Catapults, slingshots, and linear punchers all use elastic energy — the difference from flywheels is how energy is stored and released.
Catapult: arm rotates through an arc to launch — large travel distance, good for high-arc shots
Slingshot: similar arm mechanism but with bands doing more of the work — often faster reset
Puncher: linear motion, no arc — best for flat, fast, repeatable shots at a fixed target
// Section 01
Launchers vs Shooters
Flywheels and launchers both launch game pieces — but through completely different physics. Understanding the difference tells you which to choose for a given game and robot.
Elastic potential energy in rubber bands or springs
Motor role
Motor keeps flywheel spinning — continuous power
Motor loads the elastic — power only during reset
Cycle time
Depends on spin-up and feed rate; can be very fast
Fixed by reset time; typically slower per shot
Power per motor
Moderate — limited by flywheel inertia
High — all elastic energy releases at once
Tuning complexity
RPM, compression, hood angle
Elastic count/type, hard stop position, release timing
Best for
High fire rate, consistent velocity-controlled shots
High-power single shots; large or heavy game pieces
🎯 Rule of Thumb
Launchers win on power-per-motor; flywheels win on rate and adjustability. If the game requires launching a heavy piece a long distance once per cycle, a catapult or puncher is usually more motor-efficient. If you need to shoot many small pieces quickly, a flywheel is almost always the right call.
The Three Launcher Types
Catapult: A rotating arm that flings a piece at the end. High arc trajectory. Classic for large game pieces. Accuracy depends heavily on hard stop position and arm weight distribution.
Slingshot / Puncher: A linear mechanism that snaps forward to push a piece horizontally. Lower arc than a catapult. Better for flat trajectories and compact robot packaging.
Linear Puncher: A piston-style mechanism (often pneumatic or spring-loaded) that fires a piece forward in a straight line. Extremely fast cycle time when pneumatic — near-instantaneous shot.
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// Section 02
Slip Gears
A slip gear is a modified gear with teeth removed from a section of its circumference. When the gear rotates to the gap, it disengages and the launch happens. This is the most common mechanism for triggering elastic-powered launchers in VRC.
How Slip Gears Work
A slip gear drives the mechanism while its teeth engage, then releases instantly when the gap in the teeth passes the meshing point.
The gap acts as the trigger — no separate release mechanism needed
RPM of the slip gear controls how often it fires — higher RPM = faster cycle
Make the cam follower contact point smooth — rough contact causes inconsistent release timing
Number of teeth removed controls the arc of travel before release — more removed = more travel
Designing a Slip Gear
Teeth removed = release angle. A larger toothless gap gives the launch arm more time to move freely after release. Too small a gap and the gear re-engages mid-launch. Too large and the reset takes longer.
Where the gap starts matters. The gap must begin exactly at the point where the elastic is fully loaded — not before (under-loaded shot) and not after (motor fights the elastic).
Keep the slip gear axle rigid. Any flex in the axle changes the mesh depth and causes inconsistent engagement. Use short shaft spans and bearing supports on both sides.
Mark the slip gear. Put a physical mark on the slip gear aligned with the gap. During setup and calibration, you need to see exactly where the mechanism is in its cycle.
⚠️
Slip gears wear faster than full gears. The entry and exit teeth of the gap experience much higher impact loads because the gear re-engages abruptly. Check these teeth for wear or chipping after every competition day. Replace the slip gear at the first sign of chipped teeth.
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// Section 03
Elastic Loading
The elastic is your power source. How much you use, where you attach it, and how consistently you load it determine your launch power and shot-to-shot consistency.
Choosing and Configuring Elastics
Rubber band count, not brand. More bands = more power = longer reset time and more motor load during reset. Start with fewer bands and add until you hit your required distance. Document the count.
Attachment point position. Moving the attachment point on the arm changes the moment arm — further from the pivot = more torque = more power. But it also increases reset motor load proportionally.
Preload vs stretch-load. Elastics that start pre-tensioned (loaded at rest) provide faster initial launch but also require the motor to fight them from the very beginning of reset. Find the balance.
Replace on a schedule. Rubber bands fatigue and stretch permanently over time. After every competition day or 100+ cycles, replace all elastics with fresh ones of the same type and count. A tired band loses 15–20% of its energy.
Consistent Loading = Consistent Shots
Shot consistency depends on the elastic being loaded identically every cycle.
Use hard stops to define the exact load position — never rely on driver feel
Elastic bands fatigue over time — inspect and replace them on a set schedule (every N cycles or every practice)
Identical band count, stretch, and attachment points are required for shot repeatability
Document your band configuration in the notebook — it will change and you'll want to track what worked
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// Section 04
Hard Stops, Release Timing & Reset Speed
The hard stop is where your launcher stops. The release timing is when the slip gear lets go. Reset speed determines your fire rate. All three must be designed together.
Hard Stop Design
Physical hard stop, not code stop. Your arm’s fully loaded position must be defined by a physical stop — a screw, a standoff, or a machined surface the arm physically contacts. Code limits drift over time; physical stops don’t.
Cushion the hard stop. The arm impacts the stop at high speed on every cycle. A rubber pad or Delrin cushion absorbs impact and prevents the stop from loosening. Check the stop for movement after 50 cycles.
The launch arc must be clear. From fully loaded to full extension, the arm must not contact any part of the robot. Model this in Onshape and verify the full range of motion.
Reset Speed and Friction Reduction
After launch, the motor must pull the arm back to the loaded position against gravity and the elastic. This is the limiting factor on fire rate. Reduce reset time by:
Minimizing friction. Every bearing, pivot, and sliding surface on the reset path adds resistance the motor must overcome. Use bearing flats at every pivot, smooth surfaces on sliders, and remove unnecessary contact points.
Reducing arm weight. Lighter arm = motor does less work on reset. Remove material wherever the arm is not structural. Every gram saved on a long arm reduces the reset motor load significantly.
Increasing motor reduction. More gear reduction on the reset motor increases torque and therefore reset speed under elastic load — at the cost of increased reset time per revolution. Find the balance between torque and angular speed.
⚠️ Stop Building If…
×
Hard stop loosens after 20 cycles Apply Loctite and check the mount. A moving hard stop means inconsistent shots.
×
Motor stalls during reset Too many bands, too much friction, or motor reduction too low. Reduce elastic or add reduction.
×
Slip gear re-engages mid-launch Toothless gap too small. Remove more teeth or increase the gap angle.
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// Section 05
Common Mistakes
Launchers are simpler than flywheels in some ways — and trap teams in different ways. These are the failure modes that show up under match pressure.
Not replacing elastics on schedule. A stretched band from 200 cycles has noticeably less energy than a fresh one. Teams tune carefully at the start of a season and wonder why accuracy degrades by week 6. Replace on a fixed schedule and document it.
Slip gear teeth chipping unnoticed. The high-impact entry and exit teeth on a slip gear can chip after extended use. A chipped tooth causes the gear to skip or jam. Inspect visually after every competition day.
Hard stop drifting without Loctite. The hard stop screw or standoff takes thousands of impacts. Without thread locker, it backs out. Use Loctite Blue on the stop fasteners and check them weekly.
Firing before the piece is seated. A launcher that fires before the game piece is fully in position launches it off-center or not at all. Add a sensor or driver protocol — piece must be fully loaded before cycle begins.
Reset speed inconsistency. If reset time varies, shot timing from auton or from rapid driver input will be inconsistent. Measure reset time in cycles and verify it is constant.
Launch arc not cleared in design. The arm hits part of the robot at full extension because the range of motion was never modeled. Always animate the full arc in Onshape before building.
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// Section 06
Testing Checklist & Notebook Evidence
Launchers are sensitive to elastic fatigue and hard stop drift over time. Test before every competition day, not just when something seems wrong.
🔬 Launcher Testing Checklist
20 consecutive launch cycles — record landing positions
Mark each landing. Average distance from target and variance.
Reset time measured and consistent
Time 10 resets with stopwatch. Variance should be <5%.
Hard stop position verified with a measurement
Measure arm angle at hard stop with a protractor or encoder count.
Slip gear tooth inspection — no chips or cracks
Visual inspection of entry and exit teeth on every competition day
Elastic replaced if >100 cycles since last replacement
Document replacement date and cycle count in pit log
Motor current during reset is within expected range
Higher than normal = more friction or weaker battery. Find cause before competing.
Launch arc confirmed clear at all robot positions
Test while robot is turning, reversing, and at bumper contact
Notebook Evidence
Elastic configuration diagram: rubber band count, type, attachment points, preload distance — reproducible by anyone
Hard stop position documented with encoder count and physical measurement — so it can be restored if disturbed
After 50 launches, your catapult is consistently releasing 10° earlier than its original release point. What is the most likely cause?
PID drift in the reset position sensor
Elastic band fatigue — elastics lose stored energy over repeated cycles, changing the force at the release point
Battery voltage drop causing the motor to reset slower
Hard stop drift from the mounting screws loosening
Correct. Elastic bands fatigue with use. The energy stored at a given stretch decreases over cycles, which shifts the effective release point. Test with fresh vs. used bands and document the accuracy difference. Judges will ask how you accounted for mechanism wear.
📝
Notebook color guide for this mechanism: ■ Purple — Decision matrix comparing this mechanism against alternatives. Include weighted criteria and selection conclusion. ■ Orange — CAD screenshot, build notes, and any design changes made during assembly. ■ Cyan — Test protocol with hypothesis, data table (n≥5), and conclusions. Include the failure mode you found and how you addressed it.
⚙ STEM Highlight
Physics: Elastic Potential Energy & Projectile Motion
A catapult or puncher converts elastic potential energy (stored in bands or springs) into kinetic energy of the game element. The stored energy follows Hooke's Law: E = ½kx², where k is the spring/band stiffness and x is the stretch distance. At the release point, this energy transfers to the projectile and determines launch velocity. Launch angle and velocity together determine the projectile's trajectory via kinematics: range = v²sin(2θ)/g.
Elastic band fatigue is an engineering reliability concern — bands lose stored energy over repeated cycles because polymers undergo stress relaxation. Testing with fresh vs. used bands and documenting the accuracy difference is engineering evidence, not just troubleshooting.
🎤 Interview line: "We characterized elastic band fatigue as a reliability variable — we tested accuracy with fresh bands versus bands after 100 cycles and documented the degradation. Our maintenance schedule is based on that data, not just habit."