Rapid Fire is an upgrade for the Monkey Ace in BTD4, BTD5 and BTD6. It allows the Monkey Ace to shoot approximately twice as fast. The Monkey Ace also drops Pineapples a lot faster with this upgrade. In BTD4, its first appearance, it is the third upgrade. In BTD5 and BTD6, it is the first upgrade for the first path.
In BTD6, it increases attack speed by 67%, except for Spectre and Flying Fortress, which attack 33% faster with the upgrade.