BUG: Fix incorrect Jacobian in only_radial_burn branch of SolidMotor.evaluate_geometry (#944)

Copilot · MateusStano · web-flow · commit d9c79bae48a4 · 2026-03-19T23:09:22.000-03:00
* Initial plan

* BUG: Fix incorrect Jacobian in only_radial_burn branch of evaluate_geometry

Co-authored-by: MateusStano &lt;69485049+MateusStano@users.noreply.github.com&gt;

---------

Co-authored-by: copilot-swe-agent[bot] &lt;198982749+Copilot@users.noreply.github.com&gt;
Co-authored-by: MateusStano &lt;69485049+MateusStano@users.noreply.github.com&gt;
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -80,6 +80,8 @@ Attention: The newest changes should be on top -->
 - BUG: Fix parallel Monte Carlo simulation showing incorrect iteration count [#806](https://github.com/RocketPy-Team/RocketPy/pull/806)
 - BUG: Fix missing titles in roll parameter plots for fin sets [#934](https://github.com/RocketPy-Team/RocketPy/pull/934)
 - BUG: Duplicate _controllers in Flight.TimeNodes.merge() [#931](https://github.com/RocketPy-Team/RocketPy/pull/931)
+- BUG: Fix incorrect Jacobian in `only_radial_burn` branch of `SolidMotor.evaluate_geometry` [#935](https://github.com/RocketPy-Team/RocketPy/pull/935)
+- BUG: Add explicit timeouts to ThrustCurve API requests [#935](https://github.com/RocketPy-Team/RocketPy/pull/935)
 
 ## [v1.11.0] - 2025-11-01
 
diff --git a/rocketpy/motors/solid_motor.py b/rocketpy/motors/solid_motor.py
@@ -546,13 +546,14 @@ def geometry_jacobian(t, y):
                     2 * np.pi * (grain_inner_radius * grain_height) ** 2
                 )
 
-                inner_radius_derivative_wrt_inner_radius = factor * (
-                    grain_height - 2 * grain_inner_radius
-                )
-                inner_radius_derivative_wrt_height = 0
+                # burn_area = 2π*r*h, so ṙ = -vdiff/(2π*r*h):
+                #   ∂ṙ/∂r = vdiff/(2π*r²*h) = factor * h
+                #   ∂ṙ/∂h = vdiff/(2π*r*h²) = factor * r
+                inner_radius_derivative_wrt_inner_radius = factor * grain_height
+                inner_radius_derivative_wrt_height = factor * grain_inner_radius
+                # dh/dt = 0, so all partial derivatives of height are zero
                 height_derivative_wrt_inner_radius = 0
                 height_derivative_wrt_height = 0
-                # Height is a constant, so all the derivatives with respect to it are set to zero
 
                 return [
                     [
