Saab Bofors Dynamics has announced three successful tests of an experimental guided missile capable of Mach 5.5 speeds - that's 4200 mph at sea level, for a ground-launched missile, or more than a mile per second.

The hypervelocity missile (HVM) project has been mentioned very little in the past - Sweden's defense establishment is often very successfully secretive, not talking about technology until it has been demonstrated.

In this case, the key breakthrough is the ability to control and steer the missile at high speeds. The challenge is the enormous dynamic pressure, the result of high speeds and low altitude. For example, a very small amount of sideslip generates very large lateral aerodynamic forces on the body. Academic papers also indicate that the designers have worked on lightweight, high-energy motors to sustain these high speeds.

HVMs have some useful characteristics. Saab notes that the high speed is useful against armored targets, although the test missile seems pretty big for an anti-tank weapon: however, terminal speed and energy are hard to protect against.

On the other hand, it's interesting to speculate as to whether the HVM development might originally have been associated with the now-canceled counter-stealth air defense system that Saab Microwave unveiled a couple of weeks ago.

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