Shooter Designs
Projectiles in FRC: A Practical How-To Guide
1) Pick a launcher style (pros/cons & when to use)
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Hooded flywheel (single/dual wheel): Most common, accurate over a range; add an adjustable hood for multiple shot locations. Good for balls/notes. Chief Delphi+1
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Catapult/puncher (spring/elastic/motor-latched): Great for heavier/softer objects or when you want consistent “set-shot” distances; needs careful energy storage & release sizing. Chief Delphi+1
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Turret vs. fixed: Turret simplifies driver alignment and allows shooting while driving; fixed is lighter and simpler. (See 254’s turreted designs & feeder integration.) Chief Delphi+2media.team254.com+2
2) Shooter geometry & compression (for hooded flywheels)
Goal: Control contact time and spin so the game piece leaves at a repeatable speed/angle.
How to:
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Start with compression: Distance from wheel to hood smaller than the game-piece diameter. Typical starting point many teams reported in 2020: ~1.5–2.5 in; tune per game piece. Chief Delphi+1
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Choose hood material: Polycarbonate is common; grippier liners increase spin if the piece slips on the hood. Chief Delphi
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Wheel diameter & inertia: Bigger wheels give higher surface speed at the same RPM and generally more energy storage (flywheel effect); trade against weight & packaging. Chief Delphi
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Adjustable hood: Enables one mechanism to hit multiple distances/angles; CAD your arc so the normal force and compression stay reasonable across angles. Chief Delphi
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Rear “kicker”/pre-spin roller (optional): Can improve feeding and reduce shot-to-shot variation; prototype to validate. Chief Delphi
3) Motors, gearing, and energy
How to:
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Pick motor(s) to match your RPM/torque needs (NEOs/Falcons/775pros). Prototype one motor, log RPM recovery, then scale; example community configs exist (e.g., multi-775pro setups). Chief Delphi
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Gearing: Target a no-load free-speed that’s 10–30% above your on-shot speed to allow headroom for control.
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Flywheel inertia: Add mass (steel plates or heavy hub) for better velocity hold-up during a shot—balance vs. spin-up time. (See CD compression threads discussing plate “disks”.) Chief Delphi
4) Feeding, centering, and serialization
Why it matters: Consistent entry orientation & speed reduces shot variance.
How to:
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Center & single-file: Use “serializer” rollers/geometry to turn wide intakes into a single, well-registered stream. (Great example write-ups in 254 tech binders.) media.team254.com+1
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Metering sensors: Beam breaks/hall sensors before the shooter to time the feed when RPM is on-target.
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Isolation: Use compliant wheels and passive rollers to control the ball and avoid jams before the throat. media.team254.com
5) Control: getting repeatable velocity (and fast recovery)
Core strategies you can implement in WPILib:
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Bang-bang: Simple on/off for hitting setpoint quickly—great baseline. FIRST Robotics Competition Documentation
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PID/FF velocity control: Use WPILib’s tuning flow; characterize kS/kV/kA, then add PID for disturbance rejection. FIRST Robotics Competition Documentation
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State-space (advanced): Model-based control for excellent disturbance handling and recovery. WPILib has an end-to-end flywheel walkthrough. FIRST Robotics Competition Documentation
Practical steps:
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Characterize the shooter (SysId or logged step tests) and compute feedforward. FIRST Robotics Competition Documentation
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Tune: hit open-loop near the target, then close the loop; verify recovery time between rapid shots. FIRST Robotics Competition Documentation
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Disable motor-safety for flywheels (keeps them spinning during control loops). FIRST Robotics Competition Documentation
6) Trajectory, spin, and aim
How to:
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Backspin/topspin: More backspin often stabilizes flight and can help “drop-in” style goals; tune with hood friction and wheel-hood speed ratio. Chief Delphi
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Distance tables: Build RPM/hood-angle lookup tables per range—collect data at marked distances, interpolate in code.
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Turret/hood strategy: Decide “fixed hood + turret + velocity” vs. “adjustable hood + fixed shooter” based on game tasks and protected zones. (Notes from Spectrum & 2024 meta.) Chief Delphi
7) Catapults & punchers (elastic or motor-latched)
How to:
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Pick energy storage: Surgical tubing, gas springs, torsion springs; estimate spring rate and energy (½ k x²). Old but useful CD references give ballpark tubing rates—always validate on a test rig. Chief Delphi
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Size the actuator/gearbox: Use a simple simulation or spreadsheet to match wind-up torque, angular speed, and release angle (see “Electric Catapult Design & Optimization”). Chief Delphi
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Latch & release: Robust hard latches or dog clutches; ensure pre-load can’t self-release and add physical hard-stops.
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Cycle time: Design for safe reset under defense (ratchets or worm-gear holds).
8) Prototyping plan (fast & reliable)
How to:
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Bench rigs first: 2×4 frame, adjustable hood arc, sliding wheel-to-hood distance. Swap wheels/materials quickly.
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Log everything: RPM before/after shot, time-to-recover, ball exit speed (phone high-fps), hit rate vs. distance.
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Parameter sweeps: Try compression steps (e.g., +0.25 in), wheel durometer, hood liners, release angles—record a matrix.
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Durability checks: Shoot 100+ cycles and re-measure your “dialed” values to see drift (wheel wear, liner glazing).
Resources with examples & prototyping ideas: Spectrum resources hub and build blogs. spectrum3847.org+1
9) Software integration: from sensor to shot
How to:
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Sensor suite: Encoder on flywheel; beam break at exit; gyro/odometry for range estimate; (optionally) vision for pose/aim assist.
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Shot gating: Only feed when
abs(vel - setpoint) < tolerance
for N ms. -
Auto-aim options: Turret PID to vision target or odometry-based “dead-reckon” with distance→RPM/angle tables. (WPILib control tutorials cover tradeoffs.) FIRST Robotics Competition Documentation
10) Mechanical details that matter
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Stiff mounting: Shooter & hood need rigidity (avoid deflection changing aim). Elite teams detail robust interfaces between turret and shooter. Chief Delphi
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Ball path sealing: Close gaps so you don’t lose pressure/energy.
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Serviceability: Quick-change wheels/liners; access panels for cleaning debris.
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Safety: Shields around flywheels; never stand in plane of the wheel; interlocks for test mode.
11) Strategy fit & on-field use
How to:
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Decide shot families you’ll own (e.g., protected zone, “subwoofer,” mid-field). 2024 takeaways: multi-location shooting boosts cycle flexibility but increases complexity—picking one or two money shots can still be very effective. Chief Delphi
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Warm-up & calibration: Spin up on enable; auto-zero hood; shoot a short drill to confirm RPM table after field reset.
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Maintenance: Re-index compression (liners wear), check set screws, re-true wheels weekly.
12) Build-season checklist
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Define target ranges/angles and cycle goals (with strategy).
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Select launcher type; CAD the geometry (arc, compression, packaging).
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Prototype quickly; collect a data table and pick initial control strategy.
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Integrate a reliable feed path with sensors.
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Lock in materials, fasteners, guards.
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Code gating & recovery; validate with drill cards (e.g., 10 rapid shots at 2 ranges).
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Create pit procedures for inspection, alignment, and upkeep.
Suggested “Further Reading” blocks for your wiki
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WPILib: Tuning a flywheel, state-space flywheel control, strategy choice. FIRST Robotics Competition Documentation+2FIRST Robotics Competition Documentation+2
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Spectrum 3847 resources and build blogs (design slide decks, prototyping ideas). spectrum3847.org+1
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ChiefDelphi canonical threads on hood geometry, compression & materials. Chief Delphi+3Chief Delphi+3Chief Delphi+3
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Catapult math & examples. Chief Delphi+2Chief Delphi+2
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Elite team tech binders for feeder/serializer & turret integration. media.team254.com+2media.team254.com+2