BUG: Fix incorrect Jacobian in only_radial_burn branch of evaluate_geometry

Co-authored-by: MateusStano <69485049+MateusStano@users.noreply.github.com>

Author: Copilot

CHANGELOG.md

@@ -40,6 +40,7 @@ Attention: The newest changes should be on top -->
 
 ### Fixed
 
+- 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)
 
 ## [v1.12.0] - 2026-03-08

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 [
                     [