One of the problems faced by the engineers of rockets and space shuttles was that of fuel. Sending a large and heavy craft to move around in space requires a tremendous quantity of fuel. However, the fuel — and the tanks to hold it — adds enormously to the weight of the spacecraft. This, in turn, requires even more fuel, resulting in an apparently insoluble dilemma. To resolve this, they divided the rocket into several sections. When a part of the journey is concluded, the fuel has been completely drained from that section. Then, that fuel tank can be separated from the main body of the rocket and dropped. The rocket, with the remaining fuel tanks, becomes lighter, and uses less energy for the next stage.

This concept can teach us a lesson in our divine service. G-d sent us down to this world to live a life of “form (spirituality) over matter.” That is not an easy job in this physical world, especially since we have an “evil inclination.” The physicality of the world is the "fuel" spurring our soul to its heights of accomplishment. At each stage of spiritual development, we jettison the "used" aspects of the evil inclination and move forward. There are three methods to rid ourselves of the evil inclination, once we've already passed the challenges that we needed to surmount:

The first stage of ascent in Divine service is to overcome the evil inclination. In this case, the struggle is fairly balanced between good and evil. Good’s victory, at this stage, is expressed only in the fact that the person’s actual behavior follows G-d’s command. His thought, speech, and action are in accord with Torah’s demands, but his emotions — including the attributes of the evil inclination — are as strong as they ever were.

The second stage is to destroy the evil inclination. Our Sages tell us that as a result of King David’s many fasts, he so weakened his evil inclination that he killed it. This is the intermediate stage between forcibly overcoming the evil inclination, and the much higher stage of transforming it.

The third and ultimate stage in our Divine service is transformation. This is the stage when a righteous person has completely transformed his evil inclination to good, until it has become exactly like the good inclination. We will reach this stage with the ultimate redemption, when G-d will "remove the spirit of impurity from this Earth." Then we will all achieve perfection in our divine service, without external obstacles. Our mission now is to prepare for that era by adding beauty and perfection to our divine service wherever possible, thus hastening the actual revelation of Moshiach.

(Excerpted in part from Mind over Matter, a compilation of writings by the Lubavitcher Rebbe, on the sciences, edited by Dr. Branover.)