Black powder rocket motor
A black powder rocket motor pushes a model rocket using black powder. Black powder in the rockets is made of charcoal, sulphur, and potassium nitrate. The amount of each part can be changed to change how fast the black powder burns. The black powder burns very fast. The hot gases move fast as they leave the back of the rocket. This pushes the rocket forward.
Black powder rocket motors were made in a simple form by the Chinese. They were first written about in the early 13th century. The rocket motors have gotten better over the years. They have several uses. Some can be used as weapons. Some have cameras to take pictures from a height to get information. Some are also used just for fun.
Black powder rocket motors are only made in small sizes. If they are bigger there is a bigger chance of exploding. They also do not perform as well if they are bigger. They are made in different sizes called classes. The classes are 1/4 A through E. If the motor needs to be bigger than this, a different type of fuel is used. This is called a composite fuel. Some examples of composite fuels are ammonium perchlorate or ammonium nitrate.
Black powder is the oldest type of mixed rocket fuel. It was used in rockets even before it was used in guns. The three main parts of black powder are charcoal, sulphur, and saltpetre (or potassium nitrate). The Chinese were making black powder as early as 1045, which is about 1000 years ago. In the early thirteenth century (about 800 years ago) the Chinese turned black powder into a way to power weapons. Before that it was only used for fun. The first recorded use of rockets as military weapons was in 1232. The Chinese ‘arrows of fire’ were fired from a sort of catapult. The black powder was packed in a closed tube that had a hole in one end for escaping hot gases. The tube was mounted on a long stick so it was more balanced.
Black powder had a very low specific impulse. The way rockets were made got more advanced, but only on paper. People had not really built a much better rocket yet. They only had ideas and sketches of better models. In 1591 a Belgian, Jean Beavie, wrote about and drew the important idea of rockets that had more than one stage. This is called Multistaging. This means that the fuel is burned through in more than one step. This is the answer to escaping earth’s gravity and going into space.
By 1600, rockets were being used in some parts of Europe. They were used against cavalry, which is a group of fighting soldiers on horseback. By 1688, rockets weighing over 120 pounds had been built and fired in Germany. These German rockets could carry 16-pound warheads.
During the late 1700s, a London lawyer, Sir William Congreve, wanted to try to make rockets better. He did many experiments with different fuels and cases. His experiments were careful and orderly. He improved how far rockets could go, how stable they were, and how accurate they were. The British army used Congreve's new rockets to help them in the Napoleonic wars.
In 1939 researchers at the California Institute of Technology were trying to make a motor to help launch aircraft. They mixed black powder with common road asphalt to make the first composite motor. This was the beginning of the composite motor. It also ended the use of black powder for big rockets.
How It's Made
Black powder rocket propellant is a lot like old fashioned gunpowder. It has one different part called a binder. The binder is usually a substance called Dextrin. One common model rocket engine company is Estes Industries. Their motors are made with black powder propellant. Black powder propellant must be pressed very tightly to work well. If it is not packed tightly, it will burn too fast and might explode. A simple way of combining the parts to make black powder rocket propellant is: 75% potassium nitrate, 10% sulphur, and 15% charcoal. This is the most common way of making it. Dextrin may be added as a binder (usually between 0 and 5%). Extra (coarse) charcoal or metal powders (5 - 10%) may be added to make a spark trail. Adding these extras might change how fast the fuel burns.
Thrust is how hard of a push the rocket produces. Burn time is how long the fuel in the motor takes to burn while pushing gases out of the back of the rocket. The impulse of a rocket motor is the area under the thrust-time curve. This means the impulse is a number for how much force the rocket makes and for how long. The impulse of a black powder motor is used to tell what size class it is in. Motors are divided into classes from 1/4A to E. These classes hold rockets with impulses from 0 to 40 Ns (Newtons*seconds). Other kinds of model rocket motors can be named up to an ‘H’, which is up to 320 Ns.
|Class||Total Impulse (Ns)|
|1/4A||0.001 – 0.625|
|1/2A||0.626 – 1.250|
|A||1.251 – 2.500|
|B||2.501 – 5.000|
|C||5.001 – 10.000|
|D||10.001 – 20.000|
|E||20.001 – 40.000|
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.