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I Air-powered water balloon launcher, Which weights to use?

  1. Jul 12, 2018 #1
    Hi all, first post. Bare with me.

    I've got a simple air compressor-powered water balloon launcher made using PVC. It works extremely well. But, I'm needing to make some "sighting charts" to predict where everything will land based on angle, loaded psi, size of balloon---and the wadding used. By wadding, I mean an amount of water to act as a cushion and also a seal to transfer the pressure from the air reservoir to the balloon (both to cradle it and not tear it apart).

    I ran some tests with known psi and balloon sizes and time and the physical results were close enough to the paper results to suspect friction/balloon deformation/water separation/et al.

    But I wasn't using the wadding water in the figures.

    The question I'm stuck on:

    As I'm determining Vf (muzzle exit velocity), does the mass of the 'wadding' need to be included along with the mass of the ballon?

    It's a small barrel (2" pipe). The balloons are about 1.75" diameter and weight about .15lb (.07kg) using some rough assumption that the 1.75" balloon is in fact a sphere. At 2" pipe, about 1 cup of water will cover the ballon. I know less water helped the hang times which makes me think it's included.

    Also, working out gravity on the 'weight' of the ballon to use f=ma (to get an "a" to put into Vf=Vo + (a)(t)) yield paper results that didn't match actual.

    balloon: .07kg
    pipe diameter: .002"
    psi: 50 (35153 kgf/m^2)

    Thanks for taking a look.
  2. jcsd
  3. Jul 15, 2018 #2


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    Science Advisor

    Try using a sabot (look it up). One or a few could be made from paper mache for testing.

    Yes. Everything that is moved must be included. Also, take account of the varying pressure of the air as it expands into the barrel, and perhaps the pressure buildup in the barrel in front of the accelerating projectile.
    Probably because the "wadding" is leaking past the balloon; and did you account for air drag during flight? (Not easy, that balloon will not be a sphere during flight. You will be surprised at its shape if you take a video of it and play back in slow motion!)

    Pure speculation here, but try adding a little glycerin to the "wadding"... or maybe use light vegetable oil in, or instead of, the water "wadding."
    Umm. Maybe pipe area: 0.002sq.m. ?

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