BUG: rocket with a late-starting thrust curve never leaves the rail (#411)

CHANGELOG.md

@@ -37,6 +37,8 @@ Attention: The newest changes should be on top -->
 
 ### Fixed
 
+- BUG: rocket with a late-starting thrust curve never leaves the rail [#1085](https://github.com/RocketPy-Team/RocketPy/pull/1085)
+
 ## [v1.13.0] - 2026-07-21
 
 ### Added

rocketpy/simulation/flight.py

@@ -848,6 +848,18 @@ def __setup_phase_time_nodes(self, phase):
         # Add last time node
         phase.time_nodes.add_node(phase.time_bound, [], [], [])
 
+        # A thrust curve that starts at t > 0 leaves the rocket stationary at
+        # ignition-minus, so the solver (max_step defaults to inf) can take one
+        # huge step clean over the burn and the rocket never lifts off (#411).
+        # Force solver stops at ignition and burn-out so the burn is always
+        # sampled. Guarded to burn_start > 0, so ordinary motors are untouched.
+        motor = self.rocket.motor
+        burn_start = getattr(motor, "burn_start_time", 0) or 0
+        if burn_start > 0:
+            for t_burn in (motor.burn_start_time, motor.burn_out_time):
+                if phase.t < t_burn < phase.time_bound:
+                    phase.time_nodes.add_node(t_burn, [], [], [])
+
         # Organize time nodes
         phase.time_nodes.sort()
         phase.time_nodes.merge()

tests/integration/simulation/test_flight.py

@@ -5,11 +5,72 @@
 import numpy.testing as npt
 import pytest
 
-from rocketpy import Flight
+from rocketpy import Flight, Rocket, SolidMotor
 
 plt.rcParams.update({"figure.max_open_warning": 0})
 
 
+def test_flight_with_delayed_burn_leaves_rail(example_plain_env):
+    """Regression test for #411: a motor whose thrust curve starts at t > 0 must
+    still lift the rocket off the rail. Previously the solver stepped over the
+    burn (``max_step`` defaults to ``inf``) so the rocket never left the pad
+    (``out_of_rail_time`` came out as 0).
+
+    Parameters
+    ----------
+    example_plain_env : rocketpy.Environment
+        A plain environment object, this is a pytest fixture.
+    """
+    motor = SolidMotor(
+        thrust_source=[(8.0, 1500.0), (9.0, 2000.0), (14.0, 2000.0), (20.0, 0.0)],
+        burn_time=(8, 20),
+        dry_mass=1.815,
+        dry_inertia=(0.125, 0.125, 0.002),
+        nozzle_radius=33 / 1000,
+        grain_number=5,
+        grain_density=1815,
+        grain_outer_radius=33 / 1000,
+        grain_initial_inner_radius=15 / 1000,
+        grain_initial_height=120 / 1000,
+        grain_separation=5 / 1000,
+        grains_center_of_mass_position=0.397,
+        center_of_dry_mass_position=0.317,
+        nozzle_position=0,
+        throat_radius=11 / 1000,
+        coordinate_system_orientation="nozzle_to_combustion_chamber",
+    )
+    rocket = Rocket(
+        radius=127 / 2000,
+        mass=14.426,
+        inertia=(6.321, 6.321, 0.034),
+        power_off_drag=0.5,
+        power_on_drag=0.5,
+        center_of_mass_without_motor=0,
+        coordinate_system_orientation="tail_to_nose",
+    )
+    rocket.add_motor(motor, position=-1.255)
+    rocket.add_nose(length=0.55829, kind="vonKarman", position=1.278)
+    rocket.add_trapezoidal_fins(
+        n=4, root_chord=0.120, tip_chord=0.060, span=0.110, position=-1.04956
+    )
+    rocket.set_rail_buttons(0.082, -0.618)
+
+    flight = Flight(
+        rocket=rocket,
+        environment=example_plain_env,
+        rail_length=5.2,
+        inclination=85,
+        heading=0,
+        max_time=60,
+    )
+
+    # Ignition is at t = 8 s; the rocket must leave the rail shortly after,
+    # not remain at the pad the whole flight.
+    assert flight.out_of_rail_time > 8.0
+    assert flight.out_of_rail_velocity > 10.0
+    assert flight.apogee - example_plain_env.elevation > 1000.0
+
+
 @pytest.mark.parametrize(
     "flight_fixture", ["flight_calisto_robust", "flight_calisto_robust_solid_eom"]
 )