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 increase the castability of the formulation.
It would be very difficult to list all of the additives that can be used in sugar based propellants. This page and its contents is intended to list those additives that have found the greatest use in sugar formulation chemistry. Others will most likely be identified and be found worthy to list on this site. A growing number of additives have personally been engine tested in various sugar formulations. The results of these tests can be viewed through this link.
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. Nakka has documented very nicely the effect of various oxides and some other additives on the burn rate, under pressure, of these materials in KNO3/dextrose (KNDX) sugar propellant. Most often used are Iron Oxides. Red iron oxide (RIO) 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. Atmospheric burn rates of various sugar propellant formulations with and without Iron Oxide additive are listed here. If a metal oxide proves effective in modifying the burn rate of a propellant it is also quite probable that other forms of that metal will also function to a greater or lesser degree in the same manner. For example other forms of Iron such as Ferrocene, Ferrous Gluconate, etc. also function to increase burn rates. Because of their soluble nature in some propellants they often function better than the oxides but are most often much more expensive. The following links highlight some engine testing performed using various Iron compounds as a burn rate accelerators. Even though the atmospheric burn strand burn rate is significantly increased by the addition of RIO, static motor testing carried out with RIO at ~1000 psi, in combination with surfactant, does not seem to increase in burn rate over the rate observed of the baseline 65:35 KNSO propellant formulation. The use of FeGluconate (1%) does increase the burn rate by about 25% over baseline.
(RIO) (FeGlu) (baseline)
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. 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 basic potassium carbonate or potassium hydroxide are found in the oxidizer. These materials, being basic, can react readily with the metal, producing hydrogen and heat, a dangerous combination. The addition of weak acids help neutralize these basic materials, greatly reducing their danger. Among such weak acids are boric acid, tartaric and citric acids, etc.
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 sparky flame/smoke on lift off. This effect greatly enhances the visual enjoyment of such lift offs, but can cause other unwanted side effects (video).
Surfactants are used to reduce the melt viscosity of sugar propellants. This topic gets a whole section in this web site and can be reached via this link. Scott Fintel has found that propylene glycol helps reduce the melt viscosity of sucrose based sugar propellants.