ENH: Rocket Axes Definition

.vscode/settings.json

@@ -14,12 +14,14 @@
     "cSpell.language": "en-US",
     "cSpell.words": [
         "Abdulklech",
+        "addsurface",
         "adjugate",
         "akima",
         "allclose",
         "altitudemode",
         "Alves",
         "amax",
+        "angularpos",
         "arange",
         "arccos",
         "arcsin",
@@ -39,6 +41,7 @@
         "axvline",
         "axvspan",
         "Barrowman",
+        "Baruh",
         "behaviour",
         "Bigl",
         "Bigr",
@@ -77,6 +80,7 @@
         "csys",
         "datapoints",
         "datetime",
+        "dcsys",
         "ddot",
         "deepcopy",
         "deletechars",
@@ -105,6 +109,7 @@
         "figsize",
         "filt",
         "firstsimulation",
+        "flightcsys",
         "Fluxogram",
         "fmax",
         "fmin",
@@ -130,6 +135,7 @@
         "Gonçalvez",
         "grav",
         "Guilherme",
+        "Haim",
         "hemis",
         "hgtprs",
         "hgtsfc",

CHANGELOG.md

@@ -32,6 +32,7 @@ Attention: The newest changes should be on top -->
 
 ### Added
 
+-  ENH: Rocket Axis Definition [#635](https://github.com/RocketPy-Team/RocketPy/pull/635)
 
 ### Changed
 

docs/user/index.rst

@@ -14,7 +14,7 @@ RocketPy's User Guide
 
    Positions and Coordinate Systems <positions.rst>
    Motors <motors/motors.rst>
-   Rocket <rocket.rst>
+   Rocket <rocket/rocket.rst>
    Environment <environment.rst>
 
 .. toctree::

docs/user/rocket/rocket.rst

@@ -0,0 +1,15 @@
+Rocket Usage
+============
+
+.. toctree::
+    :maxdepth: 2
+    :caption: Rocket Class Usage
+
+    Rocket Class Usage <rocket_usage.rst>
+
+.. toctree::
+    :maxdepth: 3
+    :caption: Rocket Class Axes Definitions
+
+    Rocket Class Axes Definitions <rocket_axes.rst>
+

docs/user/rocket/rocket_axes.rst

@@ -0,0 +1,156 @@
+Rocket Class Axes Definitions
+=============================
+
+The Rocket class has two different coordinate systems:
+
+1. **User Defined Coordinate System**: Used for geometrical inputs of the \
+   aerodynamic surfaces and motor.
+2. **Body Axes Coordinate System**: Used during the flight simulation to assess \
+   the governing equations of motion.
+
+All inputs are automatically converted from the user's coordinate system to the
+rocket body axes coordinate system for use during the simulation.
+
+Let's dive into the definitions of these coordinate systems:
+
+
+1. User Defined Coordinate System
+---------------------------------
+
+Two things are set by the user in the user input coordinate system:
+
+1. **Coordinate System Origin**: The origin of the coordinate system is set at \
+   any point along the rocket's center line. This point can be arbitrarily chosen \
+   and is not explicitly defined. All inputs must be given relative to this \
+   point.
+2. **Direction of Center Axis**: Specified by the ``coordinate_system_orientation`` \
+   argument when initializing the Rocket (:class:`rocketpy.Rocket.__init__`). This \
+   argument defines the direction of the axis that follows the rocket's center \
+   line. It can be either ``"nose_to_tail"`` or ``"tail_to_nose"``.
+
+.. tip::
+
+   If you are using some CAD software to design your rocket, you can imagine the \
+   coordinate system as the one used in the CAD software. The origin of the \
+   coordinate system is the origin of the rocket in the CAD software, and the \
+   direction of the center axis is the direction of the rocket's centerline. \
+   You don't need to worry about the exact position of the origin, as long as \
+   all inputs are given relative to this point.
+
+.. seealso:: 
+
+   See `Positions and Coordinate Systems <positions.rst>`_ for more \
+   information on the definitions of the rocket's aerodynamic surfaces and motor.
+
+The ``x`` and ``y`` axes are defined at the plane perpendicular to the center axis,
+while the ``z`` axis is defined along the center axis. Depending on the choice of
+``coordinate_system_orientation``, the ``x`` axis and ``y`` axis can be inverted.
+
+The following figure shows the two possibilities for the user input coordinate system:
+
+.. figure:: ../../static/rocket/3dcsys.png
+  :align: center
+  :alt: Rocket axes
+
+.. note::
+
+   When ``coordinate_system_orientation`` is set to ``"tail_to_nose"``, the direction \
+   of the ``x``, ``y``, and ``z`` axes of the **User Defined Coordinate System** is \
+   the same as the **Body Axes Coordinate System**. The origin of the coordinate \
+   system may still be different.
+
+Angular Position Inputs
+~~~~~~~~~~~~~~~~~~~~~~~
+
+Angular position inputs (``angular_position``) refer to the roll angle position
+of that surface along the rocket's tube. The roll angle is defined as the angle 
+from the ``y`` axis to the surface.
+Currently, only the :class:`rocketpy.RailButtons` class uses this kind of input.  
+
+The following figure shows the roll angle
+definition for both ``coordinate_system_orientation`` options:
+
+.. figure:: ../../static/rocket/angularpos.png
+  :align: center
+  :alt: Angular position
+
+
+.. note::
+
+   The positive direction of the roll angle is defined as the direction that \
+   rotates the surface in the positive direction of the ``z`` axis.
+
+.. _rocket_axes_body_axes:
+
+2. Body Axes Coordinate System
+------------------------------
+
+The body axes coordinate system is used inside the simulation to assess the
+governing equations of motion. The body axes coordinate system is defined as follows:
+
+- The origin is at the rocket's center of dry mass (``center_of_dry_mass_position``).
+- The ``z`` axis is defined along the rocket's centerline, pointing from the center of dry mass towards the nose.
+- The ``x`` and ``y`` axes are perpendicular.
+
+3. Relation to Flight Coordinates
+---------------------------------
+
+The ``Flight`` class uses a coordinate system defined as follows:
+
+- The origin is at the launch rail.
+- The ``Z`` axis is positive upwards.
+- The ``X`` axis is position eastwards.
+- The ``Y`` axis is positive northwards.
+
+The following figure shows the rotational relationship between the
+**Body Axes Coordinate System** and the **Flight Coordinate System**:
+
+.. figure:: ../../static/rocket/flightcsys.png
+  :align: center
+  :alt: Flight coordinate system
+
+In the figure above, :math:`\bf{i}` is the ``inclination`` and :math:`\bf{h}`
+is the ``heading`` of the launch rail.
+
+The heading and inclination can be described in terms of Euler angles.
+The relation is given by:
+
+.. math::
+    \begin{aligned}
+        &\text{Precession:} \quad &\psi &= -\bf{h} \\
+        &\text{Nutation:} \quad &\theta &= \bf{i} - 90° \\
+   \end{aligned}
+
+A last rotation is defined by the ``angular_position`` of the rocket's rail buttons.
+This is a rotation around the rocket's centerline, and describes the last 
+Euler angle:
+
+.. math::
+    \begin{aligned}
+        &\text{Spin:} \quad &φ & \\
+    \end{aligned}
+
+If no rail buttons pair ir present, the spin angle is set to **0°**.
+
+.. note::
+
+   With spin angle set to **0°**, if the launch rail ``heading`` is set to \
+   **0°** and rail ``inclination`` to **90°**, the **Body Axes Coordinate \
+   System** is aligned with the **Flight Coordinate System**.
+
+Rocket's initial orientation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The initial orientation of the rocket is expressed in Euler parameters (quaternions).
+The Euler parameters are defined using the 3-1-3 rotation sequence:
+
+.. math::
+
+   \begin{aligned}
+      e_{0} &= \cos\left(\frac{φ}{2}\right) \cos\left(\frac{θ}{2}\right) \cos\left(\frac{ψ}{2}\right) - \sin\left(\frac{φ}{2}\right) \cos\left(\frac{θ}{2}\right) \sin\left(\frac{ψ}{2}\right) \\
+      e_{1} &= \cos\left(\frac{φ}{2}\right) \cos\left(\frac{ψ}{2}\right) \sin\left(\frac{θ}{2}\right) + \sin\left(\frac{φ}{2}\right) \sin\left(\frac{θ}{2}\right) \sin\left(\frac{ψ}{2}\right) \\
+      e_{2} &= \cos\left(\frac{φ}{2}\right) \sin\left(\frac{θ}{2}\right) \sin\left(\frac{ψ}{2}\right) - \sin\left(\frac{φ}{2}\right) \cos\left(\frac{ψ}{2}\right) \sin\left(\frac{θ}{2}\right) \\
+      e_{3} &= \cos\left(\frac{φ}{2}\right) \cos\left(\frac{θ}{2}\right) \sin\left(\frac{ψ}{2}\right) + \cos\left(\frac{θ}{2}\right) \cos\left(\frac{ψ}{2}\right) \sin\left(\frac{φ}{2}\right) \\
+   \end{aligned}
+
+

docs/user/rocket.rst → docs/user/rocket/rocket_usage.rst

@@ -209,7 +209,7 @@ to the rocket in one step:
 
 .. caution::
 
-    Once again, pay special attention to the ``position`` parameter. Check
+    Once again, pay special attention to the ``position`` parameter. Check \
     the :meth:`rocketpy.Rocket.add_surfaces` method for more information.
 
 .. seealso::

rocketpy/mathutils/vector_matrix.py

@@ -955,6 +955,10 @@ def transformation(quaternion):
         -------
         Matrix
             The transformation matrix from frame B to frame A.
+
+        Reference
+        ---------
+        https://en.wikipedia.org/wiki/Conversion_between_quaternions_and_Euler_angles
         """
         q_w, q_x, q_y, q_z = quaternion
         return Matrix(

rocketpy/rocket/aero_surface/fins/elliptical_fins.py

@@ -13,9 +13,11 @@ class EllipticalFins(Fins):
 
     Note
     ----
-    Local coordinate system: Z axis along the longitudinal axis of symmetry,
-    positive downwards (top -> bottom). Origin located at the top of the root
-    chord.
+    Local coordinate system:
+        - Origin located at the top of the root chord.
+        - Z axis along the longitudinal axis of symmetry, positive downwards (top -> bottom).
+        - Y axis perpendicular to the Z axis, in the span direction, positive upwards.
+        - X axis completes the right-handed coordinate system.
 
     See Also
     --------

rocketpy/rocket/aero_surface/fins/fins.py

@@ -11,9 +11,11 @@ class Fins(AeroSurface):
 
     Note
     ----
-    Local coordinate system: Z axis along the longitudinal axis of symmetry,
-    positive downwards (top -> bottom). Origin located at the top of the root
-    chord.
+    Local coordinate system:
+        - Origin located at the top of the root chord.
+        - Z axis along the longitudinal axis of symmetry, positive downwards (top -> bottom).
+        - Y axis perpendicular to the Z axis, in the span direction, positive upwards.
+        - X axis completes the right-handed coordinate system.
 
     Attributes
     ----------

rocketpy/rocket/aero_surface/fins/trapezoidal_fins.py

@@ -13,9 +13,11 @@ class TrapezoidalFins(Fins):
 
     Note
     ----
-    Local coordinate system: Z axis along the longitudinal axis of symmetry,
-    positive downwards (top -> bottom). Origin located at the top of the root
-    chord.
+    Local coordinate system:
+        - Origin located at the top of the root chord.
+        - Z axis along the longitudinal axis of symmetry, positive downwards (top -> bottom).
+        - Y axis perpendicular to the Z axis, in the span direction, positive upwards.
+        - X axis completes the right-handed coordinate system.
 
     See Also
     --------

rocketpy/rocket/aero_surface/nose_cone.py

@@ -15,9 +15,9 @@ class NoseCone(AeroSurface):
 
     Note
     ----
-    The local coordinate system has the origin at the tip of the nose cone
-    and the Z axis along the longitudinal axis of symmetry, positive
-    downwards (top -> bottom).
+    Local coordinate system:
+        - the origin at the tip of the nose cone and
+        - the Z axis along the longitudinal axis of symmetry, positive downwards (top -> bottom).
 
     Attributes
     ----------

rocketpy/rocket/aero_surface/rail_buttons.py

@@ -1,3 +1,5 @@
+import numpy as np
+
 from rocketpy.mathutils.function import Function
 from rocketpy.prints.aero_surface_prints import _RailButtonsPrints
 
@@ -15,6 +17,8 @@ class RailButtons(AeroSurface):
         Angular position of the rail buttons in degrees measured
         as the rotation around the symmetry axis of the rocket
         relative to one of the other principal axis.
+    RailButtons.angular_position_rad : float
+        Angular position of the rail buttons in radians.
     """
 
     def __init__(
@@ -40,11 +44,6 @@ def __init__(
             Radius of the rocket at the location of the rail buttons in meters.
             If not provided, it will be calculated when the RailButtons object
             is added to a Rocket object.
-
-        Returns
-        -------
-        None
-
         """
         super().__init__(name, None, None)
         self.buttons_distance = buttons_distance
@@ -56,6 +55,10 @@ def __init__(
 
         self.prints = _RailButtonsPrints(self)
 
+    @property
+    def angular_position_rad(self):
+        return np.radians(self.angular_position)
+
     def evaluate_center_of_pressure(self):
         """Evaluates the center of pressure of the rail buttons. Rail buttons
         do not contribute to the center of pressure of the rocket.

rocketpy/rocket/aero_surface/tail.py

@@ -12,9 +12,9 @@ class Tail(AeroSurface):
 
     Note
     ----
-    Local coordinate system: Z axis along the longitudinal axis of symmetry,
-    positive downwards (top -> bottom). Origin located at top of the tail
-    (generally the portion closest to the rocket's nose).
+    Local coordinate system:
+        - Z axis along the longitudinal axis of symmetry, positive downwards (top -> bottom).
+        - Origin located at top of the tail (generally the portion closest to the rocket's nose).
 
     Attributes
     ----------