As early as the 1940s the rocket scientist Samuel Herrick first coined the term “Astrodynamics”, as a way to describe the fusion of dynamical astronomy, space propulsion, celestial navigation and mission planning under a single subject. In this book we do not plan to discuss astrodynamics in detail, since it is a complicated subject that goes far outside the scope of this text. That said, there is a part of the theory of astrodynamics that is highly relevant to the physics of advanced space propulsion and for which needs discussion. The work presented in this chapter is an original derivation by the author which can also be expanded upon by others to include a more sophisticated model.