Furtivity

History

Aeronautics

Until the end of the Second world war, before the invention of the Radar, the Avion S could be detected only at sight and the sound. The furtivity consisted in painting the aircraft with colors enabling him to melt itself in the environment: camouflage of color brown-green or brown-yellow on the top of the apparatuses used above cultivated or desert zones and gray painting or gray-blue below.

The improvement, for the period of the Cold war, of the means of electromagnetic detection then infra-red involved the need for decreasing the emissions in these two frequency spectrums.

The first aircraft designed right from the start to be most discrete possible, therefore furtive , and built in series are the fighter-bomber F-117 and the strategic bomber Northrop B-2 Spirit developed during the years 1970.

All the aircraft of military use are now designed with for objective obtaining a signature the radar and infra-red weakest possible.

The furtivity radar is characterized by a figure: the equivalent Surface radar (SER). It is the value of the plane surface which would return same energy as the aircraft. In the absence of treatment the SER of a plane is about several tens of m; after treatment one obtains values about the m even less. Certain planes such as the F-22 of USAF would have SER equivalent to those of the birds but the real figures are obviously held secret by the manufacturers.

Technique

Aeronautics

There do not exist means of removing the electromagnetic or thermal energy emitted or re-emitted by the plane. The furtivity thus consists in absorbing this energy (makes some convert it into heat) or returning it in a privileged direction or distributing this energy in all the directions. The choice depends on the profile of mission of the apparatus (penetration very low altitude or flight with very high-altitude in the case of a plane) and thus of the relative position of the threat compared to the apparatus. It also depends on the type of threat, for example in the case of the radar of the frequency or the waveband of detection or its distance.

The principal treatments used are:

of a geometrical nature : suppression of the shapes in dihedron which concentrate energy and replacement by round forms (diffusion of energy) or by plane forms (emission in a privileged direction); their goal is to return the waves radar in other directions that those of the transmitter.

use of materials absorbents (Radar Absorbs Materials). Received energy is converted in another form (heat), which limits the power of the considered waves radar. These materials are Isolants which have dielectric or magnetic properties specific. They are quantified by their electric Permittivité (noted $\ epsilon$ ) or them magnetic Perméabilité (noted $\ mu$ ). These parameters, which depend on the frequency, are Complex numbers (with the mathematical direction, i.e. one represents them with a real part and an imaginary part). The imaginary part represents the losses, therefore the incidental electromagnetic energy transformation into heat.
of the active devices is also being studied, it acts to treat the signals received to return them in a form which will not allow the system detection to identify the threat. These means are closer to the techniques of deception than of the furtivity.
jamming , by disseminating metal objects (spangles, corners, etc) aiming to create additional echoes and to draw the attention of the radar to those (especially for the embarked radar of a missile) and to make him lose the track of its target.
reduction of the thermal signature on combat helicopter (standard Tiger): the exhaust fumes at exit of conduit are first of all mixed (under a cap called diluter/thrust reverser , DDJ) with cold air collected in a dynamic way by separate air intake of those of the engines then directed upwards vis-a-vis the breath of the principal Rotor which dilutes them once again.

Of course, the development of the techniques of furtivity has as a corollary of new developments of the techniques of detection. One among it consists in designing a radar multi-transmitter (passive radar) to even use the waves of the multiple transmitters existing (television, etc).

Warship

At the end of the years 1980, studies related to the Bateau X furtive. Most known is the Sea Shadow , furtive catamaran developed by Lockheed Martin Missiles and Space in 1985. Of a 50 meters length, it can reach the speed of 13 knots. France has since 1997 its furtive frigates, of Classe Fayette. In both cases, the reduction in the SER is obtained by the use of large metal or covered plane plates absorbing painting, which return the electromagnetic energy of the radar in other directions. They are directed to as well treat the radars of other ships as those of planes or air-to-sea missiles. The use of electronic devices is possible but no information on this subject is available.

Submarines

The nuclear Sous-marin S launchers of machines constitute an excellent example of furtivity, their principal function being to remain undetectable several weeks or several months of continuation.

The electromagnetic furtivity (including thermics and optics, which is electromagnetic demonstrations at particular frequencies) is simply obtained by the maintenance of the submarine in immersion, the sea water not making it possible the electromagnetic waves to be propagated with more few meters, and by the complete absence of indiscretions (radio transmissions, radar, use of the periscope, etc).

The acoustic furtivity is obtained by the combination of several techniques.

The furtivity with respect to the active sonars is obtained by the use of anechoic coatings.

The reduction of the noises of flow is obtained by a perfectly shaped hull where all the equipment of constraint is integrated into the profile or is masked under trap doors themselves integrated into the profile.

The reduction of the noises of propulsion was initially obtained by the multiplication of the number of blades of the propeller and the optimization of their form (aiming to decrease or cancel the very noisy phenomenon of Cavitation), then by the use of a ducted propeller (propeller pump). The electrical motor, directly coupled with the driveshaft, is by itself considerably quieter than a diesel or a turbopropeller and moreover does not require a reducer whose toothed wheels would have a characteristic acoustic signature.

The noises coming from the interior of SNLE are reduced on the one hand with the source, on the other hand by preventing them from exciting the hull:

preferential use of the D.C. current compared to the alternative course,
cooling of the electronic devices by thermal drains cooled by a slow water circulation rather than of the radiators cooled by ventilators,
suspension on silent-blocks of all the equipment likely to emit vibrations,
no equipment is directly fixed at the hull: all the external equipment (antennas, etc) is assembled on silent-blocks, all the internal equipment are fixed on one of the bridges of the ship,
all the bridges of the ship are suspended on the hull by a small number of giant silent-blocks,
a monitoring system of the acoustic state makes it possible to detect any vibration which reaches the hull and to locate its origin,
…

Particularly quiet razors would even have been developed especially for the crew of these submarines.

The result is a ship quieter than the background noise of the sea per good weather.

Disadvantages and compromised

The furtivity has a direct cost of study, very high development and production. The B-2 Spirit costs approximately 2 G€ is the price of 12 B-1B which integrates to him also elements to reduce its signature radar but to a less level. The program of helicopter of furtive attack RAH-66 Comanche was abandoned following the inflation of the budget necessary to its realization.

It requires a severe maintenance : paintings and the coatings of the apparatuses must be maintained and changed regularly to remain effective. Those are very fragile, and the bad weather, for example, can compromise the furtivity even during a mission.

It has also an indirect cost on the operation of the aircraft and ships. The shapes in facets of the planes, for example, degrade the performances Aérodynamique S, outcome with planes slower, flying less higher, manoeuvrable, consuming more Kérosène for a weaker payload. The forms in facets of the dead works of the corvettes and furtive frigates make the bridge unusable for handling of the tackle which must be done starting from windows of reduced size, and induce important constraints on the geometry and the positioning of the air ones.

But the indirect cost can even result in conflicts between several forms of furtivity. The shape in facets could be used to reduce the signature of the submarines with respect to the active sonars, but it would result in such a fall of the navigability and especially such a degradation of the signature with respect to the passive sonars which it is never used. More astonishing, the capacity to reflect very little energy, important against the active sensors, now reached a stage where the submarines and aircraft can be betrayed by the hole that they cause in the background noise recorded by the passive detectors.

External bonds

National office of Studies and Aerospace Research: Furtivity of the military aircrafts
'' the end of the furtivity is only one matter of time after a British discovery '', published the 17.06.01

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