ENH: time based triggering of parachute

rocketpy/rocket/parachute.py

@@ -47,14 +47,21 @@ class Parachute:
         - The string "apogee" which triggers the parachute at apogee, i.e.,
           when the rocket reaches its highest point and starts descending.
 
+        - The string "launch + X" where X is a number in seconds. The parachute
+          will be ejected X seconds after launch (t=0). This is useful for
+          simulating delay charges that activate at a fixed time from launch.
+
+        - The string "burnout + X" where X is a number in seconds. The parachute
+          will be ejected X seconds after motor burnout. This is useful for
+          simulating delay charges in motors with delay elements.
 
     Parachute.triggerfunc : function
         Trigger function created from the trigger used to evaluate the trigger
         condition for the parachute ejection system. It is a callable function
-        that takes three arguments: Freestream pressure in Pa, Height above
-        ground level in meters, and the state vector of the simulation. The
-        returns ``True`` if the parachute ejection system should be triggered
-        and ``False`` otherwise.
+        that takes six arguments: Freestream pressure in Pa, Height above
+        ground level in meters, the state vector of the simulation, sensors
+        list, current time t, and rocket object. It returns ``True`` if the
+        parachute ejection system should be triggered and ``False`` otherwise.
 
         .. note:
 
@@ -153,7 +160,14 @@ def __init__(
                 height above ground level.
             - The string "apogee" which triggers the parachute at apogee, i.e., \
                 when the rocket reaches its highest point and starts descending.
-
+            - The string "launch + X" where X is the delay in seconds from launch. \
+                For example, "launch + 5" triggers 5 seconds after launch. This is \
+                useful for simulating delay charges that activate at a fixed time \
+                from launch.
+            - The string "burnout + X" where X is the delay in seconds from motor \
+                burnout. For example, "burnout + 3.5" triggers 3.5 seconds after \
+                motor burnout. This is useful for simulating delay charges in motors \
+                with delay elements.
             .. note::
 
                 The function will be called according to the sampling rate specified.
@@ -232,35 +246,93 @@ def __evaluate_trigger_function(self, trigger):
             sig = signature(triggerfunc)
             if len(sig.parameters) == 3:
 
-                def triggerfunc(p, h, y, sensors):
+                def triggerfunc(p, h, y, sensors, t=None, rocket=None):
                     return trigger(p, h, y)
+            elif len(sig.parameters) == 4:
+
+                def triggerfunc(p, h, y, sensors, t=None, rocket=None):
+                    return trigger(p, h, y, sensors)
 
             self.triggerfunc = triggerfunc
 
         elif isinstance(trigger, (int, float)):
             # The parachute is deployed at a given height
-            def triggerfunc(p, h, y, sensors):  # pylint: disable=unused-argument
+            def triggerfunc(p, h, y, sensors, t=None, rocket=None):  # pylint: disable=unused-argument
                 # p = pressure considering parachute noise signal
                 # h = height above ground level considering parachute noise signal
                 # y = [x, y, z, vx, vy, vz, e0, e1, e2, e3, w1, w2, w3]
                 return y[5] < 0 and h < trigger
 
             self.triggerfunc = triggerfunc
 
-        elif trigger.lower() == "apogee":
-            # The parachute is deployed at apogee
-            def triggerfunc(p, h, y, sensors):  # pylint: disable=unused-argument
-                # p = pressure considering parachute noise signal
-                # h = height above ground level considering parachute noise signal
-                # y = [x, y, z, vx, vy, vz, e0, e1, e2, e3, w1, w2, w3]
-                return y[5] < 0
+        elif isinstance(trigger, str):
+            trigger_lower = trigger.lower().strip()
+
+            if trigger_lower == "apogee":
+                # The parachute is deployed at apogee
+                def triggerfunc(p, h, y, sensors, t=None, rocket=None):  # pylint: disable=unused-argument
+                    # p = pressure considering parachute noise signal
+                    # h = height above ground level considering parachute noise signal
+                    # y = [x, y, z, vx, vy, vz, e0, e1, e2, e3, w1, w2, w3]
+                    return y[5] < 0
+
+                self.triggerfunc = triggerfunc
+
+            elif "+" in trigger_lower:
+                # Time-based trigger: "launch + X" or "burnout + X"
+                parts = trigger_lower.split("+")
+                if len(parts) != 2:
+                    raise ValueError(
+                        f"Invalid time-based trigger format for parachute '{self.name}'. "
+                        + "Expected format: 'launch + delay' or 'burnout + delay' "
+                        + "where delay is a number in seconds."
+                    )
+
+                event = parts[0].strip()
+                try:
+                    delay = float(parts[1].strip())
+                except ValueError:
+                    raise ValueError(
+                        f"Invalid delay value in trigger '{trigger}' for parachute '{self.name}'. "
+                        + "Delay must be a number in seconds."
+                    )
+
+                if event == "launch":
+                    # Deploy at launch time + delay
+                    def triggerfunc(p, h, y, sensors, t=None, rocket=None):  # pylint: disable=unused-argument
+                        if t is None:
+                            return False
+                        return t >= delay
+
+                    self.triggerfunc = triggerfunc
+
+                elif event == "burnout":
+                    # Deploy at motor burnout time + delay
+                    def triggerfunc(p, h, y, sensors, t=None, rocket=None):  # pylint: disable=unused-argument
+                        if t is None or rocket is None:
+                            return False
+                        burnout_time = rocket.motor.burn_out_time
+                        return t >= burnout_time + delay
+
+                    self.triggerfunc = triggerfunc
+
+                else:
+                    raise ValueError(
+                        f"Invalid time-based trigger event '{event}' for parachute '{self.name}'. "
+                        + "Supported events are 'launch' and 'burnout'."
+                    )
 
-            self.triggerfunc = triggerfunc
+            else:
+                raise ValueError(
+                    f"Unable to set the trigger function for parachute '{self.name}'. "
+                    + "Trigger string must be 'apogee', 'launch + <delay>', or 'burnout + <delay>'. "
+                    + "See the Parachute class documentation for more information."
+                )
 
         else:
             raise ValueError(
                 f"Unable to set the trigger function for parachute '{self.name}'. "
-                + "Trigger must be a callable, a float value or the string 'apogee'. "
+                + "Trigger must be a callable, a float value or a string. "
                 + "See the Parachute class documentation for more information."
             )
 

rocketpy/simulation/flight.py

@@ -731,6 +731,8 @@ def __simulate(self, verbose):
                         height_above_ground_level,
                         self.y_sol,
                         self.sensors,
+                        node.t,
+                        self.rocket,
                     ):
                         # Remove parachute from flight parachutes
                         self.parachutes.remove(parachute)
@@ -800,6 +802,125 @@ def __simulate(self, verbose):
                     if self.__check_simulation_events(phase, phase_index, node_index):
                         break  # Stop if simulation termination event occurred
 
+                    # List and feed overshootable time nodes
+                    if self.time_overshoot:
+                        # Initialize phase overshootable time nodes
+                        overshootable_nodes = self.TimeNodes()
+                        # Add overshootable parachute time nodes
+                        overshootable_nodes.add_parachutes(
+                            self.parachutes, self.solution[-2][0], self.t
+                        )
+                        # Add last time node (always skipped)
+                        overshootable_nodes.add_node(self.t, [], [], [])
+                        if len(overshootable_nodes) > 1:
+                            # Sort and merge equal overshootable time nodes
+                            overshootable_nodes.sort()
+                            overshootable_nodes.merge()
+                            # Clear if necessary
+                            if overshootable_nodes[0].t == phase.t and phase.clear:
+                                overshootable_nodes[0].parachutes = []
+                                overshootable_nodes[0].callbacks = []
+                            # Feed overshootable time nodes trigger
+                            interpolator = phase.solver.dense_output()
+                            for (
+                                overshootable_index,
+                                overshootable_node,
+                            ) in self.time_iterator(overshootable_nodes):
+                                # Calculate state at node time
+                                overshootable_node.y_sol = interpolator(
+                                    overshootable_node.t
+                                )
+                                for parachute in overshootable_node.parachutes:
+                                    # Calculate and save pressure signal
+                                    (
+                                        noisy_pressure,
+                                        height_above_ground_level,
+                                    ) = self.__calculate_and_save_pressure_signals(
+                                        parachute,
+                                        overshootable_node.t,
+                                        overshootable_node.y_sol[2],
+                                    )
+
+                                    # Check for parachute trigger
+                                    if parachute.triggerfunc(
+                                        noisy_pressure,
+                                        height_above_ground_level,
+                                        overshootable_node.y_sol,
+                                        self.sensors,
+                                        overshootable_node.t,
+                                        self.rocket,
+                                    ):
+                                        # Remove parachute from flight parachutes
+                                        self.parachutes.remove(parachute)
+                                        # Create phase for time after detection and
+                                        # before inflation
+                                        # Must only be created if parachute has any lag
+                                        i = 1
+                                        if parachute.lag != 0:
+                                            self.flight_phases.add_phase(
+                                                overshootable_node.t,
+                                                phase.derivative,
+                                                clear=True,
+                                                index=phase_index + i,
+                                            )
+                                            i += 1
+                                        # Create flight phase for time after inflation
+                                        callbacks = [
+                                            lambda self,
+                                            parachute_cd_s=parachute.cd_s: setattr(
+                                                self, "parachute_cd_s", parachute_cd_s
+                                            ),
+                                            lambda self,
+                                            parachute_radius=parachute.radius: setattr(
+                                                self,
+                                                "parachute_radius",
+                                                parachute_radius,
+                                            ),
+                                            lambda self,
+                                            parachute_height=parachute.height: setattr(
+                                                self,
+                                                "parachute_height",
+                                                parachute_height,
+                                            ),
+                                            lambda self,
+                                            parachute_porosity=parachute.porosity: setattr(
+                                                self,
+                                                "parachute_porosity",
+                                                parachute_porosity,
+                                            ),
+                                            lambda self,
+                                            added_mass_coefficient=parachute.added_mass_coefficient: setattr(
+                                                self,
+                                                "parachute_added_mass_coefficient",
+                                                added_mass_coefficient,
+                                            ),
+                                        ]
+                                        self.flight_phases.add_phase(
+                                            overshootable_node.t + parachute.lag,
+                                            self.u_dot_parachute,
+                                            callbacks,
+                                            clear=False,
+                                            index=phase_index + i,
+                                        )
+                                        # Rollback history
+                                        self.t = overshootable_node.t
+                                        self.y_sol = overshootable_node.y_sol
+                                        self.solution[-1] = [
+                                            overshootable_node.t,
+                                            *overshootable_node.y_sol,
+                                        ]
+                                        # Prepare to leave loops and start new flight phase
+                                        overshootable_nodes.flush_after(
+                                            overshootable_index
+                                        )
+                                        phase.time_nodes.flush_after(node_index)
+                                        phase.time_nodes.add_node(self.t, [], [], [])
+                                        phase.solver.status = "finished"
+                                        # Save parachute event
+                                        self.parachute_events.append(
+                                            [self.t, parachute]
+                                        )
+
                     # Process overshootable time nodes if enabled
                     if self.time_overshoot and self.__process_overshootable_nodes(
                         phase, phase_index, node_index
@@ -946,6 +1067,8 @@ def __check_and_handle_parachute_triggers(
                 height_above_ground_level,
                 self.y_sol,
                 self.sensors,
+                node.t,
+                self.rocket,
             ):
                 continue  # Check next parachute
 
@@ -1355,6 +1478,8 @@ def __check_overshootable_parachute_triggers(
                 height_above_ground_level,
                 overshootable_node.y_sol,
                 self.sensors,
+                overshootable_node.t,
+                self.rocket,
             ):
                 continue  # Check next parachute