Hybrid Rocket Propulsion

Most conventional spacecraft use one of two chemical rocket designs, liquid fuel or solid fuel. Both designs rely on the same core principle, a highly reactive fuel is supplied with an oxidising agent and then ignited. For a liquid rocket both the fuel and the oxidiser are kept in liquid or gaseous forms and then mixed together before ignition. A solid rocket has the two components premixed and compacted into a solid core. Both designs have advantages and disadvantages, but did you know that there exists a third type of chemical rocket?

This rocket is known as a hybrid rocket and it takes the advantages of the previous two types and mixes them together.

Diagram of a hybrid rocket engine. It shows fuel flowing from the oxidser tank to the pumping unit then through the flow channel that contains the solid fuel core.
Hybrid Rocket Engine


The hybrid rocket has a solid core of fuel which has a oxidiser pumped through it. Typical oxidisers include gaseous oxygen or liquid oxygen that is vaporized. As the oxidiser is in either a gaseous or vaporised form, it is easily able to cover the surface area of the fuel core. The fuel in the presence of this oxidiser is now able to be ignited, generating thrust. As you can see in the diagram below, such a rocket is an exceptionally simple device, requiring minimal pumping or mixing.

So why choose this hybrid engine over conventional designs? As I mentioned earlier a hybrid rocket takes many of the advantages offered by its two colleagues. A few of the most notable are listed below.

  • Simplicity of design – Whilst not as simple as a solid rocket, a hybrid rocket is far less complex in design than an equivalent liquid rocket. This is due to its single flowing fluid and lack of mixing chamber.
  • Controllability – Unlike a solid rocket, a hybrid rocket can easily be controlled by the flow of oxidiser within the system. This gives it similar characteristics to liquid rockets.
  • Safety – The clear mechanical separation, and different phase states of the oxidiser and fuel allow hybrid rockets to be far safer than either solid or liquid fuel designs.
  • Port design and custom regression rate – By changing the geometry of the oxidisers flow channel, different fuel regression rates may be achieved with minimal changes to the greater system.

However the most interesting advantage offered by these rockets is something quite new to the market, 3d printing. You see the fuel in a hybrid rocket can be almost any polymer. This means that materials such as abs plastic or petg can be used as fuel. Not only are these readily accessible, but they can also be 3d printed with almost any home setup.

Yes that’s right, you can 3d print a rocket at home. In fact 3d printed hybrid rockets are becoming very common amongst both universities as well as actual spacecraft companies. For instance Gilmour space, an Australian rocket company, has been developing such a rocket for several years now and plans to launch in 2018. 3d printing offers a world of new possibilities for hybrid rockets with the ability to custom design thrust profiles and times for any rocket using hybrid propulsion. An example of the complexity that 3d printing can offer is shown in the picture below.

3D Printed Rocket Fuel for a hybrid rocket.
3D Printed Rocket Fuel <https://en.wikipedia.org/wiki/File:3D_Printed_Hybrid_Rocket_Fuel_Grain.jpg>

Hybrid rockets are a fantastic propulsion method, and there are many new end exciting ways that they will be developed in the coming years. My mechanical engineering honours thesis will see me conduct more research into this area, likely looking into different 3d printed designs for these rockets. Stay tuned to these blogs in 2018 for more hybrid rockets!

Author: Thomas Renneberg

Thomas Renneberg is a 4th-year Mechanical Engineering student at UNSW. Thomas assumed the Off-World Robotics CTO position after a year of being the mechanical team lead and laying the foundation for BLUEsat's new rover. Aside from developing BLUEsat's new rover "NUMBAT", Thomas enjoys cooking and maintaining his vegetable garden.

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