From DT Online
The Onager was a type of Catapult with a heavy base frame and a strong vertical frame fixed part way along it. They were powered by the Torsion provided by inserting a single strong arm into a twisted skein of rope, hair, or sinew held between the frames. At the end of the arm was a short Sling.
In operation, the arm would be pulled down against the twisted skein using Winches against a Ratchet and Pawl and held down by a large iron catch or clasp. The Sling loaded with stones, fire pots or rotting corpses then released by striking against the catch with a heavy Mallet. It was called an Onager (i.e. a type of wild donkey) because as the arm hits against the vertical frame on release, the back end would kick up.
Onagers were deployed to the rear of an advancing army, so they could throw over the heads of the front ranks. When besieging a city, the Battering Ram was employed in destroying the lower part of the wall, the Ballistae were used to attack troops and knock down towers or parts of towers and Onagers as the ‘heavy artillery’ to try and destroy the defences completely.
Stripwood Model Onager
A model Onager can be made using Stripwood Technology techniques with card and 8mm x 8mm square timber. Start by making a base frame, 210mm long and 100mm wide :
- Draw the frame full-size on centimetre squared paper.
- Hold strips of timber in a small Clamp-on Vice and cut to length using a Junior Hacksaw.
- Assemble the strips together by placing them over the drawing.
- Glue 30mm x 30mm triangles of medium thickness card over each join as shown.
- Turn the frame over and add card triangles to the opposite side.
- Similarly, make the central vertical frame 100mm x 100mm and assemble with angled Stripwood bracing pieces.
- Cut a piece of Stripwood about 210mm long to make the throwing arm and fix a paper clip to one end - either tape or bind with cotton and glue.
- Ensure half the paper clip is over-hanging the end of the throwing arm so that it can be bent to an angle which releases the Sling.
- Make the Sling from a strip of cloth and use cotton or twine to tie one end to the throwing arm and the other to a keyring which is hooked over the paper clip.
- Wrap elastic around the lower half of the vertical centre frame, insert a short length of Stripwood to twist the elastic then replace this with the throwing arm to make the Torsion spring.
- An alternative construction is to start with a rectangle of thick card reinforced with lengths of Stripwood.
- A clothes peg can be added as a release trigger and, if the Onager is to be put on a wheeled Chassis, clothes pegs can be used as wheel bearings (see the DT Online article Rolling Chassis for other ways to make the Onager mobile).
- When released, the throwing arm would quickly swing upwards, powered by the Torsion spring. As the arm approaches the top of its travel, the keyring is released from the paper clip hook and continues to swing upwards until the ammunition is thrown forwards when the arm hits against the vertical frame.
Safety Point! Use table tennis balls, squash balls or wooden beads as ammunition for example - NEVER use anything sharp or heavy
Activity: Find out about Roman Siege Equipment (e.g. Vinea, Pluteus, Mantelet, Testudo, Musculus, Helepolis, Turris, Corvus, Ballista, Onager, Mangonel, Trebuchet, Aries (Battering Ram) and construct a working model or models using Stripwood Technology.
Safety Point! Take great care with hot glue guns and all hot-melt glues. The melted glue can stick to the skin and cause severe burns.