Thrust Vector Control

Thrust Vector Control (propulsion/rocket/thrust-vector-control)

Use when the task is sizing or analyzing a rocket thrust vector control system: deflection-driven side force, control torque about the center of gravity, axial thrust loss, and the actuator authority the gimbal actuators must deliver.

Domain quick reference

• TVC mechanisms: a gimbaled engine or nozzle pivots the whole thrust chamber on a gimbal bearing; a flex-seal nozzle bends a flexible joint between the fixed and the movable nozzle sections (common on solid rocket motors); jet vanes are heat-resistant vanes in the exhaust stream; liquid injection injects a secondary fluid into the diverging section to deflect the flow with an oblique shock.
• The deflection angle delta is the angle between the deflected thrust vector and the vehicle axis.
• Side force: F_side = T * sin(delta), perpendicular to the vehicle axis.
• Control torque about the center of gravity: M = F_side * L, where L is the moment arm from the gimbal point to the center of gravity.
• Axial thrust loss from the deflection: T * (1 - cos(delta)); for small angles this is about T * delta^2 / 2.
• Actuator authority has three components: angular authority (max deflection), rate (deg/s, sets the achievable attitude bandwidth), and torque (overcomes bearing friction, hinge moments, and inertia).
• TVC produces control force only while the engine thrusts, but it works at zero dynamic pressure and in vacuum; aerodynamic control surfaces produce force from q * S * CN and lose authority as dynamic pressure falls, so upper stages rely on TVC alone.
• Practical deflection limits are roughly +/-5 to +/-15 deg; beyond that the cosine loss and actuator loads grow quickly.

Workflow

1. Collect the engine thrust T, the gimbal deflection angle delta, and the moment arm L from the gimbal point to the center of gravity.
2. Compute the side force with side_force(thrust, deflection_rad).
3. Compute the control torque with control_torque(thrust, deflection_rad, moment_arm).
4. Compute the axial thrust loss with axial_thrust_loss(thrust, deflection_rad) and the retained fraction with axial_thrust_ratio.
5. Size the actuator: actuator_authority_required(required_torque, moment_arm) gives the side force the actuator must deliver, and deflection_angle_for_side_force gives the deflection needed for a demanded side force; compare it with the actuator angular authority to check saturation.

Pitfalls

• Routing nozzle flow questions here: area ratio, exit Mach, choked mass flow, and ideal thrust belong to nozzle-design; TVC works with the already-designed thrust vector.
• Routing aerodynamic control questions here: hinge moments, control surface effectiveness, and aero control authority belong to the control-surface-effectiveness and control-surface-sizing leaves; TVC sizing is the thrust-driven analog.
• Routing delta-v and stage questions here: rocket equation, mass ratio, and staging belong to rocket-sizing and rocket-staging; TVC is attitude control, not trajectory energy.
• Using degrees in the trigonometry: the logic module takes radians; convert deg * pi / 180 before calling.
• Dropping the cosine loss at large deflection: a 15 deg deflection loses about 3.4% of the axial thrust, which matters for ascent performance budgets.
• Mixing up the moment arm: the arm is from the gimbal point to the center of gravity, not the nozzle length or the vehicle length.
• Assuming control without thrust: TVC produces no side force when the engine is off; coast phases need other attitude control.
• Treating rate and torque as interchangeable: a slow high-torque actuator cannot recover from a fast disturbance; both specs must close the loop with the control law.

Behavior contract (gate 3)

The side force, control torque, axial thrust loss, deflection-for- side-force, and actuator authority logic is exercised by the gate 3 contract test: scripts/test_thrust_vector_control.py against scripts/thrust_vector_control_logic.py (stdlib unittest, offline). Run: python3 skills/propulsion/rocket/thrust-vector-control/scripts/test_thrust_vector_control.py

Compliance

• Standards referenced, not reproduced: ECSS is a free ESA download (ecss.nl/standards); TVC force and torque geometry is standard mechanics, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.