Lecture delivered at Supaéro
June 10, 2003
MHD and hypersonic flight
J.P. Petit
page 1
Recap of the history of MHD in France.
The inventor of MHD (magnetohydrodynamics) was the Englishman Michael Faraday. This discipline has two aspects:
- MHD accelerators, or the art and technique of moving fluids using Laplace forces ("Lorentz forces" in English), J × B
- MHD generators, or the art and technique of converting the kinetic energy of a moving fluid into electricity.
Faraday experimented with both principles. In both cases, he implemented a linear converter, which bears his name. Schematically, a linear converter is a nozzle equipped with segmented electrodes (to achieve better current distribution in the fluid stream), flanked by coils generating a transverse magnetic field. The axis of the device, the direction of the magnetic field, and the direction of the electric field created by the electrodes form a right-handed orthogonal triad.
Faraday Converter
In the early 1960s, the British were the first to consider electricity generation via MHD, without moving parts, through "direct conversion." On paper, it seemed very simple. A fluid enters at high speed into a nozzle and cuts across the lines of force of a magnetic field B. This results in an electromotive field V × B, which generates a current J (current density, in amperes per square meter), collected by the electrodes and flowing through load resistances. These MHD generators had several advantages. They could be brought online very quickly. Moreover, they circumvented the Carnot efficiency limit, which at the time restricted gas turbine efficiency to around 40%. Theoretical calculations indicated that, on paper, overall efficiencies approaching 60% could be achieved. If these machines had functioned, this would have meant that, using the same amount of fossil fuel, we could have produced 50% more electrical energy.
However, gases are poor conductors of electricity. Consider a gaseous mixture resulting from hydrocarbon combustion. Its components have ionization potentials. But even at the highest temperatures permitted by current technology, the electrical conductivity of the medium remains low. Only a small fraction of the gas’s enthalpy would be converted into electricity, with the majority dissipated in the flow via Joule heating.
Therefore, researchers considered increasing the gas’s conductivity by adding a substance with a low ionization potential, primarily an alkali metal. This problem of enhancing conductivity was so critical that the most easily ionizable substance was immediately considered: cesium. The first MHD conversion experiments were thus conducted by adding a Faraday linear generator downstream of a combustion chamber burning hydrocarbons. The results were disappointing. Temperatures approaching 3000°C were required—similar to those of an incandescent light bulb filament. Efforts focused on the thermal endurance of materials: walls and electrodes. In the early 1960s, it was not uncommon during experiments for electrodes to shatter, as did the plates designed to maintain the thermal integrity of the walls. These studies on what were called "open cycles" continued in numerous laboratories worldwide throughout the 1960s. In France, participation included EDF at its research center in Renardières, near Moret-sur-Loing, the French Institute of Petroleum, and CGE (General Electric Company). The international civil MHD effort mobilized up to 5,000 researchers across dozens of laboratories scattered worldwide. The lack of success eventually led to the gradual termination of research. The Russians were the last to persist, until the mid-1970s, with an experimental generator named "U-25," located near Moscow.
Russian MHD Generator U-25. In the foreground, the electromagnet.
Impressive dimensions of the U-25 generator’s flow channel. Electrodes are on the right and left.
Another approach was quickly considered, based on what was called "nonequilibrium (thermodynamic) electrical conduction." This situation, where electron temperature Te exceeds gas temperature Tg, will be detailed later. These conditions prevail inside a neon tube. The basic idea is as follows: in a neon tube, an electric field E, created by electrodes, accelerates free electrons along their mean free path (between collisions with neutral atoms or ions). If the mean free path is sufficiently long, the kinetic energy gained by electrons can reach the ionization energy Ei of an atom. During a collision, an "electron avalanche" occurs. The resulting electric current creates an ionized state within the tube. Conversely, ions attract relatively slow free electrons and tend to capture them (radiative recombination).
I have already posted two MHD-related dossiers on my website, presented at a popular science level. There will be links to these in the following sections.
In the mid-1960s (specifically at the 1964 Newcastle conference in England), a young Russian researcher, Velikhov, predicted the emergence of an extremely violent ionization instability (occurring in a few microseconds). The theory behind this phenomenon is far from obvious. Its mechanism defies intuition. Here is an image from the 1960s showing (at the time, numerical simulations required the most powerful systems, and these images come from the USSR). It illustrates how this instability develops, locally compressing the electric current lines. This local increase in J triggers a response from the gas in the form of ionization. The medium thus stratifies, forming alternating layers of high electrical conductivity and low electrical conductivity.
Evolution of the electrothermal instability in a Faraday converter (1968)
It was precisely this instability, which no one could remedy, that caused the complete collapse of the entire global civil MHD effort (dozens of labs, 5,000 researchers). By the end of the 1960s, the situation was settled in Europe. All teams were dismantled, despite a single success at the Marseille Institute of Fluid Mechanics between 1966 and 1970. Two significant results emerged.
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First stable operation of a nonequilibrium generator, immune to ionization instability (J.P. Petit, 1967, 7th International Congress in Warsaw). Gas temperature: 6,000°C, electron temperature: 10,000°C, power extraction: 2 megawatts. Notable current flow up to 4,000 A.
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Acceleration of an argon plasma. Input parameters: pressure, 1 bar; velocity, 2,700 m/s; temperature, 10,000°C; electrical conductivity: 3,000 mhos/m; magnetic field: 2 teslas. Nozzle length (Faraday-type accelerator): 10 cm. Exit velocity: 8,000 m/s. MHD lab at the Institute of Fluid Mechanics, Marseille. Thesis by Bernard Forestier.
These two results passed completely unnoticed at the time. As mentioned in my book, the Americans discovered in the mid-1970s the fantastic military potential of MHD. They then launched highly advanced research in various secret facilities, including the infamous Area 51. In 1980, an American hypersonic MHD torpedo reached 2,000 km/h. The Russians possess a similar device, one meter in diameter (code name "The Big One"). I will later publish a dossier explaining the direct link between this device—intended for sale to China—and the sinking of the Kursk.
The Americans quickly advanced research focused on hypersonic flight. The Russians had a similar project: "Ajax," which failed due to lack of funding. Research was pursued intensively in the strictest secrecy. However, in 1986, I learned from Professor Kunkle (director of the Plasma Physics Laboratory at Berkeley) that part of this program was managed at the Lawrence Livermore Laboratory (California) and at the Sandia Laboratories near Albuquerque. These efforts rapidly yielded major breakthroughs. The Americans devoted resources and funding comparable to those of projects like Manhattan and Apollo. The difference was that this research was conducted in absolute secrecy. Not only was the existence of these projects hidden from the rest of the world, but misinformation was systematically spread. Masterfully, the Americans deliberately allowed their civil MHD program to wither. No more contracts, no more PhDs, no more... courses. A fundamental discipline, destined to play a major role, sank into... oblivion (and nobody cared, at least not in France). All Westerners fell for the ruse, thinking: "If the Americans abandoned this research, it must have no value." Only in Europe did I continue fighting for ten years to push for research—until I was finally defeated in 1987.
I revealed in my book, published in January 2003, the existence of three vehicles:
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A hypersonic MHD torpedo, operational in the USA and Russia since 1980.
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A U.S. spy aircraft cruising at 80 km altitude at 10,000 km/h, capable of orbit (a necessary successor to the SR-71, Mach 3, now museum-bound). Operational since 1990.
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A U.S. "antipodal" bomber, operational since 1987, which is in fact the "true B-2"—the aircraft displayed publicly are merely decoys. See the dossier.
I know these revelations are still met with skepticism in French aeronautical circles. My book sparked the creation of a "French MHD program." At first glance, the measures taken seem ridiculous compared to what should have been done. Isn’t it equally absurd that CNRS goes to Russia to seek expertise (presented as "pioneers in wave drag reduction") while deliberately suppressing all such efforts within its own institutions for fifteen years? To those who ask what should be done to catch up with the Americans, I reply:
- 1,000 researchers and technicians at the highest level, for 30 years.
The European situation regarding MHD would be comparable to that of countries that never developed a space industry, only to discover that other nations have established permanent space stations and landed on the Moon—then turning to Ruggieri’s company! Or imagine France has no airplanes, and someone says: "When I look at photos of these 'airplanes,' the front part looks quite like the front of the TGV. I have a friend at SNCF working on aerodynamics for high-speed trains. I suggest we put him in charge of an 'aviation program.'"
Don’t laugh. That’s exactly what’s happening. I believe it will still take years before my words are taken seriously (it doesn’t matter—it’s already too late).
I conjecture that no genuinely high-level MHD education will ever emerge at Supaéro, or in other grandes écoles, or in university DEA programs.
A few days ago, a Swiss researcher invited me to give a lecture intended to "raise awareness" among the staff at the École Polytechnique Fédérale de Lausanne. The creation of a small MHD lab with a few people seemed possible—a beginning. I smiled. It felt like hearing:
"The people at Ruggieri told me they don’t rule out someday using their firework rockets to carry instruments to high altitude. Some are even ready to design large-diameter rockets to explore this problem."
I am 66 years old and retired. For 25 years, I repeatedly said that MHD was something important. No one believed me, and worse, many rightly believed that allowing a "shock wave suppression experiment" to succeed would lend credibility to the UFO dossier. The articles in Air et Cosmos published early in 2003, and the complete lack of reception I received at Supaéro from both the administration and teaching staff, do not encourage me to re-launch such a crusade. Worse still, it is almost unbearable to know that the current French MHD program is being driven by the same fools responsible for the 25-year delay and major blunders (the National School of Aeronautics was directly involved in one of them, as were CERT and DERMO of Toulouse in the early 1980s). What can we expect from the current CNRS director or our Minister of Research and Technology, Claudie Haigneré? The enthusiasm of young students will not change this absurd stagnation. A "small beginning" would serve no purpose, not even at the European level. My stance can be summed up in one word: impavidity. I wrote a book, "Les Enfants du Diable," published by Albin Michel, beginning with a reminder of the myth of Cassandra. Twenty years later, nothing has changed. Why rebel? It’s useless. In France, inertia is compounded by institutional loyalty (primarily from the École Polytechnique, a true plague in this country), vanity, short-sightedness, and a certain fashionable skepticism.
When you know this, there is probably nothing left to do but sit back and laugh at the inadequacy of the measures taken, the incompetence of those "put in charge," and the certain failure of these endeavors, regardless of funding, due not only to a lack of essential knowledge but also to ignorance of some closely guarded secrets that alone make hypersonic flight possible.
I was invited by Supaéro students (not their teachers) to give a lecture at the school on June 10, 2003. I did so. Here, I leave a written record of this talk, in a tone now addressed more to the grande école student than to a general audience.
Before beginning, I wish to emphasize the crucial importance of MHD, which will inevitably permeate many areas of scientific and technical activity worldwide. Everything was tied to progress in superconductors. Indeed, conventional tesla-level fields are extremely heavy. Anecdotally, the magnetization setup we used in 1976, Maurice Viton and I, to eliminate the bow wave in front of a cylindrical model, generated a constant 1-tesla field in a volume of about ten cubic centimeters and weighed 200 kilograms (the same setup that sent me to the hospital for six months due to a spinal injury). The solenoids generating 2 teslas in a volume of a few liters, which equipped my shock tube in 1965, carried a current of 54,000 amperes for a few milliseconds.
In the early 1970s, a German civilian installation had superconducting magnets cooled by liquid helium, producing 4 teslas. In that case, it wasn’t the solenoids that were heavy, but the cryogenic system. Later, two researchers received the Nobel Prize for developing superconductors operating at higher temperatures, in liquid nitrogen, at around -190°C instead of -270°C. At the time, various opinions were expressed. Aigrin, former Minister of Research, stated that we would eventually have superconductors operating at room temperature. He concluded this with a quip:
"One day, when someone goes to buy electrical wire at a hardware store, the shopkeeper will reply: 'Do you want it normal, or superconducting?'"
I immediately shared Aigrin’s view. Our era, despite decades of stagnation in fundamental knowledge, constantly produces new technological advances. Why not this one? It is absurd to declare there is a maximum temperature beyond which superconductivity cannot occur. Especially since this phenomenon is still poorly understood theoretically. That said, if high-temperature superconductors appear, they would immediately be classified as defense secrets, despite the immense profit industrialists and even ordinary citizens could gain. Just imagine the staggering energy savings. Defense secrecy due to MHD and its multiple applications—not only hypersonic flight, but also space weapons (like Star Wars), propulsion systems for space cruises with very high specific impulse, plasma compressors, underwater propulsion systems, etc.
Technology progresses relentlessly. For example, the ticonal magnets we used in the 1960s in our experiments produced 900 gauss. Today, permanent magnets producing 1 tesla—ten times stronger—are available at affordable prices. By spending a bit more, one can obtain permanent magnets producing 2 teslas! How many teslas are there in "defense secrets"? No idea. But even a factor of twenty would be significant. If 2-tesla magnets are already in commerce, there must be more in "sanctuaries" (though this doesn’t automatically mean that advanced magnetization systems used on hypersonic vehicles are permanent magnets!).
The Americans have always dominated the field of superconductivity. Conversely, in the early 1970s, the Russian U-25 MHD generator near Moscow operated with conventional magnetization, using ordinary copper solenoids. The Russians have always lagged behind the Americans in superconductivity. I believe a significant qualitative and quantitative leap occurred across the Atlantic, but this decisive progress remains under the thickest technological secrecy. I think the Americans possess high-temperature superconductors that don’t require cryogenic systems. But they certainly won’t shout it from the rooftops. How many teslas? Impossible to say, but ten to twelve would suffice to operate hypersonic vehicles (and viable underwater propulsion systems). This technological reality will eventually emerge, even if the secrecy is tightly maintained. Then, the global technological-industrial landscape will be completely transformed. If line losses vanish entirely, electrical energy could be transported over unlimited distances. Consider for a moment the phenomenon of superconductivity. If this had not been discovered at the beginning of the century, and someone claimed we could one day pass electric current through a conductor without any Joule heating...
No one would believe it; they’d think the person insane.
Yet it has been possible for so long that we no longer marvel at it—though it remains either a miracle or a physical absurdity, depending on one’s perspective. So why not high-temperature superconductivity?
I am addressing primarily students of the National School of Aeronautics and Space. To you, I say: MHD is not a "subset" of classical fluid mechanics. It is not a matter of "adding a term" to the Navier-Stokes equations.
Classical fluid mechanics is itself a subset of MHD.
Until the advent of MHD, we employed what could be called "passive fluid mechanics," where the flow itself dictated the rules. Flying machines merely adopted the best shapes to slip through the gas. We hunted down anything that could create turbulence. We accepted shock waves as an unavoidable evil. We "optimized." With MHD, you enter the world of "active fluid mechanics," where we forcibly impose the path the gas must follow. I believe the first step is to grasp the immense strength of the Laplace force. Imagine passing a current of one ampere per square centimeter through air, in a direction orthogonal to a magnetic field of ten teslas. One ampere per square centimeter equals ten thousand amperes per square meter. Multiplying J by B gives a volumetric force:
J × B = 10,000 × 10 = 100,000 newtons per cubic meter, or... ten tons per cubic meter of air.
In the following sections of this talk, you will discover machines that seem utterly absurd from the standpoint of classical fluid mechanics—machines that force the flow to make a 150° turn around a sharp corner. But do you really think that with a force of ten tons per cubic meter, this is unrealistic? You’re surprised because you hadn’t previously imagined that fluids could be forced—told "go here, go there," without asking for their opinion.
Once you begin to accept this idea, you realize that flying machines will undergo a complete transformation. First comes the era of hypersonic flight (operational spy planes in the USA since 1990, bombers since 1997). Do today’s subsonic aircraft consume more energy than those of yesterday? Our grandparents flew from Paris to London in airplanes at 150 km/h, counting blades of grass one by one. These aircraft had a "ceiling." Today’s airliners fly at ten thousand meters in much less dense air, but at much higher speeds. If those stupid shock waves didn’t exist, the flight would have been even longer. We would have flown even higher and faster. With Concorde, we tried to bypass this, but ended up with an astonishing fuel consumption per passenger. A reader might provide the fuel cost per passenger per kilometer for a subsonic airliner and an Airbus.
Thus, hypersonic projects at Mach 3 remained on paper. Flying at long duration at Mach 3, the SR-71 was barely a flying fuel tank. Shock waves create two barriers: one synonymous with wave drag and fuel consumption, and what is known as the "heat barrier." Was this barrier surmountable? The answer lies within MHD.
Thus, our flying machines will change profoundly, and Supaéro’s teachings must follow (they could have... anticipated these changes, but let’s not dream). The school has existed for decades. In its early days, it spent long hours studying propellers. Today, do students choosing the "propulsion" option still care about propellers? They think in terms of fans and compressors. Only tourist aircraft still use propellers. These are the only ones occupying the lower atmospheric layers. Everything else—turbojets, with or without dual flow—needs to fly higher. This also has the advantage of no longer suffering from the meteorological constraints of the past, when Mermoz, flying over the South Atlantic, had to face "the black pot." A sign of the times: I’m sure most Supaéro students have never even heard that word.
Prepare yourselves for the idea that one day, even the smallest flying machine performing a transatlantic flight will climb not to ten thousand meters but ten times higher, and travel ten to fifteen times faster than our current airliners. The most astonishing part is that it will use almost conventional turbojets, as I will explain later. It’s no more than comparing an Airbus to Lindbergh’s plane. The differences are comparable.
Now to space. Do you know a more expensive way to place a payload into orbit than a rocket? It’s so costly that multi-stage vehicles had to be invented, with the first stages jettisoned and lost en route. How many tens of tons of sophisticated alloys, expensive electronics, solid or cryogenic fuels, just to put one kilogram into space? Mind-boggling. If compared to aviation, it’s like building a 747-sized, multi-element vehicle to carry three passengers to New York, jettisoning 90% of its mass into the ocean before releasing a toy-sized aircraft that lands at Kennedy Airport. That’s essentially what we’d have to do if we decided to eliminate wings and rely entirely on lift engines for the entire cruise. A fun idea for a "design office" for Supaéro students:
- Today, design me a transatlantic vessel using current propulsion technology, but where the use of wings is strictly forbidden.
Conversely, how pleasant it would be to be able to lie down calmly on the air, like a swimmer on the water in a pool, and start moving by pushing off a lifting surface. Just accelerate, accelerate, while ascending. Yes, specialists will reply, but what do you do at Mach 3? What kind of engine do you use? And how do you handle wave drag and heat influx?
The answer exists. It is already in use by the Americans, who operate hypersonic bombers at Mach 12, 80 km above the Earth, capable of circumnavigating the globe. But the future "launcher" is an extension of the Aurora vehicle, which can be made