How Do Hypersonic Weapons Work? And Your Other Questions Answered.

July 12, 2021
a hypersonic model test at NASA LangleyNASA
Cameron Tracy
Kendall Fellow

Hypersonic weapons—missiles that fly low-altitude trajectories at more than five times the speed of sound—are the focus of a burgeoning arms race between the United States, Russia, and China. To date, no nation has widely deployed these weapons, but development budgets have grown and testing activities have accelerated over the past several years.

Despite growing interest in these weapons, among both militaries and the media, much of the discourse regarding hypersonic missiles is based on unsubstantiated claims about their supposedly “revolutionary” performance. Evidence-based assessments are rare, but recent technical analysis has shown that many common beliefs regarding hypersonic weapon capabilities range from exaggerations to outright falsehoods.

To address this misinformation, UCS recently hosted a discussion of the technical and social dimensions of hypersonic weapon development. Participants submitted many questions, far more than could be addressed at the event. Here, we provide answers to some of the most frequently asked questions (FAQ). Part one of this FAQ deals with hypersonic weapon technology, while Part two addresses the roles of these weapons in global security policy.

The term “hypersonic weapon” commonly refers to two distinct categories of missile technology: boost-glide weapons and hypersonic cruise missiles. These technologies differ primarily in the manner in which they produce the thrust necessary to propel themselves to distant targets.

Boost-glide missiles consist of gliding vehicles mounted on the front of rocket boosters, much like those that launch spacecraft into orbit. These boosters contain large quantities of propellant—fuel and a chemical oxidizer. The reaction between these substances releases an intense burst of energy, accelerating the missile to high speeds.

Once they run out of propellant, typically a few minutes into flight, these rockets detach from the glider and fall back to Earth (many missiles feature multiple rocket stages that detach in sequence). The glider continues towards its target in unpowered flight. While the glider can take advantage of aerodynamic forces to generate lift and maneuver, it does not carry an engine or propellant and therefore cannot generate additional thrust.

Because boost-glide weapons carry their rocket boosters for only a short time, these boosters can be quite large and heavy. The use of large rockets allows boost-glide weapons to achieve very high speeds (up to more than twenty times the speed of sound), making them the fastest hypersonic weapons.

Unlike boost-glide weapons, hypersonic cruise missiles carry their engines with them for the duration of their flight. These engines must therefore be relatively small and light, limiting the maximum speeds these missiles can attain. As such, hypersonic cruise missiles might travel up to around ten times the speed of sound, much slower than what boost-glide weapons can achieve.

The engines that power hypersonic cruise missiles are known as scramjets (supersonic combustion ramjets). These are airbreathing engines, meaning they continuously draw in nearby air, which then reacts with fuel through combustion before it is expelled from the engine. Hypersonic cruise missiles therefore need not carry oxidizer, as a rocket does, nor do they need to actively compress incoming air, as their rapid forward motion does so. These engines generate thrust for nearly the entirety of hypersonic cruise missile flight.

Compared with rocket boosters, scramjets are a less-developed technology. Thus, in the near-term, most deployed hypersonic weapons will likely be of the boost-glide variety.

2. How do hypersonic boost-glide weapons compare with ballistic missiles?

Ballistic missiles, like hypersonic boost-glide weapons, are accelerated on the front of rocket boosters that detach once they run out of propellant. Ballistic missiles carry their explosive warheads in reentry vehicles which, like hypersonic gliders, proceed through the remainder of their trajectories in unpowered flight.

Because ballistic missiles and hypersonic boost-glide weapons are launched on similar or identical rockets, they reach the same maximum speeds. The primary area in which these two classes of missile technology differ is in flight altitude. After booster burn-out, ballistic missile reentry vehicles continue high into outer space on a trajectory controlled almost entirely by gravity. They can reach altitudes on the order of 1000 kilometers, for intercontinental flight, before gravity pulls them back to Earth’s surface.

Hypersonic gliders, on the other hand, dive back into the atmosphere early in flight, then pull up into a level trajectory and glide through the atmosphere at altitudes of tens of kilometers. This allows for more direct flight to their targets than ballistic missiles can achieve, since gliders stay close to Earth’s surface. However, low altitude flight also subjects them to atmospheric drag because, unlike the vacuum of outer space, Earth’s atmosphere is filled with relatively dense air.

Calculations show that the slowing effects of drag are so great that existing, currently-deployed ballistic missiles can fly to distant targets in less time than it would take a hypersonic boost-glide weapon to reach the same target. Thus, despite their more direct flight paths, hypersonic weapons cannot match the short delivery times of ballistic missiles.

3. Can hypersonic weapons carry nuclear warheads?

To deliver a nuclear warhead, a vehicle need only be large enough to hold the explosive device and powerful enough to carry its mass to a distant target. Both are achievable with many current hypersonic weapon designs. Modern warheads can be quite small, weighing on the order of 100 kilograms. This is a fraction of the mass of a typical hypersonic glider, which might weigh around 1000 kilograms.

Not only are hypersonic weapons capable of carrying nuclear warheads, they are currently deployed for precisely this purpose. Russia’s Avangard, an intercontinental-range hypersonic boost-glide missile, is deployed as a nuclear-armed system. According to US defense officials, China’s DF-17 missile, which carries the DF-ZF hypersonic glider, is designed to carry either conventional or nuclear explosives.

Current US hypersonic programs are, according to the Department of Defense, strictly non-nuclear. Yet leaked documents indicate that the department has solicited contract work on a hypersonic modification to the nuclear-armed ICBMs it is currently developing. This suggests at least some interest in nuclear-armed hypersonic weapons.

4. How are hypersonic weapons guided to their targets?

Like all missiles, hypersonic weapons can use a variety of techniques to determine and track their position and motion, enabling accurate guidance. One common means is inertial guidance, in which on-board accelerometers and gyroscopes record a missile’s every move during flight, so as to track its motion without the need to look at any external reference.

Better positional accuracy can be obtained by determination of a missile’s position relative to external reference points such as terrain near the missile’s target, stars in the sky, or satellites like those comprising the Global Positioning System (GPS). Unlike inertial guidance, these techniques require access to the outside world: sensors that can spot terrain or stars, or the ability to pick up radio signals.

These forms of communication are uniquely challenging during hypersonic flight. High speed flight through the atmosphere causes intense heating of the surrounding air. At sufficiently high speeds (above around twelve times the speed of sound), heating can be sufficient to ionize the atoms that make up the surrounding air, exciting electrons such that they detach from these atoms. This transforms the nearby air into a plasma in which charged particles tend to absorb radio waves, causing communication blackouts. In prior testing of hypersonic flight through the atmosphere, such blackouts have caused loss of communication with GPS satellites, degrading missile guidance.

5. Are hypersonic weapons more accurate than existing missiles?

Hypersonic weapons possess no intrinsic accuracy advantage over current missile technologies.

Consider first the hypersonic boost-glide missile. These face the same obstacles to accurate delivery as do ballistic missiles: wind, variations in air density, damage to aerodynamic surfaces from atmospheric heating, etc. In fact, the hypersonic vehicle will experience many of these for a greater duration than a ballistic missile would, because it stays within the atmosphere for a greater portion of its flight path.

Hypersonic weapons are capable of maneuvering to correct for deviations from an ideal flight path, and can do so for a much longer duration than ballistic missiles. But the corrective maneuvers required for accurate delivery tend to be small. Ballistic missiles equipped with maneuverable reentry vehicles (MaRVs), a technology the United States developed decades ago, are capable of executing these minor course adjustments in the brief period of atmospheric flight as they approach their targets. Maneuvering time is not the limiting factor in missile accuracy.

Consider next the hypersonic cruise missile. These are essentially faster versions of existing cruise missiles, and greater speed does not increase accuracy. They face the same challenges as boost-glide vehicles. Furthermore, one of the major challenges in the development of the scramjet engines that power these missiles is the difficulty of maintaining stable airflow through the engine at hypersonic speeds. This could constrain maneuvering and course correction, as large maneuvers might disrupt this airflow.

6. Are there effective defenses against hypersonic weapons?

Existing missile defenses generally fare poorly against fast and long-range ballistic missiles, particularly when decoys or other countermeasures are employed. The situation is broadly similar when it comes to hypersonic weapons.

The most widely-deployed variety of defensive system seeks to intercept missiles in the terminal phase of flight, as a missile approaches its target. Here, hypersonic weapons might be at a slight disadvantage relative to ballistic missiles. Both hypersonic weapons and ballistic missiles have similar maneuvering capability in this phase of flight, assuming the latter is equipped with a maneuvering reentry vehicle (MaRV). Thus, both could turn in flight, attempting to evade interceptors. A hypersonic weapon, however, will typically fly much slower than a ballistic missile at this stage, as its long-term flight through the atmosphere will have robbed it of much of its initial velocity. This would leave it more vulnerable to interception.

There also exist mid-course defenses, meant to intercept ballistic missiles in the intermediate stages of flight, before they approach their targets. At this point in flight, ballistic missiles travel through outer space, so existing mid-course defenses are designed to intercept outside the atmosphere. Hypersonic weapons, which travel within the atmosphere, would therefore fly under the reach of existing mid-course defenses, bypassing them.

Ultimately, these distinctions between hypersonic and ballistic missiles matter less than it might seem. Any missile travelling at very high velocity and designed to counter interceptors, be it a ballistic missile or a hypersonic weapon, would have a good chance of bypassing existing defensive systems.

7. Do hypersonic vehicles have peaceful applications?

Vehicles able to fly at hypersonic speeds for long durations have many current and potential applications, beyond their use as weapons. Perhaps the most notable of these peaceful applications is in the exploration of outer space. Spaceplanes—vehicles which can both travel through outer space and glide within a planetary atmosphere—enable the safe return of space missions to Earth, and the reuse of the vehicle for future missions. The US Space Shuttle was an example of such a system, one which carried out numerous hypersonic reentries.

Given the importance of these peaceful applications of hypersonic technologies, efforts to limit the development or deployment of hypersonic weapons must be carefully crafted so as to minimize their effects on peaceful hypersonic research.

The dual-use nature of this technology also represents an opportunity for efforts to slow or halt the hypersonic arms race. Investments to date, including in training and infrastructure, could be quickly converted to use in peaceful applications, such as the development of better spacecraft.