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, in particular, the Dutch group NERO. In correspondence with this group, 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. 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:

• Neosorb
• Sorbite
• Karion
• Sorbit
• D-Sorbol
• D-Glucitol

Sorbitol is widely used in industry, in particular, in the manufacture of processed items such as confections, toothpaste, 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 from 1-Stop-Sugarless-Shop, which specializes in sugar-free products and supplies. Other sources for Sorbitol are listed in my Links webpage. Nowadays, I get all my sorbitol from Sam & Vici's Rocket Company.

The molecular structure, which is shown in Figure 1, is a chain rather than the cyclic form of the sugars. The Sorbitol molecule is much smaller than the sucrose molecule. Smaller organic molecules often have a lower melting point and also have less tendency to decompose upon heating, which is a result of the molecule having less tendency to break up as it "thrashs around" due to thermal agitation. Testing of the Sorbitol-based propellant confirmed that the melting point is significantly lower than sucrose-based, and, significantly, no caramelization (decomposition) occurs as a result of normal heating. As far as hygroscopicity is concerned, there does appear to be some improvement over sucrose-based and dextrose-based. The mechanical properties (such as tensile strength and elastic modulus) of the Sorbitol-based propellant are similar to sucrose-based and dextrose-based propellant.

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.

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.

Table 1 -- Comparison of Properties

	Sucrose	Sorbitol	Dextrose
Chemical Formula	C12H22O11	C6H14O6    [1]	C6H12O6
Molecular Weight (g/mole)	342.3	182.2       [1]	180.16
Melting Point (C.)	185 (d)	110-112    [2]	146 (d)   [3]
Density (g/cm3)	1.581	1.489	1.562
Enthalpy of Formation (kJ/mol)	-2221.2	-1353.7	-1274.5
Enthalpy of Formation (cal/gram)	-1549.9   [4]	-1774.8   [4]	-1689.7   [4]
Appearance	white granular or cohesive powder	white granular or cohesive powder	dry white powder

d = decomposition initiates upon melting
[1] Anhydrous; hydrated form has ½ H₂O or 1H₂O
[2] Anhydrous; 98-100 C. for hydrated form
[3] Anhydrous; 86 C. for hydrated form
[4] Units of calories/gram used for GUIPEP. Conversion shown here

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:

From the graph, the following observations may be made:

1. Melting of the sample was first noticed after about 8 minutes of heating, at a temperature above 91 C. deg.

2. Sample was fully melted after about 18 minutes, at a temperature of 123 C. deg. As would be expected, a temperature plateau is observed until the melting is largely complete.

3. The temperature of the sample continued to rise with further heating, although at a decreasing rate.

4. There was no indication of decomposition as the temperature increased further. A faint yellow colour was detected, although this could very likely be a result of impurities in the sample. Since sorbitol is synthesized from glucose, it is conceivable that the trace amounts of glucose were responsible for the slight caramelization.

5. Very little change in mass occurred. A total of 4 grams reduction represents a percentage mass loss of about 1.5%. Since the resolution of the scale is 2 grams, the actual loss may be slightly more or less. The percentage of water bound in the ½ H₂O hydrated sorbitol is 4.7%. Therefore, it would appear that the sorbitol that was tested was not the hydrated form, and that the water present was simply absorbed moisture. This is contradicted, however, by the melting temperature of the sample, which corresponded to that of sorbitol hydrate.

The 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 (the method I currently use). If the oil-bath method is used, the bath temperature should be between 125-135 C. Instead of using shortening for the bath, I've used paraffin wax (m.p. 50-60 C), which is odourless, non-degradable and smokeless at this relatively low temperature.
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 preheating the oil (or deep fryer) to the required temperature, and maintaining this temperature +/- 5 degrees. 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. Once all the powdered mixture has been incorporated into the melt, it is further heated and stirred, to eliminate any lumps that may be present. An additional five to ten minutes of heating will bring the slurry up to the casting temperature. Note that the slurry will not become fluid enough to pour. The consistency will remain quite thick, and must be simultaneously poured and scooped with a spoon or spatula onto the funnel, and allowed to flow into the mould. Immediately after loading, the coring tool would be inserted into the mould, making certain it is fully inserted all the way down. This completes the casting operation. The mixture is then allowed to cool and harden.

Note -- if the colour of the slurry becomes an amber colour, this is most likely due to impurities in the potassium nitrate. The pH of the potassium nitrate must be neutral (6.5 - 7); impurities tend to make the potassium nitrate strongly alkaline, which leads to this discolourization.

As can be seen from Table 2, the melting temperature of the KNSB slurry is much lower than KNSU, and no caramelization occurs. The cast Sorbitol-based propellant is slightly translucent, and is 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 12 to 24 hours before removing it from the mould. The bore 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 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 (65/35 O/F) 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.
Details

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 absolute 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.

Burn Characteristics of Sorbitol Based Propellants
Kappa-SB rocket motor -- Static Test KSB-001 Report
Kappa-SB rocket motor -- Static Test KSB-002 Report

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.
Experience has proven that, configured as a freestanding grain, a brittle propellant can perform very reliably with no structurally related problems. A brittle material is one that is strong in compression, but may have poor capability to withstand tension loading. But can a brittle propellant, such as KNSB, alternatively be configured as case-bonded? To answer this question, it is first necessary to know two important mechanical properties:

1. Tensile strength (Ftu). This is the force per unit area (stress) that a material loaded in axial tension ultimately can carry before fracture (breakage) occurs.
2. Elastic modulus (E). Also known as Young's modulus, this term defines how much strain (stretch) occurs when a material is subjected to stress.

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.
Since the moment of inertia is given by I = 1/12 w h³, the elastic modulus may be expressed as   
A straight line was fit through the linear portion in order to calculate elastic modulus. From this, the value of elastic modulus was determined to be approximately E = 850 000 lb/in² (5.84 GPa). This is similar to bakelite. For further comparison, Plexiglas, 450 000 ; Nylon, 260 000; Polyethylene, 35 000; Aluminum, 10 000 000 lb/in².

After completion of the elastic modulus experiment, the beam was used to determine the modulus of rupture. The modulus of rupture (F_b) is the breaking strength of the material, when subjected to bending, and is defined as F_b = 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 F_b 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.

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 the 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 results in a 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.

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 prudent to add an opacifier, such as lampblack, to absorb the heat of reaction and reduce infrared transmission of heat through the grain (which may result in accelerated burning 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, very large grains are likely feasible and capable of withstanding the high acceleration loads imparted by flight.

The hygroscopic nature of this propellant appears to be significantly less than that of the sucrose-based propellant, although further testing needs to be conducted to quantify this. I recently 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.