It is evident that, as worldwide stealth and counter-stealth capabilities improve, America is gradually losing the strategic advantage that its stealth warplanes have long provided. In addition, the United States can no longer rely on its cruise missiles (such as the Tomahawk) as these weapons are simply too slow (the Tomahawk flies at 880 km/h) to meet the demands of the War on Terror. For example, during the hunt for Osama Bin Laden there were repeatedly opportunities to eliminate him, but there were no weapons able to reach him fast enough. What the United States now needs is a prompt global strike capality; a weapon system that can deliver conventional explosives anywhere in the world within two hours.
One way for America to obtain this capability would be to refit existing intercontinental ballistic missiles (ICBMs) by swapping their nuclear warheads with standard explosives. The problem with this is that since ICBMs are usually armed with nuclear warheads, and since an ICBM launched at a target in Asia or the Middle East would initially be indistinguishable from one launched at Russia or China, the launch might very well provoke a nuclear counterattack. This however, has not stopped China from developing the DF-21D, a nuclear ballistic missile, refitted with conventional explosives, designed to drop from space at hypersonic speeds and destroy aircraft carriers in the Western Pacific Ocean.
Incidentally, hypersonic speed is also the solution to America's problem. Military planners, like those at DARPA, the research arm of the United States’ armed forces, have their eyes on superfast unmanned vehicles that could strike quickly by flying through the atmosphere (rather than through space), and which therefore cannot be mistaken for nuclear missiles. These vehicles would travel at hypersonic speed, defined as speed in excess of Mach 5 (a mile a second or 6,200 km/h at sea level, or 5,300 km/h in the upper atmosphere where sound travels more slowly), without relying on a rocket (like an ICBM) but rather by using a jet engine capable of "breathing" supersonic air. While rockets carry their fuel as well as the oxidants required to burn it in the vacuum of space, all of which is very heavy, this new breed of hypersonic missiles will draw their oxygen from the atmosphere like a turbofan or turbo-jet engine.
However, a jet engine capable of propelling a missile at hypersonic speeds requires a totally different design from a turbofan or turbojet engine. The latter engines are only capable of operating up to speeds a little beyond Mach 2; any faster and their assemblies of spinning blades would no longer be capable of slowing incoming air down to the subsonic speed needed for combustion. Faster propulsion necessits a different design, such as a ramjet engine, devoid of moving parts, which slows incoming air down to subsonic speed with a precisely shaped inlet, and which can operate at Mach 3. Once again, reaching faster speeds with an air-breathing engine means another new design. Speed in the hypersonic realm of Mach 5 and beyond requires sustained combustion in a stream of supersonic air, a feat only supersonic-combustion ramjets, or scramjets, are capable of.
Injecting and igniting fuel in supersonic air is much more difficult than it already sounds, and has been compared by hypersonic-propulsion experts to "lighting a match in a hurricane and keeping it lit". Techniques to mix the fuel with the air include using injectors that protrude into the supersonic airstream to produce shockwaves that do the mixing. Ways to then ignite this mixture include using larger shock waves produced by the entrance to the combustion chamber or using laser blasts to produce pockets of hot plasma. Of course the ignition of improperly mixed fuel would cause the entire thing to blow up. Additional difficulties include the impossibility of controlling a scramjet's internal temperature and air pressure by mechanically adjusting the air intake. Instead as a scramjet accelerates it must also ascend into thinner air to, again, avoid the entire thing blowing up. Lastly, since a scramjet engine needs to be moving quickly to compress air for combustion, it cannot begin flight on its own power and instead must piggyback on a jet plane or rocket.
Despite all of these formidable obstacles, being able to build a missile capable of hypersonic speeds in the atmosphere would yield many benefits other than giving America the strategic edge in the War on Terror and in the Pacific. For one thing, such missiles would not be subject to existing ballistic-missile treaties. In addition, manoeuvering in air is much easier than in space, so these vehicles could have a greater facility dodging interceptors and changing trajectory. Finally,the creation of hypersonic vehicles would also cut the cost of flying into the upper atmosphere, allowing greater military and civilian access to space. Though true hypersonic vehicles are still years away, early tests have proven promising. In November 2004, NASA's X-43A accelerated to a record Mach 9.83. In May 2013, NASA's X-51A flew at Mach 5.1 for a record four minutes.
Tests are also being run with simpler boost-glide vehicles, which piggyback on modified ICBMs into the lower reaches of space and then simply glide back down to Earth at more than Mach 16. This kind of thing has been done before; NASA’s space shuttles used to glide back down to Earth at Mach 25. This sort of blistering speed presents the additional problem of creating so much friction that the layer of air around the vehicle turns into a superhot plasma. Nonetheless, with the ongoing invention of new heat-proof materials, such as ceramic matrix composites, and testing equipment, such as wind tunnels that can generate blasts of air at Mach 14, the Chuch Yeager-esque quest for hypersonic flight is progressing apace. With this technology, it may be possible in our lifetimes to board a plane in New York and arrive in London one hour later, or even to board a plane and arrive in low Earth orbit.