The space industry is going through a transformation phase: in the next years a significant increase in the market of small satellites is expected and current launch capacities cannot cover needs. A dedicated service is necessary to launch small satellites to specific orbits and time windows. HyImpulse intends to bring the key technologies for a game-changing, unique, low risk, innovative small launcher to the market, with a 500 kg payload capability to LEO. The launcher will be based on a combination of novel and established technologies, using inherently safe hybrid propulsion (paraffin/LOX propellants). The distinct advantages of hybrid rocket engines are summed up as follows: inert fuel and liquid oxidizer never form an explosive mixture. The fuel regression rate does not depend on the chamber pressure, hybrid engines can be throttled and the possibility to shut-down and re-ignite a hybrid motor is a major point for precise orbit insertion. High safety and low costs: inert fuels reduce costs for safety precautions and only one feedline system is required for the engine. Green propellants: the hybrid motor that will be developed will use liquid oxygen and paraffin. System complexity: even if a turbopump is required for a small launcher, the system complexity is still reduced compared to bi-liquid motors. In that context the overall objectives of the project were to develop and exploit a new fuel regression rate measurement method, to optimize the fuel grain production process and to study and improve the hybrid motor performance (paraffin composition, stability, combustion efficiency). All these scientific objectives have been achieved during the period of the action. The regression rate measurement method has been employed intensively on the medium and large-scale motors developed at HyImpulse to determine the regression rate performance of the fuel in the early stage of the development. At the end of the action period, that measurement system is available and exploited from time to time when there is a need of detailed investigations on the fuel regression rate. The wax-based fuel composition has been extensively investigated, from material characterization (mechanical properties, viscosity, influence of temperature) and from behavior during real combustion tests at different scales. Good composition and operating conditions have been identified and demonstrated to be efficient regarding both aspects: mechanical integrity (avoiding material degradation or mechanical failure under loads and combustion environment) and regression rate (in order to match the desired oxidizer to fuel ratio). Fuel grain production process has been regularly improved and has obtained a good level of maturity. The quality and reproducibility of the manufactured grains reached satisfying levels. The engine performance has been improved over the action duration and many different pre- and post-combustion chambers configurations have been tested and evaluated at medium and large scales. The motors have demonstrated very good ignition characteristics and combustion efficiencies that make them very close to the qualification for the rocket maiden launch. The stability topic has been a major part of the scientific work performed. It is a very complex topic involving many different processes interacting together and is the biggest issue regarding a rocket motor: an unstable rocket motor cannot fly since it would risk damaging either the rocket, or the payload, or both. After deep investigations involving combustion testing, literature analysis, and CFD calculations, a good understanding of the key phenomena and parameters related to that aspect has been reached and the stability issues have been mitigated.
