This 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. This propellant is usually refered to by the acronym KNSU. I first became familiar with the Potassium Nitrate/Sugar propellant way back 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 oxidizer/fuel (O/F) ratio, measured burning rate at various temperatures, pressures and O/F ratios, tried various grain configurations, performed combustion product analysis (using the now-antiquated Orsat apparatus, as well as apparatus of my own design), measured the propellant "impetus" by use of "closed vessel" combustion. I performed further investigation on this propellant during my final year at university in 1984, 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.
And of course, I launched many rockets powered by this propellant, over 57 flights in total, between the years 1972 and 1984. So I got to know this classic propellant quite well indeed! Modern sugar propellants using low melting-point sugars such as dextrose and sorbitol have made KNSU largely obsolete. Nevertheless, it is worthwhile to discuss this notable propellant that served the author and many other experimenters very well. Advent of the potassium nitrate/sucrose propellant led to a much safer era of amateur rocketry compared to the earlier use of hazardous propellant materials that were tried by enthusiastic, but ill-informed, rocketeers at the dawn of the space age.
The makeup of KNSU, and its behaviour in a rocket motor is, in essence, the same as professional rocket propellants. Of course, it has certain shortcomings such as much lower performance. However the way the propellant is prepared (cast), the form of its grain (cylindrical), its burning behaviour in a rocket motor (recedes normal to exposed surface and follows St.Robert's burning law), its internal ballistics behaviour (follows steady, one-dimensional compressible fluid flow behaviour of an ideal gas) and its well-known thermochemical properties allow for rational rocket motor design that can deliver predictable results. In other words, KNSU (and the other sugar propellants) provide for a bonafide introduction to rocket engineering.
The standard ratio of constituents for KNSU 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. Granular sugar is also useable, 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"). 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.
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. 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 2). 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.
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 sucrose (186C or 367F). The potassium nitrate, which has a much higher melting point, remains as solid particles that partly dissolve in the sucrose. The result is a slurry of solid oxidizer particles suspended in liquid sucrose medium. Sucrose tends to decompose (caramelize) at this temperature. Caramelization has been found to be a function of both temperature and time. Since caramelization is detrimental to the performance of KNSU, keeping the temperature as low as possible is important. As well, heating time should be minimized, although this has been found to be far less critical than temperature.
Heating the mixture is done using a thermostatically controlled electric "deep fryer", with the thermostat set to 380F/193C. (Figure 4). 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)
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. The casting procedure involves adding about half the powdered mix to the heating vessel, and stirring often to assist melting. Once this has melted, the remaining powdered mix is added, and heating is continued. 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 or at too high a temperature.
The consistency of standard KNSU slurry will remain quite viscous and must be simultaneously poured and scooped with a spatula 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 entire casting operation, from start of melting, to final pouring, usually took me about fifteen minutes for a B-200 grain.
I found that it was very important to remove the KNSU grain from the 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, the grain may still be flexible and could become deformed. I found that 45 minutes was the required cooling period. Following coring tool removal, the grain is trimmed as needed, and (importantly) accurately weigh and measured. I would also typically record colour and amount and type of defects (such as any bubbles or voids), then place it inside a plastic bag with a dessicant packet. 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. Two finished grains for the B-200 motor are shown in Figure 6.
Notes on casting KNSU
The cast propellant is typically within the colour range of the two grains shown in Figure 6 below:
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 viscosity of the 65/35 rato is definitely thicker than that of the melted 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 may 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 due to humidity. Indeed, the hygroscopic nature of the propellant seems to be reduced...a sample strand that had been left in the open air for 6 months, at humidity levels ranging from 50 to 70%, 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 left a reduced amount 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 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. I have not tried Dan's method but I know it is popular with some rocketry experimenters.
James Yawn ( www.jamesyawn.net) has also developed a method of producing sucrose-based propellant using a recrystallization technique. With James's method, instead of casting in the style of KNSU, a putty-like consistency is produced, which can be packed into the mould. James's method is quite popular, however, I have never tried this technique.
Melting of the mixture was rapid, owing to the high output of the heat gun, and brown coloured regions soon appeared within the melted mixture, as 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.
Heating the propellant mixture must only be done using a thermostatically controlled heating vessel, such as deep fryer or skillet. Heating propellant over an electric or gas stove element is unsafe and must never be attempted (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.
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.
Drawbacks of the KNSU propellant
There are a number of drawbacks to the KNSU propellant, the most significant of which are:
The KNSU propellant may not be a particularly "sophisticated" one, nor is it a high-performance propellant. It does have certain drawbacks especially in comparison to modern sugar propellants such as 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. In fact, I never use KNSU anymore for my rocket motors. Nevertheless, KNSU is a propellant that can be used with good success under certain circumstances, such as when alternative sugars are not locally available.