Optimization of hybrid rocket motors for small launchers’ upper-stage

The study of rocket engines and motors has become fundamental during last years. In particular the simple idea of “space exploration”, with its political power, has been joined by other more economic aspects. The telecommunications industry is increasingly oriented towards the development of new satellites. That’s why the private industry of launchers is growing more and more, creating a new important market. The necessity of powerful, efficient, environmentally friendly, cheap and safe rockets is the new target in order to send into orbit the heaviest payload at the minimum cost. Just the small launchers industry needs nowadays new solutions, so far little considered, to be optimized. In particular the frontier of hybrid rockets allows a new approach to propellant combustion that could be revolutionary for future commercial space flights. The thesis work here reported analyses the small launcher Vega, actually in use, modifying the 3rd solid and 4th liquid stage with a unique hybrid upper-stage. The hybrid rocket motor (HRM) described uses a solid grain of fuel and a liquid oxidizer, whose tank has a feed system with partially regulated pressure. The throttle control through the liquid oxidizer mass flow should be possible, but it’s not used because it would need some heavy and expensive control systems (aft-end injector and mixer). This new type of solution for a rocket motor upper-stage is cheaper, lighter and simpler than the previous one, despite the typical HRM’s problems like mixture ratio shifting and low combustion efficiency. The goal of the work is to optimize the motor parameters in order to maximize the payload and to place it into a 500 km orbit. This is possible with a code developed by Politecnico di Torino and written in Fortran programming language, which realizes a coupled optimization of motor design (direct method used) and trajectory (indirect approach based on thrust direction and motor switching times). The thesis paper is divided into different sections. The chapter 1 in the first part describes the general subdivision of rocket motors and engines, focusing on the HRM, while in the second part outlines the characteristics of Vega and its mission phases, focusing on the new solution with modified upper-stage. The next section deals with the comprehension of the Fortran code, dwelling on the subroutine dedicated to mass budget (chapter 2). The last part (chapter 3) describes in detail the work done to achieve the final optimization and to obtain the results, illustrated in tables and plots.