Since the 1960s, academic, commercial and military researchers have attempted to realize a novel form of maritime propulsion involving no moving parts – no propeller, no drive shaft, no seals – just magnets and an electric current that silently propel a boat or submarine through water. It has been an elusive goal but tests are being conducted at Navy facilities this summer with an aim toward developing a practical, workable system.
Since the 1960s, academic, commercial and military researchers have attempted to realize a novel form of maritime propulsion involving no moving parts – no propeller, no drive shaft, no seals – just magnets and an electric current that silently propel a boat or submarine through water. It has been an elusive goal but tests are being conducted at Navy facilities this summer with an aim toward developing a practical, workable system.
Novelists and moviemakers, incidentally, have had greater success, albeit fictional but worthy of blockbuster status. In the 1990’s classic Cold War thriller written by novelist Tom Clancy and adapted into the film, The Hunt for Red October, a silent “caterpillar drive” was a fictional propulsion system on the top-secret Soviet sub that was responsible for much of the tension undersea. While sometimes depicted as an MHD drive, actually, the conception in Clancy’s work uses a long tunnel of interconnected stages with multiple rotors to move the water.
Real-world developers, however, have had some success over the decades demonstrating magnetohydrodynamic (MHD) drive technology for maritime use on a small scale, but it has been inefficient and impractical for full-scale systems due to a couple big tech hurdles. The first challenge has been 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.
To address the materials challenge, DARPA initiated 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 modeling 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.
We checked with Dr. Christine Sanders, PUMP Program Manager in DARPA’s Defense Sciences Office for an update on the program and got this progress report from her on July 31. Here is what she told Magnetics Magazine:
“Our PUMP performers have completed initial electrode and system development, along with internal testing,” said Dr. Sanders. “The PUMP DARPA and Government Independent Verification Team will begin testing the designs and materials later this summer at Navy test facilities to validate the performers models and assess the technical viability of each team’s device. Late this fall, we should have more to provide regarding those test results and the next steps for the PUMP Program.”
General Atomics and HRL Laboratories are key corporate participants
Participants in the program include General Atomics, responsible for designing and building the high-temperature superconducting magnets that are crucial for the MHD drive’s magnetic field, also HRL Laboratories which has developed a prototype MHD pump using a recirculating electrochemical hydrogen cell that allows for efficient and quiet pumping, with minimal bubble generation. Based in Malibu, California, HRL is a private R&D company owned jointly by Boeing and GM that focuses on physical and information science technologies for automotive, aerospace and defense applications.
Another notable partner is the University of Illinois, providing experience in electrochemical and corrosion modeling to develop a modeling and simulation toolset to guide the electrode design.
In April, HRL announced that it has demonstrated proof-of-concept on a unique approach to achieve a silent pumping system that replaces traditional mechanical moving parts with an electric current and a magnetic field, in work being performed for the PUMP program.
HRL’s new device uses a recirculating electrochemical hydrogen cell which enables a prototype magnetohydrodynamic (MHD) pump that could be 70% efficient as well as highly reliable – with a lifespan of more than 5 years. Key design benefits include:
- Nearly eliminates gas bubbles – producing 95% fewer bubbles than traditional electrolysis cells – to deliver quiet, gas-free pumping
- Produces no oxidative or corrosive elements (O2 or Cl2) which degrade electrode performance over time
In a typical MHD pump, a DC electrical current is passed through a volume of seawater, which interacts with an applied magnetic field, resulting in a Lorentz force on the ions in the water. As the ions accelerate, they drag the water molecules and generate thrust.
HRL’s concept includes uniquely tailored gas-diffusion electrodes in its MHD model. The innovation ensures that the hydrogen gas generated at the cathode does not form bubbles but instead diffuses-out to the back side of the electrode. The resulting H2 gas is then routed to the back side of the anode where it diffuses-in and is consumed. This completes the recirculation loop while preventing corrosive oxygen and chlorine bubbles from forming at the anode.
“With the successful demonstration of a viable method to achieve an efficient, quiet and reliable MHD pump, we hope that HRL will next have the opportunity to build a complete prototype test system for the U.S. Navy for further testing,” said Jason Graetz, principal investigator at HRL.
Yamato-1 on display in Kobe, Japan
“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,” according to Dr. Susan Swithenbank, former PUMP program manager, who earlier explained the PUMP program.
“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,” said Swithenbank.
A major problem when electric current, magnetic field, and saltwater interact 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 modeling of interactions between the magnetic field, the hydrodynamic, and the electrochemical reactions, which all happen on different time and length scales.
“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,” Swithenbank said. “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 a 42-month program begun in mid-2023. 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.
Magnetohydrodynamic drive proposed for hydrogen & oxygen production in Mars transfer
There are other applications for MHD drives. They are currently used in nuclear engineering and metallurgy to transport molten metals. An application for future space exploration is being considered by NASA.
Research at the Georgia Institute of Technology was recently selected for a NASA Innovative Advanced Concepts program to assess the technical feasibility of their magnetohydrodynamic water electrolysis technology. Giner Labs, a leader in space flight electrolysis R&D, is serving as a commercial partner in the program, led by principal investigator Alvaro Romero-Calvo, an assistant professor at Georgia Tech.
The production and management of oxygen and hydrogen are of key importance for long-term space travel, particularly for human transfer to Mars, explain the researchers. They have proposed an efficient water-splitting architecture that combines multiple functionalities into a minimum number of subsystems. Their approach employs a magnetohydrodynamic electrolytic cell that extracts and separates oxygen and hydrogen gas without moving parts in microgravity, hence removing the need for a forced water recirculation loop and associated ancillary equipment such as pumps or centrifuges.
Preliminary estimates indicate that the integration of functionalities leads to up to 50% mass budget reductions with respect to the oxygen generation assembly architecture for a 99% reliability level. Successful development could enable the recycling of water and oxygen in long-term space travel. Applications such as water-based SmallSat propulsion or in-situ resource utilization might also benefit.
For more info, see www.darpa.mil, www.nasa.gov, www.ga.com, www.hrl.com, www.ae.gatech.edu.
