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This Defense Intelligence Reference Document from the Defense Intelligence Agency, dated 21 November 2010, was produced in FY 2010 under the Advanced Aerospace Weapons System Applications (AAWSA) Program. It reviews propulsion concepts that use plasmas and magnetohydrodynamics. It finds the Ajax MHD energy bypass concept meaningless below Mach 12 but calls a reverse energy bypass with a Virtual Cowl potentially practical. It also predicts that electric propulsion will become standard for spacecraft.
UNCLASSIFIED//F&II. &FFll!l*L 1!1!11!! 8HL"i' the shock or boundary layer with a modest amount of alkali metal vapor helps in getting the conductivity close to the maximum achievable level of ~3,000 mho/m. At this level of conductivity, a modest magnetic field, B~0.1-0.3 Tesla, is sufficient for substantial MHD effects (power generation, flow acceleration, or aerodynamic control). However, at gas temperatures of "only" 1,500-2,000K or so typical for scramjet combustors, even seeding the flow with alkali vapor results in conductivities no higher than 10-30 mho/m, in which case the strength of magnetic field required for substantial MHD performance at L=l meter or less is quite high: 8=3-10 Tesla. The weight, volume, and complexity associated with such a strong magnetic field that must be created in such a large volume make this application very problematic. NONEQUILIBRIUM MHD IN COLD AIR FLOWS The situation becomes worse in relatively cold air. Indeed, static gas temperatures at Mach number less than about 12 are quite low (10-100 Torr) are much higher than those in typical glow discharges (1 Torr or less) resulting in much higher power required to sustain plasmas and to severe problems with arcing instabilities. • The ionization fraction needed for a good electrical conductivity and acceptable MHD performance is much higher than that required for a fluorescent light, again resulting in high power budget and overheating. For cold nonequilibrium plasmas, the power budget is determined by the average energy cost (usually expressed in eV), Wi, of ionization (i.e., of producing an electron-ion pair), and the rate at which the electron-ion pairs must be generated in order to compensate for electron losses in recombination, attachment, and other processes. The recombination is the dominant loss mechanism at reasonably high electron densities, and its rate is proportional to the product of electron and ion number densities. Since in quasineutral plasmas the number densities of electrons and ions are close to each other, the recombination rate (per unit volume) is equal to kdrne2, where kdr is the dissociative recombination rate coefficient and ne is the electron number density. Note that the characteristic plasma decay time due to recombination is almost always very short, typically ~1-10 microseconds, so that the flow moves only a very short (~1 cm) distance during the decay time. This is why schemes with pre-ionization upstream of the MHD region with no ionization in the MHD region itself are not viable; the ionization must be done continuously throughout the MHD region. 13 UNCLASSIFIED/FOA OFFIGl,lals. W&li 8HLY
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 32 pages are in the text index: search them above, or from the library's search.