Since the 1960s, researchers across academic, commercial, and military sectors have pursued a propulsion method that operates without moving parts—no propellers, shafts, or seals—relying instead on magnets and electric currents to drive vessels through water with minimal noise. This approach, known as magnetohydrodynamic (MHD) propulsion, has been demonstrated in small-scale experiments, but scaling it to practical, high-performance systems has proven elusive.
Two primary technical barriers have hindered progress. The first is the difficulty of generating magnetic fields strong enough to achieve high-efficiency pumping of seawater. The second is the lack of electrode materials capable of withstanding the harsh combination of corrosion, hydrolysis, and erosion induced by the interaction of magnetic fields, electrical currents, and saltwater. While recent advances in magnet technology have addressed the first challenge, the second remains unresolved.
To tackle the materials problem, the Defense Advanced Research Projects Agency (DARPA) has launched the Principles of Undersea Magnetohydrodynamic Pumps (PUMP) program. This initiative aims to develop novel electrode materials suitable for a militarily significant MHD drive. It will integrate multi-physics modeling and simulation tools—spanning hydrodynamics, electrochemistry, and magnetics—to guide the scaling of MHD designs. The ultimate objective is to identify an electrode material system and produce a prototype drive capable of being scaled to operational use.
Susan Swithenbank, PUMP program manager in DARPA’s Defense Sciences Office, highlighted a historical benchmark: “The best efficiency demonstrated in a magnetohydrodynamic drive to date was 1992 on the Yamato-1, a 30m vessel that achieved 6.6 knots with an efficiency of around 30% using a magnetic field strength of approximately 4 Tesla.” She noted that recent developments in rare-earth barium copper oxide (REBCO) magnets within the commercial fusion industry have achieved large-scale magnetic fields up to 20 Tesla. Such fields could theoretically enable efficiencies approaching 90% in MHD propulsion. “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,” she said.
One persistent issue in MHD systems is the formation of gas bubbles over electrode surfaces due to electrochemical reactions in saltwater. These bubbles reduce efficiency and, when they collapse, can erode the electrodes. PUMP will explore methods to mitigate hydrolysis and erosion, including advanced coatings and surface treatments. The program will also develop models that capture the interplay between magnetic fields, fluid dynamics, and electrochemical processes—phenomena that occur across disparate time and length scales.
Swithenbank emphasized the value of cross-disciplinary input: “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 realize a militarily relevant scale magnetohydrodynamic drive.”
PUMP is structured as a 42-month effort. It will investigate multiple system architectures, including conductive and inductive approaches. In the conductive configuration, an electric current flows between electrodes immersed in a magnetic field, directly propelling the fluid. The inductive approach, by contrast, employs a time-varying magnetic field to induce currents in the seawater, generating thrust without direct electrode contact.
The program’s timeline includes a hybrid Proposers Day scheduled for May 31, 2023, with registration closing on May 24. A Broad Agency Announcement detailing the full scope is expected in late May or early June 2023. By combining advances in high-field magnet technology with breakthroughs in electrode materials, DARPA aims to move MHD propulsion from decades of experimental promise toward operational viability.