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It is the technology behind some of Tom Clancy’s greatest spy and submarine thrillers and now, the US Defense Advanced Research Projects Agency (DARPA) is taking a renewed look at finally making efficient, scalable magnetohydrodynamic drive for submarines a reality.
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Since the 1960s, academic, commercial, and military researchers have attempted to realise a novel form of maritime propulsion involving no moving parts.
Despite some success over the decades demonstrating magnetohydrodynamic (MHD) drive technology on a small scale, it has so far eluded the world’s experts, leaving the transformative technology still out of grasp.
The first challenge has been the inability to generate powerful enough magnetic fields to enable high-efficiency pumps. The second has been lack of electrode materials that can withstand corrosion, hydrolysis, and erosion caused by the interaction of magnetic fields, electrical current, and saltwater. In recent years, breakthroughs in generating high magnetic fields have been demonstrated, but the electrode materials problem remains.
In order to address this, DARPA recently announced the launch of the Principles of Undersea Magnetohydrodynamic Pumps (PUMP) program that seeks to create novel electrode materials suitable for a militarily significant MHD drive.
The program will assemble and validate multi-physics modelling and simulation tools including hydrodynamics, electrochemistry, and magnetics for scaling MHD designs. The goal of the program is to determine an electrode material system and prototype an MHD drive that could be scaled up.
Susan Swithenbank, PUMP program manager in DARPA’s Defense Sciences Office, said, “The best efficiency demonstrated in a magnetohydrodynamic drive to date was 1992 on the Yamato-1, a 30-metre vessel that achieved 6.6 knots with an efficiency of around 30 per cent using a magnetic field strength of approximately 4 Tesla.
“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 per cent 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,” Swithenbank explained further.
Previous attempts encountered a major problem — when electric current, magnetic field, and saltwater interact — which is the development of gas bubbles over the electrode surfaces. The bubbles reduce efficiency and can collapse and erode the electrode surfaces. PUMP will address different approaches to reduce the effect of hydrolysis and erosion. The program also will enable modelling of interactions between the magnetic field, the hydrodynamic, and the electrochemical reactions, which all happen on different time and length scales.
Swithenbank added, “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. We’re looking for expertise across all fields covering hydrodynamics, electrochemistry, and magnetics to form teams to help us finally realise a militarily relevant scale magnetohydrodynamic drive.”
PUMP is a 42-month program. There are multiple potential approaches to the MHD system including conductive and inductive approaches. The conductive approach involves a conductive current between a pair of electrodes within a magnetic field. The inductive approach uses a time-varying magnetic field and electric current.
