The Pentagon Wants to Make Magnet
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The Pentagon Wants to Make Magnet

Apr 30, 2023

Warships, both on and under the sea, could someday sail without moving parts.

The Pentagon's advanced science division is taking a new look at a technology that promises to revolutionize how warships travel and fight in the world's oceans.

The Defense Advanced Research Projects Agency (DARPA) is challenging industry to see if development of state-of-the-art magnetohydrodynamic (MHD) drive systems could become reality. MHDs, which use electromagnetism instead of physical propellers, promise near-silent propulsion, a boon to submarines—and the ships that hunt them.

DARPA's program, known as Principles of Undersea Magnetohydrodynamic Pumps (PUMP), kicked off in late May. As the agency describes in its industry solicitation, the goal of the program is to develop and demonstrate MHD pumps that match the efficiency and exceed the reliability of conventional impeller-based pumps, while reducing noise. In other words, the new PUMP propulsion tech should be just as good as conventional propeller- and impeller-based tech. With its emphasis on reducing noise generation, the program is also pretty squarely aimed at submarines, and to a lesser extent other warships.

Traditional ship-propulsion systems use propellers to push water or pump-jets to pump water, in both cases generating thrust in the opposite direction. While generally efficient—a single propeller screw can easily move a large ship through water—the system is noisy. The machinery that powers the propulsors, engines, transmissions, pumps, and other equipment makes noise that can escape the ship and reverberate through the water column. This helps sonar systems, designed to detect noise, pick up on and track moving ships.

Another issue with conventional mechanical propulsion is cavitation. At higher speeds, an underwater propeller or pump jet impeller will generate gas bubbles that trail the vessel, which then pop and make a cacophonous noise. This can be a dead giveaway for a submarine trying to sneak around in the presence of enemy warships. The risk of cavitation can be minimized through proper design of the propeller or impeller, but there are trade-offs that can reduce overall ship performance and speed.

Magnetohydrodynamic drive-based propulsion avoids all of that. MHDs use the conductivity of seawater and an applied magnetic field to produce a Lorentz force, which is strong enough to move a ship. As this feasibility study produced by Argonne National Lab described it more than 30 years ago, "the MHD thruster is an electromagnetic pump which accelerates the fluid [seawater] to provide thrust."

MHD drives rely on magnetic fields and electric currents in a process that has far fewer noisy moving parts than other propulsion systems. This promises to make them far quieter than existing propulsion systems—an attractive feature for submarines. MHD drives also lack a physical propulsor physically moving water, so cavitation is minimized. Proponents believe MHDs would also make ships more maneuverable than traditional propulsion methods.

Submarines would benefit the most from MHD propulsion technology; they’re engineered to be as quiet as possible to keep passive sonar systems from detecting them. Submarine-hunting surface ships would also benefit, allowing them to quietly hunt subs and catch them off-guard.

MHD drives have been around for about 60 years. In 1991, Japan's Japanese Ship and Ocean Foundation completed the Yamato-1, a research and development test bed that used MHD propulsion technology. Yamato-1 was 110 feet long, had a displacement of 185 tons, and was powered by twin thrusters with superconducting magnets generating the magnetic field. The ship was capable of a top speed of eight knots.

MHD propulsion is perhaps best known from its appearance in the book and film The Hunt for Red October. In the novel, the Soviet Navy submarine Red October, a modified, larger Typhoon-class submarine, is outfitted with an MHD propulsion system to make it nearly silent. As a result, Red October was capable of creeping close to the Eastern Seaboard of the United States, evading U.S. anti-submarine patrols, then launching a crippling nuclear first strike.

As large as Yamato-1 was, technological constraints meant that 100 of its 185 tons were devoted to the propulsion system, limiting the usefulness of the ship. According to DARPA's PUMP solicitation, in order for MHD propulsion to be more practical, there needed to be "a higher magnetic field to increase efficiency; and (2) an electrode material system that addresses the effects of hydrolysis, bubble collapse erosion, and corrosion and can reliably operate with current and high magnetic fields in seawater."

DARPA thinks that a new effort at MHD propulsion could result in systems three times as efficient as those that powered Yamato-1. That could be enough to make such systems practical, and allow future warships to take full advantage of all that MHDs have to offer.

If you have some brilliant idea that will put magnetic drive systems on every attack submarine in the U.S. Navy, the contractor solicitation is here, and the agency wants proposals back from industry by July 31, 2023.

Kyle Mizokami is a writer on defense and security issues and has been at Popular Mechanics since 2015. If it involves explosions or projectiles, he's generally in favor of it. Kyle's articles have appeared at The Daily Beast, U.S. Naval Institute News, The Diplomat, Foreign Policy, Combat Aircraft Monthly, VICE News, and others. He lives in San Francisco.

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