I first became aware of the Potassium Nitrate/Sorbitol (KNSB) rocket propellant some years ago, when I learned that it was being used as a replacement for the KN-Sucrose (KNSU) propellant by some European rocketry organizations. From correspondence with the Dutch group NERO, I learned that Sorbitol was chosen as a replacement for sucrose in an attempt to alleviate certain problems that were attributed to the brittle nature of the KNSU propellant. I more recently learned that sorbitol propellant was experimented with as early as 1977 by the Belgian group VRO, who successfully test-fired KNSB motors. Although similar in many respects to the "classic" KNSU propellant, KNSB has a number of significant advantages, which are detailed further on in this web page.
For convenience, I use the acronym KNSB to refer to the standard Potassium Nitrate/Sorbitol propellant, comprised of 65% Potassium Nitrate (KN) and 35% Sorbitol.
Now the obvious question -- what is Sorbitol?
Sorbitol is an artificial sweetener, very similar in appearance and with similar physical characteristics to both sucrose and dextrose sugars. However, unlike the sugars, Sorbitol is not a carbohydrate. Rather, Sorbitol is a hexahydric alcohol, or an "alcohol sugar". The chemical formula is C6H14O6, which is similar to dextrose, except with 2 additional hydrogens atoms. Although sorbitol occurs in nature, it is manufactured commercially by the reduction of glucose (dextrose). Some other names for Sorbitol are:
Sorbitol is widely used in industry, in particular, in the manufacture of processed items such as confections, toothpaste, chewing gum, candy and syrups. It is also used as a sweetener for diabetics. The nonvolatile, plasticizing properties of Sorbitol are exploited to impart texture to tissue paper, paper napkins, and wax papers. Sorbitol also imparts body and texture to frozen desserts.
At a consumer level, however, Sorbitol is less well known, and is less easy to obtain than dextrose. Some health food stores carry it, although a survey of such stores here in my city came up empty-handed. A local pharmacy was willing to order some, but at an exorbitant price. In other countries, however, Sorbitol may be more widely available. Eventually, I obtained Sorbitol powder by ordering on-line.
Sorbitol has one very odd property. When Sorbitol powder is melted, then allowed to cool, it does not harden immediately. Rather, it forms a transparent mass that is fairly soft and may be cut with a knife. If pulled, it stretches in the form of long strands. After two or three days, the surface hardens, but the inside remains soft. After 5 days, the mass completely hardens and attains a brittle nature. Interestingly, the material in this "cured" state is non-hygroscopic, at least at moderate humidity levels. Sorbitol-based propellant seems to cure sooner, requiring only two or three days to completely harden.
Properties of Sorbitol
The following is a brief comparison of some physical and thermodynamic properties of Sucrose, Sorbitol, and Dextrose . Note that Sorbitol may also exist in a hydrated state, with molecular bound water present.
d = decomposition initiates upon melting
 Anhydrous; hydrated form has ½ H2O or 1H2O
 Anhydrous; 98-100 C. for hydrated form
 Anhydrous; 86 C. for hydrated form
 Units of calories/gram used for GUIPEP. Conversion shown here
Effects of Heating Sorbitol
An experiment was performed to determine the effects of elevated temperature on Sorbitol (alone). This was done by slowly heating a 250 gram batch of Sorbitol in a stainless steel bowl, over an electric element. The effects of heating were noted (qualitatively, such as appearance, colour change, smell, texture, etc) as well as quantitative changes such as mass and temperature, as a function of time. Key results of this testing are summarized in the graph below:
Casting Sorbitol-based PropellantThe process of casting Sorbitol-based propellant is similar to the casting process used for KNSU or KNDX, with a few exceptions. Either an oil-bath may be utilized, or a thermostatically controlled deep-fryer (preferred method).
Preparation and mixing of the two constituents prior to casting, as well as storage of the finished grain, is also similar to that of the KNSU or KNDX propellants.
The casting procedure involves first setting the deep fryer thermostat to the required temperature. Then, starting with a small amount, the powdered mix is added, and stirred often to assist melting. One this has melted, more powdered mix is added. Stirring often helps facilitate melting. The initial colour of the melted slurry is nearly colourless, but begins to turn off-white colour as heating continues. Eventually, the colour will become that of light ivory, as the whole mixture becomes fully molten, and is ready to be cast. If the sorbitol contains residual moisture (which is commonly the case), melting occurs quite rapidly and the slurry has a pure white, almost translucent appearance. Some tiny bubbling may be noticed, as the residual water boils away. When nearly all the residual water has been driven off, the colour will begin to change to a light ivory.
Note -- if the colour of the slurry becomes an amber colour, this is most likely due to impurities (such as anti-caking additive) in the potassium nitrate. The pH of the potassium nitrate should be neutral or slightly acidic (6 - 7); impurities tend to make the potassium nitrate strongly alkaline, which leads to this discolourization, and indeed may affect burn rate and performance.
As can be seen from Table 2, the melting temperature of the KNSB slurry is much lower than KNSU, and no caramelization occurs. Cast KNSB is slightly translucent, and can be nearly pure white in colour.
There are two significant advantages to casting this propellant over casting sucrose-based propellant. One is that the heated slurry does not 'freeze' nearly as quickly, resulting in a better cast grain . This is due to the lower temperature of the slurry, which consequently has a lower heat loss rate to the surroundings. Secondly, since no caramelization (decomposition) occurs, the "pot life" of the molten propellant is not limited, and thus the casting procedure can be performed at a more casual pace.
After casting, the grain must be allowed to cure for 24 hours before removing it from the mould. The core rod should not be extracted before this duration. As previously stated, Sorbitol remains pliable even after cooling, and as such, may become deformed if handled. A full day after casting, the exposed surfaces of the grain are quite hard. The inside, while still "waxy" and easily cut with a knife, is sufficiently rigid to allow handling. Care should be exercised, as the material does tend to chip quite easily. After complete curing, however, the material becomes hard and relatively strong.
A finished cast grain is illustrated in Figure 3 below:
The theoretical performance of the KNSB propellant is very similar to that of both the KNSU propellant and the KNDX propellant. For example, the specific impulse is less than 1% lower than Sucrose-based, the combustion temperature is about 100 C. lower, and the products of combustion are very similar, including the mass fraction of particles in the exhaust. The ideal density of the cast propellant is 2.5 % lower than Sucrose-based propellant.
Effects of Overheating during CastingSince the casting process of this propellant involves operation at an elevated temperature, it is important to know how much of a "safety margin" one has, with regard to possible hazards associated with inadvertent overheating.
To determine the effects of overheating during the casting process, an experiment was performed in which a sample of KNSB propellant was overheated. This involved placing a small sample (16 grams) of the KNSB powdered mixture into an aluminum pan, and heating the sample with a forced-air heat gun (1400 watt; 1100 deg.F max. rating). A type-K thermocouple (chromel-alumel) was inserted into the mixture to monitor temperature. The setup for this experiment is illustrated in Figure 4.
Figure 4 -- Setup for propellant "overheating" experiment (left); Remains after conclusion of experiment (right). Bottom layer (exposed for clarity) was severely charred, yet failed to ignite.
Soon after the sample became fully melted, the material around the edges of the pan began to turn a slight yellowish colour. The temperature of the sample, at this point, was 175 C. The colour eventually became a greyish-amber, after 11 minutes of heating with the temperature around 225 C. Bubbles began to be noticed , and shortly after, puffs of smoke, accompanied by a strong burning caramel odour. The temperature was recorded at 250 C. These indicators of strong decomposition continued, until after 19 minutes, at a temperature of just over 300 C. it was decided to terminate further heating. This was done to allow for an examination of the sample, to see in detail the degree of decomposition that had occurred to this point. Some charring had been visible, and upon close examination, it was seen that the entire bottom layer of the sample was severely charred. As such, it would seem improbable that accidental ignition during casting would occur due to overheating, given the obvious indicators of overheating, in particular, the colour change from white to greyish-amber, bubbling combined with smoke, and the strong burning odour. As well, auto-ignition occurs at a temperature greater than 300 C., whereby the normal casting temperature is below 135 C.,
Needless to say, appropriate safety precautions must always be taken when casting the propellant. Inadvertent ignition due to other unforeseen causes (e.g. electrical short, static electricity, etc.) must be considered as a possibility. Appropriate apparel must be worn when casting the propellant, such as face & hand protection, as well as body & arm protection. Face shield or welders facemask, leather gloves, long-sleeve leather jacket are the minimum. The fact that the KNSB propellant is highly tolerant of overheating must not lead to lax standards of safety, rather, this characteristic should only be considered to be a valuable margin of safety.
The burn rate of a propellant is an important parameter for rocket motor design. This is explained in detail on another web page (Details). Briefly, burn rate will vary dependant upon ambient pressure and temperature. A rather extensive experimental investigation was conducted to determine the effect of pressure upon the burn rate by utilizing a Strand Burner apparatus. Details of this investigation are presented on another web page, but results are summarized in Figure 4. The effect of temperature upon burn rate was also explored, although not extensively. Details are found on the same web page.
It should be noted that KNSB can, under certain conditions, exhibit odd burning rate behaviour. This has been revealed from static testing of KNSB motors by the author and by several other experimenters. The thrust and pressure profiles may not follow theortical predictions based on strand burner data for KNSB for certain motor configurations, especially those with high L/D ratios. Such aberrant thrust and pressure profiles tend to have a triangular shaped profile. This topic is discussed in more detail on the following webpages:
The mechanical properties of a propellant pertain to its behaviour under the condition of loading. Such loading may be tension, compression, shear, bending, torsion etc. A propellant grain may be subjected to one or more of these loads during operation, resulting from, for example, the forces associated with acceleration (flight), or even during static testing, as a result of being subjected to high pressure that exists in the combustion chamber. The grain configuration is significant for the latter condition. If a grain is configured to be freestanding, it is subjected almost exclusively to compressive loading. A case-bonded grain with a central core, on the other hand, is predominantly subjected to tension loading.
D = deflection at beam free end
P = applied load
L = beam length
I = area moment of inertia of beam
It is apparent that this deflection is inversely proportional to E, for a given beam geometry and applied load. This provides a simple means to estimate E for the KNSB propellant. A beam of rectangular cross-section was cast, of approximate dimensions 12 x 0.53 x 0.50 inches. One end was held firmly in a vise, a force was incrementally applied to the end, and the deflections recorded. The setup is shown in Figure 5 and the results shown in Figure 6.
The deflection is seen to be initially linear, becoming less so as the magnitude of the deflection increases.
After completion of the elastic modulus experiment, the beam was used to determine the modulus of rupture. The modulus of rupture (Fb) is the breaking strength of the material, when subjected to bending, and is defined as Fb = M/S where M is the applied bending moment (M = F L for an end-loaded cantilever beam), and S is the section modulus (S = w h2/6 for a rectangular beam). The beam was cut into three specimens to allow for three separate measurements to be taken. The resulting values determined for Fb were 839, 1102 and 1072 lb/in2. Note that for the first test, the broken beam cross-section revealed a small bubble that would have reduced the bending strength somewhat. Therefore, the modulus of rupture for pristine KNSB would be approximately 1100 lb/in2 (7.6 N/mm2).
The pieces of the beam that remained after the previous tests were next used to obtain an estimate of the tensile strength of the material. Tensile specimens were prepared as shown in Figure 6. Holes were drilled through both ends and a ty-rap used to transfer tension loading without inducing any bending to the specimen.
Four tests were conducted. In the first, the specimen broke at the hole location. The resulting values for Ftu determined for the other three tests were: 436, 874 and 1050 lb/in2. These results suggest that the Ftu of pristine KNSB is (at least) 1050 lb/in2 (7.2 N/mm2) . For comparison, this is similar to particleboard. Examination of the broken cross-section did not reveal any significant flaws to account for the large scatter in results. The results show that, as is the case with any brittle material, even tiny flaws act as stress raisers which may result in tensile failure at nominal stress levels well below the strength of pristine material. As such, a propellant grain should be designed to avoid subjecting the grain to tensile stress. If not possible, the allowable tensile stress should be taken as no greater than 20% of the Ftu of the pristine material.
One final note about the mechanical properties of KNSB. In comparison with KNSU and KNDX, it has been noticed that the Sorbitol-based propellant is far more susceptible to "chipping". This is apparent when cutting the material with a hand saw. If too coarse a blade is utilized, the parted material has a tendency to chip.
To case-bond or not to case-bond, that is the question...
The performance of KNSB propellant is theoretically similar to that of both the KNSU and KNDX propellants. Actual static testing of the Kappa-SB rocket motor gave a delivered specific impulse lower than that obtained by either KNSU or KNSB, however. The lower performance figure is likely due to an unusual burning characteristic of the KNSB propellant, which often results in an off-nominal triangular shaped thrust curve. The explanation for this odd behaviour, which has been encountered by other experimenters as well, is a subject of much debate. A contributing factor appears to stem from the fact that it is rather difficult to initiate combustion of KNSB propellant, as determined by a series of Igniteability experiments. More recent experience suggests that this off-nominal performance may be reduced or eliminated by painting the exposed surfaces of the propellant with ignition primer and by having adequate spacing between grain segments (BATES configuration). I typically use no less than 1 cm spacing between segments.
This propellant has a big advantage over the sucrose-based propellant with regard to casting, and has certain advantages over dextrose-based propellant, as well. The temperature of the melted mixture is significantly lower than sucrose-based, which results in an improved safety margin with regard to overheating. Lower casting temperature also leads to the significant advantage of less rapid freeze-up of the melted mixture, which results in a better cast product with fewer voids or air bubbles. Since sorbitol-based propellant does not caramelize, the pot life is not limited. This make it very suitable for casting large propellant grains. The largest KNSB grain that I am aware of had a mass of a 40 kg. (88 lbs.), although it was cast as four separate segments. The motor was developed by the Norwegian group NEAR and has been successfully static test fired a number of times, and subsequently flown in a rocket vehicle.
The cast KNSB grain is somewhat translucent with the colour being nearly pure white. As a result of the translucent nature, it may be advantageous to add an opacifier, such as lampblack, to absorb the heat of reaction and reduce infrared transmission of heat through the grain. However, the addition of opacifier may affect the burn rate. This is an aspect of the propellant that requires further investigation.
The mechanical properties of cast sorbitol-based propellant appear to be similar to sucrose-based and dextrose-based propellants (detail mechanical tests on the latter two propellants have not yet been conducted), that is, rigid and brittle. The tensile strength, however, is appreciable. As such, the structural integrity of a grain made from this material is substantial, and if reasonable care is taken in the design and support of the grain in the motor chamber, large grains may be feasible and capable of withstanding the high acceleration loads imparted by flight.
The hygroscopic nature of this propellant appears to be less than that of the sucrose-based propellant, although further testing needs to be conducted to quantify this. As an ad-hoc experiment, I left a KNSB grain exposed to ambient air, and after several weeks, it was still perfectly dry. However, the ambient humidity may play a role -- the relative humidity (R.H.) had remained quite low over this duration, at between 55-60%. There may well be a R.H. threshold that determines whether or not the grain will be hygroscopic.