Water balloon ballista

[Ryan M.]

I'm looking for some advice on what type of ballista (A launcher type thing, just in case ya didn't watch enough Junkyard Wars growing up. ) design would be best for launching a water balloon a minimum of 20 yards. Accuracy is important, as we have to hit him for full credit. Also, compressed air, chemicals, and motors are disallowed. The water balloon will be roughly softball size (the teacher hasn't specified exactly yet).

I only have 10ish days to build and test it, so simple is good.

The classic hand slingshot hasn't been specifically disallowed yet, but assume that you can't use that. (But we can probably get away with a mounted version of that with some special mechanical release.)

What sort of device would you recommend and is there anything we should look out for?

[EricH]

Advice: make sure you have a good release. My Scout troop once built a trebuche and it only launched a tennis ball two feet due to the release (a ballcap). Other than that, try a lot of rocks in a bag for a counterweight and a nice long lever arm. Oh, and make sure that you have a sturdy support. Someone from 842 might be able to help you a bit better; they have done things like this in the past.

[karinka13]

trebuchets actually work pretty well. me and alicia (looneylin) did something like that for a physics project freshman year. we got tennis balls to go pretty far, but then again we used a pretty good amount of weight.

[EricH]

Originally Posted by karinka13

trebuchets actually work pretty well. me and alicia (looneylin) did something like that for a physics project freshman year. we got tennis balls to go pretty far, but then again we used a pretty good amount of weight.

Must have been a better release than ours. We had a stuff sack full of rocks, and some of them were pretty big ones.
Ryan, if you do build a trebuchet, I recommend the following: Make everything strong, brace it solidly, and have a Boy Scout on your build team if you can't use bolts or something similar .

[Tytus Gerrish]

make a crossbow. it's hard to make somthing better than that

[ChrisH]

Originally Posted by Tytus Gerrish

make a crossbow. it's hard to make somthing better than that

Nobody has suggested the perennial favorite, the "cone cannon". A funnel and two lengths of surgical tubing (length varies by desired range) are all you need. Poke a couple of holes near the edge of the funnel, tie on the surgical tubing, a couple of friends to hold the ends of the tubing and you're ready to launch. With a little work you can get the accuracy pretty good too.

[Simon Strauss]

the only experience with things like this that i have had was with a trebuchet for an elective in the 8th grade, it was three and a half feet tall when not cocked and used 15lbs of counter weight, i managed to launch a tennis ball 42 ft at my best and 36 on average. As far as a ballista goes id stick with the classic design (just like the one used in the ostrich egg launch episode of junkyard wars) and just modify it by a muzzle to guide perhaps. by classic i mean this (you have to give some credit to history that even though they didn't have what do tech wise, that they perfected their weaponry to an extent.) the muzzle i have in mind should be containing the water balloon at its cocked state(this can be done by having it cut so that the top half covers and entraps the balloon and still allows for movement of the sling, then becomes a full pipe after the head part) also if you haven't seen it already http://www.trebuchet.com/ is a great site and should help

[Not2B]

Originally Posted by Ryan M.

Accuracy is important, as we have to hit him for full credit.

Accuracy and repeatable I would assume. Imagine a dart board.

Someone who is accurate will get all around the bulls-eye, maybe some in the ring outside of that. Let's say within a 3 inchs of the bulls-eye.

Someone who is repeatable might aim for the bulls-eye, but always hit the 20. The zero in the 20. Let's say all within 0.1 inch of each other.

Some one who is accurate AND repeatable will hit the bulls-eye, and only the bulls-eye, everytime.

Repeatable might be more useful for this project, since you say a minimum of 20 yards, which leads me to think you can adjust the exact distance as necessary based on experimentation.

SO....... whatever you use.... make sure there is some way of measuring the input energy for every launch. Pull the rubber band back the same amount every time, use the same counterweight every time, place a stop to stop the catapult arm in the same spot everytime, whatever... Also try to keep the water ballon shape and size and weight consistant. (If under your control.)

And experiement.

(I make our students do a ping pong ball launch every year to learn about this kind of stuff. It's a good thing.)

[mechanicalbrain]

When I was younger (funny coming from a high school student ) I had a thing for medieval and roman siege weapons. Have you considered a mangonel (also called a onager). They employed woven cable to act as a spring. Very accurate. They were introduced by the Romans after the design of the balistia. A personal favorite of mine.

[Ryan M.]

Originally Posted by mechanicalbrain

When I was younger (funny coming from a high school student ) I had a thing for medieval and roman siege weapons. Have you considered a mangonel (also called a onager). They employed woven cable to act as a spring. Very accurate. They were introduced by the Romans after the design of the balistia. A personal favorite of mine.

Currently, our design does use woven cable, but it's more of a crossbow:


I'm afraid of the cables streching, so I'm hoping to change it to make the lines holding the cup the part which actually provide the force, which should mostly circumvent that problem. Any recommendations on preventing the cables from streching?

Thanks to everyone so far.

[Ian Curtis]

In case you haven't seen it, which you probably have, there is this. But it is complicated, but I've seen one in action and it went far, really far.

[Matt Leese]

Originally Posted by iCurtis

In case you haven't seen it, which you probably have, there is this. But it is complicated, but I've seen one in action and it went far, really far.

Trebuchets really aren't that complicated. I built a computer-targetted one for my senior project. They're simpler in that they don't use tension; they use a counterweight instead. The only real hard part about them is getting a proper sling design.

Matt

[mechanicalbrain]

Originally Posted by Ryan M.

I'm afraid of the cables streching, so I'm hoping to change it to make the lines holding the cup the part which actually provide the force, which should mostly circumvent that problem. Any recommendations on preventing the cables from streching?

Thanks to everyone so far.

Have you actually tested to see if your cable is stretching? I what you are using is having trouble staying taught then maybe a tighter weave. I can't think of the material but lassos (the real ones) are like cable when it comes to bending....

[wingnut1705]

You might want to try a simple catapult. For my Boy Scout troop I built a two foot tall catapult that launched a bean bag around 150 feet. I utilized two rat traps and several rubber bands. It worked really well.