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So You Want to Build an Onager Brian Lapham Lord Doughal Stewart

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So You Want to Build an Onager Brian Lapham Lord Doughal Stewart Catapulta So you want to Build an Onager By Brian Lapham Also known as: Lord Doughal Stewart Copyright ©2003 Fowl Farms Publishing A division of Fowl Farms Inc. 1 Contents Getting Started 1 Firing 23 Background 1 Preparing for a live shoot 23 What do you want your catapult to do? 2 Tensioning the skein 23 How will you transport it? 2 Aiming and firing 23 Where will you store it? 2 Tweaking 24 What are those parts called, anyway? 3 What will it look like? 4 Calculating the dimensions 4 Making plans 6 Conclusion 25 Choosing materials 7 Tools 8 Book sources A Construction 9 Internet sources A Main frame 9 Subsidiary structure and buffer 11 Parts list B Throwing arm 12 Torsion hardware 14 Additional schematics C Attaching the skein and arm 15 The winch 16 Sling 18 Pouch shape 18 Sling length 19 Sling stability 19 How to build a sling 19 Projectile ramp 20 Wheels 21 Projectiles 21 Finishing work 22 Getting Started So how long have you wanted to fling things? I’ve had the urge for this since I was in elementary school. My interest was re-peaked when I saw a Saturday Night Live sketch which was a commercial spoof for a backyard catapult to launch your trash into your neighbor’s yard. But it wasn’t until 1994, after I’d been in the Society for Creative Anachronism for a couple of years, when I went to my first Estrella war and saw a catapult powered by garage door springs. That was when I decided to make catapults the focus of my SCA hobby. I’ve been hooked ever since. I currently have two catapults in operation. Odin’s Pride is a 2’x 3’ catapult which can shoot a single tennis ball 50 yards. Gungnir is 4’x 6’ and can throw most one pound objects 100 yards, provided the object has some aerodynamic properties. Gungnir Odin’s Pride Background When researching ancient artillery, there are two main sources of information: Greek and Roman Artillery, by Eric Marsden and The Book of the Crossbow, by Ralph Payne-Gallwey. This is where the bulk of my information comes from. However, there are other good sources, which include: Latin siege Warfare in the Twelfth Century, by Randall Rogers and Medieval Siege Warfare, by Christopher Gravett. Plus there are many web sources two of which are Trebuchet.com and the Grey Company Artillery site. 1 The catapult was one of the most continually used pieces of artillery until the adaptation of gunpowder. The Greeks called their version one-arm. The Romans referred to this device as the scorpio, catapulta or the onager. In the Medieval ages it was described as the catapult and mangonel. The catapult waxes and wanes throughout history until the 4th century AD when it becomes the primary form of artillery until the 12th century with the addition of the trebuchet to the medieval arsenal. The catapult is a torsion device. This means that the throwing arm is powered by the elastic tendencies of a skein of twisted fibrous material, usually hemp rope, horsehair, womens’ hair, animal sinew, or a combination of any of these. The catapult is in the same family of artillery as the ballistae, but instead of two skeins powering two arms, a single larger skein powers a single arm. This makes it a somewhat easier machine to build and maintain. The ballistae is a much more sophisticated device due to its dual arm design requiring a complex balancing of the power in each arm in combination with the bowstring. Exact details of catapult construction are sketchy at best. Period illustrations can be notoriously wrong in their representations on engines. This is attributed to two reasons. First, the artist was an artist and not an engineer. Therefore he drew what he thought he saw, sometimes omitting important components. The second is that these were most likely military state secrets. The pictures were purposely drawn wrong so as not to give away technologies that may make an opponent’s engines more effective than your own. Marsden could only find three main sources covering catapult construction. Ammianus, Apollodorus of Damascus, and interestingly, Anonymus Byzantinus are Marsden’s only written sources concerning the catapult and of these, Amianus’ brief non-technical description is the best source, the other two describe some sort of torsion device added to the front of a battering ram. This apparent lack of source material is probably why there exist so many variations of the catapult. For the purposes of this booklet, I will guide you through conception and construction of a catapult similar to that in Marsden’s book, pointing out where applicable, variations which you may wish to choose. What do you want your catapult to do? How will you transport it? Where will you store it? These are probably the three most important questions you will have to answer before you begin building anything. The larger the object you wish to toss, the larger the catapult you will need to build, the larger the vehicle you will need to transport it, the larger the structure you will need to store it in. I highly recommend storage in a shed or garage or something, above storage outside or even under a tarp. Weathering leads to accelerated deterioration of the wood. Since the main scope of my endeavors is for SCA combat, then the bulk of this booklet will be in that context. However, the formulas and proportions I will give should work with larger scale engines. So if you have a 10,000 square foot warehouse, own a flatbed tractor-trailer, and want to chuck 250 pound rocks, go for it! But if your like me and have a small work shed, a pickup truck, and shoot at you friends, then this is the guide for you. 2 What are those parts called, anyway? A lot of people make up their own names for the parts of the catapult. That’s fine if you’re talking to your buddies who are helping you build the thing. But to keep us all on the same page I’ll be using the names the ancients gave them in most cases. We’ll start at the ground. The bottom most framework, in the shape of a rectangle as seen from above, is called the main frame. The two longer sides, where the holes will be for the skein, are the main ground joists. The shorter sides are the front and rear crossbeams. Any framework above the main frame is called the subsidiary framework. The subsidiary framework supports the pad that the throwing arm strikes, called the buffer, and consists of front and rear legs. The throwing arm is pretty self-explanatory. The ring around the throwing arm that the winch is hooked to is called, of course, the winch attachment. The two fixed points where the sling is attached are the primary sling attachment(s). And the rod sticking out the end is the nock point. The hardware used to tension the skein is called the torsion hardware and is broken down into two sets of four parts each. Each skein hole has what Marsden calls a metal ring or flange. But this terminology is a bit confusing so I call it a bushing as this more accurately describes its actual purpose. What Marsden calls a washer, which is more akin to what a modern person would call a flange, is the modiolus. The next piece is a hollow or solid bar that rests in the notches across the open cylinder of the modiolus. Marsden calls this the tightening lever, but the ancient sources and I call this the epizigus. The final piece is the locking pin. This goes through the modiolus, holds tension in the skein, and keeps the whole thing from unraveling. modiolus, epizigus, and locking pin The winch consists of three main parts. A bar or pole is suspended about half the length of the throwing arm’s length, between the main ground joists. On the outside of the joists at either side on the bar is the ratchet and drum. The drum is where the winch lever is used to crank down the throwing arm. The ratchet keeps the arm at tension by the use of a prawl, or clicker, which 3 falls into the notches of the ratchet. The winch attaches to the throwing arm by means of a rope with a quick release, or slip hook. What will it look like? The more common image of the catapult is one that has the buffer mounted to a subsidiary structure so that the rear legs are perpendicular to the main frame and parallel to the upright throwing arm. The skein is usually mounted at a point equidistant from either end of the main ground joists. I call this the “classical” frame. The only real problem with this is that the buffer has to span the entire width of the engine and therefore has to be a fairly stout piece of lumber. With the classical frame, the force of impact from the throwing arm is first diverted 90 to the dual front and rear legs, then down to the mainframe joists. Thor’s hammer, classical design The design we will be using is attributed to Roman sources and therefore I call “Roman” style. I prefer this style because it uses less lumber and all of the force of impact is directed straight forward through the buffer, the front leg, then through the front cross beam and directly to the ground.
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