Shooter hood geometry

I am trying to design an adjustable hood for a shooter but I am unsure of the geometry that goes into it. Other than trying to figure out compression, I don’t know what else goes into it if anything. Thanks in advance.


How deep down the rabbit hole you wanna go?

This is a bit of a brain-dump, so sorry in advance if any of the bullet points are confusing, I can eventually get around to making a sketch of the degrees of freedom for each of these.

(other than bullet point #2, all of these are adjustments to the profile of the shooter (when viewed from the side))

  • Compression
  • Ball centering geometry (TBH, your ball should always be centered going into the shooter anyway, you probably just want a flat back plane (hood) (flat, as in normal to the side plates))
  • Flywheel offsets (hood profile and flywheel are not concentric (if hood is a arc))
  • Hood profile geometry is not a circle, rather it’s some spline (control compression at different points throughout the shooter.
  • Any number of things around the release angle and the trajectory
  • Flywheel/hood sizing and magnus effect
  • Adjustability (particularly in a proto)
  • Do you want to carry in momentum from your feeding assembly? What types of momentum? or stop the ball dead then run it through the shooter).
  • Fixed or adjustable hood?

Could you go farther down the rabbit hole? I think any information would be useful.

So just a bunch of examples, a sketch is going to be more useful here than paragraphs:

1" compression, fixed geometry (45 deg angle) 4 in flywheel:

Same as above with a hood segment over top of the fixed bit (you will lose compression here) the flywheel, fixed portion of the hood, and movable portion all share the same center point:

Same a #1, but flywheel is lower, so compression starts at 1" then drops off as the ball is accelerated (flywheel and hood do not share same centerpoint) differences in compression are shown with the driven dimensions (the ones in parentheses) :

Larger flywheel, therefor higher surface speed for the same flywheel rpm as the 4" (more energy too!) the ball will come out spinning faster and have a greater magnus effect (theoretically), the other obvious solution is to up the speed of the 4" version, but there are reasons you may not want to do that:

Finally, using something other than an arc as the hood profile (this is just a quick and dirty spline for illustrative purposes only) same kind of idea at the flywheel offset, except you have even more control over compression as it passes through the shooter:

Also consider the size of the contact patch/time of the ball (under compression on the wheel and on the shooter hood). In the sketches they are just tangent circles, but that flywheel isn’t going to have instant grip on the ball. If you go too small with the flywheel you just won’t have the time to get the ball up to speed.


There are some factors here that most teams this season considered (compression, release angle, for example), but the concept of changing compression is not something that I have heard teams discuss.

Our shooter hood, initially, had some flexibility in the joints between the adjustable hood and the fixed part of the hood. This flexibility had a bad effect on our shooting power and once we fixed this flexibility such that the fixed compression that we had originally designed was maintained in the actual hardware, our shooting was much stronger and more consistent. So, my first reaction is that relaxing the compression near the exit is not a good thing.

It is intriguing, to be sure, but the normal 6 week build season does not offer the time to systematically explore such nuances. I am wondering whether any teams, now that the the build season has been extended to ~9 months will spend the time to systematically investigate different hood geometries to see if there is an advantage to variable geometry versus the more typical fixed geometry.

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@BenV and @agunal as with any engineering problem, you must have some firm objectives i.e. what are the characteristics of the object you want to shoot, how far do you have to shoot it, how high do you have to shoot it, how accurately do you have to shoot it, how often do you have to shoot it, etc.?

If you study the shooter designs for different games, you will find that they will generally be optimized differently for each game.

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Okay thanks, I’m doing a redesign of our current hood which worked but had problems with friction. What I am wondering is just how to do compression I guess. I know there needs to be an increase in compression towards the end but other than that I don’t know much. I’m going to be constantly using the indexer and not stop the balls before they leave but it’ll be constantly moving. Thank you all for your responses.

I can’t help with the compression for the 2020 game piece since my team built a low goal bot.

From studying 6-8 of the top Steamworks shooters at Houston Champs, I concur with what @Skyehawk posted. The second point in his first post was one of the things that separated those top shooter designs from those that sprayed Fuel over too wide an area to score effectively.

A second feature those top shooters had was that the game pieces were introduced into the shooter at regular and well defined time intervals i.e. they all used some sort of indexer/singulator. This may mean that you may want to redesign your indexing system.

Not 100% sure what you mean by this, most teams (for 2020/2021) will probably be using fixed compression for the whole shabang. The only “variable compression” most from this season will have is from how the ball initial contacts the flywheels, and as the ball leaves the exit “edge” of the shooter for acceleration purposes we can treat the later as neglagable. As for the former point: How you introduce a ball into the flywheel is important. as @philso pointed out in the post prior w/ regard to the 2017 game.

This year’s ball is quite a bit different than 2017, so specifics will vary wildly, but take a moment to consider the following:

  • Speed and feed of the input to the shooter
  • What does the handoff from the feed of the shooter look like? (are you forcing the ball (always under compression) into the flywheel with the feed?, are you gingerly handing it off into the shooter flywheel? Are you launching it into the flywheel with the feed?
  • How does do the above two points handle non ideal balls?

Also keep in mind that energy you are using to compress the ball is energy that could be going towards accelerating the ball, so there is reason to work towards the lower end of compression that works with your setup.

Now, I know the OP was on hood geometry, particularly that of an adjustable hood. There has been a bit of theorycrafting in this thread and no data, so as a next step I propose you do the following. Figure out what your tolerances are.
How accurate do you need the shooter? What rate of fire are you interested in/capable of? Where are you interested in shooting from? etc. In an ideal situation you want a mechanism that meets (or exceeds) those requirements while at the same time capable of taking a hit and still being fine (operating well under loose tolerances). You shouldn’t desire a system that only accuratly puts a ball in the goal when the compression is within 1/8" of nominal and the balls have x amount of wear on them.

There is a reason that “groups**” the world over are famous for using the Avtomat Kalashnikova rifle (AK47), it can get the snot beat out of it and still throw a round downrange. You will find this same design philosophy in loads of industrial or professional equipment, Farm equipment, professional cameras/lenses, a lot of automotive engineering, etc.

** Don’t @ me, this is a fact not a political statement.


This is a bit late, but I have a question: what is the purpose of having the flywheel and hood be nonconcentric?

It varies the compression on the ball, which can be helpful in certain situations.

Could you please give an example?

From what I can tell, the “why” behind some of these things is very much a black box. I think people just end up trying things and seeing what works (though, it’s very possible that some people actually understand the physics and I just don’t).

If you’re asking what it means to vary the compression, it’s basically saying that the ball fits into a 6" gap at the beginning of the shooter and fits out of a 5" gap on its way out.

My hypothesis on the variable compression is that during a rapid compression cycle, the ball actually acts more like a air-filled ball, but during a slower or non-changing compression cycle the ball just acts like a static spring.

So basically the variable compression is constantly ‘increasing the pressure’ inside the ball, attributing to a higher hood frictional force. The constant compression shooters do all the compression at the beginning and then hold it constant so your hood friction is totally dictated by the condition of the internal foam.

We were just beginning our deeper investigation of this before quarantine.


The pictures at the top of Cybertooth’s reveal thread show a variable compression of 1/2" over the first 10 degrees of the shooter hood travel. After that the compression stays the same. @Nick_Lawrence claims it is an inch of compression change, but also states that the holes are 1/2" apart and they only move from the outer hole to the middle hole in that picture. It is likely that it was changed after the original picture was taken to add more compression.

But at least in the pictures they show, the variation in the compression is all accomplished early in the hood travel. I’m guessing that the positive effect that was produced is that the ball ends up getting compressed more slowly than it would if you just forced it into the final compression right at the entry to the hood. The one thing that we have noticed about damaged balls is that they let the air out very quickly whereas undamaged balls let it out more slowly (the thick zesty skin doesn’t let the air out, but as soon as you cut a slit in it, the air can escape). The effect of this is that an undamaged ball will put up more of a fight when entering the hood than a damaged ball which can effect the resulting speed that it achieves while passing through the hood. By spreading out the compression over a longer segment of the hood arc, you probably get more consistent acceleration between damaged and undamaged balls and therefore more consistent exit velocity.


Another possible reason for increasing compression along the ball path that I came up with my limited physics knowledge:

Keeping in mind that a shooter is just transferring kinetic energy from the flywheel to the ball, the rate of energy transfer should be proportional to the difference in kinetic energy between the ball and flywheel if I’m not mistaken (this is the basis for my idea and im not a 100 percent sure of it so correct me if I’m wrong). As the KE of the ball gets closer to the KE of the flywheel, the transfer of energy between the two gets slower. This energy transfer rate is also affected by the amount of compression on the ball. So, at the start when the ball and flywheel have a big difference in KE, the extra compression isn’t needed for faster energy transfer and only makes the process less efficient. Towards the end of the balls path through the shooter, the KE of the flywheel and ball are closer in magnitude so more compression is necessary to maintain the same energy transfer rate.

For shooters this is absolutely the case.

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This is a bit of a tangent, but is it worth investing into a turret shooter (one that can turn independently of the robot)?

I think design needs to follow strategy. The benefits of a turreted system is the ability to track a target and perform a game task from multiple locations. If this is part of your strategy (shooting from multiple locations under multiple conditions) then now (offseason) seems like a decent time to look into it.

Do be aware that additional degrees of freedom require additional software to control.

A jack of all trades robot that is less consistent due to things such as lack of driver practice, lack of software optimization, or increased complexity typically does not fare as well as a simpler more consistent robot. My $0.02


In that case, I will probably forget about the turret for now and work on other improvements to the robot. Thank you!