This paper is based on the concepts of lagrangian Reynolds transport equation (LRTE), momentum, and the generalized total kinetic power of a rocket. The wood rocket model is used as a vivid model to explain propulsive efficiency of a rocket. We prove that the novel U-turn launch mode can convert total kinetic power and gravitational potential energy into rocket propulsive power. Furthermore, we use the first stage rocket engine of the Saturn V as an example to simulate and analyze its propulsive efficiency computationally with Matlab software. In conclusion, this revolutionary launch mode has the potential to dramatically improve the propulsive efficiency of launch vehicles.