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 an electric charge and is then accelerated by an electromagnetic field, generating thrust. Basically, an MHD drive consists of a hollow tube with electrodes at one end and magnetic coils around it. Since the device doesn't have any shafts, gears, propellers, turbines, or jets, it produces very little noise and even the small amount it does generate can be attributed to natural sources.
Such a stealth drive would be invaluable for submarine warfare. Not only would it allow submarines to remain hidden from hunters, it would also be a big help on reconnaissance and intelligence missions by removing the boat's interfering audio signal as its sonar gathers data.
The question is, if this technology is so valuable, why hasn't it been used for over 60 years except in a couple of experimental surface boats? The answer is twofold. First, the electromagnetic coils need to be extremely powerful and making ones that are light enough and efficient enough to install in a submarine isn't easy. The second is that the electrodes must stand up to a lot of wear due to corrosion, hydrolysis, and erosion caused by the interaction of the magnetic fields, electrical current, and saltwater.
In recent years, there have been huge strides in the development of magnets, but there's still room for improvement and finding the right materials for making the electrodes remains a problem.
To overcome this, DARPA has established its 42-month Principles of Undersea Magnetohydrodynamic Pumps (PUMP) program that will take multiple approaches to resolving these issues to create a practical military MHD drive, though without Mr Connery's glowering presence.
"The best efficiency demonstrated in a magnetohydrodynamic drive to date was 1992 on the Yamato-1, a 30-m (100-ft) vessel that achieved 6.6 knots with an efficiency of around 30% using a magnetic field strength of approximately 4 Tesla," said Susan Swithenbank, PUMP program manager in DARPA’s Defense Sciences Office. “In the last couple years, the commercial fusion industry has made advances in Rare-Earth Barium Copper Oxide (REBCO) magnets that have demonstrated large-scale magnetic fields as high as 20 Tesla that could potentially yield 90% efficiency in a magnetohydrodynamic drive, which is worth pursuing. Now that the glass ceiling in high magnetic field generation has been broken, PUMP aims to achieve a breakthrough to solve the electrode materials challenge."
In the case of the electrodes, a major obstacle is that gas bubbles tend to form over the electrode surfaces. This insulates them, reducing efficiency, and when the bubbles collapse they can damage the electrodes as if they'd been hit repeatedly with hammers. By developing computer models that evaluate the interactions of the magnetic field, the hydrodynamic, and the electrochemical reactions on different time and length scales, it may be possible to adjust the hydrodynamics, electrochemistry, and magnetics to decrease the damage while boosting efficiency.
"We’re hoping to leverage insights into novel material coatings from the fuel cell and battery industries, since they deal with the same bubble generation problem," said Swithenbank. “We’re looking for expertise across all fields to form teams to help us finally realize a militarily relevant scale magnetohydrodynamic drive."
Source: DARPA