I need to build a hacky-sack launcher

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http://party.windycitynovelties.com/search?w=bls042dz2&asug=&submit.x=0&submit.y=0" is the hacky sack, it weighs 50-60g and is about 2" in diameter
The launcher has to fit inside a 15" cube when set, weigh less than 15 lbs. and has to use purely mechanical power, compressed air not included. It will have to be accurate to within 1 foot from 10-35 feet away. It also needs an internal trigger.

 

[Danger]

Welcome to PF, Advertisehere.
So what's the question? Nobody here will just up and give you blueprints for a class problem. You have to tell us what you think, and we'll guide you. What sort of mechanisms have you looked into so far? Which one do you favour, and why?

 

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Well, I've looked into a trebuchet assisted by a spring at the base of the end with the hacky sack and and an elastic band at the other end.
I've also considered a catapult with adjustable launch angle, as well as a sling-shot type thing.

 

[Danger]

Good start. What do you consider to be the advantages and disadvantages of each? Does the accuracy have to be repeatable, or is it a one-shot deal?

 

[advertisehere]

Accuracy is very important, it basically needs to be calibrated to each foot with pretty much 100% accuracy.
The trebuchet might be too complex, and maybe not accurate enough.
The catapult may not give good range.
The sling shot is simple but the accuracy may not be too good

 

[Danger]

Well... to start with, forget the 100% accuracy. That just isn't going to happen. I do agree that the trebuchet has serious issues in that regard, but the throwing capacity is amazing. If memory serves, one of those can heave a cow about half a mile. That's a bit of overkill in your situation. (And one of that scale wouldn't meet your size/weight requirements.)
Catapults can be very accurate given a constant payload, which you have. It takes a while to tune them in, but things are good after that. Remember that part of that constancy relates to exactly how the load is seated in the cradle, so you have to be able to duplicate it repeatedly.
The term 'slingshot' actually covers a fair bit of real estate. You can have anything from a 'Dennis the Menace' tree branch with a bit of bicycle inner tube up to 'Wrist Rockets' that can almost rival the power of a small handgun. One of my school buddies made one using surgical tubing that could put a 1/4" steel bearing through the side of a 50 gallon oil drum. (It was a rusty old drum, but that's still pretty impressive.) He actually killed a crow at about 100 feet with it. His neighbour on the next farm over was going nuts trying to figure out where all of the dents in his barn were coming from.

 

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Im thinking of having a design like this with a spring or elastic band with adjustable mounting points to adjust for distance.
Im not sure if it would fit into the 15" cube though.

 

[Danger]

You can always scale it down to fit. Lack of lever-arm length can be compensated for by increased spring power.

 

[Bob S]

How about a big coil-spring rat trap or similar design. They are bigger than mousetraps. You have to launch the hackysack at about 14.6 meters per sec at an angle of 45 degrees to reach 35 feet.

 

[blimkie.k]

I like the idea of a slingshot, Similar to a water balloon sling shot but designed to launch a hacky sack. Now if you could take the sling shot on a cross structure so that the bucket holding the hacky sack is propelled along a strait line you could control the angle the projectile (hacky sack) would leave the slingshot. You could use bungie chords for the elastics and have it so the bungie chord could be locked at various tensions thus controlling the intitial velocity of the hacky sack.

With a changeable angle and being able to change launch speeds by tensing the bungie chords more of less you could run some tests take some range measurements to find out which settings will fire your bean bag the farthest.