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Advancing the Depths: DARPA’s APEX Propulsion Technology for Submarines and UUVs


The world beneath the ocean’s surface is a realm of mystery and intrigue, home to some of Earth’s most uncharted territories. In this underwater world, submarines play a pivotal role in scientific exploration, defense, and more. However, to unlock the full potential of this hidden domain, propulsion technology needs to continually evolve. The Defense Advanced Research Projects Agency (DARPA) has stepped up to the challenge with its APEX program, aimed at revolutionizing propulsion systems for both crewed submarines and unmanned underwater vehicles (UUVs). In this article, we explore the significance of DARPA’s APEX program, the breakthroughs it promises, and the potential implications for undersea exploration


The Need for Speed:

Traditional underwater propulsion systems, such as screw-type axial propellors, convert torque into thrust; in other words, power from an engine turns the propellers and generates force by moving the flow of water downward and behind the blade. Autonomous underwater vehicles (AUVs) are playing an ever-growing role in modern subocean operations, generating a demand for faster, more manoeuvrable designs capable of deployments of increasingly longer durations.


Traditionally, underwater vehicles, especially submarines, are known for their stealth and ability to operate silently beneath the waves. However, there are scenarios where speed is of the essence, such as responding rapidly to threats or reaching critical locations quickly. The APEX program addresses this need for speed while maintaining the stealth and efficiency that submarines and UUVs are renowned for.



U.S. military researchers are asking the industry to develop enabling technologies for next-generation propulsion for crewed submarines and unmanned underwater vehicles (UUVs) that will be quieter and more efficient than ever before. Officials of the U.S. Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., issued a solicitation (HR001122S0046) in August for the Advanced Propulsor, Experimental (APEX) project.


The APEX program will design, build, and test classified submarine propulsor technology. The BAA seeks full proposals for Phase 1 (Phase 1A Base, Phase 1B Option, and Phase 1C Option) and a rough order of magnitude (ROM) cost estimate for Phase 2 (Phase 2A and Phase 2B).


Performers will explore the propulsor design space of potential solutions for a submarine or unmanned underwater vehicle (UUV). Relevant areas of expertise include hydro-dynamics, hydro-acoustics, mechanical engineering, naval architecture (submarine), electro-mechanical, and other disciplines. The Design Space Exploration (DSE) process is expected to explore the design space with metrics related to efficiency, signature, mechanical design and limits, and operational considerations.


Traditional underwater propulsion systems, such as screw-type axial propellers, are relatively noisy and inefficient. They also have limitations in terms of maneuverability. DARPA is exploring a variety of new propulsion technologies, including:

  • Biomimetic propellers: These propellers are inspired by the fins and flukes of marine animals, such as dolphins and whales. They are designed to be quieter and more efficient than traditional propellers.
  • Undulating membranes: These propulsion systems use undulating membranes to generate thrust. They are similar to the way that jellyfish swim. Undulating membranes are very quiet and can produce high levels of thrust, but they are also relatively complex and expensive to build.
  • Microfluidic propulsion: Microfluidic propulsion systems use tiny channels to control the flow of water. They are very efficient and can be used to create very quiet propulsion systems. However, they are still in the early stages of development and are not yet ready for use in operational submarines or UUVs.

DARPA is also developing new materials and manufacturing processes for propellers and other propulsion components. The goal is to create propulsion systems that are lighter, stronger, and more durable than current systems.

Phase 1A Base is a 12-month period in which performers are expected to quantitatively explore the APEX design space via a proposed DSE process. Proposals should describe the planned approach to trade space exploration and the design evolution process to assess multiple potential APEX designs and culminate in a conceptual design review (CoDR), which may include multiple potential designs. Proposers should describe the design process they intend to use, and the tools and techniques they plan to employ to conduct their DSE, as well as starting points, points of departure, and any candidate designs they intend to evaluate and why they are of interest in the trade space. CoDR level designs should only consider the full-scale designs.

Phase 1B Option is a 9-month period which may be exercised dependent upon performance and funding availability. In Phase 1B, performers will work toward defining a single APEX design approach based on the CoDR results, then refine the design through engineering design and analysis, and risk reduction activities. Phase 1B will include a System Requirements Review (SRR) and culminate with a preliminary design review (PDR) of the APEX design. PDR should include the selected full-scale objective design and a quarter-scale demonstration design variant intended for fabrication and testing in Phase 2. Proposers will be expected to show traceability between the objective and demonstration designs.


Through an updated BAA during Phase 1B with limited competition among Phase 1B performers, a single performer and single concept may be selected for Phase 2 of the program where a detailed design will be finalized and a quarter-scale demonstration unit will be built and tested.


Officials of the U.S. Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., announced an $8.6 million order to General Dynamics Applied Physical Sciences in Sep 2023 to develop models for the Advanced Propulsor, Experimental (APEX) program.


This order to General Dynamics Applied Physical Sciences brings the value of the company’s APEX contract to $18 million. The company won a $9.4 million contract last April for the APEX project.


Potential benefits of the DARPA APEX program:

Enhanced Tactical Mobility: The integration of high-speed propulsion technology into crewed submarines offers a remarkable boost to their tactical mobility. With the capability for rapid deployment to strategic areas and swifter evasion of potential threats, these submarines empower naval forces to maintain a decisive advantage in critical situations.

Reduced Vulnerability: Speed plays a pivotal role in the realm of submarine warfare, contributing to the avoidance of detection and the ability to preempt enemy actions. A faster submarine significantly reduces its vulnerability to potential adversaries, transforming it into a highly secure and elusive platform.

Stealthier Propulsion Systems: The development of quieter propulsion systems represents a game-changing advancement, rendering submarines and UUVs exceptionally challenging to detect. This enhanced stealth capability provides a substantial tactical advantage, enabling covert operations and increasing the element of surprise.

Enhanced Efficiency for Extended Operations: More efficient propulsion systems translate to extended operational capabilities, allowing submarines and UUVs to operate for prolonged periods without the need for frequent refueling. This increased endurance substantially augments their range, enabling them to cover larger areas and complete extended missions effectively.

Greater Maneuverability: Propulsion systems designed for enhanced maneuverability equip submarines and UUVs with superior agility and control. This increased maneuverability makes them exceptionally versatile, capable of excelling in a wide array of mission profiles and effectively adapting to evolving challenges.

Empowering Unmanned Underwater Vehicles: UUVs are becoming increasingly critical in both military and civilian applications. They are used for a wide range of missions, from intelligence gathering to environmental monitoring. The APEX program can revolutionize UUVs in the following ways:

Swift Response to Environmental Disasters: UUVs equipped with high-speed propulsion can quickly respond to environmental disasters, such as oil spills or natural disasters, helping mitigate their impact.

Improved Data Collection: UUVs are often tasked with data collection missions, including mapping the ocean floor or monitoring marine life. Faster propulsion can allow them to cover larger areas and collect more data in less time.


The program is expected to continue for several years, with the first prototypes of new propulsion systems expected to be tested in the early 2030s.



The APEX program is still in its early stages, but it has the potential to revolutionize the way submarines and UUVs operate. Quieter and more efficient propulsion systems would allow submarines to operate with greater stealth and range. They would also allow UUVs to operate for longer periods of time and in more challenging environments.

DARPA’s APEX program represents a significant leap forward in the realm of undersea vehicle propulsion. Whether for crewed submarines or UUVs, the development of high-speed, efficient propulsion systems promises to reshape the capabilities of these underwater vehicles. As this technology matures, it will not only enhance defense capabilities but also facilitate more efficient undersea exploration, scientific research, and disaster response.

In the not-so-distant future, we may witness a new generation of undersea vehicles that can move faster, respond more rapidly, and operate with greater precision in the mysterious depths of our oceans.



About Rajesh Uppal

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