GGDT Comparison Guide

1. What is GGDT?
2. Installation
3. Parameter Reference
4. Golf Ball Launcher Walkthrough
5. Valve Data Reference
6. Physics Equations Used
7. Experimental Validation Data
8. Sources

1. What is GGDT?

Gas Gun Design Tool (GGDT) is a simulation program developed by David Hall (D_Hall) for designing pneumatic cannons. It's considered the gold standard in the spudgun community for predicting pneumatic launcher performance.

Accuracy: GGDT typically outputs numbers within 5-10% of measured values when proper input parameters are used. The tool has been verified many times with actual testing.

What GGDT Models

Five different valve types with opening behavior
Pressure drop calculations during expansion
Flow choking limitations
Projectile leakage around bore (blow-by)
Pilot chamber behavior
Temperature effects on gas properties
Real gas equation of state (Redlich-Kwong)

What GGDT Does NOT Model

Turbulence or frictional losses between gas and walls
Pressure drops due to bends or rough edges
Reservoir geometry effects (fineness ratio)
Freezing or liquification of gases
External ballistics (trajectory after muzzle)

2. Installation

Download

GGDT is available from SpudFiles. The current version is 4.4 or 4.6.

Required Files

Place these files in the same folder:

ggdt4.4.exe (or ggdt4.6.exe)
COMDLG32.OCX
MSCHRT20.OCX

Windows 7+: Right-click on ggdt.exe and select "Run as Administrator". You may also need to enable XP compatibility mode.

Linux Users

GGDT can run under Wine:

wine ggdt4.4.exe

3. Parameter Reference

Reservoir Data (Chamber)

Parameter	Description	Notes
Outer Diameter	Outside diameter of chamber pipe	Use pipe OD from specifications
Inner Diameter	Diameter of any pipe running through chamber	Set to 0 for most designs! Only non-zero for coaxial designs where barrel runs through chamber
Length	Length of the pressure chamber	Internal length, not including fittings
Pressure	Operating pressure	Gauge pressure (psig)
Gas Type	Propellant gas	Usually "Air"

Barrel Data

Parameter	Description	Notes
Bore Diameter	Inside diameter of barrel	Use pipe ID from specifications
Length	Barrel length	From valve seat to muzzle

Projectile Data

Parameter	Description	Notes
Mass	Weight of projectile	In grams
Diameter	Projectile diameter	Affects blow-by calculation
Friction	Friction force	Typically 0-5 PSI equivalent for golf balls

Valve Data

Parameter	Description	Notes
Valve Type	Configuration of valve	Chamber sealing pilot, Barrel sealing pilot, Burst disk, Hammer, Generic
Seat Diameter	Effective opening diameter	Determines flow area
Flow Coefficient	Valve flow restriction	% of ideal flow (not manufacturer Cv!)
Opening Time	Time to fully open	In milliseconds - critical parameter!
Dwell Time	Time valve stays open	Set to 999 for valves that stay open
Dead Volume	Volume between projectile base and valve	In cubic inches

4. Golf Ball Launcher Walkthrough

Real-World Test Data (Your Launcher)

Parameter	Value
Barrel Type	SDR26 1.5" (ID = 1.754")
Barrel Length	30 inches
Chamber	2" x 40" (ID = 2.067")
Pressure	25 psi
Launch Angle	40 degrees
Projectile	Golf ball (45.93g, 42.67mm / 1.68")
Measured Range	~25 yards (75 feet)
At 125 psi	Close to 200 yards

1Reservoir Data

For a 2" Schedule 40 chamber (40 inches long):

GGDT Field	Value to Enter	Why
Outer Diameter	2.375 in	2" Sch40 OD
Inner Diameter	0 in	Not a coaxial design
Length	40 in	Chamber length
Initial Pressure	25 psig	Your test pressure
Gas	Air	Standard propellant

Chamber Volume: ~134 cubic inches
V = π × (2.067/2)² × 40 = 134.2 in³

2Barrel Data

For SDR26 1.5" barrel (30 inches long):

GGDT Field	Value to Enter	Why
Bore Diameter	1.754 in	SDR26 1.5" ID
Length	30 in	Barrel length

Barrel Volume: ~72.5 cubic inches
V = π × (1.754/2)² × 30 = 72.5 in³

3Projectile Data

GGDT Field	Value to Enter	Why
Mass	45.93 g	Standard golf ball mass
Diameter	1.68 in	Golf ball = 42.67mm
Friction	0-2 psi	Golf balls have low friction; some blow-by helps

Blow-by Gap: 1.754" - 1.68" = 0.074" total gap
Gap Ratio = (barrel area - ball area) / ball area = ~8.9%
This is a loose fit with significant blow-by!

4Valve Data

For QEV (Quick Exhaust Valve)

GGDT Field	3/4" QEV	1/2" QEV
Valve Type	Barrel Sealing Pilot	Barrel Sealing Pilot
Seat Diameter	1.25 in	0.75 in
Flow Coefficient	45-60%	45%
Opening Time	2-5 ms	2-3 ms
Dwell Time	999 ms	999 ms
Dead Volume	1.178 in³	0.5 in³
Piston Diameter	1.75 in	1.0 in
Piston Mass	8.3 g	4 g
Pilot Volume	0.98 in³	0.5 in³

For Sprinkler Valve (Modified)

GGDT Field	1" Sprinkler (modded)	3/4" Sprinkler
Valve Type	Generic	Generic
Seat Diameter	1.0 in	0.75 in
Flow Coefficient	35-45%	35%
Opening Time	15-30 ms	20-40 ms
Dwell Time	999 ms	999 ms
Dead Volume	2-4 in³	1-2 in³

Critical Note: Sprinkler valves are 50-60ms to open with stock solenoid. Modified with blowgun pilot: 15-30ms. This slow opening drastically reduces performance compared to QEV!

5Run Simulation

After entering all values, click "Calculate" or "Run" in GGDT.

Key Output Values to Compare:

Muzzle Velocity - in ft/s or m/s
Muzzle Energy - in Joules
Exit Pressure - remaining chamber pressure
Time in Barrel - in milliseconds

Expected GGDT Output (approximate):
At 25 psi with SDR26 barrel and QEV, expect ~80-120 fps muzzle velocity.
At 125 psi, expect ~300-400 fps.

5. Valve Data Reference

Valve Opening Times

Valve Type	Opening Time	Performance Impact
QEV (1/2" - 1")	1-5 ms	Excellent - snap-open design
Custom Piston Valve	1-5 ms	Excellent if well designed
Spring Ball Valve	20-30 ms	Good for slow projectiles
Hand Ball Valve	100+ ms	Poor - significant loss
Sprinkler (stock)	50-60 ms	Poor
Sprinkler (modded)	15-30 ms	Moderate

Why Opening Time Matters

For a projectile with 500 fps exit velocity in a 36" barrel, transit time is only ~12 milliseconds. If your valve takes 30ms to open, the projectile exits before the valve is fully open!

GGDT Comparison (same launcher):
30ms valve: 172 m/s, 295.8 J
Fast piston valve: 236.7 m/s, 560.2 J
Nearly 2x the energy just from valve speed!

Flow Coefficient (Cv) Estimation

Use this formula for quick Cv estimates:

Cv = K × D² (where D = seat diameter in cm)

Valve Type	K Value	Example
Sprinkler / Ball Valve	K = 1	1" valve → Cv ≈ 6.5
Piston / QEV	K = 2	1/2" QEV → Cv ≈ 3.4
Burst Disc	K = 3	1" disc → Cv ≈ 19.5

Note: GGDT uses Flow Coefficient as a percentage (0-100%), not the actual Cv number from manufacturer datasheets!

6. Physics Equations Used

Basic Hand Calculation (D_Hall's Method)

For quick estimates without GGDT:

1. Exit Pressure (Isothermal Expansion)

P_exit = P_start × V_chamber / (V_chamber + V_barrel)

2. Muzzle Energy

E = (P_start + P_exit) / 2 × L_barrel × π/4 × D_bore²

3. Muzzle Velocity

v = √(2 × E / m)

Limitations of Basic Equations:

Assumes instant valve opening
Ignores blow-by around projectile
Ignores valve flow restriction
Uses isothermal instead of adiabatic expansion
Ignores friction

These simplifications can lead to 2-5x overestimation of actual performance!

More Accurate Model (What GGDT Does)

Adiabatic Expansion

P_exit = P_start × (V₁/V₂)^γ

Where γ = 1.4 for air (heat capacity ratio)

Work Done by Gas

W = P₁V₁/(γ-1) × [1 - (V₁/V₂)^γ-1]

Valve Flow Restriction

Mass flow rate through valve is limited by:

Valve opening percentage (time-dependent)
Flow coefficient (Cv)
Choked flow at sonic velocity

Blow-by (Projectile Leakage)

Air escaping around projectile reduces effective pressure:

Gap ratio = (A_barrel - A_projectile) / A_projectile
Higher gap ratio = more blow-by = lower efficiency
Effect is amplified at low pressures (slower acceleration = more time for blow-by)

7. Experimental Validation Data

Your Golf Ball Launcher Data

Test Conditions	Measured Result	Notes
25 psi, 40° angle, 30" SDR26 barrel	~25 yards	Plus ~25 yards roll
125 psi, similar setup	~200 yards	Close but shy

Velocity Back-Calculation from Range

For a 40° launch angle and 25 yard (75 ft / 22.9m) range:

Range = v² × sin(2θ) / g
22.9 = v² × sin(80°) / 9.8
v ≈ 15.2 m/s ≈ 50 ft/s

This is a very rough estimate ignoring drag, but suggests muzzle velocity around 50-70 fps at 25 psi.

SpudFiles Community Data

Test	Velocity	Conditions
3/4" Sprinkler	388 fps	40 CI chamber, 5' CPVC, 100 PSI
1/2" QEV	517 fps	Same setup - 33% higher!
4.5mm barrel (tight)	279 fps	Lead BBs, 100 psi
5mm barrel (loose)	242 fps	Same - 33% less energy from blow-by

8. Sources

Primary References

Academic References

Internal ballistics of a pneumatic potato cannon (USNA) - European Journal of Physics, 2009

Other Tools

SpudFiles - Computer Applications - List of other simulation tools

Next Steps

Download and install GGDT from SpudFiles
Enter the golf ball launcher parameters from this guide
Run simulation and note the predicted velocity
Compare with our web calculator results
Use GGDT's numbers to calibrate our physics model

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