Next month, the Air Force will take a step toward an entirely new weapon that might solve such dilemmas: a hypersonic cruise missile that flies at over six times the speed of sound. That's when the first of four flight tests of the X-51 WaveRider is scheduled, Air Force chief scientist Werner Dahm said in an interview.
The X-51 test vehicle will be launched from a B-52 bomber, after which the vehicle's rocket booster will take it to speeds up to Mach 4.5. At that point, the supersonic combustion ramjet -- known as a "scramjet" -- will kick in. (A scramjet works by sucking in oxygen from the atmosphere, which allows it to be smaller and lighter than models with an onboard fuel tank.)
U.S. Air Force
An illustration of the X-51 WaveRider.
If all goes well, the X-51 will fly for five minutes at speeds of up to Mach 6.5.
A few minutes aloft may sound modest, but it's an impressive feat given that NASA's X-43A hypersonic vehicle, flown in 2004, provided only seconds' worth of flight time. A critical advance for the X-51 is the use of ordinary hydrocarbon jet propellant, which cools the walls of the combustor, allowing the engine to run as long as desired. "These are huge steps toward having a practical hypersonic capability for the Air Force," Dahm says.
If the X-51 does lead to a deployable weapon, it could fulfill what the Pentagon calls "prompt global strike" capability: the ability to hit any target in the world within about an hour.
The Pentagon has been looking at providing this capability by replacing the nuclear warheads on submarine-launched ballistic missiles with conventional ones. Those efforts, however, have been hampered by concerns that Russia would interpret any such launch as a possible nuclear attack, thus risking an accidental nuclear confrontation.
The "prompt global strike" capability is critical, advocates argue, if the U.S military is to be prepared to seize opportunities to take out top terrorist leaders or deal with some other immediate threat.
One hypothetical outlined by former defense secretaries Harold Brown and James Schlesinger in a Washington Post op-ed involved spotting a terrorist group with a nuclear weapon at a specific location. "A delay of even an hour or two in launching a U.S. strike on that location could mean the group would depart, contact might be lost, and the weapons would be smuggled into the United States or an allied nation and detonated," they wrote.
A hypersonic cruise missile also offers a distinct advantage as an alternative to an intercontinental ballistic missile: Because it would have a very different trajectory, and not leave the earth's atmosphere as an ICBM does, there's no risk of it being mistaken for a nuclear weapon.
Dahm stresses that the X-51 is merely a test vehicle, and that the Air Force still has a lot of work to do before it can add a hypersonic weapon to its arsenal. But after next month's test flight, it should have a clearer sense of just how long that journey might be.
