Thrust vector control

A solid motor burns until it is empty, whatever direction the vehicle points. The gimbal points the thrust instead: a two-axis mount, two servos, pushrods, and a controller that works only while the motor burns.

Two-axis gimbal inside the airframe, cutaway Cutaway of the aft airframe. The motor sits in a two-axis gimbal; both servos, the gimbal ring and the pushrods are inside the tube. When the vehicle is pushed off vertical, the pitch servo swings the motor through its pushrod so that the thrust brings the vehicle back. A small view from above shows where the two servos and the two pivot axes sit in the tube. pitch servo yaw servo from above
  1. 01

    The mechanism

    The only control authority a solid motor offers is the direction of its thrust. The motor sits in a two-axis gimbal: two nested rings, each turned by one servo through a pushrod. Tilting the motor tilts the thrust line, and that turns the vehicle.

    The thrust vector control gimbal: printed mount, two servos and pushrods
    Fig. 2.1  —  Gimbal mk10 · assembled hardware and CAD
  2. 02

    Sensing

    Every cycle of the burn the flight computer takes pitch and yaw from the IMU. The measured attitude is compared with the target: straight up, zero degrees on both axes.

  3. 03

    Control

    Each axis has its own PID controller. The error between the measured angle and the target becomes a servo command: a proportional part for the present error, a derivative part that reacts to how quickly it changes, and an integral part for lasting offsets. The gains are tuned on the bench and read from the SD card at boot, so the vehicle can be retuned at the pad without a toolchain.

    The animation is a schematic. Its gains and limits are illustrative, not the flight values.

  4. 04

    Limits and self-test

    Both axes are limited in software to a travel window around centre before anything is written to the servos, so a controller fault cannot drive the gimbal into its mechanical stops. At power-up the controller sweeps both axes through their range and back, a visible confirmation that the actuation path is live before the motor is armed. The gimbal is live only during powered flight: the servos attach at liftoff and are released at burnout.

  5. 05

    Specification

    Revision
    mk10  (mk1 → mk10)
    Airframe
    75 mm
    Axes
    2 — pitch, yaw, independent
    Controller
    PID per axis, setpoint 0°
    Active
    powered flight only
    Tuning
    set from the SD card at boot
    Travel
    clamped each cycle before write
    Actuation
    2 servos, pushrod-driven
The mk1 gimbal parts and two servos on a cutting mat next to pliers
mk1
The mk2 gimbal on a test stand next to a laptop and a monitor
mk2 on the test stand
The mk6 gimbal with two servos and their wires on a wooden desk
mk6
Revision history
VehiclePeriodGimbal
EA-32021First gimbal and steering unit
EA-42022 →Gimbal mk10 with two servos and pushrods, the current test article

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