Hypergolic Ignition by Fuel Gellation and Suspension of Reactive or Catalyst Particles

M ODERN propulsion systems require the development of energetic propellants that satisfy the increasingly stringent requirements of safety and being environmentally friendly. Issues, such as propellant toxicity, storability, and density, should be taken into account in the evaluation of the overall performance of a space system and the possibility of success in its mission. Presently, hypergolic propellants, i.e., propellants that ignite upon contact, are used in many systems. Traditional storable hypergolic bipropellants are hydrazine-based fuels, such as monomethyl hydrazine (MMH), combined with nitrogen tetroxide (NTO) or inhibited red fuming nitric acid (IRFNA) [1]. However, both NTO and the hydrazine-based fuels pose significant health hazards. NTO has a permissible exposure limit of 5 ppm but a very high vapor pressure of 720 mmHg, whereas MMH, a carcinogenic liquid, has a permissible exposure limit of 0.2 ppm and a relatively low vapor pressure of 36 mm Hg. Significant reduction of the health hazards is obtained by gelling the liquids. Gels are liquids whose rheological properties have been altered by the addition of certain gelling agents (gellants) and, as a result, their behavior resembles that of solids. The gel surface hardens in contact with gaseous environment; hence, in cases of failure in the feeding system or during storage, the leakage rate is reduced in comparison with liquids. In cases of accidental spillage due to damage in the fuel and oxidizer tanks, burning will occur only at the fuel–oxidizer interface, if they are hypergolic. When contact ceases, the rheological nature of the gels will prevent further flow and chemical reaction. The volatility of gels is significantly lower than the volatility of liquids and in case of leak or spill, much less vapors will be released, thus reducing toxicity hazards. Several studies were conducted on hypergolic gel propellants [2–4]. Rahimi et al. [2] examined the rheological characteristics of gelled IRFNA and MMH and conducted firing tests. They achieved c efficiencies of 90% and significant improvements in storage and handling. Other hypergolic bipropellants are based on hydrogen peroxide (HP) [3,4]. HP decomposes exothermically into steam and oxygen, it is environmentally safe and relatively easy to handle. Mostly, decomposition of HP is achieved using catalyst beds [5–8] based on silver, platinum and other materials. Catalyst beds produce high temperature decomposed HP that can burn with a hydrocarbon fuel; however, the system complexity and weight are both increased. Another method is based on the idea of using catalytic or reactive material (such as metal oxides—MnO2, PbO2, F2O3, etc.) that is dissolved in a liquid fuel. The reactive material, decomposes HP and ignites the fuel, so hypergolic ignition is achievedwithout the use of a catalyst bed. However, this method requires fuels such as ethanol or methanol that serve as solvents for the reactive material. All these solvents either used alone or with kerosene-based fuels, have relatively low heat of combustion; therefore, the energetic performance of the system is low. Figure 1 shows the performance of several hypergolic bipropellants at standard conditions (chamber pressure 68 atm [1000 psia] exit pressure 1 atm, adapted nozzle), using the Gordon and McBride NASA CEA thermochemical code [9]. The highest performance is achieved by the IRFNA-N2H4 combination, which gives an Isp of 279 s. Gels are non-Newtonian fluids and contrary to Newtonian fluids, their viscosity depends on the applied shear rate. Moreover, gels for applications in rocket and airbreathing propulsion systems can be designed, by choosing the appropriate gelling agent, to have a yield point in order to enable the suspension of particles. In addition, they have a shear-thinning behavior to provide a reasonable pressure drop when pumped to the chamber. The phenomena involved in the combustion of gelled fuels were investigated for both organic and inorganic gellants. Nachmoni and Natan [10], and Arnold and Anderson [11] conducted an experimental study on the ignition and combustion characteristics of inorganic-gellant, nonmetallized, kerosene-based, gel fuels. The authors indicated that, in general, gels obey the d law of diffusioncontrolled combustion. The heat of vaporization of gels was found to increase with increasing gellant content in the liquid fuel and gels burned at lower burning rates than the pure liquid. Increasing the gellant content resulted in an increase in the ignition delay time and the higher heat input was required for ignition. Solomon et al. [12] studied the combustion of organic-gellantbased gel fuel droplets. The research revealed that during the combustion, after the vaporization of part of the fuel, an elastic layer of gellant is formed around the droplet that prevents further fuel vaporization from the outer surface of the droplet. This causes the fuel to evaporate below the droplet surface producing bubbles, which results in droplet swelling, fuel jetting, and collapse of the remaining droplet. The process repeats itself until complete consumption of the fuel and gellant. This is actually a periodic combustion phenomenon. The scope of the present research was to propose a nontoxic propellant, based on gelled kerosene in which catalyst or reactive particles are suspended and ignite hypergolically with an oxidizer. In the present study, the hypergolic ignition of gelled kerosene with hydrogen peroxide was investigated. The outcome of the present research has been patented [13].