Me and my science "buddy" are building a catapult for science class and I was wondering if anyone had any suggestions for a basic design. (dimensions should be about 3' high and 2' x 3' for a base.

 

All Suggestions are greatly appreciated...

Catapult or trebuchet?

Trebuchets are better, IMO.

How far does it have to be able to shoot things, and what are you going to try to shoot?

 

I've built a trebuchet that can launch a tennis ball more than 75 feet using a bucket filled with 50 pounds of lead as a counterweight. That may be overkill for what you need to do.

 

With more details on what you need we might be able to help more.

You guys couldn't have waited until tomorrow, could you. <sigh>

 

Anyway.

 

Overkill? NO SUCH THING ! Go with the trebuchet.

We want pictures!

my bro a friend and I built a trebuchet. It was really cool. It flung a baseball about 50+ feet.

 

It's just as easy to build a trebuchet as a catapult, plus it looks cooler and the ammo travels farther.

We want pictures!

 

Excuse me while I whip this out

Catapult or trebuchet?

 

Catapult.. I would like to keep this rather simple

 

How far does it have to be able to shoot things, and what are you going to try to shoot?

 

I've built a trebuchet that can launch a tennis ball more than 75 feet using a bucket filled with 50 pounds of lead as a counterweight. That may be overkill for what you need to do.

 

With more details on what you need we might be able to help more.

 

A bit.. this is just a simple science project, and the main objective is just to use three simple machines. I would say we only need to be able to launch like a 5 pound object about 30 feet, although being able to launch a tennis ball 75 feet would probably get us an A...

 

Excuse me while I whip this out

 

"Bought this trebuchet as a kit several years ago, and put it together over the Memorial day weekend"

 

Kits are lame..

Have a quick look through:

http://www.mlahanas.de/Greeks/war/CatapultTypes.htm

 

First thing you'll need to decide is what you're going to use to power the thing, whether you're going to use twisted rope, some kind of rubber or wood...

 

I've only ever built quite simple ones using either bit of building or a quickly lashed together frame using bungie cords as the power....

 

http://www.mandp.co.uk/productinfo.aspx?catRef=500285

 

I prefer trebuchets, and have my own design for a combination device but I really need to test that before I start talking about it..

Kits are lame..

 

When you don't have a workshop, you take what you can get.

The most successful catapult designs I've seen use several bungee cords stretched out to power the arm. You could easily lob a tennis ball out to 75 feet with one of them, depending on how many bungee cords you use.

Catapult.. I would like to keep this rather simple

A trebuchet uses gravity via a counterweight as opposed to the tension a catapult utilizes. I don't think a trebuchet is that much more difficult to build.

 

I would say we only need to be able to launch like a 5 pound object about 30 feet, although being able to launch a tennis ball 75 feet would probably get us an A...

Five pounds is quite a lot actually.

 

 

Kits are lame..

So is criticizing the advice you get after you asked for it in the first place. While a kit may not be acceptable for your school project, I see nothing lame about copying a proven design. It takes a lot of energy to redesign the wheel... or the trebuchet.

most of the siege weapons I've built have been built as pioneering projects, and then trebuchets are far easier to build, as there's less continued stress that you can't easily deal with, harder to master though, in terms of accuracy, especially range.

most of the siege weapons I've built have been built as pioneering projects, and then trebuchets are far easier to build, as there's less continued stress that you can't easily deal with, harder to master though, in terms of accuracy, especially range.

i think the coolest use of a trebuchet in modern times was on the TV show Northern Exposure. Chris the DJ built one for fun and ended up using it to fling the coffin of a friend way out over a lake to bury him at sea. Completely, totally awesome.

I think that violates some sort of funeral protocol. Especially if the trebuchet malfunctions and sends the coffin flying backwards.

Cap'n: it may be slightly 'illegal'...but awesome none-the-less if it gets flunk backwards.

 

I personally think that a trebuchet it the easiest, most effective design for what you're thinking, Dark matter.

The most successful catapult designs I've seen use several bungee cords stretched out to power the arm. You could easily lob a tennis ball out to 75 feet with one of them, depending on how many bungee cords you use.

 

hmm.. bungie chords, that's a good idea. Are you talking about suspension or tension?

You use them for tension. You have an arm mounted on a fulcrum point, and attached to the arm are bungee cords stretched out and attached to a part of your frame that sticks up. The cords pull the arm forward and launch your projectile.

Basically a larger, more structurally sound slingshot... You really want a launch shelf that'll slide fowards and then stop allowing something in it to carry on going....

Maybe, but the trebuchets I have looked at look fairly complex and expensive. I have made a rather... primitive drawing of my idea of a catapult as follows:

 

i think the coolest use of a trebuchet in modern times was on the TV show Northern Exposure. Chris the DJ built one for fun and ended up using it to fling the coffin of a friend way out over a lake to bury him at sea. Completely, totally awesome.

 

Jeez... wouldn't the coffin break? What's the upward velocity on that?

 

You use them for tension. You have an arm mounted on a fulcrum point, and attached to the arm are bungee cords stretched out and attached to a part of your frame that sticks up. The cords pull the arm forward and launch your projectile.

 

Question: is there a way to hold down the arm once it's at full extension and then use the same device to let it go?

Edited May 29, 200817 yr by Dark matter
multiple post merged

Maybe, but the trebuchets I have looked at look fairly complex and expensive.

Check this out:

 

I made a small version of this once using some Kinex and a D battery taped to a paper clip.

 

 

 

A catapult is just different, not simpler:

 

 

Here's an analogy: hold a baseball in your hand and have someone hold your wrist so you can't throw it. Gradually build up the tension, then have the person release your wrist. The ball goes flying. *That's* a catapult.

 

Now hold the baseball up in front of you like you're offering it to a giant. Let the weight of your arm and the ball make your arm drop down and turn a complete circle with the ball (like a reverse softball pitch), releasing it at the top of the arc. *That's* a trebuchet.

 

 

Jeez... wouldn't the coffin break? What's the upward velocity on that?

Imagine a coffin positioned in the sling on that groove in the first pic above, so that when the coffin reached the top of it's arc, the coffin continues outward and the sling continues downward. The velocity was tremendous but I think the only stress on the coffin would be on impact with the water. It probably would have shattered completely, but this was a TV show so it just went under after being flung so majestically.

Question: is there a way to hold down the arm once it's at full extension and then use the same device to let it go?

You can use a small eye-hook on the arm and a matching one on the frame. When you pull the arm back far enough, the two hooks line up and you stick a metal bit through the two to hold the arm back. Tie the metal bit to some string and yank it out when you want to fire.

You can use a small eye-hook on the arm and a matching one on the frame. When you pull the arm back far enough, the two hooks line up and you stick a metal bit through the two to hold the arm back. Tie the metal bit to some string and yank it out when you want to fire.

 

You want two eye-hooks on the frame, on either side of the arm. Unless the pin is very thick (i.e. about the diameter of the hole) it will tend to slip and fire accidentally.

That just makes it more exciting.