My 13 year old daughter designed this project for the Science Olympiad competition. Although designed to meet certain requirements, it is a nice project for any STEM, 4H, Boy/Girl Scouts activity.
This was my daughter’s plan; so I had to resist the urge to make it “better”. I did try to convince her to add a few enhancements but they failed (see below). So the old adage (KISS) still applies. My daughter and her teammate are currently taking data by adjusting the launch angle and weight height. The results will be posted after the competition.
Video of it in action
Launch at Competition
Step 1: Competition Overview
In preparation for competition, the teams will design, construct, and calibrate a device capable of launching projectiles. At the competition, each team will be shooting at targets 2 to 8 meters away.
Rules:There is a two page description of the rules/contest. Below is a simplified version.
- Launching force must supplied by gravitational potential energy (PE) from a falling mass (5kg max).
- Gravitational PE must be converted to air pressure, which is used to launch the projectile.
- Max size: 1m x 1m x .8m
- Mass must be detachable (impounded prior to event)
- The launcher must have a triggering device. Competitors must remain at least 1m away from launch area during launch.
- Tennis, racquet, ping-pong and /or plastic golf balls can be used as projectiles.
- Launch area is 1m x 1.5m.
- The competitors must not be within 1m of the launch area while triggering the device.
- Targets are placed in front of the launch area at distances between 2m and 8m.
- Shooting into a 1 to 5 gallon bucket gives bonus points.
- As is typical with scoring, high score wins.
- Final score = Best Close Target Score + Best Short Target Score + Graph Score – Penalties + Bucket Shot. See website for complete list.
Step 2: Build Overview
A weight (wood block) drops on a 2 liter bottle. The bottle is connected to a PVC pipe through a flexible hose. Air from the bottle propels a ping-pong ball out through the pipe. Weight height and launch angle are adjustable.
Step 3: Tools
- Drill or Drill Press
- Hole Saw (2 1/2” Diameter)
- Screw Driver
Step 4: Materials
- Plywood: 34” x 20” x .5”
- 2x4 by 20” Long
- 1x6 by 18” Long
- PVC Pipe: 1.5” nominal size by 12’
- End Cap, PVC, 1.5” (x6)
- 90 Degree Fitting, PVC, 1.5” (x6)
- Tee Fitting, PVC, 1.5” (x2)
- PVC Pipe Cement
- Pipe Clamps (x9)
- 5/16” Diameter threaded rod x 8” Long
- 5/16” Flat Washer (x2)
- 5/16” Rubber Washer (x2)
- 5/16” Wing nut (x2)
- 2 liter bottle w/cap
- Flexible hose (2” inner diameter) – used old shop vac hose
- Ping-Pong Ball (40mm)
- Duct tape
- Clear Tape
- Paper Ruler
Step 5: Build
You will be building to this drawing.
Step 6: Base
Cut plywood for the base of the launcher. We used 1/2" thick to keep the weight down. Keep in mind the overall size requirements (1m x 1m x .8m) during this step.
Step 7: PVC Pipes
Cut pipes as shown. You might want to hold off on the 3” long pieces until final fit check.
Step 8: Drill Holes
Drill 5/16” clearance holes through the center of 3 of the 12” pipes
Step 9: PVC Assembly
Assembly the PVC pipes/fittings as shown - don’t glue yet! For the launcher tube, feed the 5/16” threaded rod through the holes. We used rubber spacer for the inner interface. Flat washers and wing nuts were used on the outside. Note that this rod also is used for the ball stop. Optional – add another through rod to the a few inches from the bottom. This will give the vertical sections more support.
Step 10: Weight
Depending on PVC fit, the dimensions might not match the drawing. Remove the top piece and use it as a template for the slider holes on the 1x6. Use a drill and hole saw to make these holes. Note that it took us two iterations for this step. For the first try, we had holes that were only slightly bigger than the outside diameter of the PVC pipe. This seemed nice until the trial phase. The board would catch and not fall about 25% of the time. We increased the diameter to 2 ½” and it now works every time.
Step 11: Additional Weight
Attach the 2x4 to the bottom side of the slider for additional weight.
Step 12: Eyehook/Cord
Add the eyehook to the top side (centered in both directions). Tie a cord to the eye hook.
Step 13: Hose
Run a flexible hose between the bottle and PVC launching tube. Duct tape was used to attach the hose to the PVC tube.
Step 14: Bottle Connection
The bottle will most likely need to be replaced (good for ~ 100 hits). You can either use tape or try the replaceable feature which we modified based on seamster’s instructable. We chose the replaceable option. Connect the end cap assembly to the other end of the hose with duct tape.
Step 15: Failure
I tried to make a tape free version using only fittings. Although it looks nice, the flow restrictions limited the performance of the launcher. So, we were back to my daughter’s original idea with a “Dad, I told you so!” thrown in.
Step 16: Glue It Together
Once you are happy with the setup, glue pieces together with PVC cement. Note that we didn’t include this step since the assembly needed to be broken down for transport. Each joint was pressed together tightly and duct tape used for additional strength.
**DON'T GLUE THE TOP SECTION - THIS PART NEEDS TO BE REMOVABLE SO THE MASS CAN BE IMPOUNDED**
Step 17: Attach PVC to Base
Attach the completed assembly to base as shown with pipe clamps.
Add an additional clamp a few inches away from the 2 liter bottle. This does two things: locates the bottle under the weight and helps with angle adjustment.
Step 18: Launch Angle
Again, we took the simple approach for this step. A 2x4 was placed between the launching tube and bottle as shown. The closer the 2x4 was place to the bottle, the lower the launch angle. Four board locations were marked off.
Step 19: Weight Height
A paper ruler covered in clear packing tape was added to the rear to mark the weight height. An optional approach would be to use a sharpie to mark the tube directly.
Step 20: Ready to Fire
Set the angle.
Drop the ball onto the tube.
Raise/drop the weight
Step 21: Update 1 - Added a Trigger Mechanism
Add two eyehooks to the weight. Pull pin used for the release.
Step 22: Update 1 - Added a Trigger Mechanism
A cord/latch was used from an old book bag to lock in the weight height.
Video of the trigger: