NOVEL ORGANOMETALLIC PROPELLANTS FOR HYPERGOLIC APPLICATIONS

Transcription

1 NOVEL ORGANOMETALLIC PROPELLANTS FOR HYPERGOLIC APPLICATIONS T. L. Pourpoint, J. J. Rusek Swift Enterprises, Ltd., West Lafayette, Indiana, U.S.A. Tel: (765) ; Fax: (765) ; Abstract Chemical propulsion engineers at Swift Enterprises, Ltd. are developing an analytical and experimental database of hypergolic rocket fuels with highly reduced toxicity and greater energy density than currently used rocket propellants. The fuels of interest in this study are those designed around alkaline metallic species which render existing hydrocarbon fuels reactive with Rocket Grade Hydrogen Peroxide (RGHP) while maintaining exceptional energy. The proposed fuels have delta V and energy density values greater than NTO/MMH bipropellants. Introduction The genesis of non-toxic high performance propellants constitutes an important step towards safer and logistically superior propulsion systems of interest to civilian and military applications. The non-cryogenic fuels used with today's chemical propulsion systems are mainly kerosene or hydrazine-like fuels. Swift Enterprises, Ltd. is investigating advanced non-toxic fuels that could be added to kerosene with significant delta V and energy improvement, while maintaining short ignition delay parameters. The higher density of these new fuels would reduce propellant tank and vehicle size and impart higher energy density to the system. The proposed reactive fuels are comprised of light metal complex hydrides dissolved in appropriate enhanced energy organic fuels. The oxidizer of choice is either anhydrous or 98% hydrogen peroxide. Background The toxicity and performance levels of rocket propellants are key factors in the success of any chemical propulsion system. As an example, the orbital maneuvering engines of the Space Shuttle currently operate with hydrazine-based fuels. Such fuels are highly toxic and require the decontamination of the vehicle after each and every flight. Non-toxic propellants could eliminate this process and significantly reduce the cost of a Space Shuttle mission. As a further example, improved performance would allow telecommunication or surveillance satellites to operate in space longer and would enhance their maneuverability. Storage of non-toxic propellants eases logistics and monitoring concerns while higher energy densities promise an enhanced mission capability. For the last decade, researchers working at the United States Air Force (USAF), the United States Navy (USN), Purdue University and, more recently, at Swift Enterprises, Ltd. have made significant progress towards the development of hypergolic rocket propellants with reduced toxicity and greater performance than traditional hydrazine-based compounds 1,2,3. Based on the USN patent entitled Non-toxic Hypergolic Miscible Bipropellant 4 by Rusek, et al., researchers at Swift Enterprises, Ltd. continue to study renewable hypergols, which are non-toxic and are catalytic with Rocket Grade Hydrogen Peroxide (RGHP). The patent describes a class of fuels called Non-Toxic Hypergolic Miscible Fuels (NHMFs). These fuels are hypergolic with RGHP. "Block 0 is the most widely used heterogeneous NHMF tested with RGHP by institutions such as the USN at China Lake, the USAF Academy and Purdue University. When compared in terms of delivered vacuum specific impulse with the conventional storable propellant combination nitrogen tetroxide and monomethyl hydrazine (NTO/MMH), the performance of the combination RGHP/NHMF is only 7% lower. The combustion chamber temperatures and the density specific

2 impulse values follow the same trend. Research conducted at Swift Enterprises, Ltd. has led to the development of the Advanced-NHMFs, based on advanced energy organic fuels. A further enhancement was seen with the inclusion of organometallic fuels, such as the quadrasilane family developed and marketed by Organic Technologies, Incorporated of Coshocton, Ohio. When used with RGHP, the Advanced-NHMFs had a vacuum specific impulse of 97% of NTO/MMH. Current research at Swift Enterprises, Ltd. is leading to improved versions of the previously defined NHMFs and most recently, to the development of reactive fuels based on light metal compounds dissolved or dispersed in high-energy renewable organic fuels. These latter fuels are the focus of this paper. Fuel Selection parameters A basis of comparison for liquid propellants can be arrived at from the following three qualities: energy, kinetics and utilization 5. Current research at Swift Enterprises, Ltd. is aimed at determining the energy merit of reactive fuels within a propellant couple from a purely thermodynamic perspective. As described below, future research will take into consideration the kinetics and utilization qualities of the fuels. The best values of specific impulse are obtained from high exhaust-gas temperatures, and from exhaust gases having low molecular weight. Therefore, to be efficient, a propellant should have a large heat of combustion to yield large available energy, and should produce combustion products containing simple light molecules embodying such elements as hydrogen, carbon, oxygen, and lighter metals (lithium, aluminum, etc.). The constituent elements of the burnt gases further need to be chosen with a view to the substances formed during combustion having heats of formation as low as possible. In order to meet these criteria, researchers at Swift Enterprises, Ltd. and KB Sciences have been looking at light metal compounds such as lithium as the basis for the next generation of rocket fuels. Starting in 1998, Dr. Ron Humble of the United States Air Force Academy and KB Sciences studied fuel candidates for hybrid grain applications. Thanks to its molecular geometry (strength of the molecular bonds) and the high number of hydrogen atoms, lithium hexahydridoaluminate (Li 3 AlH 6 ) proved to have a high heat of formation (+ 85 kcal/mol) and a high density (0.994 g/cm 3 ) making it a very high-performance propellant. Current research at Swift Enterprises, Ltd., under contract to KB Sciences, is leading toward making pound quantities of lithium hexahydridoaluminate and conducting a series of chemical and engineering evaluations to further determine its applicability as a rocket fuel. Despite the untimely death of Ron, the research continues. After an extensive literature review and a careful selection of commercially available lithium compounds with favorable molecular geometries and heats of formation, the USAF Isp Code was used to determine the optimum vacuum Isp and vacuum density Isp of seven lithium compounds. Prior calorimetric and physical property analyses were conducted at Swift Enterprises, Ltd. on quadricyclane and numerous quadrasilanes. Again the USAF Isp code was invoked to determine optimum energy values. Along with the physical properties of the fuels, Table 1 depicts the performance of these initial candidates used in combination with anhydrous hydrogen peroxide. The table also includes system performance data assuming a 60s burn time of a 10,000 lbf propulsion system with an inert weight of 300 lb. The performance of these fuel components are compared with five reference fuels: USN Block 0, used in combination with anhydrous hydrogen peroxide. As mentioned previously, Block 0 is the first non-toxic miscible fuel hypergolic with RGHP that has actually been tested by several institutions in several different rocket engine configurations. While its performance in terms of vacuum specific impulse is 7% lower than that of NTO/MMH, it is a reliable baseline. NTO/MMH, currently one of the most widely used bipropellants for Divert and Attitude Control System (DACS).

3 ClF5/Hydrazine, used in the Aerojet version of the Advanced Liquid Axial Stage (ALAS) program and currently the state of the art bipropellant solely in terms of specific impulse. Quadricyclane, investigated by the Air Force Research Lab at Edwards AFB and the NASA Marshall Propulsion Research Center. It has better performance than RP-1 but is expensive to make and has recently been rated by the Department of Defense as mildly toxic. Quadrasilanes, developed by Organic Technologies, Coshocton Ohio. Included in this study is the highest performer of the quadrasilane family. When used with RGHP its performance in terms of vacuum specific impulse and delta V are comparable to NTO/MMH. Theoretical performance results The results of the performance calculations are given in Table 1 and represented in Figures 1, 2, and Density Isp (kg*s/m 3 ) Better than NTO/MMH Methoxide Amide Block 0 n-butyllithium Quadricyclane MMH Quadrasilane Hydrazine Borohydride Aluminum Aluminum H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 NTO H2O2 ClF5 H2O2 H2O2 H2O2 Figure 1: Density Isp (kg*s/m 3 )

4 Table 1: Fuel Properties and Performance Fuel Properties Propellant Performance System Performance Oxidizer Fuel Methoxide Chemical Formula Molecular Weight (g/mole) Li-OCH 3 38 Amide Li-NH 2 23 Density (g/cm 3 ) Heat of Formation (kcal/mole) Heat of Combustion (BTU/gal) Optimum Mass O/F for max Ispvac Optimum Volumetric O/F for max Ispvac Optimum Flame Temperature (K) for max Ispvac < < 0 Optimized Vacuum Isp (s) Optimized Vacuum Density Isp Propellant Mass (kg) Initial Mass (kg) V (m/s) Delta V (m/s) vs. NTO/MMH Block 0 n-butyllithium Quadricyclane Mn C H O LiH < < 0 C 4 H 9 Li < 0 C 7 H < 0 NTO MMH CH 6 N Baseline Quadrasilane SiC 12 H ClF5 Hydrazine N 2 H Borohydride Aluminum Aluminum LiBH 4 22 LiAlH 4 38 Li 3 AlH Assumptions for the calculations: Chamber pressure = 500 psia, Expansion to vacuum, Nozzle area ratio = 250 Final Mass = 300 lb, Thrust = lbf, Burn time = 60 s

5 Flame temperature (K) Better than NTO/MMH Methoxide Amide Block 0 n-butyllithium Quadricyclane MMH Quadrasilane Hydrazine Borohydride Aluminum Aluminum H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 NTO H2O2 ClF5 H2O2 H2O2 H2O2 Figure 2: Flame temperature (K) Better than NTO/MMH V (m/s) Methoxide Amide Block 0 n-butyllithium Quadricyclane MMH Quadrasilane Hydrazine Borohydride Aluminum Aluminum H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 NTO H2O2 ClF5 H2O2 H2O2 H2O2 Figure 3: V (m/s)

6 Among the seven lithium-based compounds, lithium borohydride (LiBH 4 ), lithium aluminum hydride (LiAlH 4 ) and lithium hexahydridoaluminate (Li 3 AlH 6 ) not only have higher performance than NTO/MMH but also higher performance than the Aerojet ALAS propellant in terms of vacuum specific impulse and vacuum density specific impulse. Of particular interest is Li 3 AlH 6 used in combination with anhydrous hydrogen peroxide. This propellant combination has performance characteristics 30% greater than NTO/MMH while maintaining virtually the same combustion chamber temperature. It can be further noticed that all of the candidate combinations have lower flame temperatures than NTO/MMH, important for structural engine design. From a delta V standpoint and using the baseline system conditions outlined previously, LiBH 4, LiAlH 4 and Li 3 AlH 6 respectively provide 300, 410 and up to 960 m/s more delta V than NTO/MMH. Mission Improvements The design of a rocket and, in particular, of its propulsive system has to be optimized to satisfy the mission requirements while providing as much payload mass and volume as possible. Representing up to 90% of the mass of a rocket and a sizable portion of the volume, rocket propellants have to be carefully selected. The density of the propellants as well as material compatibilities issues have to be considered. Calculations were performed in order to evaluate the gain in payload mass and tank volume that the lithium-based fuels would provide compare to NTO/MMH. The results are presented in Table 2 assuming a mission from Low Earth Orbit (LEO) to Geostationary Earth Orbit (LEO) for which the required delta V is equal to 3801 m/s. The assumptions included a chamber pressure of 500 psia, and an expansion to vacuum with a nozzle area ratio of 250. Furthermore, the calculations were performed at the optimum volumetric O/F of NTO/MMH; using baseline propulsive hardware. Table 2: Mission Specific Fuel Performance Oxidizer NTO ClF5 Fuel Assumed V (m/s) Propellant Mass (kg) Initial Total Mass (kg) Delta Mass compared to NTO/MMH % Fuel Tank Volume of NTO/MMH % Oxidizer Tank Volume of NTO/MMH Methoxide Amide Block 0 n-butyllithium Quadricyclane 3801 MMH Quadrasilane Hydrazine Borohydride Aluminum Aluminum BASELINE

7 Taking into account all the assumptions stated previously, the performance of LiBH 4 and Li 3 AlH 6 respectively allow for total mass gains of 24 and 59kg. In addition, both the fuel and the oxidizer tanks could be reduced by up to one quarter with the propellant combination Li 3 AlH 6 /. Such reductions would directly benefit the payload of a spacecraft using of these propellants. Future Research The performance and physical properties of the proposed hypergolic fuels are investigated using a selection process developed by research engineers at Swift Enterprises, Ltd. The selection process is based upon three tasks. The first task, called preliminary selection, is composed of the following three subtasks: Theoretical Performance Prediction of developed fuel mixtures The USAF Isp Code is used to determine the energy merit of composite fuels within a propellant couple from a purely thermodynamic perspective. The following criteria should be satisfied: o Delta V, Vacuum specific impulse and density specific impulse greater than those of NTO/MMH. o Combustion chamber temperature lower than NTO/MMH. Solubility and Toxicity Studies Test solutions of 3 to 10 grams will be prepared using standard laboratory equipment. Sample vials are then stored in an open rack at room temperature and stability is studied over time. Fuel components and fuel mixtures are to be non-carcinogenic and not particularly hazardous in their manufacturing, transport, handling and use. Hypergolicity and Ignition Delay Measurements The hypergolicity of the candidate fuels with RGHP will be investigated with the Hypertester (Figure 4) and a recently developed Ignition Tester (patent pending). Originally conceived by the United States Navy, the Hypertester has been continuously improved over time at Swift Enterprises, Ltd. Figure 4: Hypertester The Hypertester provides a rapid and low cost method for determining the ignition delay values of rocket propellants. It is easy to set up and requires very low maintenance. The most common configuration of the Hypertester is one in which a drop of oxidizer comes into contact with the fuel originally placed in a crucible. A velocity transducer records the time at which the drop of oxidizer impacts the crucible, and a photodiode records the time at which ignition occurs. The volume of the fuel in the crucible is approximately equal to the volume of the drop of oxidizer so that the volumetric mixture ratio is close to unity.

8 The Ignition Tester is another device that helps researchers at Swift Enterprises, Ltd. to assess as accurately and easily as possible the ignition delay of rocket fuels in a spray configuration. Adding to the reproducibility and multiplicity of the test results, the Ignition Tester allows for the testing of propellants at different mixture ratios, temperatures, pressures and humidity levels. The destructive power of the propellant mixtures is greatly limited by the very low volume of materials used. The second task of the selection process is a detailed investigation of the chemical and physical properties of the fuel candidates. It is composed of: The measurement of the ignition delays of the fuel candidates that passed the preliminary selection. A detailed investigation of properties such as polarity, molecular geometry, molecular weight of the exhaust products, stability over time and storage conditions of the fuel candidates. The determination of the cost and ease of production of the remaining fuel candidates. It should be noted at this point that a strategic alliance with GFS Chemicals of Columbus, Ohio, is a great asset towards the assessment of the production of these fuels. The third and final task of the selection process consists of preparing experimental amounts of the fuel mixtures that satisfy the criteria of the first two tasks of the process. At this point, the final fuel compositions will be ready for rocket testing. Summary Non-toxic high performance propellants have an important role to play towards safer and logistically superior propulsion systems of interest to civilian and military applications. Swift Enterprises, Ltd. is focused on fuels designed around alkaline metallic species to render existing hydrocarbon fuels reactive with RGHP. Research performed up to the end of the summer 2002 has shown that these fuels have delta V and energy density values greater than NTO/MMH bipropellants. Used in existing rocket engines, these high-density fuels would reduce propellant mass and tank volumes allowing for larger payloads. Progressing towards the ultimate verification of the fuels in a rocket engine firing, researchers at Swift Enterprises, Ltd. are currently preparing test solutions of the fuels. Aging and stability studies will be performed in sequence with hypergolicity and ignition delay measurements. High performance propellants having a low ignition delay allow for precise control of exo- and endo-atmospheric vehicles. Scaled versions of such engines could be used as a means of primary propulsion on board a rocket and for Emergency Power Units on aircraft. References 1) Rusek, J. J., Lormand, B. M., Purcell, N. L., and Pavia, T. C., "Non-Toxic Hypergolic Propellant Demonstrations," AIAA 1998 Missile Sciences Conference, Monterey, CA, Nov ) Rusek, J. J., and Lormand, B., Non-Toxic Hypergolic Miscible Fuels for In-situ Decomposition of Rocket-Grade Hydrogen Peroxide, 1st Annual International Symposium of High Test Peroxide, Surrey, UK, August ) J.E. Funk and J.J. Rusek, "Assessment of United States Navy Block 0," 2nd Annual Hydrogen Peroxide Propulsion Conference, Purdue University, IN, November ) U.S. Patent #5,932,837, "Non-Toxic Hypergolic Miscible Bipropellant", John J. Rusek, Nicole Anderson, Bradley M. Lormand, and Nicky L. Purcell, Aug. 1, ) M. Roy, Thermodynamique des systèmes propulsifs à réactions, Dunod, Paris, 1947.