This article covers it in more detail: https://www.defenseone.com/technology/2024/05/giant-military-manta-ray-drone-passes-first-ocean-test/396244/ https://www

This article covers it in more detail: https://www.defenseone.com/technology/2024/05/giant-military-manta-ray-drone-passes-first-ocean-test/396244/ https://www.defenseone.com/technology/2024/05/giant-military... “One thing that you notice in underwater vehicle design is you typically can have either a vehicle that lasts very long periods of time but can't really carry anything with it,” Woerner said on a podcast in 2022. “But if you want something that can carry a sensor or payload that is perhaps a larger size or mass, or has a larger energy draw, you tend to need a more traditional underwater vehicle, propeller driven in most cases. And those tend to not have anywhere near the endurance” that the military is looking for. The ocean itself is full of potential sources of energy, such as currents, waves, and even subtle differences in water temperature or salt levels. But there’s no single perfect source of ocean energy for what DARPA is trying to accomplish. “If you're interested in maybe closer to surface transport, wave energy is a really great resource, most of the wave energy is distributed near the surface. If you want to go into deeper water, right? That means that wave energy wouldn't be a great resource for that,” Sandia National Laboratory engineer Kelley Ruehl, an advisor on the program, said on the podcast. Similarly, current energy is a very localized resource, where we have tidal streets— those are unique locations in the world. So it's a very specific place that one would need to harvest tidal-energy resource.” DARPA said PacMar Technologies, another contractor on the Manta Ray program, will spend the rest of this year testing a full-scale energy-harvesting system.