All solid propellant contains at least two components–an oxidizer and a fuel. Chemically speaking, the oxidizer accepts electrons, the fuel supplies them. In double-base propellants, oxidizer and fuel are contained within the same molecule. A common double-base propellant is nitrocellulose (smokeless powder) plasticized with nitroglycerin. There is little amateur experimentation with double-base propellant, for obvious reasons.

Blackpowder (typically 75% potassium nitrate oxidizer, 15% charcoal, 10% sulfur) is still used for small motors, and has some desirable characteristics. It is quite cheap and has a high burn rate suitable for endburning motors. It can be “toned down” for coreburning motors. However, it has a fairly low specific impulse of about 80 seconds (meaning that one pound of the propellant configured to produce one pound of thrust will burn for 80 seconds). Blackpowder motors are made by pressing the ingredients into the paper casing at ca. 1 ton/sq.in., which gives many amateurs pause. And blackpowder is much more sensitive to spark and consequent accidental ignition than some other solid propellants.

Some amateurs work with potassium nitrate-sugar mixtures, sometimes referred to as “candy” propellant. The mixture is usually melted at a low temperature. Candy propellant has higher specific impulse than blackpowder, up to 130-140 seconds. Its burn rate is slower than that of blackpowder, but is still fast enough to be used in coreburning motors. Richard Nakka’s site gives a very comprehensive description of this and related propellants.

Composite propellant is made using a rubbery binder-fuel (sometimes just called a binder) and a granular oxidizer. As the name implies, the binder-fuel holds the propellant together and also acts as a fuel. Like epoxy glue, a binder-fuel has two ingredients that, when mixed, harden into a rubbery mass.

To prepare the propellant, the resin part of the binder-fuel is mixed with the oxidizer and other solid ingredients. The curative part of the binder-fuel is then added, the mixture is packed, poured or extruded into paper tubes or directly into the motor, and the mixture is then cured (sometimes at elevated temperatures). A core may be formed before curing or drilled/cut after curing.

Most rocketeers who experiment with solid propellant, work with composite propellant, as it is fairly simple to make and presents less danger from accidental ignition than does blackpowder.

The most commonly-used oxidizer in composite propellant may be ammonium perchlorate, NH4ClO4, also known as AP (composite propellant is sometimes–erroneously–referred to as AP). AP produces the highest practical specific impulse obtained for typical experimental propellant, 140-250 seconds or so. It is relatively simple to use, ignites easily, has a reasonable burn rate, does not require a burn rate catalyst or a thermic agent, and is nonhygroscopic. Many AP compositions burn well only under pressure, and the motor must be designed to take this into account. The burn rate of most AP compositions means that they are best suited to coreburning, rather than endburning, motors.

Ammonium nitrate (AN) is another oxidizer sometimes used in experimental motors. AN compositions have lower specific impulse than comparable AP compositions. AN propellant is also less dense than AP propellant. A 29mm motor 10” long that uses AP propellant would likely be a full-H impulse motor. With AN propellant it is likely to be a low-H or even high-G motor. AN propellant generally requires higher solids loading, around 80% for proper operation than does AP propellant. Either a burn rate catalyst (speeds the burn rate; ammonium dichromate) or a thermic agent (most commonly magnesium) is required; however, AN propellant still burns more slowly (0.1”/sec) than most AP compositions (0.3”/sec). AN undergoes a phase change that can render a propellant unuseable from temperature cycling over time, however, this is not a major concern for propellant that will be properly stored and used within a few days or weeks. AN is very hygroscopic and propellant containing it must be prepared and stored under low-humidity conditions. It is quite cheap and readily available in many US localities, though it may require recrystallization and milling for propellant use. Phase-stabilized AN, already milled to size and suitable for propellant use, is available from CP Technologies.

Other oxidizers are used by the experimental rocketry community, but not as commonly as are AP or AN.

Submitted by: Terry McCreary