Romans invented ballistae to use in their wars as siege engines. These giant crossbows could lob large flaming bolts and small rocks at enemy ships and fortifications. Ballistae are powered by torsion, which is the tension formed when cordage is wound up very tightly. Torsion is easy to create at any level, from using large ropes to using cotton string. I was tired of all the mini siege engines that used rubber bands, as no full size siege engines used rubber to power them. Therefore, I decided to make this instructable showing how to make a simple ballista that is powered as it traditionally would be and can be made using crafting materials commonly found around the house. I hope you enjoy!

Supplies

You will need:

Wooden Popsicle sticks (10 - 15)
Glue gun and glue sticks
Cotton twine or yarn
Bamboo skewer
Wire cutter
Utility knife
Drill with 3/16" and 1/16" drill bits
Two toothpicks
Fishing line or sewing thread

Step 1: Prepare the Torsion Frame

Use the wire cutter to cut the rounded edges off two of the popsicle sticks.
Cut four pieces just over an inch long.
Mark the two longer popsicle sticks the width of one and a half popsicle sticks from the middle, so that the space between the marks is the width of three popsicle sticks. Refer to the fourth photo to see what I mean.
Mark two circles on each long popsicle stick about half way between the mark and the edge of the stick, as shown in the final photo.

Step 2: Glue the Torsion Frame

With your hot glue gun, apply glue to one of the longer popsicle sticks along either end and on the two lines marked out.
Stick the small popsicle stick pieces vertically onto the glued popsicle stick, as shown in the second photo.
Glue the ends and marked lines of the other long popsicle stick as above, and stick this onto the frame as shown on the last photo.

Step 3: The Support Strutts

Cut two 2" pieces and one 3/4" piece as shown.
Glue the two longer struts on the bottom of the torsion frame in an "A" shape, as shown.
Glue the smaller one between the two longer struts for support , and add extra glue to the joints for support if necessary.

Step 4: The Stock

Cut the rounded ends off of five popsicle sticks.
Coat one side of one of the popsicle sticks with glue as shown in the second picture.
Take two more popsicle sticks and attach them to the glued one side by side as shown.
Apply glue to the outward side of the two popsicle sticks from the previous step.
Attach the two final popsicle sticks to the glued edges, so that there is a space the width of a popsicle stick between them.
Glue the finished stock to the torsion frame in the middle as shown in the photo.
Add an extra piece of popsicle stick to the underside of the stock as shown in the final photo.

Step 5: Add the Torsion Strings

Cut an 8" piece of cotton twine or yarn.
Tie the two ends of the string together.
Take the string loop and fold it in half, so that you have a double strength smaller loop.
Stick one end of this loop through the top and bottom holes on one side of the torsion frame, making sure that the back end doesn't go through either hole.
Cut one toothpick in half and stick the first half through the small loop that didn't go through either hole, then stick the second half through the loop that went through both holes, as shown in the last picture.
Repeat steps 1-5 on the other side of the torsion frame.

Step 6: The Arms

Cut a popsicle stick in half and drill 1/16" or smaller holes through the end of each half.
Insert each arm into the torsion string so that there are two strands on either side of it, as shown in the second picture.
Twist the toothpicks on top and bottom away from the center of the ballista, so that the arm is forced outwards, causing tension. Continue twisting until it is hard to move the toothpicks anymore, as it will be more powerful the more you twist it. Be careful not to twist so far that you snap the string.
The toothpick may want to untwist. If so, add a drop of hot glue that it will catch on, as shown in the final photo.

Step 7: Bow String and Trigger

Cut a piece of fishing line or sewing thread about eight inches long. Tie it to both arms through the drilled holes, making sure it is taut. (Note: I used sewing thread as it is more visible when photographing, but I recommend using fishing line as it is much stronger.)
Hot glue the popsicle stick sticking off the back of the stock, and attach a metal paper clasp to it as shown. This is your trigger.

Step 8: The Bolt & How to Use

Cut down a bamboo skewer to four inches long, or cut a popsicle stick lengthwise as I do.
Cut a notch in the back and sharpen the front using the utility knife.
To load, insert the bolt through the center hole in the torsion frame so that the notch catches the bowstring, then pull back until you can insert the back of the bolt into the paper clasp.
To fire, push down on the back of the paper clasp so that it releases the bolt.
Congratulations! You've successfully made a Roman-style mini ballista! Do not shoot at people, animals, or glass objects.