Enormous underwater military drone won't be waterproof

US military contractor Anduril has signed a US$100 million contract with the Australian Navy, to design and deliver three iterations of an Extra Large Autonomous Undersea Vehicle (XL-AUV). These drones will be as big as buses, but not watertight.

The XL-AUV is pitched as a long-endurance, multi-mission capable platform that can be decked out with all sorts of gear and sent out on "a wide range of military and non-military missions such as advanced intelligence, infrastructure inspection, surveillance, reconnaissance and targeting," New Atlas reported on November 3.

Above all else, though, Anduril says it'll be "affordable" – or at least, a relative hundred-million dollar bargain in the context of specialized military drone development. And to keep costs low and timelines to a tight three years for all three iterations of this machine, Anduril will do away with the pressure hull and let water freely flood through the vehicle.

This is by no means unprecedented – indeed, it's a common decision made by many radio-controlled submarine hobbyists once they start working with bigger machines. Dry-hull radio-controlled submarines with sealed, waterproof exteriors are nice to work on, according to the Nautilus Drydocks, because you've got lots of room inside and plenty of access to the components. But they're a huge pain to seal up tight enough to handle deep-water levels of pressure, and equally annoying to open up and work on. They're also extremely heavy; they need to carry much more fixed ballast weight to fight against the buoyancy of the air they keep trapped inside them.

Wet hull designs, on the other hand, simply let the water in and out freely, so they're easier and faster to build, and they're much lighter, making them significantly less of an operation to launch and retrieve. The key systems within are sealed up in their own, much smaller watertight pressure modules, and while this does make individual components more of a pain to get into, it's much cheaper to build a bunch of small pressure hulls than one big one, and if one of them fails, only the equipment in that module will be damaged.

In the XL-UAV's case, this means a series of propane gas tank-sized pressure vessels to house all the sensitive electronics, and pressure-resistant battery packs, according to The Australian.

No specifications are available at this point, but the XL-UAV is likely to be capable of operating autonomously for missions over a week long, cut off from communications simply due to the depths involved. So it'll need to be capable of identifying, tracking and analyzing targets on its own, making certain decisions and taking actions of its own accord.