I assume the parabolic path you are talking about is the path of the object taken when it is subject to gravity forces only, as would occur if the rocket, assumed not in orbit, ran out of fuel. This is a conventional parabolic motion problem, in which case, since only a conservative force acts (gravity), total mechanical energy is conserved (KE + PE is constant, that is , the change in KE plus the change in PE sums to zero). But in the more general case when the rocket is subject to other forces besides gravity, like the propelling force from the rockets escaping gasses, its motion could be in any shaped curve. The point I am trying to make is that when an object is subject to non conservative forces that do work, total mechanical energy is not conserved (delta KE plus delta PE is not equal to zero). Incidentally, if you look up the definition of a non conservative force in Wiki, it is likely to confuse the living daylights out of you. Basically, in Mechanics, gravity and ideal springs/ideally elastic bodies, exert conservative forces. Most every other force is non conservative. The literature usually talks about friction being a non conservative force. Correct. But tension, normal forces, pushing forces, applied forces, and all other so called 'contact' forces, are also non conservative in nature.