What is a Hybrid Engine?
Hybrid rocket engines use solid fuel and liquid oxidizer. They are an attempt to combine the advantages of solid and liquid engines. Here the constructive simplicity, storage properties and reliability of solid engines is joined by the efficiency, high specific impulse and reignition capability which liquid engines offer. The attainable specific impulse of a hybrid engine can be comparable to those from liquid engines, under the use of certain propellants. Until now only small to medium sized hybrid engines have been used, primarily on sounding rockets. However, several plans to implement them on orbital class rockets have been proposed in recent years. The most recognizeable applications of hybrid engines include the spacecraft SpaceShipOne and SpaceShipTwo from Scaled Composites, an American company planning to offer commercial spaceflights in the future.
How does a Hybrid Engine Work?
Oxidizer and fuel grain are stored separately; the chamber with the fuel grain is directly connected to the combustion chamber. Using a injection nozzle, the oxidizer is getting conveyed into the combustion chamber (mainly pushed only by its weight). The burn-off behaviour can be manipulated by the geometry of the propellant. Varying thrust and specific impulses can be achieved by using different propellants. For our HEROS and MIRAS rockets, paraffin wax was used as fuel grain and nitrous oxide as oxidizer.
Why do we use Hybrid Engines?
Hybrid engines are especially suitable for academic and educational purposes as they offer a relatively high safety margin compared to liquid engines. Furthermore, they offer students to work with both solid and liquid propellants, while allowing the omittion of complicated fuel conveying components used in liquid rocket engines. This way hybrid engines present a great opportunity for students to get realistic hands-on experience on rocket engines. With a hybrid engine we were able to set the European altitude student rocket record with an altitude of 32.3km under the sponsorship of the STERN program.