IntroductionThis webpage describes a set of on-going experiments that have been conducted over the past five years relating to the goal of developing an amateur rocket propellant based on ammonium nitrate (AN) oxidizer. Objectives of the experiments include development of a propellant that is relatively simple to manufacture, safe to manufacture & handle, low cost, and employ materials that are relatively easy to obtain. In other words, one that is suitable as a relatively high-performance "amateur rocket propellant". With regard to performance, a goal of attaining a delivered specific impulse of 200 seconds had been set.AN has a number of qualities that make it particularly appealing for use as a rocket propellant oxidizer. It is readily available and is low in cost with annual global consumption being over 20 million tones as an agricultural fertilizer [1]. It is chemically stable at room temperature [2], does not burn on its own, and has very low sensitivity to friction and shock [3]. The decomposition temperature is quite high (200oC.) AN, which has the chemical formula NH4NO3, contains no metal ions and when heated decomposes solely to gaseous products. This contributes to low molecular weight combustion products, which is desirable, and does not inherently suffer from two-phase flow (condensed particle) losses. These factors provide for a specific impulse potential ranging from very good to excellent. AN does have a couple of drawbacks. One is related to changes in crystalline phase with changing temperature, possessing five distinct phase changes. One of the phase changes occurs at 32oC, which is associated with a sizeable volume increase of approximately 4%. Since this temperature is in the range that may be experienced during storage, this needs to be taken into consideration in manufacture and storage. Exposure to repeated cycles of phase change could be potentially damaging to the structural integrity of a propellant grain, depending upon various factors discussed later in this article. A second drawback is the hygroscopic nature of AN. However, as the humidity threshold is around 70% at room temperature (similar to certain sugar propellants such as KNSB), this is not necessarily a harsh drawback. Understanding the Chemical Behaviour of ANOne of the first steps taken in the approach to tackling the challenge of developing a practical and safe rocket propellant was to study technical reports relating to AN chemistry and decomposition behaviour. Several dozen of such reports were studied and a great deal was learned in the process, which helped in the understanding of this common yet remarkable material.Knowledge of the chemistry and in particular, the decomposition processes of pure AN and catalysed AN is important for the experimenter to understand. Not only for the obvious sake of safety, but also to allow a more rational approach to tackling the challenges associated with propellant developmental work. The most common use of AN is as an agricultural fertilizer. The agricultural designation is 35-0-0, which refer to the amount of nitrogen, phosphorus and potassium (known as NPK) contained in the product. For example 8-8-8 signifies that a fertilizer contains 8% elemental nitrogen (N), 8% elemental phosphorus (P), 8% elemental potassium(K) by weight. AN has zero percent of both phosphorus and potassium, and 35% nitrogen. This latter value can be easily reproduced knowing the atomic mass of elemental nitrogen contained in AN:
NH4NO3 molecular weight = 14 + 4 (1) + 14 + 3(16) = 80 g/mole At ambient conditions, AN is chemically stable and can be stored in large amounts without fear of self-ignition or spontaneous combustion [7]. AN by itself does not burn. AN is extremely soluble in water, increasing exponentially with temperature. When dissolved in water, heat is absorbed to the tune of 79 cal/gram at room temperature. This property is exploited by "instant cold packs" sold in pharmacies to provide pain relief for soft-tissue injuries. One significant challenge to be overcome in utilizing AN as a propellant oxidizer is to deal with its natural tendency to self-extinguish. This tendency is a result of the large amount of water that forms upon decomposition, which slows down the combustion process (burn rate) to such an extent that combustion tends to be non-sustained. Although not directly applicable to the propellant research being conducted, it is interesting to examine what happens when a sample of AN is heated, in order to gain insight which could prove of value in tackling the problem of propellant development. When pure AN is heated to a temperature range of 169o to approximately 200oC, essentially no decomposition occurs. When heated further, to the range of 200-250C, the following exothermic reaction (heat is released) primarily occurs: NH4NO3 => N2O + 2 H2O, delta H= -8.8 kcal/mol From the above equation, for 100 grams of AN, 45 grams of H2O is produced, with the remaining 55 grams being gaseous nitrous oxide. This reaction is exothermic, with the release of 110 calories/gram. Simultaneous to this reaction, a dissociating reaction occurs endothermically (heat is absorbed) whereby the AN breaks down into ammonia and nitric acid.: NH4NO3 => NH3 + HNO3, delta H= +44.6 kcal/mol The combination of these two effects results in a steady-state, or self-limiting temperature, provided the decomposition process is carried out with the gaseous reaction products allowed to freely escape (in particular the HNO3). As such, if pure AN is heated at a moderate rate in the open air with no confinement, the temperature cannot rise appreciably beyond its melting point. Under steady-state conditions, the endothermic dissociation of AN into gaseous NH3 and HNO3 absorbs all the heat available from decomposition. Thus, when heat is added to AN at atmospheric pressure and even from a very hot source, the temperature of the AN is limited by its own dissociation to values at which decomposition rate is comparatively moderate. At elevated pressures, however, the dissociation reaction is repressed and the rate of decomposition accelerates. [5] When AN is heated very rapidly, such as would occur in a rocket motor, the decomposition process is notably different than that of the bulk decomposition reaction . Under such condition, the decomposition is chiefly this dissociation reaction with NH3 and HNO3 as the products [10]. Certain substances are known to have a catalytic effect on the decomposition of AN. Water, chlorides and chromates are notable ones. The presence of even a minute amount of water causes decomposition to begin at 180oC. Ammonia and alkali substances such as urea have an inhibiting effect on decomposition. Chorides are particularly effective in speeding up the decomposition rate of AN. When catalyzed with 1% NaCl (table salt), the decomposition rate at 175oC was found by researchers to be 1000 times that of pure AN. Small quantities of chlorides (in presence of free acid) may cause decomposition at temperatures as low as 140oC. [5]. Powdered aluminum added to molten AN is non-reactive, but powdered zinc reacts violently [8]. When heated to melting, AN tends to react exothermically with organic substances, which appear to have a catalytic effect. For example, with carbon: 2NH4NO3 + C => 2N2 + CO2 + 4H2O, delta H= -75.2 kcal/mole Interestingly, a gun propellant invented in the 1880's, Ammonpulver, used 15% charcoal (largely carbon), combined with 85% AN. This mixture was pressed into grains. Although difficult to ignite, it was vastly more powerful than blackpowder. Two serious drawbacks limited its practicality - the hygoscopic nature of AN, and the tendency for the grains to crack due to the crystalline phase-change. The latter resulted in over-pressurization and with resultant damage to the gun barrel. Specific details of the phase changes of pure crystalline AN are provided in Table 1.
Early experiments performed by the author confirmed that AN mixed solely with typical fuel/binders such as epoxy, sucrose, polyester, polyurethane and silicone would generally not sustain combustion. If a small amount of NaCl were incorporated, sustained burning (smouldering) would occur, however, very slowly, and the resulting low combustion temperature would tend to produce voluminous amounts of carbon-rich ash. One successful AN-based amateur rocket propellant is the well-known "Wickman" formula, comprised of PSAN (phase-stabilized AN), magnesium powder, and R-45 polymer. ( CP Technologies composition): Wickman Propellant The key to this propellant is the use of a significant mass fraction of a "thermic" agent, magnesium. Combustion of magnesium is highly exothermic, and provides the thermal energy to flash the released water as steam, which then reacts with the metal in a self-sustaining manner. References [3] and [4] discuss a number of experimental rocket propellant formulations that utilize magnesium as an effective thermic. The inclusion of elemental silicon, to the tune of 0.4-6.0%, has been suggested as a performance enhancer [6]. The drawbacks to using magnesium as a constituent in a propellant include the safety concerns associated with handling, the high cost of this material (especially when hazmat shipping fees are factored in), and the lack of easy availability. After much pondering, a rather interesting alternative thermic agent came to light. Aluminum, which has a greater heat of reaction than magnesium, was then tried. The difficulty with combusting particles of aluminum is due to the tough shell of aluminum oxide (alumina) that encases the readily oxidized metal. Initial attempts at simply blending aluminum powder with AN and a binder were fruitless. The aluminum particles did not burn satisfactorily, being well protected by the tough alumina shell. Reference [4] also describes attempts at using powdered aluminum, but the studied formulations failed to burn. Earlier experiments performed by the author relating to the doping of KN-based propellants had indicated a similar difficulty in getting aluminum to combust. It was eventually discovered that the addition of a sizeable amount of sulfur aided the combustion of aluminum particles. This was initially discovered when preparations of KN and silicone rubber were investigated. The addition of 5-10% of sulfur allowed these preparations to burn quite vigorously. A similar phenomenon was observed when RNX propellant was doped with aluminum. This approach was tried with AN, however, the results were not as successful. Nevertheless, sulfur was found to aid the ignition of the experimental AN formulations and may also serve to increase the efficiency of aluminum combustion. Various additives and different binders were tried in attempts to aid the reaction of aluminum with AN. Polyurethane initially appeared to be promising, but efficient combustion of the stubborn metallic aluminum was elusive. Having researched dozens of technical papers on AN combustion, it was decided to employ the use of a chlorine donor such as NaCl, initially, and later NH4Cl. Results of these experiments were more promising. It later occurred to the author that the Spitfire igniter pyrolant, which contains an appreciable amount of aluminum, burned exceptionally vigorously for some reason. This pyrolant utilizes Neoprene-based contact cement as a binder. Interestingly, Neoprene is a DuPont trade name for polychloroprene, which has the chemical formula [C4H5Cl ]n and having an elemental chlorine mass fraction of 39%. Experiments that followed employed contact cement as a binder, producing result that were considered to be a breakthrough. Although difficult to initiate combustion, once ignited, these trial formulations burned in a very stable manner with an intensely hot flame and essentially none of the white "sparklers" that are indicative of incomplete metal combustion. In the open air, magnesium ribbon (or shavings made by cutting magnesium on the metal lathe) proved to be effective in igniting these formulations. A hot burning pyro composition such as thermite was likewise found to be an effective combustion initiator. If ignited with a standard propane torch, the compositions merely burned in a smouldering, flameless fashion (so called "cigar-burning"). It was found out quite early in this experimentation that an appreciable percentage of aluminum was essential. If not enough aluminum was present, the resulting formulation did not generate enough heat to sustain efficient combustion. Typically the formulations burned fiercest with an aluminum content in the range of 15-25%. The bare minimum was found to be around 10%, depending on the specifics of formulation. Although AN is considered to be a "low energy" oxidizer (heat of explosion only 300-400 calories/gram), when used with a thermic agent such as aluminum, the theoretical performance can be quite impressive. GUIPEP runs indicate a theoretical Isp for an AN/Al/Neoprene composition to be in the range of 220-250 seconds at a chamber pressure of 1000 psi. Figure 1 shows excerpts from results of a GUIPEP run for a typical "high aluminum content" composition based on AN, aluminum, and Neoprene (chloroprene).
Safety of AN as a propellant oxidizerAs explained in the preceding section, the author's research indicated that AN does not pose any undue safety concerns as a constituent of a rocket propellant, being similar in this manner to another commonly used amateur propellant oxidizer, potassium nitrate (KN). As pointed out earlier, AN is chemically stable at room temperature, does not burn on its own, and has very low sensitivity to friction and shock. As well, AN is non-toxic. There is one particular characteristic of AN, however, that sets it apart from an oxidizer such as KN. AN is known to be capable of detonation under the "right" conditions. As such, investigation of the safety of AN would not be complete without considering this characteristic. Detonation is best described as a nearly instantaneous decomposition (typically measured in microseconds) of a mass of material. Propagation of decomposition is by means of a shock wave of sufficient energy level. Since detonation is something to be avoided due to its destructive nature, it is necessary to ascertain whether or not detonation is a factor. This is relevant since a special form of AN is used in blasting operations due to its capacity to detonate. ANFO and ammonal (AN with aluminum powder) are two examples. It should be noted that AN is commonly used as a blasting agent simply because it is very inexpensive, readily available, and is safe to handle and transport, requiring a powerful initiator charge.Low bulk density is crucial for detonation of AN to occur. A porous nature of the reacting material is needed to provide the "reaction centres" whereby adiabatic compression, due to a propagating shock wave, heats the air pockets several thousand degrees Celsius. This generates a reaction front which provides energy to propagate a detonation wave. A lack of sufficient voids (bulk density > 1 gram/cc), so called "dead-packing", makes detonation of AN impossible [8]. Detonation sensitivity is dependant upon many factors, especially bulk density for a low-energy material such as AN. Without the presence of voids (air pockets), detonation is not possible [8]. Another important criterion for detonation of AN is heavy confinement [8]. Although not directly applicable here, the molten form of AN, when not "aerated" by bubbles, can withstand considerable hydrodynamic shock without undergoing detonation [9]. In the temperature range of 169oC to 190oC, at which the rate of thermal decomposition is negligible, AN is virtually non-detonatable [9]. Reference [8] describes testing of mixtures of AN/C/Al, when initiated in a confinement, deflagrated, but did not detonate.
* critical diameter is a measure of detonation sensitivity, referring to the minimum diameter of a mass of an explosive that can be detonated without being heavily confined. Critical diameter greater than 1 inch is generally considered to be "insensitive". A commonly used "professional" rocket oxidizer, AP, is significantly more sensitive to detonation than AN. AP is also used for commercial High Power rocket motors and is used by many rocketry enthusiasts for "experimental" motors. Reference [7] provides a minimum "critical diameter*" of ¼ inch for AP. This compares to a critical diameter of several inches for AN in low-density, prilled form (the most sensitive form). AP propellants of similar formulations to those discussed here (e.g. with appreciable aluminum content) have been used in safety with regard to detonation concerns by HPR and the experimental rocketry community over many years. Interestingly, the Space Shuttle Boosters utilize an AP-based aluminized propellant (Table 1) not unlike many of the AN/Al formulations being studied, with the exception of the oxidizer being the more-sensitive AP rather than AN.
Space Shuttle Booster Propellant
A24 Propellant Another possible concern that was addressed relates to the safety of compressing AN formulations under hydraulic pressure. This technique is used to form the propellant grains for motor testing. Reference [3] describes the manufacture of pellets used for researching burning rate measurements of AN/TNT mixtures by compressing the combined powdered mixture to 0.2 Gpa (29000 psi). Experimental AN / Aluminum FormulationsTo date, 33 different formulations have been experimented with to various degrees of diligence. In least rigorous cases, a small sample batch of a given formulation was prepared and burned in the open air to qualitatively assess its combustion characteristics. If this suggested unsatisfactory combustion behaviour, no further experimentation was conducted. For the more promising formulations, propellant grains have been produced and test fired in rocket motors. For the most promising formulations, chamber pressure and thrust curves were obtained from static firings in order to assess key parameters such as delivered specific impulse and characteristic velocity (c-star).The earliest formulations used either polyurethane or epoxy as a binder. Both proved to be less than satisfactory for various reasons. Later formulations used Neoprene as a binder, which proved to be suitable. A complete listing of all 33 formulations including pertinent details is provided in Table 2.
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