Workshop › Lunar Lander › Step 15 of 31

The engine

What we are doing

The engine is under the lander and always pushes the way the lander points. Upright it pushes straight up; tilted, part of the push goes sideways. sin and cos split the push into its two parts.

```js
const THRUST = 0.1 // speed gained per frame of engine, along the direction the lander points

function burning() {
  return state === 'flying' && keys.ArrowUp
}

  if (burning()) {
    // The engine pushes along the direction the lander points: angle 0 is straight up.
    lander.vx += Math.sin(lander.angle) * THRUST
    lander.vy -= Math.cos(lander.angle) * THRUST
  }
```

What it means

burning() is true while flying with the up arrow held.
For an angle measured from straight up, sin is the sideways part and cos the upward part of a push of 1.
The engine changes the speed, not the position: the lander keeps moving after you let go.
vy -= because up is a smaller y.

Your turn

Above const SPIN write the THRUST constant.
Above function touchdown() {, write burning, followed by an empty line.
In update, under the two arrow lines, write the if (burning()) block.

Skills: Velocity and gravity

← Tilt A flickering flame →

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