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The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy while the fusion of nuclei heavier than iron absorbs energy. The opposite is true for the reverse process, nuclear fission.
In the simplest case of hydrogen fusion, two protons have to be brought close enough for the weak nuclear force to convert either of the identical protons into a neutron forming the hydrogen isotope deuterium. In more complex cases of heavy ion fusion involving two or more nucleons, the reaction mechanism is different, but the same result occurs–one of combining smaller nuclei into larger nuclei.
Nuclear fusion occurs naturally in all active stars (see astrophysics). Synthetic fusion as a result of human actions has also been achieved, although this has not yet been completely controlled as a source of nuclear power. In the laboratory, successful nuclear physics experiments have been carried out that involve the fusion of many different varieties of nuclei, but the energy output has been negligible in these studies. In fact, the amount of energy put into the process has always exceeded the energy output.
Uncontrolled nuclear fusion has been carried out many times in nuclear weapons testing, which results in a deliberate explosion. These explosions have always used the heavy isotopes of hydrogen, deuterium (H-2) and tritium (H-3), and never the much more common isotope of hydrogen (H-1), sometimes called "protium".
Building upon the nuclear transmutation experiments by Ernest Rutherford, carried out several years earlier, the fusion of the light nuclei (hydrogen isotopes) was first accomplished by Mark Oliphant in 1932. Then, the steps of the main cycle of nuclear fusion in stars were first worked out by Hans Bethe throughout the remainder of that decade.
Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project, but this was not accomplished until 1951 (see the Greenhouse Item nuclear test), and nuclear fusion on a large scale in an explosion was first carried out on November 1, 1952, in the Ivy Mike hydrogen bomb test. Research into developing controlled thermonuclear fusion for civil purposes also began in the 1950s, and it continues to this day.
Fusion reactions power the stars and produce virtually all elements in a process called nucleosynthesis.
Generally, when dealing with elements lighter than iron, the ratio of atomic mass to mass number is lower the heavier the nucleus is. This is known as mass defect. So fusion of lighter nuclei into heavier nuclei leads to loss of mass, even though no nucleons are lost. This lost mass is released as energy in accordance with E=mc2 (actually the mass is not changed into energy, rather the released energy carries the lost mass along with it).
Although the fusion of lighter elements in stars releases energy, production of elements heavier than iron absorbs energy.
When the fusion reaction is a sustained uncontrolled chain, it can result in a thermonuclear explosion, such as that generated by a hydrogen bomb. Non-self sustaining reactions can still release considerable energy, as well as large numbers of neutrons.
Research into controlled fusion, with the aim of producing fusion power for the production of electricity, has been conducted for over 50 years. It has been accompanied by extreme scientific and technological difficulties, but has resulted in progress. At present, break-even (self-sustaining) controlled fusion reactions have not been demonstrated in the few tokamak-type reactors around the world. Workable designs for a reactor that theoretically will deliver ten times more fusion energy than the amount needed to heat up plasma to required temperatures (see ITER) were originally scheduled to be operational in 2018, however this has been delayed and a new date has not been stated.
It takes considerable energy to force nuclei to fuse, even those of the lightest element, hydrogen. This is because all nuclei have a positive charge (due to their protons), and as like charges repel, nuclei strongly resist being put too close together. Accelerated to high speeds (that is, heated to thermonuclear temperatures), they can overcome this electrostatic repulsion and get close enough for the attractive nuclear force to be sufficiently strong to achieve fusion. The fusion of lighter nuclei, which creates a heavier nucleus and often a free neutron or proton, generally releases more energy than it takes to force the nuclei together; this is an exothermic process that can produce self-sustaining reactions. The National Ignition Facility, which uses laser-driven inertial confinement fusion, is thought to be capable of break-even fusion.
The first large-scale laser target experiments were performed in June 2009 and ignition experiments are beginning early in 2011.
Energy released in most nuclear reactions is much larger than in chemical reactions, because the binding energy that holds a nucleus together is far greater than the energy that holds electrons to a nucleus. For example, the ionization energy gained by adding an electron to a hydrogen nucleus is 13.6 eV—less than one-millionth of the 17 MeV released in the deuterium–tritium (D–T) reaction shown in the diagram to the right. Fusion reactions have an energy density many times greater than nuclear fission; the reactions produce far greater energies per unit of mass even though individual fission reactions are generally much more energetic than individual fusion ones, which are themselves millions of times more energetic than chemical reactions. Only direct conversion of mass into energy, such as that caused by the collision of matter and antimatter, is more energetic per unit of mass than nuclear fusion.
A substantial energy barrier of electrostatic forces must be overcome before fusion can occur. At large distances two naked nuclei repel one another because of the repulsive electrostatic force between their positively charged protons. If two nuclei can be brought close enough together, however, the electrostatic repulsion can be overcome by the attractive nuclear force, which is stronger at close distances.
When a nucleon such as a proton or neutron is added to a nucleus, the nuclear force attracts it to other nucleons, but primarily to its immediate neighbours due to the short range of the force. The nucleons in the interior of a nucleus have more neighboring nucleons than those on the surface. Since smaller nuclei have a larger surface area-to-volume ratio, the binding energy per nucleon due to the nuclear force generally increases with the size of the nucleus but approaches a limiting value corresponding to that of a nucleus with a diameter of about four nucleons. It is important to keep in mind that the above picture is a toy model because nucleons are quantum objects, and so, for example, since two neutrons in a nucleus are identical to each other, distinguishing one from the other, such as which one is in the interior and which is on the surface, is in fact meaningless, and the inclusion of quantum mechanics is necessary for proper calculations.
The electrostatic force, on the other hand, is an inverse-square force, so a proton added to a nucleus will feel an electrostatic repulsion from all the other protons in the nucleus. The electrostatic energy per nucleon due to the electrostatic force thus increases without limit as nuclei get larger.
The net result of these opposing forces is that the binding energy per nucleon generally increases with increasing size, up to the elements iron and nickel, and then decreases for heavier nuclei. Eventually, the binding energy becomes negative and very heavy nuclei (all with more than 208 nucleons, corresponding to a diameter of about 6 nucleons) are not stable. The four most tightly bound nuclei, in decreasing order of binding energy, are 62
Fe, and 60
Ni. Even though the nickel isotope, 62
Ni, is more stable, the iron isotope 56
Fe is an order of magnitude more common. This is due to a greater disintegration rate for 62
Ni in the interior of stars driven by photon absorption.
A notable exception to this general trend is the helium-4 nucleus, whose binding energy is higher than that of lithium, the next heaviest element. The Pauli exclusion principle provides an explanation for this exceptional behavior—it says that because protons and neutrons are fermions, they cannot exist in exactly the same state. Each proton or neutron energy state in a nucleus can accommodate both a spin up particle and a spin down particle. Helium-4 has an anomalously large binding energy because its nucleus consists of two protons and two neutrons; so all four of its nucleons can be in the ground state. Any additional nucleons would have to go into higher energy states.
The situation is similar if two nuclei are brought together. As they approach each other, all the protons in one nucleus repel all the protons in the other. Not until the two nuclei actually come in contact can the strong nuclear force take over. Consequently, even when the final energy state is lower, there is a large energy barrier that must first be overcome. It is called the Coulomb barrier.
The Coulomb barrier is smallest for isotopes of hydrogen—they contain only a single positive charge in the nucleus. A bi-proton is not stable, so neutrons must also be involved, ideally in such a way that a helium nucleus, with its extremely tight binding, is one of the products.
Using deuterium-tritium fuel, the resulting energy barrier is about 0.01 MeV. In comparison, the energy needed to remove an electron from hydrogen is 13.6 eV, about 750 times less energy. The (intermediate) result of the fusion is an unstable 5He nucleus, which immediately ejects a neutron with 14.1 MeV. The recoil energy of the remaining 4He nucleus is 3.5 MeV, so the total energy liberated is 17.6 MeV. This is many times more than what was needed to overcome the energy barrier.
If the energy to initiate the reaction comes from accelerating one of the nuclei, the process is called beam-target fusion; if both nuclei are accelerated, it is beam-beam fusion. If the nuclei are part of a plasma near thermal equilibrium, the process is called thermonuclear fusion. Temperature is a measure of the average kinetic energy of particles, so by heating the nuclei they will gain energy and eventually have enough to overcome this 0.01 MeV. Converting the units between electronvolts and kelvin shows that the barrier would be overcome at a temperature in excess of 120 million kelvins.
There are two effects that lower the actual temperature needed. One is the fact that temperature is the average kinetic energy, implying that some nuclei at this temperature would actually have much higher energy than 0.01 MeV, while others would be much lower. It is the nuclei in the high-energy tail of the velocity distribution that account for most of the fusion reactions. The other effect is quantum tunneling. The nuclei do not actually have to have enough energy to overcome the Coulomb barrier completely. If they have nearly enough energy, they can tunnel through the remaining barrier. For this reason fuel at lower temperatures will still undergo fusion events, at a lower rate.
The reaction cross section σ is a measure of the probability of a fusion reaction as a function of the relative velocity of the two reactant nuclei. If the reactants have a distribution of velocities, e.g. a thermal distribution with thermonuclear fusion, then it is useful to perform an average over the distributions of the product of cross section and velocity. The reaction rate (fusions per volume per time) is <σv> times the product of the reactant number densities:
f = n_1 n_2 \langle \sigma v \rangle.
If a species of nuclei is reacting with itself, such as the DD reaction, then the product n1n2 must be replaced by (1 / 2)n2.
\langle \sigma v \rangle increases from virtually zero at room temperatures up to meaningful magnitudes at temperatures of 10–100 keV. At these temperatures, well above typical ionization energies (13.6 eV in the hydrogen case), the fusion reactants exist in a plasma state.
The significance of \langle \sigma v \rangle as a function of temperature in a device with a particular energy confinement time is found by considering the Lawson criterion.
One force capable of confining the fuel well enough to satisfy the Lawson criterion is gravity. The mass needed, however, is so great that gravitational confinement is only found in stars–the least massive stars capable of sustained fusion are red dwarfs, while brown dwarfs are able to fuse deuterium and lithium if they are of sufficient mass. In stars heavy enough, after the supply of hydrogen is exhausted in their cores, their cores (or a shell around the core) start fusing helium to carbon. In the most massive stars (at least 8-11 solar masses), the process is continued until some of their energy is produced by fusing lighter elements to iron. As iron has one of the highest binding energies, reactions producing heavier elements are generally endothermic. Therefore significant amounts of heavier elements are not formed during stable periods of massive star evolution, but are formed in supernova explosions. Some lighter stars also form these elements in the outer parts of the stars over long periods of time, by absorbing energy from fusion in the inside of the star, by absorbing neutrons that are emitted from the fusion process.
All of the elements heavier than iron have some potential energy to release, in theory. At the entremely heavy end of element production, these heavier elements can produce energy in the process of being split again back toward the size of iron, in the process of nuclear fission. Nuclear fission thus releases energy which has been stored, sometimes billions of years before, during stellar nucleosynthesis.
Electrically charged particles (such as fuel ions) will follow magnetic field lines (see Guiding center). The fusion fuel can therefore be trapped using a strong magnetic field. A variety of magnetic configurations exist, including the toroidal geometries of tokamaks and stellarators and open-ended mirror confinement systems.
A third confinement principle is to apply a rapid pulse of energy to a large part of the surface of a pellet of fusion fuel, causing it to simultaneously "implode" and heat to very high pressure and temperature. If the fuel is dense enough and hot enough, the fusion reaction rate will be high enough to burn a significant fraction of the fuel before it has dissipated. To achieve these extreme conditions, the initially cold fuel must be explosively compressed. Inertial confinement is used in the hydrogen bomb, where the driver is x-rays created by a fission bomb. Inertial confinement is also attempted in "controlled" nuclear fusion, where the driver is a laser, ion, or electron beam, or a Z-pinch. Another method is to use conventional high explosive material to compress a fuel to fusion conditions. The UTIAS explosive-driven-implosion facility was used to produce stable, centered and focused hemispherical implosions to generate neutrons from D-D reactions. The simplest and most direct method proved to be in a predetonated stoichiometric mixture of deuterium-oxygen. The other successful method was using a miniature Voitenko compressor, where a plane diaphragm was driven by the implosion wave into a secondary small spherical cavity that contained pure deuterium gas at one atmosphere.
Some confinement principles have been investigated, such as muon-catalyzed fusion, the Farnsworth–Hirsch fusor and Polywell (inertial electrostatic confinement), and bubble fusion.
A variety of methods are known to effect nuclear fusion. Some are "cold" in the strict sense that no part of the material is hot (except for the reaction products), some are "cold" in the limited sense that the bulk of the material is at a relatively low temperature and pressure but the reactants are not, and some are "hot" fusion methods that create macroscopic regions of very high temperature and pressure.
Muon-catalyzed fusion is a well-established and reproducible fusion process that occurs at ordinary temperatures. It was studied in detail by Steven Jones in the early 1980s. It has not been reported to produce net energy. Net energy production from this reaction cannot occur because of the energy required to create muons, their 2.2 µs half-life, and the chance that a muon will bind to the new alpha particle and thus stop catalyzing fusion.
Generally cold, locally hot fusion
Accelerator-based light-ion fusion is a technique using particle accelerators to achieve particle kinetic energies sufficient to induce light-ion fusion reactions. Accelerating light ions is relatively easy, and can be done in an efficient manner—all it takes is a vacuum tube, a pair of electrodes, and a high-voltage transformer; fusion can be observed with as little as 10 kV between electrodes. The key problem with accelerator-based fusion (and with cold targets in general) is that fusion cross sections are many orders of magnitude lower than Coulomb interaction cross sections. Therefore the vast majority of ions end up expending their energy on bremsstrahlung and ionization of atoms of the target. Devices referred to as sealed-tube neutron generators are particularly relevant to this discussion. These small devices are miniature particle accelerators filled with deuterium and tritium gas in an arrangement that allows ions of these nuclei to be accelerated against hydride targets, also containing deuterium and tritium, where fusion takes place. Hundreds of neutron generators are produced annually for use in the petroleum industry where they are used in measurement equipment for locating and mapping oil reserves. Despite periodic reports in the popular press by scientists claiming to have invented "table-top" fusion machines, neutron generators have been around for half a century. The sizes of these devices vary but the smallest instruments are often packaged in sizes smaller than a loaf of bread. These devices do not produce a net power output.
Sonofusion or bubble fusion, a controversial variation on the sonoluminescence theme, suggests that acoustic shock waves, creating temporary bubbles (cavitation) that expand and collapse shortly after creation, can produce temperatures and pressures sufficient for nuclear fusion.
The Farnsworth–Hirsch fusor is a tabletop device in which fusion occurs. This fusion comes from high effective temperatures produced by electrostatic acceleration of ions. The device can be built inexpensively, but it too is unable to produce a net power output.
The Polywell is a concept for a tabletop device in which fusion occurs. The device is a non-thermodynamic equilibrium machine that uses electrostatic confinement to accelerate ions into a center where they fuse together.
Antimatter-initialized fusion uses small amounts of antimatter to trigger a tiny fusion explosion. This has been studied primarily in the context of making nuclear pulse propulsion, and pure fusion bombs feasible. This is not near becoming a practical power source, due to the cost of manufacturing antimatter alone.
Pyroelectric fusion was reported in April 2005 by a team at UCLA. The scientists used a pyroelectric crystal heated from −34 to 7 °C (−29 to 45 °F), combined with a tungsten needle to produce an electric field of about 25 gigavolts per meter to ionize and accelerate deuterium nuclei into an erbium deuteride target. Though the energy of the deuterium ions generated by the crystal has not been directly measured, the authors used 100 keV (a temperature of about 109 K) as an estimate in their modeling. At these energy levels, two deuterium nuclei can fuse together to produce a helium-3 nucleus, a 2.45 MeV neutron and bremsstrahlung. Although it makes a useful neutron generator, the apparatus is not intended for power generation since it requires far more energy than it produces.
In hot fusion, the fuel reaches tremendous temperature and pressure inside a fusion reactor or nuclear weapon (or star).
The methods in the second group are examples of non-equilibrium systems, in which very high temperatures and pressures are produced in a relatively small region adjacent to material of much lower temperature. In his doctoral thesis for MIT, Todd Rider did a theoretical study of all quasineutral, isotropic, non-equilibrium fusion systems. He demonstrated that all such systems will leak energy at a rapid rate due to bremsstrahlung produced when electrons in the plasma hit other electrons or ions at a cooler temperature and suddenly decelerate. The problem is not as pronounced in a hot plasma because the range of temperatures, and thus the magnitude of the deceleration, is much lower. Note that Rider's work does not apply to non-neutral and/or anisotropic non-equilibrium plasmas.
Astrophysical reaction chains:
The most important fusion process in nature is the one that powers stars. The net result is the fusion of four protons into one alpha particle, with the release of two positrons, two neutrinos (which changes two of the protons into neutrons), and energy, but several individual reactions are involved, depending on the mass of the star. For stars the size of the sun or smaller, the proton-proton chain dominates. In heavier stars, the CNO cycle is more important. Both types of processes are responsible for the creation of new elements as part of stellar nucleosynthesis.
At the temperatures and densities in stellar cores the rates of fusion reactions are notoriously slow. For example, at solar core temperature (T ≈ 15 MK) and density (160 g/cm³), the energy release rate is only 276 μW/cm³—about a quarter of the volumetric rate at which a resting human body generates heat. Thus, reproduction of stellar core conditions in a lab for nuclear fusion power production is completely impractical. Because nuclear reaction rates strongly depend on temperature (exp(−E/kT)), achieving reasonable energy production rates in terrestrial fusion reactors requires 10–100 times higher temperatures (compared to stellar interiors): T ≈ 0.1–1.0 GK.
Criteria and candidates for terrestrial reactions
In man-made fusion, the primary fuel is not constrained to be protons and higher temperatures can be used, so reactions with larger cross-sections are chosen. This implies a lower Lawson criterion, and therefore less startup effort. Another concern is the production of neutrons, which activate the reactor structure radiologically, but also have the advantages of allowing volumetric extraction of the fusion energy and tritium breeding. Reactions that release no neutrons are referred to as aneutronic.
To be a useful energy source, a fusion reaction must satisfy several criteria. It must
* Be exothermic: This may be obvious, but it limits the reactants to the low Z (number of protons) side of the curve of binding energy. It also makes helium 4
He the most common product because of its extraordinarily tight binding, although 3
He and 3
H also show up.
* Involve low Z nuclei: This is because the electrostatic repulsion must be overcome before the nuclei are close enough to fuse.
* Have two reactants: At anything less than stellar densities, three body collisions are too improbable. In inertial confinement, both stellar densities and temperatures are exceeded to compensate for the shortcomings of the third parameter of the Lawson criterion, ICF's very short confinement time.
* Have two or more products: This allows simultaneous conservation of energy and momentum without relying on the electromagnetic force.
* Conserve both protons and neutrons: The cross sections for the weak interaction are too small.
Few reactions meet these criteria. The following are those with the largest cross sections:
1D + 3
1T → 4
2He ( 3.5 MeV ) + n0 ( 14.1 MeV )
1D + 2
1D → 3
1T ( 1.01 MeV ) + p+ ( 3.02 MeV ) 50%
(2ii) → 3
2He ( 0.82 MeV ) + n0 ( 2.45 MeV ) 50%
1D + 3
2He → 4
2He ( 3.6 MeV ) + p+ ( 14.7 MeV )
1T + 3
1T → 4
2He + 2 n0 + 11.3 MeV
2He + 3
2He → 4
2He + 2 p+ + 12.9 MeV
2He + 3
1T → 4
2He + p+ + n0 + 12.1 MeV 51%
(6ii) → 4
2He ( 4.8 MeV ) + 2
1D ( 9.5 MeV ) 43%
(6iii) → 4
2He ( 0.5 MeV ) + n0 ( 1.9 MeV ) + p+ ( 11.9 MeV ) 6%
1D + 6
3Li → 2 4
2He + 22.4 MeV
(7ii) → 3
2He + 4
2He + n0 + 2.56 MeV
(7iii) → 7
3Li + p+ + 5.0 MeV
(7iv) → 7
4Be + n0 + 3.4 MeV
(8) p+ + 6
3Li → 4
2He ( 1.7 MeV ) + 3
2He ( 2.3 MeV )
2He + 6
3Li → 2 4
2He + p+ + 16.9 MeV
(10) p+ + 11
5B → 3 4
2He + 8.7 MeV
For reactions with two products, the energy is divided between them in inverse proportion to their masses, as shown. In most reactions with three products, the distribution of energy varies. For reactions that can result in more than one set of products, the branching ratios are given.
Some reaction candidates can be eliminated at once. The D-6Li reaction has no advantage compared to p+-11
5B because it is roughly as difficult to burn but produces substantially more neutrons through 2
1D side reactions. There is also a p+-7
3Li reaction, but the cross section is far too low, except possibly when Ti > 1 MeV, but at such high temperatures an endothermic, direct neutron-producing reaction also becomes very significant. Finally there is also a p+-9
4Be reaction, which is not only difficult to burn, but 9
4Be can be easily induced to split into two alpha particles and a neutron.
In addition to the fusion reactions, the following reactions with neutrons are important in order to "breed" tritium in "dry" fusion bombs and some proposed fusion reactors:
n0 + 6
3Li → 3
1T + 4
n0 + 7
3Li → 3
1T + 4
2He + n0
To evaluate the usefulness of these reactions, in addition to the reactants, the products, and the energy released, one needs to know something about the cross section. Any given fusion device has a maximum plasma pressure it can sustain, and an economical device would always operate near this maximum. Given this pressure, the largest fusion output is obtained when the temperature is chosen so that <σv>/T² is a maximum. This is also the temperature at which the value of the triple product nTτ required for ignition is a minimum, since that required value is inversely proportional to <σv>/T² (see Lawson criterion). (A plasma is "ignited" if the fusion reactions produce enough power to maintain the temperature without external heating.) This optimum temperature and the value of <σv>/T² at that temperature is given for a few of these reactions in the following table.
fuel T [keV] <σv>/T² [m³/s/keV²]
1T 13.6 1.24×10−24
1D 15 1.28×10−26
2He 58 2.24×10−26
3Li 66 1.46×10−27
5B 123 3.01×10−27
Note that many of the reactions form chains. For instance, a reactor fueled with 3
1T and 3
2He creates some 2
1D, which is then possible to use in the 2
2He reaction if the energies are "right". An elegant idea is to combine the reactions (8) and (9). The 3
2He from reaction (8) can react with 6
3Li in reaction (9) before completely thermalizing. This produces an energetic proton, which in turn undergoes reaction (8) before thermalizing. Detailed analysis shows that this idea would not work well, but it is a good example of a case where the usual assumption of a Maxwellian plasma is not appropriate.
Neutronicity, confinement requirement, and power density
Any of the reactions above can in principle be the basis of fusion power production. In addition to the temperature and cross section discussed above, we must consider the total energy of the fusion products Efus, the energy of the charged fusion products Ech, and the atomic number Z of the non-hydrogenic reactant.
Specification of the 2
1D reaction entails some difficulties, though. To begin with, one must average over the two branches (2) and (3). More difficult is to decide how to treat the 3
1T and 3
2He products. 3
1T burns so well in a deuterium plasma that it is almost impossible to extract from the plasma. The 2
2He reaction is optimized at a much higher temperature, so the burnup at the optimum 2
1D temperature may be low, so it seems reasonable to assume the 3
1T but not the 3
2He gets burned up and adds its energy to the net reaction. Thus we count the 2
1D fusion energy as Efus = (4.03+17.6+3.27)/2 = 12.5 MeV and the energy in charged particles as Ech = (4.03+3.5+0.82)/2 = 4.2 MeV.
Another unique aspect of the 2
1D reaction is that there is only one reactant, which must be taken into account when calculating the reaction rate.
With this choice, we tabulate parameters for four of the most important reactions
fuel Z Efus [MeV] Ech [MeV] neutronicity
1T 1 17.6 3.5 0.80
1D 1 12.5 4.2 0.66
2He 2 18.3 18.3 ~0.05
5B 5 8.7 8.7 ~0.001
The last column is the neutronicity of the reaction, the fraction of the fusion energy released as neutrons. This is an important indicator of the magnitude of the problems associated with neutrons like radiation damage, biological shielding, remote handling, and safety. For the first two reactions it is calculated as (Efus-Ech)/Efus. For the last two reactions, where this calculation would give zero, the values quoted are rough estimates based on side reactions that produce neutrons in a plasma in thermal equilibrium.
Of course, the reactants should also be mixed in the optimal proportions. This is the case when each reactant ion plus its associated electrons accounts for half the pressure. Assuming that the total pressure is fixed, this means that density of the non-hydrogenic ion is smaller than that of the hydrogenic ion by a factor 2/(Z+1). Therefore the rate for these reactions is reduced by the same factor, on top of any differences in the values of <σv>/T². On the other hand, because the 2
1D reaction has only one reactant, the rate is twice as high as if the fuel were divided between two hydrogenic species.
Thus there is a "penalty" of (2/(Z+1)) for non-hydrogenic fuels arising from the fact that they require more electrons, which take up pressure without participating in the fusion reaction. (It is usually a good assumption that the electron temperature will be nearly equal to the ion temperature. Some authors, however discuss the possibility that the electrons could be maintained substantially colder than the ions. In such a case, known as a "hot ion mode", the "penalty" would not apply.) There is at the same time a "bonus" of a factor 2 for 2
1D because each ion can react with any of the other ions, not just a fraction of them.
We can now compare these reactions in the following table.
fuel <σv>/T² penalty/bonus reactivity Lawson criterion power density (W/m3/kPa2) relation of power density
1T 1.24×10−24 1 1 1 34 1
1D 1.28×10−26 2 48 30 0.5 68
2He 2.24×10−26 2/3 83 16 0.43 80
3Li 1.46×10−27 1/2 1700 0.005 6800
5B 3.01×10−27 1/3 1240 500 0.014 2500
The maximum value of <σv>/T² is taken from a previous table. The "penalty/bonus" factor is that related to a non-hydrogenic reactant or a single-species reaction. The values in the column "reactivity" are found by dividing 1.24×10−24 by the product of the second and third columns. It indicates the factor by which the other reactions occur more slowly than the 2
1T reaction under comparable conditions. The column "Lawson criterion" weights these results with Ech and gives an indication of how much more difficult it is to achieve ignition with these reactions, relative to the difficulty for the 2
1T reaction. The last column is labeled "power density" and weights the practical reactivity with Efus. It indicates how much lower the fusion power density of the other reactions is compared to the 2
1T reaction and can be considered a measure of the economic potential.
Bremsstrahlung losses in quasineutral, isotropic plasmas
The ions undergoing fusion in many systems will essentially never occur alone but will be mixed with electrons that in aggregate neutralize the ions' bulk electrical charge and form a plasma. The electrons will generally have a temperature comparable to or greater than that of the ions, so they will collide with the ions and emit x-ray radiation of 10-30 keV energy (Bremsstrahlung). The Sun and stars are opaque to x-rays, but essentially any terrestrial fusion reactor will be optically thin for x-rays of this energy range. X-rays are difficult to reflect but they are effectively absorbed (and converted into heat) in less than mm thickness of stainless steel (which is part of a reactor's shield). The ratio of fusion power produced to x-ray radiation lost to walls is an important figure of merit. This ratio is generally maximized at a much higher temperature than that which maximizes the power density (see the previous subsection). The following table shows the rough optimum temperature and the power ratio at that temperature for several reactions.
fuel Ti (keV) Pfusion/PBremsstrahlung
1T 50 140
1D 500 2.9
2He 100 5.3
2He 1000 0.72
3Li 800 0.21
5B 300 0.57
The actual ratios of fusion to Bremsstrahlung power will likely be significantly lower for several reasons. For one, the calculation assumes that the energy of the fusion products is transmitted completely to the fuel ions, which then lose energy to the electrons by collisions, which in turn lose energy by Bremsstrahlung. However, because the fusion products move much faster than the fuel ions, they will give up a significant fraction of their energy directly to the electrons. Secondly, the plasma is assumed to be composed purely of fuel ions. In practice, there will be a significant proportion of impurity ions, which will then lower the ratio. In particular, the fusion products themselves must remain in the plasma until they have given up their energy, and will remain some time after that in any proposed confinement scheme. Finally, all channels of energy loss other than Bremsstrahlung have been neglected. The last two factors are related. On theoretical and experimental grounds, particle and energy confinement seem to be closely related. In a confinement scheme that does a good job of retaining energy, fusion products will build up. If the fusion products are efficiently ejected, then energy confinement will be poor, too.
The temperatures maximizing the fusion power compared to the Bremsstrahlung are in every case higher than the temperature that maximizes the power density and minimizes the required value of the fusion triple product. This will not change the optimum operating point for 2
1T very much because the Bremsstrahlung fraction is low, but it will push the other fuels into regimes where the power density relative to 2
1T is even lower and the required confinement even more difficult to achieve. For 2
1D and 2
2He, Bremsstrahlung losses will be a serious, possibly prohibitive problem. For 3
3Li and p+-11
5B the Bremsstrahlung losses appear to make a fusion reactor using these fuels with a quasineutral, isotropic plasma impossible. Some ways out of this dilemma are considered—and rejected—in Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium by Todd Rider. This limitation does not apply to non-neutral and anisotropic plasmas; however, these have their own challenges to contend with.
Air-to-Air (A2A) Missiles:
An air-to-air missile (AAM) is a guided missile fired from an aircraft with the purpose of destroying another aircraft or helicopter. It is typically powered by one or more rocket motors, usually solid fuelled but sometimes liquid fuelled. Ramjet engines, as used on the MBDA Meteor (currently in development), are emerging as propulsion that will enable future medium-range missiles to maintain higher average speed across their engagement envelope.
Guided missiles operate by detecting their target (usually by radar or infra-red methods, sometimes by laser guidance or optical tracking), and then "homing" in on the target on a collision course. The target is usually destroyed or damaged by means of an explosive warhead, often throwing out fragments to increase the lethal radius, typically detonated by a proximity fuse (or impact fuse if it scores a direct hit).
Note that although the missile may use radar or infra-red guidance to home on the target, this does not necessarily mean that the same means is used by the launching aircraft to detect and track the target before launch. Infra-red guided missiles can be "slaved" to an attack radar in order to find the target and radar-guided missiles can be launched at targets detected visually or via an infra-red search and track (IRST) system, although they may require the attack radar to illuminate the target during part or all of the missile interception itself.
Radar guidance is normally used for medium or long range missiles, where the infra-red signature of the target would be too faint for an infra-red detector to track. There are two major types of radar-guided missile - active and semi-active.
Active radar(AR)-guided missiles carry their own radar system to detect and track their target. However, the size of the radar antenna is limited by the small diameter of missiles, limiting its range which typically means such missiles have to use another method to get close to the target before turning their radar set on, often inertial guidance).
Semi-active radar (SAR) homing missiles are simpler and more common. They function by detecting the radar energy reflected from the target, the radar energy is emitted from the launch aircraft's own radar signal. However, this means the launch aircraft has to maintain a "lock" on the target (keep illuminating the target aircraft with its' own radar) until the missile makes the interception, limiting the attacking aircraft's ability to maneuver, which may be necessary should threats to the attacking aircraft appear.
An early form of radar guidance was "beam-riding" (BR). In this method the attacking aircraft directed a narrow beam of radar energy at the target. The air-to-air missile was launched into the beam where sensors on the aft of the missile controlled the missile, keeping it within the beam. So long as the beam was kept on the target aircraft, the missile would ride the beam until making the interception. While simple in concept, the difficulty of simultaneously keeping the beam solidly on the target (which couldn't be relied upon to cooperate by flying straight and level), continuing to fly one's own aircraft, all the while keeping an eye out for enemy countermeasures, can be readily appreciated. Although radar beam-riding air-to-air missiles are obsolete, the technology has since evolved toward laser-beam guided air-to-ground munitions, such as laser-guided bombs (LGB). These precision-strike munitions are sometimes called "smart weapons" by the press.
Radar guided missiles can be countered by rapid manoeuvring (which may result in them "breaking lock", or may cause them to overshoot), deploying chaff or using electronic counter-measures.
Infrared guided (IR) missiles home on the heat produced by an aircraft. Early infra-red detectors had poor sensitivity, so could only track the hot exhaust pipes of an aircraft. This meant an attacking aircraft had to maneuver to a position behind its' target before it could fire an infra-red guided missile. This also limited the range of the missile as the infra-red signature soon become too small to detect with increasing distance and after launch the missile was playing "catch-up" with its' target.
More modern infra-red guided missiles can detect the heat of an aircraft's skin, warmed by the friction of airflow, in addition to the fainter heat signature of the engine when the aircraft is seen from the side or head-on. This, combined with greater maneuverability, gives them an "all-aspect" capability, and an attacking aircraft no longer had to be behind its target to fire. Although launching from behind the target increases the probability of a hit, the launching aircraft usually has to be closer to the target in a tail-chase engagement.
An aircraft can defend against infra-red missiles by dropping flares that are hotter than the aircraft, so the missile homes in on the brighter, hotter target. Towed decoys and infra-red jammers can also be used. Some large aircraft and many combat helicopters make use of so called "hot brick" infra-red jammers, typically mounted near the engines. Current research is developing laser devices which can spoof or destroy the guidance systems of infra-red guided missiles.
However, the latest missiles such as the ASRAAM use an "imaging" infra-red seeker which "sees" the target ( much like a digital video camera), and can distinguish between an aircraft and a point heat source such as a flare. They also feature a very wide detection angle, so the attacking aircraft does not have to be pointing straight at the target for the missile to lock on. The pilot can use a helmet mounted sight (HMS) and target another aircraft by looking at it, and then firing. This is called "off-bore sight" launch. The Russian Su-27 is equipped with an infrared search and track (IRST) system with laser rangefinder for its HMS-guided missiles.
A recent advancement in missile guidance is electro-optical imaging. The Israeli Python-5 has an electro-optical seeker that scans designated area for targets via optical imaging. Once a target is acquired, the missile will lock-on to it for the kill. Electro-optical seekers can be programmed to target vital area of an aircraft, such as the cockpit. Since it doesn't depend on the target aircraft's heat signature, it can be used against low-heat targets such as UAV's and cruise missiles.
Air-to-air missiles are typically long, thin cylinders in order to reduce their cross section and thus minimize drag at the high speeds at which they travel. At the front is the seeker, either a radar system, radar homer, or infra-red detector. Behind that lies the avionics which control the missile. Typically after that, in the centre of the missile, is the warhead, usually several kilograms of high explosive surrounded by metal that fragments on detonation.
The rear part of the missile contains the propulsion system, usually a rocket of some type. Dual-thrust solid-fuel rockets are common, but some longer-range missiles use liquid-fuel motors that can "throttle" to extend their range and preserve fuel for energy-intensive final manoeuvring. Some solid-fuelled missiles mimic this technique with a second rocket motor which burns during the terminal homing phase. There are missiles in development, such as the MBDA Meteor, that "breathe" air (using a ramjet, similar to a jet engine) in order to extend their range. Modern missiles use "low-smoke" motors - early missiles produced thick smoke trails, which were easily seen by the crew of the target aircraft alerting them to the attack and helping them determine how to evade it.
Missiles are often cited with their maximum engagement range, which is very misleading. A missile's effective range is dependent on factors such as altitude, speed, position, and direction of target aircraft. For example the Vympel R-77 has stated range of 100 km. That's only true for a head-on, non-evading target at high altitude. At low altitude, the effective range is reduced by as much as 75%-80% to 20-25 km. If the target is taking evasive action, or in sterm-chase position, the effective range is even further reduced. The effective range of an air-to-air missile is known as the 'no-escape zone', noting the range at which the target can not evade the missile once launched.
Name: AIM-120A AMRAAM
Manufacturer: Hughes Missile Systems Division
Date Deployed: 1991
Range: 72 km (Some sources claim 48 km)
Speed: Mach 4
Propulsion: One solid-propellant rocket motor
Guidance: Mid-course inertial navigation and Hughes active radar.
Warhead: 20 kg proximity and impact delay fused blast/fragmentation
Launch Weight: 152 kg
Length: 3.66 m
Diameter: 0.178 m
Fin Span: 0.63 m
Platforms: F-15 Eagle, F-16 Falcon, F/A-18 Hornet, F-4F Phantom, JAS-39 Gripen, Tornado, Sea-Harrier
Remarks: Raytheon is also integrating the AIM-120 on the Eurofighter Typhoon, F/A-22A and Harrier II+
Name: AIM-54A/C Phoenix
Manufacturer: Hughes Missiles Systems
Date Deployed: 1973
Range: 180 km
Speed: Mach 4.3+
Propulsion: One Aerojet Mk 60 Mod 0 or Rocketdyne Mk 47 Mod 0 solid-propellant rocket motor
Guidance: Hughes DSQ-26 system using inertial, semi-active and active radar
Warhead: 59.9 kg Bendix IR and Downey Mk 334 radar proximity and impact delay fused continuous rod blast/fragmentation
Launch Weight: 446.8 kg
Length: 4.01 m
Diameter: 0.38 m
Fin Span: 0.925 m
Platforms: The F-14 Tomcat navy planes
Name: AIM-7F/M Sparrow
Manufacturer: Raytheon Co.
Date Deployed: July 1956
Range: 100 km
Speed: Mach 3.7
Propulsion: One Hercules Mk 58 Mod 0 or Aerojet Mk 65 Mod 0 dual-thrust solid-propellant rocket motor
Guidance: Raytheon Advanced Monopulse Seeker inverse-monopulse semi-active radar homing
Warhead: 39.9 kg proximity and impact delay fused Mk 71 continuous- rod blast/fragmetation
Launch Weight: 228.2 kg
Length: 3.68 m
Diameter: 0.203 m
Fin Span: 1.02 m
Platforms: F-14 Tomcat, F-15 Eagle, F-16 Falcon, F/A-18 Hornet
Name: AIM-9L/M Sidewinder
Manufacturer: Raytheon Co. and Ford Aerospace and Communications Co.
Date Deployed: 1976 for L 1983 for M
Range: 29.03 km
Speed: Mach 2.5
Propulsion: One Thiokol or Bermite Mk 36 Model 7/8 solid-propellant rocket motor or ( later ) One reduced-smoke Thiokol Mk 36 Mod 9 ( TX-683 ) solid-propellant rocket motor
Guidance: DSQ-29 IR homing
Warhead: 10.2 kg Hughes DSU-15/B active laser-fused WDU-17 annular blast/fragmentation
Launch Weight: 85.3 kg
Length: 2.85 m
Diameter: 0.127 m
Fin Span: 0.63 m
Manufacturer: British Aerospace
Date Deployed: 1978
Range: 45 km
Speed: Mach 4
Propulsion: One Aerojet Mk52 Mod 2 or Rocketdyne Mk38 Mod 4 solid-propellant rocket motor
Guidance: Marconi XJ521 monopulse Semi-Active Radar Homing
Warhead: 39.5-kg HE fragmentation with contact, delay action fuses.
Launch Weight: 192.8 kg
Length: 3.68 m
Diameter: 0.203 m
Fin Span: 1.02 m
Name: AIM-132 ASRAAM
Country: UK, Germany and Norway
Manufacturer: British Aerospace
Date Deployed: 1998 ?
Range: 300 m to 15 km
Speed: Mach 3+
Propulsion: One dual-thrust solid-propellant rocket motor
Guidance: strapdown inertial and Imaging Infrared
Warhead: 10 kg blast/fragmentation
Launch Weight: 100 kg
Length: 2.73 m
Diameter: 0.168 m
Fin Span: 45 cm
Military jet engines
A jet engine is an engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton's third law of motion. This broad definition of jet engines includes turbojets, turbofans, rockets and ramjets and water jets, but in common usage, the term generally refers to a gas turbine used to produce a jet of high speed exhaust gases for special propulsive purposes.
In the 1930s, the piston engine in its many different forms (rotary and static radial, aircooled and liquid-cooled inline) was the only type of powerplant available to aircraft designers. However, engineers were beginning to realize conceptually that the piston engine was self-limiting in terms of the maximum performance which could be attained; the limit was essentially one of propeller efficiency. This seemed to peak as blade tips approached the speed of sound. If engine, and thus aircraft, performance were ever to increase beyond such a barrier, a way would have to be found to radically improve the design of the piston engine, or a wholly new type of powerplant would have to be developed. This was the motivation behind the development of the gas turbine engine, commonly called a "jet" engine, which would become almost as revolutionary to aviation as the Wright brothers' first flight.
The key to a practical jet engine was the gas turbine, used to extract energy to drive the compressor from the engine itself. In 1929, Aircraft apprentice Frank Whittle formally submitted his ideas for a turbo-jet to his superiors. On 16 January 1930 in England, Whittle submitted his first patent (granted in 1932). The patent showed a two-stage axial compressor feeding a single-sided centrifugal compressor. Whittle would later concentrate on the simpler centrifugal compressor only, for a variety of practical reasons. In 1935 Hans von Ohain started work on a similar design in Germany, seemingly unaware of Whittle's work. Whittle had his first engine running in April 1937. It was liquid-fuelled, and included a self-contained fuel pump. Von Ohain's engine, as well as being 5 months behind Whittle's, relied on gas supplied under external pressure, so was not self-contained. Whittle unfortunately failed to secure proper backing for his project, and so fell behind Von Ohain in the race to get a jet engine into the air.
One problem with both of these early designs, which are called centrifugal-flow engines, was that the compressor worked by "throwing" (accelerating) air outward from the central intake to the outer periphery of the engine, where the air was then compressed by a divergent duct setup, converting its velocity into pressure. An advantage of this design was that it was already well understood, having been implemented in centrifugal superchargers. However, given the early technological limitations on the shaft speed of the engine, the compressor needed to have a very large diameter to produce the power required. A further disadvantage was that the air flow had to be "bent" to flow rearwards through the combustion section and to the turbine and tailpipe.
Austrian Anselm Franz of Junkers' engine division (Junkers Motoren or Jumo) addressed these problems with the introduction of the axial-flow compressor. Essentially, this is a turbine in reverse. Air coming in the front of the engine is blown towards the rear of the engine by a fan stage (convergent ducts), where it is crushed against a set of non-rotating blades called stators (divergent ducts). The process is nowhere near as powerful as the centrifugal compressor, so a number of these pairs of fans and stators are placed in series to get the needed compression. Even with all the added complexity, the resulting engine is much smaller in diameter. Jumo was assigned the next engine number, 4, and the result was the Jumo 004 engine. After many lesser technical difficulties were solved, mass production of this engine started in 1944 as a powerplant for the world's first jet-fighter aircraft, the Messerschmitt Me 262. After the end of the war the German Me 262 aircraft were extensively studied by the victorious allies and contributed to work on early Soviet and US jet fighters.
Centrifugal-flow engines have improved since their introduction. With improvements in bearing technology, the shaft speed of the engine was increased, greatly reducing the diameter of the centrifugal compressor. The short engine length remains an advantage of this design. Also, its engine components are robust; axial-flow compressors are more liable to foreign object damage.
British engines also were licensed widely in the US. Their most famous design, the Nene would also power the USSR's jet aircraft after a technology exchange. American designs would not come fully into their own until the 1960s.
A turbojet engine is a type of internal combustion engine often used to propel aircraft. Air is drawn into the rotating compressor via the intake and is compressed, through successive stages, to a higher pressure before entering the combustion chamber. Fuel is mixed with the compressed air and ignited by flame in the eddy of a flame holder. This combustion process significantly raises the temperature of the gas. Hot combustion products leaving the combustor expand through the turbine, where power is extracted to drive the compressor. Although this expansion process reduces both the gas temperature and pressure at exit from the turbine, both parameters are usually still well above ambient conditions. The gas stream exiting the turbine expands to ambient pressure via the propelling nozzle, producing a high velocity jet in the exhaust plume. If the jet velocity exceeds the aircraft flight velocity, there is a net forward thrust upon the airframe.
Under normal circumstances, the pumping action of the compressor prevents any backflow, thus facilitating the continuous-flow process of the engine. Indeed, the entire process is similar to a four-stroke cycle, but with induction, compression, ignition, expansion and exhaust taking place simultaneously, but in different sections of the engine. The efficiency of a jet engine is strongly dependent upon the overall pressure ratio (combustor entry pressure/intake delivery pressure) and the turbine inlet temperature of the cycle.
It is also perhaps instructive to compare turbojet engines with propeller engines. Turbojet engines take a relatively small mass of air and accelerate it by a large amount, whereas a propeller takes a large mass of air and accelerates it by a small amount. The high-speed exhaust of a jet engine makes it efficient at high speeds (especially supersonic speeds) and high altitudes. On slower aircraft and those required to fly short stages, a gas turbine-powered propeller engine, commonly known as a turboprop, is more common and much more efficient. Very small aircraft generally use conventional piston engines to drive a propeller but small turboprops are getting smaller as engineering technology improves.
The turbojet described above is a single-spool design, in which a single shaft connects the turbine to the compressor. Higher overall pressure ratio designs often have two concentric shafts, to improve compressor stability during engine throttle movements. The outer high pressure (HP) shaft connects the HP compressor to the HP turbine. This HP Spool, with the combustor, forms the core or gas generator of the engine. The inner shaft connects the low pressure (LP) compressor to the LP Turbine to create the LP Spool. Both spools are free to operate at their optimum shaft speed.
Most modern jet engines are actually turbofans, where the low pressure compressor acts as a fan, supplying supercharged air to not only the engine core, but to a bypass duct. The bypass airflow either passes to a separate 'cold nozzle' or mixes with low pressure turbine exhaust gases, before expanding through a 'mixed flow nozzle'.
Forty years ago there was little difference between civil and military jet engines, apart from the use of afterburning in some (supersonic) applications.
Civil turbofans today have a low specific thrust (net thrust divided by airflow) to keep jet noise to a minimum and to improve fuel efficiency. Consequently the bypass ratio (bypass flow divided by core flow) is relatively high (ratios from 4:1 up to 8:1 are common). Only a single fan stage is required, because a low specific thrust implies a low fan pressure ratio.
Today's military turbofans, however, have a relatively high specific thrust, to maximize the thrust for a given frontal area, jet noise being of little consequence. Multi-stage fans are normally required to achieve the relatively high fan pressure ratio needed for a high specific thrust. Although high turbine inlet temperatures are frequently employed, the bypass ratio tends to be low (usually significantly less than 2.0).
Comparative suitability for turboshaft, low bypass and turbojet to fly at 10 km attitude in various speeds.
Comparative suitability for (left to right) turboshaft, low bypass and turbojet to fly at 10 km attitude in various speeds. Horizontal axis - speed, m/s. Vertical axis carries only logical meaning.
Efficiency as a function of speed of different jet types Efficiency as a function of speed of different Jet types. Although efficiency plummets with speed, greater distances are covered, it turns out that efficiency per unit distance (per km or mile) is roughly independent of speed for Jet engines as a group.
The main manufactures of military jet engines today are :
* Pratt & Whitney (US; F-16, F-22)
* General Electric (US; B-1, B-2)
* Rolls-Royce (UK; Harrier)
* Tumansky (Soviet Union; Mig-25, Mig-29)
* Lyulka/Saturn (Soviet Union; SU-27, SU-37)
* Klimov (Soviet Union; Mig-17)
* Turbo-Union (UK, Germany, Italy; Tornado)
* EuroJet (UK, Germany, Italy, Spain; Eurofighter Typhoon)
* SNECMA (France; Mirage-2000, Rafale)
Military aircraft jet-engines in more detail
"After World War Two, piston engines continued to power civil airliners for many years, but in the field of military aircraft they were rapidly displaced by the gas turbine. Fighters and bombers switched to the turbojet, transports and maritime-patrol aircraft used turboprops, and helicopters benefited greatly from changing to turboshaft engines. The change meant more power for less weight, far greater reliability, no cooling problems and safer kerosene-type fuels.
With extraordinary reluctance, designers eventually recognized that the turbofan, offering a wide choice of bypass ratio (BPR - the mass flow of air in the bypass duct divided by that through the core), could with advantage replace the turbojet. In supersonic aircraft the need to minimize frontal area means that BPR is seldom as high as 1, and even then the installation must be done with great care. When the J79 turbojet of 79.63 kN thrust installed in the McDonnell Douglas F-4 Phantom was replaced in the British versions by the Rolls-Royce Spey turbofan of 91.25 kN the change made the aircraft slower in level flight, while giving improvements in take-off and climb performance!
Today the turbojet is almost extinct, except for some countries like China, where different criteria apply. Elsewhere, the trend has been towards achieving greater power with engines that are not only lighter but also smaller and dramatically simpler. For example, the Spey Mk 202, the engine of the RAF Phantoms, had a total of 17 stages of blading in the compressors (5+12 flow pressure+high pressure) and four stages of blading in the turbines (2+2). The next-generation RB. 199, engine of the Tornado, has 12 stages of compression (3+3+6) and again four stages of expansion through the turbines (1+1+2), whereas today's Eurojet EJ200, engine of the Eurofighter, has only eight compressor stages (3+5) and two turbine stages (1+ 1).
In general, the more stages of blading an axial-flow compressor has, the greater the overall pressure ratio (OPR) and the better the fuel economy (and thus, for a given aircraft tankage, the greater the range and endurance). One might therefore think that the simpler compressors have been achieved at the expense of more rapid fuel burn, but in fact the reverse is true. The OPR of the Phantom's Spey was 20, the figure for the Tornado engine is 23, and for the Eurofighter it has gone up to 26. Indeed, the next-generation fighter engine could have an OPR of 35, with only six or seven stages of blading.
Benefits of Simplicity
Simpler engines mean greater reliability, better resistance to battle damage, easier maintenance, and several other advantages including lower cost, though cost is not as dominant as it is in the civil sector. In the immediate postwar era, up to 1970, it was normal practice not to introduce an engine to the airlines until hundreds or even thousands had gained experience in fighters and bombers. The two families then diverged. Airliners needed engines offering the lowest possible fuel consumption and lowest possible noise at airports, and these (surprisingly slowly) eventually led to today's engines with a BPR of from 5 to 9, with enormous fans. Combat aircraft need slim engines, as already noted, so military experience is seldom much help to civil engines (though the best-selling CFM56 has the core of a long-established military engine, the F101 used in the Rockwell B-1B Lancer).
Today, the military trend towards greater simplicity is being echoed by civil engines. Nearly 30 years ago, special turbojets and turbofans were being produced purely to lift VTOL (vertical take-off and landing) aircraft. They were used only at take-off and landing, so were made as simple as possible. Like other engines, they sometimes had two spools (low-pressure and high-pressure compressors, each driven by its own turbine), and the aerodynamicists found that by making the spools rotate in opposite daemons, it was possible to do away with at least some of the stator (fixed) blades ahead of the turbine rotors.
Apart from Concorde, which has a low-augmentation form of reheat (afterburning), civil aircraft do not burn fuel in the jetpipe downstream of the turbines. Supersonic aircraft have afterburning engines in order to increase the energy in the jet so that, properly expanded in a special nozzle, it can be ejected at highly supersonic speed, in order to achieve the highest flight Mach number possible. Such aircraft as the MiG-25 Foxbat and Lockheed SR-71 Blackbird can fly at Mach 3 (three times the speed of sound).
Today's fighters have augmentation, the name given to burning extra fuel downstream of the turbines of a turbofan engine. With a turbofan there is abundant oxygen in the mixed flow in the jetpipe, much of which has not passed through the core and thus has had no fuel already burned in it. In any case, the latest engines are so powerful that augmentation is needed only on rare occasions (for example, in close combat) when maximum thrust is needed, because it burns fuel rapidly and also shortens engine life.
Thus, many modem fighters are capable of making a cold (unaugmented) take-off. The first to do this were the Grumman F-14B and F-14D re-engined versions of the Tomcat naval fighter. Fitting the General Electric F110-400 engine not only transformed reliability but also increased dry thrust to 71.6 kN (16,080 lb), not far short of the thrust of the original TF30 engine in full afterburner. As a result, the F-14 can be catapulted off a carrier without using afterburner, giving an increase in mission range of no less than 62%. Moreover, the greater dry thrust results in a reduction in time to climb to patrol altitude of 61%!
Supercruise and stealth
Going on from there, military engines are now so powerful that the latest fighters can supercruise, the term for flying at sustained supersonic speed without the use of the afterburner. Earlier supersonic aircraft could exceed Mach I on the level only in maximum afterburner, when the rate at which fuel was burned was so high that supersonic speed could not be sustained for longer than about a minute. Today such aircraft as the Lockheed Martin F-22 Raptor or Eurofighter Eurofighter Typhoon can accelerate to supersonic speed (with or without using augmentation) and then sustain such a speed indefinitely in dry thrust.
Apart from dramatically reducing the rate of fuel consumption, the ability to supercruise also reduces the IR (infrared) signature by some 75%. Clearly there is little point in making a 'stealth' aircraft, almost invisible to hostile radars, if its IR emissions proclaim its presence like a lighthouse. Many (indeed, most) of today's air-to-air guided missiles (AAMs) home in on a source of IR radiation, and a fighter in afterburner finds it much more difficult to throw a heat-homing AAM off the scent than a stealthy one in dry thrust, even in supercruise.
Of course, there is a relationship between the speed of an aircraft and that of its propulsive jet. For over 40 years, visionaries - and even a few professional aircraft designers - have considered military aircraft that are capable of hypersonic speed. This term is usually taken to mean Mach numbers several times greater than 1, such as Mach 5, which at high altitude equates to 2868 kts (5310 km/h). In my opinion there is no way a Mach 5 aircraft could supercruise, if by that it meant using a turbofan in dry thrust.
A Mach 5 aircraft will have to have an engine running continuously in full augmentation or, preferably, a ramjet. The trouble with a ramjet is that such engines cannot start from rest. Back in 1951, the Republic Aviation began beavering away at the XF-103, a fighter to cruise at Mach 3.7 (3,930km/h). This would have had a Wright J67, an afterburning turbojet based on the Bristol Siddeley Olympus, installed in a vast duct with a valve which, at high airspeed, could be switched over to bypass the J67 and convert the propulsion system into a ramjet. In August 1957 program was cancelled.
The SR-71 Blackbird had Pratt & Whitney J58 engines, which at Mach 3 behaved like ramjets, the J58s merely getting in the way of the hurricane passing through the nacelles. Whatever kind of engine might be invented, it is sure to have a nozzle whose profile and area can be varied. This by itself is quite a challenge, but beautiful examples can be seen on the latest fighters. Dr Viktor Chepkin is sad that his ALAI engines in the MiG 1.42/1.44 have so far stayed on the ground, because in his opinion, this augmented turbofan is a world-beater. Anyone who has watched the Su-27 Flanker perform, will know that his engines are pretty impressive, and he says "The AL-41 is a totally new generation engine".
Dr Chepkin is one of the Russians who have taken the bull by the horns and boldly combined a fully variable nozzle with the ability to vector (point in different direction). Vectored thrust was pioneered 50 years ago, initially by simply having a switch-in deflector to direct the jet either to go out the back or else through a separate nozzle pointing downwards. This was flown in Meteor RA490 in 1954, but it was a brutishly crude arrangement, and I would have hated to be the pilot if one engine had diverted while the other did not.
Since then, vectored thrust has taken many forms. Some aircraft, such as the Bell Model 65 of 1954 and the supersonic German VJ 101C of 1963, adopted the seemingly obvious method of mounting complete engines on pivots, so that they could point downwards or backwards. Bristol Siddeley designers adopted the more subtle method of fitting a turbofan with two pairs of nozzles, two for fan air and two more at the back for the hot jet. All four were mechanically linked (by motorcycle chain!) to swivel in unison, and the result was the Harrier. Despite the scorn of the USAF, which apparently thought there would always be a handy 10,000ft (3km) runway in any future war, some Harriers actually got into service, and proved so crucial in the Falklands that vectored-thrust versions of the Joint Strike Fighter have equal priority with those needing long runways.
Of course, another form of vectored thrust is to fit a thrust reverser, to slow the aircraft rapidly after a conventional high-speed landing. Reversers are universal on big jetliners (even on small ones, except the BAe I46/RJ and Fokker F28), but are rare on combat aircraft. Offhand, I can think of only the Viggen and Tornado. So far nobody has been clever enough to make an engine that can vector its thrust in all three modes: for combat agility, for VTOL or STOVL (short take-off and vertical landing) and to slow a conventional landing.
Today's supersonic fighters use vectoring purely to enhance in-flight agility. indeed, Eurofighter GmbH (it is a German company) is even at the Millennium still desperate to try to avoid putting vectoring nozzles on their otherwise superb aircraft. Despite presumably having watched Comrade Pugachev and his colleagues demonstrate the superb maneuverability of the MiG-29 Fulcrum and Su-27 Flanker, their position in late 1999 purported to be "We think, in the fullness of time there may well be a naval Eurofighter, and if so, that version might be improved by fitting vectored nozzles". Fortunately ITP in Spain has developed an excellent vectoring nozzle for the EJ200 engine, so it will be available when the penny finally drops. Indeed some Eurofighter folk have told me such nozzles might come "at the first mid-life update".
Size / unmanned aircraft
The technology of gas-turbine engines has never shown the slightest sign of approaching a plateau, far less a barrier. Since the dawn of gas-turbine aviation in 1940, the power available from a given bulk of engine has doubled roughly every 30 years, while specific fuel consumption (rate of fuel consumption for a given power output) has consistently fallen below the most sanguine expectations. If you look at a Gloster Meteor you see nacelles that housed engines of 15.6kN (3,500 lb) thrust. Today, nacelles of the same overall size could house engines with a dry thrust of 156 kN (35,000 lb).
Such power has led to modern fighters becoming impressively large. The McDonnell Douglas F-15 Eagle seemed big, with a wing area of over 55.7m2, but today the F-22 has a wing with an area of 78 m2, precisely the same as that of the World War Two Vickers Wellington heavy bomber. The wing of the MiG 1.44 has an area of 90.5m2. My own feeling is that future fighters ought to be smaller, and Saab (now in partnership with British Aerospace) has shown the way to go with the Gripen. This has the same engine as the Boeing F/A-18 Hornet, but half as many (just as its predecessor the Saab Draken had the same engine as the English Electric Lightning but half as many).
The easiest way to make fighters even smaller is to leave out the pilot. This has a considerable effect, because replacing the cockpit by 'black boxes' not only saves space and weight, and eliminates the environmental system, but also gives the designer greater flexibility in the overall layout. For example, he can put the engine inlet where the windscreen used to be, which most designers consider is a route to enhanced stealth characteristics, without significantly harming pressure recovery in the inlet during fight maneuvers.
Equally important, leaving out a human crew enables the whole aircraft to be approximately half as big, whilst at the same time allowing the designers to go for a maximum acceleration in the vertical plane of at least 20g. Such an aircraft could literally 'fly rings round' a fighter limited to today's 9g. Just how such aircraft would fly their missions depends on the task, and is outside the scope of this article. Suffice it to say, the future UAV (unmanned air vehicle) would almost certainly be single-engined, and could have an engine with a maximum thrust anywhere from 35,000lb (156kN) down to 350lb (1.56kN). Indeed, aircraft used solely as sensor platforms or decoys might have engines of a mere 35 lb (0.156kN) st, replacing today's much slower UAVs powered by tiny piston engines.
Eliminating the pilot, as well as any fins, will do much to enhance stealth qualities. This will focus increased pressure on the need to devise truly stealthy propulsion system For many years, designers have made it impossible for hostile, to 'see' the face of the engine, by suitably kinking the inlet duct (or, as noted, putting it above the fuselage). The propulsive nozzle is harder, and here there is a need to minimize thermal, visual and even acoustic signatures. The Lockheed Martin F-117 Nighthawk nozzles are flattened slits in the trailing edge of the wing, while those of the B-2 are tucked inside deep channels above the rear part of the wing, upstream of movable trailing edges.
I have numerous documents, all published openly in the United States, which purport to explain how the B-2 is even stranger - far stranger - than it appears. Most are articles published in commercial magazines, some are openly published US Patents, while a few are open USAF publications by Wright Aeronautical Laboratory and Air Force Systems Command's Astronautics Laboratory. They deal with such topics as electric-field propulsion, and electrogravitics (or anti-gravity), the transient alteration of not only thrust but also a body's weight. Sci-Fi has nothing on this stuff.
The literature goes back to Faraday, but the idea of electrogravitics really took off in the 1920s when an American physicist, Townsend T. Brown, carried out extensive experiments. He may have been the first to recognize that a capacitor (a dielectric material sandwiched between positive and negative plates) experiences a force tending to move it in the direction of the positive face. He found that the electrostatic charge induced a gravity field between the two plates. Soon he was making capacitors rotate on whirling arms, and measuring the loss in weight of capacitors with the positive face turned uppermost.
In 1953, Brown demonstrated to the USAF a whirling rig of 50ft (15.2m) diameter, which at 150,000 volts (150kV) became a mere blur. The subject was immediately classified, and for the next 40 years, while 'black' research in this field made astonishing progress, it was not reported. Though private individuals continued to experiment, and to take out unclassified patents, not much surfaced. Exceptions were Electrogravitics Systems (February 1956) and The Gravitics Situation (December 1956), published for subscribers only by Aviation Studies (International). This was a London-based 'think tank' run by two very bright young men: R G 'Dicky' Worcester and John Longhurst. Unlike the established journals, they published reports and informed comment without the slightest regard for questions of 'security'. The only time they were taken to court, they won their case and collected heavy damages.
I was fascinated to read those reports, but had no wish to reside in The Tower, so I refrained from discussing clever airplanes with leading edges charged to millions of volts positive and trailing edges at millions of volts negative. In any case, it all seemed a bit farfetched, especially as it appeared that the gravity field could not only propel aircraft to supersonic speed with propulsive efficiency greater than / but could also lift them independently of the atmosphere.
Various snippets appeared suggesting that electrostatic fields could not only do wondrous things in the field of propulsion but could also reduce aerodynamic turbulence (at any Mach number), reduce radar cross-section and even virtually eliminate the sonic boom Indeed, back in 1952, Dr M Rose had noted in unclassified literature: "The positive field.. travelling in front... acts as a buffer which starts moving the air out of the way. This field acts as an entering wedge which softens the supersonic barrier..." From 1985, the name P A LaViolette emerges as author of a shoal of interesting electrogravitics articles in professional literature.
The first Northrop Grumman B-2 Spirit stealth bomber was rolled out on November 22,1988, and anyone with the slightest interest in aircraft could not fail to have noticed the unbelievable leading edge, with a deep profile coming to a knife-edge almost in line with the upper surface. In 1990, a NASA 'boffin' retired and perhaps foolishly talked to The Arkansas Democrat who did not understand his story and ran it under the headline "Ex-NASA expert says Stealth uses parts from UFO".
What really put the cat among the proverbial pigeons was a feature published in a March 1992 issue of Aviation Week & Space Technology, entitled "Black world engineers, scientists, encourage using highly classified technology for civil applications". For the first time in open literature, this article explained how the B-2's sharp leading edge is charged to "many millions of volts", while the corresponding negative charge is blown out in the jets from the four engines. There is more: though the General Electric F118 engines can operate as ordinary turbofans, in flight they act as flame-jet generators, pumping out gas greatly diluted by fresh air, all at millions of volts negative. The word 'flame' gives a rather false picture, because in fact the jet comes out not very much hotter than the surrounding atmosphere.
Unclassified articles have described in some detail how the leading edge is divided into eight sections, each individually ionized. The section on each wing immediately upstream of the engines cannot be thus ionized, because the air would then enter the engines and cancel out the negative charge in the jets. Accordingly, this is where the Hughes covert strike radars are installed. They would not be able to 'see' forwards if they were anywhere else.
Take-off thrust of the F118-100 at sea level is given as '19,000 lb (84.5 kN) class' by Northrop Grumman and as '17,300 lb (77.0 kN)' by the USAF. These are startlingly low figures for an aircraft whose take-off weight is said to be 336,500 lb (152,635 kg) and which was until recently said to weigh 376,000 lb (170,550 kg). Aircraft usually get heavier over the years, not 20 tones lighter. Even at the supposed reduced weight, the ratio of thrust to weight is a mere 0.2, an extraordinarily low value for a combat aircraft.
The USAF has never said anything about B2 speed. It has been tacitly assumed to be in the Mach 0.8 class, but according to the extensive open literature, the four FI 18 engines equate to about 25 MW (megawatts) of electrical power at take-off, but under the influence of the electrogravitic field the speed could soon become supersonic, the output of the air-diluted exhaust then rising to at least 100 MW.
Everyone who has heard a B-2 take off has been astonished at the quietness. Obviously the noise would not be in the same class as the F101 engines of the B-1B in full afterburner, but writers have used the words 'shocking', 'uncanny' and 'incredible' in describing B-2 departures. As for elimination of contrails (condensation trails) (normally a giveaway even for a stealth aircraft), the USAF said chlorofluorosulphonic acid was injected into the jets to eliminate contrails. Later it said this was done by 'regulating exhaust temperatures'. Such an explanation is nonsense; contrails are ice crystals from water vapor left when hydrocarbon fuel is burned, and can never be eliminated by 'regulating exhaust temperatures'. Another point to note is that the channels downstream of the jetpipes appear to be carbon-fiber composite, which is incompatible with normal jet temperatures (not because of the fiber but because of the adhesive sticking them together).
Other writers have commented on the size of the B-2 wing and noted that its stealth depends on the huge black skin being made of RAM (radar-absorbent material). This, say the physicists, is 'a high-k, high-density dielectric ceramic, capable of generating an enormous electrogravitic lift force when charged'. I could go on and on. We have come some way from the Lancaster and B-17, and I seem to have strayed some way from traditional jet engines."
1940 - 1950 Supermarine Spitfire, P-47 Thunderbolt, P-51 Mustang, Mosquito, F-4U Corsair, P-38J Lightning, F-86 Sabre, P-40 WarHawk, FW-190 Focke Wulf, Messerschmitt ME-262, F-6F Hellcat, P-80 Shooting Star, B-25 Mitchell, F-8F Bearcat, Mig-15 Fagot, A1 Skyraider, Macchi M.C.205 Veltro, Gloster Meteor F Mk.8, B-17 Flying Fortress, de Havilland Vampire, MiG-3, B-29 Superfortress, F4F Wildcat, B-24 Liberator, Me-163 Komet, Yakovlev Yak-9
1950 - 1960 MiG-21 Fishbed, F-104 Starfighter, F-8 Crusader, B-52G Stratofortress, CF-105 Avro Arrow, F-106 Delta Dart, F-84F Thunderstreak, MiG-17 Fresco, F-105 ThunderChief, F-100 Super Sabre, J-32 Lansen, F-101 VooDoo, B-58 Hustler, MiG-19 Farmer, J-29 Tunnan, Super Mystere B2, Hawker Hunter, F-102 Delta Dagger, Fiat G.91
1960 - 1970 Buccaneer, F-4 Phantom II, SR-71 Blackbird, A-7D Corsair II, Hawker Harrier, Mirage F1, A-6 Intruder, U-2 Dragon Lady, J-35 Draken, Avro Vulcan, A-4 Skyhawk, SU-17/22 Fitter, English Electric / BAC Lightning, A-5 Vigilante, Mirage III, XB-70 Valkyrie
1970 - 1980 F-14 Tomcat, F-15 Eagle, F-5E Tiger II, MiG-27 Flogger, Jaguar GR1, SU-24 Fencer, MiG-25 Foxbat, A-10 Thunderbolt, F-16 Falcon, Alpha Jet, SU-25 Frogfoot, Super Etandard, JA37 Viggen, F-21 Kfir, BAe Hawk, L-39 Albatros
1980 - 1990 EF111 Raven, F-18 Hornet, Panavia Tornado, Mirage 2000, Mirage 4000, MiG-29 Fulcrum, SU-27 Flanker, MiG-31 Foxhound, F-117 NightHawk, B-1B Lancer, F-20 Tigershark, AMX, Tupolev Tu-160 Blackjack
1990 - 2000 SU-35 Super Flanker, Yak-141 Freestyle, JAS-39 Gripen, Rafale, B-2 Spirit, YF-23, SU-37 Terminator, Ching-Kuo Indigenous Defense Fighter (IDF), FA-18E Super Hornet
2000 - 2010 F/A-22 Raptor, EuroFighter Typhoon, Chengdu J-10, JF-17 Thunder (Chengdu FC-1), V-22 Osprey, Mig-35 Fulcrum-F, LCA
2010 - SU-47 (S-37 Berkut), MiG/MAPO 1.42 MFI, F-35 Lightning II (X-35 Joint Strike Fighter), Sukhoi PAK-FA (T-50), Aurora
If you can picture the cerulean blue waters of the Mediterranean, you will understand why the birthstone for March is named Aquamarine. Derived from the Roman word "Aqua," meaning water, and "mare," meaning sea, this pale blue gem does indeed resemble the color of seawater. The ancient Romans believed that the Aquamarine was sacred to Neptune, the god of the sea, having fallen from the jewel boxes of sirens and washed onto shore. Early sailors wore aquamarine talismans, engraved with the likeness of Neptune, as protection against dangers at sea.
The association with water led to the belief that the Aquamarine was particularly powerful when immersed. Water in which this gemstone had been submerged was used in ancient times to heal a variety of illnesses of the heart, liver, stomach, mouth and throat. Aquamarines were also used to reverse poisoning and to aid in fortune telling.
The Aquamarine is a member of the beryl family. Beryl is a mineral that crystallizes within large grained igneous rocks on the earth's crust. It varies in color from clear to vibrantly colored gemstones such as the Emerald. Beryl was used as far back as 2,000 years ago to correct vision, and it continues to be used today in the manufacture of eyeglasses. It is a very hard mineral, making the Aquamarine a durable gemstone for use in jewelry.
Aquamarine varies in color from blue-green to a light sky blue, but gems containing green are often heat-treated to remove this less desirable color. The majority of Aquamarines, unlike other gemstones, are flawless. It is a relatively abundant gem, the largest deposits being in Brazil, but other sources of Aquamarine are in China, India, Australia, Africa, and the United States.
A gift of Aquamarine symbolizes both safety and security, especially within long standing relationships. Some people even say that the Aquamarine reawakens love in a tired marriage, so if you want to bring back that spark in your partner's eyes, you might consider this gem as an anniversary gift!
An alternate birthstone for March is Bloodstone, a dark green opaque quartz flecked with red spots. The name Bloodstone derives from the legendary belief that the red coloration resulted from the blood of Christ spilling onto green jasper during the crucifixion. For this reason, too, the stone has been called "martyr's stone." Bloodstone is mined in India, Brazil, Uruguay, Australia, and the United States. Ideal for carving into cameos and beads, this stone symbolizes courage.