often, however, a small amount of carbon is used as an opacifier, making a visible smoke trail. The carbon acts as a heat sink, keeping a portion of the heat of combustion located in the propellant rather than having it transferred quickly to the motor casing. If metallic fuels such as aluminum or magnesium are used in a sugar formulation, a danger exists if traces of acids are found in the oxidizer. Acidic materials can react readily with the metal, producing hydrogen and heat, a dangerous combination. The addition of weak bases helps to neutralize these acidic materials, greatly reducing their danger. Titanium metal flake or sponge (about 20 mesh in size) is often added to sugar formulations at levels from 5 to 10% in order to produce a sparking flame and smoke on lift off. Surfactants are used to reduce the melting viscosity of sugar propellants. For example, propylene glycol helps reduce the melt viscosity of sucrose based propellants. Formulations A typical sugar propellant formulation is typically prepared in a 13:7 oxidizer to fuel ratio (weight ratio). However, this formulation is slightly fuel rich., and can be varied by up to 10%. There are many different possible formulations that will allow for flight in amateur rocketry. Preparation There are a number of different methods for preparing a sugar-based rocket propellant. These methods include dry compression, dry heating, and dissolving and heating. The latter two methods involve heating the propellant. In dry compression, the sugar and potassium nitrate are individually ground as finely as possible, and then mixed in a ball mill or tumbler to ensure uniform mixing of the components. This mixture is then compressed into the motor tube, similar to the method for packing black powder into a muzzle loading rifle. However, this method is rarely used for serious experiments, and careful safety considerations should be made before deciding to employ this method. Another, more common, and safer method of preparing a sugar-based rocket propellant is dry heating. First, the potassium nitrate is ground or milled to a fine powder, and then thoroughly mixed with powdered sugar which is then heated. This method does not actually melt the potassium nitrate, as the melting temperature of KNO3 is , but it melts the sugar and coats the grains of KNO3 with the melted sugar. An alternative to this method used by Rick Maschek of the Sugar Shot to Space