Aerodynamic Surfaces#

This page provides an overview of the aerodynamic surfaces available in RocketPy and explains how they connect to the rocket’s simulation.

RocketPy models the aerodynamic forces and moments generated by three types of surfaces: nose cones, fins, and tails. Each surface is defined by its geometric parameters and, optionally, by an airfoil profile. The aerodynamic coefficients are computed internally using the Barrowman method and are used during the flight simulation to evaluate the rocket’s stability and control.

See also

For the full mathematical derivations of the Barrowman method applied to individual fins, see Individual Fin Model.

For the elliptical fin equations, see Elliptical Fins Equations.

For roll dynamics and cant angle conventions, see Roll Equations.

For the coordinate systems used by the Rocket class, see Rocket Class Axes Definitions and Positions and Coordinate Systems.

Nose Cones#

The nose cone is the forward-most aerodynamic surface of the rocket. It contributes to the normal force and the center of pressure position.

Available Nose Cone Kinds#

RocketPy supports the following nose cone shapes, specified via the kind parameter:

  • "conical": A simple cone with a straight taper from base to tip.

  • "ogive": A tangent ogive shape, defined by the ratio of the tip radius to the base radius (the bluffness parameter).

  • "elliptical": An elliptical cross-section nose cone.

  • "tangent": A tangent ogive with a rounded tip.

  • "von karman": A Von Karman ogive, a common choice for supersonic rockets.

  • "parabolic": A parabolic nose cone shape.

  • "powerseries": A power series nose cone, controlled by the power parameter (must be between 0 and 1). When this kind is used, the bluffness parameter must be None or 0.

  • "lvhaack": A LV-Haack series nose cone.

Note

The bluffness parameter controls the ratio between the radius of the rounded tip and the radius of the base. It is only used for ogive nose cones. For all other kinds, it should be None or 0.

Caution

When kind="powerseries", the power parameter is required and must satisfy 0 < power <= 1. A value of 1 produces a conical shape, while lower values produce blunter shapes.

Geometric Parameters#

All nose cones share the following parameters:

  • length: The length of the nose cone along the rocket’s centerline.

  • base_radius: The radius of the nose cone at its base, which must match the rocket’s radius at the nose cone position.

  • position: The position of the nose cone along the rocket’s centerline, relative to the coordinate system origin. See Positions and Coordinate Systems for details.

The center of pressure of the nose cone is computed internally and contributes to the overall aerodynamic model. The normal force coefficient derivative (CLalpha) depends on the nose cone kind and geometry.

See also

For the class API, see rocketpy.NoseCone.

Fins#

Fins are the primary stabilizing surfaces of a rocket. RocketPy supports three fin geometries and two ways to define them: as a fin set (the usual approach) or as an individual fin.

Fin Sets vs. Individual Fins#

RocketPy distinguishes between two levels of fin definition:

See also

For the mathematical model of individual fins, including the moment coefficient derivations, see Individual Fin Model.

Fin Geometries#

Trapezoidal Fins#

Trapezoidal fins are defined by a root chord, tip chord, span, and optionally a sweep length. They are the most common fin geometry in model rocketry.

Parameters:

  • root_chord: The chord length at the fin’s root (where it meets the rocket body).

  • tip_chord: The chord length at the fin’s tip. Setting this to 0 produces a triangular fin.

  • span: The span of the fin, measured from the rocket body to the fin tip.

  • sweep_length (optional): The distance from the root chord leading edge to the tip chord leading edge, measured along the rocket’s centerline. If not provided, it is computed from the root chord, tip chord, and span.

  • cant_angle (optional): The angle at which the fin is canted relative to the rocket’s centerline, in radians. A positive cant angle produces a negative roll moment at zero angle of attack.

Caution

The cant_angle convention is important for control surface design. See Roll Equations for details on how cant angles affect roll, pitch, and yaw moments.

Elliptical Fins#

Elliptical fins are defined by a root chord and a span. The chord length varies elliptically from root to tip.

Parameters:

  • root_chord: The chord length at the fin’s root.

  • span: The span of the fin.

See also

For the full mathematical derivation, see Elliptical Fins Equations.

Free Form Fins#

Free form fins allow arbitrary fin shapes defined by a list of (x, y) coordinates. This is useful for non-standard fin geometries that cannot be represented by trapezoidal or elliptical shapes.

Parameters:

  • coordinates: A list of (x, y) tuples defining the fin shape in the fin coordinate frame.

Common Fin Set Parameters#

In addition to the geometry-specific parameters above, all fin sets share the following:

  • n: The number of fins in the set (for fin set classes only).

  • position: The position of the fin set along the rocket’s centerline, relative to the coordinate system origin.

  • angular_position (optional): The roll angle of the first fin in the set, in degrees. Measured from the y-axis of the rocket’s coordinate system.

  • rocket_radius: The radius of the rocket at the fin’s position. This is usually set automatically when adding fins to a rocket.

  • airfoil (optional): A tuple (path, units) specifying an airfoil profile. See Airfoil Profiles below.

Airfoil Profiles#

Fin aerodynamic coefficients can be enhanced by specifying an airfoil profile. Without an airfoil, fins are treated as flat plates.

The airfoil parameter accepts a tuple of the form (path, units):

  • path: Path to a CSV file containing the airfoil’s lift coefficient curve. The first column is the angle of attack, the second column is the lift coefficient (CL).

  • units: The unit of the angle of attack in the CSV file. Must be either "radians" or "degrees".

The CSV file should contain angle of attack points up to the stall point. The data is used to compute the fin’s CLalpha (normal force coefficient derivative) during the simulation.

Note

If airfoil is not provided or is None, the fin is modeled as a flat plate. This is a reasonable approximation for many model rockets but may underestimate the normal force coefficient.

See also

Airfoil data files can be obtained from Airfoil Tools.

Adding Fins to a Rocket#

Fins can be added to a rocket using the Rocket class methods:

from rocketpy import Rocket, TrapezoidalFins

rocket = Rocket(
    radius=0.0635,
    mass=14.426,
    inertia=(6.321, 6.321, 0.034),
    power_off_drag="data/powerOffDragCurve.csv",
    power_on_drag="data/powerOnDragCurve.csv",
    center_of_mass_without_motor=0,
    coordinate_system_orientation="tail_to_nose",
)

fin_set = rocket.add_trapezoidal_fins(
    n=4,
    root_chord=0.120,
    tip_chord=0.060,
    span=0.110,
    position=-1.04956,
    cant_angle=0.0,
    airfoil=("data/airfoils/NACA0012-radians.txt", "radians"),
)

See also

Tail#

The tail is a transitional surface at the aft end of the rocket, typically used to reduce base drag. It is defined by a top radius, bottom radius, and length.

Parameters:

  • top_radius: The radius of the tail at its top (where it meets the rocket body).

  • bottom_radius: The radius of the tail at its bottom (aft end).

  • length: The length of the tail along the rocket’s centerline.

  • position: The position of the tail along the rocket’s centerline.

The tail contributes to the normal force and center of pressure calculation. Its effect is generally smaller than that of the nose cone and fins.

See also

For the class API, see rocketpy.Tail.

Generic Surfaces#

For advanced use cases, RocketPy provides the rocketpy.GenericSurface and rocketpy.LinearGenericSurface classes. These allow you to specify aerodynamic force and moment coefficients directly, rather than relying on the built-in geometric models.

This is useful when:

  • You have aerodynamic data from CFD simulations or wind tunnel tests.

  • You want to model aerodynamic surfaces that do not fit the built-in geometries (e.g., canards with unusual shapes, air brakes).

See also

For more information, see Generic Surfaces and Custom Aerodynamic Coefficients.

Note

This section provides a brief overview of generic surfaces. A more detailed treatment of coordinate frames and coefficient conventions for generic surfaces is available in the referenced user guide page.

Reference Area and Length#

The aerodynamic forces and moments computed by each surface are scaled by a reference area and a reference length:

  • Reference area: The cross-sectional area of the rocket, computed as \(\pi r^2\) where \(r\) is the rocket’s radius.

  • Reference length: The diameter of the rocket (\(2r\)).

These values are used consistently across all aerodynamic surfaces and are set automatically from the rocket’s radius.

See also

For more details on how forces and moments are applied during the simulation, see rocketpy.Rocket and rocketpy.Flight.

What’s Next#

This page covers the available surface types and their parameters. For deeper understanding of the underlying math:

For coordinate system conventions: