Introduction: Trebuchet Catapult Using Only Fast Food Straws - Straw Builder Project #3

About: Hi. I am a Dad, software product manager and avid runner. I enjoy building projects with my kids which generally means approaching design using inexpensive materials. Simplicity in design and build processe…
Video Link of Trebuchet in action:  Roughly 15' shot across the hallway.

During the Middle Ages, a Trebuchet was the ultimate siege weapon.  Let's see if we can replicate some of its basic functionality using nothing more than fast food straws.  When complete, just add rocks or spit wads.  Yes, I am fully aware that this project is completely ridiculous. I just couldn't resist the engineering challenge.

The world certainly doesn't need another Trebuchet.  A simple Internet search will fill your screen with countless variations of catapults and Trebuchets.  If you don't already know, a Trebuchet is a specific class of catapult.  It doesn't use a tension system but rather a counterweight.

If you want to build a Trebuchet that can launch a piano or pumpkin, this project is not for you.  If you have a science fair project due in two days and you have no budget, this might be perfect.  

One major plus is that this device is relatively safe to study and operate with your kids.  It would be nearly impossible for this to hurt someone making it the ideal way to explore the mechanics of a Trebuchet without an ambulance on stand-bye.  You'll still want to use common sense and never direct it towards another person.  A pebble in the eye wouldn't be fun.

On a scale of 1 to 10 for difficulty, I would rate this at about a 6 and the mechanical hand at about a 5.  It's only slightly more difficult then the mechanical hand because of the need to ensure a bit of frame symmetry.

So, intrepid straw builders, let's start building this fast food siege weapon for the zombie apocalypse.

Step 1: Materials

You will need about 35 straws to complete this project.  I hadn't bothered to sketch out a design before building and made a bunch of mistakes along the way.  If you are lucky enough to have closer to 45 straws, consider building a slightly wider Base and much taller Frame.  If you do, you'll be able to accommodate a longer Beam throwing arm and Swinging Bucket Counterweight.

All of the straws used in this project were from McDonalds except for one smaller diameter red straw I sourced from Carls Jr.   As I've mentioned in other projects, the McDonalds straws have a few advantages.  They have a yellow and red color band which helps you stay orientated when cutting.  They also seem to be a little bit more malleable.  That could be my imagination but they seem too have the perfect amount of flex.

As a note, the smaller diameter red straw isn't required.  I only used it as a temporary way to hold the Beam throwing arm into the frame while I measured the length needed for the Counterweight.  It made quick assembly and dis-assembly easier.  You can just as easily substitute a toothpick for that task.

You will also need some Ballast for the Counterweight.  You could source just about anything here but I chose to use rock.  Other substitutes could be ketchup packets, a roll of pennies or sand.  If you are a straw builder purist and only want straws, I suppose you could fill straws with water and crimp the ends similar to the way I demonstrated in the carpenters level project.

Step 2: Tools

You will need a hand held paper punch, pair of scissors, permanent marker and ruler.  

I also used a wooden skewer (not pictured).  A long wooden skewer is really helpful for reaching inside of a straw.  There are portions of the Frame build that would be impossible without it.

You will find this project extremely difficult to build without a ruler.  If you don't ensure a rough symmetry in the Base and Frame, the Beam throwing arm and counterweight will likely not operate efficiently.

Step 3: Basic Components of the Trebuchet

The following Trebuchet elements were considered for this project.  Most of them made it into the final design.

1. Base
2. Frame
3. Axle Rod on Fulcrum Axis
4. Beam (throwing arm)
5. Counterweight (static or swinging)
6. Sling
7. Pouch
8. Wheels (not used)
9. Release Pin (not used)

Step 4: Bloopers

I'm showing you the design bloopers first.  

Swinging Bucket Counterweight
If you are truly intent on building this machine, I urge you to read this and consider building a slightly wider base and much taller frame.  This would have been spectacular, had I planned properly.  Instead, I have an ok-ish Trebuchet.

I originally wanted a swinging bucket counterweight.  The swinging bucket would have made the machine more historically accurate.  Unfortunately, I didn't begin constructing the Swinging Bucket Counterweight until after the frame was completed.  This proved to be disastrous.  As I progressed on the Swinging Bucket, it became obvious it would be too large to fit below the main Beam throwing arm and within the confines of the Frame width.  What was I thinking?  Tip to self, before committing to a final framing dimension, first understand the maximum range-of-motion of all moving parts within the machine.  I should have first built the Swinging Bucket Counterweight and Beam throwing arm, extended them to full range, measured, and only then begin building the Frame.  Too much time had been committed already so I opted to keep the frame as-is and engineer a new Counterweight.

My second blooper was my ambition.  I was determined to build the frame platform with four wheels.  I honestly thought wheel construction would be straight-forward.  I was going to build them similar to wagon wheels.  I did ultimately create wheels but I'm not happy with them so they don't appear in the model.  

I'll continue to refine wheel construction for a different project involving a car with a tension engine.  This also explains why the frame platform is perfectly flat versus just letting the straws project out through the bottom.  So I did end up with a very stable platform minus the wheels.  

Having watched a bunch of videos on the subject, I know this machine is less efficient without the wheels.  You'll notice that a Trebuchet with wheels is able to throw farther because it rolls forward as the arm reaches it's zenith above the Axle Rod.  This is also why a baseball pitcher steps into the pitch.

Step 5: Base

Let's begin by building the Base.  We want our Base to be as perfectly flat and square as is reasonably possible for straws.  

Everything builds on top of the Base so getting it right is worth the effort.  Sloppiness in the Base will trickle through the rest of the project and haunt the final machine.

Punch holes on both ends of two full length straws.  Lay the two full length straws parallel to one another.

Cut a third straw in half and punch holes on both ends of the two shorter straws.  Insert these shorter straws into the longer straws to form a rectangle.

Cut a fourth straw in half and use one member to create a mid section support.  Punch holes to connect it with the base frame.

Punch two holes in the mid section support as shown in the picture.  The two holes will also be repeated on the outer two shorter straws.  All of these holes need to be in alignment.

Note that the shorter pieces are inserted into the longer pieces.  Later in the build, we'll be adding additional rigidity to the Base by inserting more straws inside these shorter straws in later stages.

Step 6: Frame

The Frame is the complicated part of the build.  It needs to handle a lot of stresses, transfer weight to the Base, act as the Fulcrum for the axis point of the Beam, and allow free movement of the Axle Rod.

Building the Frame uses only seven short straws.  Sounds simple - right?  

When working in angles with straws, you'll find that you need to create oval shaped openings instead of circular openings.  You'll get the hang of it but plan on mistakes along the way while you become competent in the technique.  

Another reason this is especially challenging is that this model uses tabs to connect the frame with the base.  Tabs or flaps offer a very secure connection without requiring a straw to jet-out beyond the intersection point.  While this gives you a nice flat base, it will confound your patience.  This is where the wooden skewer will help you.  You be using the wooden skewer to flip the flaps of the tabs into orientation while you slide a second filleted straw into the base supports thus securing the tabs permanently into position.

The pictures show all of the major stages from various angles.  You should be able to work out hole locations using the pictures far better than I could describe in written essay.  If not, post a question and I'll try to answer it for you.

As you build, always be checking for symmetry.  View your progress from multiple angles to spot problems as early as possible.  One of the pictures shows a credit card being used to check for a 90 degree angle.

Notice the three full length straws that are installed in the base.  Those three straws were filleted and inserted into the base straws to hold the tabs into place, act a lateral support, and attach point for wheels (which didn't happen).

Step 7: Beam Throwing Arm

At its essence, the Beam is two straws joined together and acting as the throwing arm of the machine.  Normally, two straws can be joined with a simple outer sleeve and two rivets.  Because the Beam will be under load, we'll need to reinforce it or the Beam will bend.  If you follow these steps, you will have an exceptionally strong union of two straws that form the Beam.

Outer Jacket
Begin by punching two holes in a short piece of straw.
Punch two holes.
Filet this straw.

Use the Outer Jacket as a guide as to where to punch holes in the two full length straws that are to become the Beam.

Cut two short straw lengths.
Punch a hole in each.
Do not filet the rivets.

Inner Sleeve
Cut a medium length straw length.
Use the Outer Jacket as a guide as to where to punch holes.  You will need to account for the location of the Counterweight rivet so don't make it excessively long.

Assemble Beam
Insert  Inner Sleeve into the two straws to form one continuous Beam.
Align the holes
Slide the outer jacket over these two members.
Align the holes
Install the rivets.

Step 8: Counterweight

You may recall that I mentioned that I originally wanted to add a Swinging Bucket Counterweight.  You can see more about that in the Blooper section.  Since that wasn't meant to be, I settled for a Counterweight that exploited as much leverage as I could squeeze into the design.  

This particular Counterweight is designed to act as a sleeve that holds a zip-lock bag holding stone for Ballast.

The weight is always 100% forward of the fulcrum and axis.  It also manages to allow the main beam to move forward of the fulcrum by a few precious minor degrees.

It's not ideal but it gets the job done.  More importantly, its easy to build.

Step 9: Axle Rod

Remove the red straw that you were using as a temporary Axle Rod.  You will no longer need this piece and it isn't strong enough to be used for the final Axle Rod.

The Axle Rod bears the full weight of the Counterweight and payload.  In addition, it is a moving part.  It is critical that it is able to move freely and maintain its shape.  You could say it is the single most important piece you need to make correctly.

Cut two short lengths that are roughly 75% the inside width of the Fulcrum and Beam assembly.

Filet and remove the red band so that the each of the pieces are now smaller in diameter.

Insert one straw into its partner and try to maintain overlap.  You do not want their seam to line up.  We want to overlap the seams so that we have roughly uniform strength in the Axle Rod in all directions.

Insert the newly created Axle Rod into the Fulcrum and Beam assembly.  It should be a nice snug fit.

Insert one end of the Axle Rod into the Frame.  Repeat for the other side to complete the installation of the Fulcrum and Beam into the Frame.

At this point the Trebuchet should start to take on its final form.

Step 10: Sling and Pouch

The sling and pouch are created using only one straw.  The sling needs to be very flexible in order to create a whipping action once the Beam has reached its zenith.  We are trying to emulate an overhand cast of a fishing pole.  We don't have strings or ropes to work with so we will creatively fashion a very narrow band of a straw for a similar effect.

Filet the full length of one straw down the middle of the yellow band.  

Form a barrel on each end of the straw by cutting away and leaving only the red band to serve as the Sling (rope).  

One of the barrels will act as the anchor to the Beam.  The other end of the straw will act as the Pouch.

The Sling anchor barrel side of the Sling requires no additional work.  It is done

The Pouch requires some simple cutting to complete.  I was going for more of a fanged cup for the Pouch that would release the stone with minimal effort.  This one performs perfectly.  It looks really sloppy but I was anxious to try firing the machine (blushing).

Slightly crimp or pinch the Sling anchor and insert it into the top of the Beam throwing arm.  It should be snug.  It can easily be adjust inside the Beam for length and twist but it will still remain in place when released.  If you find it isn't staying inside the Beam, you likely made the anchor barrel too short and may need to fabricate a new Sling.