What is a Hybrid?

A hybrid rocket engine is one that uses a combination of solid and liquid propellants: Typically, a fuel is stored in the combustion chamber as a solid tube, into which a liquid or gaseous oxidiser is injected (although in a few cases solid oxidisers have been used with liquid fuels). The sketch below shows the layout of a typical hybrid rocket.

Once ignited, the hot gasses flowing down the chamber erode fuel off the inner surface of the fuel grain

(known as the 'port') in the same way that the draught of air across a log fire increases the burning; a chimney fire is a large hybrid!

The eroded fuel vaporises, and this fuel gas mixes with the vaporising liquid that was injected at the head of the port. This then burns to greatly expand the gasses and eventually produce thrust at the nozzle. The hot gasses caused by the burning erode yet more fuel from the grain, and so the process is self-sustaining until the fuel runs out or the injected propellant is shut-off.

This arrangement means that the fuel and oxidiser only come into contact with one another during the combustion process, unlike a solid rocket in which the fuel and oxidiser are bound together in an explosive mixture. Safety is further enhanced by the use of inert fuels such as polyethylene or rubber, which are extremely safe to store and handle.

A further benefit of hybrid engines is that their thrust can be throttled by controlling the flow of oxidiser, which can be shut-off in an emergency. This is an important advantage over solid motors, which are very difficult to control once lit.

If a suitable propellant combination is selected, hybrids can also be extremely clean, since the major combustion products are typically carbon dioxide and water. This exhaust causes far less environmental damage than the toxic mixtures produced by many traditional rockets.

Hybrid rocket engines provide amateur rocketry groups such as Aspirespace with a safe and reliable alternative to traditional rockets, combining many of the advantages of a liquid rocket with the simplicity of a solid rocket. Aspirespace members have demonstrated that it is possible to design, construct, and launch hybrids with only limited resources.

Detailed information about hybrids can be found in our Technical papers.