Hybrid Rocket - Hybrid Safety

Hybrid Safety

Generally, well designed and carefully constructed hybrids are very safe. The primary hazards associated with hybrids are:

• Pressure vessel failures - Chamber insulation failure may allow hot combustion gases near the chamber walls leading to a "burn-through" in which the vessel ruptures.

• Blow back - For oxidizers that decompose exothermically such as nitrous oxide or hydrogen peroxide, flame or hot gasses from the combustion chamber can propagate back through the injector, igniting the oxidizer and leading to a tank explosion. Blow-back requires gases to flow back through the injector due to insufficient pressure drop which can occur during periods of unstable combustion. Blow back is inherent to specific oxidizers and is not possible with oxidizers such as oxygen or nitrogen tetroxide unless fuel is present in the oxidizer tank.

• Hard starts - An excess of oxidizer in the combustion chamber prior to ignition, particularly for monopropellants such as nitrous oxide, can result in a temporary over-pressure or "spike" at ignition.

Because the fuel in a hybrid does not contain an oxidizer, it will not combust explosively on its own. For this reason, hybrids are classified as having no TNT equivalent explosive power. In contrast, solid rockets often have TNT equivalencies similar in magnitude to the mass of the propellant grain. Liquids typically have TNT equivalencies calculated based on the amount of fuel and oxidizer which could realistically intimately combine before igniting explosively; this is often taken to be 10-20% of the total propellant mass. For hybrids, even filling the combustion chamber with oxidiser prior to ignition will not generally create an explosion with the solid fuel, the explosive equivalence is often quoted as 0%.