About Trebuchets. WarSerpent Construction. Paul Schmidt s WarSerpent trebuchet construction article. Left:

Transcription

1 Paul Schmidt s WarSerpent trebuchet construction article Left: Two full sized trebuchets, reenacting the historic construction of the famous WarWolf on NOVA s video documentary Secrets of Lost Empires : Medieval Siege The trebuchet at left has just hurled a 300 pound rock; this one has a fixed counterweight. The trebuchet at right is closer to the original machine, and has a hanging counterweight; this is the model for the WarSerpent "Cum catapultae proscriptae erunt tum soli proscripti catapultas habebunt." ("When catapults are outlawed, only outlaws will have catapults.") About Trebuchets The trebuchet (pronounced tray-boo-shay) is a machine capable of hurling objects over a considerable distance relative to it s size. In the Middle Ages, the trebuchet was the most powerful and most feared weapon that existed. Indeed, a famous medieval text describing the construction of trebuchets warned, He who would build a trebuchet must take care. At their height of historical development, trebuchets could hurl a spherical stone of 300 pounds [136 kg] over a distance of 300 yards [275m], and were also very accurate and repeatable, making it possible for their operators to reduce castle walls to rubble. During a siege, the defenders of the fortification knew the end was at hand when the attackers started erecting a trebuchet nearby. Making matters worse, the machine s range exceeded defender s retaliatory efforts by arrows or any other weapon. Before trebuchets, attackers used other hurling machines that were important but much less effective. The Greek ballista (machine that throws) could hurl stones, the Roman mangonel (engine of war), also nicknamed onager (donkey, for it kicked like one when triggered), threw various objects, and the French catapulte (to hurl against) launched arrow-like bolts. During the Crusades, Europeans who visited China learned about a weapon with a sling at the end of a mechanical arm, and operated by people pulling down on ropes. They brought this design back to France, the WarSerpent prototype Page 1

2 where it was enlarged and improved to operate by gravity; this was the trebuchet (a beam that falls over a pivot). A trebuchet works similarly to the hand-held sling used by people since time immemorial; a projectile is carried in a pouch that is in turn suspended at each end from two cords. Initially, both cords are held together and the sling is moved rapidly through an arc to build up momentum in the projectile. Then, one of the cords is released, the pouch opens and the projectile departs the sling on a trajectory towards the target. This works fine if the target is a foe or an animal, but is impractical for payloads heavy enough to do damage to fortifications. With a trebuchet, the sling is swung at the end of a mechanical arm, with the advantage of mechanical leverage being brought to bear by the fact that the arm s pivot is off center; size being limited only by strength of available materials. With the Chinese predecessor of the trebuchet, the operators pulled on the short end of the arm to move the long end with the sling. With the refinement of the trebuchet, a heavy counterweight was attached to the short end, removing human strength from the equation. Large trebuchets are exceedingly dangerous weapons, but small model trebuchets are fun to build and use. The WarSerpent is an easy to build and inexpensive working model that can throw an object like a golf ball (or similar sized rubber ball) a distance of at least 30 feet [9m] (up to 50 feet [15m] with lighter payloads). This model is called the WarSerpent, henceforth referred to as WS in this document, in part because it s construction was inspired by a serpent player s suggestion, in part because the construction article follows a series of build-it-yourself serpent plans. For more information on trebuchets, refer to and to the video documentaries NOVA: Secrets of Lost Empires - Medieval Siege and A&E History Channel: Modern Marvels - Siege Machines. WarSerpent Construction Frame The WS is comprised of a frame, a throwing arm, and a sling. Starting with the frame, obtain suitable wood from which all sections will be made. The prototype used strips of finished wooden trim strips from the local hardware store, with an approximate 1/2 x 3/4 [1.3cm x 1.9cm] cross section, and about 17 feet [5.2m] of this was required. In addition, several feet of 1/4 [0.63cm] diameter wooden dowel is needed for axles and pins. The frame is assembled by using wood glue; at each joint a hole is drilled through both pieces of frame, sized to just fit the dowel; the fit should be tight enough that the dowel cannot wobble. At each joint, put a dot of glue, fit the pieces together, then put another dot of glue into the hole, smear more glue on the dowel, and twist and/or tap the dowel into the hole. one side of frame during gluing both frame sides after trimming The horizontal frame section is the rail, and it is 18 [45.7cm] long. The vertical frame section is the arm support, and it is 16 [40.6cm] long. At the top of the arm support will be a 3 [7.6 cm] horizontal piece called the bearing. Start by joining the arm support to the rail at it s midpoint, making sure that the two are 90 degrees square to each other before the glue sets. Use the bearing to support the other end of the arm support. Once this angle is established, join the bearing to the top end of the arm support, also at 90 degrees. Mark the rail at a point 3/4 Page 2

3 Cut two 6 [15.2cm] pieces and one 14 [35.4cm] piece for the frame beams. With the two shorter beams laying flat on their wider sides, place the two frame sections on top of them and join with glue and dowel pieces, making sure that 90 degree angles are maintained so the assembly is square. Also use clamps to hold a piece of 4 [10.1cm] wood (or clamp a ruler in place as shown in the photo), to make sure that the frame sides are perfectly vertical. Once the glue is set, add the longer beam at the midpoint of the rail, extending 4 [10.1cm] to either side beyond the arm supports. Glue this beam first without dowel pins, then turn the frame over and drill holes through the center beam and about 1/3 of the way into the rails (not the arm supports), then glue and insert dowel pins into the holes. Once the glue has completely set, use a saw to trim any protruding dowels and in general clean up the frame assembly. Some sanding may be required as well, but make sure to always keep the top of the frame at the bearings supported and separated by the temporary wood block, or clamped ruler, whenever the arm is not in place on the frame. frame sides with braces and ruler clamped across bearings [1.9cm] from each end, and attach two 18 frame diagonals so that they cross the rail at the marks and also touch the arm support at the top on the bearing. These diagonals will extend slightly beyond the the end of the arm support on the bearing and the bottom end of the rail, so trim them flush after the glue has set completely. Make two frame sections in this manner. closeup of frame stabilizer brace closeup of ruler clamped across bearings Finish the frame assembly by adding diagonal braces from the protruding ends of the long middle beam to the sides of the arm supports. It will be necessary to cut these to the correct angles, then glue them in place, then drill holes and Page 3

4 glue in dowel pins once the first glue joints are dry. These braces will help to stabilize the sides of the frame, but it is still a good idea to always protect the frame by keeping either the arm or the temporary wood block (or ruler) between the bearings. WarSerpent Construction stocks with their securing dowel pins drilling stocks sections, 1-1/2 [3.8cm] long each, into the new holes in the bearings, wipe away any excess glue, then put the stocks down over the dowels to keep them in alignment while the glue sets. Once the glue is dry, the stocks should slide on and off the dowels with only a small amount of effort. This completes the assembly of the frame. stocks after forming U cutouts The arm pivots on an axle made from a section of dowel. The dowel rests across the two bearings, and is held in place by two stocks ; the stocks do not hold the axle down, as gravity accomplishes this, but they keep the axle from moving out of position on the horizontal plane. Make two stocks from 2-3/8 [6cm] sections of wood, and drill holes positioned so that their edges almost cut through the side of the wood. Then use a saw and knife or file to finish the U shaped cutout as shown in the photos. Cut a 6 [15.2cm] piece of dowel for the axle, and lay it across the top of the bearings. Place the stocks over the axle, and position them carefully above the bearings. Drill two holes down through each stock and bearing, with the holes positioned in between the three dowels that join the frame sections to the bearing on each side of the frame. Wiggle the drill inside the holes in the stocks to make them slightly larger. Glue four dowel throwing arm assembly with axle and sling pins in place Page 4

5 Throwing Arm WarSerpent Construction The throwing arm is made from two 28 [71cm] lengths of wood, laminated together to produce a 1 x 3/4 [2.5cm x 1.9cm] cross section. Smear an even layer of wood glue along the wide side of one section, then clamp the other section to this glued area, and clamp firmly at many points along the length. On the prototype, it was necessary to clamp sideways as well, since the two lengths had a slight warp and did not align over their entire length. Once the glue has dried, sand the arm and trim the ends square. At a point 19 [48.3cm] from one end (marked as the sling end), drill a hole for the dowel axle. The hole should start on the narrower side of the arm and pass through in the wider direction. Make sure to drill this hole so that it is square both in reference to the length and width of the arm, so that it will pivot true on the axle. throwing arm with axle and axle spacers into the hole; it should go completely through the entire length of the dowel. On the prototype, the wire was coated with epoxy first, and this had the dual function of lubricating the wire during insertion and securing it afterwards. Once the wire is fitted to the dowel, insert this release pin assembly into the hole at the end of the arm. Cut two 1-1/2 [3.8cm] sections of wood, and drill them through lengthwise to fit the dowel axle. Sand the ends smooth, then assemble to the throwing arm by passing the axle through all three pieces, as shown in the photos. These spacers and the axle may be glued to the arm, but this is not required, and will make it impossible to perform any repairs to the arm or axle if needed later on. This completes the throwing arm. axle spacers Drill similar holes at points 1-1/4 [3.2cm] from the sling end and 1 [2.5cm] from the other end (marked as the weight end). These holes will be for the fixed sling attachment and counterweight hanger, respectively. Finally, drill another hole directly into the end of the arm at the sling end, to a depth of 1 [2.5cm] (stopping before drilling into the sideways hole already drilled there); this will be the receiver for the release pin that holds the free end of the sling. Cut three sections of dowel, 3.5 [8.9cm], 2 [5.1cm] and 1 [2.5cm], for the counterweight pin, the fixed sling pin and the release pin, respectively. Fit the first two pins into their holes; they should fit with just enough friction to prevent them falling out easily. Obtain a 2 [5.1cm] length of stiff steel wire; for the prototype, marker flags (of the type made from wire and topped with small colored plastic flags, and used for marking underground cables, pipes, etc.) were obtained from the local hardware store, and cannibalized for their wire only. Drill a hole, of the same diameter as the wire, into the end of the third dowel section, and press fit the wire simple dowel release pin (top) and adjustable release pin with steel wire (bottom) Counterweight The force that moves the throwing arm is provided by a counterweight suspended from it s short end. In the WarWolf the counterweight was a large wooden box filled Page 5

6 with rocks, dirt and sand. In the WS, it is also a box, built up from sections of the same wood used for the frame, plus sides of either cardboard or thin plywood; it can be filled to the desired weight by placing various heavy objects inside. WarSerpent Construction prototype s counterweight parts, including thin plywood (0.2 [0.5cm] thick) sides metal pin of adjustable release finger extending from sling end of throwing arm; this picture also shows how the fixed end of sling is rigged with it s dowel pin tall plus overlap between the hangers and top of the box, plus the necessary extension past the dowel pin going through the arm. The box should have angled sides, so that it does not interfere with the arm in the cocked position (see photo), and at least one side should be removable or hinged to allow for easy addition or removal of weight. The prototype had two sloped sides with hinges at the bottom and small nails at the top; a rubber band stretched between the nails, holding the doors shut. axle with spacers on arm between bearings and stocks Since available materials for building the weight box will vary from case to case, these instructions will be general in nature; refer to the photos for additional ideas. The prototype s box was 6 [15.2cm] long, 2-1/2 [6.3cm] wide, and 7-1/2 [19cm] tall from the bottom of the box to the center of the dowel pin where it passes through the hole in the short end of the throwing arm. The objective is to have a box that will swing through the frame with plenty of clearance, will not hit the short end of the arm as it pivots around, and will not strike the bottom of the frame when the arm is pivoted fully up. On the prototype, the box was made 6 [15.2cm] tall and then had two 3 [7.6cm] long wooden hangers to suspend it from the arm; this added up to 7-1/2 box s cross pieces cut to shape, with three bottom pieces being glued and clamped together The prototype used various steel tools and pipe fixtures, etc; to achieve a total loaded box weight of 43.3oz [1.228kg]. The dowel that connects the box hanger to the throwing arm should be somewhat longer than the minimum width, in order to have enough sticking out to grip with the fingers for adjustment or removal. Page 6

7 box bottom and cross pieces being glued to one side of box Sling The projectile, or payload, is carried in a cloth or leather pouch pulled by a pair of cords, comprising the sling. The pouch can take many forms (originals were usually a net woven from rope), but due to the small scale the WS uses a cloth pouch; on the prototype, the cloth selected was felt. To help keep the spherical payload in place during the hurl, the pouch has a slightly cupped contour when bent around the projectile. Start by drawing and cutting the pattern for the pouch. Refer to the photo below of the prototype pattern for dimensions. completed box with door hinges, hanger dowel and other hardware visible pouch pattern layout and dimensions pattern is 2.1 [5.3cm] by 6 [15.2cm] center part of pattern is 1.2 [3cm] wide three points on each edge are 0.3 [0.8cm] wide, with spaces between of 0.6 [1.5cm] Page 7

8 Step 5 Step 6 pouch shape cut from felt cloth, with marks for slots visible Step 7 Step 8 Step 1 Step 2 Step 9 Step 10 Step 3 Step 4 Completed sling, showing cupped shape when bent in an arc Last step With sling cord drawn up tight and knotted, cut off excess cord See completed sling at far left Page 8

9 Place the pattern on the cloth, trace, and cut out the cloth shape. At each of the six points, three on each end of the pattern, cut small slots for the sling cable. Each slot should be just large enough to pass the selected cord twice, and should be positioned on the cloth so that the center of the slot is about 3/8 [1cm] from the point. Any razor-sharp hobby or modeling knife (X-Acto or similar) will work for making these cuts. Cut two lengths of nylon cord, about 2 feet [61cm] long each. Hold one end of each cord over a flame until the fibers are fused, to prevent fraying. While still soft, quickly roll the end of the cord between two flat surfaces, restoring roundness and preventing the cord from being wider at the ends than elsewhere along the length. Rigging Prepare a launch platform, or skid, from thin plywood or cardboard, and place between the frame rails, resting on the three beams. Glue the skid to these beams. The sling attaches to the throwing arm by way of two loops tied in the sling cords. Cut another length of nylon cord, about 18 [46cm] long, for the arm stay. Tie one end of the cord around the vertical arm support on one side of the frame, and tie a loop in the other end, positioned so that it goes over the dowel pin on one side of the end of the arm, and holds the arm fully down (the cocked position). With the arm secured in this position (be careful that the arm stay Refer to the sequence of photos on page 8, and thread the fused end of one cord through the three cut slots at one end of the pouch, loop around and back through, then draw tight and make a knot. Repeat at the other end of the pouch with the other cord. This completes the sling. sling pouch positioned on the launch platform / skid, with sling cords rigged to the end of the throwing arm...also visible in this view is the arm stay cord that holds the arm in it s cocked position sling cords attached to end of throwing arm; fixed end of cord is tied with a loop large enough to fit around arm, and secured by passing around opposite sides dowel on opposite sides of arm; free end of cord is tied in similar loop, and fits around metal release pin as shown cord does not slip off the dowel, as the arm could suddenly flip up and inflict injury), position the sling pouch on the skid at a point about 12 [30cm] from the end of the arm. Tie a loop in one of the sling cords, large enough to fit around the arm and adjusted so that with the loop firmly against the arm s dowel pin, the cord is tight between this attachment and the properly positioned pouch on the skid. Tie a similar Page 9

10 knot in the other sling cord, sized to fit loosely over the metal release pin and positioned so that the cord is tight back to the positioned pouch. Refer to the photo on page 9, and with the two sling loops positioned on the arm as shown, verify that the pouch is evenly restrained by the two cords when tension is put on it. If one cord or the other is the wrong length, make adjustments now. Rig the fixed end of the sling cord around the dowel pin, so that on one side of the arm the loop is in front of the pin, and on the other side of the arm the loop is behind the pin; this simple rigging will keep the sling from slipping free of the arm during operation of the trebuchet. This completes the construction of the WS. end of arm end of arm release pin free end of sling release pin free end of sling release pin angle adjustments top: straight pin will release sooner bottom: angled pin will release later projectile, such as a rubber ball of golf ball size (an actual golf ball will also work, but it can inflict more pain and/or damage if it hits something). Take a length of cord or fishing line, and tie it to the loop in the end of the arm stay cord. This will allow the operator to pull the loop free of the dowel pin on the arm (from a safe distance to one side), triggering the WS. Cock the WS and load the projectile into the pouch, and position the pouch with the sling cords tight and the pouch centered sideways. Check that the coast is clear, and tug lightly on the trigger line. The arm should release, the counterweight should descend, the arm whipping to the vertical position, and the sling releasing the projectile somewhere near to top of the arc. This SHOULD happen, but probably will not. On the first hurl, the WS is quite likely to fling the projectile straight up into the air, or fling it right down on the ground in front of the trebuchet, or it might even possibly throw it backwards a few feet. This is why the WS needs to be tuned. Tuning counterweight box loaded with steel objects Open the counterweight box and load it with weighty objects until the total box weight is about 43oz [1.2kg]. This is a good starting weight, and may be increased or decreased later as required. Take the WS to a flat location where at least 50 feet down range is clear of people or objects that might be damaged by the projectile. Take at least one payload There are three basic adjustments for a given payload weight, and they interact with one another. A heavier counterweight will cause the arm to accelerate more quickly, which will cause the sling to whip around faster, and this will influence where in the arm s arc the free end of the sling releases, and hence the angle at which the projectile departs the pouch. If the weight is too light, the projectile might barely leave the trebuchet and might then depart backwards. Too heavy and the sling could whip around too fast and plant the projectile right in front of the trebuchet. Sling length is another adjustment, but is much harder to make. Finally, the angle of the release pin, relative to the arm, can be adjusted by bending it with a pliers where it exits it s piece of dowel. Page 10

11 Probably the simplest way to get satisfactory hurls is to first adjust the counterweight for a nice, energetic hurl either upwards or forwards. With this much going right, start making small adjustments to the release pin angle. If the hurl is too vertical, this indicates that the free end of the sling is slipping off the pin too early, and the pin should be bent forwards (in the direction of throw). If the hurl is too low or even downwards towards the front, this indicates that the free end of the sling is staying on the pin too long, and the pin should be bent backwards, making it straighter relative to the arm. Once good hurls are happening, further tuning can take place. Try adding or subtracting weights to change the speed of the arm, and making a few different slings, with different cord lengths, might be worthwhile too. For most situations, the best hurl range will occur with a high arm speed towards the top of the arc, and a high trajectory combined with good forward speed of the projectile. Up to a point, a higher trajectory will keep the projectile in the air longer, and it will go a longer distance down range. But if the trajectory is too high, there will be insufficient forward speed, and the projectile will not travel very far from the trebuchet. WarSerpent Construction completed WarSerpent trebuchet completed WarSerpent trebuchet Page 11

12 Analysis of the WarSerpent trebuchet, provided by the ATeb program, written by Les Scholz. This is a Windows program that will run in most versions of the operating system from Win95 and up, and reportedly runs on some Apple computers with Windows emulators. The program and it s manual are available for download, for a nominal fee, from The program allows a trebuchet builder to test various designs before building them, and also allows existing machines to be optimized for best range. While designed primarily for larger machines (all inputs are in kg (Kilograms) and m (meters), it will still work for model sized versions. The image above was generated by ATreb, based on the WarSerpent dimensions as documented in this article, assuming counterweight mass of 43.3oz [1.228kg], payload mass of 2oz [56g], and a sling length of 14 [36cm], with a release finger angle of 35 degrees relative to the arm. The program predicts a range, under these conditions, of about 30 feet [9 meters], which is close to accurate. Better range has been obtained through further tuning. Neither the author, or The Serpent Website, make any guarantee that a trebuchet built from this article will perform at any level the builder might expect. The author also reminds the reader that any copies of the WarSerpent trebuchet are large enough to cause minor damage to property if used carelessly. Finally, the author cautions the reader that a projectile hurled from a trebuchet of this size can impact a person or animal with painful force, and should never be used with anyone in line with the throwing arm, either in front or behind the machine. Page 12