Add ClusterMotor for symmetric multi-engine clusters (port of upstream #924)#4
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April 25, 2026 17:28
Port of upstream RocketPy PR RocketPy-Team#924 (RocketPy-Team#924). ClusterMotor wraps a single SolidMotor and replicates it N>=2 times symmetrically around a circle of radius R. Inherits from Motor so it plugs into Rocket.add_motor() unchanged. Inertia is aggregated using the parallel-axis theorem (Huygens-Steiner): Izz = N*Izz_single + N*m*R^2; Ixx = Iyy = N*I_trans + 0.5*N*m*R^2. Thrust, dry mass, and propellant mass scale linearly with N. Files: - rocketpy/motors/cluster_motor.py (new): ClusterMotor class. - rocketpy/motors/__init__.py: export ClusterMotor. - rocketpy/__init__.py: re-export at package root. - rocketpy/plots/rocket_plots.py: render N nozzles/grains around the circle for cluster motors. - tests/integration/motors/test_cluster_motor.py (new): PR test suite. Notes: - The PR's own test_cluster_mass_and_thrust_scaling and test_cluster_propellant_inertia_dynamic fail with "ValueError: truth value of an array..." on rocketpy.Function comparisons. This is a PR-side test bug (np.isclose against a Function object) and not specific to this port. Cluster construction, geometry, dry inertia, methods, and setters tests all pass (4/6). - When upstream PR RocketPy-Team#924 merges, swap this commit for the upstream version during the next fork sync.
The two failing assertions compared rocketpy.Function objects against scalars. np.isclose triggers Function.__array__, returning the source array, which makes np.isclose return an array and crashes assert with "truth value of an array with more than one element is ambiguous". Call propellant_mass(0), propellant_I_11(t), propellant_I_33(t) so np.isclose receives scalars. Matches the pattern already used at line 61 (cluster.thrust(1)). All 6 cluster tests pass after this fix.
raytracerfvf
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May 9, 2026
…e, unused-argument) Three issues introduced when ClusterMotor (PR #4) and the parachute trigger signature tests landed: 1. Cyclic import in `rocketpy/motors/cluster_motor.py`: importing `Function` and `Motor` from the package facade re-entered `rocketpy.__init__` while the module was still being initialized. Switch to canonical module paths (`rocketpy.mathutils.function`, `rocketpy.motors.motor`). 2. `not-callable` on `cluster.propellant_mass(0)` and `cluster.propellant_I_11(t)` in `tests/integration/motors/test_cluster_motor.py`: a later test in the same file assigned a raw `float` through the property setter (`cluster.propellant_mass = 50.0`), which made pylint infer the attribute as `float | Function` for every read in the file. Exercise the setters with values of the documented `Function` type instead. 3. `unused-argument` on parachute trigger lambdas in `tests/unit/rocket/test_parachute.py`: `Parachute.__evaluate_trigger_function` introspects only `len(sig.parameters)` (parachute.py:300), so parameter names are irrelevant. Rename unused trigger params to `_p`/`_h`/`_y`/ `_sensors` to match the project's `dummy-variables-rgx` convention. Repo-wide pylint score: 9.98 → 10.00.
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Summary
Port of the open upstream RocketPy PR RocketPy-Team/RocketPy#924 onto our v1.12.0-synced fork. Adds a new `ClusterMotor` class that wraps a `SolidMotor` and replicates it `N >= 2` times symmetrically around a circle of radius `R`. Because it inherits `Motor`, it slots into `Rocket.add_motor()` unchanged.
Approach
Skipped from upstream PR
Known issues
The PR's own tests `test_cluster_mass_and_thrust_scaling` and `test_cluster_propellant_inertia_dynamic` fail with `ValueError: truth value of an array with more than one element is ambiguous` because they use `np.isclose` against `rocketpy.Function` objects directly instead of evaluating them. This is a bug in the upstream PR test code, not in our port — cluster construction, geometry, dry inertia, methods, and setters tests all pass (4/6). Reported here so we can re-verify when we re-sync after upstream PR RocketPy-Team#924 merges.
Test plan
Follow-up
When upstream RocketPy#924 merges, swap this commit for the upstream version during the next fork sync.