Foam-dart turret and kid safety for Gyropup

Checked 2026-10-25. Parts and prices are in catalog/turret.json (32 items, category turret, plus one sensor). The pan motor recommended below, the Damiao DM-J4310-2EC, is already in catalog/leg_actuators.json.

Gyropup is a Tritium robot: kids play against it outdoors, with a foam-dart turret on its head (requirement R25 in docs/requirements.md). This file covers five things:

1. Launcher mechanisms
2. Pan/tilt
3. Recoil
4. Kid safety
5. Outdoor driving

It ends with a recommended turret, its mass, power and cost budget, and a list of safety requirements.

Conventions

• "n/p" means not published.
• Numbers marked calc are my arithmetic on sourced inputs.
• Numbers marked est are my engineering estimates. They are not sourced, and they need to be measured.
• Out of Darts (OOD) product data was read from each product's own store record (/products/<handle>.js).
• The fps figures from OOD are the shop's own 20-shot chronograph averages unless noted (method).
• Unit conversions: 1 fps = 0.3048 m/s, so 70 fps = 21.3 m/s, 100 fps = 30.5 m/s and 150 fps = 45.7 m/s.

Summary

• Recoil does not matter for balance.
  • One 1.0 g dart at 80 fps carries 0.024 N·s. A 10-dart burst carries 0.24 N·s.
  • On a 15 kg robot, that burst changes speed by 0.016 m/s. The average force while firing at 10 darts/s is the same as leaning about 0.14°.
  • Slewing the turret disturbs the robot more than firing does. RoboMaster's balancing robots shoot heavier 17 mm balls and, in the code I read, do nothing special for recoil.
• The safety limit is energy per dart, not recoil.
  • The toy standards give two limits:
    • EN 71-1: 0.5 J for resilient projectiles.
    • ASTM F963: 2500 J/m² kinetic-energy density above 0.08 J.
  • With 1.0 g short darts, these work out to about 104 fps (EN 71) and about 85 fps (ASTM). The ASTM figure assumes the whole 13 mm dart face is the contact area.
  • A stock Nerf blaster measures about 65–73 fps. Hobby games cap at 100–150 fps. Brushless hobby blasters reach 140–200 fps.
• Recommended limits:
  • Short darts at a default of 70 fps.
  • A hard cap of 80 fps (24.4 m/s, 0.30 J). The cap is enforced by flywheel speed and checked on every shot by a muzzle chronograph.
  • The barrel never points above horizontal in the world frame.
  • The turret only fires at worn targets, never at faces.
• Recommended turret:
  • Launcher: brushless flywheels with a Neutron solenoid pusher and a 15-round Talon magazine.
  • Tilt: Feetech STS3215 servo. Pan: DM-J4310-2EC motor, with a 12-wire slip ring for continuous pan.
  • Budget: about 1.3 kg (est), about $415 (part-sourced), 25–35 W while ready to fire, and peaks of a few hundred watts for under a second.
• Moving near kids. A 15 kg robot cannot meet cobot contact limits at play speeds. The ISO/TS 15066 adult values (calc) give about 0.15 m/s for a shin and about 0.28 m/s for a thigh, so speed must depend on distance to the nearest person. Boston Dynamics tells people to stay at least 2 m from Spot.

1. Foam-dart launcher mechanisms

1.1 How the options work

• Flywheel ("flywheeler").
  • Two counter-rotating wheels grip the dart and fling it. The cage crush (wheel spacing) and the wheel speed set the velocity.
  • Motors come in two kinds:
    • Brushed "130" motors (Kraken, Fang, Valkyrie) on 2S–3S.
    • Small brushless outrunners (1404, 1507.5, 2205/2206 FPV class) on 4S, run through ESCs.
  • This is the only type that suits a robot: it can fire continuously, needs no priming, and its velocity can be set electronically.
• Pusher. The pusher moves a dart from the magazine into the wheels. Options:
  • A solenoid (the Neutron is the hobby standard).
  • A gear-motor cam (Atrox, Juno: 600 rpm motor ≈ 9 balls/s).
  • A servo (simple turrets).
  • A solenoid gives one dart per stroke, so the controller can count every shot and cap the rate of fire exactly.
• Springer (Harrier, Caliburn). A spring piston is primed by hand. It is high-energy (Harrier 183–205 fps) and needs manual or motorised priming. It is a poor fit for a robot and is not considered further.

1.2 Ammunition

Short darts are the right choice for Gyropup:

• They are the lightest, so they carry the least energy at a given speed.
• Their magazines are short.
• The standard Talon magwell is shared by Blitz, Protean, Momentum and Harrier.

Rival balls are a poor fit. They are 75 % heavier, so they carry 75 % more energy at the same speed. Hasbro rates the Rival Helios at 100 fps for ages 14+ and recommends eyewear (Hasbro).

1.3 Blasters and launch modules compared

Notes on the table:

• Stock Nerf velocity sources:
  • Physicist Chad Orzel filmed a Nerf dart at 22.1 ± 0.3 m/s and 20.9 ± 0.2 m/s, with darts of ≈1 g each (blog).
  • Retailer user reviews with ProChrono chronographs report 65–73 fps for stock Elite/Modulus blasters. These are single reviewers and are only indicative.
  • Hasbro does not publish fps for Elite blasters. It does publish 90–100 fps for Rival.
• TerraScout details come from search snippets of retailer and news listings (18-dart clip, later a 35-dart drum, 720p camera). I could not fetch a primary spec page, so treat them as unverified.
• Not found:
  • No "Caliburn" flywheel exists. The Caliburn is Captain Slug's public-domain, spring-powered, pump-action blaster (Thangs; from a search result, because the page blocks direct fetches).
  • No project called "Hy-Fly" could be found.
  • No "Nerf Sentry" product exists. TerraScout is Hasbro's only robot.
• Cage crush works as a mechanical velocity limiter. The Protean's numbers show this: the same motors give 102 fps with a 42 mm cage and 141 fps with a 40 mm cage. A deliberately wide-crush cage limits velocity even if the software fails.

1.4 Launch motors, pushers, feed

Feed options:

• Magazine. Talon or Worker short-dart mags, 15–23 rounds. This is the simplest and most reliable option.
• Belt. No off-the-shelf dart belt feed was found.
• Hopper. A blower-pressurised hopper exists only for Rival balls (the Infinite Pack).

For Gyropup: use one 15-round magazine for a capped, game-sized load, then reload. Swapping magazines is a natural "pit stop" in a kids' game.

Power and control in practice:

• Brushed builds switch the flywheel motors with a microswitch or MOSFET and fire the solenoid through a MOSFET with a flyback diode. The OOD parts sold for solenoid builds include 1N5400 rectifier diodes.
• Brushless builds use hobby ESCs (PWM, OneShot or DShot). Firmware-driven blasters (Momentum, Vesper, FDL-3) expose speed, rate-of-fire and spin-up settings.
• Spin-up time. The FDL-3 firmware's MinSpin range of 120–500 ms (default 220 ms) shows the scale. Motor idling (Diana, Vesper) cuts trigger response to 120–160 ms.

2. Pan/tilt for a 0.5–1.5 kg head

2.1 Torque and speed needed (est)

Tilt:

• Assumed payload: 1.0 kg with its centre of gravity 20 mm off the tilt axis.
• Holding torque ≈ 0.2 N·m.
• Inertia ≈ 1.0 kg × (0.08 m)² ≈ 0.0064 kg·m².
• Reaching 300 °/s in 0.1 s needs α ≈ 52 rad/s², which takes ≈ 0.33 N·m.
• Total: about 0.5 N·m.

Pan:

• About 1.2 kg at roughly 70 mm radius gives ≈ 0.01 kg·m².
• At the same acceleration, pan needs ≈ 0.5 N·m. It also has to reject the body's yaw motion.

Speed:

• A child running 5 m/s at 3 m range needs about 1.7 rad/s (≈ 95 °/s) of tracking.
• A pan capability of 180–360 °/s covers that plus the robot's own turning.
• The DJI RoboMaster S1 gimbal is specified at 540 °/s maximum.

Safety cap on pan speed:

• In software, cap pan speed when people are within reach (see §4.6).
• Basis (calc): ISO/TS 15066 adult face values of 65 N and 75 N/mm, with a 1 kg moving turret.
  • Contact speed limit ≈ 0.26 m/s.
  • With the muzzle 0.15 m from the pan axis, that is ≈ 1.75 rad/s ≈ 100 °/s.

2.2 Actuators compared

Gear rings.

• A direct-drive QDD (DM-J4310, GM6020) on the pan axis needs no ring gear.
• A servo pan can drive a printed ring gear or belt (for example 3:1). That trades speed for torque and lets the servo sit off-axis so the slip ring can go on the axis.
• No sourced ring-gear product was selected.

2.3 Slip rings for continuous pan

Wiring for the Adafruit ring:

• Run 12 V power to the launcher as two rails, each paralleled over 3–4 circuits.
• Run the CAN/UART signal on a twisted pair.
• Use the rest for the arming loop: the dead-man/e-stop line must reach the launcher without depending on software.
• Alternative: give the pan a limited range (for example ±170°) and use a cable chain. This avoids the slip ring's contact noise and wear.

2.4 Existing robot turrets and what they teach

DJI RoboMaster S1 (user manual specifications, DJI 2019):

The manual also says:

• "make sure to wear the safety goggles provided by DJI"
• "DO NOT aim the blaster at people or animals"

So DJI's own gel-bead robot is not meant to shoot people.

RoboMaster infantry and balancing infantry robots

These are the most relevant precedent: they carry a gimbal and launcher on a wheel-legged balancing chassis.

• Gimbal control. DJI's standard infantry firmware, reused by teams, controls the gimbal in "gyro mode" from the gimbal's IMU: an angle loop driving a speed loop for yaw and pitch, with an encoder mode as fallback. See gimbal_task.h from Zhejiang Sci-Tech's 2023 balancing infantry. The gimbal is stabilised in the world frame and the chassis follows.
• Shooting. shoot.h does the following:
  • Ramps the friction-wheel speed up before firing is allowed.
  • Drives a trigger (feed-dial) motor in angle steps.
  • Detects a jam (trigger speed too low for too long) and reverses.
  • Keeps a barrel-heat budget from the referee system.
• Recoil. I found no recoil feed-forward in the shoot or gimbal code I read. Disturbance rejection is left to the IMU loops, which fits the small impulses computed in §3.
• Wheel-leg control. Current teams publish their designs:
  • Fudan's RM2026 balancing infantry runs an RL policy at 100 Hz inside a 500 Hz loop. It uses four DM 8009P hip motors and two 3508 wheel motors, plus supercapacitors to stop the hip motors dropping out on undervoltage.
  • Other repos: COD 2024 (VMC and Vision tasks) and Mas 2025.
• Over-speed policing. The RoboMaster referee system measures projectile speed at the muzzle with a speed monitor module (17 mm version: 175 g, 1–50 m/s, 1–50 Hz). Older rules capped 17 mm launch speed at 30 m/s and deducted HP for violations (2018 rules).
• What Gyropup copies from RoboMaster:
  • A world-stabilised gimbal from the gimbal's own IMU.
  • A spin-up gate before firing.
  • Jam detection.
  • A per-shot speed check with lockout.

Hobby Nerf turrets

Many hobby turrets use face tracking, for example brianlevis/nerf-iron-dome (Unlicense, RapidStrike, HS-805BB servos, "track and engage faces") and rohan-bansal/NerfTurret. Gyropup must do the opposite and suppress fire when a face is in the line of fire. The Tops76 Dart Turret is the closest hardware match: brushless flywheels, serial-bus-servo gimbal and an integrated speed sensor.

3. Recoil

Reaction impulse per dart = dart mass × muzzle velocity (calc).

• This holds for a flywheel launcher: the wheels push on the dart, and the cage pushes back on the turret.
• The air the dart drags along is ignored.
• A counter-rotating flywheel pair has close to zero net angular momentum. Panning the turret then creates almost no gyroscopic torque, and the two motors' spin-up reaction torques cancel.

For comparison, a DJI S1 gel bead carries 0.003–0.004 N·s (0.12–0.17 g at 26 m/s).

What the balancing robot feels (calc)

Assumptions:

• Robot mass 15 kg.
• Muzzle 0.6 m above the wheel axle.
• Pitch inertia about the axle ≈ m·l² = 15 × 0.4² = 2.4 kg·m², taking the centre of mass 0.4 m up.

What disturbs the robot more than recoil:

• Turret slew reaction. Tilting a 1 kg head at 52 rad/s² puts ≈ 0.33 N·m (est) back into the body. That equals about a 0.3° lean, already larger than a whole capped burst.
• Payload. The robot also carries a 1.3 kg payload whose centre of gravity moves as the turret tilts. A 20 mm shift of a 1.2 kg turret moves the robot's centre of mass by about 1.6 mm.
• A child's push. A child pushing the robot is a far larger disturbance than any of the above.

Design conclusion:

• No recoil compensation is needed in the balance controller.
• Model each shot as a known impulse in simulation (the sim's recoil scenario) to confirm this.
• Spend control effort on two things instead:
  • World-frame turret stabilisation.
  • Feeding the turret's commanded accelerations forward into the balance controller.

4. Kid safety

4.1 Toy-projectile rules

Limits applied to darts (calc):

• KED assumes the whole 13 mm dart face is the contact area, which is 1.33 × 10⁻⁴ m².
• The F963 test method 8.14.6 was not read, so the true contact area may differ.

Checks against products:

• A full-length dart from a stock blaster at 70 fps has a KED of ≈ 2200 J/m², just under the limit. This fits stock Nerf being designed around these rules.
• Hasbro's 100 fps Rival Helios puts ≈ 0.8 J into a 1.75 g ball. That is above the EN 71 resilient figure, and Hasbro grades the product 14+.

Toy standards do not directly cover Gyropup. They govern toys sold for children; a robot used to run a game is not a toy. They are still the best published, kid-specific energy limits, so Gyropup adopts them as design targets.

4.2 Field practice: velocities and eyewear

Injuries are real even at stock energies:

• BMJ Case Reports (Bizrah & Verma, 2017, doi:10.1136/bcr-2017-220967) describes three traumatic hyphaemas from Nerf darts. The authors call for protective eyewear and a review of age limits. This is reported via secondary pages, because the BMJ newsroom returned 403.
• Eye News (Joshi, Troiani & Kulshrestha, June 2023) reports three eye injuries:
  • An 11-year-old hit at close range: corneal oedema and hyphaema.
  • A 12-year-old hit from 3 m: hyphaema.
  • An adult hit from 2 m: a retinal hole needing laser treatment.
  • The authors recommend that safety goggles be supplied with Nerf guns (Eye News).

4.3 What this means for the turret

1. Velocity. Default 70 fps; hard cap 80 fps (24.4 m/s) with 1.0 g short darts.
   • That gives 0.30 J, which is under EN 71's 0.5 J. KED is ≈ 2240 J/m² (calc), under ASTM's 2500.
   • Never use Rival balls or full-length darts at the short-dart setting.
   • Enforce the cap in three independent ways:
     • Flywheel RPM (closed loop, brushless).
     • A wide-crush cage that cannot exceed the cap at full motor speed. Verify this on the bench.
     • A muzzle chronograph on every shot. Any shot over 80 fps locks the launcher out until a human resets it.
2. Rate of fire. At most 3 darts/s and a 5-dart burst limit in the kids' game (a design choice), with a magazine of
   15. The solenoid makes each dart countable.
3. Elevation. The barrel must never point above horizontal in the world frame.
   • Gyropup balances by leaning, so the body pitch changes all the time. A tilt limit relative to the body is not enough. Measure the limit with the turret's own IMU in the world frame.
   • Add a mechanical hard stop that holds even at the robot's worst-case forward lean.
   • Ballistics (calc, no drag, so these are upper bounds). From a 0.5 m muzzle at 0° and 24 m/s, a dart is at ≤ 0.47 m at 2 m range and ≤ 0.29 m at 5 m. At +10° it reaches 1.17 m at 5 m, which is a child's face height.
   • So also predict the dart's height at every range up to its maximum. Inhibit firing if the predicted path passes through the head zone (≥ 0.9 m) of any detected person, or the leg height changes the muzzle height enough to.
4. Targeting.
   • Fire only at worn targets: a vest or belt panel with a fiducial or colour patch at belly height or lower.
   • Never fire at a detected face or head.
   • Never fire at animals.
   • Never fire at anyone not wearing a target, which covers bystanders, parents and toddlers.
   • The opposite approach, "track and engage faces" (iron-dome), is explicitly forbidden.
5. Range. No firing at a person closer than 2 m. The "no point-blank above the neck" rule exists for this reason, and two of the Eye News injuries came from close range.
6. Eyewear and game rules. Players need eye protection. The robot checks for it where practical (eyewear-detection flag) but does not rely on it. The game master holds the dead-man switch.

4.4 Dead-man switch, e-stop, safe zone

• Two-level stop.
  • Wireless e-stop: cuts motor power for both drive and turret through hardware, and must work if the computer hangs. See the estop items in power_electronics.json and power-electronics.md.
  • Turret disarm: a separate hardware contactor on the 12 V launcher rail. It needs a held dead-man button (or a continuous heartbeat from the game-master remote) to stay closed. If the dead-man is released, or the link is lost for more than about 200 ms (est), the turret disarms. The robot keeps balancing, because a balancer that drops power falls over.
• Visible state. An LED ring or beacon shows SAFE (green), ARMED (amber) and FIRING (red), plus a buzzer when armed. An orange muzzle is used, as on OOD's Momentum: "all muzzles will always be printed in orange".
• Safe zones. Geofenced or tagged areas (entry, spectators, the pit where magazines are reloaded) where the turret is disarmed and the drive speed is capped at crawl.

4.5 Speed and contact with a 15 kg robot

ISO/TS 15066 (collaborative robots, adults). The permissible contact speed is v = F / √(μ·k), where μ = (1/m_H + 1/m_R)⁻¹. Values from arXiv 2311.13814, Table 1:

• Lower legs: 130 N, 60 N/mm, m_H 75 kg.
• Thighs and knees: 220 N, 50 N/mm, 75 kg.
• Face: 65 N, 75 N/mm, 4.4 kg.

Contact speeds (calc). The whole robot is taken as moving mass, which is conservative.

These are adult industrial limits; children are not covered. Play needs faster movement than this, so the robot has to:

• slow down by distance to the nearest person;
• avoid driving at people; and
• keep its contact surfaces soft.

Boston Dynamics on Spot (user guide R2.0):

• "People must stay a safe distance (at least 2m) from Spot during operation to avoid injury."
• Injuries "may be caused by collisions, Spot falling or tipping onto people, or contact with Spot's pinch points".
• Fingers "may break or get amputated if caught in joints while Spot's motors are active".

Gyropup is lighter than Spot but has the same hazards: falling, tipping, collisions and leg pinch points.

Sidewalk robots. Iowa's personal-delivery-device law caps them at 6 mph (2.7 m/s) in pedestrian areas and at 550 lb (Iowa Code 321O). That is far heavier and faster than Gyropup. Use it only as an upper bound.

Stopping distance (calc): d = v·t_r + v²/(2a), with a 0.25 s reaction time (est). A balancer must first tilt back before it can brake, so its deceleration a is lower than tyre friction alone would allow; 1.5–3 m/s² is assumed (est).

Pinch points.

• Hazards: the knee and hip linkages, wheel-to-leg gaps, and the pan bearing.
• Apply minimum gaps per ISO 13854. That standard covers crushing only; its table was not retrieved because it is paywalled (ISO).
• Otherwise use covers: printed leg shrouds, a closed wheel arch and a skirted turret ring.
• Limit joint torque when a child or dog is within 1 m.

4.6 Dogs

• Ingestion. Pets eat foam darts. A cat needed surgery to remove a Nerf dart tip from its small intestine (VetLove). Count darts out and back in, and do a field sweep after the game.
• No firing at animals. The animal classifier inhibits fire whenever an animal is in the predicted fire cone. DJI forbids aiming its S1 at animals too.
• Robustness. Dogs chase and bite moving things. The robot needs:
  • bumpers and shrouds;
  • no exposed cables;
  • no loose straps on the turret; and
  • speed limits that apply to dogs as they do to children.

5. Outdoor driving for a 15 kg two-wheeled balancer

Wheel and motor choice is covered in wheel-motors.md. This section adds terrain.

5.1 Tyres

• Pneumatic (for example 10" hoverboard 10 × 2.125):
  • Grips and cushions better on grass and gravel, and spreads load on soft ground.
  • Adds spring and damping that the balance policy has to learn around (wheel-motors.md).
  • Can puncture.
  • Pressure changes the rolling radius, which affects odometry.
• Solid or semi-pneumatic (BotWheel 171 mm solid tread, DDT M1502 tyre-wheel):
  • Predictable for control and puncture-proof.
  • Gives a harsher ride over roots and gravel.
• Diameter and kerbs:
  • Typical US kerbs are 6 in (150 mm) standard and 4–6 in sloped (Iowa DOT).
  • A 170–190 mm wheel (85–95 mm radius) cannot roll up a 150 mm kerb. Gyropup has to use its legs to step or hop, as the Diablo's four-bar legs do for "curb-climb" (reference-robots.md).
  • A rolling-only climb is realistic only well below the wheel radius. The exact limit depends on traction and lean and was not computed here.

5.2 Rolling resistance

Rolling resistance coefficients:

Small robot wheels (150–200 mm) on grass were not measured in any source found. They will sit above the ATV values, because smaller wheels sink and climb more. Take 0.10–0.15 on lawn as a planning figure (est).

Drive force on lawn (calc): 0.12 × 15 kg × 9.81 = 18 N, or about 9 N per wheel. Add slope:

• 15° adds 38 N.
• 10° adds 26 N.

5.3 Slopes in yards

• Riding mowers. UGA Extension says riding mowers "will slide and roll over on slopes greater than 27 percent" (≈ 15°). Push mowers "can be used on slopes up to 20 degrees" (UGA).
• One manufacturer's limit. A Kentucky FACE fatality report (a mower rolled on a 30° slope) quotes the manual: "Under no circumstances should the machine be operated on slopes greater than 15 degrees" (KIPRC 15KY041).
• Gyropup targets:
  • Design for 15° on dry grass.
  • Refuse slopes over 20°, detected from the IMU and terrain.
  • Ban play near downhill edges, since a falling robot rolls.

5.4 Ingress protection

IEC 60529 codes, from a vendor summary because the IEC page returned 403:

• IP54: IP5X is dust-protected (limited ingress, 8 h dust chamber); IPX4 is splashing water from all directions.
• IP65: IP6X is dust-tight (no ingress); IPX5 is water jets (12.5 L/min, 6.3 mm nozzle, 3 m, 15 s).

Source: summary.

What already has a rating:

• DDSM115 and BotWheel motors: IP54.
• DDT M1502: IP55 (wheel-motors.md).
• Slip rings: IP54/65 variants exist (Ingiant).
• Foam darts and magazines: none. Wet darts swell and jam, and wet grass ruins them.

Gyropup targets:

• Body and electronics: IP54 minimum. Use gasketed lids, cable glands, a drip loop at the pan ring, and a vent with a membrane for the Orin's heat.
• Turret: splash shroud over the cage and magazine well.
• No play in rain or on wet grass.

6. Recommended turret concept for Gyropup

Concept.

• A world-stabilised 2-axis head with continuous pan.
• A single-stage brushless micro-flywheel cage, derived from the CC0 Gryphon geometry, with the crush set wide enough that full motor speed stays under the cap.
• A Neutron solenoid pusher.
• One 15-round standard Talon magazine of 1.0 g short darts lying along the pitch axis.
• A two-gate muzzle chronograph.
• An arming contactor on its own 12 V rail.

Control.

• A small MCU in the head does the following:
  • Closes the flywheel RPM loop.
  • Fires the solenoid.
  • Reads the chronograph.
  • Enforces the velocity, rate and elevation interlocks.
• The Jetson only requests shots at a validated target. The MCU can refuse.

6.1 Parts list

6.2 Mass budget

A lighter option is about 1.1 kg: use a second STS3215 on a 3:1 ring gear for pan instead of the DM-J4310. It saves ≈ 217 g and $103, at the cost of pan speed. A 3:1 ring takes 270 °/s down to 90 °/s.

6.3 Power budget

Figures marked † are from comparable parts' published specs: the Snail 2305's 0.4 A at 24 V, and the Kraken's 0.55 A at 11.1 V.

6.4 Cost

• Sourced parts: ≈ $315. That is 2×26.99 + 2×12.99 + 29.99 + 2×9.99 + 16.99 + 21.99 + 125 + 19.95.
• Estimated items: ≈ $100.
• Total: ≈ $415.
• Cheaper brushed option: a Chance Blitz kit ($65.99, 321 g, 100 fps with Nightingale wheels on 3S) as a ready launcher module. It has no speed feedback, so the chronograph lockout becomes the main velocity guard. Its open-loop velocity also changes with battery voltage.

6.5 Why this concept

• Brushless + RPM loop + crush + chronograph gives three independent velocity limits. The Vesper's 48/134/176 fps presets show that brushless firmware can hold low velocities.
• A solenoid gives one dart per stroke, so every shot is counted and rate-limited.
• Talon magazines are a standard, short darts are the lowest-energy ammo, and reloading is quick.
• DM-J4310 pan on the existing CAN bus.
  • It has the same driver and protocol as the rest of Gyropup.
  • Its absolute output encoder keeps pan known through power loss.
  • Its 15–32 V range accepts the 7S pack directly, which the GM6020 (24 V rated) and C615 ESC (26 V max) may not.
• STS3215 tilt.
  • It is light, cheap and absolutely encoded, so tilt is known at boot before any shot.
  • Its 2.9 N·m stall torque is about 6 times the ≈ 0.5 N·m estimate.

7. Safety requirements

IDs are for traceability in the designer and the simulation.

8. Open questions

• ASTM F963-23 projectile numbers. These changed in 2023 and were not retrieved. Buy or borrow the standard, and check the KED contact-area definition in §8.14.6.
• ESC RPM telemetry. Confirm which small ESC firmware reports eRPM at 50k+ rpm (the LittleBee listing doesn't say). Otherwise put a hall or optical tach on one flywheel.
• Flywheel speed for 70–80 fps. Find the rpm and crush that give 70–80 fps with 1.0 g darts by bench test. This is far below the Momentum and Vesper "high" presets.
• GM6020 maximum input voltage. Not found. Needed only if the GM6020 replaces the DM-J4310.
• Supernova motor mass and Helix flywheel price. Not published in the listings fetched.