Because the oxidizer has no significant structural strength, the fuel must not only perform well but must also supply the necessary form and rigidity to the grain. Much of the research in solid propellants is devoted to improving the physical as well as the chemical properties of the fuel. Ordinarily, in processing solid propellants the fuel and oxidizer components are separately prepared for mixing, the oxidizer being a powder and the fuel a fluid of varying consistency. They are then blended together under carefully controlled conditions and poured into the prepared rocket case as a viscous semisolid. They are then caused to set in curing chambers under controlled temperature and pressure.
Solid propellants offer the advantage of minimum maintenance and instant readiness.
However, the more energetic solids may require carefully controlled storage conditions, and may offer handling problems in the very large sizes, since the rocket must always be carried about fully loaded. Protection from mechanical shocks or abrupt temperature changes that may crack the grain is essential. Typical fuels include kerosene, alcohol, hydrazine and its derivatives, and liquid hydrogen. Many others have been tested and used. Oxidizers include nitric acid, nitrogen tetroxide, liquid oxygen, and liquid fluorine.