FRC 95 The Grasshoppers 2020 Build Thread

We have done some high speed camera analysis of our shooter. I’ve edited two of the videos together and analyzed the data. Presenting all of them would be too time-consuming for us.

This compares two hood backer materials on our shooter, HDPE (unmodified note) and PTFE (noted as PTFE).

Shot at 480fps with a cell phone. This is an accessible test for most teams.

Results:

HDPE - 38mph, 25hz rotation
PTFE - 42mph, 23hz rotation

PTFE shoots ~10% faster, and with ~10% less spin (let’s not talk in more precise numbers, that would imply a level of accuracy our data doesn’t have).

Things get fun when you compute the ‘rolling speed’ of the power cell. If rolling speed = exit speed then the power cell was not slipping along the hood at all. If rolling speed = 0 the power cell slipped perfectly along the hood

rolling speed = pidiameterrolling frequency

HDPE rolling speed = pi*7in*25hz = 31mph
PTFE rolling speed = pi*7in*23hz = 29mph

We could say that…
HDPE had 1-31/38 = 18% slippage
PTFE had 1-29/42 = 31% slippage

[I know this math is a little sloppy, but it will ballpark things to the level of accuracy I think we need]

What can we conclude?

A slipprier hood allows more of the shooter’s energy to be converted into translational kinetic energy instead of rotational kinetic energy. Assuming the reducing in backspin is tolerable (ours was) then shot range is improved. This is what we have observed anecdotally with our testing and theorized about, but it’s nice to have real numbers on it: slippry => less spin => more exit velocity.

41 Likes