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In this report, the Congressional Budget Office analyzes the hypersonic weapons being developed by the U.S. military and compares them with less expensive existing or potential weapons that might fill similar roles, such as ballistic missiles or cruise missiles. CBO reached the following conclusions:

After many decades of conducting basic research, the Department of Defense (DoD) recently increased its spending to develop technology for hypersonic weapons. The Air Force, Army, and Navy all plan to field hypersonic missiles within the next few years. China and Russia have stated that they are also fielding such weapons.

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Hypersonic missiles with those ranges, however, would be more expensive than similar ballistic missiles and pose much greater technical challenges (see Figure S-1). CBO estimates that hypersonic missiles would cost roughly one-third more than ballistic missiles with maneuverable warheads that had the same range and accuracy and traveled at similar speeds. (The United States does not currently field such ballistic missiles, but the technology for them is well developed.)

Hypersonic weapons could be better than ballistic missiles at penetrating long-range missile defenses that operate outside the atmosphere. So far, however, no potential U.S. adversaries have succeeded in developing such defenses. Against shorter-range defenses, it is unclear whether hypersonic missiles would have an advantage over ballistic missiles with maneuverable warheads.

Given their cost, hypersonic weapons would provide a niche capability, mainly useful to address threats that were both well-defended and extremely time-sensitive (requiring a strike in 15 minutes to 30 minutes). If time was not a concern, much cheaper cruise missiles could be used. If targets were time-sensitive but were not protected by defenses that effectively intercept incoming ballistic missiles in the middle of their flight, less costly ballistic missiles with maneuverable warheads could be used.

Of the potential alternatives that CBO examined for those types of missions, only ballistic missiles equipped with maneuverable reentry vehicles (MaRVs) and hypersonic boost-glide missiles would have the combination of speed and range to strike targets under the strict time constraints associated with the most challenging A2/AD scenarios.

The United States does not currently field intermediate-range ballistic missiles, in part because some of those missiles were prohibited by the Intermediate-Range Nuclear Forces treaty, which the United States was a party to from 1988 to 2019. Over the years, however, the United States has mastered the technological challenges of operating such missiles. For example, in the 1980s, the U.S. military deployed Pershing II medium-range ballistic missiles with maneuverable warheads. Both the hypersonic missiles that DoD is currently developing and the ballistic missiles with MaRVs that CBO included in its analysis would probably be accurate enough to strike many fixed targets. However, both missiles would need improved targeting technology (such as a homing seeker or other type of sensor) to improve their accuracy or to enable them to strike moving targets.

Realizing the full potential of hypersonic weapons will depend on future success in achieving some of the improvements now being researched and developed, including missile components such as transparent communications windows that can withstand the high heat of extended flights. DoD is taking a phased approach to fielding hypersonic missiles. Early versions of those weapons will have capabilities at the lower end of the desired ranges and will not be able to locate targets autonomously or maneuver in response to attacks by missile defenses. However, those early hypersonic missiles may be able to carry out small maneuvers on preplanned trajectories to make them more difficult for defenses to track and intercept. Further enhancements would increase the risks of cost increases and schedule overruns.

The United States has not yet fielded such weapons, for both scientific and policy reasons. Hypersonic missiles are expensive, and there have been questions about the value of the capabilities they might provide. Achieving the desired performance for those weapons would require significant investment in additional research. Nevertheless, because of their potential, the U.S. military has established accelerated programs for hypersonic missiles, sharply increasing its support and funding, testing prototypes, and creating units in anticipation of fielding the weapons in the next few years.

Missiles are considered to be hypersonic not only on the basis of their speed but also on the basis of their flight profile and means of control. The harshness of the environment around a hypersonic missile as it flies makes designing such missiles challenging. (For explanations of missile speeds and other terminology, see Box 1-1.)

hypersonic missile: a missile that travels within the atmosphere (rather than above it) at speeds greater than Mach 5 and that can maneuver in the atmosphere for much of its flight. Hypersonic missiles can take various forms, such as boost-glide missiles or cruise missiles.

cruise missile: a missile that typically flies at low altitudes (ranging from less than 100 meters for a subsonic cruise missile to about 30 km to 40 km for a hypersonic cruise missile) and that is powered throughout its flight. A hypersonic cruise missile is initially accelerated to speeds approaching hypersonic by a rocket booster, then accelerates and maintains speed throughout its flight using a jet engine called a supersonic combustion ramjet (or scramjet) that operates at speeds above Mach 4.

Another defining characteristic of hypersonic missiles is that they depend on aerodynamic control surfaces (such as wings or tail fins) to glide and maneuver, like an aircraft, rather than using thrusters, like a spacecraft. Air is necessary for control surfaces to function, so a hypersonic missile must be within the atmosphere to maneuver.

The U.S. military is developing two types of hypersonic missiles: boost-glide missiles and cruise missiles. Both types need air to operate for reasons besides maneuvering. A hypersonic boost-glide missile consists of a rocket motor that accelerates the missile to a high altitude and speed and a glide body that detaches from the spent rocket. In addition to the kinetic and potential energy from its initial acceleration, the glide body uses lift generated by its movement through the air to extend its range and maneuver to hit its target (see Box 1-1).

A hypersonic cruise missile is also initially accelerated to a high speed by a rocket booster. After that, it accelerates and maintains speed throughout its flight by using a type of jet engine known as a supersonic combustion ramjet (or scramjet). A scramjet uses oxygen from the air to burn its fuel, rather than carrying an oxidizer, as a rocket does, so it is known as an air-breather. Scramjets require supersonic air flow and only begin to operate at speeds above Mach 4. Although air-breathing engines tend to be smaller and lighter than rocket engines that carry both fuel and oxidizer, that size advantage is lessened by the size of the rocket booster used for the initial acceleration.

Heating and Thermal Shielding. Moving at very high speeds within the atmosphere creates thermal challenges for an object such as a hypersonic glide body. The leading edge (the nose cone or the front edge of a wing) compresses the air ahead of the object, heating the air. The more rapidly the air is compressed, the hotter it will become. Drag also generates heat along any exposed surfaces, but to a lesser degree than air compression does. As the object moves through the heated air, its surfaces will begin to warm.

Flying at hypersonic speeds through the atmosphere for more than a few minutes presents challenges greater than those of reentering the atmosphere. Materials routinely used on reentry vehicles (such as ballistic missiles and the space shuttle) work over the shorter time frame associated with reentry to absorb heat or dissipate it through chemical processes. Such materials are not sufficient for hypersonic flight because they will eventually conduct heat inward or be used up.

Hypersonic flight continues to be extraordinarily challenging. But in recent years, computational science and materials science have advanced enough to transition to developing practical technologies for use in operational hypersonic missiles.

The research from those programs is being applied to several current efforts to develop an operational hypersonic cruise missile. Although scramjet technology appears to be maturing, integrating the engine into a missile that is small enough to be tactically useful will require significant further efforts. Trade-offs exist between speed, size, and range that might be addressed with improvements to fuel efficiency. Such improvements are being explored in laboratory testing and with modeling and simulation.

Much of the research into hypersonic cruise missiles being conducted worldwide is exploring the use of hydrogen fuel. Hydrogen has many characteristics that make it desirable for use in scramjets, such as high flammability and high caloric value (a measure of the energy that can be extracted during combustion). Hydrogen-fueled scramjets could theoretically operate at speeds higher than Mach 20.4 And many researchers prefer hydrogen to hydrocarbon-based fossil fuels for environmental reasons, because it produces only water as a byproduct. But hydrogen fuels also have downsides. Their extreme volatility increases safety concerns during the storage and handling of a missile, and their lower density than hydrocarbon fuels makes it harder to design a missile with the sort of range that DoD wants.5 For those reasons, DoD is likely to use hydrocarbon fuels in hypersonic cruise missiles, which would limit their maximum speed to about Mach 9. be457b7860