Pull request type

• Code changes (bugfix, features)

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

In the Flight class, during two-stage rocket simulations, the second-stage motor is temporarily turned off after separation (except in hot separation cases). However, the current condition for applying power-on drag (t < self.rocket.motor.burn_out_time) incorrectly keeps power-on drag active during this period.

New behavior

The condition has been updated to self.rocket.motor.burn_start_time < t < self.rocket.motor.burn_out_time in u_dot and u_dot_generalized to ensure proper handling of drag forces for both single- and multi-stage rockets. This ensures that power-off drag is correctly applied when the second-stage motor is off.

Breaking change

• No

Additional information

Firstly reported here: https://discord.com/channels/765037887016140840/1334492994631372810