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What is a Hybrid Rocket Engine?

Despite the use of the term ‘hybrid’, a hybrid rocket engine (HRE) is a distinct form of rocket engine that uses both solid and liquid propellants. ‘Liquid-Solid’ Rocket Engine or ‘L-SRE’ would be a more accurate term.


Classically designed HRE’s using a liquid oxidizer and a solid fuel operate more closely to a LRE than a solid rocker motor (SRM). In a SRM, oxidizer and fuel are intimately blended and formed into a propellant grain; whereas, a HRE fuel grain contains no oxidizer and the port(s) dually serves as the engine’s combustion chamber; and through a phase-change ablation process, the source of fuel.


A significant advantage of an HRE compared to other forms of chemical rockets is its immunity to accidental detonation. This immunity is achieved by the fact that the propellants are stored on-vehicle in two different states of matter; and thus, it is virtually impossible to create a detonable oxidizer/fuel mixture outside of the rocket engine’s fuel grain port(s) or combustion chamber(s). In most designs, HRE propellant density is higher than most common liquid bi-propellant systems. Despite their enhanced safety and denser propellants, HRE are less developed and historically suffer from a range of performance disadvantages, however those performance disadvantages have been solved with Firehawk’s 3D printed rocket fuel. Our fuel allows us to build an engine with the safety of an HRE and the performance of a liquid bi-propellant engine. The benefits of our fuel grain are listed in our past blog post.

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