rubber. Complex geometries, which are driven by the need for higher fuel mass flow rates, makes casting fuel grains for hybrid rockets expensive and time-consuming due in part to equipment costs. On a larger scale, cast grains must be supported by internal webbing, so that large chunks of fuel do not impact or even potentially block the nozzle. Grain defects are also an issue in larger grains. Traditional fuels that are cast are hydroxyl-terminated polybutadiene (HTPB) and paraffin waxes. Additive manufacturing Additive manufacturing is currently being used to create grain structures that were otherwise not possible to manufacture. Helical ports have been shown to increase fuel regression rates while also increasing volumetric efficiency. An example of material used for a hybrid rocket fuel is acrylonitrile butadiene styrene (ABS). The printed material is also typically enhanced with additives to improve rocket performance. Recent work at the University of Tennessee Knoxville has shown that, due to the increased surface area, the use of powdered fuels (i.e. graphite, coal, aluminum) encased in a 3D printed, ABS matrix can significantly increase the fuel burn rate and thrust level as compared to traditional polymer grains. Oxidizer Common oxidizer choices Common oxidizers include gaseous or liquid oxygen, nitrous oxide, and hydrogen peroxide. For a reverse hybrid, oxidizers such as frozen oxygen and ammonium perchlorate are used. Proper oxidizer vaporization is important for the rocket to perform efficiently. Improper vaporization can lead to very large regression rate differences at the head end of the motor when compared to the aft end. One method is to use a hot gas generator to heat the oxidizer in a pre-combustion chamber. Another method is to use an oxidizer that can also be used as a monopropellant. A good example is hydrogen peroxide, which can be catalytically decomposed over a silver bed into hot oxygen and steam. A third method is to inject a propellant that is hypergolic with the oxidizer into the flow. Some of the oxidizer will decompose, heating up the rest of the oxidizer in the flow. 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