Ballast is often used to adjust a rocket's CG and target altitude. It's especially useful because as parts are added and removed from a rocket, the mass and CG can change drastically. In the case of IREC, where there is a target altitude, this ballast is essential to be able to get closer to the target altitude for maximum points. The CG position is critical for rocket stability.
Ballast is usually some kind of dense material that ......