He built his arm first out of plywood and I’m pretty sure you could easily build the same design out of a high quality single piece of wood. It really comes down to an extremely well-optimized design.
They could be replaced with either a weight, steel springs, or perhaps rope or sinew cords like the Romans used. Rubber is more effective and compact, but watch the video and you see there’s nothing particularly special about using rubber & he could’ve picked something else. The rubber puller itself does not exceed 10 m/s.
I think there is something particularly special about rubber in the design. He stretches it extremely tight. Even with the leverage of a crank, he strains to compress it to the length required, and the rubber holds up. It might be possible to achieve the same result with older materials, but the design would likely be much less compact.
No, I don't think so. His design respects the inherent latency to accelerate a looooog set of elastics. Instead of stretching a set of elastics over a long distance, he uses a heavy bundle and stretches them over a short distance. It still takes a lot of effort to get that stretch, but all that energy is released extremely fast..as the elastics don't have to travel very far to do so.
I agree he could have used some other spring, but I don't think he could have used a weight. The acceleration of springs/rubber is potentially much greater than a weight (at least one starting at rest).
Which means a giant physical structure to support that giant lever arm. When you scale up to things that can knock down castle walls I suspect the difference is incredible.
