## 5 Conclusions  
The PHAEDRA program, funded by the Italian Space Agency, aims at improving the understanding and the technology readiness level of hybrid propulsion, with a specific focus on paraffin-based fuels and re-ignitability. The final target is the development and testing of a medium size, scaled-down demonstrator (DEMO) of a reference larger upper stage motor using hydrogen peroxide as an oxidizer. In the frame of this program, a series of preliminary tests campaigns have been performed at lab-scale in order to characterize the behavior of different paraffin blends and vortex injections. In the first campaign, a strengthened paraffin has been tested with swirl numbers from 0 (axial) to 3.33, showing regression rates up to 15 times higher than classical ones. These notable regression rates are ideal for single port boosters' applications at larger scales and/or shorter burning times than previously considered. Vortex injection provides also high efficiencies (>90%) even at the very small chamber volumes tested, with the expectation to obtain further improvements at the scales of real applications. In the second campaign, swirl numbers lower than previously investigated have been tested with a conventional plastic fuel, i.e., HDPE. The results confirmed the possibility of raising combustion efficiency to optimal values (>96%) without excessive increase in heat transfer, limiting the effect on the regression rate and nozzle erosion. This combination is instead ideal for smaller sizes and/or longer burning times. Therefore, the two campaigns together proved the possibility of extending the range of applications where hybrid rockets can perform effectively. The final test campaign combined the low swirl numbers with another paraffin blend loaded with more additives and was able to provide an intermediate regression rate between the plastic fuel and the baseline paraffin. The results confirmed the findings obtained with HDPE, even if with slightly lower efficiencies (generally > 93%), again with the expectation that the results should improve at the targeted sizes. Concluding, the combination of all the test campaigns have proven the possibility of using two different design variables, namely the swirl number and fuel composition, to optimize hybrid rocket performance and envelope in a wide range of different scenarios. Figure 16: The graphic illustrates how different mission profiles commonly encountered in space applications may be accomplished by hybrid motor technology. In particular, it relates them to the values of regression rates achieved by coupling different types of fuels with different swirl number values. The potential of selecting alternative configurations for the same regression rate values is shown by the overlaps between different fuel compositions. Moreover, the explored range of combinations between swirl numbers and paraffin types should allow the possibility of fulfilling the requirements set by Avio for the reference upper stage motor and the corresponding scaled-down DEMO test-bed. However, this expectation has to be confirmed by following tests at the increased scale. Conceptualization, M.S. and F.B. ; Writing, M.S. and F.B. ; Resources (paraffin grains), C.P. and L.G., Investigation, M.S., L.Z. and L.N. ; Software (CFD), M.T.M., M.F., D.B. and F.N. ; Project administration, A.C. and N.B. ; Supervision, D.P., R.C.P. and E.C. All authors have read and agreed to the published version of the manuscript. This research was funded by Agenzia Spaziale Italiana, grant: 2019-5-I.0. The datasets presented in this article are not readily available because the data are classified within the PHAEDRA Consortium. Requests to access the datasets should be directed to the Italian Space Agency. The authors declare no conflicts of interest. The following abbreviations are used in this manuscript: