Top/bottom roller shooter vs. bottom roller only

[ToddF]

John V.,
Our prototyping experience supports your statements regarding shot to shot consistency.

For any particular motor selected to power your wheeled shooter, you can choose a gear ratio to get your desired shot speed. A single wheeled shooter needs to spin ~2X as fast as a double wheeled shooter. A speed difference between your double wheels, either with speed controlled motors on each wheel, or mechanical coupling, gives a selectable amount of backspin. A single wheeled shooter is slightly less "efficient" in that much more energy is expended putting a higher, non-selectable, amount of backspin on the ball.

Energy transfer happens nearly instantaneously, and is governed by the equations derived in a different thread. The most important factors affecting ball speed are rotational speed of the wheels and their rotational moment of inertia. Motor power has little effect on the shot itself. Rather, the more motor power, the faster the initial spin-up of the wheels, and the shorter the time between shots.

The majority of the time, the motor runs at nearly its free speed, inhibited only by the energy losses in the power transmission mechanics. The reduction in speed after a shot takes the motor closer to its max power speed. This helps the motor power the wheels back up to their full free speed. The higher the rotational inertia of the wheels, the less reduction in speed during a shot.

We found that our single wheeled prototype was more consistent, shot-to-shot, than our double wheeled prototype. Both types of wheeled shooters depend on ball friction and compliance for ball speed. The balls vary in both of these aspects right out of the bag. We expect these variations to only get worse over time. So, there is going to be a limit to the ball to ball repeatability of wheeled shooters that is out of our control. We also expect that the accuracy of a wheeled shooter will decrease over time, as the balls degrade.

A well executed catapult should be less subject to ball to ball variation as long as the shape and mass of the ball is consistent. Our catapult prototype keeps being "almost there". So, we have decided to go with a single wheeled shooter as our "plan of record" and proceed full speed ahead with detailed design and fab. At the same time, we are letting a couple people finish the catapult in case it proves to be a revolutionary breakthrough in terms of repeatability.