Super Soaker Central

Some sort of sabot is necessary to prevent the balloon from popping as far as I know, so yes.

Edit: I've been doing a little thinking, and I've come to the conclusion that water balloons can be launched much more efficiently with some sort of elastic system. Catapults can have efficiencies of 80% from what I understand--this is a huge advantage because I'd be struggling to get 30% efficiency with a pneumatic gun. I'm thinking something like a crossbow with a balloon clip could possibly be the ultimate water balloon gun.

This isn't to say that this project isn't going to be made. I've already ordered the parts and have nothing better to do.

How far are you willing to stretch the elastic? That affects how close together the two ends can be. If they're far apart, then a lot of the tension in the elastic comes from the two posts pulling away from each other, and not from them pulling in the direction of the shot. That extra force then requires a stronger frame, etc.

Given how much the elastic stretches, and given how far back you'll stretch it, you should be able to find how far apart to put the posts.

The loading shouldn't be too much of a problem. I intend to do tests to see how thick an elastic band is necessary and work from the minimum, so the forces will not be particularly high. And I can easily design a frame to support the load with minimum deflection and material. Taking a few solid mechanics classes help. A simple and cheap box beam of the correct dimensions should be more than adequate.

With that being said, I haven't decided how far to set the posts. I'll take a look at the force-deflection curves for latex tubing to get an idea of how much deflection is wanted. I can work from there.

Edit: Let me also say that while a crossbow will be more efficient than a pneumatic gun, that doesn't mean I won't work to improve the efficiency further. Relatively simple engineering like this is very satisfying.

A catapult may be more efficient, but an air cannon will be more accurate.

Maybe my comment about the efficiency of a catapult was confusing. I'm considering crossbows and remember reading something about the efficiency of a catapult being really high. Crossbows shouldn't be too much less accurate than an air cannon, especially with guides.

After some thought, I've decided to change the water balloon launcher I had in mind to a semi-automatic tennis ball launcher. The design probably won't be able to work as a WBL due to the fragility of water balloons, but it'll be interesting nonetheless.

Good luck with it!

Actually, a ping-pong-ball launcher would be cool too. You could use that in mock warfare safely.

Ping-pong-balls can't fly past, what, 20 feet, right? I'm guessing that since they weigh practically nothing, their momentum will be reduced very quickly due to air resistance...

I wish this design would work with waterballoons, though, since it'd be nice to have something that shoots "friendly projectiles" (if one can say tat WBs are friendly...).

The design I had in mind (I've put it off until next summer at the earliest) might be adaptable to water balloons with a solid sabot. I'll think about it. The design is very simple and much like my FANG Nerf gun if anyone wants to go try it.