Achieving a satisfactory casting of a sugar propellant (KNSU, KNDX & KNSB) grain can be something of a challenge, especially for the neophyte. Indeed, a certain amount of practice may well be required to develop the proper technique. Due to the relatively high viscosity of the melted propellant slurry, the actual process of loading the material into the casting mould can be tricky. This is especially true since the slurry tends to rapidly thicken as it cools, so the process must be done in an efficient and timely matter. Usually, a combination of pouring and scooping is employed. The elimination of trapped air in the cast material is also an essential part of the process, so care must be taken to minimize the inclusion of air pockets. The particular technique that works best in achieving a satisfactory product may differ due to operator preferences and available facilities. Certainly, practice helps to develop casting skill and to pinpoint process deficiencies, and allows for modifying the technique to make the necessary improvements.|
Instead of practicing this process (or experimenting with modifications to the process) using actual propellant, it is rather convenient, less expensive, and indeed more safe to cast INERT simulated propellant instead. INERT propellant is prepared in exactly the same manner as genuine propellant, except that instead of using potassium nitrate (KN) as the oxidizer constituent, common table salt (sodium chloride) is substituted. The O/F ratio remains the same (65/35) as well as all other aspects of the propellant preparation, described in detail in the respective pages on this web site.
Both the melting rate and the viscosity of the melted slurry of the INERT propellant has been found to be quite similar to genuine propellant. The one difference that has been observed is that both the sucrose-based and the dextrose-based INERT propellants tend to caramelize to a greater extent (illustrated in Figure 1). This is most likely due to the differences in the thermochemical properties between the potassium nitrate and the sodium chloride. This tendency to caramelize to a greater degree does not adversely affect the simulated behaviour of the INERT propellant. In fact, this feature is beneficial since it allows for the INERT propellant to be readily identified. It is important to be able to differentiate between the simulated and genuine propellants for obvious reasons. It'd not bode well to inadvertently load an INERT grain into a rocket motor. Of more serious concern, a grain of genuine propellant must never be inadvertently mistaken as a simulated grain! As such, it would be a good idea to add a drop of food dye or a pigment to the mixture to readily identify the cast product as an INERT grain.
Figure 1 -- Slug of INERT dextrose propellant (left) and genuine KNDX propellant grains (right).
Undoubtedly, there are other non-oxidizing chemical compounds that may be used in place of sodium chloride that will work as well or better. To date, I have only tried sodium chloride and potassium chloride. It would be interesting to find a commonly available compound that will more closely match the thermochemical properties of potassium nitrate in order to even better simulate the propellant in all respects with regard to casting and post-casting properties.
Why does the sodium chloride cause the greater degree of caramelization? The answer likely lies in the different specific heat capacity (Cs) of the sodium chloride in comparison to the potassium nitrate. The specific heat capacity of a particular material relates the amount of thermal energy required to raise the temperature of the material a given amount. Figure 2 shows a comparison of specific heats for various substitute compounds relative to potassium nitrate. Note that specific heat capacity is a function of temperature for most materials.
Figure 2 -- Comparison of heat capacities of various compounds
Not only is the specific heat capacity of sodium chloride less over the full temperature range of interest, it is seen that the potassium nitrate experiences a step increase at approximately 125oC, making the difference even more pronounced. This is due to a phase change of the solid material. The specific heat capacity of potassium chloride (KCl) is notably lower. When KCl was tried as a substitute, it was found that the caramelization was more severe than with sodium chloride as a substitute. This is consistent with the hypothesis that the specific heat capacity of the compounds explains the behaviour. Magnesium sulphate (MgSO4) may prove to be an even better substitute material than sodium chloride. I have not tried this compound, but it is very commonly available as epsom salts. However, epsom salts are hydrated, containing approximately 30% molecular-bound water, and as such, must first be dessicated.