Our competitors Defense News report that the Air Force has handed Lockheed's Skunk Works a contract to develop "a stealthy 4,000-mph plane capable of flying to altitudes of about 100,000 feet, with transcontinental range." The rest of the story is subscription-only.

If this sounds to you a lot like the "Aurora" stories of the early 1990s, you're right. However, early last year I had a conversation with a senior Skunk in which he talked about the company's proposal for a new high-speed, high-altitude X-plane.

The X-plane would be the size of a fighter and would be designed for a speed of Mach 6.5 -- 4300 mph -- at 100,000 feet. (The SR-71 Blackbird, retired in 1990, could manage up to Mach 3.3 in sprints at 85,000 feet). It would be powered by two jet engines -- bigger versions of the engine used on the Skunk Works' RATTLRS (Revolutionary Approach To Time-critical Long Range Strike) cruise missile -- integrated into ramjets.

The speed -- less than DARPA'S Falcon Hypersonic Cruise Vehicle project or the USAF's X-51 scramjet demonstrator -- is important. At Mach 6.5, the vehicle can be powered by ramjets, rather than having to incorporate a scramjet (supersonic combustion ramjet) mode into the system. It would take off from a runway and land under power, not as a high-speed glider. It can burn near-standard hydrocarbon fuel, not hydrogen or a similarly exotic propellant. It could be made from conventional materials -- even composites -- with heat-resistant materials confined to the leading edges.

This is important because the idea of the X-plane is not to prove that such an airplane can fly at 4,300 mph but that it is "doable, practical and will work like a regular airplane." (Conspiracy theorists may choose to speculate about why the Skunks regard Mach 6.5, in itself, as No Big Deal.)

And why? The senior Skunk explains that high-fast stresses the defenses in a completely different direction from a stealthy airplane. Stealth aircraft are hard to detect -- but they tend to be slow and easy to hit. A high-fast aircraft may be easy to detect but it is a bugger to hit. Any missile has to lead the target -- or it will never have the energy to catch it -- and it has to lead the target by a long way because the target is covering more than a mile every second as the missile ascends. And at the same time, even a wide turn by the target causes the predicted impact point to move by miles.

In the present budget environment it's unlikely that the Skunk Works has been handed a blank check to build an X-plane, let alone an operational aircraft -- but its seems that the Mach 6 proposal is gaining traction.