On 13 October Pakistan announced a test of its Fatah-2 ER and called it a hypersonic missile. Russia calls the Kinzhal hypersonic. So does China with the DF-17, and so does the US Air Force with the HACM it hopes to field later this decade. The word is everywhere, and it is doing a lot of work.

Strictly, "hypersonic" just means faster than Mach 5, five times the speed of sound. By that measure a ballistic missile warhead from the 1950s qualifies. What defence planners actually mean is narrower: a weapon that flies and manoeuvres above Mach 5 for a sustained part of its flight. Under that definition there are three quite different machines hiding behind one word. Here is how to tell them apart.

Type One: The Ballistic Missile, Fast but Predictable

Every long-range ballistic missile has been hypersonic since the 1950s. An intercontinental warhead comes back into the atmosphere at many times Mach 5. Speed was never the problem for defenders. The problem the newer weapons try to solve is predictability.

Once a ballistic missile's motor burns out, its warhead follows an arc set by gravity, like a thrown stone. A radar that tracks the first part of that arc can work out where it will land, and an interceptor can be sent to meet it. Some designs add a manoeuvring re-entry vehicle that can jink in the final seconds, such as the US Pershing II or China's DF-21D anti-ship missile, but most analysts still file them under ballistic rather than hypersonic.

The most famous member of this family is Russia's Kh-47M2 Kinzhal, carried by the MiG-31K of the Russian Aerospace Forces. Russia markets it as a hypersonic weapon. A May 2023 Brookings Institution analysis titled "Don't believe the hypersonic hype" described it instead as an air-launched ballistic missile: very fast, but flying a largely ballistic path rather than gliding or cruising. In May 2023 Ukraine said a US-made Patriot battery had shot one down over Kyiv, the first reported interception of a Kinzhal.

Type Two: Boost-Glide, the Skipping Stone

A boost-glide weapon starts like a ballistic missile. A rocket carries a glide vehicle to the edge of space and releases it at very high speed. Instead of falling back along a ballistic arc, the vehicle pulls up and glides through the thin upper atmosphere, using its own aerodynamic lift to stretch its range and to steer.

That flatter, lower path is the whole point. It keeps the vehicle below the altitudes where many missile-defence radars look for warheads for much of its flight, and because it can turn, a defender cannot simply extrapolate where it is going. The idea is old. Eugen Sänger's 1941 "Silbervogel" design for Nazi Germany was a rocket-boosted glider meant to skip along the top of the atmosphere, and the US Air Force's X-20 Dyna-Soar spaceplane of the early 1960s was a crewed boost-glider until it was cancelled in December 1963.

Making it work is brutal. DARPA's Falcon HTV-2 was boosted to near orbital speed on a Minotaur IV Lite rocket, and both of its test flights, in 2010 and 2011, ended early when the vehicle was lost. At those speeds the air around the vehicle becomes hot enough to burn through its skin, and control surfaces have to work in a regime wind tunnels can barely reproduce.

DARPA footage from the August 2011 flight of the HTV-2 boost-glide test vehicle. Video: DARPAtv

Russia and China got there first operationally. Russia says its Avangard glide vehicle, carried on an intercontinental missile, entered service in December 2019. China showed the DF-17, a road-mobile missile topped with a glide vehicle, at its October 2019 National Day parade. The United States is now fielding its own Common Hypersonic Glide Body on the Army's Long-Range Hypersonic Weapon and the Navy's Conventional Prompt Strike.

Type Three: The Scramjet Cruise Missile

The third type flies like an aircraft rather than a thrown object. A hypersonic cruise missile breathes air and burns fuel the whole way, using a scramjet: a supersonic-combustion ramjet with no moving parts, in which air enters, is compressed by the shape of the inlet, mixes with fuel and burns while still moving faster than sound. A scramjet produces no thrust at a standstill, so a rocket booster first has to accelerate the missile to roughly Mach 4 to 5 before the engine can light.

Afterburner readers have met the test vehicles that proved it could be done. NASA's X-43A reached Mach 9.6 in 2004, and the Air Force's X-51A Waverider flew on scramjet power for 210 seconds in 2013, both launched from under the wing of a B-52. The American weapon that follows them is the Hypersonic Attack Cruise Missile (HACM). Russia says its ship-launched 3M22 Zircon is already in service. Cruise missiles fly lower and with less speed than glide vehicles, but they can be smaller, fit on aircraft and ships, and steer throughout their flight.

Russian footage of a Zircon hypersonic cruise missile test launch. Video: news.com.au

Why Everyone Wants One, and Why Critics Are Unimpressed

The selling point of a true hypersonic weapon is the combination of speed and manoeuvre. Speed shrinks the time a defender has to react. Manoeuvre makes the target point uncertain until late, and the low flight path hides the weapon from distant radars for longer than a ballistic warhead. Together, the argument goes, they can beat today's missile defences.

Why hypersonic missiles are hard to stop. Video: The Washington Post

Not everyone accepts that. Physicists at the Union of Concerned Scientists argued in a study publicised in January 2021 that a glide vehicle loses so much speed to drag that a ballistic missile on a depressed trajectory can often arrive faster and be seen just as late.

His co-author was blunter about where the money goes.

So Which One Is the Fatah

Back to the headline that started this. Pakistan's military media wing, ISPR, described the Fatah-2 ER announced on 13 October as a hypersonic ballistic missile, an extended-range version of the Fatah-2 guided rocket. By the strict definition that makes it type one: very fast, guided, but not a glider or an air-breather. When the next country announces a "hypersonic" test, that is the question worth asking: does it glide, does it breathe, or does it simply fall very fast?

fast?

Sources: Wikipedia (Hypersonic weapon; Boost-glide; DARPA Falcon Project; Scramjet); Brookings Institution, "Ukraine and the Kinzhal: Don't believe the hypersonic hype" (23 May 2023); Union of Concerned Scientists (Tracy and Wright, January 2021); DARPA; Inter-Services Public Relations, Pakistan.

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What is a hypersonic weapon?

A hypersonic weapon is a missile that flies and manoeuvres above Mach 5, five times the speed of sound, for a sustained part of its flight. The term usually covers boost-glide vehicles and scramjet-powered cruise missiles. Ballistic missiles also exceed Mach 5 but follow a predictable arc, so most analysts do not count them.

What is the difference between a boost-glide weapon and a ballistic missile?

Both start on a rocket. A ballistic warhead then falls along a predictable gravity-driven arc. A boost-glide vehicle instead pulls up and glides through the upper atmosphere on its own lift, flying a lower, flatter path and steering as it goes, which makes it harder for radars to track and predict.

How does a scramjet hypersonic cruise missile work?

A scramjet is a supersonic-combustion ramjet with no moving parts. Air is compressed by the inlet shape, mixed with fuel and burned while still moving faster than sound. It produces no thrust at rest, so a rocket booster first accelerates the missile to roughly Mach 4 to 5 before the scramjet lights.

Is the Kinzhal a hypersonic missile?

Russia calls the Kh-47M2 Kinzhal hypersonic, and it does exceed Mach 5. A 2023 Brookings Institution analysis described it as an air-launched ballistic missile, carried by the MiG-31K, flying a largely ballistic path rather than gliding or cruising. Ukraine reported shooting one down with a Patriot over Kyiv in May 2023.

Which countries have hypersonic weapons in service?

Russia says its Avangard glide vehicle entered service in December 2019 and that the Zircon cruise missile is operational. China showed the DF-17 boost-glide missile in October 2019. The United States is fielding a common hypersonic glide body for the Army and Navy and is developing the HACM scramjet missile.

Why are hypersonic weapons hard to intercept?

Hypersonic weapons combine high speed with manoeuvre. Speed cuts the reaction time available to defenders, the low flight path keeps them below many radars for longer, and the ability to turn means their target cannot be predicted from early tracking. Critics argue a ballistic missile on a depressed trajectory can be just as hard to stop.

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