Hypersonic missiles: Speed is the new stealth

Hypersonic weapons: Building vehicles that fly at five times the speed of sound is amazingly hard, but researchers are trying

ON AUGUST 20th 1998 Bill Clinton ordered American warships in the Arabian Sea to fire a volley of more than 60 Tomahawk cruise missiles at suspected terrorist training camps near the town of Khost in eastern Afghanistan. The missiles, flying north at about 880kph (550mph), took two hours to reach their target. Several people were killed, but the main target of the attack, Osama bin Laden, left the area shortly before the missiles struck. American spies located the al-Qaeda leader on two other occasions as he moved around Afghanistan in September 2000. But the United States had no weapons able to reach him fast enough.

After the terrorist attacks of September 11th 2001, American officials decided that they needed to obtain a “prompt global strike” capability, able to deliver conventional explosives anywhere on Earth within an hour or two. One way to do this would be to take existing intercontinental ballistic missiles (ICBMs) and replace the nuclear warheads with standard explosives. The hitch is that ballistic missiles are usually armed with nuclear warheads. A launch could therefore be misconstrued as the start of a nuclear strike, says Arun Prakash, a former Chief of the Naval Staff, the top job in India’s navy.

Moreover, ICBMs carrying conventional explosives towards targets in Asia or the Middle East would at first be indistinguishable from those aimed at China or Russia, according to a paper issued by the Congressional Research Service, an American government-research body. This uncertainty might provoke a full-scale nuclear counterattack. In the years after 2001 funding for non-nuclear ballistic missiles was repeatedly cut by Congress, until military planners eventually gave up on the idea. Instead, they have now pinned their hopes on an alternative approach: superfast or “hypersonic” unmanned vehicles that can strike quickly by flying through the atmosphere, and cannot be mistaken for a nuclear missile.

These hypersonic vehicles are not rockets, as ICBMs are, but work in a fundamentally different way. Rockets carry their own fuel, which includes the oxygen needed for combustion in airless space. This fuel is heavy, making rockets practical only for short, vertical flights into space. So engineers are trying to develop lightweight, “air breathing” hypersonic vehicles that can travel at rocket-like speeds while taking oxygen from the atmosphere, as a jet engine does, rather than having to carry it in the form of fuel oxidants.

The term hypersonic technically refers to speeds faster than five times the speed of sound, or Mach 5, equivalent to around 6,200kph at sea level and 5,300kph at high altitudes (where the colder, thinner air means the speed of sound is lower). Being able to sustain flight in the atmosphere at such speeds would have many benefits. Hypersonic vehicles would not be subject to existing treaties on ballistic-missile arsenals, for one thing. It is easier to manoeuvre in air than it is in space, making it more feasible to dodge interceptors or change trajectory if a target moves. And by cutting the cost of flying into the upper reaches of the atmosphere, the technology could also help reduce the expense of military and civilian access to space.

All this, however, requires a totally different design from the turbofan and turbojet engines that power airliners and fighter jets, few of which can operate beyond speeds of about Mach 2. At higher speeds the jet engines’ assemblies of spinning blades can no longer slow incoming air to the subsonic velocities needed for combustion. Faster propulsion relies instead on engines without moving parts. One type, called a ramjet, slows incoming air to subsonic speeds using a carefully shaped inlet to compress and thereby slow the airstream. Ramjets power France’s new, nuclear-tipped ASMPA missiles. Carried by Rafale and Mirage fighter jets, they are thought to be able to fly for about 500km at Mach 3, or around 3,700kph.

It’s not rocket science

But reaching hypersonic speeds of Mach 5 and above with an air-breathing engine means getting combustion to happen in a stream of supersonic air. Engines that do this are called supersonic-combustion ramjets, or scramjets. They also use a specially shaped inlet to slow the flow of incoming air, but it does not slow down enough to become subsonic. This leaves engineers with a big problem: injecting and igniting fuel in a supersonic airstream is like “lighting a match in a hurricane and keeping it lit,” says Russell Cummings, a hypersonic-propulsion expert at California Polytechnic State University.

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