Rocket Candy, or R-Candy, is a type of rocket propellant for model rockets made with sugar as a fuel, and containing an oxidizer. The propellant can be divided into three groups of components: the fuel, the oxidizer, and the 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). Potassium nitrate is most commonly found in household 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.
There are many different methods for preparation of a sugar-based rocket propellant. Dry compression does not require heating, only the grinding of the components and then packing into the motor. However, this method is not recommended for serious experimenting. Dry heating does not actually melt the KNO3, but it melts the sugar and then the KNO3 grains become suspended in the sugar.
The specific impulse, total impulse, and thrust 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. Users may also launch using these motors by applying for an FAA flight waiver. Similar laws apply in Canada, the UK, and Australia.
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.
Many different sugars can be used as the fuel for rocket candy, including glucose, fructose, and sucrose; however, sucrose is the most common. Sorbitol, a sugar alcohol commonly used as a sweetener in food, produces a less brittle propellant with a slower burn rate. This reduces the risk of cracking propellant grains. Sugars with a double bonded oxygen, such as fructose and glucose, are less thermally stable and tend to caramelize when overheated, but have a lower melting point for ease of preparation. Sugars that only have alcohol groups, like sorbitol, are much less prone to this decomposition. Some other commonly used sugars include erythritol, xylitol, lactitol, maltitol, or mannitol.
The oxidizer most often used in the preparation of sugar motors is potassium nitrate (KNO3). Other oxidizers can be used as well, such as sodium and calcium nitrates as well as mixtures of sodium and 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. This can be done using a coffee grinder. Rock-tumblers can also be used to mill into a fine grained well mixed powder.
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 only 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 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.
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.
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 a method for loading black powder.[further explanation needed] However, this method is rarely used for serious experiments, and careful safety considerations should be made before deciding to employ this method. There is a significant chance for self-ignition while mixing, which could lead to serious injury.
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 613 °F (323 °C), but it melts the sugar and coats the grains of KNO3 with the melted sugar. The melting process must be performed using a heat spreader, so as to avoid creating autoignition hot-spots.
James Yawn advocates for the dissolving and heating method. Dissolving and heating the propellant actually dissolves both elements of the propellant and combines them. First, the KNO3 and sugar are placed in a pot or saucepan. Then, just enough water is added to be able to completely dissolve the KNO3 and the sugar. The mixture is then heated and brought to a boil until the water evaporates. The mixture will go through several stages: first boiling, then bubbling and spitting, then it will turn to a smooth creamy consistency. There are several advantages to dissolving the sugar and KNO3 in water before heating. One advantage is that the KNO3 and the sugar do not have to be finely powdered, because they both end up completely dissolved. This method of preparation also causes the resultant propellant to resist caramelization in the pot, giving more time to pack it into the motors.
Sugar based rocket propellants have an average Isp(specific impulse) of between 115 and 130 seconds. Compare that to the average Isp of an APCP (Ammonium perchlorate composite propellant), which is 180 to 260 seconds. Sorbitol and KNO3 based propellants with a typical 35:65 ratio are capable of an Isp of between 110 and 125 seconds. However, sorbitol and KNO3 rockets with additives have been recorded as having specific impulses of up to 128 seconds.
Xylitol and KNO3 based rocket propellants are capable of a specific impulse of ~100 seconds. These have an unconfined burn rate of about 1.3 mm/s. Overall, sugar rockets can compete[clarification needed] fairly well.
Dextrose and KNO3 based fuels are capable of an Isp of 118 seconds.
Rocket candy is also occasionally known as "caramel candy", a term that was popularized by Bertrand R. Brinley, in his pioneering book on amateur rocketry, Rocket Manual for Amateurs, published in 1960. This propellant was used in some of the amateur rockets described by Homer Hickam in his best-selling memoir Rocket Boys.
Rocket candy was also employed in a small amateur rocket described by Lt. Col. Charles M. Parkin in a lengthy Electronics Illustrated article that continued over several issues, beginning in July 1958. Parkin described how to prepare the propellant mixture by using an electric frying pan as a heat source for the melting operation. This article was reprinted in Parkin's book, The Rocket Handbook for Amateurs, which was published in 1959. Parkin's article contributed to the increasing popularity of the rocket candy propellant among amateur rocket groups beginning in the late 1950s and early 1960s.
The Sugar Shot to Space program was formed[by whom?] with the goal "to loft a rocket powered by a ‘sugar propellant’ into space" equivalent to 100 kilometres (62 mi) in altitude. The Double Sugar Shot rocket will reach[when?] 33 kilometres (21 mi), or one third of the goal altitude. The first Mini Sugar Shot rocket, a prototype of the Extreme Sugar Shot rocket, reached an altitude of 4 kilometres (2.5 mi) before a catastrophic motor malfunction occurred; contact with the second Mini Sugar Shot rocket was lost at an altitude of nearly 6 kilometres (3.7 mi) going in excess of Mach 1. The Extreme Sugar Shot rocket, the rocket expected to meet the goal of entering space, has not yet been completed.[when?]