Solid propellants come in two types: double-base and composite. Double-base propellants have oxidizer and fuel in the same molecular compound. They tend to be more explosive and have lower specific impulse than composite propellants, which are more commonly used. In composite propellants, the fuel and oxidizer are separate but intimately mixed together. The organic fuel material is initially in a liquid or semiliquid form; it gets mixed with a solid oxidizer, typically ammonium perchlorate, and cured to produce a solid binder.
Powdered aluminum is often mixed with the binder. The burning aluminum particles increase overall energy and performance while improving combustion stability. Propellants are often quoted in terms of their solid loading—that is, the percentage of propellant (by weight) that is solid ammonium perchlorate or aluminum. Typical loadings are 80–88 percent solids. In addition to oxidizer and fuel, solid propellants also contain a small amount of additives that help with the curing process.
Advances are being made in the areas of smokeless propellants, formulations with low infrared signature, high-energy additives, and formulations that are less prone to combustion instability. For example, scientists are working on a new propellant made up solely of nitrogen atoms. The energy release from such a compound can be quite high and at the same time have a low infrared signature.