Editing Magnetohydrodynamic drive (section)

===Aircraft propulsion===
{{see also|Flow control (fluid)}}

====Passive flow control====
First studies of the interaction of plasmas with [[Hypersonic speed|hypersonic flows]] around vehicles date back to the late 1950s, with the concept of a new kind of [[Atmospheric entry#Thermal protection systems|thermal protection system]] for [[space capsule]]s during high-speed [[Atmospheric entry|reentry]]. As low-pressure air is naturally ionized at such very high velocities and altitude, it was thought to use the effect of a magnetic field produced by an electromagnet to replace [[Atmospheric entry#Ablative|thermal ablative shields]] by a "magnetic shield". Hypersonic ionized flow interacts with the magnetic field, inducing eddy currents in the plasma. The current combines with the magnetic field to give Lorentz forces that oppose the flow and detach the [[Bow shock (aerodynamics)|bow shock wave]] further ahead of the vehicle, lowering the [[heat flux]] which is due to the brutal recompression of air behind the [[stagnation point]]. Such passive [[Flow control (fluid)|flow control]] studies are still ongoing, but a large-scale demonstrator has yet to be built.<ref name="NASA reentry">{{cite report
 |last1=Sterkin |first1=Carol K.
 |date=December 1965
 |title=Interactions of spacecraft and other moving bodies with natural plasmas
 |id=19660007777. NASA-CR-70362. JPLAI/LS-541
 |publisher=NASA
 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660007777.pdf
 }}</ref><ref name="ESA">{{cite web
 |author=<!--Not stated-->
 |title=Magnetohydrodynamic flow control during reentry
 |website=European Space Agency
 |url=https://www.esa.int/gsp/ACT/pro/projects/mhd_reentry.html
 |access-date=2018-04-13
}}</ref>

====Active flow control====
Active flow control by MHD force fields on the contrary involves a direct and imperious action of forces to locally accelerate or slow down the [[airflow]], modifying its velocity, direction, pressure, friction, heat flux parameters, in order to preserve materials and engines from stress, allowing [[hypersonic flight]]. It is a field of magnetohydrodynamics also called '''magnetogasdynamics''', '''magnetoaerodynamics''' or '''magnetoplasma aerodynamics''', as the working fluid is the air (a gas instead of a liquid) ionized to become electrically conductive (a plasma).

Air ionization is achieved at high altitude (electrical conductivity of air increases as atmospheric pressure reduces according to [[Paschen's law]]) using various techniques: [[high voltage]] [[Electric arc|electric arc discharge]], [[Radio frequency|RF]] ([[microwave]]s) electromagnetic [[glow discharge]], [[laser]], [[Cathode ray|e-beam]] or [[betatron]], [[radioactive source]]... with or without seeding of low [[ionization energy|ionization potential]] [[alkali]] substances (like [[caesium]]) into the flow.<ref name="Froning 1999">{{cite conference
 |last1=Froning |first1=H. D.
 |last2=Roach |first2=R. L.
 |date=November 1999
 |title=Influence of EM discharges on hypersonic vehicle lift, drag, and airbreathing thrust
 |conference=9th International Space Planes and Hypersonic Systems and Technologies Conference
 |location=Norfolk, VA
 |book-title=AIAA-99-4878
 |doi=10.2514/6.1999-487
 |url=http://ayuba.fr/pdf/ajax/froning1999.pdf
 }}</ref><ref name="Lineberry 2000">{{cite conference
 |last1=Lineberry |first1=John T.
 |last2=Rosa |first2=R. J.
 |last3=Bityurin |first3=V. A.
 |last4=Botcharov |first4=A. N.
 |last5=Potebnya |first5=V. G.
 |date=July 2000
 |title=Prospects of MHD flow control for hypersonics
 |conference=35th Intersociety Energy Conversion Engineering Conference and Exhibit
 |location=Las Vegas, NV
 |book-title=AIAA 2000-3057
 |doi=10.2514/6.2000-3057 
 |url=http://ayuba.fr/pdf/ajax/lineberry2000.pdf
 }}</ref>

MHD studies applied to [[aeronautics]] try to extend the domain of hypersonic [[Fixed-wing aircraft|planes]] to higher Mach regimes: 
* Action on the boundary layer to prevent laminar flow from becoming turbulent.<ref name="Ullah2021">{{cite journal 
 | last1 = Ullah | first1 = L.
 | last2 = Samad | first2 = A.
 | last3 = Nawaz | first3 = A.
 | date = 2021
 | title = The convective instability of the boundary-layer flow over a rotating cone in and out of a uniform magnetic field
 | journal = European Journal of Mechanics B/Fluids
 | volume = 87
 | pages = 12–23
 | doi = 10.1016/j.euromechflu.2020.12.013
 | bibcode = 2021EuJMB..87...12U
 | url = https://doi.org/10.1016/j.euromechflu.2020.12.013
 }}</ref>
* Shock wave mitigation for thermal control and reduction of the wave drag and form drag. Some theoretical studies suggest the flow velocity could be controlled everywhere on the wetted area of an aircraft, so shock waves could be totally cancelled when using enough power.<ref name="Petit 1983">{{cite conference
 | last1 = Petit |first1 = J.-P.
 |author-link1=Jean-Pierre Petit
 | date = September 1983
 | title = Is supersonic flight without shock wave possible?
 | conference = 8th International Conference on MHD Electrical Power Generation
 | location = Moscow, Russia
 | url = http://www.jp-petit.org/papers/MHD/1983-Moscow-shockwave.pdf
 }}</ref><ref name="Petit 1989a">
{{cite journal 
 | last1 = Petit | first1 = J.-P.
 | last2 = Lebrun | first2 = B.
 | date = 1989 
 | title = Shock wave annihilation by MHD action in supersonic flow. Quasi one dimensional steady analysis and thermal blockage
 | journal = European Journal of Mechanics B
 | series = B/Fluids
 | volume = 8
 | issue = 2
 | pages = 163–178
 | url = http://www.jp-petit.org/papers/MHD/1989-EurJMech-2.pdf
 }}
</ref><ref name="Petit 1989b">
{{cite journal 
 | last1 = Petit | first1 = J.-P.
 | last2 = Lebrun | first2 = B.
 | date = 1989 
 | title = Shock wave annihilation by MHD action in supersonic flows. Two-dimensional steady non-isentropic analysis. Anti-shock criterion, and shock tube simulations for isentropic flows
 | journal = European Journal of Mechanics B
 | series = B/Fluids
 | volume = 8
 | issue = 4
 | pages = 307–326
 | bibcode = 1989EuJMB...8..307L
 | url = http://www.jp-petit.org/papers/MHD/1989-EurJMech-2.pdf
 }}
</ref>
* Inlet flow control.<ref name="Lineberry 2000" /><ref name="Sheikin 2005">{{cite conference
 |last1=Sheikin |first1=Evgeniy G.
 |last2=Kuranov |first2=Alexander L.
 |date=2005
 |title=Scramjet with MHD Controlled Inlet
 |conference=AIAA/CIRA 13th International Space Planes and Hypersonics Systems and Technologies Conference
 |location=Capua, Italy
 |book-title=AIAA 2005-3223
 |doi=10.2514/6.2005-3223 
 |url=http://ayuba.fr/pdf/ajax/sheikin2005.pdf
 }}</ref><ref name="Petit 2008">{{Cite journal 
 |last1=Petit |first1=J.-P.
 |last2=Geffray |first2=J.
 |title=MHD flow-control for hypersonic flight
 |date=June 2009
 |journal=Acta Physica Polonica A
 |volume=115
 |issue=6
 |pages=1149–1513
 |doi=10.12693/aphyspola.115.1149
 |bibcode=2009AcPPA.115.1149P
 |url=http://przyrbwn.icm.edu.pl/APP/PDF/115/a115z667.pdf
 |doi-access=free
 }}</ref>
* Airflow velocity reduction upstream to feed a scramjet by the use of an MHD generator section combined with an MHD accelerator downstream at the exhaust nozzle, powered by the generator through an MHD bypass system.<ref name="Bityurin 1996">{{cite conference
 |last1=Bityurin |first1=V. A.
 |last2=Zeigarnik |first2=V. A.
 |last3=Kuranov |first3=A. L.
 |date=June 1996
 |title=On a perspective of MHD technology in aerospace applications
 |conference=27th Plasma Dynamics and Lasers Conference
 |location=New Orleans, LA
 |url=https://www.researchgate.net/publication/271369148
 |format=PDF 
 |doi=10.2514/6.1996-2355
}}</ref><ref name="Bityurin 1997">{{cite conference
 |last1=Bityurin |first1=V. A.
 |last2=Lineberry |first2=J.
 |last3=Potebnia |first3=V.
 |last4=Alferov |first4=V.
 |last5=Kuranov |first5=A.
 |last6=Sheikin |first6=E. G.
 |date=June 1997
 |title=Assessment of hypersonic MHD concepts
 |conference=28th Plasmadynamics and Lasers Conference
 |location=Atlanta, GA
 |doi=10.2514/6.1997-2393
 |url=http://ayuba.fr/pdf/ajax/bityurin1997.pdf
 }}</ref><ref name="Fraishtadt 1998">{{cite journal
 |last1=Fraĭshtadt |first1=V. L.
 |last2= Kuranov |first2=A. L.
 |last3=Sheĭkin |first3=E. G.
 |date=November 1998
 |title=Use of MHD systems in hypersonic aircraft
 |journal=Technical Physics
 |volume=43
 |issue=11
 |pages=1309–1313
 |doi=10.1134/1.1259189
 |url=http://ayuba.fr/pdf/ajax/fraishtadt1998.pdf
 |bibcode=1998JTePh..43.1309F|s2cid=122017083
 }}</ref><ref name="Sheikin 2003">{{cite conference
 |last1=Sheikin |first1=E. G.
 |last2=Kuranov |first2=A. L.
 |date=October 2003
 |title=Analysis of Scramjet with MHD bypass
 |conference=3rd workshop on Thermochemical processes in plasma aerodynamics
 |location=Saint Petersburg, Russia
 |s2cid=10143742
 |url=https://pdfs.semanticscholar.org/925b/e7242750df9e92a7d39fc4248826da59ff5f.pdf |archive-url=https://web.archive.org/web/20180412082744/https://pdfs.semanticscholar.org/925b/e7242750df9e92a7d39fc4248826da59ff5f.pdf |url-status=dead |archive-date=2018-04-12 }}</ref>

The Russian project [[Ayaks]] (Ajax) is an example of MHD-controlled hypersonic aircraft concept.<ref name="NAB" /> A US program also exists to design a hypersonic MHD bypass system, the [[Hypersonic Vehicle Electric Power System]] (HVEPS). A working prototype was completed in 2017 under development by [[General Atomics]] and the [[University of Tennessee Space Institute]], sponsored by the US [[Air Force Research Laboratory]].<ref name="General Atomics">{{cite web
 |author=<!--Not stated-->
 |date=21 March 2017
 |title=General Atomics Scores Power Production First
 |website=General Atomics
 |url=http://www.ga.com/general-atomics-scores-power-production-first
 |access-date=2018-04-13
}}</ref><ref name="UTSI">{{cite web
 |last1=Whorton |first1=Mark
 |date=2 July 2017
 |title=Hypersonic Vehicle Electric Power System (HVEPS)
 |website=The University of Tennessee Space Institute
 |url=http://www.utsi.edu/hypersonic-vehicle-electric-power-system/
 |access-date=2018-04-13
}}</ref><ref name="AFRL">{{cite web
 |author=<!--Not stated-->
 |date=7 June 2017
 |title=Scramjet MHD System Generates Electrical Power
 |website=Wright-Patterson Air Force Base
 |url=https://www.wpafb.af.mil/News/Article-Display/Article/401319/scramjet-mhd-system-generates-electrical-power/
 |access-date=2018-04-13
}}</ref> These projects aim to develop MHD generators feeding MHD accelerators for a new generation of high-speed vehicles. Such MHD bypass systems are often designed around a [[scramjet]] engine, but easier to design [[turbojet]]s are also considered,<ref name="Adamovich 2003">{{cite conference
 |last1=Adamovich |first1=Igor V.
 |last2=Rich |first2=J. William
 |last3=Schneider |first3=Steven J.
 |last4=Blankson |first4=Isaiah M.
 |date=June 2003
 |title=Magnetogasdynamic Power Extraction and Flow Conditioning for a Gas Turbine
 |conference=34th AIAA Plasmadynamics and Lasers Conference
 |location=Orlando, Florida
 |book-title=AIAA 2003-4289
 |doi=10.2514/6.2003-4289
 |url=http://ayuba.fr/pdf/ajax/adamovich2003.pdf
 }}</ref><ref name="Blankson 2003">{{cite conference
 |last1=Blankson |first1=Isaiah M.
 |last2=Schneider |first2=Stephen J.
 |date=December 2003
 |title=Hypersonic Engine using MHD Energy Bypass with a Conventional Turbojet
 |conference=12th AIAA International Space Planes and Hypersonic Systems and Technologies
 |location=Norfolk, Virginia
 |book-title=AIAA 2003-6922
 |doi=10.2514/6.2003-6922 
 |url=http://ayuba.fr/pdf/ajax/blankson2003.pdf
 }}</ref><ref name="Schneider 2011">{{cite conference
 |last1=Schneider |first1=Stephen J.
 |title=Annular MHD Physics for Turbojet Energy Bypas
 |conference=17th AIAA International Space Planes and Hypersonic Systems and Technologies Conference
 |location=San Francisco, California
 |book-title=AIAA–2011–2230
 |doi=10.2514/6.2011-2230
 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110016528.pdf
 |hdl=2060/20110016528
 |hdl-access=free
 }}</ref> as well as subsonic [[ramjet]]s.<ref name="Chase 1998">{{cite conference
 |last1=Chase |first1=R. L.
 |last2=Boyd |first2=R.
 |last3=Czysz |first3=P.
 |last4=Froning, Jr. |first4=H. D.
 |last5=Lewis |first5=Mark
 |last6=McKinney |first6=L. E.
 |date=September 1998
 |title=An AJAX technology advanced SSTO design concept
 |conference=AIAA and SAE, 1998 World Aviation Conference 
 |book-title=Anaheim, CA
 |doi=10.2514/6.1998-5527
 |url=http://ayuba.fr/pdf/ajax/chase1998.pdf
 }}</ref>

Such studies covers a field of [[Magnetohydrodynamics#Ideal and resistive MHD|resistive MHD]] with [[magnetic Reynolds number]] ≪ 1 using [[Nonthermal plasma#Aerospace|nonthermal]] [[Degree of ionization#Physics usage|weakly ionized]] gases, making the development of demonstrators much more difficult to realize than for MHD in liquids. "Cold plasmas" with magnetic fields are subject to the [[electrothermal instability]] occurring at a critical Hall parameter, which makes full-scale developments difficult.<ref name="Park 2007">{{cite journal
 |last1=Park |first1=Chul
 |last2=Bogdanoff |first2=David W.
 |last3=Mehta |first3=Unmeel B.
 |date=July 2003
 |title=Theoretical Performance of a Magnetohydrodynamic-Bypass Scramjet Engine with Nonequilibrium Ionization
 |journal=Journal of Propulsion and Power
 |volume=19
 |issue=4
 |pages= 529–537
 |doi=10.2514/2.6156
 |url=http://ayuba.fr/pdf/ajax/park2003.pdf
 }}</ref>

==== Prospects ====
MHD propulsion has been considered as the main propulsion system for both marine and space ships since there is no need to produce lift to counter the [[gravity of Earth]] in water (due to [[buoyancy]]) nor in space (due to [[weightlessness]]), which is ruled out in the case of [[flight]] in the [[atmosphere]].

Nonetheless, considering the current problem of the [[Electric power|electric power source]] solved (for example with the availability of a still missing multi-megawatt compact [[Fusion power|fusion reactor]]), one could imagine future aircraft of a new kind silently powered by MHD accelerators, able to ionize and direct enough air downward to lift several [[tonne]]s. As external flow systems can control the flow over the whole wetted area, limiting thermal issues at high speeds, ambient air would be ionized and radially accelerated by Lorentz forces around an [[Rotational symmetry#Rotational symmetry with respect to any angle|axisymmetric]] body (shaped as a [[cylinder]], a [[cone]], a [[sphere]]...), the entire [[airframe]] being the engine. Lift and thrust would arise as a consequence of a [[pressure]] difference between the upper and lower surfaces, induced by the [[Coandă effect]].<ref name='"Coanda patent">{{cite patent
 |country=US
 |number=2108652
 |status=patent
 |title=Propelling device
 |pubdate=1936-01-15
 |gdate=1938-02-16
 |fdate=1936-01-10
 |url=https://patentimages.storage.googleapis.com/d9/67/6d/6cbdb5f33cc76e/US2108652.pdf
}}</ref><ref name="Coanda saucers">{{cite journal
 |last1=Petit |first1=J.-P.
 |date=August 1974
 |title=Flying saucers R&D: The Coanda effect (English version)
 |journal=Science & Vie
 |issue=683
 |pages=68–73
 |url=http://ayuba.fr/pdf/coanda_disc.pdf
 }}</ref> In order to maximize such pressure difference between the two opposite sides, and since the most efficient MHD converters (with a high [[Hall effect]]) are disk-shaped, such MHD aircraft would be preferably flattened to take the shape of a [[Lens (optics)#Types of simple lenses|biconvex lens]]. Having no [[wing]]s nor [[airbreathing jet engine]]s, it would share no similarities with conventional aircraft, but it would behave like a [[helicopter]] whose [[Helicopter rotor|rotor blades]] would have been replaced by a "purely electromagnetic rotor" with no moving part, sucking the air downward. Such concepts of flying MHD disks have been developed in the [[peer review]] literature from the mid 1970s mainly by physicists [[Leik Myrabo]] with the [[Lightcraft]],<ref name="Myrabo 1976">
{{cite journal
 | author = Myrabo, L.N.
 | author-link = Leik Myrabo
 | date = 1976
 | title = MHD propulsion by absorption of laser radiation
 | journal = Journal of Spacecraft and Rockets
 | volume = 13
 | issue = 8
 | url = http://ayuba.fr/pdf/myrabo1976.pdf
 | doi = 10.2514/3.27919
 | pages=466–472
 | bibcode = 1976JSpRo..13..466M
 }}</ref><ref name="Myrabo 1999">{{cite conference
 | last1 = Myrabo | first1 =  L. N.
 | last2 = Kerl | first2 = J.M.
 | display-authors=etal
 | date = June 1999
 | title = MHD slipstream accelerator investigation in the RPI hypersonic shock tunnel
 | conference = 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit
 | book-title = AIAA-1999-2842
 | location = Los Angeles, CA
 | url = http://ayuba.fr/pdf/myrabo1999.pdf
 | doi = 10.2514/6.1999-2842 
}}</ref><ref name="Myrabo 2000a">{{cite conference
 | last1 = Myrabo | first1 =  L. N.
 | display-authors=etal
 | date = January 2000
 | title = Experimental investigation of a 2-D MHD slipstream generator and accelerator with freestream Mach = 7.6 and T(0) = 4100 K
 | conference = 38th Aerospace Sciences Meeting and Exhibit 
 | book-title =  AIAA-00-0446
 | location = Reno, NV
 | url = http://ayuba.fr/pdf/myrabo2000a.pdf
 | doi = 10.2514/6.2000-446 
}}</ref><ref name="Myrabo 2000b">{{cite conference
 | last1 = Myrabo | first1 =  L. N.
 | display-authors=etal
 | date = July 2000
 | title = Experimental Investigation of a 2-D MHD Slipstream Accelerator and Generator
 | conference = 36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit
 | book-title = AIAA-00-3486
 | location = Huntsville, AL
 | url = http://ayuba.fr/pdf/myrabo2000b.pdf
 | doi = 10.2514/6.2000-3486
}}</ref><ref name="Myrabo book">{{cite book
 |last1=Myrabo |first1=Leik N.
 |last2=Lewis |first2=John S.
 |title=Lightcraft Flight Handbook LTI-20: Hypersonic Flight Transport for an Era Beyond Oil
 |date=May 2009
 |publisher=Collector's Guide Publishing
 |isbn=978-1926592039
}}</ref> and [[Subrata Roy (scientist)|Subrata Roy]] with the [[Wingless Electromagnetic Air Vehicle]] (WEAV).<ref name="Roy 2011">{{cite report
 |last1=Roy |first1=Subrata
 |last2=Arnold |first2=David
 |last3=Lin |first3=Jenshan
 |last4=Schmidt |first4=Tony
 |last5=Lind |first5=Rick
 |last6=Durscher |first6=Ryan
 |last7=Riherd |first7=Mark
 |last8=Houba |first8=Tomas
 |last9=Anderson |first9=Richard
 |last10=Zito |first10=Justin
 |last11=Casanova |first11=Joaquin
 |last12=Thomson |first12=Carlton
 |last13=Blood |first13=Daniel
 |last14=Tran |first14=Dong
 |display-authors=5
 |date=20 December 2011
 |title=Demonstration of a Wingless Electromagnetic Air Vehicle
 |id=AFRL-OSR-VA-TR-2012-0922
 |publisher=Defense Technical Information Center
 |editor1=Air Force Office of Scientific Research
 |editor2=University of Florida
 |asin=B01IKW9SES
 |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a564120.pdf
 |archive-url=https://web.archive.org/web/20130517063830/http://www.dtic.mil/dtic/tr/fulltext/u2/a564120.pdf
 |url-status=live
 |archive-date=May 17, 2013
 }}</ref><ref name="Roy patent 2013">{{cite patent
| country = US
| number = 8382029
| status = patent
| title = Wingless hovering of micro air vehicle
| gdate = 2013-02-26
| fdate = 2008-12-23
| pridate = 2006-07-31
| invent1 = Subrata Roy 
| assign1 = University of Florida Research Foundation Inc
| url = https://patentimages.storage.googleapis.com/0b/a8/52/3c6718c040ad54/US8382029.pdf
}}</ref><ref name="Roy patent 2015">{{cite patent
| country = US
| number = 8960595
| status = patent
| title = Wingless hovering of micro air vehicle
| gdate = 2015-02-24
| fdate = 2012-12-19
| pridate = 2006-07-31
| invent1 = Subrata Roy 
| assign1 = University of Florida Research Foundation Inc.
| url = https://patentimages.storage.googleapis.com/25/43/bb/2bdc198ea976a9/US8960595.pdf
}}</ref>

These futuristic visions have been advertised in the media although they still remain beyond the reach of modern technology.<ref name="Science & Vie 1976">{{cite magazine
 |last1=Petit |first1=Jean-Pierre
 |title=Un moteur à plasma pour ovnis
 |trans-title=A plasma engine for UFOs
 |language=fr
 |date=March 1976
 |magazine=Science & Vie
 |issue=702
 |pages=42–49
 |url=http://ayuba.fr/pdf/S&V702-moteur_plasma_ovnis.pdf
 }}</ref><ref name="Popular Mechanics 1995" /><ref name="Scientific American">{{cite web
 |last1=Greenemeier |first1=Larry
 |date=7 July 2008
 |title=The World's First Flying Saucer: Made Right Here on Earth
 |website=Scientific American
 |url=https://www.scientificamerican.com/article/worlds-first-flying-saucer/
}}</ref>