Answer:
In a two particle system, the center of mass lies on the center of the line joining the two particles.
The shuttles acceleration in the creases as the fuel is burned because the acceleration of the obect as produced by net force is directly proportional to the magnitude of the net force.
It's velocity is not constant as direction is changing.
We know, velocity is speed with direction, so if direction is changing, velocity can't be constant, doesn't matter that speed is constant.
Hope this helps!