pid-rocket-landing

A PyBullet simulation of an autonomous rocket booster performing a powered vertical landing, using cascaded PID control for attitude and horizontal position, with a full landing-leg touchdown sequence.

Current Status: complete. the rocket lands successfully and consistently.

Overview

The rocket starts airborne with an initial tilt disturbance and must:

1. Null out its attitude error (pitch/roll) using thrust-vector gimbaling
2. Correct horizontal drift (X/Y) back toward the landing pad
3. Descend under a hover-thrust controller with velocity damping
4. Detect ground contact and hold thrust through touchdown, then cut engines once contact is sustained

Control Architecture

Cascaded PID, two loops deep:

        Position error (X, Y)
                |
        [ x_pos_pid / y_pos_pid ]
                |
        target Pitch / Roll  (clamped to MAX_TARGET_TILT)
                |
        [ pitch_pid / roll_pid ]
                |
        gimbal angle (X, Y)   (clamped to +/-15 deg)
                |
        Thrust vector applied at nozzle

Altitude is handled separately by a hover-thrust + vertical-velocity damper, with a ground-contact hold-down that cuts thrust after landing.

Files

File	Purpose
sim.py	Main simulation: PID controllers, physics loop, telemetry
rocket.urdf	Rocket body (1m cylinder), 4 landing legs,

A red landing pad marker is created procedurally inside sim.py.

Key Fixes Along the Way

• Gimbal Y sign bug — thrust applied below the center of mass couples X-force into Y-torque and Y-force into X-torque with opposite signs (an artifact of the r x F cross product). The pitch loop's sign correction doesn't carry over to roll; roll needed the opposite sign, or it drove itself into positive feedback and tipped over.
• Position loop Y sign bug — same root cause one level up: pitch and roll couple into world-frame X/Y translation with opposite sign conventions, so the Y position loop needed its own sign flip even after the gimbal fix.
• Ground contact height — recalculated after adding landing legs; contact now happens at leg-tip height (~0.72m at the COM), not the bare cylinder's base (0.5m).

Live Telemetry

Position (X, Y, Z) and orientation (roll, pitch) are rendered as a floating in-sim panel that tracks the rocket, updated in place each frame via addUserDebugText with replaceItemUniqueId.

Possible Extensions

Not needed for this project's goal, but natural next steps if picked back up later:

• A proper descent trajectory / altitude-scheduling loop (currently just hover-thrust + damping, with no planned descent profile)
• Disturbance rejection (wind, sensor noise) to stress-test the gains