KCaSO

Potassium Nitrate/Calcium Ammonium Nitrate Sorbitol Eutectic Propellant (KCaSO)

An intriguing low melting Nitrate based Eutectic uses KNO₃ and Ca(NO₃)₂ in a 54/46 ratio. Such a mixture melts at around 145°C, (eutectic MP table) which is a far lower melting point than the individual components. Calcium Nitrate turns out to be a reasonably available material which is used as a specialty fertilizer. One can purchase it in one of two different hydrated forms. The most readily available form is actually a CalciumAmmonium Nitrate double salt with a formula of 5Ca(NO₃)₂NH₄NO₃.10H₂O. Thus the amount of nitrate supplied by ammonium nitrate is quite small. The presence of the ammonium nitrate does, undoubtedly, depress the melting point of this propellant even lower than the 145°C melting point of the simple KNO₃/Ca(NO₃)₂ mixture. Because this type of calcium nitrate has so much water of hydration associated with it the melting point of this component is already quite low. I am able to prepare the propellant melt by first heating up just the calcium ammonium nitrate to about 100°C. When this material is completly liquid then the KNO₃ and sorbitol are added and the temperature elevated to ~140 - 150°C. The mixture soon becomes very runny. The mixture does not, however become completly clear but remains cloudy. It takes quite a while to remove all of the water of hydration from this mixture. I found that it took a good hour at 150°C. It is interesting to note, however, that the removal of the water from the formulation turns out to have minimal effect on the performance of the propellant. I routinly now launch one or two rockets a month using the propellant listed below without making any effort to allow the water of hydration to escape from the formulation. The ratio of ingredients that delivers the same amount of nitrate as the potassium nitrate delivers in a 65/35 ration formulation is as follows:

Potassium Nitrate - 38%
CalciumAmmonium Nitrate Hydrate - 32%
Sugar - 35%

Some outlets do sell Calcium Nitrate Hydrate which is not associated with ammonium nitrate. I have had less success using this oxidizer than the double salt. The final propellant made from this eutectic is very hygroscopic, especially if time is taken to remove the water of hydration. Grains will need to be put in a dry place as soon as possible after pouring. A burn strand does not burn at atmospheric pressure. I ensured that my test stand grains lit by "priming" them at the forward end of the grain stack with a bit of regular KNO3/sorbitol propellant which I daubed, in molten form, on the inside of the core face. Poured and hardened grains from this propellant are very amorphous and "glassy" compared to the eutectic propellant prepared from a mixture of potassium and sodium nitrates. A burn rate accelerator is also necessary in the formulation. I used 0.5% RIO in this propellant, but typicaly use 1%. An attempt to get test stand data on a simple KCaNH4SO propellant with no RIO resulted in a motor which did burn but produced no thrust. The burn was more like a volcano spewing liquid propellant out of the nozzle. The addition of RIO did allow the motor to develop some pressure and thrust. This initial test stand result is given below. As can be seen this burn developed only 100 psi chamber pressure even though the initial Kn was high at 500. ISP was thus relativly low. Burn time for this 38mm motor was a whopping 10 seconds. The burn was however, steady and complete. It appears that a better burn rate accelerator will be needed in order to use sorbitol as a fuel with this oxidizer mix.