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Accurate Projectile Launcher

  1. Jan 27, 2009 #1
    I am building a projectile launcher for school and was wondering what is the best way to go about it. I was thinking about a trebuchet but am unsure about how accurate it will be. The launcher does not need to be very powerful, it only needs to launch a 250g mass roughly 1.5m, but has to be very accurate, i.e land in a designate area which consists of a circle about 10cm in diameter. Would a spring powered cannon be better suited for this application?
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  3. Jan 28, 2009 #2


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    A trubuchet is a tricky thing. From the ones I have seen by people I know, they were not very consistent.

    I would say a spring loaded or pressurized air launcher would provide more "tunability" and reliability.
  4. Jan 28, 2009 #3


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    What is the shape of the projectile? If it's mostly round, think about how pitching machines are typically constructed......
  5. Jan 28, 2009 #4


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    I did an exercise recently with an elastic band powered catapult. After some optimisation we were accurate to within a few millimetres either side of the target at a distance of a few metres.
  6. Jan 28, 2009 #5
    The projectile can be any shape I want, ideally it would be a shape that bounces as little as possible, I was thinking sort of like a short cylinder, i.e. pancake.

    I went and measured the precise distances and it needs to launch 1.31 +/- .07m. The mass is also actually more like 50 to 100 g.

    Is there any plans available on the internet that you started from, or did you just design it from scratch?

    Edit: I was searching for catapult ideas and came across this, which I think would be a good place to start and modify a bit, in comparison to the catapult you made which was very accurate, are there any major deficiencies in this design?

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    Last edited: Jan 29, 2009
  7. Jan 28, 2009 #6


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    So the projectile cannot bounce? What about roll? If the projectile is not measured by where it actually hits, then maybe the idea is just to "shoot" it as short as possible and let it roll to where you want it to.
  8. Feb 9, 2009 #7
    I've built a trebuchet before. The one fatal flaw that cost us was the sling, it needed a four point support system instead of just two. The projectile kept falling out of the side XD.

    Catapults are good, but don't really take full advantage of the physics behind a giant falling mass.

    A few things to go for with a trebuchet:

    - Make the arm as light as possible. For the arm on my small one we used a 1x4. We left the end holding the counterweight full width, but tapered it about 1.5 cm past the axle on the side with the projectile. Barely even took a tap to move that arm.

    - Wheels make your trebuchet shoot farther, provided it actually moves. It allow the counterweight to drop in a straighter path.

    - Keep the projectile going in a straight line off the base. Otherwise it will veer. Use a curved trough to help centre it.

    - Spherical shaped projectiles work the best with either a trebuchet or catapult. You don't really know what orientation the shape will leave the machine at and so it's better to have an aerodynamically consistent projectile, instead of a pointy one that heads out the wrong way and creates so much drag it falls short.

    - When you pick a sling length, experiment. If it's too long the projectile will still be in the sling when your counterweight stops giving power, too short and you're wasting some of the energy. You need to find a perfect balance.

    Trebuchets are plenty accurate, even more over long distances. They can throw much farther then catapults. They're a bit more complicated but when done right they work wonders. I can tell you the specifics about how to put it together if you wish.
  9. Nov 27, 2010 #8
    please tell me the specifics? :)
  10. Nov 27, 2010 #9
    TREBUCHETS RULE ... just sayin ... we made one in robotics. it is definitely worth it.
  11. Nov 28, 2010 #10
    What else do you want to know? It was a trebuchet.
  12. Nov 28, 2010 #11
    Why build such an indepth tool such as a trebuchet, to launch a small 50 to 100g projectile the short distance of 4 feet or so? Is this just an exercise in scaling or model building? A small slingshot made out of surgical tubing might work, or as FredGarvin said a spring loaded or air powered cannon would be easy, tuneable and reliably repeatable as long as they were set up to lob, a catapault would have a better suited trajectory so the projectile would be less prone to roll after landing. You could also make your projectile into a bolt and make a ballista, then it would stick into the ground where it hit instead of rolling.
  13. Nov 28, 2010 #12
    Are you kidding? The one I made fit inside of a one metre cube, and it was firing stuff over half way down the college parking lot. The issue with the one I built was the sling, failed more often than not, but when it worked it was spectacular.
  14. Nov 29, 2010 #13
    No I wasnt kidding, the OP only has to throw the projectile 1.3 m, not halfway across a parking lot. You may have mis-read, I said indepth not inept.
  15. Nov 30, 2010 #14
    I see what you were saying now. I thought you had grossly underestimated the range a small trebuchet could achieve.
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