Rocket 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). The fuel is a sugar; sucrose is the most commonly used. The most common oxidizer is potassium nitrate (KNO3). 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 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. Dissolving and heating the propellant actually melts both of the components and combines them.
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. Preparing rocket candy in a microwave oven is very dangerous because localized hotspots can ignite. Because rocket candy is extremely flammable, it should be prepared in small batches, out of doors or in an outbuilding, and using adequate personal protective equipment (eye protection at the very least).
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.
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, however sucrose is the most common sugar. 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, like sodium and calcium nitrates as well as mixtures of sodium and potassium nitrate or potassium chlorate (KClO3). 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.
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. These materials, being acidic, 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 65:35 oxidizer to fuel ratio. However, this formulation is slightly fuel rich. This ratio allows for the formulation to be more fluid in its melted state. Additives could take up between zero and ten percent of the total fuel. There are many different possible formulations that will allow for flight in model rocketry.
There are a number of different methods for preparing a sugar-based rocket propellant. Other than dry compressed, all of these methods involve heating the propellant. These various methods include: dry compressed, dry heated, and dissolved and heated.
In dry compression, the sugar and potassium nitrate are 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. However, this method is rarely used for serious experiments, and careful considerations should be made before deciding to employ this method.
Another, more common 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 then heated. This method does not actually melt the potassium nitrate, as the melting temperature of KNO3 is 613 degrees Fahrenheit, but it melts the sugar and coats the grains of KNO3 with the melted sugar.
James Yawn, a well known amateur rocket experimentalist, 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 last longer in the pot before it caramelizes. This means that the person preparing the fuel has 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 150 to 180 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 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.
Today there is a program called the Sugar Shot to Space Program. The “underlying goal of the Sugar Shot to Space program is to loft a rocket powered by a ‘sugar propellant’ into space.” The goal is for a rocket with a sugar-based motor to make it into space, or 100 km (62.137 mi) high. The Double Sugar Shot rocket has reached 33 km, or one third of the goal altitude. The Mini Sugar Shot rocket, a prototype of the Extreme Sugar Shot rocket, reached an altitude of 12 km before a catastrophic motor malfunction occurred. The Extreme Sugar Shot rocket, the rocket expected to meet the goal of entering space, has not yet been completed and is a work in progress.