A black powder rocket motor propels a model rocket using black powder. Black powder rocket propellants consist of charcoal, sulfur, and potassium nitrate. Adjustments can be made to the amount of each component to change the rate at which the black powder burns.
Black powder rocket motors were created in a primitive form by the Chinese in the early 13th century, and through the years refinements have been made and several uses created. They have been used for weapons and surveillance devices as well as recreation.
Black powder rocket motors are only produced in small sizes, to reduce the risk of explosion and a loss of efficiency. Black powder rockets are produced in classes 1/4 A through E. Larger sizes of model rocket motors use ammonium perchlorate composite propellant, or other composite fuels that contain ammonium nitrate.
Black powder is the oldest composite propellant. Its use in rockets actually preceded its use in guns. The three main components of black powder are charcoal, sulphur, and saltpetre (or potassium nitrate). It is known that by 1045, the Chinese were producing black powder, because many references to the subject were found in The Wu-ching Tsung-yao (Complete Compendium of Military Classics). In the early thirteenth century the Chinese turned black powder propelled objects, formerly only used for entertainment, into weapons of war. The first recorded use of rockets as military weapons was in 1232. The Chinese ‘arrows of fire’ were fired from a sort of catapult launcher. The black powder was packed in a closed tube that had a hole in one end for escaping hot gases, and a long stick as an elementary stability and guidance system.
Black powder had a very low specific impulse, however. Refinements in rocket design were made over the next few hundred years, at least on paper. In 1591 a Belgian, Jean Beavie, described and sketched the important idea of multistage rockets.
By 1600, rockets were being used in various parts of Europe against cavalry. By 1688, rockets weighing over 120 pounds had been built and fired with success in Germany. These German rockets, carrying 16-pound warheads, used wooden powder cases reinforced with linen.
Black powder rockets reached a new level of performance with the introduction of iron hulls and high pressure combustion, developed in India by the engineers of Tipu Sultan. With a range of a kilometer, his rocket-propelled grenades and incendiaries took British invaders by surprise during the Anglo-Mysore Wars. Impressed by these weapons, a London lawyer, Sir William Congreve, became fascinated by the challenge of improving rockets. He made extensive experimentation with propellants and case design. His systematic approach to the problem resulted in improved range, guidance (stabilization), and incendiary capabilities. The British armed forces used Congreve's new rockets to great advantage during the Napoleonic Wars.
In 1939, researchers at the California Institute of Technology seeking to develop a high performance solid rocket motor to assist aircraft take-off, combined black powder with common road asphalt to produce the first true composite motor. This was the birth of the true composite motor and marked the end of the use of black powder in major rocketry applications.
Black powder rocket propellant is very similar in makeup to old-fashioned gunpowder. The main difference is the presence of a binder, usually dextrin. The commonly used Estes model rocket engines are made with black powder propellant. Black powder propellant must be pressed very tightly in order to function well. Motors designed with black powder are most often end-burners, due to the fast burn rate of this propellant. A simple dextrin-free version (the most commonly used formulation) incorporates 75% potassium nitrate, 10% sulphur, and 15% charcoal. Dextrin may be added as desired (usually between 0 and 5%). Additional (coarse) charcoal or metal powders (5 - 10%) may be added to obtain an interesting spark trail. However, this may alter slightly the burn rate of the mixture.
The impulse (area under the thrust-time curve) of a black powder motor is used to determine its class. Motors are divided into classes from 1/4A to E, which covers an impulse range of 0 to 40 N·s (newton-seconds). Other types of model rocket motors can be classified up to an ‘H’, which is up to 320 Ns, and even further in some cases. Each class's upper limit is double the upper limit of the previous class.
Figures from tests of Estes rocket motors are used in the following examples of rocket motor performance.
For miniature black powder rocket motors (13 mm diameter), the maximum thrust is between 5 and 12 N, the total impulse is between .5 and 2.2 Ns, and the burn time is between .25 and 1 second. For Estes ‘regular size’ rocket motors (18 mm diameter), there are three classes: A, B, and C. The A class 18 mm motors have a maximum thrust between 9.5 and 9.75 N, a total impulse between 2.1 and 2.3 Ns, and a burn time between .5 and .75 seconds. The B class 18 mm motors have a maximum thrust between 12.15 and 12.75 N, a total impulse between 4.2 and 4.35 Ns, and a burn time between .85 and 1 second. The C class 18mm motors have a maximum thrust from 14 – 14.15 N, a total impulse between 8.8 and 9 Ns, and a burn time between 1.85 and 2 seconds.
There are also 3 classes included in Estes large (24 mm diameter) rocket motors: C, D, and E. The C class 24 mm motors have a maximum thrust between 21.6 and 21.75 N, a total impulse of between 8.8 and 9 Ns, and a burn time between .8 and .85 seconds. The D class 24 mm motors have a maximum thrust between 29.7 and 29.8 N, a total impulse between 16.7 and 16.85 Ns, and a burn time between 1.6 and 1.7 seconds. The E class 24 mm motors have a maximum thrust between 19.4 and 19.5 N, a total impulse between 28.45 and 28.6 Ns, and a burn time between 3 and 3.1 seconds.