This paper illustrates the cost advantage of a space transportation system that uses pressure-fed technology and describes pertinent aspects of scaling the propulsion technology for light-lift through medium-and heavy-lift vehicles. Microcosm has used this approach of scaling the propulsion system to support the design and development of a number of lowcost, pressure-fed launch vehicles for various defense, scientific, and commercial applications, particularly the Scorpius ® family of launch vehicles. The application of this innovative and low-cost propulsion approach to various pressure-fed space transportation systems, including those for thrust augmentation boosters and upper stages, will be discussed. In addition, the enabling propulsion technologies, their development status and the results of trade studies associated with deriving the optimum propulsion parameters will be discussed. The paper discusses our approach to achieving higher reliability from our simple but robust, lightweight and scalable critical technologies such as the low-cost ablative combustion chambers, all-composite propellant tanks, the High Performance Pressurization System (HPPS) that also provides for an innovative RCS/ACS systems for the upper stages, a low-cost TVC, and a GPS-based GN&C system. The results of trade studies will be presented to demonstrate our approach to providing a balance between costs and performance. Pressure-fed, liquid rockets have the potential to significantly lower the cost of delivering payload to orbit. While pressure-fed propulsion Systems result in higher stage dry mass fractions and slightly lower specific impulse, their very low costs more than offset the weight penalties, resulting in lower cost as measured by cost per pound of payload over the life cycle of the vehicle system. Therefore, for a given payload, gross weight and size of a pressure-fed, expendable vehicle will be somewhat larger than those of a pump-fed vehicle, but cost per pound of stage will be much lower. In addition, because pressure-fed stages are relatively inexpensive, more stages can be employed for the pressure-fed system, partially offsetting the effect of high dry mass fraction. Finally, pressure-fed stages are inherently more robust during manufacture and ground handling, providing another means for reducing cost. Moreover, as the vehicle is scaled up, development costs for a pressure-fed system do not rise as fast as those for a pump-fed system because of the complexity of the latter system's engine design and its cost of development. This implies that pressure-fed vehicles become more cost-effective as the propulsion systems and vehicles are scaled to progressively larger configurations. Nomenclature r = characteristic linear dimension of vehicle
