Building a Trebuchet

[jenarelJAM]

Got another picture for you guys. Turns out we're thinking of creating 3-4 smaller catapults to go with her, so here you go:

<a href="http://s43.photobucket.com/albums/e383/jenarelJAM/?action=view&current=catapult.jpg" target="_blank"><img width=50% src="http://i43.photobucket.com/albums/e383/jenarelJAM/catapult.jpg" border="0" alt="Photobucket"></a>

It will be powered by surgical tubing that goes between the two 2x4's in a "tower", loops around the arm, and goes back up to the other tower. Probably we'll have 3-5 lengths of surgical tubing to give it enough strength. This is meant to throw a water balloon a shorter distance, but possibly full size water balloons.

One of our number is a blacksmith, and he'll be hammering out the buckets.

Note: I still need to provide support for the arm, it's a little flimsy right now.

[pmstc]

sodium polyacrylate is scentless as far as I can remember. we did a little lab with it in my organic chemistry class where we were given a gram of it and able to soak up some ridiculous amount of water like a liter with it.

you might be able to dilute it so that its just a more cohesive form of water. I do know that the more water you add, the less gelly it gets. which also leads me to believe that it would wash away with rain. but i wasnt really testing for that when i used it.

http://cgi.ebay.com/2oz-Sodium-Polya...QQcmdZViewItem

that amount of sodium polyacrylate should absorb about 1.5 gallons of water. its cheaper if you buy more, but that might be a bit expensive. i dunno. its worth a try though i guess?

[trevorjk]

for ammunition

http://cgi.ebay.com/BULK-LOT-OF-STRE...QQcmdZViewItem

but seriously, look for lots of foam stressballs ect. most lots are probably $20 shipped for 25 balls.

[Ninjeff]

HA! thats looks like hella fun!

[jenarelJAM]

Go Nor*Cal! (you said hella, heh)

On a side note: We're looking for some 1 1/4" to 1 1/2" steel bar for the hinge, roughly 10' length, cut into a 3' section and 3 2' sections. Anyone know of a cheap place for us to get this? We can cut it ourselves.

[thejere]

Woot wooden artillery.

Some things to consider for catapult design (I've made a twisted rope balista and twisted rope catapult)

Spend as much time as you can perfecting the release mechanism for you sling/shape of the bucket. If your projectile doesn't get released right you lose a lot of range, worse case includes throwing it straight at the ground or backwards. Best thing to do is design it to be adjustable from the get go than having to recut hooks or hammer out buckets every time it doesn't go right.

Add wheels, big ones. Not because it will make it mobile (although good) but you can get a nice roll back on a trebuchet and increase you range without adding weights.

Best of luck, nothing is quite like watching a trebuchet action.

[acropilot19]

...its nice to see the sport that my son & I invented really taking off. I have fond memories of beating the crap outa him with an 8 foot long noodle many years ago. Once he learned how to fight, we had some epic battles. That is, until the incident with grandma & her false teeth becoming wedged tightly in the ceiling fan, but she talked alot of trash anyway...
Rock On!

[jenarelJAM]

Woot wooden artillery.

Some things to consider for catapult design (I've made a twisted rope balista and twisted rope catapult)

Spend as much time as you can perfecting the release mechanism for you sling/shape of the bucket. If your projectile doesn't get released right you lose a lot of range, worse case includes throwing it straight at the ground or backwards. Best thing to do is design it to be adjustable from the get go than having to recut hooks or hammer out buckets every time it doesn't go right.

Add wheels, big ones. Not because it will make it mobile (although good) but you can get a nice roll back on a trebuchet and increase you range without adding weights.

Best of luck, nothing is quite like watching a trebuchet action.

For the release mechanism, it will be adjustable. We will have a rope that attaches the bucket to the back of the catapult. At the specified length, it will go taut and stop the bucket arm, releasing the projectile.

Thanks for the tip on the trebuchet and wheels. I'll check it out.

[ScatterPlot]

Do a little math on there first. You're not going to get really much of anything to go 150 feet with that short of a drop and that little weight.

Range = (v^2 * sin(2(theta)))/g
Theta=45 degrees (sin 90 = 1) is optimal for range, reducing the equation to:
R=v^2/g

To get R=150 feet, you'd need v to be:
v^2=Rg
v=sqrt(Rg)
v=sqrt(150 feet * 32.2 ft/sec^2)
v=sqrt(4830) =~ 70 ft/sec

Now to find your initial velocity, you need to convert energies here. The potential energy at the start will be the kinetic energy at the end.
PE=counterweight*height=160 lbs * 3 feet = 480 ft-lbs
KE = .5*projectilemass*v^2

These have to equal. A 10 lb projectile has a mass of .31 slugs.
480 ft-lbs = .5*.31 slugs*v^2
v=55 feet/sec.

That is -perfect- conditions. Wind resistance is one HUGE factor, losses in the trebuchet (non-linear falling weight for one) will be another large loss. Not to mention the energy needed to rotate your trebuchet arm to go fast (like spooling up a tire, takes rotational energy to move it quickly)

To fire a projectile 150 feet, the weight would have to be:
480ft-lbs=.5*m*70^2
Weight ends up being about 6.3 lbs.

From previous experience, I've built a trebuchet to launch cantaloupes, about 5 lbs each. We used probably around 200 lbs. The first run broke our 2x4 beam the -strong- way. We also dropped ours maybe 5 feet. It maybe went 60-70 feet tops. Not in a perfect arc mind you, but still I wouldn't think it would be too easy to snipe at anything with it. Ours ended up being about 6 feet to the pivot and with the arm up, closer to 15.


Good luck with yours, I don't mean to bust any balls here but I just want you to know what you're getting into before you make all this. Just don't hurt yourself




(BTW before you ask, the sling will help range, but doesn't factor into the calculations I have up there)

[doc_Zox]

i have watched punkin chunkin misfires that lob the pumpkin straight up

it took the people watching a moment to realize that the return vector was 10 feet behind the launcher

[jenarelJAM]

Do a little math on there first. You're not going to get really much of anything to go 150 feet with that short of a drop and that little weight.

Range = (v^2 * sin(2(theta)))/g
Theta=45 degrees (sin 90 = 1) is optimal for range, reducing the equation to:
R=v^2/g

To get R=150 feet, you'd need v to be:
v^2=Rg
v=sqrt(Rg)
v=sqrt(150 feet * 32.2 ft/sec^2)
v=sqrt(4830) =~ 70 ft/sec

Now to find your initial velocity, you need to convert energies here. The potential energy at the start will be the kinetic energy at the end.
PE=counterweight*height=160 lbs * 3 feet = 480 ft-lbs
KE = .5*projectilemass*v^2

These have to equal. A 10 lb projectile has a mass of .31 slugs.
480 ft-lbs = .5*.31 slugs*v^2
v=55 feet/sec.

That is -perfect- conditions. Wind resistance is one HUGE factor, losses in the trebuchet (non-linear falling weight for one) will be another large loss. Not to mention the energy needed to rotate your trebuchet arm to go fast (like spooling up a tire, takes rotational energy to move it quickly)

To fire a projectile 150 feet, the weight would have to be:
480ft-lbs=.5*m*70^2
Weight ends up being about 6.3 lbs.

From previous experience, I've built a trebuchet to launch cantaloupes, about 5 lbs each. We used probably around 200 lbs. The first run broke our 2x4 beam the -strong- way. We also dropped ours maybe 5 feet. It maybe went 60-70 feet tops. Not in a perfect arc mind you, but still I wouldn't think it would be too easy to snipe at anything with it. Ours ended up being about 6 feet to the pivot and with the arm up, closer to 15.


Good luck with yours, I don't mean to bust any balls here but I just want you to know what you're getting into before you make all this. Just don't hurt yourself




(BTW before you ask, the sling will help range, but doesn't factor into the calculations I have up there)

Agreed. I hadn't actually done the math, but what you've done seems reasonable. And I also agree that it doesn't take into account any frictional forces and inefficiency of the contraption. We've got all the parts built for the wooden frames for the trebuchet and 3 catapults, and we're hoping to start assembliung them some this weekend.

Yeah, we're probably throwing 1-2 lb MAX water balloons a farther distance like that, not 10 lbs.

This is kinda a tester project for us. We're doing it for fun and for a purpose, but we'll probably refine our ideas quite a bit while building and testing it, and we'd then know what to do better for when we make a second, bigger, one.