One show at DARPATech was a conceptual model of Underwater Express, a radical submersible designed to blast through the depths at 100 knots - much faster than any underwater object apart from Russia's insane Shkval rocket torpedo.
The secret to Underwater Express, says program manager Khime Latt, is supercavitation - the same phenomenon as Shkval uses but applied in a different way. The sub's conical nose or "cavitator" generates a low-pressure pulse behind it that causes air dissolved in the water to cavitate, or form bubbles. Gas is then added to the stream to maintain cavitation along the length of the hull, dramatically reducing drag.
Low drag is important because the speed of a submarine vehicle basically maxes out when the propulsion plant is as big as the craft itself, Latt says. The model shows that much of the hull is filled with fuel tanks and motor, but that there's still a useful payload.
A 15-month Phase 1 program, with teams led by Northrop Grumman and GD Electric Boat, wraps up in November 2007 after water tunnel tests to prove the basic hydrodynamics. That is due to be followed by a 15-18-month Phase 2 in which subscale prototypes will be tested in open water. Finally, Phase 3 - possibly starting in 2009 - will lead to a demonstrator "of realistic size", Latt says. It should be capable of a 100 kt, ten-minute run.
Underwater Express would be powered by a high-energy-density, air-independent power system - such as a torpedo engine on steroids or a hydrogen peroxide turbine (like the WW2 German Walther submarines). The model shows a waterjet with a ventral scoop, but those details are likely to change.
The craft could be used for inserting teams into denied areas or planting sensors ahead of hostile ships, according to DARPA videos. What it won't be is precisely stealthy, and other challenges will include detecting and avoiding obstacles. Nobody wants to center-punch a whale at 100 knots.