Mixed Nitrate Oxidizer Propellant Formulations
Traditional “sugar” propellant formulations use KNO3 as the oxidizer. Some usage of NaNO3 also takes place. These oxidizers, because of the production of atomized carbonate solids when burning, do not give propellants which have high Isp values when used in rocket fuels. Ammonium perchlorate and ammonium nitrate, which generate all gaseous byproducts when burning, are more efficient oxidizers. The biggest upside to the use of potassium and sodium nitrates are in their thermal stability (i.e. safety) and cost. Indeed, these two oxidizers can be heated significantly above their melting points without decomposition. Another fascinating property that many inorganic salts possess is the melting point depression one observes when mixtures of these salts are melted. This is especially true with the alkali and alkaline earth nitrates. Both KNO3 and NaNO3 have melting points above 300°C by themselves. When mixed at a weight ratio of 54:46 KNO3 to NaNO3 the melting point of the salt mixture drops to 221°C. This 54:46 ratio generates the lowest melting point that can be obtained when these two salts are mixed. This ratio is known as the eutectic point. This eutectic temperature is low enough that the thermal stability of some sugars such as the sugar alcohols (sorbitol, mannitol, erythritol, etc.) is not exceeded. Of even more interest is the fact that when this eutectic oxidizer mixture and a thermally stable sugar are mixed in a traditional weight ratio of 65:35, the sugar is completely or almost completely soluble in the liquid salt. The sugar used also adds a third component to the propellant lowering the melting point even further. Where the melting point of the KNO3/NaNO3 eutectic is 221°C the melting point of this oxidizer mix and sorbitol or mannitol drops to the 180°C range. Thus molten, completely liquid sugar propellant formulations can be prepared. These liquid formulations have very low viscosities, not much greater than water. They pour so easily that with a proper pour spout 18mm grains with central core in place can be poured.
A significant number of “eutectic” nitrate salt mixtures have been discussed in the literature. There is considerable interest in using these “eutectics” as heat exchange fluids in high temperature power generation. The following link (EutecticMix1) connects to a table listing some information found in the literature on some of these eutectic mixtures, the weight percentage of each component present and the melting point of the eutectic. Some of the melting points of these eutectics are quite low. Many of them use nitrates which have considerable tendency to pick up water. Thus propellants made with these nitrates will need to be kept dry. Many of the mixtures use nitrates which produce formulations that give burn strands that do not support combustion at atmospheric pressure. Not all of the eutectic mixtures listed in this table have been evaluated as potential oxidizers in sugar propellant formulations at this time. The links below connect to engine test stand data that has been generated to date on eutectic mixtures that do perform well enough to function as useful propellants.