True catapult that uses a extremely tightly wound string to power it, it fits in the palm of your hand, it cost under $20. all supplies can be purchased at Michaels. fires small foam objects 40 feet aprox 1053 feet to scale, about 7.5FPS when firing a 14g weight (twice the weight of the catapult its self) (i know cause i did a school project on it. This is my design and i didn't copy it) it can fire other objects farther and faster such as rocks but you can not easily recover them cause they go to far and fast to visually track.
this is for the competition so please comment and rate (this is the 4th catapult i have made they all worked good yours should to)
This is technically an [http://en.wikipedia.org/wiki/Onager_(siege_weapon) onager] but it is still in the catapult family.

this is a rather complicated instructable and is an example on how to build one yours dosent have to be exactly the same i use cm for most measurements but some are in inches ask if you get confused

use safety glasses when drilling cutting wire and when firing
this is safe when fire foam objects when firing rocks never aim at a person animal car personal property it fires hard enough to break a paint ball if used for office war only use foam light weight objects and make sure every one is wearing safety glasses

build this in a room with good ventilation the Ca glue and accelerator smoke when they dry
Ca glue bonds to skin on contact use un-cure do not pull apart. the glue will not separate but the skin on your fingers will.
The accelerator causes chemical heat and you can get a chemical burn from it which hearts so dont touch it

it has a range between 15-50 feet depending on ammunition and angle of trajectory

Other videos on step 4 and 9

I have pics of my 6th grade Sci fair project back when i was 11

Step 1: Get Your Stuff

i went to Michaels and spent a little over 3 on:

wood, midwest 3/18 X 3 X 36100% polyester string 500yds

you will need other balsa wood i have a huge box of scrap so i didnt need to get any thing else

you could use cotton, or any sting thats not hemp or nylon thats around the same diameter as kite string or you could just use kite string.

you will also need:

wood glue 19 gauge steel wiresawexacto  kniferuler square (a right angle thing)sand paper bamboo skewers Ca glue and accelerator  (with out accelerator its pointless)5min epoxy the kind that you mix two elements together

you only need epoxy one time the when your gluing the arm, you could use wood glue or hot glue as a substitute but i dont recommend it.

Step 2: Mesure Then Cut Main Frame

you will need to measure the midwest 3/18 X 3 X 36 piece

See Pictures
measure
first draw a line using your square at 5cm it needs to be 42 cm long
at 17 cm draw a line using the square repeat once draw a line at 9cm repeat once

cut
your saw probably can cut the long 17cm long cut so carefully use a razer and a metal strait edge like in the picture

Step 3: Glue Main Frame

this step will probably take the longest time because you have to wait for the glue to dry

line the pieces up and make sure every thing is strait and at a right angle

use Ca/accelerator, put a little drop on the top corner of the joint and spray with accelerator wait 30 sec and carefully pick it up and flip it over put it another drop on other side doing this makes using wood glue so much easier.
Then use the same proses to glue the other two pieces into place
use wood glue to finish the job

why not just use Ca? Ca can take some stress and shaking but hard shocks and impacts weaken it. so we need wood glue not hot glue, hot glue will flex way to much (as i learned from my first catapult)

let it dry then proceed on

Step 4: Mesure the Frame for the Holes and Other Stuff

decide what will be the front and top docent really matter which just chose

repeat all this for the other side
see pictures
make a line 3.5 from bottom
make another line 6.5 from end
where the cross is where a hole will be drilled for the torsion spring tightener

Step 5: Strengthening

on my first and second attempts the catapult riped its self in half to to extreme forces at work so we need to add reinforcements directly under where the holes will be this is where the most stress will be.
a cross will also help prevent warping.

the pictures should help the directions make sense

Measure.
the inside width of your frame mine was exactly 8cm

Mark
on a thick piece of wood (mine was 1X1 cm) mark at 8cm cut then cut another piece 8cm long

Measure across the frame mine 20cm long do it again for other direction

Cut the two pieces

Measure.
slide one piece under the frame make sure its the bottom of frame and no the top then trace the inside corners Repeat for other piece

Cut
along both lines on both side repeat for other piece

Sand
you will probably need to do a little sanding to make them both fit in

Mark
mark where the two pieces meet on both sides

Cut
use a wide exacto knife like a chisel to carefully cut half way through on both pieces

Pre glue
Put them back in make sure they fit
Glue using Ca glue to temporarily hold together put a dot at each corner and where the two pieces cross over dry with accelerator

Go back to the other 2 pieces we cut out earlier
Select one of them at random and:

Sand
i had to do a little sanding before it would fit in

Slide
slide it down so that its 6.5 Cm from the front

Mark where the 2 cross pieces cross over it

Cut about half way through like you did before with the cross

Pre glue
use Ca/ accelerator to glue the brace in

Now the remaining piece

repeat what you just did with the first one but do it 13.5 cm from front and pre glue with Ca

Add more

get a scrap piece from cutting out frame measure/cut it to fit Ca glue it 2cm from top 2.7 cm from front

Glue
use wood glue to glue every joint let it dry then proceed on

Step 6: Build Upper Frame

the upper frame needs to be extremely strong
this is complicated because their are to many variables in the design

You need a base to mount the two 45 angle supports i used some scrap from the frame
Ca glue that for now

now measure how long the vertical supports will be cut them and Ca glue in place

measure the length of the cross bar cut and glue into place

use pictures as guidlines

i also added a square support on the inside sides to help support once you have every thing Ca glued use wood glue to finish the job

Step 7: Drill/string/sling/ring

i am using the arm from an older design but ill still show you how to make your own.

start by using a dremel tool or drill press to drill the 2 holes use a drill bit close to the same size i did. use safety glasses when drilling

cut about 2yds and 6inch of string.
tie it together into loop and cut of the excess string
double the loop once, then again

After Step 8 come back to here and make sling/ring (do to tecnical dificulties i can not have another step)

for the ring cut about 7cm of wire and twist it into a loop like on a key ring (or you could just use a key ring)
then attach it to the arm like in the last pictures using a drop of Ca/ accelerator

now for the sling (i am using the sling from 3.0 so this one is just a model)

using a sharpie or marker make a diagram similar to the picture below (with out the numbers/words)
cut it out and punch holes on each corner like in pictures

tie an even amount of string on each hole

tie the two ends evenly together and make a loop with the remaining string

Look at the pictures for the rest of instructions its a lot easier to explain and understand

Step 8: Making the Arm

i had a throwing arm laying around so i used it but ill still show you how to make one.

First cut on thin piece of balsa wood 18cm long now cut another 18cm long piece using thicker balsa (i didnt have 18cm of thick so i had two glue two together)
mark 1cm on the thin piece
glue togeather using 15min epoxy (glue so that 1 cm of thin is infront of thick)
allow to dry.

now we need to drill a hole to but the hook into
using safety glasses
cut about 1.5 INCHs of wire bend it so it kinda resembles the picture.
get a drill bit little thicker than wire
carfully drill a hole 2 inches deep into thicker piece of wood

glue use Ca glue to fill the hole you drilled, insert the hook so its facing the same way as picture
do not use accelerator give it 5 min to dry on its own

now that thats done flip back to 7 to complete the sling and ring (sorry bit of technical difficulties)

Step 9: Rigging the Arm

we are to the part of the project where the torsion spring and arm will be installed into frame!

the pictures should explain them selfs

Here is a video to help explain (the microphone on camera is messed up so all it gets is static) yes i intended it to jump out of my hands i was trying to show how powerful it is

After its twisted really tight let it sit over night the spring will relax, and when you tighten it again it will be even strong than before (getting the string a little bit wet will make it relax too)

Step 10: Adding the Catch

the catch this is the firing mechanism you push it down or pull a string an it fires its pretty simple you already made the ring so now all you need is a hook made out of the same 19 gauge wire.

first get some of that scrap left over from the frame cut it at about 4 cm
now cut 2, 2cm pieces i used 1X1cm
position the pieces like in pic 3 and glue them with Ca/accelerator
then i just snapped a bamboo skewer so that it was about 5cm long (i just snapped it no need to use saw or measure)
put dots of glue down, glue the piece of skewer on and spay with accelerator

now Ca/accelerator glue the whole thing about 1 cm from top

this has no safety the hook is a kinda sharp and it could go off at any moment if it hits you in the eye it will cause severe damage i have had the tip jab my finger it sinks in deep and it hearts

Here it is (i did not load the catapult in video)

Step 11: Load and Fire!!!

to load simply pull down arm hook the hook through the ring. loop the string loop around the other hook
place ammo in sling aim in safe direction push down lever and Fire!!!
If you read the bold on first page you would know that this is a temporary video and one that meets the criteria will soon follow