At a meeting of the Presidium of the Academy of Sciences discussed the complex from a technical point of view and important from the standpoint of achieving military advantage in the air to reduce the problem of radar visibility of aircraft - a powerful striking force of modern armies. The search for the solution together are representatives of basic and applied science. Their work was a radical reduction of radar visibility serial fourth-generation aircraft such as Su-27. How this was achieved, explains below, published an article based on any scientific report on RAS Presidium.
FUNDAMENTAL AND APPLIED PROBLEMS
stealth technologies
AN Lagar'kov, MA Pogosyan
Lagar'kov Andrei - corr. RAS, Director of the Institute of Theoretical and Applied Electrodynamics, Joint Institute for High Temperatures RAN.
Mikhail Pogosyan Aslanovich - corr. RAS, director of PMT Air Industrial Complex Sukhoi.
Contrary to recent trends in Russian language, we still decided to use the English term "stele" - "secretive", "secret" in the title of our report, primarily because he had already taken root in national literature and is widely used in our press, and in the foreign, including in non-English speaking countries. Under the stealth technology is now involve complex technical solutions, which resulted in reduced levels of the signals coming from the military facility to the receiving system, trying to object to detect and destroy. These signals are transferred by acoustic and electromagnetic waves in a wide range of frequencies. We shall deal here only one problem, namely: reducing the signal level resulting from the reflection object of the electromagnetic waves emitted by the radar. Radar station - the most "distant" means detection and guidance, and the first boom in the press about the stealth technology was associated with the emergence of the American the F-117, was created primarily to successful overcoming air defense system of the Soviet Union, equipped with powerful radars.
Ben Rich, former head of the plant firm Lockheed, in his memoirs about creating the F-117 maintains a history of stealth technology since 1975, linking it to begin with this aircraft. In fact, first steps were made in Germany at the end of World War II. As a result, the British night attack aircraft equipped with radar, have risen sharply loss of German submarines. It was then that their cabin began to apply radio-absorbing coating and was developed by serial production technology fine ferromagnetic fillers used in coatings. Since then, the problem of reducing the radar visibility of various objects weapons and military equipment attracts serious attention in industrialized countries.
main characteristic determining the properties of plane as reflecting electromagnetic radiation of the object is the effective scattering surface. It characterizes the ability to convert incident electromagnetic wave to the scattered wave propagating in the direction of the receiver. In the future we will consider monostatic location, when the transmitter and receiver are located in one place. Effective scattering surface (ESR) is defined as:
s = 4 p R 0 2 S s / S i ,
where R 0 - the distance between the transmitter and the object; S s - the energy flux density of the scattered wave near the receiver; S i - density energy flux of the incident wave near the body.
value of the ESR depends on the orientation of the object relative to the radar beam and the wavelength of electromagnetic radiation. For physicists, the best analogy of the EPR - is the differential scattering cross back. If we consider the frequency dependence of ESR metal ball (Fig. 1) a range of short wavelengths EPR coincides with an area of its cross section. For wavelengths comparable with the dimensions of the ball, there are diffraction minima and maximums.
Fig. 1. Frequency dependence of the effective scattering surface of a metallic sphere a - radius sphere, l - wavelength
Figure 2 shows the ESR of the cylinder depending on the angle of incidence of electromagnetic waves for the wave vector, lying in a plane that passes through the axis of the cylinder in Figure 3 - EPR modern aircraft for different directions of incidence of electromagnetic wave in the horizontal plane passing through the axis of the aircraft. Due to the complex shape of the object to its ESR showed significant fluctuations in the values very small change in the angles of the electromagnetic wave. Such a pattern is due to diffraction phenomena and the subsequent interference of electromagnetic waves reflected from different parts of the aircraft with the appropriate phase delay of each reflecting element.
Fig. 2 . The angular dependence of the effective scattering surface of the cylinder
D and L - the diameter and length of the cylinder, q - the angle of incidence of electromagnetic waves; arrow indicates the direction of its fall
Fig. 3. "facet" model of the Su-27 (top) and diagram of the effective surface scattering plane
depending on the angle in the horizontal plane (bottom)
solid line corresponds to the scatterplot of real plane, dashed - aircraft with reduced radar visibility, 1 and 2 - averaged over a range of angles ± 30 ° the values of the EPR
Naturally, in practice, are interested in the values of ESR, averaged over a relatively small range of angles because of the inevitable oscillation trajectory precisely determine the average level of the signal being processed in the receiver. The task of the stele, the technology is possible a possible decrease in the EPR aircraft. For example, a qualitative leap in military capabilities into the aircraft, there would be the case if, instead of ESR, shown in Figure 3, solid line, we could obtain the EPR, as shown in the same figure the dotted line.
Stealth technology includes the following main areas: theory of diffraction by complex bodies, development and study of radar absorbing materials, coating technology, and finally, Radiophysics experiment used to control each of these areas.
currently stealth technology starts with the mathematical modeling of scattering electromagnetic wave on an object, the radar visibility of which must be reduced. This step is crucial for a preliminary assessment of the achievable results and to optimize the shape and electrical characteristics of the object. Mathematical and computational models based on solving the boundary diffraction problems of electromagnetic waves on bodies of complex shape, having in its composition, special materials and coatings. Capabilities of modern computing techniques allow the creation of software for modeling the scattering of electromagnetic waves, even in such complex objects such as airplanes and ships, given the variety of suspended equipment, crevices, covers and many other seemingly minor details of the construction.
on the world market offers several software packages that are suitable for such modeling. All of them are under development using the "facet" model of the object it which will be lower, and the method of the physical theory of diffraction. In the well-known reasons, due to closeness of such works, until recently, domestic print (as opposed to the foreign press) are not published information about a specific application of mathematical modeling of scattering by objects of weaponry and military equipment. However, the analysis of scientific literature on the theory of diffraction of specialists in the field of electrodynamics, it was easy to assess the level of development of the industry.
Contribution of Russian scientists in the theory of diffraction is not only significant, but in many important ways the fundamental. Suffice it to mention the names of academician VA Fock, Professor E. N. Vasilyev and P. J. Ufimtseva. VA Fock, not just the first to formulate the integral equations relative to the current on the surface of the irradiated metal body, but also suggested that an approximate solution for bodies with a large radius of curvature. Pioneer were the work of E. Vasilieva on numerical solution of integral equations in the problem of electromagnetic excitation of metallic bodies. PY Ufimtsev - Founder method of edge waves in physical theory of diffraction, which is of exceptional importance in the development of stealth action recognized by the international scientific community. That numerical methods for solving scattering problems using integral equations, estimation of scattering properties of the method of edge waves, and the combination of these methods have been used by us in many models of electrodynamic objects with low radar visibility.
Institute's staff Theoretical and Applied Electrodynamics RAS for many years, developing numerical methods, relying on the achievements of both Russian and foreign Schools diffraction theory. The application of asymptotic and hybrid algorithms requires a clear understanding of the physical phenomena that lead to the formation of the scattered field in each case: specular reflection, diffraction at the edges, running waves, multiple reflections inside cavities, etc. Accounting mirror reflections of real objects is realized through the development of "facet" model, where the surface of the object is in the form of a large number of separate flat areas of simple shape (see Fig. 3).
When calculating the EPR large (in wavelengths) complex bodies is essential that the relatively high frequencies can distinguish the individual fragments of the structure of objects, which in this perspective, the main contributors to the formation of scattered field. This makes it possible to perform calculations for these scattering centers independently, if necessary, taking into account their mutual influence on subsequent stages. The accuracy of calculating the characteristics of the scattering centers is of particular importance in the analysis of subtle details, so great attention has been given to the development and improvement of relevant electrodynamic models. Classical model of this kind is considered a wedge (edge). In this model, infinite boundaries are accounted for using the results of a physical theory of diffraction, and the fields and currents in a finite region of space near the edge are defined by solving the equations of the electromagnetic field. Select individual scattering centers can also analyze specific diffraction phenomenon peculiar to this structure, and identify their dependence on a small number of local parameters.
role of numerical methods exclusively large when the characteristic size of the fragments of the object is much larger than the wavelength, but because of the complexity of the diffraction process high-frequency approximation do not provide the desired accuracy. Such a fragment, creating the main contribution to the scattered field in the forward hemisphere plane intake is the power install the system blade inlet of the engine. Using the hybrid approach allows to consider whether the passage of electromagnetic waves via the air intake and diffraction on a system of blades. However, the calculation of the scattering wave at the inlet of the engine having a transverse dimension of tens of wavelength and consisting of many elements of complex shapes - the independent task that requires the use of rigorous methods and their effective numerical implementation. The developed software allows you to perform the appropriate calculations with sufficient accuracy (Fig. 4).
Fig. 4. Model inlet plane (a) and diagram of the effective surface scattering intake (b)
Comparison calculated and experimental data shows good accuracy of the software
creating an adequate electrodynamic model such a complex object like an airplane, requires a combination of methods of Radiophysics, electrodynamics, computational mathematics, signal processing. In result of extensive experience in programming and computational tasks of the technology effective use of modern computing technology. In particular, the importance of the integration of specialized software products: the computational core, designed to determine scattering properties, and engineering systems design, through which the transfer of design documentation (electronically) Preparation and modification of data on the form, structure, facility, the analysis output. Widespread system of AutoCAD, from which the data are transferred in electromagnetic simulation models through a special interface.
In the short term progress in the development of methods for calculating the scattering properties of complex systems is largely due to good organization works to raise the necessary specialists, good information and technical support, since it seems to us, many fundamental issues that existed previously in this area have been resolved.
inflicted on the various elements aircraft radio-absorbing coating differ from those used 10-15 years ago. Modern coatings have variable thickness along the profile, complex structure with varying values of permittivity and permeability both in thickness (normal to the surface) and along the surface sheathing. Solving the fundamental problems associated with the passage of an electromagnetic wave through a heterogeneous environment, studying the behavior of heterogeneous mixtures near the percolation threshold, the researchers learned how to create a substance with any frequency behavior of the real and imaginary parts of permittivity, Naturally, under the principle of the Kramers-Kronig relations. Creating materials with an arbitrary behavior of the magnetic permeability is associated with some additional constraints. Modern Designer, optimizing the EPR of a fragment of the plane and choosing for this purpose, the dielectric and magnetic permeability substance, the distribution of its thickness over the surface of the fragment can be sure that the technology in the future will reproduce the desired settings. Note also that an appropriate optimization requires significant prior experience, because of the need continuously to find a compromise between the desire to obtain the minimum reflection, the thickness of the material, its strength and, finally, the aerodynamic properties of the fragment.
For application coatings are used completely different technology. Most of the surface plane is covered by paint technology. Despite the apparent simplicity, this technique is most responsible, because of its quality depends on the contribution to the ESR of one of the key elements of the scattering plane - its air intake, In addition, the violation of coating adhesion in the air intake can lead to a plane crash. Multilayer coating radioabsorbing sprayed with as robotic systems for serial production of aircraft, or manually when you create experimental models of equipment.
daunting goal is the production of fine ferromagnetic fillers with the given values of magnetic permeability. Currently, applying specific technology, we can fairly wide range, changing the spectra of ferromagnetic fillers in the microwave region. Combining the ferromagnetic inclusions with inclusions of other types, create the desired set of broadband radar absorbing coatings. Their further development is impossible without solving many fundamental issues.
The first step is to study in detail the factors that determine the value of the permeability of fine particles and thin magnetic films, which constitute the basis for a radar absorbing coatings. These factors include: the effect of mechanical stresses that form the magnetostriction of the samples; effect of small amounts of ferromagnetic materials, resulting in a change in its magnetic anisotropy; flow at high frequencies, the relaxation processes in the created structures, characterized by the damping parameter in the Landau-Lifshitz-Gilbert. In some cases, improve the radio-absorbing coating may be using ferromagnetic conductive particles of very small size. However, in this way appear two principal obstacles: first, these particles become piroformnymi (probably - pyrophoric - VV ), and secondly, they could not enter with a sufficiently high density of the liquid polymer matrix due to increased adsorption capacity with decreasing particle size. This well-known chemical problem to still not resolved.
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