2. MHD generator
3. Combustion chamber
4. MHD accelerator
2. MHD generator
3. Combustion chamber
4. MHD accelerator
Hypersonic AeroPropulsion Integration Course, AE P 452-50, a student design team took on the task of analyzing Ajax. The resulting performance increase reduced the size and weight of the performance-sized aircraft [Esteve et al., 1977].The student team members were Yago Sanchez, Maria Dolores Esteve, Alfonso Gonzalez, Ignacio Guerrero, Antonio Vicent, Jose Luis Vadillo. Professor Mark A. Prelas, Department of Nuclear Engineering, University of Missouri-Columbia, was an advisor to the student team. After touring a number of Russian nuclear facilities, he provided first-hand knowledge of the ionization devices that are reported to be key components of the Ajax system.
From Novichokv [Novichokv, 1990a], comes a sketch of the propulsion system concept with the coupled MHD generator-accelerator showing the energy bypass concept, Figure 4.40. The simple sketch gives a cross-section similar to any totally integrated propulsion system in which the bottom of the vehicle hosts the propulsion system, and the forebody is indeed the front part of the inlet. Figure 4.40 clearly shows the energy bypass concept associated with the Ajax propulsion system. Also from Novichokv [Novichokv, 1990b] are the features of the Ajax system and reasons the Ajax system was developed. They are as follows:
(1) Energy bypass: via a coupled MHD generator-accelerator system [Gurianov and Sheikin, 1996; Carlson, 1996; Lin and Lineberry, 1995], a portion of the free stream kinetic energy bypasses the combustion chamber to reduce the entropy rise associated to aerodynamic diffusion and to the combustion process.
(2) Reforming of hydrocarbon fuel via a thermal decomposition process followed by an electrical arc process into a high hydrogen fraction fuel, with about
20,200 Btu/lbm heat of combustion. It is assumed that the products are gaseous hydrogen, ethylene and other combustible species, and possibly carbon monoxide. The quantity of water used or the disposal of the excess carbon for this process is unclear (experimental data and analyses from various sources, including Russian, support qualitatively the relevance of this feature).
(3) Ionization of the airflow at the nose of the aircraft and of the airflow entering the engine, probably generated by the Russian-developed Plasmatron. One of these Plasmatron devices is operating in the plasma wind tunnel test facility at the von Karman Institute (VKI) in Brussels. The former may alter the shock system surrounding the aircraft to reduce drag and to permit the MHD nose generator to extract kinetic energy from the flow. The latter permits the MHD generator-accelerator to function with the magnetic field strengths possible with superconducting magnets and the flow velocities present within the engine module to produce a flow energy bypass system [Tretyakov, 1995; Gordon and McBride, 1993; Gorelov et al., 1995, 1996], (Russian information supported by analysis and available databases.)
(4) Powering of the fuel reforming process by an MHD generator in the nose of the vehicle [Batenin et al., 1997], that with a particle beam generator in the nose, produces a plasma cloud at the vehicle nose and results in a reduction of the vehicle total drag [Gurijanov and Harsha, 1996; Tretyakov, 1995; Zhluktov, 1996; Gorelov et al., 1996, 1995; Smereczniak, 1996]. (Russian information with experimental data obtained, under an Italian research collaboration effort with the Russian Academy of Sciences (RAS)-Novosibirsk).
(5) Increase in the combustion efficiency within the engine by means related to injection of plasma or hydrogen ahead of the fuel injector struts [Tretyakov et al., 1995]. (Russian information with experimental data obtained under Italian collaboration research effort with RAS-Novosibirsk.)
(6) Diversion of the bypassed energy to a directed energy device on an intermittent basis for peaceful purposes. Purposes listed are: reduction of the ice crystal formation over Antarctica to reduce the size of the ozone hole, space debris burning, ionosphere and upper atmosphere research, ozone generation, communication with artificial satellites, water surface and atmosphere ecological conditions diagnostics, ore deposits prospecting, earth vegetation research and monitoring, seismic conditions and tunnel monitoring, ice conditions and snow cover monitoring, and long-range communication and navigation.
In January 2001, Alexander Szames of Air et Cosmos interviewed Nikolai Novitchkov and Vladimir L. Freishtadt [Szames, 2001]. The article states that the project originated in the State Hypersonic Systems Research Institute (GNIPGS) in St Petersburg. Vladimir Freishtadt was the OKB Director, with members Viktor N. Isakov, Alexei V. Korabelnikov, Evgenii G Sheikin, and Viktor V. Kuchinskii. It is clear in the literature that Ajax is primarily a global range hypersonic cruise vehicle. All the discussions with individuals about Ajax stress both the global range capability at hypersonic speeds and the directed energy device for peaceful purposes. When the illustration (Figure 4.41) was published in Paris in December 1999 it
showed a vehicle concept that corresponded to correct hypersonic design criteria, and a flow field significantly modified by MHD interaction. A paper presented in the 1997 IAF Congress held in Turin, Italy, provided details of an axisymmetric MHD nose generator. Its intent is to drive the device that creates plasma ahead of the nose. Researchers from Novosibirsk have stated such tests have been conducted in their hypersonic, high-temperature wind tunnels and presented very similar pictures. An AIAA paper by Dr J. Shang of the Air Force Research Labs has similar data. One of the difficulties with the MHD propulsion system analysis is the only analyses possible are for aircraft in a free stream flow field without any ionization. As the Szames illustration shows, and Russian researchers have stated, the propulsion system and aircraft operate as if they were in a modified Mach number and gas flow field. In fact the flow around the aircraft and entering the engine is a plasma flow. None of the aircraft or propulsion analyses these authors have done have considered this plasma flow field. The plasma effect is not the same as a simple thermal modification of the gas properties. Since the atmosphere ahead of the aircraft has the lowest density, MHD interaction with the flow field ahead of the aircraft is the greatest and covers the greatest extent. An IAF paper presented in Turin, Italy, describes the nose MHD device that reportedly powers a fuel-reforming process of unknown description [Batenin et al., 1997].
The reported performance includes a 13,812 km (7458 nautical miles) range at Mach 8 and 33 km altitude, and the mission time, 129 minutes. Cruise speed is then 8,005 ft/s. From historical aircraft performance correlations, the climb and descent time and distance are 46 min and 1250 nautical miles, respectively. With ground operation, that yields a cruise distance of 6,208 nautical miles (11,497 km) and a mission time of 130 minutes. For a fuel fraction of 50% the range factor is 16,590 km (8,960 nautical miles). The sketch of Ajax (Figure 4.41) indicates a Kuchemann's tau of about 0.10. That yields an aerodynamic lift-to-drag ratio (L/D) of 4.1. The integrated propulsion system and gravity relief results in a final L/D of 4.7. The reported heat of combustion for Russian reformed kerosene is about 30,000 Btu/lbm. With a 50% propulsion energy conversion efficiency the V/sp is 1,920 nautical miles (3,557 km) and the Isp is 1,457 s. The resulting range factor is 9,024 nautical miles (16,712 km). If low-level ionization were employed to reduce the cruise drag, then the mission range would be 25,309 km (13,666 nautical miles) in 204 minutes. So the reported Ajax performance is an Earth-circling range in three and one-half hours [Bruno et al., 1998].
For a cruise system the total heat load can be an order of magnitude greater than for an atmosphere-exit trajectory, so some form of continuous energy management is required to prevent the airframe thermal capacitor from absorbing excess energy [Anon., 1970]. The heat capacity of some of the reformed hydrocarbon fuels can be greater than hydrogen. From the Szames article the heat of formation is given as 62,900 kJ/kg or 59,620 Btu/lb for the case of reformed methane. In the case of Ajax the thermal energy is not discarded but used to create thrust. As indicated in the Introduction, the Ajax system is an energy management system that minimizes the shock losses (entropy rise of the total aircraft system in hypersonic flight) and makes converted kinetic energy available for applications. The fraction of the thrust energy provided by the recovered aerodynamic heating reported in the Russian references, 30%, is in agreement with prior analyses [Czysz, 1992; Ahern, 1992].
MHD flows are governed by the interaction of aerodynamic and electromagnetic forces. As a result the key MHD parameter contain elements of both. The seven most important considerations and parameters are cyclotron frequency and collision frequency, the MHD interaction parameter, the load parameter, the Hall parameter, the Hartmann number, and the gas radiation losses; they characterize and also constrain the performance of a MHD system. The bold-faced parameters are the four discussed in this chapter. One of the authors (CB) provided information related to the impact of each of these parameters. Four of them are critical to the operation of the MHD generator and accelerator in determining the existence and intensity of the Lorentz force [Bottini et al., 2003]. That is the force that accelerates or decelerates the airflow via electromagnetic energy interaction with the ions in the flow. If the Lorentz force is not present, there is no electromagnetic acceleration of the gas.
Consider the motion of a single charged particle in a magnetic field B. A single charged particle spirals around the B field lines with the electron cyclotron frequency. The charged particle of an ionized gas is thus guided ("confined", in plasma parlance) by the magnetic field (and thus can be separated by ions and create an E field and a voltage), but only on condition its mean free path (the distance a particle travels between collisions) is greater than the cyclotron radius. If this were not the case, after a collision with another particle, the particle would be scattered away from its spiral trajectory and "diffuse" across the field lines. This condition is the same as saying that the collision frequency must be less than the cyclotron frequency. The condition for guidance, accounting for collision frequency and cyclotron frequency, scales with B, pressure and temperature as the following equation:
where B = magnetic field strength (in tesla), T = gas static temperature (K), p = static pressure (atm) and a = ionization fraction. The LHS of equation (4.21) is the Hall parameter. Since the numerical factor in front of equation (4.21) is on the order of 10~3, it is clear that this condition requires very high B or very low pressure. Very high (nonequilibrium) electron temperature Te can satisfy this condition, provided B is on the order of 1 tesla or greater, and pressure is on the order of 0.1 atmosphere. This puts a stringent condition on the operation of a MHD device. It is clear that this rules out equilibrium ionization for all practical purposes (the equilibrium temperature would have to be unrealistically high, many thousand K), and that extraction can work efficiently after a certain amount of dynamic compression, but not inside combustion chambers, where pressure is of the order of 1 atm for a supersonic though-flow combustor and 10 to 20 atmospheres for subsonic through-flow combustor. This condition favors hypersonic cruise vehicles, as their typical dynamic pressure (hence internal pressures) are at least one-third that of an accelerating launcher.
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