Historical research guide · 0330 · 2010
DIRD MHD Air Breathing Propulsion and Power for Aerospace Applications
DIA / AAWSAP contractor
In one minute
Magnetohydrodynamics is the trick of taking hold of a moving gas with a magnetic field — but it only works if the gas conducts electricity, and ordinary air does not. This Defense Intelligence Reference Document works through everything that single fact allows and forbids. Above about Mach 12, and everywhere on a reentry vehicle, the air is hot enough to ionise itself: a light magnet of a tenth of a tesla and one square metre of skin can then generate megawatts with no moving parts, and a plasma laid ahead of the nose returns 40 to 50 times the power spent on it by cutting drag, lifting the lift-to-drag ratio from about 1 to somewhere between 3 and 10. Below Mach 12 the air has to be ionised artificially, which costs more than the device returns, so the well-known Russian Ajax energy-bypass scheme does not pay there. The authors’ own answer, a reverse energy bypass with a plasma virtual cowl, sends the power upstream instead and models 20 to 30 percent more thrust.
Connection
Chapter 9 treats plasma as an engineering material rather than a curiosity, and this is the government’s own inventory of what plasma can already do to a vehicle: make its own electricity from the airflow, reshape the air ahead of it, and steer without moving a surface. Chapter 8’s transmedium question — how a craft moves without shoving air aside in the ordinary way — begins with virtual surfaces of exactly this kind.
From the source
The basic principles of magnetohydrodynamics are understood very well: a conducting fluid crossing magnetic field lines induces a Faraday electromotive force equal to flow velocity times field strength times channel width, and the induced current both delivers power to an external load and produces a retarding jXB body force plus Joule heating in the fluid.Chapter 2, Basic Principles of Magnetohydrodynamics, pp. 11–12
The strength of an MHD interaction is set by the Stuart number, conductivity times field squared times length over density times velocity; in reentry shock and boundary layers at Mach 12 to 25 thermal ionisation alone gives 100 to 3,000 mho per metre, so 0.1 to 0.3 tesla suffices, whereas in a scramjet combustor at 1,500 to 2,000 K even alkali seeding yields only 10 to 30 mho per metre and 3 to 10 tesla would be required.Chapter 2, pp. 12–13
In cold air below about Mach 12 the ionisation must be made artificially, and even with electron beams — the most efficient method, at about 34 eV per electron-ion pair — the ionisation fraction caps near one in a million and conductivity near 1 mho per metre, so the Ajax MHD energy bypass is not a meaningful technology there; above Mach 12, where the air ionises itself, NASA Ames modelling shows the same bypass can raise specific impulse.Chapter 2, Nonequilibrium MHD in Cold Air Flows, pp. 13–14; The Ajax Concept, pp. 17–18
The reverse energy bypass places the MHD generator downstream of the combustor, where alkali-seeded combustion products conduct at 10 mho per metre or better without any ioniser, and sends the electricity upstream; paired with an optimised plasma virtual cowl that scoops cold air into an off-design inlet, the combined system can increase thrust by 20 to 30 percent.The Reverse Energy Bypass, pp. 18–20; Figures 8 and 9
On a reentry vehicle a surface-integrated MHD device can generate megawatt-scale power from one square metre of skin, and spending part of it on a plasma ahead of the nose returns 40 to 50 times the power used — more than 100 times if the plasma region is made long and thin — raising the hypersonic lift-to-drag ratio from about 1 to between 3 and 10 and enabling aerodynamically assisted, on-demand orbital inclination changes.MHD Applications to Reentry and Near-Orbital Flight, pp. 20–22; Figure 10
The outlook for MHD propulsion changes dramatically if hypersonic vehicles begin to carry powerful onboard electricity sources such as nuclear fission or fusion reactors, and the reentry, global-strike glider and aeroassisted orbital manoeuvring applications are within reach of several other nations inside a decade.Summary, p. iv; Chapter 4: Summary and Predictions, p. 25
The way in
https://documents2.theblackvault.com/documents/dia/AAWSAP-DIRDs/DIRD_33-DIRD_MHD_Air_Breathing_Propulsion_and_Power_for_Aerospace_Applications.pdfHow to cite it
DIA / AAWSAP contractor (2010) DIRD MHD Air Breathing Propulsion and Power for Aerospace Applications https://documents2.theblackvault.com/documents/dia/AAWSAP-DIRDs/DIRD_33-DIRD_MHD_Air_Breathing_Propulsion_and_Power_for_Aerospace_Applications.pdf