9. Propulsion Systems

Propulsion systems in space transportation systems have to impart the necessary energy to the vehicle to achieve the desired orbital conditions for the specified satellite. Different categories of propulsion systems such as chemical propulsion, electric propulsion, nuclear propulsion, solar sail, etc. are being used in various applications of space missions, depending on the requirements. For the boost phase with the present day technologies, chemical propulsion systems are generally being used and the energy source is the chemical reaction. There are two types of chemical propulsion systems like air breathing propulsion and non-air breathing propulsion. In air breathing propulsion, the oxygen available in the atmosphere and the fuel stored vehicle onboard are used for the combustion process, whereas in the case of non-air breathing propulsion system, both oxidizer and fuel stored vehicle onboard are used for generating the necessary thrust. Depending on the type of propellants used, the chemical rocket propulsion systems are classified into solid motors rockets, liquid engines and hybrid propellant rockets. In solid propellant rocket motor, the propellant is stored in combustion chamber and propellant burns from the surface. The liquid propellants are categorized into bipropellant and monopropellant. Depending on the propellants used, the bipropellant systems are further classified into Earth storable systems, cryogenic systems and semi-cryogenic systems. In the hybrid rocket propulsion system, generally the fuel is solid and the oxidizer is liquid. The payload capability of a multi-stage vehicle has to be maximized and in achieving this objective the propulsion module for each stage has to be carefully selected. The selection of the number of stages and type of propulsion system depend on the mission objectives, state-of-the-art technologies in propulsion, technology base available, development lead time, and cost and reliability requirements. The selection and design of a suitable propulsion system has to be carried out by considering various factors and also its close interactions with several other major subsystems of the vehicle. This chapter addresses important system engineering aspects of propulsion systems like the selection and design of propulsion modules for a STS, the basics of rocket propulsion, several propulsion options and their relative merits and demerits. The staging aspects and criteria for selection of suitable propulsive modules are highlighted. The qualification process for the propulsive stages is also included. A brief discussion on air breathing propulsion system also outlined.