Open Shot Golf Simulator

Table of Contents

• Overview
• Current State
• Feature Highlights
• Ball Physics and Distance Calculation
• Aerodynamics and Reynolds Number Modeling
• Surface and Rollout Tuning
• Launch Monitor and Networking
• Data Sequence Diagram
• Sample Data Payload
• Build and Run
• Controls
• Project Layout
• Future Plans

Overview

Open Shot Golf (formerly JaySimG) is an open source golf simulator built with the Godot Engine. It is designed to work out of the box with the PiTrac Launch Monitor and any GSPro-style interface that sends ball data to the configured port. PiTrac project: https://github.com/jamespilgrim/PiTrac

Current State

• Launch monitor support: Officially tested with PiTrac; other GSPro interfaces should work when pointed at the correct port.
• Game modes: Driving range with data readouts, club selection, and range session recording.
• Platforms: Linux and Windows confirmed; macOS is untested but expected to work.

Feature Highlights

• GSPro-compatible TCP listener for incoming ball/club data.
• Physics based ball flight with drag, lift (Magnus), grass drag, and friction modeling.
• On-range telemetry: carry, total, apex, offline, and shot trails.
• Environment tuning for temperature and altitude, impacting air density and flight.
• Range session recorder and basic UI for club selection and shot playback.

Ball Physics and Distance Calculation

• Ball flight is driven by Player/ball.gd using force/torque helpers in physics/ball_physics.gd (gravity, drag, Magnus lift, grass drag, and frictional torque for bounce and rollout).
• Spin, launch angle, and ball speed are applied in hit_from_data, and the ball transitions through FLIGHT, ROLLOUT, and REST states.
• Distance metrics come from Player/player.gd: horizontal distance is Vector2(x, z).length() in meters, converted to yards in range UI when needed (Courses/Range/range.gd). Carry, apex, and offline distances are tracked until the ball rests.

Aerodynamics and Reynolds Number Modeling

• Drag (Cd) and lift (Cl) coefficients are calculated in physics/aerodynamics.gd based on Reynolds number (Re) and spin ratio (S).
• Reynolds number determines flow regime: Re = (air_density × velocity × diameter) / viscosity
  • Re < 50k: Low Reynolds regime (slow wedges/chips < 77 mph) - constant Cl = 0.1
  • 50k < Re < 75k: Polynomial interpolation between Re-specific models
  • 75k < Re < 200k: Linear model for most normal golf shots (77-155 mph)
  • Re > 200k: Very high Reynolds (extreme long drive competition) - clamped linear model
• A Python script (assets/scripts/reynolds_calculator.py) is provided to analyze Reynolds numbers for different shot speeds and validate aerodynamic regime assignments.
• This implementation ensures physically realistic behavior across the full range of golf shot speeds, from chips to drivers.

Surface and Rollout Tuning

• Range settings expose a surface preset (Firm/Fairway/Soft Fairway/Rough) that maps to ground friction and grass drag parameters in physics/surface.gd (u_k, u_kr, nu_g).
• Firm uses lower friction/grass drag for faster rollout; Rough uses higher values to shorten rollout; Fairway sits between.
• These were limited tested with PiTrac hits with limited ball speeds between 40-80mph. Also compared and tested against what a player would expect on GSPro Practice session rollout (FIRM). These numbers are always subjected to weather (morning dew), slightly longer grass in rough vs shorter, etc. Overall, its a good starting point to give options. In the future the code leaves room to scale to sand, and different types of grass (e.g. FIRM_FESCUE vs FIRM_BERMUDA)
• Defaults are heuristic (tuned for believable rollout) and can be adjusted in the range settings UI. They are not direct measurements from a single study but informed by typical rolling/sliding friction ranges on turf and the drag curve below.
• References:
  • USGA Green Speed Physics (Stimpmeter deceleration): https://www.waddengolfacademy.com/putting/USGA%20Green%20Speed%20Physics.pdf
  • Jenkins et al., “Drag Coefficients of Golf Balls,” World Journal of Mechanics 2018 (Cd vs Re): https://www.scirp.org/pdf/WJM_2018062515520887.pdf
  • USGA Stimpmeter Booklet (green speed measurement): https://www.usga.org/content/dam/usga/pdf/imported/StimpmeterBookletFINAL.pdf

Launch Monitor and Networking

• A TCP server in TCP/tcp_server.gd listens on port 49152 for GSPro-style JSON payloads. When ShotDataOptions.ContainsBallData is true, ball data is emitted to the gameplay layer.
• Good data responses return { "Code": 200 }; malformed data returns a 50x response. Adjust your launch monitor to target the host IP and port 49152.
• Keyboard shortcuts remain available for local testing without hardware (see Controls).

Data Sequence Diagram

Sample Data Payload

Example GSPro-style message used for socket testing (assets/data/drive_test_shot.json):

{
    "DeviceID": "GSPro LM 1.1",
    "Units": "Yards",
    "ShotNumber": 13,
    "APIversion": "1",
    "BallData" : {
        "Speed": 147.5,
        "SpinAxis": -13.2,
        "TotalSpin": 3250.0,
        "BackSpin": 2500.0,
        "SideSpin": -800.0,
        "HLA": 2.3,
        "VLA": 14.3,
        "CarryDistance": 256.5
    },
    "ClubData": {
        "Speed": 0.0,
        "AngleOfAttack": 0.0,
        "FaceToTarget": 0.0,
        "Lie": 0.0,
        "Loft": 0.0,
        "Path": 0.0,
        "SpeedAtImpact": 0.0,
        "VerticalFaceImpact": 0.0,
        "HorizontalFaceImpact": 0.0,
        "ClosureRate": 0.0
    },
    "ShotDataOptions": {
        "ContainsBallData": true,
        "ContainsClubData": false,
        "LaunchMonitorIsReady": true,
        "LaunchMonitorBallDetected": true,
        "IsHeartBeat": false
    }
}

Build and Run

Install Godot

Download and install Godot 4.5 for your operating system: https://godotengine.org/download

Clone Repository

• Clone repository into a local folder:
  git clone https://github.com/jhauck2/OpenShotGolf.git

Import Project

• Open Godot.
• In the Project Manager window, select Import.
• Navigate to the OpenShotGolf folder and select project.godot.

Run

• Press the play button or F5 to start the project.
• When opening the project for the first time, Godot errors may appear due to importing add-ons. Simply close and re-open.
• Set your launch monitor to send data to port 49152, or use the local hit/reset shortcuts below.
  • Python script ~/Resources/SocketTest/SocketTest.py could be used to test TCP functionality (defaults to assets/data/drive_test_shot.json).

Controls

• h: Simulate a built-in hit with sample ball data.
• r: Reset the ball and clear the shot trail.

Project Layout

• Player/: Ball physics, player controller, and shot metric tracking.
• physics/: Shared physics modules (BallPhysics, Aerodynamics, surfaces, docs).
• TCP/: TCP server and GSPro-style JSON handling.
• Courses/Range/: Range scene, UI, and yardage output.
• Resources/, UI/, Utils/: Art assets, UI components, and helper scripts.

Future Plans

• Full course play (currently in early design).
• Additional range features and recording improvements.
• Mobile (Android/iOS) builds once platform pipelines are tested.