The vast expanse of the world’s oceans has always posed a unique set of challenges for transportation and defense. Marine propulsion systems move ships through the water, ensures a better safety standard for the marine ecosystem and are cost-efficient. Marine propulsion is the mechanism or system used to generate thrust to move a ship or boat across water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of an electric motor or engine turning a propeller, or less frequently, in pump-jets, an impeller.
Traditional methods of underwater propulsion, reliant on propellers, drive shafts, and seals, have served us well, but we are now on the verge of a revolutionary breakthrough. Imagine a propulsion system that allows boats and submarines to glide silently through water, without any moving parts. This groundbreaking technology is becoming a reality with Magnetohydrodynamic (MHD) drive, and the Defense Advanced Research Projects Agency (DARPA) is at the forefront of this transformative endeavor with their PUMP program.
Magneto Hydrodynamic Propulsion for Ocean Vehicles
Magnetohydrodynamic drive or MHD accelerator is a method for propelling vehicles using only electric and magnetic fields with no moving parts, accelerating an electrically conductive propellant (liquid or gas) with magnetohydrodynamics. The fluid is directed to the rear and as a reaction, the vehicle accelerates forward.
The working principle involves the acceleration of an electrically conductive fluid (which can be a liquid or an ionized gas called a plasma) by the Lorentz force, resulting from the cross product of an electric current (motion of charge carriers accelerated by an electric field applied between two electrodes) with a perpendicular magnetic field. The Lorentz force accelerates all charged particles (positive and negative species) in the same direction whatever their sign, and the whole fluid is dragged through collisions. As a reaction, the vehicle is put in motion in the opposite direction.
The concept of Magneto-Hydrodynamic (MHD) propulsion can be used to implement a propeller-less propulsion system for marine vehicles. Electrodes are lined up along the walls of the duct which act as the source of the electric field. Seawater acts as the conducting medium for the current when it passes through the duct. This medium is then subjected to a strong magnetic field within the duct, thereby producing an axial force, i.e., an axial thrust. Propulsion systems based on MHD require virtually no mechanical components, therefore a good application would be to design a propulsor which produces very little noise for small underwater vehicles.
Recognizing the immense potential of MHD drives, DARPA launched the Principles of Undersea Magnetohydrodynamic Pumps (PUMP) program. This ambitious initiative aims to tackle the material challenge head-on by developing novel electrode materials capable of withstanding the demanding conditions of undersea propulsion. Drawing upon expertise from various fields, including hydrodynamics, electrochemistry, and magnetics, the program seeks to foster collaboration and innovation in order to achieve its goals.
For decades, researchers in academia, commercial enterprises, and the military have been exploring the potential of MHD drive technology. While early prototypes demonstrated its feasibility on a small scale, there were significant obstacles to overcome before implementing it in full-scale systems. Generating powerful magnetic fields and finding suitable electrode materials that could withstand the corrosive effects of saltwater were among the primary challenges.
Efficiency has been a historical challenge for MHD drives, but recent breakthroughs in generating high magnetic fields have rekindled hope. Historically, the efficiency of magnetohydrodynamic drives has been limited. The most efficient demonstration to date was achieved in 1992 on the Yamato-1, a 30-meter vessel that reached a speed of 6.6 knots with an efficiency of around 30%, using a magnetic field strength of approximately 4 Tesla.
However, recent advancements in rare-earth barium copper oxide (REBCO) magnets by the commercial fusion industry have shown the potential for large-scale magnetic fields of up to 20 Tesla. This breakthrough opens up the possibility of achieving 90% efficiency in magnetohydrodynamic drives, making further exploration of the technology worthwhile.
The comprehensive PUMP program, spanning 42 months, provides ample time for in-depth exploration of MHD systems. To solve the electrode materials challenge, DARPA is seeking expertise from various fields, including hydrodynamics, electrochemistry, and magnetics.
MHD corrosion challenges
One critical issue when dealing with the interaction of electric current, magnetic fields, and saltwater is the formation of gas bubbles over the electrode surfaces. These bubbles can negatively impact efficiency and lead to the erosion and collapse of the electrodes. The PUMP program aims to address this challenge by exploring different approaches to mitigate the effects of hydrolysis and erosion. Additionally, the program will enable modeling of the complex interactions between the magnetic field, hydrodynamics, and electrochemical reactions that occur on different time and length scales. Researchers are leveraging insights from industries working with fuel cells and batteries, aiming to adapt novel material coatings that have proven effective in mitigating bubble generation.
By leveraging multi-physics modeling and simulation tools encompassing hydrodynamics, electrochemistry, and magnetics, the aim is to design novel electrode materials capable of withstanding the corrosive and erosive effects of magnetic fields, electrical current, and saltwater. The ultimate goal of the program is to develop a prototype MHD drive that can be scaled up.
The agency hopes to leverage insights from industries working with fuel cells and batteries, as they face similar issues related to bubble generation. By forming interdisciplinary teams, DARPA aims to pool knowledge and experience to finally achieve a scalable and militarily relevant magnetohydrodynamic drive.
It encompasses both conductive and inductive approaches. The conductive approach involves a current flowing between a pair of electrodes within a magnetic field, while the inductive approach utilizes a time-varying magnetic field and electric current. By thoroughly investigating these methods and their applications, researchers can unlock the true potential of MHD drives.
The implications of successful MHD drive implementation go beyond revolutionizing underwater transportation; they extend to the realm of defense strategies as well. Silent and efficient undersea propulsion can significantly enhance surveillance capabilities and submarine operations. The ability to maneuver stealthily through the depths of the ocean provides a distinct advantage in military and security applications.
Revolutionizing underwater transportation and defense through the power of MHD drives represents a paradigm shift in maritime technology. As breakthroughs in generating high magnetic fields continue to unfold, DARPA’s PUMP program is actively addressing the remaining challenges associated with electrode materials. The collaborative efforts of researchers and experts from diverse disciplines are bringing us closer to realizing the full potential of MHD propulsion.
The future of underwater exploration, surveillance, and security is within reach. With the successful development and deployment of efficient and scalable MHD