Rocket candy

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Rocket candy is a term used to describe the practice of manufacturing rocket fuel from a formulation using sugar as a fuel and binder.

This formulation is used primarily by amateur rocketry hobbyists due to the ease of obtaining the ingredients. The sugar fuel/binder is mixed with the oxidizer potassium nitrate, which is also easily obtainable and reasonably safe to work with. Engines using this type of fuel are sometimes termed "sugar motors" or K/S motors (for KNO₃/Sucrose).

The KNO₃ is commonly mixed with molten sugar and poured into the rocket casing, sometimes using various centre molds to obtain different cross-sections. These cross-sections dictate the burning pattern of the poured grain. Another method is to make a water solution of the two components, in the same manner as making toffee. This mixes the two parts completely, and it occurs at a lower temperature, providing a safer margin below the auto-ignition point. Open flame should never be used to melt the propellant, and the mix should always be heated in an oil bath, never over direct heat, to avoid hot spots causing auto-ignition or caramelization and degradation of the propellant.

Rocket candy is hygroscopic and needs to be sealed from moisture.

A common variation of this formulation replaces sugar (sucrose) with sorbitol, a common artificial sweetener. Sorbitol produces a less brittle propellant with a somewhat reduced burn rate. This reduces the risk of cracking of the propellant grains, and makes the motor less prone to overpressurization.

Other sugars, particularly dextrose, have also been used, with varying degrees of success.

A typical mix consists of 35% sugar, 65% KNO₃. Sometimes additives are used to change the processing or ballistic characteristics of the propellant; common additives include corn syrup, carbon black, red iron oxide, and others. Sometimes additives like metal flakes are added for sparks or other effects.

Rocket candy was used in the amateur rockets in the book Rocket Boys and its film adaption October Sky. It was also featured on the Mentos and Coke episode of MythBusters, though it was contained in a paper cupcake wrapper and called a "smoke bomb".