# What is the best shooter design for a straight line trajectory?

Hi, our team is working on a robot that could play dodgeball–and to minimize headshots we thought any shooter that achieves a near straight line trajectory with the contact point under 5’ would be ideal.

We looked into concepts like back spin with a single flywheel turret, but I was wondering if anyone knew of a better design? I just feel like someone out there knows of some insanely shooting method that no-one’s known.

Our projectiles are, for simplicity’s sake, 5" dodgeballs in an environment where drag and aerodynamics is significant.

In my opinion, the best shooters for straight shots are two wheeled shooters. Things like baseball pitching machines or football launchers. Like this. Or this

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Agreed.

That said, if you want a turret, a hooded shooter with a powered top roller which was popular in 2020 also works pretty well.

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Bill is correct here - if you are limited to a fixed surface speed (likely due to safety or motor efficiency constraints), a double-roller design will get you double the linear speed of the ball compared to a fixed-hood single-roller shooter design like those we’ve seen for the last couple years. The typical tradeoff is that your ball won’t have any spin on it, which can result in differing aerodynamic effects (no Magnus effect, for instance) and also change how it bounces off of things.

If your two wheels have differing surface speeds, you get to trade a little of that linear speed for some rotational speed.

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Physics teacher brain melting…

Seriously though, just shoot it with no spin at an insane speed and you’ll be ok.

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Did you read the first paragraph of this post?

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This is key. Shoot the projectile with a low launch angle at high velocity, and it’s gonna have a flat trajectory. Easy peasy.

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Thanks for pointing that out, I completely forgot to do that. Sorry.
One difficulty with a straight-line trajectory, especially for an application that involves hitting humans, is the speed required to do it. In addition, most double-flywheel shooters, or at least every one we have built, have had consistency issues. Some teams, most noticably 971’s 2016 robot solved this problem by adding side by side wheels. Not only does this make the shooter more consistent by giving it more time to accelerate the ball, it might also allow you to accelerate the ball faster by gearing the wheels at different speeds (so the ball accelerates even more into the second wheel.)

Engineer’s proverb #47362894:

A finite segment of a very large parabola is well approximated by a line.

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You want a ballistic object to travel flat, throw it infinitely fast. There is only 2 ways around this. Add wings to make it fly or back spin it (Magnus).

If I was to build a ball shooter designed to target humans and do minimal damage, I would build a 2 wheel shooter (side by side). Depending on the ball, getting hit with a ball with tremendous backspin is painful. Even those silly 2017 whiffle balls hurt the hands when trying to catch.

The shooter in the link could chuck the ball full field and hit the target and it did this using 2 RS550s. 971s shooter is really compact and appears to have wicked fast ball speed.

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If you never tried to catch a full court frisbee shot in 2013, it’s worth noting that the insane spin on those made them almost impossible to catch. May be fun to explore for dodge ball too depending on what you’re building this for, you’d probably want them to drop when they touch someone so back spin would be appropriate, this may help the ball ‘float’ due to the magnus effect too, but does make it harder to get fast velocities. Partial backspin is an option too similar to how many teams ran their hood rollers this year.

Long linear nearly straight line acceleration paths will likely yield the most accuracy and let you consistently get the ball to the speed you’d need for a straight line launch.

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