Rocket candy, or R-Candy, is a type of rocket propellant for model rockets made with a form of sugar as a fuel, and containing an oxidizer. The propellant can be divided into three groups of components: the fuel, the oxidizer, and the (optional) additive(s). In the past, sucrose was most commonly used as fuel. Modern formulations most commonly use sorbitol for its ease of production. The most common oxidizer is potassium nitrate (KNO3), a compound commonly found in tree stump remover. Additives can be many different substances, and either act as catalysts or enhance the aesthetics of the liftoff or flight. A traditional sugar propellant formulation is typically prepared in a 65:35 (13:7) oxidizer to fuel ratio. This ratio can vary from fuel to fuel based on the rate of burn, timing and use. There are many different methods for preparation of a sugar-based rocket propellant. Dry compression does not require heating; it requires only grinding the components and then packing them into the motor. However, this method is not recommended for serious experimenting, this is because dry compression is less saturated, and can be dangerous if it falls out of the rocket. Dry heating does not actually melt the KNO3, but it melts the sugar and then the KNO3 grains become suspended in the sugar. Alternatively, the method of dissolving and heating involves both elements being dissolved in water and then combined by boiling the water off, creating a better mixture. The total impulse and thrust provided are generally lower for the same amount of fuel than other composite model rocket fuels, but rocket candy is significantly cheaper. In the United States, rocket candy motors are legal to make, but illegal to transport without a low explosives users permit. Since they count as amateur motors, they are typically launched at sanctioned Tripoli Rocketry Association research launches which require users to hold a Tripoli Rocketry Association high power level 2 certification, however, as long as the mass of the motor is kept under 125 grams, it can still be launched without an FAA flight waiver.
Components Rocket candy can be broken down into three major groups of components: fuels, oxidizers, and additives. The fuel is the substance that burns, releasing rapidly expanding gases that provide thrust as they exit the nozzle. The oxidizer provides oxygen, which is required for the burning process. The additives can be catalysts, to speed up or make the burning more efficient. However, some additives are more aesthetic, and can add sparks and flames to liftoff, or add smoke for ease of following the rocket in the air.
Fuels Many different sugars are used as the fuel for rocket candy. The most common fuel is typically sucrose, however, glucose and fructose are sometimes used. As an alternative, sorbitol, a sugar alcohol commonly used as a sweetener in food, produces a propellant with a slower burn rate and is less brittle when made into propellant grains. Sugars with a double bonded oxygen, such as fructose and glucose, are less thermally stable and tend to caramelize when overheated. Sugars that have alcohol groups, like sorbitol, are much less prone to this decomposition. Some other commonly used sugars include erythritol, xylitol, lactitol, maltitol, or mannitol.
Oxidizers The oxidizer most often used in the preparation of sugar motors is potassium nitrate (KNO3). Other oxidizers can be used as well, like nitrates of sodium and calcium, as well as mixtures of sodium + potassium nitrate. KNO3 can be acquired through purchasing a granular "stump remover" from stores that carry garden supplies. Other rarely used oxidizers are ammonium and potassium perchlorate. Two main issues need to be addressed with respect to the oxidizer if one is using potassium nitrate. The most important issue is the purity of the material. If a purchased material does not perform satisfactorily it may be necessary to recrystallize the KNO3. The second important issue with respect to the oxidizer portion of a propellant is its particle size. Most propellant makers prefer their KNO3 ground to a small particle size, such as 100 mesh (about 150 μm) or smaller, which can be done using a basic coffee grinder. Rock-tumblers can also be used to mill into a fine grained well mixed powder.
Additives Additives are often added to rocket propellants to modify their burn properties. Such additives may be used to increase or decrease the burn rate of the propellant. Some are used to alter the color of the flame or smoke produced. They can also be used to modify a certain physical property of the propellant itself, such as plasticizers or surfactants to facilitate the casting of the formulation. There are many types of experimental additives; the ones listed here are the most commonly used. Metal oxides have been found to increase the burn rate of sugar propellants. Such additives have been found to function best at levels from 1 to 5 percent. Most often used are iron oxides. Red iron oxide is used most often as it is somewhat easier to obtain than the yellow, brown, or black versions. Brown iron oxide exhibits unusual burn rate acceleration properties under pressure. Carbon in the form of charcoal, carbon black, graphite, etc; can be and sometimes is used as a fuel in sugar formulations. Most 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 in the form of flakes 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.
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