From 34353ee23f5a678c929b13890b25c334bef70478 Mon Sep 17 00:00:00 2001
From: thc1006 <84045975+thc1006@users.noreply.github.com>
Date: Sat, 8 Aug 2026 20:00:59 +0800
Subject: [PATCH] The grain's outer radius was smaller than its bore

grain_outer_radius was 0.00843 where the official scenario has 0.0843. A hollow
grain's mass goes as (outer^2 - inner^2), so an outer radius under the inner one
is a negative mass, and RocketPy 1.13 builds it without a word:

  solid grain mass   -0.623 kg  ->  4.861 kg
  total propellant   15.407 kg  ->  20.891 kg
  apogee              199.8 m   ->  130.5 m

The apogee was 53% high because the rocket was carrying negative fuel.

Since nothing downstream will catch this, the radii are now named constants with
a check in front of them. It rejects an inner radius that is zero, negative,
equal to the outer one, or larger.

Also center_of_dry_mass_position, 0.015 where the scenario has 0.15. That one is
inert today because dry_mass is 0 and it is a mass-weighted position, so it
changes no number in this diff. Aligned anyway: it is the same decimal slip, and
it stops being inert the day dry_mass does not stay zero.

Every other parameter in this file already matches the official scenario.

Signed-off-by: thc1006 <84045975+thc1006@users.noreply.github.com>
---
 custom_rocket.py | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/custom_rocket.py b/custom_rocket.py
index c8d3819..afbf653 100644
--- a/custom_rocket.py
+++ b/custom_rocket.py
@@ -3,12 +3,30 @@
 from rocketpy.motors.tank import MassFlowRateBasedTank
 from rocketpy.sensors import Accelerometer, Barometer, GnssReceiver, Gyroscope
 
+# A hollow grain's mass goes as (outer^2 - inner^2), so an outer radius under
+# the inner one is a negative mass. RocketPy 1.13 accepts it in silence: the
+# motor carried -0.623 kg of fuel and the flight apogeed at 200 m, not 131 m.
+GRAIN_OUTER_RADIUS = 0.0843
+GRAIN_INITIAL_INNER_RADIUS = 0.0295
+
+
+def _check_grain_geometry(outer_radius, inner_radius):
+    """A hollow grain needs a positive bore strictly inside the outer wall."""
+    if not 0 < inner_radius < outer_radius:
+        raise ValueError(
+            "grain geometry is impossible: need "
+            "0 < grain_initial_inner_radius < grain_outer_radius, got "
+            f"inner={inner_radius}, outer={outer_radius}"
+        )
+
 
 def create_custom_rocket():
     """
     Create a custom rocket with a hybrid motor and an oxidizer tank.
     """
 
+    _check_grain_geometry(GRAIN_OUTER_RADIUS, GRAIN_INITIAL_INNER_RADIUS)
+
     tank_shape = CylindricalTank(0.133, height=0.83)
     oxidizer_tank = MassFlowRateBasedTank(
         name="oxidizer_tank",
@@ -28,13 +46,16 @@ def create_custom_rocket():
         thrust_source=1080,
         dry_mass=0,
         dry_inertia=(0, 0, 0),
-        center_of_dry_mass_position=0.015,
+        # 0.15 in the official scenario, not 0.015. Inert while dry_mass is 0,
+        # since this is a mass-weighted position, but a 10x error waiting for
+        # the day that mass stops being zero.
+        center_of_dry_mass_position=0.15,
         burn_time=(0, 30),
         reshape_thrust_curve=False,
         grain_number=1,
         grain_separation=0,
-        grain_outer_radius=0.00843,
-        grain_initial_inner_radius=0.0295,
+        grain_outer_radius=GRAIN_OUTER_RADIUS,
+        grain_initial_inner_radius=GRAIN_INITIAL_INNER_RADIUS,
         grain_initial_height=0.2757,
         grain_density=900,
         nozzle_radius=0.04425,