24 hour flight and Boeing's plan for a five year flight. So it should come as no surprise the DARPA is trying to develop a pilotless robot ship that could automatically chase enemy subs around without needing control or direction. It would need to be light and carry a lot of fuel for this kind of stamina which it does and will be made from aluminum. But at 62 ft long with a high point of 45 ft it doesn't sound like it will be too sneaky. It'll use an obstacle avoidance system which is pretty common for autonomous vehicles in other mediums. I suspect in a few years we'll see requests for proposal of versions of this craft that are mostly imperceptible to sonar or otherwise difficult to track by the subs they are chasing. The general idea is shooting it down will only create a red flag for someone to chase down thereby giving away the location of the escaping sub. But from what I know of the military they don't like their expensive unmanned crafts to suffer enemy fire if they can help it.