SilentGuy wrote:I was thinking about a more heavy-duty solution; for example, a wooden piston with a concave face carved into one base. If the balloon contacts the piston with greater area due to a more similar curve, then there will be less pressure and a lesser chance of rupture. Of course, this is all speculation.
Ductile materials typically fail due to shear stress (if you look at Von Mises/distortion energy theory, etc.), but that's beyond the scope of the forum. If you don't understand solid mechanics, just trust me on this one when I say.............
As long as the balloon can mold itself into the shape of whatever container you're putting it in (with nothing too sharp to puncture the balloon), all you need to do is match the volume of water with the volume of the container. Imagine the container sitting on a table, and imagine if gravity were suddenly increased at and around the table. A balloon sitting in a can with more volume than the balloon's water is just going to sit down into the can more, but the balloon won't break because there will be equal compressive forces on either side of the rubber where the water and can meet.
If the volume of the holder is less than the balloon, our table experiment would yield a balloon that sags over the edges. In a barrel, this will cause parasitic loss as the balloon rubs the edges. (This is why I recommend a lubricated barrel, especially for a deformable projectile. Is a little bit of oil heading toward your target going to really ruin your water fight anyway?)
A soup can is quite a bit more durable than you might think. One key advantage over the wood idea is that it has a very good specific strength (high strength with a low mass). As far as the water balloon launching properties of it, I don't know... As I said, I'm not that interested in launching water balloons. My next launcher is going to be for water (so that's why I'm here), but my interest in launching solid objects extends to other things.
IF I were interested in designing a water balloon launcher, I'd be interested in something LIGHT and STRONG to hold the balloon during launch, and I'd want a parachute behind the balloon holder so the holder doesn't hit the intended target.
The weight of the balloon holder is important for more than one reason. First, you don't want the extra weight sucking up the finite amount of energy coming from your chamber. Second, you don't want your launcher to be cumbersome or tiring to use. Third, you don't want a heavy holder to hit a human target in the unlikely event that your balloon doesn't leave the can and the 'shute doesn't open.
I make no warranty, express or implied, on the safety of launching anything at a human. I would recommend paintball-style protection if you're crazy enough to do anything at close-range. (Close-range isn't why we're building our own launchers, from what I understand here.)