IntroductionThis web page describes the "classic" sugar-based rocket propellant comprised of a fused mixture of Potassium Nitrate serving as the oxidizer, and Sucrose (table sugar) serving as the fuel and binder. I first became familiar with the Potassium Nitrate-Sugar propellant in the early 1970's from Captain B.R.Brinley's book Rocket Manual for Amateurs. In his book, Brinley briefly describes this propellant, focusing mainly on its drawbacks. To his credit, however, he did state that "it might be worth your while to undertake a development project on this particular propellant...". And so this is exactly what I proceeded to do, and spent many years experimenting with this propellant. I conducted numerous static motor tests in order to determine the optimum O/F ratio, measured burning rate at various temperatures, pressures and O/F ratios, tried various grain configurations, performed combustion product analysis (using the old ORSAT apparatus, as well as apparatus of my own design), measured the propellant "impetus" by use of "closed vessel" combustion. And of course, I launched many rockets powered by this propellant, over 57 flights in total. I performed further investigation on this propellant during my final year at university, in particular, the theoretical performance, and wrote my graduation thesis Solid Propellant Rocket Motor Design and Testing based on both my theoretical and experimental work. So I got to know this propellant quite well indeed!
For convenience, I now use the acroynm KNSU in reference to the contemporary formulation of this propellant, which is 65% Potassium Nitrate and 35% Sucrose.
Although not a high performance propellant, KNSU delivers a fair specific impulse. Its main advantage over many other amateur formulations is the relative ease and safety of preparation and usage. Another factor that makes this propellant popular is the ingredients, both of which are commonly available.
The propellant described in this web page is heat cast, using a melting technique, to form the propellant body, or grain. Although some experimentalists have utilized the combination of KN and sucrose in the powdered, pressed form, or cold cast (using a solvent such as water), I have avoided such for a number of reasons. The two main reasons, however, are superior mechanical properties of the fused form, and the consistency of performance that is ensured by an essentially invariable final product that results from the melting method.
As mentioned earlier, the standard ratio of constituents is 65% Potassium Nitrate and 35% Sucrose, by mass. This ratio has proven to give the best overall performance combined with acceptable casting qualities. Theoretically, the highest specific impulse is delivered at a 66/34 ratio, although the standard 65/35 ratio tends to be used by most experimentalists. There are three reasons for this:
The Sucrose that I used was the very fine powdered form, commonly referred to as icing (or confectioners) sugar, rather than the common granular (table) sugar. This form contains up to 5% cornstarch. Since cornstarch and sucrose are chemically similar, the effect of this impurity may be neglected. Granular sugar may be used, it simply needs to be ground to a fine powder first.
Potassium nitrate, also known as saltpetre, is a commonly used chemical (used, for example, for pickling meats, and in toothpaste for "sensitive teeth") and as such is quite readily available. Other uses for potassium nitrate are for hydroponics, and in gardening for raising the nitrate level of soils. Iíve purchased potassium nitrate in 2 kg. lots at a veterinary chemical supply store. It is also available at many pharmacies, sold typically in 100 gram or 8 oz. tins. 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 no different that purer grade. Other commercial sources for potassium nitrate are listed in my Links webpage.
Recent experimentation has shown that stump-remover is a viable source of potassium nitrate, at least the particular brand tested (Wilson's). According to the MSDS's, Spectracide, Later's and Dragon brands of Stump Remover are essentially pure potassium nitrate. Stump-remover is readily available at garden shops, home renovation outlets, and some hardware stores. Before use, however, the product may need to be washed in methanol to remove any caustic coating.
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 2. The hopper should be half filled, then run the grinder for about 30-40 seconds. To facilitate milling, the grinder should be slowly "gyrated" about its base. Milling as such will reduce the particle size to an average of 50-100 microns. As obtained, the potassium nitrate particles are typically 150-250 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 alot more fluid and easier to cast if the potassium nitrate is milled for a shorter time, such as 10 seconds. The potassium nitrate will consequently have a larger particle size. The drawback is slightly reduced performance due to less efficient combustion.
If granulated sugar is used rather than icing sugar, this should also be milled. A second grinder should then be used, rather than the one used for the potassium nitrate. The reason is that the seal that separate the grinding compartment from the motor compartment is not perfect. Over time, some fine powder finds its way into the motor compartment, and slowly accumulates. If the accumulated powder is solely sucrose, or solely potassium nitrate, the risk of the accumulation igniting is minimal. However, if the same grinder were used for both sucrose and oxidizer, the risk of combustion is much greater, creating an obvious hazard.
Following the milling process, the two constituents are carefully weighed out using an accurate scale or balance beam. 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% for small batches, less for larger batchs. For the B-200 motor, which has a grain capacity of 225 grams, I would prepare 182 grams potassium nitrate, and 98 grams sucrose, for a total of 280 grams powdered mix. For the C-400 motor, with a grain capacity of 380 grams, the mixture would be 297 grams potassium nitrate, and 160 grams sucrose, totaling 457 grams dry mix. For the smaller A-100 motor, with a grain capacity of 100 grams, 131 grams of mix would be prepared, consisting of 85 grams potassium nitrate, and 46 grams of sucrose.
It should be noted that both sucrose and potassium nitrate, particular the finely divided form, are slightly hygroscopic and therefore a small percentage of the mass is due to moisture. If this percentage were sufficiently high, this could affect the final O/F ratio. However, testing that was conducted on sucrose showed that less than one percent moisture is present. Similar testing indicated the same is true for potassium nitrate, and thus moisture content may be neglected with little error in the final product.
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 rotating mixer for this purpose (Figure 2). The powdered mixture is loaded into a tupperware container, then secured to the rotating drum with rubber bands, and, then allowed to mix for several hours. As a guideline, I allowed one hour per hundred grams of powdered mixture.
MouldThe mould is the "form" into which the propellant is cast to make the grain of the desired shape. All of my motors have had a "hollow cylindrical" shaped grain. This is simply a cylinder with a round, centrally located hole, or core. With such a grain, the mould can be any circular form, such as a metal tube. The core is formed in the grain by inserting a coring tool into the mould after the propellant has been loaded.
For the A-100, B-200 and C-400 motors, the motor casing (with attached nozzle) served as the mould. Since the grain is to be removed after casting, the inside surface of the motor is first sprayed with light oil. In order to form the grain with a diameter slightly less than that of the inside diameter of the motor casing, a spacer is placed inside the motor (the grain is required to be a slightly loose fit in the motor in order to have all surfaces exposed for burning). The spacer consists of a sheet of medium weight paper (24 lb/90g/sq.m.), well lubricated first with oil, then with a light coating of grease. The paper should be sized such that it forms two or three layers. The grease that I used was automotive wheel bearing grease, which retains its viscosity at elevated temperature. For the B-200 motor, the paper size was 6.35 x 8.75 in., large enough to form a double layer around the inside perimeter of the casing. The sheet is carefully rolled and placed inside the motor, tightly against the walls. |
The nozzle throat must be plugged to prevent the molten mixture from escaping. A cork plug provides a simple solution. Prior to casting, the motor must be held in a fixture, with an open, lightly lubricated metal funnel positioned to guide the molten mixture into the motor. I found that it is not necessary to preheat the mould.
The coring tool is a steel or aluminum rod, tapered at one end. The other end is fitted with a handle, and an adjustable guide that allows for exact centering of the coring tool in the mould. Prior to casting, the coring tool should be lightly coated with silicone lubricant, then preheated in an oven set at 300 F (150C.).
Another tool that aids the casting process is a plunger. This is simply an aluminum or steel rod of a diameter slightly less than the grain diameter. This tool is used to press down on the molten mixture once it is poured into the mould in order to compact the slurry and squeeze out any trapped air. Prior to casting, the plunger should be placed in a freezer for an hour or so. This is important and serves to prevent the molten mixture from sticking to the plunger.
CastingThe "slurry" 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 sucrose. The potassium nitrate, which has a much higher melting point, remains as solid particles. The result is a slurry of solid oxidizer particles suspended in liquid sucrose medium.
The vessel which I originally used for heating the mixture was a cast iron pot, of about one litre capacity, modified to be held partly submersed in an oil bath. Cast iron retains heat, which is beneficial for maintaining the mixture in a fluid state during the actual pouring operation, important, since the mixture "freezes" rapidly once it begins to cool. More recently, I've been using an oil-free method, which involves heating the mixture directly in a thermostatically controlled electric "deep fryer", with the thermostat set to 380 F. (Figure 5).
It is essential, from a long-term safety consideration, that only these methods be employed for heating propellant, the critical intent being that no exposed heating surface being of higher temperature than the melting point of the propellant. For this reason, heating the mixture in a container over an electric or gas stove element is absolutely unacceptable and must never be attempted (actually, common sense tells us this!). Although this propellant is highly tolerant of overheating, we want to keep this feature solely as a valuable safety margin. Suitable protective gear must be worn during the casting operation (see Effects of Overheating during Casting).
(click for larger image)
The oil bath method utilizes a deep fryer, half filled with oil. Vegetable shortening works well, as it does not smoke at the required temperature, and becomes solid once cooled, convenient for storage. Other oils or fats with a high smoke point may alternatively be used. To monitor the temperature of the oil (or mixture), I used a Taylor, dial-type candy thermometer with a range of 100 - 400 deg. F. A better solution is to use a digital thermometer equipped with a probe (many of today's multimeters have this capability).
The casting procedure involves first preheating the oil/deep fryer to a temperature of 380 F, and maintaining this temperature +/- 5 degrees (for the 65/35 ratio; for 60/40 the temperature should be 370 F). 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, and such is continued until all has been incorporated. Stirring often is important to prevent overheating , which leads to a greater degree of caramelization, and consequently reduced performance of the propellant. Caramelization is characterized by a darkening of the melt. The initial colour of the melted slurry is nearly colourless, but begins to turn tan colour as caramelization commences. Eventually, the colour will become that of peanut butter, 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. The entire casting operation, from start of melting, to final pouring, usually took me about 1/2 hour with the oil bath method, twenty minutes if heated directly in the deep fryer.
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.
I found that it was very important to remove the grain from the motor mould after an exact amount of time had elapsed. If allowed to cool too long, it would be difficult to remove the grain. If removed too soon, it would still be flexible and could become deformed. I found that 45 minutes was the required cooling period. Using the handle of the coring tool, the entire grain could be quite easily extracted. The paper spacer could then be peeled off, and with a twist of the handle, the coring tool would snap loose and slide out of the grain. I would then use mineral spirits to clean the oil and grease from the grain, trim it to size, and (importantly) accurately weigh and measure the grain, record colour and amount and type of defects (such as bubbles), then place it inside a plastic bag. Better yet, vacuum seal the grain. At this point, the grain would be put inside a protective container and stored in a deep freeze until ready to be loaded into the motor. Loading the grain into the motor is one of the last steps before a flight (or static) test is performed. Two finished grains for the B-200 motor are shown in Figure 6.
For the neophyte who has never cast any sugar-based propellants, it is suggested that the grain casting process be first practiced using INERT propellant. The INERT propellant is prepared in exactly the same manner as KNSU, with the important exception that table salt (sodium chloride) is substituted for the potassium nitrate. This "propellant" melts and casts in a similar manner to the genuine thing, however, INERT propellant is completely non-combustible and inexpensive.
Notes on casting sucrose-based propellant
The cast propellant should be within the colour range of the two grains shown in Figure 6 below:
Alternative Casting MethodsThere are alternative methods of casting KNSU propellant that other rocketry experimenters have developed. Two methods that I am aware of deserve particular attention. Both involve the inclusion of water and/or corn syrup to the basic propellant ingredients. One method involves dissolving the potassium nitrate and sucrose in water, then boiling until all the water has been eliminated. The suggested ratio of water to potassium nitrate/sucrose is 2 to 3. Interestingly, the thermostat of the heating vessel may be set to a lower temperature than with the slurry method, typically 130 to 150oC. (260 to 300o F.).
Although I have not used this technique to cast an actual propellant grain, I have conducted preliminary experimentation of both the standard 65/35 ratio, as well as the 60/40 ratio. For both, I have have produced small samples of propellant, including strands for burn rate measurement. It was found that the water boils away quite rapidly. An off-white slurry results, which must be heated further to drive out all residual water. This condition is indicated by a slight odour of caramelization. Further heating is done until the colour of the slurry is a light tan. The consistency of the 65/35 rato is definitely thicker than that of the slurry method, and the propellant needs to be packed into the mould rather than poured or scooped. However, it was found that the viscosity of the 60/40 ratio is sufficiently lower such that it can be cast in a manner similar to regular KNSU. Curiously, when the propellant cools and hardens, it is completely dry, unlike propellant prepared by the slurry method, which tends to develop a sticky film immediately after casting. Indeed, the hygroscopic nature of the propellant seems to be reduced...a sample strand that has been left in the open air for 6 months, at humidity levels ranging from 50 to 70%, has remained bone dry.
There are interesting advantages to using water as a medium for the propellant constituents during the heating process. There is no need to premix or to grind up the constituents, as both fully dissolve in the water. Combustion appears to be more efficient -- samples burned showed no sign of residue. Importantly, the water solution would clearly add additional safety to the process, at least during the initial phase where the water content is great. The casting temperature may also be lower. As well, the fact that both potassium nitrate and sucrose are soluble in water should produce a more homogenious propellant, which differs from the "composite" mixture of the heated slurry technique. Burn rate measurements at ambient pressure indicate a burn rate slightly greater than the "composite" version (0.45 cm/s versus 0.40 cm/s, typical). The difference in burn rate at elevated pressure, if any, is currently not known.
Dan Pollino ( www.inverseengineering.com) has developed a similar technique, with certain differences. Dan's formulation consists of 60% KN, 36% sucrose, and 4% corn syrup. To this, water is added to the tune of 10% of the constituents mass. The inclusion of the corn syrup apparently improves the fluidity of the melted mixture, which improves castability. This innovative casting method is detailed in Dan's web site.
Effects of OverheatingAs 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 possible overheating during the casting process, an experiment was performed in which a sample of KNSU propellant, of 65/35 O/F ratio, was overheated. This involved placing a small sample (approx. 10 grams) of the propellant 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 propellant 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 sucrose began. The sample soon fully caramelized, with the temperature recorded at 200 C. (395 F.). Interestingly, the temperature of the propellant reached a "plateau" at this point, and would not rise even after several minutes of further heating, At this point, the propellant was a highly decomposed, with the appearance being that of a charred mixture of brown and black fused together, with some smoke emanating. Since the temperature of the mixture was not increasing beyond this plateau, it was decided to "force" ignition of the mixture by placing the tip of the heat gun about 2-3 cm. away from the sample, directed down from above. This caused a black carbon "skin" to form on the propellant closest to the heat source. After about 10 seconds, ignition occurred, and propellant sample burned nearly instantly, due to its greatly elevated temperature.
Needless to say, appropriate safety precautions must always be taken when casting the propellant. Inadvertent ignition due to other unforseen 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 KNSU 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.
Heated propellant is inherently more hazardous than cool propellant for two reasons. For one thing, being already hot, less energy is required to ignite it. Cool propellant, even if exposed to a flame, takes time to ignite, as the absorbed energy first goes into melting. Hot propellant, being already melted, ignites readily if exposed to a flame. Secondly, the burn rate of hot propellant is much greater. Cool KNSU propellant will burn in the open air at a rate of 0.15 in/sec. Although I have not quantified the burn rate of molten propellant, ad hoc testing indicated it is much greater, perhaps by a factor of five.
During the entire casting operation, as a safety precaution, I would wear protective clothes. These included long leather welder's gloves, leather welder's arm protection and apron, and complete face protection, consisting of an arc welder's head and face shield. (of course, I only used the clear glass in the viewing port, not the darkened glass!). Other sensible precautions were also taken, such as having a fire extinguisher available (although it may not extinguish burning propellant) and keeping anything at all flammable away from the casting operation. I always had a large pail of water nearby (water can extinguish burning propellant by means of heat absorption).
In all the years of my rocketry work, 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.
Propellant Chemistry and Performance CharacteristicsDetails of the KNSU propellant chemistry, combustion equation, and performance characteristics are detailed here.
Drawbacks of the KNSU propellant
There are a number of drawbacks to the KNSU propellant, the most significant of which are:
1) Casting at an elevated temperature. Since hot propellant ignites and burns more readily than cold propellant, this requires that special precautions be taken during the casting process to avoid any potential ignition sources. The propellant rapidly begins to harden as it cools, requiring that the pouring operation be conducted quickly. The melted mixture is also quite sticky (as one might expect!). I found it necessary to lubricate anything the mixture was to come into contact with during the casting procedure, such as the pouring funnel, and the coring tool. The sucrose in the mixture, once heated to melting temperature, begins to caramelize (decompose), which reduces the performance. Therefore, the heating process must be conducted over the minimum possible time duration.
2) High elastic modulus and brittle nature of the propellant grain. Although the material has reasonable strength and toughness, it is brittle in nature, meaning that if overstressed, will crack or break, rather than plastically deform to absorb energy. Care must be taken during handling to avoid cracking a grain. Before loading a grain into the motor, it should be inspected for cracks. I have found that a visual inspection is sufficient to detect cracks, owing to the translucent nature of the propellant which tends to make a crack readily visible. My experience has shown, however, that if a grain is overstressed, it will almost always break rather than crack, owing to the high elastic modulus.
3) Hygroscopic nature of the propellant grain. In its cast form, the KNSU propellant is hygroscopic, meaning that it readily absorbs moisture from the air. Of course, the more humid the air, the more significant the problem. My experience has indicated, however, that the effects are relatively minor, and more of a nuisance than anything else. Only the outer surface will become "sticky", if exposure to the air is kept to a minimum. There is no noticeable performance reduction. Ignition of the grain becomes somewhat more difficult, but if anything, I consider this to be an advantage on the side of safety.
4) Corrosive nature of exhaust products. One of the products of combustion is potassium carbonate, which is hygroscopic, absorbing moisture from the air to form an aqueous solution of hydroxide ions which can be rather corrosive. As well, the trace amount of KOH that forms exacerbates this. Although this is not a serious problem, it is a nuisance. Metals, especially aluminum, become etched or corroded if not cleaned immediately with warm water. It is best to shield any equipment or other components to prevent exposure, or to paint any surfaces that cannot be shielded. As well, the skin may can be irritated by the residue, if exposure is prolonged.
The KNSU propellant may not be a particularly "sophisticated" one, nor such a high-performance propellant. As well, it does have its drawbacks. However, it is also clear that it does have significant merit, such that it meets or exceeds any reasonable expectation of the requirements set forth for an amateur rocket propellant. It is, after all, important to recognize that amateur requirements for a rocket propellant are not necessarily the same as the requirements for a professional rocket propellant, so lack of sophistication or performance may not be an important concern.
As a final note, I should mention that I rarely use this propellant anymore for my rocket motors. Rather, I now use the more contemporary sugar propellants, namely the KN-Dextrose Propellant (KNDX) and the KN-Sorbitol Propellant (KNSB). Although very similar, these propellants have notable advantages over the "classic", such as a much lower casting temperature and minimal caramelization.