Simulating Zero-Gravity Thrusters in Matter.js

Simulating zero-gravity astronaut maneuvering in Matter.js involves disabling engine gravity, eliminating atmospheric friction, and applying localized vector forces to a rigid body to emulate compressed gas thrusters. By applying directional forces at the center of mass for linear translation and offset forces for angular rotation, you can recreate the authentic inertia and physics of a manned maneuvering unit (MMU) operating in a frictionless vacuum.

1. Configure the Zero-Gravity Environment

Matter.js applies vertical gravity by default. For a space environment, you must set both horizontal and vertical gravity components to zero in your engine instance. Additionally, to simulate the vacuum of space, you must disable air friction on the moving body so it maintains its momentum indefinitely until an opposing force is applied.

const { Engine, Render, Runner, Bodies, Composite, Body, Vector } = Matter;

const engine = Engine.create();
engine.gravity.x = 0;
engine.gravity.y = 0;

2. Define the Astronaut Body

Create the astronaut as a rigid body. Set frictionAir to 0 to prevent environmental deceleration, and configure an appropriate mass and moment of inertia to ensure realistic resistance to linear and rotational acceleration.

const astronaut = Bodies.rectangle(400, 300, 40, 70, {
    mass: 120, // Astronaut plus gear in kg
    frictionAir: 0,
    friction: 0,
    restitution: 0.1
});

Composite.add(engine.world, astronaut);

3. Implement Thruster Dynamics

Compressed gas thrusters operate through Newton’s third law: expelling gas in one direction accelerates the astronaut in the opposite direction. These maneuvers are divided into translation (linear movement) and attitude control (rotation).

Forward and Backward Translation

To fire thrusters along the astronaut's current orientation without causing rotation, apply the force vector directly at the astronaut's center of mass (astronaut.position).

function fireLinearThruster(magnitude) {
    const angle = astronaut.angle;
    // Calculate the directional vector based on body orientation
    const force = {
        x: Math.sin(angle) * magnitude,
        y: -Math.cos(angle) * magnitude
    };
    
    Body.applyForce(astronaut, astronaut.position, force);
}

Rotational Control (Attitude Thrusters)

To induce rotation, fire thrusters perpendicular to the center of mass. You can either directly modify astronaut.torque or apply a force at an offset position (e.g., at the head or feet).

function fireRotationalThruster(torqueMagnitude) {
    // Directly adjusting torque provides clean RCS thruster behavior
    astronaut.torque = torqueMagnitude;
}

Alternatively, to simulate off-center physical thrusters using applyForce:

function fireLeftHandThruster(magnitude) {
    // Vector offset from center of mass to thruster location
    const offset = Vector.rotate({ x: -20, y: 0 }, astronaut.angle);
    const thrusterPosition = Vector.add(astronaut.position, offset);
    
    // Direction of thrust
    const forceVector = Vector.rotate({ x: 0, y: -magnitude }, astronaut.angle);

    Body.applyForce(astronaut, thrusterPosition, forceVector);
}

4. Continuous Input Loop and Gas Mechanics

To handle sustained burns, listen for keyboard inputs and apply small, continuous forces per physics tick using the beforeUpdate event.

const keys = {};

window.addEventListener('keydown', (e) => { keys[e.code] = true; });
window.addEventListener('keyup', (e) => { keys[e.code] = false; });

let gasReserve = 100; // Total gas units
const consumptionRate = 0.05;

Matter.Events.on(engine, 'beforeUpdate', () => {
    if (gasReserve <= 0) return;

    const thrustForce = 0.002;
    const rotationalForce = 0.05;

    // Linear Forward (W key)
    if (keys['KeyW']) {
        fireLinearThruster(thrustForce);
        gasReserve -= consumptionRate;
    }
    // Linear Reverse (S key)
    if (keys['KeyS']) {
        fireLinearThruster(-thrustForce);
        gasReserve -= consumptionRate;
    }
    // Rotate Counter-Clockwise (A key)
    if (keys['KeyA']) {
        fireRotationalThruster(-rotationalForce);
        gasReserve -= consumptionRate;
    }
    // Rotate Clockwise (D key)
    if (keys['KeyD']) {
        fireRotationalThruster(rotationalForce);
        gasReserve -= consumptionRate;
    }
});

5. Adding Damping Mechanics (Optional Stabilization)

Real-world space suits utilize an automatic stabilization system to halt unwanted drift. You can simulate an active stability assist mode by checking current velocities and applying counter-forces:

function stabilizeAstronaut() {
    // Counteract linear velocity gradually
    Body.setVelocity(astronaut, {
        x: astronaut.velocity.x * 0.98,
        y: astronaut.velocity.y * 0.98
    });

    // Counteract angular drift
    Body.setAngularVelocity(astronaut, astronaut.angularVelocity * 0.95);
}

Enabling this function dynamically lets the astronaut switch between full Newtonian coasting and computer-assisted maneuvering.