eli5 How does a spacecraft slingshotting around a planet increase it’s speed?

Question

Shouldn’t the gravity of the planet force the craft to expend fuel to escape the gravity?

In: 1

Answer 1

Slingshotting is essentially falling towards a planet and missing. As you fall, you gain speed, but since you aren’t actually hitting the planet, you end up with all this extra speed, and go flying past the planet instead.