Taking a plot point from the 1990 Sean Connery movie thriller The Hunt for Red October, DARPA is working on a super-silent submarine drive that has no moving parts and provides propulsion through the water using magnets and electricity. In Red October, the titular Soviet super-submarine was equipped with a fictional stealth drive that was based on a very real technology. Since the late 1950s, engineers have been interested in an exotic concept called magnetohydrodynamics (MHD). It's a very simple principle that produces a very simple propulsion mechanism. In an MHD drive, a fluid, like air or water, is given...