Catapults Away!

May 8, 2006 9:15 PM

How do I build a catapult, trebuchet or ballista?

I am a high school senior. My physic's teacher has allowed us to choose a senior project. A catapult (or some contraption like it, if you can think of something more original) always seemed like a cool project. However, after searching, I found too many sites offering products, and not that many conclusive designs that I could actually use. Keeping in mind I am a high school senior, this is a school physics project, and I have ample but limited resources, what could I realistically build and where can I find the designs? Since we are on the projectile motion area, my other friend would like to know designs for a potato gun.

posted by GMD1987 to grab bag (11 comments total)

Trebuchet Plans

Free Trebuchet simulator
posted by pgoes at 9:31 PM on May 8, 2006

Blogger Murray Hill sells a set of plans for a trebuchet, and for a bit more money he'll also sell you the most difficult-to-make hardware.

I don't know for sure if this stuff is still in stock; probably ought to send him email and check. Also, he's in New Zealand, so the shipping won't be cheap. But it's still a pretty good price (in NZ dollars).
posted by Steven C. Den Beste at 9:53 PM on May 8, 2006

I think a trebuchet is the way to go, because anything involving springs or other forms of torsion as a way to store energy is potentially very dangerous if you have equipment failure. If you get a structural failure of a trebuchet, everything tends to go down instead of sideways.
posted by Steven C. Den Beste at 9:55 PM on May 8, 2006

A friend and I did this very same project for our senior high school physics class. We ended up choosing the ballista, because it was the simplest and easiest to implement. Don't spend time looking for designs, though. Either use the following as a base for your own design, or come up with another. It's much more fun. And don't buy stuff online! Just go to a Home Depot or something.

There are three ways to design a device of this type:
1) Fixed launch angle, variable power
2) Variable launch angle, fixed power
3) (predictably) variable angle, variable power

The one we built was pretty simple, actually. I wish I could sketch you something, but this will have to do. I'll just go over the gist of it, and let you sort out the details. Our design was a metal tube mounted on a platform that let us change its angle. A wooden dowel was inserted into the back of the tube, and bungee cords were attached (very strongly, use nuts and bolts) to the dowel and a secure base. Pull back dowel, insert marble, take cover!

To get accurate results, you need to calibrate the device indoors (hint: gym after school), or in a windless area (try beside a big wall). Graph pullback distance vs how far the projectile goes for a couple of angles (pick easy ones). Then, use the graphs to extrapolate a spring constant for your bungee cords. At that point, simple kinematics will let you come up with launch formulas. It will depend on how carefully you build it, but ours was accurate to about plus or minus 0.1m (we could nail a piece of paper from 25-30m, easy)

Things to consider:
-Use a trigger system. You introduce human error when releasing the elastics by hand. We did this by using a notched board for the dowel to latch onto, and a garden gate opening thing as the trigger (but not by directly putting the cord into the mechanism - use the trigger to unlatch the dowel, instead).
-Grease the dowel! Friction is not your friend. It's always borrows your energy, and never gives it back. Bastard.
-Get a right sized launch tube. Otherwise the marble might hit the lip of the tube on exit, which will wreck your accuracy.
-Try not to use pulleys, they have a tendency to fail. (Ours did, anyway.)
-Always have someone stand on the base of the launcher. Otherwise it has a tendency to flip over (and break... funny story, we built ours twice!)
-Insert discreet, but hilarious comments into your final report. High school teachers love this. Do not do this in university. TAs do not love it.

And finally. If you really want to build a trebuchet, here's what my friend did. He was having trouble getting the release mechanism timing to work right. His solution was to use his digital camcorder to film the thing in action, then play it back in slo-mo. This was he could tweak his build, and the judge the results.

But really, this is a super fun project. Probably the best one I ever did in high school. And hitting your partner in the back of the head with a marble from 30m away? Classic.
posted by Drunken_munky at 10:36 PM on May 8, 2006

You might want to enquire to be sure your school would allow such a project. I doubt they would, any more than they'd support the synthesis of cyclonite or the construction of a heavy mortar. These things are military apparatus, and the fact that they're outdated doesn't mean they're no longer capable of killing people. I doubt your school wants to take responsibility for supervising you in this.
posted by ikkyu2 at 11:04 PM on May 8, 2006

"I have ample but limited resources"

This does not make sense.

Good luck if you do try to embark on this - you'll probably learn a lot of physics out of it. Of course, you'd get exactly the same lesson out of a tenth size one, which if I was a publicly liable teacher I would restrict you to.
posted by wilful at 12:45 AM on May 9, 2006

Check out trebuchet.com

Also: The Art of the Catapult: Build Greek Ballistae, Roman Onagers, English Trebuchets, and More Ancient Artillery.
posted by zanni at 1:34 AM on May 9, 2006

potato guns are really simple. also, fairly dangerous. it's an explosive, and PVC pipe is not built to be a combustion chamber. they are pretty fun though.

btw, if you want to get really destructive, and probably learn a lot more physics than some fuckin catapult, how bout a railgun?
posted by sergeant sandwich at 2:37 AM on May 9, 2006

You might want to read Catapult: Harry and I build a Seige Weapon. It's a nonfictional account of two men's attempts to build a full-sized catapult.
posted by malp at 6:17 AM on May 9, 2006

These things are military apparatus, and the fact that they're outdated doesn't mean they're no longer capable of killing people.

Unless he or the school have an army of serfs and some tree trunks, he ain't gonna be building a full-size siege engine. A wee desktop model or waist-high model from 2x4's is all that's called for.

I would assume from the school's point of view that smaller is better.
posted by ROU_Xenophobe at 7:03 AM on May 9, 2006

If you could build one powerful enough, and precise enough, to shoot full court shots with a basketball in the gym I think you would be pretty much guaranteed an A, especially if one or more of those shots actually goes in.
posted by caddis at 7:36 AM on May 9, 2006