A rocket engine is designed for maxium efficiency at a particular thrust and fuel consumption. It is at this power level where it works best, so its fuel consumption, energy production and thrust is essentially constant while it is operating. However, the rocket takes off slowly, and accellerates throughout its flight. In fact, it is experiencing a change in acceration, or "jerk", to use the technical term. How can this be?

By the time the engine is operating at peak efficiency, while still on the pad, the thrust of the engine exactly counterbalances the acceleration of gravity on the rocket, the two forces cancel out and the rocket goes nowhere, its velocity and accelleration are zero. However, the rocket is consuming fuel at an enormous rate and it is getting lighter, so that constant engine thrust soon exceeds the force of gravity acting on the rocket and a net force on the rocket+fuel mass produces an accelleration. Not only does the velocity increase (acceleration), but the accelleration increases too (jerk) as the fuel mass changes. The velocity and accelleration are lowest at the start of the trip because the vehicle is heavy and in a thick atmosphere, it would break up if it were violently accellerating.This continues until the last of the fuel runs out, at which instant the velocity and accelleration are both at their maximum. At that time, the vehicle is in free fall and subject to gravitational accellerations and aerodynamic forces only.

Consider the difference between a rocket and an artillery shell. In the barrel of the cannon, the gases of combustion from the burning propellant push the shell out the barrel. Accelleration is high, but not instantaneous, the propellant is usually designed to continue burning while the projectile is still in the barrel, the increasing pressure forcing the shell out the muzzle. Maximum accelleration and velocity occurs somewhere inside the barrel, depending on the combustion rate of the propellant and how the gases expand into the ever-increasing volume between the projectile and the breach of the gun. Unlike the rocket, fuel is not carried in the projectile, so accellerations are phenomenally high and propulsion is highly efficient. This is why artillery shells are built so robustly compared to flimsy rockets. They get their accelleration all at once ,the moment the projectile is out the muzzle it is in free fall.

As Jules Verne knew perfectly well, spacecraft fired from cannons need carry no fuel at all! No fuel energy is wasted accelerating unconsumed fuel. The key to spaceflight is to having the working fluid or propellant not stored aboard the spacecraft.