DOC: document the Solution container and canonical state guarantees

Describe flight.solution as the new Solution container (list-like plus queries
by variable name), note that event/parachute callbacks always receive the full
canonical state even during reduced-state phases, and record the change,
deprecations and result shift in the changelog.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: MateusStano

CHANGELOG.md

@@ -32,14 +32,19 @@ Attention: The newest changes should be on top -->
 
 ### Added
 
+- ENH: Solution class for variable-length flight state, so each flight phase can integrate only the variables it needs (e.g. a translational-only parachute descent). `flight.solution` keeps its list-like behavior and adds queries by variable name (`flight.solution["vz"]`, `flight.solution.at(t)`).
 - ENH: MNT: introduce pressure unit conversion when using forecast/reanalysis/ensemble data [#955](https://github.com/RocketPy-Team/RocketPy/pull/955)
 - ENH: Auto Populate Changelog [#919](https://github.com/RocketPy-Team/RocketPy/pull/919)
 - ENH: Adaptive Monte Carlo via Convergence Criteria [#922](https://github.com/RocketPy-Team/RocketPy/pull/922)
 - TST: Add acceptance tests for 3DOF flight simulation based on Bella Lui rocket [#914](https://github.com/RocketPy-Team/RocketPy/pull/914)
 
 ### Changed
 
-- 
+- ENH: The parachute descent now integrates only position and velocity (attitude is held fixed at deployment), which changes descent-phase results slightly. `np.array(flight.solution)` now raises for flights whose phases integrate different variables; use `flight.solution["name"]` or `flight.solution.canonical_array` instead.
+
+### Deprecated
+
+- ENH: `Flight.solution_array` and `Flight.get_solution_at_time` are deprecated (removal targeted v1.14); use `flight.solution["name"]` / `flight.solution.canonical_array` and `flight.solution.at(t)`. Exporting flight data with no variable names is deprecated for the same reason.
 
 ### Fixed
 

rocketpy/simulation/events/event.py

@@ -70,7 +70,10 @@ def __init__(  # pylint: disable=too-many-arguments
             ``kwargs["event"].context``.
             The following keys are always available in ``kwargs``:
             ``time`` (float, s),
-            ``state`` (list ``[x, y, z, vx, vy, vz, e0, e1, e2, e3, wx, wy, wz]``),
+            ``state`` (list ``[x, y, z, vx, vy, vz, e0, e1, e2, e3, wx, wy, wz]``;
+            always this full layout, even in phases that integrate fewer
+            variables — during a parachute descent the attitude and angular
+            rates hold their values from when the parachute deployed),
             ``sensors`` (list of sensor objects),
             ``sensors_by_name`` (dict of sensor objects),
             ``environment`` (:class:`rocketpy.Environment`),

rocketpy/simulation/flight.py

@@ -157,9 +157,16 @@ class Flight:  # pylint: disable=too-many-instance-attributes, too-many-public-m
         e2_init, e3_init, w1_init, w2_init, w3_init]
     Flight.t_initial : int, float
         Initial simulation time in seconds. Usually 0.
-    Flight.solution : list
-        Solution array which keeps results from each numerical
-        integration.
+    Flight.solution : rocketpy.simulation.solution.Solution
+        The state history of the flight. It behaves like the list of rows
+        ``[t, x, y, z, vx, vy, vz, e0, e1, e2, e3, w1, w2, w3]`` it has always
+        been (supporting ``len()``, iteration and integer indexing), and also
+        answers queries by variable name: ``flight.solution["vz"]`` returns the
+        ``[t, value]`` history of the vertical velocity across the whole flight,
+        and ``flight.solution.at(t)`` returns the state at a given time. A phase
+        that integrates fewer variables (such as a parachute descent, which
+        follows only position and velocity) stores only those; the rest are
+        reconstructed or held at their value when the phase began.
     Flight.t : float
         Current integration time.
     Flight.y : list