Storm the Castle

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Storm the Castle is an event in which participants must design and construct a device that is capable of accurately launching a previously unknown projectile the farthest distance possible using a counterweight of previously unknown mass. Prior to the competition, competitors must construct their device to meet certain size and engineering parameters, such as the energy parameter which prevents any energy from being stored in the device other than what comes from the falling counterweight. After completion, these devices are then rigorously tested with counterweights, projectiles, pin angles, release mechanisms, etc. of all types. At the tournament, a counterweight and projectile are selected by the event coordinators and each team fires with the same counterweight and projectile. The event is designed so that an accurate, well-tested and calibrated device is likely to beat a device that can fire great distances, but not accurately.

The C division Storm the Castle competition differs from division B in the size of the castle target. The C division castle is a smaller 20cm cube then the B division 40cm cube. Division C can also be considered somewhat more competitive then B.


In 2005-2006, the format for scoring was target distance (in meters) minus two times the distance that the projectile was away from the target (also in meters), plus an additional bonus of 0.1 multiplied by the target distance (also in meters) if the projectile made contact (on the fly) with the target. Teams were given 3 shots, with the highest-scoring launch being multiplied by two. An additional ten points were possible if teams created 5 charts/graphs based on their previously collected data and demonstrated how such graphs/charts could be used in competition (for example, if given a 2.3 kg counterweight and a 43g projectile, where would you set your target?). Various penalties usually result in 3 points deducted per offense (e.g. improper goggles, entering the launch area, not warning the judges etc.)


This event requires the teams to impound their devices before the competition. At that time, the legality of their device is checked(e.g. dimensional parameters, energy rule compliance). Once all the devices are impounded, the teams are notified of the counterweight and the projectile they will use at the competition. At this time a team usually determines what they want their target distance to be, based on their previously collected data from the pre-competition testing stages. When a team is called up, they first must state that distance; once the event supervisors set up the target at the said distance the 5 minute time begins. The teams are allowed to do all of their launches and/or modifications to their trebuchet during this time. Note that all of the tools that you need for this competition must fit in the 2x2 meter operating area. Also, the participants are required to wear approved goggles when the counterweight is loaded in the machine. Before firing the participants must stand outside the launch area and must give warning to the supervisor. Upon doing so they may fire their device by pulling a pin or something similar.


A multitude of devices is built for this competition, in reverse order or rarity they are:

  • The Hinged Counterweight Trebuchet (HCW) - a standard trebuchet.
  • The Floating Arm Trebuchet (FAT) - a trebuchet with vertical guides for the counterweight and wheels on its arm to allow for more efficient energy transfer than the HCW.
  • The Floating Everything Trebuchet (FET) - a poor man's FAT, without the vertical guides.
  • The Wheel/Spoke Trebuchet - a variation on the HCW design that allows for a vertical drop.
  • The Multi-Rotational Trebuchet (MRT) - a complicated design relying on very high arm ratios and a complex sling release mechanism to transfer the energy more efficiently than its progenitor the Wheel Trebuchet.
  • The Counterweight Ballista - a complicated design that first transfers the counterweight's energy to a storage system, which then accelerates the projectile.

These are, respectively:

A2j ballista.jpg

Occasionally you will see a goofy trebuchet that does not fit in those common categories; usually such designs feature some sort of a pulley system, but in general, they operate similarly to their progenitor types.

Historically the FAT type has won a significant number of competitions due to its high efficiency and relative ease of use. On the other hand, not many types of trebuchets besides the FAT and traditional have been used at Olympiad and it is debatable whether in the future the FAT will remain the best. For now, it holds a large advantage over the traditional trebuchet because the counterweight falls straight down as opposed to following a curved path. Nevertheless, many teams choose to build inferior designs because they do not have the time and/or expertise to construct such complex devices. However considering that the Traditional Trebuchet (HCW) was used by almost all teams for a great deal of time, with FAT's coming up only recently, it is conceivable that other more efficient designs exist but are as yet undiscovered.


When faced with the building of a trebuchet one must consider many factors: speed of construction, cost, adaptability, structural integrity, compliance with the rules etc... To ignore even one of those factors is often tantamount to forfeiting the competition. Let us discuss the choice of materials first(in reverse order of "goodness"):

Lab Equipment

No one should seriously use this material if they can help it. But sometimes one finds oneself with 2 hours left before the competition with no trebuchet, and at this point anything goes. Usually teams use a pair of ring stands for the base and a metric ruler for the arm with a paper cup attached at its end. There are no recorded cases of such devices getting anything above the last place. However one does get participation points since the device is usually legal(if one finds ring stands low enough) Here is a breakdown of this material's usage in various trebuchet components:

  • Arms - easy to build, actually rather good if used with a good frame
  • Frames - easy to build, but usually collapse at competition
  • Avoid this material if you can help it.
  • Speed of construction: Extreme
  • Cost: None
  • Adaptability: Low
  • Structural Integrity: Low
  • Compliance With The Rules: Moderate

Polyvinyl Chloride(PVC)

A material known for its versatility and ease of assembly, PVC is often seen used as a part of trebuchets or even complete devices. Easily bought at most hardware stores, this material is somewhat on the expensive side and it forces one to some design limitations. It is often used by the less imaginative teams operating on the correct assumption that PVC is easy to use. They do not tend to do well. Here is a breakdown of this material's usage in various trebuchet components:

  • Arms - easy to build, sturdy, but VERY heavy
  • Frames - easy to build, but are useless on all trebuchets but the HCW, light as frames go
  • Rails(FAT,FET) - probably the only valid use of PVC, take advantage of the curved cross-section
  • Speed of construction: High
  • Cost: Moderate
  • Adaptability: Low/Moderate
  • Structural Integrity: Low to High
  • Compliance With The Rules: High


A material of choice for more than 90% of teams. National winners for both divisions used this material with great success. Wood is very easy to use, provided one has some power tools, is very sturdy and highly adaptable. Choose harder woods like poplar, since they will often result in better frames. When connecting wood planks use screws, NOT nails. Use multiple screws instead of glue when possible - if one fails with the design one can always take it apart and try something else with it. The only problem with this material is its weight, it is only marginally lower than PVC's, and the fact that is is sometimes tough to find straight pieces at the store. Here is a breakdown of this material's usage in various trebuchet components:

  • Arms - easy to build, sturdy, but somewhat heavy
  • Frames - easy to build, very sturdy, highly adaptable
  • Wood is the best material if one has moderate resources.
  • Speed of construction: Moderate
  • Cost: Low
  • Adaptability: High
  • Structural Integrity: High
  • Compliance With The Rules: High


A material seldom used for trebuchets due to its high cost compared to wood. All-metal trebuchets usually catch approving stares from competitors, but usually represent a waste of money for they seldom do any better than their wooden counterparts. Metal is very hard to use, requiring drill-presses and sometimes welding equipment. However there are areas where metal shines as a material. Aluminum arms are very common among competitive teams, they are lighter and stronger than wooden arms and they last longer. Also rails for FAT's and FET's are usually made of metal strips, chosen for their strength and smoothness. Metal parts are easy to find at most hardware stores. Here is a breakdown of this material's usage in various trebuchet components:

  • Arms - rather difficult to build, sturdy, light
  • Frames - difficult to build, very sturdy, VERY heavy(add wheels)
  • Metal is a high-end material that has limited use in trebuchet building.
  • Speed of construction: Very Low
  • Cost: High
  • Adaptability: Moderate
  • Structural Integrity: High
  • Compliance With The Rules: High

Space Age Materials

A catch-all group representing materials that have arrived with the advent of space travel, and often represent the best materials that the money can buy. Examples include carbon-fiber rods for arms, Kevlar string etc. Usually only used by high-performance schools, they do not represent a wise investment by teams who know that they will not advance past some low level. These materials are almost exclusively found on the internet.

  • Arms - Carbon-Fiber arms are extremely light, extremely sturdy
  • Chose space age for victory.
  • Speed of construction: Varies
  • Cost: Low - High
  • Adaptability: Usually Low
  • Structural Integrity: High
  • Compliance With The Rules: High


Even bigger catch-all group representing various small gadgets like ball bearings, cotter pins, string meshes, nuts, bolts etc. They are critical to any trebuchet, but they are often specific to a particular design. Thus is difficult to advise one on which other material to chose. In general the more expensive the item is the better it will work, but that is not always the case. Try to use household materials first to gauge their effectiveness and then go to the store for better selection.

Planning and Safety

Once one have chosen the material, it is wise to make some blueprints before beginning construction. People use paper, CAD, Paint etc. to aid them with this but no amount of software will make one a good engineer. Thus is is helpful to build at least one trebuchet without a blueprint(time permitting of course) to figure out how joints work, what can and cannot be done in the real world. Or simply find some plans on the Internet, although they will often be illegal and require severe modification before use. Once one has designed the trebuchet and checked that it would be legal according to THIS year's rules, one may start building. On average a trebuchet may take about 10-20 man-hours to complete. Also factor in that more often than not the first prototype will not work and/or one will break some component of it during testing which will require additional time. Always allocate some time before the competition to test the contraption, even the best build device will not win if it is untested.

When building, try to not injure yourself - obey all safety protocols, often you will be the only builder who is competent at this and it will not help your team if you become crippled. Work with a partner when possible. Plan out all connections before hand, draw and measure on the pieces themselves to help you line them up. Make sure the pieces are exactly in their right positions before applying glue(or do not use glue at all, add more screws/bolts). Keep your blueprints at the end of the year to help your successors continue your(hopefully) wining streak.

Suggested materials

These are readily available materials suggested for use on your trebuchet:

Frame materials

1. lumber and deck screws- try to get premium lumber if you can afford it. Otherwise, just watch for warps. 2. Shelf braces- Shelf braces are very strong and exactly 90 degrees. Use them to hold the uprights of your frame, and you will save the time of trying to cut bracing pieces. 3. Angle irons- Found near the lumber or roofing sections, these metal brackets are premade for certain joints. They make construction of the frame much easier, although if you have a miter saw then you don't need these.

Arm materials

1. wood- a 1x2 piece of premium lumber is more than strong enough for your trebuchet, and is pretty much too heavy for competitive use. 2. metal- available in most hardware stores, look for a hollow square member of the lowest weight. Do NOT use U shaped members as they can 'twist' while square tubes won't. Aluminum tends to work well. 3. Golf club shafts- generally a bit hard to work with, these are light, and if you have one you can spare, go ahead and use it. You need to mount it in a piece of wood, though which you mount the axle. 4. Carbon fiber- Go to an archery store. Get their longest carbon fiber arrows (get more than one, since carbon fiber is brittle and will shatter if you don't treat it well. A bit expensive, these also need to be mounted in a piece of wood. do NOT hit it with a hammer.

Axle materials

1. pipes- You can use a metal pipe if you need to. They're not ideal, but they'll pull through in a pinch.

2. Rods- Get Cold Rolled steel. if you can bend the rod, it's not cold rolled, and it's probably not the best for you.

3. dowels- dowels can be used like a steel pipe.

4. threaded rod- watch out for how much weight you're putting on it. a 1/4 inch piece that's 2 feet long will easily bend. Be sure to get a large enough piece to avoid bending problems.

5. Roller blade or skateboard wheels- use them for the bearings. Also good if you choose to go with a FAT. This will probably decide what size of axle you use- mine had 5/16th holes with plastic bushings that reduced it to 1/4, giving me two options as far as axle size.

Sling materials

1. fabric. Cut it up and sew it, if you can. 2. duct tape- fashion a sling out of duct tape. heavy, but will pull through in a pinch. 3. string- you can tie a sling, but it's usually a lot of trouble.

Other hardware/ supplies to get

1. string. Mason's line will do, and it's usually brightly colored so you won't lose it. Cut it with a flame, and the ends won't fray. 2. eyelets- use these to hold the firing pin. 3. large nails for firing pins.


most important is to know where your trebuchet will fire. To do this, you need two things:

1- an adjustable counterweight. Use rolls of pennies in a can. 12 dollars of pennies should be about 3 kilograms (this is an estimate, don't' hold me to it). Only take apart one roll. Use weights that are nice numbers, 1 kilogram, 1.1 kilograms, etc. so your calculations will be easier. Since most beam balances only go up to 500, you'll have to measure your weight in batches, which makes the fact that you can just pull out half the rolls very handy.

2- weighted masses. Go out and buy a package of 50 some balloons, a funnel, and a package of rice. make projectiles- preferably in the weight range given in the rules. Nice numbers are not required, so don't spend too much time counting grains of rice. Fill up the balloons, tie them, then weight them. Hopefully you'll have a nice spread of weights. Write the weights on the balloons with sharpie. If you have many different colors, it many help to make the lightest ones all the same color, etc.

then, get use of a gym one day. Start with one counterweight mass, then fire all the balloons. have someone down range to stop where the balloon hits the ground, and move it to that spot (the balloon will roll a few feet). Record the distance of each balloon- it will probably help to have a preprinted table with a list of all the balloon weights.

then, change the counterweight, and repeat. Do this many times.

You should now have tons of data. what you want to do is put it in excel and make graphs for each counterweight, with projectile mass along the X and distance along the Y.

make sure to print out a copy for the judges as well. On competition day, pull out the graph for the assigned counterweight, and find the projectile weight. you now know exactly where to place the castle.


golf balls and racket balls behave differently in the air than balloons do. Do your tests like you did with balloons (be sure your downrange spotters are paying attention!), and make SEPARATE GRAPHS. If on competition day they have golf balls, be sure you use the golf ball graph.

if you don't have time to do this, just know that golf balls add distance to your throw. by how much varies by what your trajectory is and what distance you're throwing.

don't forget to use different types of golf balls, and weight each individually as well.

Now, if you're really really bored you can start doing tests with different sling lengths, etc. usually, just pick a three counterweights (low medium high) and three projectiles (low medium and high) and do tests before you do a large data collection. Find which sling lenght seems the best, and stick with it.

Testing is a pretty important. a well tested traditional trebuchet has a very good chance of beating an untested FAT.

How to make a Trebuchet the night before competition

Here is a quick how to for making a quick traditional trebuchet, and testing it, the night before competition.

The two stores you need to go to are target and home depot. If these are not in your area, any suitably large store that sells about the same items will do.

1) You need to have these things before you start. If you don't, get them quickly.

  1. a bathroom spring scale or package scale capable of measuring your counterweight. it needs to be able to read at most 10 pounds, and must be able to read at the smallest 1 pounds.
  2. a triple beam balance or postal scale, capable of measuring to every ten grams or quarter of an ounce
  3. a computer with excel, or another spreadsheet program on it.
  4. a large floodlight with extension cord. You will end up working in the dark outside at night- this will be helpful.
  5. a long, flat area to work in. a driveway is preferable, a sidewalk will work in a pinch. Try to stay away from the road as you'll be working in the dark.
  6. a partner- one extra person can help hold pieces and make it easier to build. Sometimes they can end up hindering you, so be careful.
  7. 15 dollars in pennies. either that, or 4 dollars in pennies and about 8 pounds of small weights.

2) You'll need these tools

  1. power drill/driver. Remember to charge it if you have a cordless. Make sure you have bits.
  2. wrenches. You need two crescent wrenches of the same size (size depends on which nuts you buy). Two pliers will work in a pinch.
  3. tape measure. Feet will work, but you'll have to convert.
  4. a wood saw. A miter saw would be good, but a simple hand saw will work.
  5. a hacksaw for cutting some metal pieces
  6. a level. Not needed but helps.
  7. a square. The kind that looks like an L.
  8. a funnel. Plastic is nice, but paper will pull through if you need it.
  9. scissors
  10. lighter or matches. Not needed, but it's pretty useful for the string.
  11. electrical tape. Duct tape or masking tape can substitute.
  12. an empty coffee can or similarly sized sturdy material
  13. some cloth scraps, some spare buttons, and a needle and thread

3) Head on to target and buy these things (unless you already have them):

  1. balloons. About 25, and brightly colored ones. Get the 10-12 inch balloons.
  2. some granular item, like rice, or birdseed, or sand.
  3. roller blade wheels. One will do, 2 are the most you will need.

4) Take your roller blade wheels. By sticking the tip of a screwdriver in the axle hole, you should be able to pop the bearing out of one side. Repeat on the other side, and you should have two bearings. There may be a plastic bushing inside- remove that if you can. Bring the bearings with you to home depot.

5) Go to home depot and buy these things:

  1. 3 eight foot long two by fours
  2. one threaded rod the same diameter as the inside of your bearings to be your axle, and about 1 to 1.5 feet long (you can cut down to this length)
  3. about 12 nuts and 12 flat washers that will fit on your axle
  4. a piece of square aluminum tubing, the smallest you can get. Make sure it's larger than your axle.
  5. a package of deckmate deck screws, 2.5 inches long, the kind with the included bit. You can use another brand, but make sure you have a bit that fits.
  6. two smooth metal nails, about 2.5 inches long
  7. two screw eyelets that your metal nails with fit through
  8. two bolt eyelets that your metal nails will fit through, and washers and nuts that will fit on it. Make sure the bolt portion is longer than your aluminum square tubing is wide.
  9. nylon twine. Get a bright color that's easier to see.
  10. a spade bit the same size as the outside diameter of your bearings
  11. a spade bit between the size of your axle and the outside of your bearings
  12. a drill bit that fits your axle

4) Go home and cut your 2x4s into the following:

  1. two pieces one inch short of your length limit (parallel to firing direction). These are your long pieces.
  2. three pieces one inch longer than your axle. These are your cross pieces.
  3. two pieces two and a half inches less than the limit of your height. These are your uprights.

5) Take your square aluminum tube and do the following to make your arm:

  1. cut it down so it will fit within your space confines, along the hypotenuse. For instance, if you're confined to 100 cm of length and 75 cm of width, you want to have your piece slightly less than 125 cm.
  2. about 1- 1.5 feet from one end, drill a hole the size of your axle all the way through both sides.
  3. about one inch from each end drill a hole the size of your bolt eyelets all the way through the top and bottom. These holes should be parallel to each other, and skew to the axle hole.

6) Take one long bottom piece of wood and two upright wood pieces along with your arm.

  1. place the long piece on the floor and place one vertical piece upright against the end. next, take your arm and hold one end to the top of your upright, and let the other end rest on the floor next to the long piece.
  2. tilt this triangle onto the floor. Make sure you have the axle hole in your arm closer to the top of the "upright" piece.
  3. lay another upright piece across the bottom piece and the arm so that it is perpendicular to the long piece and the axle hole falls on it's centerline.
  4. make sure that the overlap between the upright you have just laid and the long piece is a 3.5 inch square and comes to a perfect right angle, forming a T.
  5. remove the arm from underneath the upright, and stick another 2x4 under the upright so that it is level to the ground.
  6. drill 4 1/8th inch pilot holes in each of the 4 "corners" of the overlap, through both pieces of wood. Be careful not to let the wood shift.
  7. drive 4 deck screws into the pilot holes, through both pieces of wood. the two should now be securely joined.
  8. put back the arm, lining up the axle hole. mark this spot on the upright. with a 1/8th bit, drill all the way through the wood.
  9. take your spade bit the size of the outer diameter of your bearings. using masking tape, mark off the thickness of your bearing.
  10. use the spade bit to make two recesses on each side of the upright 2x4, centering each on the 1/8 inch hole. make each the same depth as your bearings are thick.
  11. use your slightly smaller drill bit, and drill all the way through your 2x4 at the 1/8 inch pilot hole (which should now have the two recesses in it)
  12. make a mirror copy.

7) Take your cross pieces and:

  1. place your two T assemblies next to each other so that both uprights are touching each other, and the two bottom pieces are on the ground ( if they won't touch each other you made it wrong!)
  2. move them apart so that the cross pieces lay across the bottom pieces exactly. lay one at each end making a rectangle.
  3. drill pilot holes and secure ONLY THE ONE closest to the uprights.
  4. turn this on it's side. on the bottom (which should now be on the side), place two cross pieces at each end and secure them with pilot holes and screws. Be sure not to hit the screws from the first cross piece you placed.
  5. turn it back upright. It should be stable.

8) Assemble your trebuchet as follows:

  1. put your bearings into the recesses (there should be 4 in total) of the two uprights. if you only have 2 bearings, use the outside recesses.
  2. pur your axle through one of the uprights (through the bearings. put the following on the axle in the following order: washer nut nut washer arm washer nut nut washer.
  3. put the axle through the other upright. on the outside of both put a washer and nut.
  4. tighten down all the nuts so that they push a washer against something. make sure the arm is centered on the axle.
  5. make sure the nuts against the bearings are relatively loose and the nuts against the arm are very tight.
  6. put your bolt eyelets into the two remaining holes in the arm and put washers and nuts on them. secure them tight, with the holes facing perpendicularly to the arm.
  7. take one nail and tape it on the long side of your arm, point facing off the end, and on the top side.
  8. let the arm fall. One bolt eyelet should be resting on a 2x4 (that is in turn resting on the ground). on both sides of this eyelet, put in your two screw eyelets close enough that your last nail will go all the way though all three of them.

9) Make the sling

  1. take a stripe of fabric about 8 inches by 2.5 inches. take the two ends, and scrunch them together into points. sew them together, then sew a button to it.
  1. take two pieces of string about the length of your arm. Tie each one to each button on your sling.
  2. tie the end of one string onto the eyelet on the long end of the arm.
  3. tie the other string into a loop that you slip on the nail.
  4. adjust the length of the strings in order to get an optimal sling length. Usually the length of the long side of your arm is a good start.

10) Test it!

  1. using a coffee can, pennies, and some string, make a counterweight that you can adjust to at least 5 different weights in your weight range.
  2. use your funnel and fill up your balloons to many different weight. Weight each and mark the weight on it.
  3. fire off all combinations of counterweight and projectile mass you can, recording the parameters, and distance of each test.

11) Graph it!

  1. enter your data into excel (projectile mass and distance), and use chart wizard. Make a line graph. Title it with your counterweight.
  2. repeat for all counterweights you used.
  3. print them out, and don't forget your school name on them, and an extra set for the judges.

Since storm the castle is about accuracy, testing is very important.


Storm the Castle Energy Rule Tip Sheet 2012-