Wednesday’s flight of the Air Force X-51A Waverider was certainly impressive. The X-51A – a Dustbuster-shaped air vehicle dropped from a B-52 bomber – used scramjet power to accelerate beyond a screaming-fast Mach 5 (five times the speed of sound).
That means Waverider went hypersonic. Its 200-second scramjet burn was the longest ever for that technology. Producing thrust from a scramjet (or supersonic combustion ramjet, if you prefer) is technically so challenging that it has been compared to lighting a match in a hurricane – and keeping it burning.
Well, great. But what’s the X-51A for?
The first mission for its technology likely would be use in a weapon. The X-51A could morph into a hypersonic cruise missile by as soon as 2015. That would provide the Air Force a long-sought Prompt Global Strike capability, defined as the ability to target any spot on earth with a conventional warhead within 60 minutes.
A new space vehicle?
Scramjets also hold promise for vehicles capable of taking off like aircraft and then flying into space. This could revolutionize US launch capabilities by 2025.
Unlike rockets, ramjets burn fuel mixed with oxygen from the atmosphere. They aren’t burdened with carrying their own combustion-feeding oxidizers. Unlike jet engines, air rams through their combustion chambers at supersonic speeds. That allows them to operate efficiently at much higher speeds.
That’s the theory, anyway. Scientists have tinkered with the technology for years. Air Force officials noted that Wednesday’s scramjet burn, at 200 seconds, did not go as long as they had planned. An “anomaly” terminated the flight short of its 300-second goal.
For the Pentagon, Prompt Global Strike might be the first X-51A payoff. The Air Force’s 2010 Posture Statement notes that the Department of Defense “plans to analyze conventional prompt global strike prototypes and will assess the effects that these systems, if deployed, might have on strategic stability.”
'Another weapon in the quiver'
The problem, from the point of view of the Air Force, is that current conventional systems are too slow to strike urgent targets. Say intelligence indicated that a nation in the Middle East was about to launch a ballistic missile with a chemical warhead at a neighbor. It would take hours for a US cruise missile or even a manned bomber to get to the site.
On Aug. 20, 1998, for instance, US warships in the Arabian sea launched a cruise missile attack on an alleged Al Qaeda training camp in eastern Afghanistan in the belief that Osama bin Laden was on site. The Tomahawks took two hours to fly the 1,100 miles to their target. By the time they arrived, reportedly he was gone.
One candidate for Prompt Global Strike is a Trident II submarine-launched missile equipped with a conventional warhead. ICBMs reach targets in minutes. Subs can be parked clandestinely in watery spots all over the globe.
But Trident IIs also carry nuclear warheads. Detecting a Trident launch could make other nuclear powers very nervous. Under extreme circumstances, they might consider quickly launching their own missiles, rather than have them caught by a surprise nuclear strike and destroyed on the ground.
Mach 6 a possibility
Waverider would not have this problem. Its angular snout and flattened body, designed to take advantage of lift from the shock waves generated by its hypersonic speed, look like no other air vehicle. And it might, eventually, sustain Mach 6 or greater speeds over extensive distances.
A potential weapon developed from the X-51A could reach Mach 8, enabling it to travel 600 nautical miles in eight minutes, notes a 2007 Air Force Research Laboratory briefing on “Disruptive Technology: Hypersonic Propulsion.”
Such a missile could carry a 300-pound payload – including conventional explosives, “smart” submunitions, or a penetrator to attack deeply buried targets, says the AFRL briefing.
Hypersonic scramjets might then develop along a stair-step upward path, building on previous models, according to the briefing. Medium-size scramjets could provide expendable, on-demand lift into space. Large models might make possible quick, manned space access – or a so-called “Orient Express” aircraft capable of flying from New York to Tokyo at fantastic speed.