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ExposeExciteIgniteMay2012

8 Sugar can go Whoosh!

8 Sugar can go Whoosh! Sugar-Based Rocket Fuel There is hardly an amateur science activity that gives as much satisfaction as the ‘whoosh’ launch of a home-made rocket piercing the sky! In the previous chapter, we utilized sugar’s surface area and now we will blend it with an oxidizer to speed up its combustion rate. We are out to prove that the chemical potential energy contained in the molecules of 3.5 g sugar can drive a tiny rocket to heights of almost 80 metres. The Background Rocket propellants develop a thrust by producing copious amounts of hot gases that are propelled away from the rocket body. This action alone does not propel the rocket, but the reactive force described by the Third Law of Newton, that acts as an equal and opposite thrust on the rocket body. The amount of thrust developed by a rocket engine depends, amongst other factors, mainly on • the velocity with which the burning gases leave the chamber and • the mass of the burnt expelled gases (F = ma). The Space Shuttle’s two solid rocket boosters weigh more than half a million kilograms! In order to produce hot expanding gases, propellants need to be exothermic (heat generating), act independently of the surroundings and produce the gas at a constant high rate. Most rocket fuels are made up of at least two components that act in a redox reaction: The oxidizer and the fuel. The role of the oxidizer is that of supplying oxygen to the fuel to make the combustion self-contained, 93

self-sustained and non-reliant on surrounding gases. A great rocket mix should be able to burn quickly in the complete absence of air oxygen. There are hundreds of different fuel / oxidizer mixing options. Here are five that have proven to be very popular in model rocketry: Fuel Sulphur Magnesium Powdered sugar Charcoal & Sulphur Aluminium dust Oxidizer Potassium chlorate Potassium chlorate Potassium permanganate Potassium nitrate Ammonium perchlorate Table 9.1 The Estes type of model rocket engines use black powder as a propellant. Their composition comprise of 71.8% Potassium nitrate, 13.5% Sulfur, 13.8% Charcoal and 0.95% Dextrin. Preparing any of these mixes will require previous pyrotechnic mixing experience and special safety equipment. Furthermore, the chemical instability of some of these compounds are to be taken very seriously! In this book we are focusing on safe, spectacular activities, so we will not engage in mixing any of the more risky explosive-prone compounds. We love our fingers and are after simple, easy methods. This is our challenge: Prepare a simple, safe rocket that is easily reproducible. In this project we will utilize a sugar as fuel and potassium nitrate as oxidizer. Sucrose, C12H22O11 Sucrose is comprised of one molecule of glucose linked to a fructose molecule. It occurs widely in plants and is particular abundant in sugar cane from which it is extracted and refined for table sugar. At 200! (392") it starts decomposing (caramelizing). Potassium nitrate Potassium nitrate (KNO3) also known as saltpetre, is a well-known oxidizing agent and considered very stable and safe to work with. It is the 94