Hypersonic Vehicle Electric Power System Technology
Rene Thibodeaux
Air Force Research Laboratory
1950 Fifth Str...
connected electrodes produce the optimum
configuration that takes full advantage of both the
Faraday and Hall electric fie...
A trans-atmospheric vehicle may be a very
large single-state to orbit vehicle or a two-stage
launcher and orbiter. In eith...
The generator design effort includes fluid
dynamic, thermodynamic and plasma analysis of
MHD generators as well as scaling...
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Hypersonic vehicle electric power system technology

  1. 1. 2002-2109 Hypersonic Vehicle Electric Power System Technology Rene Thibodeaux Air Force Research Laboratory 1950 Fifth Street Dayton, OH 45433 937-255-6016 Rene.Thibodeaux@wpafb.af.mil Abstract-The objective of this program is to develop magnetohydrodynamic (MHD) and other plasma technologies for hypersonic vehicles. The focus of this effort is to design, fabricate, and test a hypersonic MHD generator. In addition, MHD inlet compressors and plasma combustors for scramjets may be investigated. Hypersonic engines will need to produce power up to 10 Gigawatts to achieve the high speeds, so power extraction of l00 Megawatts s reasonable. The extreme temperatures and flow speeds at hypersonic conditions will eliminate any rotating turbines for propulsion, so non-rotating MHD power generation technology is ideal for hypersonic vehicles. The enormous kinetic energy of the hypersonic flow can produce MHD generators of extremely high power densities on the order of l00’s Megawatts per cubic meter. The high-speed flow reduces the magnetic field requirements compared to previous MHD generator efforts. These technologies will provide unprecedented mission capabilities for future military hypersonic aerospace vehicles. MHD power preserves the versatility of air-breathing hypersonic engines by avoiding the need to carry independent oxygen supplies for high power turbine- based auxiliary power units or fuel cells. Additionally, MHD and plasma technologies may be the breakthrough technologies that enable sustained combustion in scramjets, a goal that has prevented hypersonic engine development for decades. BACKGROUND Magnetohydrodynamics is a concept dating back to the time of Sir Humphrey Davy and Michael Faraday who built the first motor using a wire moving in liquid mercury. Concepts for conducting fluid MHD generators began to appear in patents as early as 1910; however, no method of ionization was suggested. The first work on a conducting gas MHD generator began with Karlovitz and Halasz in 19381 . The natural high temperature produced by the drag at hypersonic speeds lead to speculation of MHD reactive control of the vehicle by Krantowitz in 19552 . The Air Force initiated a large research program from 1960 to 1980 to develop MHD for aircraft, focusing on subsonic and supersonic flows. The largest MHD power generator proposed by the Air Force was a 30 MW /Mach 2/LOX/JP-4 MHD that was to be tested at the Arnold Engineering Development Center3 . The program was cancelled in 1978. The last known Air Force MHD effort was the testing of a PAMIR-3U unit purchased from Russian in 1995. This generator produced 10-14 MW and weighed 18,000 kg. The combustion-driven MHD power unit is the oldest MHD technology. The disadvantage of the direct combustion unit is the need for internal oxidizer and the limited operating time. Russian work on inlet ram air-driven MHD “AJAX” concept has been recently published44 . To date scramjet driven MHD has remained a theoretical concept due to the simple fact that a long duration working scramjet engine has not been developed. Ironically, MHD and plasma combustion technology may be the breakthrough technologies that produce sustained combustion in the scramjet. An MHD inlet compressor can also control the angle of the inlet shock and eliminate the need for mechanical variable geometry inlets5 . Russians research has been ongoing in these areas since the 1990’s6 . TECHNICAL DISCUSSION MHD generators produce a current in an ionized gas by passing the gas through a magnetic field. The current density is governed by the law, J = s(E + UxB), where J is current density, sigma is the conductivity TT is the gas velocity, and B is the magnetic field in Tesla. The electric field, UxB, consist of a perpendicular (Faraday) component and an axial (Hall) component relative to the direction of motion. Electrons moving because of this field form surface charges at of the MHD channel and develop an opposing electric field E. Electrodes are positioned to use the Faraday or Hall fields, but diagonally 1
  2. 2. connected electrodes produce the optimum configuration that takes full advantage of both the Faraday and Hall electric fields simultaneously. Most MHD generators have been diagonally connected generators, although some pulse power generators utilized continuous Faraday electrode designs are possible. The higher voltage requirements of DEW favor the diagonal configuration. The extracted power density of a diagonal MHD is determined by the equation, P = k (l-k)s(UB)2 , where p is the power density, k is the load factor. The conductivity itself is actually a three dimension tensor usually simplified to a scalar value, s = K ne, and composed of the mobility K, the electron (or ion) number density n, and the electron charge e. At the low subsonic and even at supersonic speeds, large power was only possible with large magnetic fields or large conductivity. The former was a major technical problem and the latter was a major physics problem. Even though MHD generators were built and worked, the difficulty of solving the magnet or conductivity properties limited the application of MHD. Operating in high-speed flow dramatically changes these requirements, reducing the required magnetic field and taking advantage of the “natural” ionization produced by high temperature hypersonic flows. Thermal ionization is a natural byproduct of the high collision rate of the atoms in the high temperature gas. Achieving natural thermal ionization in molecular gases requires extremely high temperature. For example, ionization of pure air requires temperatures over 4,000 °K. To enhance the thermal ionization, techniques of artificial nonequilibrium ionization such as adding high electron affinity alkali metals, high electric fields, particle beams, radio frequency inductive heating, or high power microwaves can be used. The techniques strip electrons from any neutral atoms or to absorb free electrons to prevent recombine with any ions. Hypersonic MHD generators can easily extract large amounts of electric power from the tremendous kinetic and thermal energy available in a hypersonic flow. Typically, the MHD becomes more efficient with size since most irreversible associated with it are wall losses that proportionally as the surface-to-volume decreases. Demonstrated enthalpy extraction MHD generators in the 0.5-2.0 MWe range has been at the order of 1.0-5.0 % and for larger units in the 10-500 MWe range, enthalpy extraction increases to the 10.0-20.0 % range. For large-scale MHD power systems, isentropic efficiency approaching 80% is theoretically possible. Load Working Fluid Out Anode Wall Working Fluid In Cathode Wall Electric Current Flow of Electrically Conductive Fluid (Plasma) Through a Magnetic Field Electric Potential (u x B) is Induced Orthogonal to the Flow and Magnetic Field Direction Tapping Across Potential through External Load Extracts Electric Power Principle of MHD Power Generation Insulating Wall Magnetic Field u x B u Four generator configurations are possible for a hypersonic vehicle. The first is a ram air driven MHD auxiliary power units using free-stream air. This either can be located at the inlet of the hypersonic engine or placed side-by-side with the hypersonic engine using an independent combustion source if necessary. The second is a scramjet integrated MHD power unit using the scramjet's combustor flow. For a space vehicle, a direct (rocket) combustion-driven MHD power unit can be fueled with traditional propellants and oxidizer. A trans-atmospheric vehicle can use a re-entry plasma- drive MHD power unit for trans-atmospheric vehicles can be integrated with the external skin. ADVANTAGES OF HYPERSONIC MHD Advantages of Hypersonic Speeds An aircraft with the combined capability of high speed and long distance weaponry would perform missions not possible before. A hypersonic vehicle cruising in the atmosphere at speeds up to Mach 12 could travel anywhere on the globe within two hours at altitudes above 100,000 ft. At that altitude, the directed energy weapon system or surveillance radar would have a virtually turbulent free atmosphere and extremely long line-of-sight distance to the horizon. A trans-atmospheric space vehicle has wider global range and is virtually a self-contained platform, capable of launching from the achieving orbit, and returning to ground. This alone is a significant improvement over the permanently orbiting weapon platforms proposed under the Strategic Defense Initiative. A major drawback of this concept was always the requirement for heavy launch vehicles and maintenance shuttles (or more likely no maintenance at all). B J Electric Current 2
  3. 3. A trans-atmospheric vehicle may be a very large single-state to orbit vehicle or a two-stage launcher and orbiter. In either case, the system is likely a multi-billion platform. A more economical platform may be a boost-glide re-entry vehicle equipped with MHD generators imbedded in the vehicle skin. Launch methods range from the old expendable vehicles similar to the 1960’s Dynasoar concept to a two-state hypersonic cruiser launcher concept. The reentry heat easily produces very high conductivity plasma flowing at extremely high velocities so that magnetic fields of less than one Tesla could be sufficient. Although the reentry plasma would restrict the use of radio and microwave devices, the plasma frequencies at reentry conditions are below the range of visible light, making visible or UV lasers viable candidates as weapons. Second, the resulting effects of turbulence and hypersonic shock waves can be moderated or even eliminated through the very application of external high magnetic fields7 . The vehicle then glides back to the launch point or a recovery area for late re-boosting. It must be considered that the great capabilities of hypersonic speeds also cause extreme environmental problems for a human pilot. The extreme temperature environment, high centrifugal turns, and the high angle of attack needed to optimize the inlet shock angle make uninhabited vehicle technology more appropriate for hypersonic vehicles. Advantages of MHD in Hypersonic Flows Due to the unique nature of hypersonic flows, MHD has several advantages over competing power generating systems. First, the high velocity and associated extreme temperatures of hypersonic flows prohibit the use of rotating turbines and instead require the use of non-rotating ramjets, scramjets, and rocket engines. The fact that an MHD generator is nothing more than a flow channel with external magnets makes it a natural companion of hypersonic engines. The lack of moving parts in an MHD generator eliminates the inertia limits inherent in heavy turbo-generators, allowing rapid repetitive turn-on and turn-off, ideal for directed energy weapon applications. The advantage of ramjets and scramjets is the fact that they are air breathing and therefore avoid the extreme weight penalty of carrying any oxidizer. An MHD generator retains the air breathing advantage of the hypersonic engine. A turbine based auxiliary power unit or fuel cells could supply power to the hypersonic vehicle, but an oxidizer must be carried on the vehicle. This would quickly become a problem if the vehicle must fly for extended time and produce Megawatts of electrical power for a directed energy weapon. MHD is an excellent match to directed energy weapons in terms of achievable power levels and duty cycles. MHD is competitive with most storage and rotating devices over durations of 1 to 1000 seconds. Chemically driven MHD pulse power systems with power density on the order 100’s MW/m3 have been demonstrated over the pass 40 years. Russian work in 1980’s was reported8 to have produced an output of 500 MW at a power density of approximately 600 MW/m3 . Power densities of hypersonic MHD generators can be made extremely high on the order from hundreds to thousands of megawatts per cubic meter. No other non-nuclear power technology can compete with this technology for hypersonic air and space vehicle power sources using directed energy weapons. PROGRAM OBJECTIVE AND GOALS The overall program objective is to perform a ground test demonstration of up to a one-megawatt MHD generator in a hypersonic wind tunnel. This ground test will determine whether MHD generators can operate effectively in hypersonic flows and if MHD adversely affect the performance of a scramjet. The effort will demonstrate insulation techniques needed for superconductor magnets to operate in the extreme temperature environment expected under hypersonic conditions. Plans for a flight demonstration of MHD technology in a prototype hypersonic test vehicle will be developed from the results of this effort, Other plasma-aerodynamic effects utilizing MHD principles such as inlet compression control, plasma combustion, and exhaust thrust vectoring could be tested in this effort. Initial work will be trade studies, analysis, modeling, and simulation of hypersonic vehicles equipped with MHD power generators. Different integration configurations between the MHD generator, engine, and vehicle will be identified and characterized. The important issue of power conditioning will be addressed in this program. particularly the optimum distribution voltage needed for DEW systems. Since the preferred fuel hypersonic vehicle above Mach 10 is 20 °K liquid hydrogen and hydrocarbon fuels below Mach 10, different thermal requirements must be considered in concert with the overall vehicle/weapon system thermal management. This will reduce the large external magnetic field leakage as well as the need for a homogeneous internal field will be important for the design of a flyable superconducting magnet. 3
  4. 4. The generator design effort includes fluid dynamic, thermodynamic and plasma analysis of MHD generators as well as scaling studies. Conceptual designs of MHD generator power systems will be produced for airborne and spaceborne systems considering the integrated support structure for the generator/magnet/engine. Using the analytical tools developed, generator modeling and simulation of both spaceborne and airborne MHD systems with parallel laboratory verification tests will be performed. The magnet development effort will begin with the analysis of various conceptual designs. Using state-of-the-art magnet field codes, the magnet conceptual designs will be modeled, leading to the eventual fabrication and testing of samples magnets. The conductors of interest include low temperature niobium based superconductors with advanced helium cryocoolers, high temperature superconducting BSCCO and YBCO, as well as cryogenic high purity aluminum. The MHD generator development effort will initially utilize smaller supersonic testing facilities. Supersonic wind tunnel test include material testing, component testing, seeding scheme testing, gas dynamic physics testing and develop diagnostics for the final hypersonic facility tests. A plasma torch device will be used to simulate the high-temperature plasma environment for testing electrodes and insulator materials. After sufficient verification tests of the conceptual designs are accomplished, the detailed design, and fabrication of the prototype generator components will begin. Preparation for the hypersonic MHD generator tests will be conducted at a hypersonic wind tunnel test facility depending on availability. Test data will include basic power extraction, component lifetimes, electrode wear rates, magnet performance, and off-design behavior. SUMMARY The objective of this program is to develop magnetohydrodynamic (MHD) power generators for integrated power production on hypersonic vehicles. MHD will provide order-of-magnitude improvement in the power capabilities of future Air Force hypersonic aerospace vehicles, enabling global missions using directed energy weapons. Proposed scramjet engines will be non-rotating engines producing Gigawatts of power. Therefore, non- rotating MHD generators are a natural match to produce efficient, high voltage power for many directed energy weapons concepts. Recent developments in lightweight superconductors and structural material now make flight-weight MHD power systems for hypersonic aircraft feasible. In addition, an inlet MHD compressor can control the engine pressure and temperature along with a plasma technique to sustain hypersonic combustion, may overcome the problem that has prevented the development of scramjet engines. REFERENCES 1 B. Karlovitz, “Process for the Conversion of Energy,” U. S. Patent No. 2,210,918 (August 13, 1940). 2 Krantrowitz, A., “A Survey of Physical Phenomena Occurring in Flight at Extreme Speeds,” The Proceedings of the Conference on High Speed Aeronautics, 1955 3 Swallom, D., et al, “Results from the PAMIR-3U Pulsed Portable MHD Power System Program,” IEEE Paper 96467 31st Intersociety Energy Conversion Engineering Conference (IECEC), August 11-16 1996. 4 Goovitchev, V. I., Hansson, J., “Some Trends in Improving Hypersonic Vehicles Aerodynamics and Propulsion,” AIAA Paper IS-090 14th International Symposium on Air-Breathing Engines, September 5-10 1998 5 Lineberry, J. T., et al, “Prospects of MHD Flow Control for Hypersonics,” AIAA Paper 2000- 3057 35th IECEC, July 24-28 2000. 6 Bityurin, V. A., Zeigarnik, V. A. ,Kuranov, A. L., “On A Perspective of MHD Technology In Aerospace Applications,” AIAA Paper No.1996- 2355 2nd AIAA Plasmadynamics and Lasers Conference, June 17-20 1996. 7 Miles, R. B., et al, “Plasma Control of Shock Waves in Aerodynamic and Sonic Boom Mitigation,” AIAA Paper No.2001-3062 33rd AIAA Plasmadynamics and Laser Conference and Weakly Ionized Conference, June 11-142001. 8 Velikov, E. P., et al., “Pulsed MHD Power System ‘Sakhalin’ -The World Largest Propellant Fueled MHD Generator of 500 Electric Power Output,” International Conference on MHD Power Generation and High Temperature Technologies, October 1999. 4