The hot exhaust gases from the secondary combustor exit through the nozzle, producing thrust for the ducted rocket engine. Fuel for the solid fuel gas generator has been a problem, and has been the subject of several research efforts. In some cases, oxidizers are mixed at low level with hydrocarbon polymeric materials to provide a solid fuel gas generator. In other approaches, energetic binders have been mixed with combustible solid materials to give a solid fuel gas generator. The most energetic solid fuel gas generators use metal particles as a means of obtaining high energy and high density.
Examples are magnesium, aluminum, boron, or zirconium powders incorporated in a binder.
In these instances, efforts have been made to incorporate carbon in various forms as a solid additive in the composition of the solid fuel gas generator. Carbon has been known to exist in two pure forms, diamond and graphite, and a number of impure forms, such as coal or soot. Previous experience has shown that solid fuel gas generators in which carbon is included have two main problems. The first problem is that the inclusion of carbon in the solid fuel gas generator introduces particles which must be ejected into the secondary chamber. This is an energy absorbing process which tends to decrease the burning rate of the solid fuel in the gas generator.