Hybrid rockets present some disadvantages, mainly low regression rate and combustion inefficiencies. A promising technology to solve both is swirling oxidizer injection, which enhances the wall heat flux and the mixing of the combustion reactants and thus increases the regression rate and the combustion efficiency. A numerical investigation is carried out with a commercial computational fluid dynamics code. This type of analysis can really help with the comprehension of the physical phenomena hidden behind the experimental measurement, and so it can be a powerful aid in the preliminary development and testing of hybrid motors. The first step of this numerical investigation is to study the initial motor geometry, increasing the complexity of the system with the addition of each component one by one to better understand which parameters influence the swirling flowfield inside the combustion chamber. Afterward, a comparison between the axial and swirl injection is done, analyzing the qualitative differences in the flowfields and the quantitative ones in the performance. The central and most important part of this numerical study is focused on the inspection of the motor performance related to several scaling parameters.

Hybrid rockets using specific oxidizer–fuel combinations are considered a green alternative to current propulsion systems, as they do not release very toxic or polluting exhausts, but only much less harmful substances such as carbon monoxide/dioxide and soot. However, in a long-term vision where space access and rocket transportation become a daily routine all around the world, the simple use of current green propellants could begin to become insufficient if the rest of the industry already follows much stricter rules, which are expected to tighten significantly in the future, thereby making emissions from rocket flights no more negligible. In this paper, the possible use of alternative sustainable solid fuels for hybrid rockets that are not derived from fossil fuels and are ideally carbon neutral is investigated and discussed based on the available data in the hybrid literature and on the literature related to renewable fuels in general. Even if this topic is apparently far away from the current necessities, as hybrid propulsion is not yet operational, it is paramount to consider a long-term vision and associated research efforts to make sure that the potential hybrid propulsion introduction to the commercial market is more than a simple flash in the pan, but offers a solid opportunity.

CFX software is used to simulate different hybrid rocket configurations, applying liquid ${\mathrm{N}}_2\mathrm{O}$ as the oxidizer and paraffin as the fuel. This work is the prosecution of a previous paper analyzing liquid injection in a lab-scale hybrid rocket. It is focused on the formulation of the most suitable simulation technique to represent another type of liquid injector, compared with the one described in the previous paper. It also aims at extending the computational fluid dynamics simulation approach to hybrid rockets of larger scales. To validate computational fluid dynamics output, experimental results coming from both a laboratory scale and an increased-scale engine have been used. The different geometries studied include an increased-scale engine with a cylindrical grain having no diaphragm, the same rocket with a one-hole diaphragm inside the fuel grain, and a lab-scale rocket with a one-hole diaphragm. Simulations are steady state, and combustion derives from a single-phase chemical reaction. Liquid injection is fully simulated for the oxidizer, but paraffin entrainment is neglected for the fuel. Computational fluid dynamics results show a good agreement with the corresponding experiments for the ballistic parameters of interest in this study: chamber pressure, efficiency, and $C^{*}$. Computational fluid dynamics results prove that the simulation technique proposed can be applied to any hybrid rocket scale.