Hybrid Rocket Propulsion Systems and Combustion Dynamics

Hybrid rockets combine a solid fuel grain with a liquid or gaseous oxidiser, offering a compromise between the simplicity of solid motors and the throttle‐and‐restart capabilities of liquid engines. Their inherent safety, cost-effectiveness and environmental advantages have renewed interest in applications ranging from sounding rockets to suborbital vehicles and upper stages. Central challenges concern the low fuel regression rates and the complex coupling between the gas–surface interaction and turbulent reacting flows. Combustion dynamics in hybrid systems are governed by boundary-layer heat transfer, phase‐change phenomena in liquefying fuels and instabilities such as shear-driven Kelvin–Helmholtz waves. Advances in propellant formulations, port geometries and feed systems are sought to enhance mass burning rates, stabilise combustion and scale performance for larger launch vehicles. Recent work also explores additive manufacturing of tailored fuel grain structures, electrically driven pump-fed cycles for improved feed reliability and green oxidisers for sustainable propulsion.

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Hybrid Rocket Propulsion Systems and Combustion Dynamics publication trend

The graph below shows the total number of articles in hybrid rocket propulsion systems and combustion dynamics across all publications each year (not limited to Nature Index journals).

Technical terms

Fuel regression rate: The linear rate at which a solid fuel surface recedes during combustion, typically expressed in mm/s.

Oxidiser mass flux: The mass flow of oxidiser per unit port cross-sectional area per unit time, influencing boundary-layer heat transfer and regression rate.

Kelvin–Helmholtz instability: A shear-driven fluid instability that arises at the interface between streaming gas and a low-viscosity liquid fuel melt layer, leading to droplet entrainment.

Liquefying fuel: A solid fuel that forms a low-viscosity melt layer under heat flux, enhancing surface regression through entrainment and improved thermal coupling.

Additively manufactured fuel grain: A solid propellant grain produced via three-dimensional printing, allowing customised internal geometries and porosity distributions for optimised performance.

References

1. Additively manufactured aluminium nested composite hybrid rocket fuel grains with breathable blades. Virtual and Physical Prototyping (2023).
2. Approaches to Low Fuel Regression Rate in Hybrid Rocket Engines. International Journal of Aerospace Engineering (2012).
3. Hybrid rocket propulsion technology for space transportation revisited - propellant solutions and challenges. FirePhysChem (2021).
4. Viability of an Electrically Driven Pump-Fed Hybrid Rocket for Small Launcher Upper Stages. Aerospace (2019).
5. Development of Green Storable Hybrid Rocket Propulsion Technology Using 98% Hydrogen Peroxide as Oxidizer. Aerospace (2021).
6. Understanding Kelvin–Helmholtz instability in paraffin-based hybrid rocket fuels. Experiments in Fluids (2018).
7. The Application of Computational Thermo-Fluid-Dynamics to the Simulation of Hybrid Rocket Internal Ballistics with Classical or Liquefying Fuels: A Review. Aerospace (2019).
8. Evaluation of Regression Rate Enhancing Concepts and Techniques for Hybrid Rocket Engines. Aerotecnica Missili & Spazio (2022).
9. Experimental regression rate profiles of stepped fuel grains in Hybrid Rocket Engines. Acta Astronautica (2023).