46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 25-28 July 2010, Nashville, TN AIAA 2010-7144 Hypergolic Ignition of Oxidizers and Fuels by Fuel Gelation and Suspension of Reactive or Catalyst Particles Benveniste Natan, Valeriano Perteghella and Yair Solomon # Faculty of Aerospace Engineering, Technion Israel Institute of Technology, Haifa, Israel The need for a storable, non-toxic and high energy hypergolic propellant points towards the combination of kerosene with hydrogen peroxide. Hypergolic ignition of almost any fueloxidizer combination can be obtained by gelling one of the liquids and adding the proper material. The rheological characteristics of gels enable the suspension of reactive or catalyst particles, uniformly distributed in the fuel, without compromising the energetic performance of the system. In the present research, hypergolic ignition was achieved for a 92% concentration hydrogen peroxide and a kerosene gel containing sodium borohydride particles. Ignition delay times of less than 10 µs were observed. I. Introduction In general, for the selection of rocket propellant, its energetic performance is the most important factor. However, modern propulsion systems require the development of propellants that may satisfy the increasingly stringent requirements of safety, security and environment friendly fuels. Other issues, such as propellant toxicity, storability and density, should be taken into consideration in the evaluation of the overall performance of a space system and the possibility of success in its mission. In the past, the solution seemed to be the utilization of hypergolic propellants, i.e., propellants that ignite upon contact. Traditional solutions requiring storable hypergolic bipropellants have used a hydrazine based fuel, 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 mm Hg whereas MMH, a carcinogenic liquid, has a permissible exposure limit of 0.2 ppm and a relatively low vapor pressure of 36 mm Hg. 2 One of the ways to handle the health hazards is to gel the liquids so they are less dangerous to handle. 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 a gaseous environment. Hence, in cases of failure in the feeding system or during storage, the leakage rate is reduced in comparison to 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. 3-5 Rahimi et al 3 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 were developed, based on hydrogen peroxide (HP), 4,5 a high density liquid oxidizer that decomposes exothermically into steam and oxygen, as well as being environmentally safe and relatively easy to handle. High concentration peroxide solutions have been suggested as oxidizers in combination with different fuels in which suitable agents were dissolved and their ignition properties were studied in order to attain the desired hypergolicity level. Most ideas regarding hydrogen peroxide for various applications are based on its catalytic decomposition using catalyst beds. 6-10 Catalyst beds produce high temperature decomposed HP that can burn with a hydrocarbon fuel, however, the system complexity and weight are both increased. International Patent Application PCT/IL2010/000527. Associate Professor, Head, Missile Propulsion Laboratory, AIAA Associate Fellow. Visiting Scientist. # Research Scientist, presently graduate student at Purdue University. 1 Copyright 2010 by B. Natan, V. Perteghella, Y. Solomon. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
Another method is based on the idea of using catalytic or reactive material that is dissolved in a liquid fuel. The reactive material, decomposes the hydrogen peroxide and ignites the fuel, so hypergolic ignition is achieved without 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 in standard conditions. The highest performance achieved for IRFNA and N 2 H 4 with an Isp of 279 s. Figure 1. The energetic performance of selected hypergolic bipropellants. A. Non-Newtonian, power-law fluids Contrary to Newtonian fluids, in a non-newtonian fluid, the viscosity, η, depends on the applied shear rate γɺ. For the characterization of Newtonian and non-newtonian fluids without a yield point, the power-law model (Ostwald-de Waele model) is the most common correlation 11 : n 1 η = Kγɺ (1) where K is the consistency and n is the rate indexes respectively. For n=1, Eq. (1) describes a Newtonian fluid. Figure 2 depicts the behavior of shear stress and viscosity as a function of shear rate for different Newtonian and non-newtonian fluids. 12 Various fluid types are presented, such as a shear-thinning fluid for a power-law exponent (rate index) 0<n<1 or shear-thickening (dilatant behavior) for n>1. Gels envisaged for the application in rocket and air breathing propulsion systems should have a yield point in order to enable the suspension of particles. In addition they should have a shear-thinning behavior to provide a reasonable pressure drop when pumped from the storage tank to the chamber. Figure 2. Shear stress and viscosity vs. shear rate for Newtonian and non-newtonian fluids. 2
B. Combustion of Gel Fuels The phenomena involved in the combustion of gelled fuels were investigated for both organic and inorganic gellants. Nachmoni and Natan, 13 and Arnold and Anderson 14 conducted an experimental study on the ignition and combustion characteristics of inorganic gellant, non-metallized, kerosene-based, gel fuels. In addition, calorimetric tests were conducted to evaluate the heat of vaporization of gels. The authors indicated that in general, gels obey the d 2 -law of diffusion-controlled combustion and that the effect of the gelling agent on the burning time can be taken into account through the heat of vaporization. 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. In gels, as in liquids, burning rate was found to increase with increasing oxygen mass fraction. As regards ignition, increasing the gellant content resulted in an increase in the ignition delay time and the higher heat input was required for ignition. Solomon, Natan and Cohen 15 studied the combustion of organic-gellant-based 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 finally collapse of the remaining droplet. The process repeats itself until complete consumption of the fuel and gellant. This is actually a periodic combustion phenomenon. C. Scope of the present study The scope of the present research was to suggest a green propellant, based on a gel kerosene-based fuel in which catalyst or reactive particles are suspended that can ignite hypergolically with an oxidizer. In the present study the hypergolic ignition of gelled kerosene with hydrogen peroxide was investigated. II. Gelled Fuel with Suspended Reactive or Catalyst Particles The present solution based on the rheological characteristics of gels. The existence of yield stress assures that particles can be added without the affect of sedimentation. Gels enable the suspension of reactive or catalyst particles, uniformly distributed in the fuel, without compromising the energetic performance of the system. The use of suspended particles enables every combination of fuel and oxidizer as a hypergolic bipropellant by gelling one of the liquids and adding the proper material. In addition the suspended particles can be energetic materials as aluminum or boron particles that can increase the energy of the combustion. Figure 3 shows the performance of several fuels with HP in standard conditions. The highest performance achieved for RP-1/Al with I sp of 281 s. Figure 3. Energetic performance of several fuels with hydrogen peroxide. 3
(a) Figure 4. Energetic performance of several propellants; I sp (a) and ρ I sp (b). The combination of nitric acid with hydrazine (IRFNA-N 2 H 4 ) represents a standard hypergolic bipropellant, which is use in several systems. It is the hypergolic propellant with the highest performance available but it poses significant health hazards. RP-1 and O 2 is one of the most common propellants for launching vehicles, it has very high performance but the use of LOX involves complicated cryogenic systems. The RP-1/H 2 O 2 combination is a storable, non-toxic and high energy pyrophoric propellant. Introduction of Al into the RP-1/H 2 O 2 combination is feasible by gelation of RP-1 that enables the suspension of Al particles and catalyst particles that can make it pyrophoric. Production of hypergolic gel based on kerosene A hypergolic bipropellant based on kerosene may be combined with most oxidizers. For the use of HP as an oxidizer a wide range of ignition materials are suitable. A wide range of metal oxides can serve as catalysts that decompose HP. Also, metal hydrides like sodium borohydride (NaBH 4 ) or lithium aluminum hydride can serve as reactive materials For the use of a NO 2 based oxidizer with kerosene, a wide range of solid products of hydrazines with aldehydes and ketones, namely, the hydrazones can serve as reactive materials. Jain 15,16 examined the hypergolicity characteristics of various hydrazones with NO 2 oxidizers as HNO3, WFNA and RFNA and got ignition delay times of 9 µs for some of the hydrazones powders. The solid hydrazone particles can be suspended in the gel and serve as reactive materials. For the use of LOX, presumably alkalic metals and pyrophoric materials can serve as reactive materials. III. Experimental System The kerosene/hp combination is non-pyrophoric by its nature and there is no catalytic material that can dissolve in kerosene. The use of silica gelant in kerosene enables the mixing of sodium borohydride particles in the kerosene. The silica based gels are non-newtonian with a long-term storage capability and silica is compatible with the sodium borohydride; therefore the gel is storable. A. Chemicals Kerosene based fuel (JP-8, JP-5 etc): Kerosene is a liquid mixture of carbon chains that typically contain between 6 and 16 carbon atoms per molecule and approximate molecular formula C 11 H 21 (Jet-A). The density of kerosene is 810 kg/m 3. The flash point is 52 C. The boiling range is 165-265 C and its autoignition temperature is 220 C. The heat of combustion of kerosene is 43.1 MJ/kg. 17 Nano-Silica fumed powder, 0.014 µm (SIGMA): Nano-silica has hydrophilic characteristics and is capable of building up hydrogen bonds to create a gel when mixed with the pure liquid do-to his large surfers area. Sodium Borohydride (NaBH 4 ) p.a., 96 % (SIGMA): Sodium borohydride is a metal hydride that reacts spontaneously with Hydrogen Peroxide. The density of NaBH 4 is 1074 kg/m 3. The flash point is 70 C. The boiling point is 500 C and its auto-ignition temperature is 220 C. (b) 4
Hydrogen Peroxide (O 2 H 2 ) p.a., 92 %: Hydrogen peroxide is a liquid with a melting point temperature of - 43 C, boiling point temperature of 150 C and density of 1460 kg/m 3. Hydrogen peroxide decomposes exothermically into water and oxygen. It has a heat of formation of 98.2 MJ/kmol. B. Combustion system In order to test the reaction characteristics of hypergolic substances, a suitable test apparatus was designed to enable reliable measurements on a drop-like quantity of fuel. Chemical reaction rates depend on temperature and pressure; therefore these parameters may affect the ignition delay time. Hence a hermetically sealed cell, in which temperature and pressure were controlled, was used to conduct the ignition tests. The test cell can be pressurized up to 20 atm using nitrogen as an inert ambient gas. Ambient temperature was controlled by an electrical resistance inside the cell and it was measured using a thermocouple located at the upper part of the cell. The gel droplet was placed on a glass plate and the hydrogen peroxide droplet is dropped from a syringe on the gel fuel droplet. Two thick circular optical glass windows have been fitted into two holes in the cell walls along the same axis in order to enable view through the cell and focus the high speed camera on the fuel and oxidizer droplet impingement point. Figure 4. The experimental system. Figure 5. Test cell view. 5
plate holder heating resistance glass plate Figure 6. Test cell interior. 1 2 3 4 5 6 Figure 7. A sequence of high-speed photographs demonstrating hypergolic ignition of hydrogen peroxide with kerosene. The time interval between sequent pictures is 2 µs. 6
The most important parameter to be evaluated is the ignition delay, defined as the time interval between the moment the oxidizer drop and the fuel drop come into contact and the moment the fuel ignites. The experimental system is presented in Figs. 4, 5 and 6. The ignition delay measured by a high-speed camera. The camera was set to a frame rate of 500 fps, i.e., the time interval between sequent pictures was 2µs. It is possible to identify the frame recording the moment the two drops come into contact and the frame recording the moment the ignition occurs. The time lapse between these frames can be accounted as the ignition delay time. IV. Experimental Results Figure 7 depicts high-speed photographs (taken at 500 fps) of a hypergolicity test of 92% HP with a kerosene gel containing NaBH 4 suspended particles. The ignition delay is less than 8µs. The experimental observations indicate that the idea of gelling kerosene and adding reactive particles that can promote hypergolic ignition with oxidizers is feasible and can be used in green rocket systems. V. Conclusion Gelation of liquid fuel and suspension of reactive or catalyst particles enables hypergolic ignition of almost any fuel-oxidizer combination. It allows using more energetic and environmentally safe hypergolic bipropellants as hydrogen peroxide with kerosene. It eliminates the need for complicated and heavy ignition systems and the danger of working with carcinogenic and toxic materials. References 1 Sutton, G. P., and Biblarz, O., Rocket Propulsion Elements, Seventh Edition, John Wiley and Sons, Inc., New York, 2001. US Department of Labor, Chemical Sampling Information (CSI), http://www.osha.gov/dts/chemicalsampling/toc/toc_chemsamp.html 2 Rahimi, S., Hasan, D., and Peretz, A., Development of Laboratory-Scale Gel-Propulsion Technology Journal of Propulsion and Power, Vol. 20, No. 1, 2004, pp. 93-100. 3 Ventura, M., and Mullens, P., The Use of Hydrogen Peroxide for Propulsion and Power, AIAA paper 1999-2880, June 1999. 4 Salem, I. A., El-Maazawi, M., and Zaki, A. B., Kinetics and Mechanisms of Decomposition Reaction of Hydrogen Peroxide in Presence of Metal Complexes, International Journal of Chemical Kinetics, Vol. 32, 2002, pp. 643-666. 5 Wernimont, E. J., and Heister, S. D., Combustion Experiments in Hydrogen Peroxide / Polyethylene Hybrid Rocket with Catalytic Ignition, Journal of Propulsion and Power, Vol. 16, No. 2, 2000, pp. 318-325. 6 Sisco, J. C., Austin, B. L., Mok, J. S., and Anderson, W. E., Ignition Studies of Hydrogen Peroxide and Kerosene Fuels, AIAA paper 2003-831, Jan. 2003. 7 Miller, K., Sisco, J. C., Austin, B. L., Martin, T., and Anderson, W. E., Design and Ground Testing of a Hydrogen Peroxide/Kerosene Combustor for RBCC Application, AIAA paper 2003-4477, July 2003. 8 Wernimont, E. J., and Durant, D., State of the Art High Performance Hydrogen Peroxide Catalyst Beds, AIAA paper 2004-4147, July 2004. 9 Wernimont, E. J., and Durant, D., Development of a 250 lbfv Kerosene 90% Hydrogen Peroxide Thruster, AIAA paper 2004-4148, July 2004. 10 Ferguson, J., and Kemblowski, Z., Applied Fluid Rheology, Elsevier Science Publishing Co., Inc., New York, NY, 1991. 11 Rapp, D. C., and Zurawski, R. L., Characterization of Aluminium/RP-1 Gel Propellant Properties, AIAA paper 1988-2821, July 1988. 12 Nachmoni, G., and Natan, B., Combustion Characteristics of Gel Fuels, Combustion Science and Technology, Vol. 156, pp. 139-157, July 2000. 13 Arnold, R., and Anderson., W.E., Droplet Burning of JP-8/Silica Gels, AIAA paper 2010-421, Jan. 2010. 14 Solomon, Y., Natan, B., and Cohen, Y., Combustion of Gel Fuels based on Organic Gellants, Combustion and Flame, Vol. 156, No. 1, 2009, pp.261-268. 15 Jain, S. R., Spontaneously Igniting Hybrid Fuel-Oxidizer Systems, Defense Science Journal, Vol. 45, No. 1, 1995, pp 5-16. 16 Jain, S.R., Vittal, J.J., and Mimani, T., Some Mechanistic Aspects of Hypergolic Ignition: Reaction of Dinitrogen Tetroxide with Solid Amines and their Mixtures with Magnesium, Defense Science Journal, Vol. 42, No 1, Jan. 1992, pp. 5-12. 17 Edwards., T., Kerosene Fuels for Aerospace Propulsion Composition and Properties, AIAA paper 2002-3874, July 2002. 7