What typically happens to the center of gravity (CG) of a rocket during a solid rocket motor's thrusting phase?

During the thrusting phase of a solid rocket motor, the center of gravity (CG) of the rocket generally shifts forward. This shift occurs due to the consumption of propellant as the motor burns. As the rocket burns fuel, the mass of the rocket decreases, and this reduction typically happens in the aft section where the motor is located.

As propellant is expended, the remaining mass of the rocket, which includes the airframe and any payloads, relatively moves forward since the weight at the rear decreases. This process results in the CG moving in the direction of the rocket's nose. A forward CG is advantageous for stability during flight as it enhances the rocket's aerodynamic characteristics, helping to maintain a proper flight trajectory and reducing the risk of tumbling or oscillation.

Factors such as altitude may affect aerodynamic forces on the rocket, but they do not directly influence the overall impact of mass loss due to propellant consumption on the CG position during the thrusting phase. Therefore, while altitude could affect flight dynamics, the primary reason for the CG shift during thrust is the reduction of aft mass from propellant burn, confirming that the correct understanding aligns with the option indicating that the CG shifts forward.