Rocket Drag Coefficient Calculator
Estimate total Cd by summing nose, body, base, fin, and interference drag using semi-empirical methods from Hoerner and Mandell/Caporaso/Bengen.
About this calculator
This calculator builds up a rocket's total drag coefficient piece by piece rather than treating drag as one black-box number. It first computes a Reynolds number from your total airframe length and flight velocity, then derives a turbulent-flow skin friction coefficient using the Schlichting formula (0.455/log10(Re)^2.58) — the standard turbulent flat-plate correlation from Hermann Schlichting's textbook "Boundary-Layer Theory," a staple reference in aerodynamics engineering — adjusted upward for rougher surface finishes like unpainted cardboard. That friction coefficient is applied to the wetted (surface) area of the nose cone and body tube separately, and combined with a pressure-drag term specific to your chosen nose shape — ogive, conical, or Von Karman — each carrying a different baseline coefficient reflecting how cleanly it parts the airflow. Fin drag combines its own friction contribution with a pressure term driven by the thickness-to-chord ratio (thicker fin stock relative to chord length costs more drag), then an interference term adds roughly 5% of fin drag per fin to account for the turbulent junction where fins meet the body tube.
Base drag from the blunt aft end and protuberance drag from launch lugs or rail buttons are held at fixed typical values rather than derived from your specific geometry. All of this is combined into a total Cd referenced to the body's cross-sectional area, then converted into an actual drag force at your specified velocity. Because the fin wetted-area calculation treats fins as simple rectangles and several terms use representative constants, treat the result as a solid subsonic estimate for design comparison — not a wind-tunnel-grade figure, and not valid once the rocket approaches transonic speeds.
Inputs
Results
Total Cd
0.89
Figures current as of 2000. Source: Hermann Schlichting (Klaus Gersten, rev.), Boundary-Layer Theory, 8th/9th ed. (Springer)
How to Use This Calculator
- Enter Body diameter (mm), Body tube length (mm), and Nose cone length (mm).
- Select the Nose type, then set Number of fins and Fin root chord (mm).
- Adjust Fin semi-span (mm), Fin thickness (mm) as needed.
- Review the Total Cd result.
- Use Nose cone Cd and Body tube Cd to inform your decision.
How the result changes with Body diameter (mm)
| Body diameter (mm) | Total Cd |
|---|---|
| 15 | 2.36 |
| 22 | 1.31 |
| 44 | 0.55 |
| 73 | 0.36 |
What each input means
- Body diameter (mm)
- Outside diameter of the body tube.
- Body tube length (mm)
- Total length of body tube (excluding nose cone).
- Nose cone length (mm)
- Length from nose tip to body shoulder.
- Nose type
- Shape of the nose cone.
- Number of fins
- Number of fins. Fewer fins = less drag.
- Fin root chord (mm)
- Length of fin at the body tube.
- Fin semi-span (mm)
- Distance from body tube to fin tip.
- Fin thickness (mm)
- Thickness of fin stock material.
- Flight velocity (m/s)
- Velocity for drag force calculation. Use expected max velocity.
- Surface finish
- Surface finish of the airframe, which affects skin friction drag.
What each result means
- Total Cd
- Total drag coefficient referenced to body cross-section area. Typical rockets: 0.3-0.8.
- Nose cone Cd
- Drag contribution from the nose cone (pressure + friction).
- Body tube Cd
- Skin friction drag from the body tube.
- Base drag Cd
- Drag from the blunt base (wake pressure).
- Fin Cd
- Combined fin friction and pressure drag.
- Drag force at speed (N)
- Total aerodynamic drag force at the specified velocity.
- Reynolds number
- Flow regime indicator. Most model rockets operate in turbulent flow (Re > 500,000).
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersBody diameter (mm) = 29, Body tube length (mm) = 400, Nose cone length (mm) = 80, Nose type = 1 = 10 input(s) provided
- Calculate Total CdTotal Cd = CdNose + CdBody + CdBase + CdFins + CdInterference + CdProtuberance0.8932 = 0.8932
- Calculate Nose cone CdNose cone Cd = CdNosePressure + CdNoseFriction0.0493 = 0.0493
- Calculate Body tube CdBody tube Cd = Cf * (bodyWettedArea / Aref)0.2387 = 0.2387
Figures and sources
- Schlichting turbulent flat-plate skin-friction coefficient correlation (2000) — Hermann Schlichting (Klaus Gersten, rev.), Boundary-Layer Theory, 8th/9th ed. (Springer)
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the Von Karman nose have the lowest pressure-drag coefficient in the calculator even though it's built for supersonic flight?
The calculator assigns each nose shape a fixed baseline pressure-drag coefficient (0.025 for ogive, 0.035 for conical, 0.02 for Von Karman) based on how cleanly each shape parts subsonic airflow. Von Karman shapes are optimized to minimize wave drag near and above the speed of sound, but that same smooth profile also happens to produce low pressure drag at subsonic speeds, which is why it shows up as the lowest of the three baseline values here even though the calculator itself is only valid for subsonic flight.
Why does adding more fins increase drag by more than just the extra fin area?
Each additional fin adds its own friction and pressure drag, but it also adds interference drag — modeled here as 5% of total fin Cd multiplied by the fin count — to represent the extra turbulent airflow junctions where each fin meets the body tube. So going from 3 fins to 4 doesn't just add one fin's worth of drag; it also increases the interference penalty because there's one more root-tube junction disturbing the airflow.
Why does a rougher surface finish increase drag even though I haven't changed the rocket's shape?
Surface roughness affects the skin friction coefficient directly, not the pressure-drag terms. The calculator multiplies the Schlichting turbulent skin-friction coefficient by a roughness factor — 1.0 for smooth/painted, 1.15 for typical paint, 1.4 for rough unpainted cardboard — so a rougher airframe increases the friction contribution to both the nose cone and body tube Cd even though the geometry inputs stay identical.
Why shouldn't I trust this Cd estimate near the speed of sound?
The entire model — the Schlichting skin-friction formula, the fixed nose-shape pressure coefficients, and the simplified rectangular fin wetted-area calculation — is built for subsonic, incompressible airflow. As a rocket approaches transonic speeds, shock waves and compressibility effects change drag behavior in ways this calculator doesn't model at all, so treat the output as a subsonic design-comparison estimate and expect it to diverge sharply once you're near Mach 0.8-1.0.
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