Since the last update I've decided to go for an entirely steel frame. Not only will I learn how to weld in the assembly process, but it'll also be much stronger without extensive supports. The downside is that the project will end up costing "a bit" more. I never ended up totalling the cost for a wooden frame, so I don't know how much more, but with a firmed up budget (~$500), I'm not to worried.
Moving from wood to steel called for a major overhaul to the frame. I've gone from four vertical members to two because I don't need four to support the load and it'll be significantly cheaper. I think I've supported the vertical members sufficiently, but we'll go over it with great scrutiny at our PDR and CDR.
The biggest issue I've been grappling with is how the two carts are going to react under load. I'm worried mostly with how the carts are going to tip or slide out under load. Obviously the casters have brakes so that helps, but not completely. I plan on connecting the two carts together with six foot sqaure steel tubing. My plan is that by making the connection between the two, I'll aleviate the moment from the rocket on the rope and cart.
The connecting rod will be 3/4" like most of the frame already, but I'll also insert 5/8" tubing that'll fit inside it. This makes connecting the carts as easy as sliding the connecting rod into the open points on the frame of the cart.
Next on the to do list is the window box frame which I haven't really thought about until now. I better get on it!