Investigation of the existing methods for designing pre-combustion chambers in hybrid rocket engines

Hybrid propellant rocket engines (HPRE) have been under development and advancement since the beginning of rocket propulsion research, alongside liquid and solid propellant rocket motors. However, its pace was much slower than that of the other propulsion types. Several companies have proposed hybrid launch vehicles in the last few years, and research and publications on such technology have increased significantly. One of the most crucial parts of HPREs is the pre-combustion chamber. This device is positioned upstream of the combustion chamber and downstream of the injection head, and it greatly affects the engine’s performance, size, weight, and cost, and combustion stability. Therefore, propulsion engineers are intensely interested in understanding the processes occurring inside pre-combustion chambers. Unfortunately, there are few published works on the design of such systems. Additionally, many references only recommend empirical size relations, which may only serve as a design starting point. Observing this issue, this paper compiles all the methods for designing pre-combustion chambers and provides rich discussions on each. These methods are: 1) experiments, 2) empirical relations, 3) numerical simulations, 4) droplet vaporization models, 5) combustion instability models, 6) droplet collision outcome prediction, and 7) molecular dynamics. The positive and negative aspects of each method are also considered. Moreover, other components that affect the calculation are discussed, such as injectors and grain design. Finally, a development workflow recommendation was constructed by applying all the discussed methods to reduce related costs and obtain an optimized design.