Having read about a number of catapults recently, I realized I should ask for clarification on the term. It tends to have two distinct definitions in English, one quite broad and one quite narrow. The broad one classifies all sorts of stuff as catapults, including trebuchets, ballistas, etc. This is probably familiar to many as it applies on aircraft carriers with the aircraft catapult. The narrower definition would be the more classical one, based on elastic/tensile forces being used, elastic potential energy is stored to be released by swinging an arm with the projectile in it. So I’m wondering what common design approaches are. Maybe some are elastic bands with a slip gear to rotate the arm down until released. Has anyone tried using nylon rope wound around an axle similarly to classical catapults? Are people just swinging arms directly from motors using gears? Etc.? Thanks.

We’re just swinging an arm from a gear, but I’ve also never used a catapult for long range flinging before so that might not be the best way to do it.

Thanks. A nice thing about that method is that it’s easy to adjust the launch speed. I suspect @meng 's students are doing it differently since he mentioned the difficulty of adjusting the launch speed.

We tried a number of things in the NbN year. I seem to remember that we tried something with nylon cord but it didn’t work out (something about not being able to wind the cord fast enough and it getting caught in the mechanisms). We also tried to use motor force directly but it did not provide nearly enough force to throw a ball the requiuired height and distance. So we ended up with rubber bands a slip gears.

We did differentaite between a classical catapult and a striking mechanism. Our definition of a catapult was a device that held the object in a basket (or something similar), moved quickly, then used a hard stop to propel the object. A striker did not hold the object but would use force to hit a stationary object. We found a striker to be much more efficient than a catapult because the catapult has the added weight of the object being propelled and the basket . Energy transfer was much more efficient with a striker.

We further increased the efficiency of the striker by using rubber bands to maximize energy transfer. In case you are interested, here are two videos we submitted for the innovate award that demonstrate the increased efficiency using a rubber band faced striker. In NbN, our robot could get full court shots using one motor. That was not very common.


It doesn’t surprise me that motor force directly is problematic. I’ve only seen direct force work well classically in limited ways. In terms of catapults, only with one of classical trebuchets (the ones you pull instead of using counterweights). And that has a lot to do with the trebuchet’s release mechanism, which is hard to mimic here.

As for the nylon cord, it sounds like perhaps you were stressing it lengthwise instead of having it wrapped around an axle. Is that right?

Nice surface. Yes, that use of rubber bands is sweet (pun intended), especially when you have something that won’t rebound fairly elastically off of a hard surface on its own. That’s why aluminum bats aren’t allowed when wooden bats still are.

This does make me wonder if we should classify different types of “punchers,” such as rotational v. translational, as well as different types of catapults.

I honestly cant remember exactly what we did with the nylon rope. We didn’t have a good EN that year so I can’t go back and look. Add to that the fact that, as a coach, it was pretty much the first time i had built anythingand was one lesson ahead of the kids - i just dont remember the details.

But, yes, i agree, some common terminology would be very helpful. I believe i see things characterized as catapults that really arent catapults in the true sense. And, as you point out, you can strike linearly or rotationally. Each has its pros and cons.

One of my teams spent a couple months working on a nylon rope, torsion catapult to work for starstruck. They did get the protoype to fire, but the side forces created during winding were so extreme that it became impractical. They ended up moving to a rubber band based catapult, which had a rotating drum for adjustable power and variable stop for launch angle. They could launch a cube from one corner to the opposite corner.

We used the slip gear release approach.

We don’t see the need to adjust the speed honestly, simply because the force of impact is already not as much as a flywheel. We try not to make that worse.

As for the angle of release, I am very sure our’s is not at the vertical. There are some forms of a restrain.
I will need to take a look to see what sort of restrain do my kids used.
For nbn (8059a worlds edition), we used nylon rope to restrict how far the arm and swing forward.

For this year singvex, i remember there was an initial discussions among the team on how to adjust the angle of release by changing the height of the stopper. But I think no matter how good is the quality of build, it is difficult to build a stopper that can withstand the force of the swinging arm.

And one more point - the 2-ball catapult is never meant to be long-range. We wanted it to be short range.

I had thought so, but it was hard to tell.

I wasn’t thinking of slowing it down so much as speeding it up (and then potentially varying it from there).

Ya, vertical arms would require height violations unless very far from the flags.

That’s a nice idea! That could stop it pretty suddenly without doing a lot of damage. I like it!

That makes sense. I did post about getting larger flexibility on launch distances, which does not require avoiding short range. It just makes for more flexibility in launch location in case you’re being interfered with.

That makes sense. There ends up being a lot of force involved.

Cool. Are there photos or video of the drum and stop? I’d love to see them.

We also had issues using nylon rope as a stop. The force was so hard that the nylon rope would stretch with each shot. We ended up using a heavily reinforced piece of steel c-channel. Actually 2 pieces filled with steel single bar.

For a stop you should probably use a material that can absorb force better, such as the foam the vex sells or the rubber from tires (The SS Ri3d used that so i assume it works). In NBN i remember using Nylon rope attached to rubber bands so it decelerated the arm a bit before it would hit the stop and prevented a lot of the stretching.

@briancole Use a slip gear or even a ratchet and pawl mechanism can work…+having 2 catapults for the different flags can also help with having a faster fire rate

We actually had a long discussion as a team and concluded on a flywheel for its variable positioning. Still not sure how I feel about that

no leeks 4 you boi

Which are at all relevant to the OP about the term “catapult” and the various mechanisms in use that are named “catapult” because… Please stay on topic. Thanks.

the definition is one google away

google says:
a device in which accumulated tension is suddenly released to hurl an object some distance, in particular.


And other definitions say other things. (E.g. ). Under the definition you found an “aircraft catapult” isn’t a catapult, for instance. I’m aware of many different definitions that can be found, which is exactly why I asked the question in the first place.

i don’t think an aircraft catapult has anything to do with vex

I don’t know if you’ve just totally ignored the original post, if you don’t understand what how examples serve to make points, if you’re being belligerent just for the sake of it. If you go back to the OP, you’ll see I specifically noted narrower and broader definitions of catapults from the get go. I simply used “aircraft catapult” as an example of something that clearly doesn’t fit the narrower catapult definition, does fit the broader definition, and is named “catapult.” It serves as a very good example of how broad the term “catapult” can be.

Now, if you look at the examples people have provided, you’ll see just what I was asking about. Some people have said “catapult” in reference to VEX designs that fit the narrower definition. Some people have said “catapult” in reference to VEX designs that don’t fit the narrower definition but do fit the broader one. So we have clearly established that when someone says “catapult,” there is not nearly the clarity of language that we have when someone says “single flywheel” or other things. The OP was very specifically asking questions to find out if we have some sort of clarity with the term or if it’s a general grab-bag term. There may be more design types that are called “catapults,” too. I’d love to hear them.