The rocket engines used in this activity are fueled by gunpowder, which is a mixture of Charcoal (C), Sulfur (S), and Nitrate (NO3). (The nitrate is normally in the form of Potassium nitrate (KNO3).
The underlying chemical reaction is the same as when you burn charcoal in a grill:
C (Carbon) + O2 (Oxygen) ---> CO2 (Carbon dioxide)
But gunpowder burns much faster than charcoal...
... that's where the nitrate comes in.
Nitrates can break down to form Nitrogen and Oxygen by this reaction:
2 NO3 (Nitrate) ---> N2(Nitrogen) + 3 O2(Oxygen)
This reaction provides a lot of oxygen. It's like blowing on a charcoal fire with a tank of pure oxygen.
Not only does the Nitrate help the charcoal burn faster, the nitrogen and oxygen gases expand out of the reaction area, making the mixture even more explosive.
The best ways to control the speed of the reaction is to control the amount of Nitrate in the mixture. A simple rule of thumb is: the more nitrate the more explosive the mixture.
The powder used in model rockets has less nitrate than that used in guns, making it less explosive.
Try this great demo to show how important it is to mix reactants in the correct proportions before a reaction can take place.
What is the role of the sulfur?
The Sulfur adds a little bit of fuel to the fire, but it's main function is to speed up the burning of the charcoal. The sulfur makes it easier for the charcoal to react with the oxygen, thereby speeding the reaction, and allowing it to happen at a lower temperature.
Most model rocket engines look like this:
The low-nitrate powder provides the energy for the rocket to go into flight. Once in flight, the rocket coasts while the fuse burns. When it's near the top if it's flight path, the fuse ignites the high-nitrate mixture which ejects the nose cone, and opens up the parachute so that the rocket can safely return to Earth.
Return from Gunpowder to Rocket Science