitudes. Understanding whether these attributes would be likely to provide a significant military advantage and, ultimately, whether the benefits of boost-glide weapons would outweigh their costs and risks raises a complex series of technical and policy questions."

* boost-glide roughly doubles the range over the purely ballistic trajectory.
* In contrast to these maneuvering warhead concepts, there has been growing interest in the traditional boost-glide concept not to extend range per se, but to allow it to reach a given range while flying at a much lower altitude. The goal in this case is to keep the reentry vehicle below radar coverage until it enters the terminal phase.

Science and Global Security: The Technical Basis for Arms Control, Disarmament, and Nonproliferation Initiatives, Hypersonic Boost-Glide Weapons

The United States, Russia and China are developing hypersonic boost-glide vehicles. A simple model of their trajectory is developed by assuming that the vehicle does not oscillate during the transition to equilibrium gliding

A model is also used to calculate the tactical warning time that a boost-glide attack would afford an adversary. Other aspects of boost-glide weapons’ military effectiveness are explored. Approximate calculations suggest that, compared to existing non-nuclear weapons, boost-glide weapons could penetrate more deeply but would be less effective at destroying silos. The distance at which a boost-glide weapon armed with a particle dispersion warhead could destroy a mobile missile is also calculated; it is expected to be significantly larger than for an explosive warhead.

The current American attempt to develop boost-glide weapons dates to 2003 when the administration of George W. Bush initiated a program that became known as Conventional Prompt Global Strike (CPGS) to develop fast, long-range, non-nuclear weapons. The United States has since tested two gliders: the Hypersonic Technology Vehicle-2 (HTV-2) and the Advanced Hypersonic Weapon (AHW).

The HTV-2, which had a planned range of 17,000 km, was tested in April 2010 and August 2011. Both tests were terminated prematurely and this program has now been effectively canceled. Instead, current U.S. efforts are focused on the AHW. According to a 2008 study by the National Research Council of the U.S. National Academies, the AHW would have a range of about 8,000 km and might, therefore, be more accurately described as a non-global Conventional Prompt Global Strike weapon. The AHW was tested successfully in November 2011. A second test, in August 2014, failed because of a booster problem.

In January 2014,Beijing tested a boost-glide system for the first time. A second test, in August 2014 over a planned range of 1,750 km, appears to have ended in failure following a booster problem. There is some evidence that, unlike the United States, China's goal is the delivery of nuclear weapons—although the overall scale and scope of the Chinese program remain extremely murky.

China just completed a seventh successful hypersonic test.

Russia has its own Conventional Prompt Global Strike program.

A standard exo-atmospheric gliding trajectory, used by the HTV-2 for example, is shown below.
Schematic diagram of the different phases of an exo-atmospheric boost-glide weapon's trajectory. The labels, tn, indicate the time at which each phase ends. For clarity, radial distances are exaggerated relative to tangential distances, making it appear as though the booster's trajectory is lofted, whereas it is actually depressed.

Alternatively, it is possible to launch the glider on such a highly depressed trajectory that it never leaves the atmosphere. This strategy appears to have been adopted for the AHW, which the National Research Council describes as “endoatmospheric.” In theory, if an endo-atmospheric booster is able to attain horizontal flight at exactly the right altitude it could inject an RV straight into equilibrium gliding without the need for a pull-up. It does not appear, however, as though the AHW test flight involved this kind of direct injection. So, in practice, the HTV-2 and AHW trajectories are probably quite similar after the start of the pull-up.

Detection and early warning of hypersonic boost glide missile attack

Three different types of early-warning system are considered:

  1. satellite based infra-red sensors designed to detect missile plumes, and
  2. ballistic missile early-warning radars and
  3. modified air defense radars designed to detect incoming re-entry vehicles (RVs).

An area for further study is the possibility of detecting the heat signal of an incoming glider by airborne or space-based infra-red detectors.

Currently, only the United States operates satellite-based infra-red sensors (known in U.S. military jargon as overhead persistent infra-red sensors) to detect ballistic missile launches. Following a series of technical failures, none of Russia’s early-warning satellites are operational at the moment, but it had a space-based early-warning capability until 2014.

Because a boost-glide weapon would be launched by a rocket very similar to a long-range ballistic missile—if not by a repurposed intercontinental ballistic missile (ICBM) or sea-launched ballistic missile (SLBM)—it would be detected by an appropriately positioned satellite very shortly after launch; at the latest immediately after penetrating through any clouds that happened to be present. For satellite-based early warning systems, therefore, the warning time would essentially be equal to the total weapon travel time.

An alternative means to detect an incoming boost-glide weapon would be radar. The United States, Russia and China all operate large land-based radars designed to detect incoming intercontinental-range missile RVs early in flight. The differences between the boost-glide and ballistic trajectories have important implications, however, for monitoring by such radars. The HTV-2, for example, is designed to glide at an altitude of less than 50 km and to approach the target at 30–40 km. By contrast, ICBMs generally “top out” at well over 1,000 km. In consequence, an incoming hypersonic glider would be located below a radar’s horizon—that is, hidden from it by the Earth’s curvature—for much more of its trajectory than a ballistic missile. Thus radars would provide much less warning time of a boost-glide weapon attack than a ballistic missile attack.

Finally, a state could attempt to use a less powerful air-defense radar to detect an incoming boost-glide weapon. In this case, the detection distance would depend on the radar cross section, σ, of the incoming RV.

Existing air-dropped penetrators are reported to be able to reach speeds of between 460 and 500 meters per second, significantly lower than is required to maximize penetration depth.

Boost-glide weapons weapons, by contrast, could deliver a penetrator at the optimum speed. In fact, they would generally need to slow down from their cruising speeds to prevent failure of the penetrator on impact.

Optimum impact speed maximizes penetration depth. This speed depends on the yield strength of the penetrator’s shell and is between about 1, 000 and 1, 200 meters per second for modern materials.

There are plans to weaponize the HTV-2 called for it to be armed with a particle dispersion warhead with a total mass of about 390 kg consisting of 70 kg to 90 kg of high explosive and “several thousand debris particles, each measuring no more than a few centimeters...in diameter.

The high explosive in this kind of weapon is detonated shortly before impact in order to create an expanding cloud of debris particles. These particles aim to damage the target through their kinetic energy (the high explosive plays no direct role in effecting this damage). Changing the height at which the explosive is detonated changes the size of the weapon’s “footprint” on the ground. Clearly, a large footprint is desirable to mitigate uncertainty about the location of a mobile target. However, increasing the size of the footprint reduces the density of debris particles, increasing the probability that, even if the weapon’s footprint overlaps the target, none of them hit it. There is, therefore, a trade-off involved in choosing the size of the footprint.

For a particle dispersion warhead mounted on a boost-glide weapon, the kinetic energy of the debris particles could be extremely large. For example, if there were 4, 000 particles in the warhead with a combined mass of 300 kg, and if the weapon reached the target at “only” 2, 000 meters per second, then the kinetic energy of each particle would be 150, 000 J. By way of comparison, a particle with an energy of 20, 000 J is required to inflict heavy damage on an aircraft. An energy of 150, 000 J therefore seems more than sufficient to penetrate the missile skin and any protective canister, providing it is not too highly armored (which seems unlikely given the weight constraints imposed by mobility).

A ballistic missile can be effectively disabled by penetrating its motor with even a single debris particle.

If the particle dispersion weapon contains 4, 000 particles then the weapon footprint should have a radius of 94 meters. Boost-glide weapons would be more effective at attacking mobile missiles if armed with a particle dispersion warhead rather than an explosive warhead.

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