Structural Design
Almost all the Autonomous Underwater Vehicles (AUVs) in the RoboSub competition use a cylindrical hull, often an acrylic tube. There are two main advantages of a cylindrical hull: it is straightforward to obtain via commercial off-the-shelf for a low price, and it distributes pressure more evenly. However, the AUV never exceeds 15 feet in depth; this reduces the advantage that a cylindrical hull would usually have.
We decided to go with a rectangular hull design to optimize the arrangement of electronics inside. With our previous vehicle, we fit square components into a round tube and ended up with a lot of extra air volume, which we had to consequently balance out with dive weights. The added weight led our team to lose a couple of points. Being able to arrange our electronics in a 'flat-pack' design made the best use of our volume.
The green areas highlight wasted volume
RoboSub's tasks change each year, so our vehicle has to adapt to different challenges. Our previous vehicle's mounting plane (the large horizontal piece) was designed with particular components in mind. As a result, the lightening holes' shapes and sizes were very odd, and it became complicated to mount any new components to our submarine.
In our new design, there are two mounting planes instead of one, doubling the mounting area without increasing the vehicle's footprint. Each mounting plane has three large rectangular cutouts which house a replaceable 'modular panel.' These panels are easily and cheaply made using 3D printing and allow for different hole patterns without altering the permanent components of the vehicle.
Upper fore modular panel
The weight load of the electronics enclosure is distributed uniquely. The electronics enclosure consists of three parts: a custom carbon fiber enclosure, bonded to a machined aluminum lip, and an aluminum lid. The carbon fiber enclosure rests on the bottom mounting plane so that there is little to no load pulling the carbon off the bonded lip. The lip also overlaps and rests on the upper mounting plane, so there is no vertical compression load on the carbon fiber. The two mounting planes are connected by four aluminum plates that also serve as a mount for the vertically-oriented thrusters.
When the vehicle is lifted for transport by the handles, part of the load goes through the thruster-mounting plates and lifts the enclosure's bottom. The remaining load is supported through lifting from the lip of the enclosure.
Distributing the weight through the entire structure of the vehicle minimizes weight, as we won't have to design components purely for structure; most of the components serve both as structural members as well as functional areas for mission tasks.