My small onager



This is my little engine that I use to move pebbles about. The model is based upon the 3rd and 4th century roman onagers, but I have not tried to make it authentic; the Roman's used sinew and horsehair instead of polypropylene, iron instead of pvc and pesca, and oak instead of tropical hardwood. The basic principles are the same though.

Why an onager? I live in a big city, and there is not enough space to shoot any hurling device without breaking the neighbours' property. So this thing needs to be so small that it can be moved on the back of a bicycle. Torsion based catapults excel at hurling small objects great distances, while trebuchets are great at hurling very heavy ones at still respectable distances. But for a trebuchet to throw large distances, it needs to be big. This catapult throws stuff at over a 100 meters and you would need treb with a 3 meter arm to throw as far!

The specs:

Weight:   3.7 Kg
Dimensions:   width: 0.24 m, length: 0.46 m, height: 0.25 m (0.071 m for the base)
thickness of the construction: 28 mm minimum
hole for the rope: 31 mm
Materials: Base: hardwood
Skein: 6 m Polypropylene woven rope (6 mm, breaking strength: 3700 N/380 Kg)
Arm: Hickory I guess (old hockeystick)
Washers: Pesca (some kind of impossible to get very hard plastic) and 40 mm PVC tube to hold it in place.
Washer plates: Pesca plate (2 mm thick) and steel plate (2mm, not in the photo)
Pouch: strong shopping bag
Range: well over 100 m for 10g pebbles (very difficult to determen, the pebbles are not visible anymore at such distances and it is impossible to find them back. One projectile, fired at half power, was recovered at 77m)
About 75 m for 20g pebbles.
Length of arm: 0.31 m
Length of sling: 0.20 m
Maximum torque: more than 45 Nm (it takes 15 Kg of force to push down the beam)
Energy storage: 40 J

Things that have broken so far:

A few things have been broken so far:
  • The 8 mm thick steel rods that hold the rope where bended the first time I launched with, even though only half of the rope was in place. They are now replaced by 10 mm ones.
  • The first pouch was riped by the ropes holding it.
  • The (said to be unbreakable) rope of the second pouch broke.
  • The third (reenforced) pouch was riped by the ropes holding it. I think it is time to look for another pouch material.
  • Shooting without using a pouch was not a good idea; the stopping bar broke in two places. image
  • Slings, slings and more slings.

    Work in progress

    I am working to get more power out of the existing torsion spring; however, this requires modifications to most parts of the onager.

    The arm is almost at its maximum tension and will be replaced by a new ash one. The sling needs to be stronger and there will be a new, winched, lever to pull the arm down as this can not be done by hand anymore. Also, I am not very happy with the washers, since they have started to break; I may need to replace them with authentic ones. (The romans figured this out a while back) The release hook needs a completely new design.

    Oct 23 2000. The thing is finished. All reenforcements are done and it can take a large torque. This also means the end of the project and I'll have to look for another hobby. It has been fun building and playing with it for the last two months.

    A word of caution

    These machines used by the romans to kill people; even small ones like mine can be dangerous. I have no doubt it will break several bones of your hand if the arm hits it at full speed.

    Be careful: don't stand in front of it, don't put your head above it and don't stand behind it. Use a safety zone of at least 200 meters in front and 75 meters behind the machine (depending on its size). Don't park your car behind it.

    And on the brighter side; don't shoot grapes at your bathroom wall; they make a real mess.


    Unsorted photo's Construction and detailed photo's.
    The onager simulator A full computer simulation of the onager


    My first onager, version 1
           My previous onager. It has a very simple design and can be build in a couple of hours.
    The physics of ballistic projectiles
           Formulas for computing height range and speed of projectiles.
    Onager online
           The inspiration for my catapult
    Jari Vaarma's siege engine page
           Jari has build a decently sized onager.
    Siege artillery
           Has plans for all kinds of siege engines, including an excellent onager plan.
    The Trebuchet Resource Site.
           Home of the catapult message board.
    The Algorithmic Beauty of the Trebuchet
           Has a trebuchet simulator
    Rüstung 3 - Onager
           A beautiful catapult but it looks like it is not very efficient.
    The Grey Company Trebuchet Page
           Great source of information on trebuchet's and related stuff. Large page of links.
    Adam Betz's onager
           Adam Betz's (large) onager
    Bob's onager
           the site is still under construction, but has nice photo's
    My other websites
           On b.ackgammon, d.raughts and a.tomic b.ombs

    (c) , , 10 June 2000