This web page describes the KN-Dextrose rocket propellant, referred to by the acronym KNDX. This propellant is formulated as a heat-fused mixture of potassium nitrate (KN), and dextrose. This propellant was developed as an alternative to the traditional "sugar-based" propellant, KN-Sucrose (KNSU), and the more contemporary KN-Sorbitol (KNSB). Although the sucrose-based propellant is, in many ways, an excellent amateur propellant, there are a number of drawbacks when using sucrose as the fuel/binder, as described on the KNSU web page. With this in mind, several years ago (early 1998) I began to search for an alternative to using sucrose as the rocket fuel. Since sucrose is a carbohydrate (interestingly, carbohydrates are the primary sources of energy of foods), I decided to investigate other forms of similar compounds, starting with other kinds of sugars. Sucrose, often referred to as table sugar, is the most common sugar. Other naturally occurring sugars are:
Dextrose monohydrate, as obtained in the white granular form, is essentially identical in appearance to sucrose. It is easily ground to a fine powder by use of a coffee grinder.
It is interesting to note that dextrose, in the form of blood sugar, is the main energy source for most living organisms.
Details of the results of testing that was conducted during the development of KNDX, as well as theoretical performance of the KNDX propellant, are presented in this web page.
Properties of some Sugar compounds
The following is a brief comparison of some physical and thermodynamic properties of Sucrose, Dextrose Monohydrate, and Anhydrous Dextrose sugars:
d = decomposition initiates upon melting
Effects of Heating Dextrose
To investigate the potential usefulness of dextrose-based propellant, I conducted a series of experiments, on dextrose alone, as well as on dextrose-based propellent. The first of the series of experiments that were performed was to determine the effects of elevated temperature on dextrose monohydrate (alone). This was done by slowly heating a 250 gram batch of dextrose monohydrate 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. Some results of this experimentation are summarized in Figure 2.
Figure 2 -- - Effects of elevated temperature on Dextrose Monohydrate
Note that the anhydrous mass of the sample was calculated to be 180.16/198.18 * 250 = 227 grams.
The formulation of KNDX propellant is the same as KNSU and sorbitol-based KNSB, that is, 65% KN (oxidizer) and 35% Dextrose (fuel/binder), by mass. This ratio represents a practical upper limit for "solids" loading of a sugar binder, while maintaining good performance and burn rate characteristics. A higher O/F ratio, and thus higher "solids" loading, may give slightly enhanced performance, but leads to a thicker consistency of the melted mixture (slurry). This makes casting more difficult. The effect of using a lower O/F ratio is reduced performance and a slower burning rate. However, the slurry has a thinner consistency, which makes casting easier. For those who are casting this propellant for the first time, it is best touse the 60/40 O/F ratio. This ratio may likewise be a better choice when casting small-diameter grains.Potassium nitrate, also known as saltpetre, is a commonly used chemical (used, for example, for pickling meats, and in toothpaste for "sensitive teeth"). Iíve purchased potassium nitrate in 2 kg. lots at a veterinary chemical supply store. Potassium nitrate is also sold as 14-0-45 fertilizer at farm supply stores, typically 98-99% pure. This is by far the most economical form, and the performance is generally no different that purer grades. Sadly, nowadays the sale of potassium nitrate has had restrictions placed on it, making is harder for prospective rocket engineers to carry on their calling.
It is important to note that 9% of the mass of dextrose monohydrate is bound water. As such, the ratio of potassium nitrate to dextrose monohydrate must be adjusted to achieve a given oxidizer/fuel (O/F) ratio. In other words, the required amount of dextrose monohydrate required in the mixture must be multiplied by 1.1, which is the ratio of Molecular Weights (198.18/180.16). For example, if 100 grams of KNDX propellant of 65/35 O/F ratio is required, the quantity of dextrose monohydrate required is 0.35 * 100 * 1.1 = 38.5 grams. Instead of using dextrose monohydrate, it has been found to be more expedient to use the dessicated form: anhydrous dextrose. Dessicating serves to drives out the water, allowing the unaltered fraction of 35% dextrose to be used. Dessicating is readily done by spreading dextrose monohydrate onto a cooking sheet, to a maximum depth of 2 cm, then placing this into a preheated oven set at 80oC (175 oF.) for between 1 and 1.5 hours. Once cooled, the resulting caked lumps may be reduced to a fine powder by using a flour sifter or by rubbing inside a strainer.
Preparation and Mixing
The first step in preparation of the propellant is to grind, or mill, the potassium nitrate to a fine texture. This may easily be done with the use of an electric coffee grinder, such as that shown in Figure 3. Adding two tablespoons (30ml) to the hopper, the grinder is then run about 20-25 seconds. To facilitate milling, the grinder is slowly "gyrated" about its base. Milling as such will reduce the particle size to an average of 50-100 microns.
It should be noted that the viscosity of the melted propellant slurry is highly dependant upon the particle size of the potassium nitrate. When prepared as described above, the slurry will not be fluid enough to cast, and as such it is necessary to scoop the propellant into the casting mould. The slurry will be more fluid and easier to cast if the potassium nitrate is milled for a shorter time, such as 10 seconds. Or if the potassium nitrate is obtained as a fine granular form (similar to table salt), it can be used "as is". The drawback is slightly reduced performance due to less efficient combustion, combined with a slightly longer burn time compared to propellant made with fine-grind oxidizer.
Following the milling process, the two constituents are carefully weighed out using an accurate scale. Enough of the powdered mixture of potassium nitrate and sucrose has to be prepared to take into account the inevitable waste resulting from the casting procedure, typically 20-25% additional for small batches, less for larger batches.
After individually weighing out the two constituents, the two are blended together in a single container. Complete mixing of the two is necessary for optimum and consistent performance. I built an electric tumbler for this purpose (Figure 4). The powdered mixture is loaded into a tupperware container, then secured to the rotating drum with rubber bands, then tumbled for several hours. The tumbler rotates at 30 RPM. As a guideline, I allow one hour per hundred grams of powdered mixture.
Once the mixing operation has been completed, the powdered mixture is transferred immediately to a closed container for safe storage. At this stage, the mixture is readily combustible amd sensible precautions are observed to keep it away from any possible ignition sources.
Casting Dextrose-based Propellant
The casting process involves heating of the powdered mixture until it becomes molten, then casting into a mould to produce the propellant grain of the desired shape. The required temperature that the mixture must attain is just above the melting point of dextrose. The potassium nitrate, which has a much higher melting point, remains as solid particles that partly dissolve in the dextrose. The result is a slurry of solid oxidizer particles suspended in liquid dextrose medium.
Heating the mixture is done using a thermostatically controlled electric "deep fryer". A thermostatically controlled skillet may also used for heating KNSU.
It is essential, from a long-term safety consideration, that only a thermostatically controlled heating vessel with no exposed elements be used for heating propellant, the critical intent being that no exposed heating surface is of significantly higher temperature than the melting point of the propellant.
(click for larger image)
For Dextrose monohydrate-based, the casting temperature should be in the range of 125-130oC., and for anhydrous Dextrose-based, it should be in the range of 145-150 oC. To monitor the temperature of the heated slurry, a dial-type candy thermometer, a digital thermometer equipped with a probe sensor, or an infrared thermometer may be used. I've used all three types and have found the infrared thermometer to be the most convenient.
The casting procedure involves first preheating the deep fryer or skillet to the required temperature, and maintaining this temperature +/- 5 degrees. Then, starting with about half the total amount, the powdered mix is added, and stirred often to assist melting. Once this has melted, the remaining powdered mix is added. Stirring helps facilitate melting. The initial colour of the melted slurry is nearly colourless, but begins to turn off-white colour as slight caramelization commences. Eventually, the colour will become that of ivory, as the whole mixture becomes fully molten, and is ready to be cast. A colour darker than this implies that the mixture has been heated too long, as caramelization is a function of time as well as temperature. 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 is 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.
The KNDX propellant should be removed from the casting mould and the bore rod extracted as soon as the material has fully solidified and cooled (typically an hour or so). It is not necessary for the cast propellant to "cure" for an extended period of time, as is required with KNSB.
Casting Dextrose monohydrate-based propellant
The heating experiments that were conducted on samples of dextrose monohydrate clearly showed that water of hydration which was released upon heating required appreciable time to fully evaporate. The implication is that propellant prepared with dextrose monohydrate will have some moisture remaining in the cast propellant unless it is heated long enough to ensure that all the moisture has been driven off. Otherwise, exactly how much water remains once the propellant has been cast cannot be known for certain. The presence of moisture in the propellant affects certain characteristics. It has been demonstrated by experiments, and by theoretical analysis, that moisture affects the following:
The following temperatures were measured during the casting process, utilizing a thermocouple, and are compared to sucrose-based:
In order to eliminate all residual water from the dextrose monohydrate, it is necessary to heat the melted propellant for a sufficiently long period of time. But how long a heating period? Fortunately, there does seem to be an indicator of the dessicated condition. In Figure 2, it is seen that when all the moisture has been eliminated (mass=anhydrous mass), decomposition commences. At this point, it was observed during the investigation that the colour of the sample showed a tinge of yellow, which soon became more pronounced as the sample was heated further. Therefore, if the propellant is heated until this colour change commences, it is assured that nearly all the moisture has been driven off.
Casting Anhydrous Dextrose-based propellant
Certainly the best way to ensure that the cast propellant has minimal residual moisture is to eliminate all the water beforehand. This is done by preparing the propellant mixture using anhydrous dextrose, rather than dextrose monohydrate. Another advantage is that no adjustment to the O/F ratio is required in weighing out the constituents, as is required with the monohydrate.
When casting the propellant, somewhat greater heating is required before melting will commence. In fact, melting initiates at the known melting point of dextrose (not surprisingly). The following temperatures have been measured during the casting process:
When casting propellant prepared with anhydrous dextrose, it is not necessary to continue heating until caramelization commences, indicated by ivory coloured slurry; it may be poured into the mould as soon as the mixture is fully melted and blended.
Notes on casting dextrose propellant
The cast propellant should be in the colour range between ivory (#FFFFF0) and beige (#F5F5DC), as illustrrated in Figure 6 below:
Water Content and Hygroscopicity
After casting, once the grain has cooled, the propellant surfaces may be sticky due to moisture deposition, depending on the relative humidity. The propellant will dry out typically after 24 hours, if placed within a dessicator. A simple dessicator can be made from a tupperware container with a layer of calcium chloride pellets or flakes spread over the bottom pan. Calcium chloride is readily available as an ice & snow melter or as a drying agent for damp environment.
It would appear that there is a relative humidity (R.H.) threshold that determines whether or not the propellant will be hygroscopic. As with the KNSU propellant, the KNDX propellant is hygroscopic (absorbs moisture from the air). However, the hygroscopic nature of the dextrose propellant is far less severe than sucrose-based. As an ad-hoc experiment, I'd exposed a grain cast from KNDX monohydrate to the open air, and after about two months, it was still bone dry. The R.H. over this duration ranged from 55-60%. At high R.H. levels (around 90%), however, this propellant is indeed very hygroscopic. Interestingly, experimentation has suggested that the resistance to moisture absorption is better if cast as dextrose monohydrate based rather than anhydrous-dextrose based. Perhaps this is a consequence of the lower casting temperature.
The theoretical performance of the KNDX propellant is essentially the same as that of the KNSU and KNSB propellants. For example, the specific impulse (65/35 O/F) is less than 1% lower than KNSU, the combustion temperature is nearly identical, 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 also nearly identical (1.879 compared to 1.888 g./cu.cm.).
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 KNDX (anhydrous) propellant, of 65/35 O/F ratio, was overheated. This involved placing a small sample (approx. 10 grams) of the KNDX 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 7.
Melting of the mixture was rapid, owing to the high output of the heat gun, and brown coloured regions soon appeared in the melted mixture, as rapid decomposition of the dextrose began. The sample soon fully caramelized, with the temperature recorded at 167 C. (333 F.). Interestingly, the temperature of the propellant reached a "plateau" at this point, and would not rise until after several minutes of further heating, after which the propellant was highly decomposed with much charring. Only then did the temperature start to climb, eventually reaching a recorded maximum of 300 C. (572 F.). Heating was stopped at this point, with the propellant failing to ignite (the charred appearance of the remaining propellant after this experiment is shown in Figure 7).
It would seem that, once decomposition of the dextrose initiates, that the added thermal energy of heating goes solely into decomposing the dextrose, rather than raising the thermal energy (temperature) of the propellant, an rather nice characteristic. As such, it would seem improbable that accidental ignition during casting would occur due to overheating, provided that the proper heating method is employed as described above in the Casting section, given the obvious indicators of overheating, such as severe caramelizing, burnt sugar odour and the fact that auto-ignition occurs at a temperature greater than 300o C.
In all the years of my rocketry propellant making, I never had an accident that resulted in injury. Mishaps, and the unexpected, certainly occurred, but because I always took sensible precautionary measures, and considered safety as the number one priority, no harm resulted.
The burn rate of a propellant is a key parameter used in rocket motor design. This is explained in detail on another web page (Details). Briefly, burn rate will vary dependant upon pressure and temperature. The effect of pressure is far more pronounced. As such, 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 8. Interestingly, KNDX was found to be exhibit a plateau burn rate behaviour, as can be seen in Figure 8, where the burn rate curve flattens off in the regime between about 800 and 1200 psi. The effect of temperature upon burn rate has also been explored, although less extensively. Details are found on the same web page.
Experimentation was conducted on KNDX made with two different burn rate modifiers . The modifiers were red iron oxide and air-float charcoal. It was found that the burn rate was increased by about 20% with the addition of 1% iron oxide, over the full pressure regime explored. The effect of doping the propellant with 1% charcoal was an increase in burn rate throughout the lower pressure regime. Perhaps the most significant effect of these modifiers was to dampen the plateau effect. The details on this experimentation are given in the Burn Rate Experimentation web page.Burn rate as a function of Oxidizer/ Fuel (O/F) ratio
Motor Design Charts for KNDX propellant
Over the past decade and a half, I have developed and successfully fired a number of successful motors which utilize KNDX propellant, including:
The delivered performance of the KNDX propellant is essentially identical to that of the KNSU and better than that of the KNSB propellant. Operating in the Kappa motor, the propellant was found to deliver a respectable specific impulse of 137 seconds.
The burn rate of this propellant (at elevated pressure) was found to be somewhat lower than sucrose-based propellant. This allows for the design of a rocket motor with a greater burn time. One drawback of KNDX propellant is the "plateau" burn rate behaviour, which tends to complicate the design process to some degree. However, recent experimentation has shown that the "plateau" behaviour can be suppressed by doping the formulation with charcoal pigment. Iron oxide provides a similar effect, and results in a greater burn rate.
The KNDX propellant has a considerable advantage over the sucrose-based propellant with regard to ease of casting. The temperature of the melted mixture is significantly lower, which results in an improved safety margin with regard to overheating. Lower casting temperature also leads to the noteworthy advantage of less rapid "freeze-up" of the melted mixture, which results in a better cast product with fewer voids or air bubbles. Decomposition (caramelization) of the propellant due to heating is minimal, and pot life of the melted slurry is superior to that of KNSU (although less than that of KNSB).
The mechanical properties of cast dextrose-based propellant are very similar to both KNSU and KNSBpropellants, that is, rigid and brittle, whether prepared using monohydrate or anhydrous form of dextrose. However, the strength of dextrose-based propellant (and of the KNSU & KNSB) is appreciable, and does not appear to pose any practical limitations on grain size or geometry.
The hygroscopic nature of this propellant appears to be less problematic than that of KNSU or KNSB propellants, particularly if cast in the monohydrate form. As well, the ambient relative humidity (R.H.) seems to play a significant role. It would appear that there is a R.H. threshold that determines whether or not the propellant is hygroscopic. It has been determined by experience that if the R.H. is maintained less than approximately 60%, the propellant will remain completely dry to the touch when exposed to the open air.
Download complete report of the dextrose heating experiments
Download complete report of the burn rate experiments