I need to build a catapult that can throw a golf ball 15 meters. We can only use those dimension: 30 cm long, 20 cm across and 40 cm high. The catapult itself should not exceed 2 kg. We are completly new to catapult building and we wondered what type of catapult should we make. It also need to be precise within the 15 m. We try a small onager, but we failed to make 4 meters. Do you have any idea of what we should do?
Date: 18 May 2011 01:12
Number of posts: 4
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How did your onager fail? Generally you can wind up an onager tighter until something breaks, fix that and repeat until you get enough range. My first onager (Onagenator 1, see my blog, a picture here threw 64 meters with a golf ball, and weights about 1KG, and is about that size. It took a lot of broken parts and fixes to get it to perform that well (about 10 slings before I figured out this way which has yet to break on me). Originally it couldn't throw a marble across a room, so don't worry if your first attempt was pretty wimpy. If you get stuck improving your onager, detail why/how you can't fix/improve its weak points, and I can give you some more advice.
Some random notes: Since you have a length limit, you probably want to get the bundle pretty close to the front to allow a longer arm. Mine has a 24 cm arm and throws further than you need though, so thats not critical. You also have a pretty big height limit, you might want legs (like this little guy I just put up a page for: Tiny Onager) so you can cock the arm pretty far back while still having a long sling. Make sure you are using a good type of rope. Nylon string is an excellent choice, and cheap too (I use it on all my torsion devices).
And by the way, an Onager is what I would recommend for this project. They tend to have the most power for their size. Good choice.
Ok I check my catapult and I think we didn't put enough torsion. I'll try later on with more torson. My teammate though the catapult would break. We use rather smal rope and I don't know what type it was. From the picture, it looks like you use a bigger rope that we did. Also, we didn't put a sling. We though it would be less precise. We will be attribute points if our catapult hit taget randomly place within 15 m. Does it affect a lot how far that we can trow?
Thank you for your help.
A good sling will get you vastly more range, especially in your case where you have a size limit (with a sling you can get more range, even with a shorter arm). A sling is somewhat like making your arm longer without adding much weight, except at the end of the throw (which is all that matters) the sling is rotating faster than the arm, and also sucking the kinetic energy out of the arm to the projectile as long as the sling hasn't passed the arm in rotation. A good sling will yield several times the range if well tuned.
Slings can be very accurate. Trebuchets used to hit castle walls consistently from hundreds of yards away with slings, and onagers are no different (I beat several trebuchets in an accuracy competition with one of my onagers). Onagers can be pretty inaccurate if you aren't careful though. Its good to make sure the sling is releasing before the arm hits the stop, or you can get inconsistent releases, and be sure that you either re-aim very well every shot or weight it down really well because onagers tend to jump around a lot when throwing. I should note that is you don't get everything right, you can end up with an inconsistent sling, I've heard of it, and seen people have issues with it, but I personally have never had the issue, and you can copy my sling design :)
Be very careful when tensioning your onager up to its breaking point, they can hurt you. As long as you arn't in the plane of rotation of the arm, and have safety glasses, you will generally be quite safe, but it varies on the design. Onagers if they break the arm, it usually goes in the plane of rotation (I'm thrown chucks of the arm backwards, but I have heard of them going forwards as well). If the rope breaks, its generally not exciting at all, my little onager has had a broken strand it its bundle for years. I've broken ropes in every single one of my onagers. If the frame breaks, it will usually just cave in a bit (then there is no more load on it), I've never had that happen though.