A rockoon (derived from the terms rocket and balloon) was an extension to the rocket, which allowed the rocket to achieve further distance. The rockoon was a solid fuel rocket that, rather than being immediately lit while on the ground, was first carried into the upper atmosphere by a gas-filled balloon, and then separated from the balloon when it had reached its maximum height and automatically ignited. This would allow the rocket to achieve a higher altitude, since the rocket did not have to move through the lower thicker air layers.