SKALSKI Paweł1
KRUPSKI Jarosław2
Stealth technology. Yesterday, today and tomorrow
INTRODUCTION
Stealth or low-observable is a military concept often misunderstood by civilians. Aircraft
incorporating this technology are often believed to be invisible to the enemy. In fact, stealth only
reduces aircraft visibility. What is more, the technology is used not only in airplane constructions, but
in many military projects, starting with previously mentioned aircraft e.g. F-117 A; ships e.g. Swedish
Visby class corvette; helicopters e.g. RAH-66 Comanche; submarines e.g. German Type 212; and
ending with subsonic missiles e.g. AGM-129 ACM.
Stealth technology includes various methods to make object less visible (detectable) from radar,
and other sensors. This technology, if on high level, provides significant advantage over an opponent
by enabling the user to carry out striking military attack with increased survivability. Stealth is not a
single technology. Under this term, there are hidden various options for making aircraft, or other
objects, low-observable. Stealth is exploited to camouflage objects, absorb or reflect some of
incoming radar waves, reduce the IR (Inherent) signature, or to avoid LIDAR (Light Detection And
Ranging). Stealth technology is also used to reduce submarines’ acoustic and magnetic signatures [1,
2].
1
CURRENTLY EMPLOYED STEALTH TECHNOLOGIES
Various stealth technologies have been developed, since there are multiple ways to detect friendly
unit and there are of many different vehicle types. I have focused on the basic types of stealth and the
prime methods of making them low-observable. The following chapter is based upon Ashish Z. work
[1].
1.1
Radar Stealth
The main mission of this technology is to reduce RCS (Radar cross section). Two techniques
described in this publication are developed to minimize RCS. One is based on vehicle shaping,
restructuring the frame and changing geometric design, to achieve the desired RCS level. The other
refers to „radar absorbent materials”, and is linked with designing materials capable of reducing the
reflectivity of the airframe. Subsequently, effective structures, known as “Radar Absorbent
Structures”, needs to be created.
Vehicle Shape is a basic aspect of every construction. It turns out that it also has huge impact on
low-observable technology. Since radars were first invented they have had a huge impact on the
progress of battle. For example, in the Battle of Britain, engineers recognized the need to reduce radar
cross-section of aircraft. Since at least 1960's it has been known that vehicle shape changes
detectability. The British Avro Vulcan bomber, had an incredibly small appearance on radar, despite
its large size. Besides eliminating propellers in a stealth airplane, the design of the internal
construction is vital.
The idea of re-entrant triangles under the skin of aircraft was first used in the F-117.
Radar waves penetrating the “skin” of an airplane get trapped in these structures and bounce
inside, losing energy. Another patent involves tilting the tail surface to reduce corner reflections that
occur between them. Going further, the best effect is achieved when the tail is eliminated completely,
as in the B-2 Spirit (Fig. 1).
1
Instytut Lotnictwa, Centrum Nowych Technologii; 02-256 Warszawa;
Al. Krakowska 110/114. Tel. 22 846 00 11; pawel.skalski@ilot.edu.pl.
2
Akademia Obrony Narodowej; 00-910 Warszawa;Al. Generała Antoniego Chruściela „Montera” 103.
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Fig. 1. B-2 Spirit - fine example of no tailplane aircraft [2]
Furthermore, stealth-designed aircraft must bury elements like the engine and fuselage, and
external weapons or fuel tanks cannot be attached to undercarriage, because such elements reflect
radar waves back very well.
Ships were also adapted to stealth technology. The aforementioned Visby corvette was the first
such ship to enter service. When designing a ship with reduced radar signature, it is crucial to remove
radar beams originating above the horizon. In addition, vertical shapes must be avoided, as they
would perfectly reflect such a beam back to the emitter. The surfaces of stealthy ship should protrude
and retrude slightly.
Adaptive Water Curtain Technology(AWCT)pertains to ships only. AWCT’s main target is to
deflect or scatter radar waves and, as a result, reduce a ship's radar cross section. It uses highconductive sea water to effectively create an angled radar reflective curtain. The process is shown in
Fig 2. The ship command can modulate the water curtain, to appear as “sea clutter”. However, satellite
data and local conditions must be taken into consideration, as trying to mimic sea clutter when enemy
while satellite data shows clear weather would be ineffective.
Fig. 2. How AWCT Technology works [3]
Radar absorbing materials and radar absorbing structures are the second solution to the stealth
problem. Radar absorbing materials (RAM) are used to diffuse the energy of the radar wave,
preventing the signal from returning to the antenna. Usually the radio frequency (RF) energy is
converted to heat. Dielectric and magnetic RAMs are the two main types, including variations of
them in current aircraft design use. However, new composites and smart materials are now being
introduced.
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1.2 Infrared Stealth
Radiation from the aircraft cannot be avoided. The energy is detected by the passive IR sensors.
The only way to defend against sonar is IR reduction, or matching it with the background. There are a
few Infrared Signature suppression systems that could help with the IR problem. Black Hole
Ocarina’s (BHO) IRSS system is an optical blocking solution to obscure hot engine parts, the cooling
exhaust duct and the plume. The system consists of a finned nozzle with internal bends to prevent the
direct view of hot internal exhaust surfaces. The whole system is designed to diffuse the exhaust
plume and reduce the radiation of those plumes. The hot exhaust sucks in the cold air from the engine
compartment, lowering out-coming gas temperatures. Film Cooled Tailpipe (FCT) IRSS system was
designed as a „mission kit”, meaning it's retrofit able on aircraft’s factory exhaust. FCT consists of an
improved nozzle, flow wedge and film cooling slots with cold air entries. This is a passive system
which depends on the pressure distribution along the length of tailpipe to draw in ambient air. The
film cooled tailpipe system is designed for helicopters and requires minimum modifications for
installation. Near-Infrared (NIR) Absorbing materials are organic solids and polymers capable of
absorbing in the near-infrared region (1000-2000 nm). These materials are emerging with great
potential for use in Telecommunications, for example. A few known types of NIR materials include:
fused phorphyrin arrays, semiquinones and mixed-valence binuclear metal complexes. Organic NIR
region materials that are electrically, optically, or thermally active at the telecommunication
wavelengths can be used in a device for optical attenuation, absorption or antireflection, thanks to
their low-cost production and their unique electrical and optical properties.
1.3
Visual Stealth
Can be achieved using camouflage. Great examples of this technology’s use, are the black painted
F-117 and B-2 Spirit designed to operate at night. The design of aircraft camouflage is complicated
because an observer might be located above, or under the airplane. Older camouflage schemes used
counter shading: light color underneath he plane, and darker on top. Some other schemes take aircraft
movement in battle into consideration and apply a chosen pattern to the entire machine. The palette of
colors in such method is a selection of two or three dull, neutral shades. Due to the, modern air
battlefield’s high speed and heavy reliance on radar, the role of camouflage has decreased and tends to
approach electronic „stealth”. Nowadays, paints are designed to be able to absorb electromagnetic
radiation of radar, thus reducing aircraft signature.
1.4
Sonar Stealth
This type of stealth technology focuses on submarine or ship sound reduction, so that the unit
remains undetected. One solution to this problem is using device named Bathytermograph(Btermograph), which records temperatures of the ocean at various depths. B-termograph can be placed
on a ship or submarine. During the measurement process, the ship's (or submarine’s) speed can be up
to 22 knots. When a submarine is submerged, it cannot use the device. Operators of bathytermograph
gather information about where thermo cline is. If a submarine is submerged at the layer of thermo
cline or immediate below it, the machine will remain undetected. However, this occurs only if the ship
uses hull mounted sonar. If tower sonar is fitted, it could be submerged under the thermo cline and
thus detect enemy unit.
Sonar absorber is a submarine covering, able to absorb some of the sonar waves. This kind of
solution presents some technical problems. The major problem is finding material which will have the
requisite impedance and would be enough. What's more, it would need to cover a broad band of
frequencies and boast high hydrostatic strength so that it could be used in deep water. The future of
this technology is composite rigid syntactic foam combined with many fillers and ingredients. A truly
broadband sonar absorber of a high hydrostatic strength can be made by dispersing suitably-sized
elastomeric particles in a syntactic foam matrix.
1.5 Plasma Stealth
This specific kind of stealth technology is based on ionized gas particles. Ab ion is an electrically
charged particle or group of atoms. A plasma cloud’s total electrical charge is zero (quasineutral group
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of free charged particles). The theoretical purpose of using a plasma cloud around aircraft is
theoretically to reduce radar cross section. However, this method may be difficult to implement.
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MATERIALS USED IN STEALTH TECHNOLOGY
First radar absorbing materials (RAM) were designed in 1930's. Material named
“NaamloozeVennootschapMachinerieen” produced since 1936 ,in Netherlands, was a quaterwave
resonant material in the 2GHz region. It consisted of carbon black to achieve dissipation and thallium
oxide, which guaranteed high dielectric constant. The need for better absorbers emerged during WW
II due to radars advancement. The German „Jaumann absorber” was designed with a multilayer
approach. This allowed it to attain reflection coefficients of 20 dB on average, over a wide band
between 2, and 15 GHz near normal incidence.
Meanwhile two types of absorbers named Halpern-Anti-Radar-Paint (HARP) were introduced in
the United States. They provided reflection reduction of 15-20 dB at resonance. Two versions of
HARP absorber were developed. The airborne MX-410 was similar to paint, it could operate in the Xband with thickness of 0.07 inch. The rugged shipboard version consisted of a high concentration of
iron particles in a neoprene binder, and showed similar performance.
In 1952, the now well-known RAM – „Salisbury screen” was developed. The system consisted of
a resistive sheet placed at quarter wavelength from the scatterer, spaced by a low dielectric material.
Another example of widely used resonant absorber was „Dallenbach layer” developed in 1970. It
is formed of homogeneous lossy layer on a metal plate. The thickness of the lossy layer was selected
to match its input impedance with the intrinsic impedance of the free space.
Scientists also came up with the idea to modify the resonant absorbers and invented „circuit
analog” RAM (CA-RAM), which were obtained as Salisbury screen derivatives. They are fabricated
by depositing lossy material in geometric patterns on a thin lossless film. The advantage of CA-RAM
is its relatively low thickness, resulting in better performance.
„Magnetic” RAMlike, „ferrites” (Fig. 3) were broadly explored. They are characterized by low
reflection coefficients and can operate at lower frequencies. Even though they are thinner compared to
dielectrics, they have the disadvantage of higher weight, and are more prone to disintegration at higher
frequencies.
Fig. 3. Example of magnetic ferrite based RAM [4]
In 1980's the huge discovery, that some biotech products have ultra-wide band absorption abilities
was made. Tests conducted on those RAMs showed that significant RCS reduction was possible by
dissolving these compounds in aircraft structural materials. The desired RAM was a retinylcompund,
which is a Schiff-base salt and is much lighter than ferrites.
The 1990's homogenous materials were taken into consideration. Studies resulted in chiral RAMs
development, which depend upon their optical activity and circular dichroism. Also backscatter radar
cross section is independent of polarization. Compared to magnetic and dielectric, chiral RAMs have
enormously better RCS reduction.
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Adaptive RAM or dynamically adaptive RAM are group of newly designed smart materials
capable of absorbing radar waves. One of the ways they deal with radar waves is absorption by
changing sheet impedance. Dynamic absorbers should be studied in order to counter frequency of
swift radars [7].
3
STEALTH TECHNOLOGY RESEARCH METHODS
Static stealth performance comes down to measuring average radar cross section value of the
circumferential area of target, or that of some important radar detecting areas under some important
radar frequencies. Such research is conducted in a controlled environment. According to these
analyses, the stealth aircraft with lower average RCS value has better stealth performance. The
analysis conclusion can just reflect several target RCS scattering characters. The problem is that the
machine with the same average RCS value can have completely different circumferential RCS
scattering characters.
A new approach in this topic is active stealth performance. The research was named „the
integrated stealth performance analysis method”. That kind of tests can provide information about
stealth performance of an aircraft, which carries out missions. This method can reflect various target
RCS scattering characters. The integrated stealth performance method takes the radar dynamic
detecting characteristics and changing rules of the target dynamic RCS scattering characters into
account, so that complete analysis is delivered [9].
However, those two methods may be taken into consideration only with a fully prepared aircraft,
with all stealth techniques applied. It's worth first testing the radar absorbing material that is intended
to be used and machine shape performance separately.
There are several methods of measuring RAMs performance and properties and there are three
forms of material, that can be measured: finished absorber panels or products; thin sheets used as
components in the design; samples of uniform bulk material. I will mention only few important ones.
The coaxial probe method is helpful with isotropic, homogeneous and nonmagnetic materials. It
requires: samples with one flat surface, and no air gaps allowed. Thickness is not limited. The coaxial
probe is fitted for liquids and inviscid fluids. The advantages of this method are a broad band and it is
simplicity and convenience [8].
In the Waveguide system method, reflectivity measurements of finished products, at lower
frequencies can be performed by placing a test sample in the apparatus. Typical frequencies oscillate
between 100 – 400 MHz. The measuring probe is not moveable and the adjustments are made at the
attached shorting stub. The apparatus is capable of a single measurement at a time. Stepped frequency
measurements are possible with additional equipment. A circular waveguide can be used more
effectively than co-axial cables for measuring the plane wave reflection properties of layered
dielectrics [6].
The cavity technique is basically a single frequency method that involves complex analysis.
However it is also suitable for anisotropic materials. In this technique a part of cavity resonator is
filled with the sample, without affecting the field distribution inside. The cavity technique is the most
accurate if we take low loss homogenous materials and liquids under consideration [8].
The free space technique is generally a high frequency, non-contacting and non-destructive
method. Typical samples are of large size, flat, and have parallel faced homogeneous materials. The
Test site is shown on Fig. 4. The measurements themselves are not always performed in a free spacelike environment as the name suggests, but are conducted in the confines of a waveguide, or a cavity.
Errors in free space technique are usually due to: finite sample size, non-plane wave illumination,
mechanical instability, misalignment between sample and antenna, or imperfect quality of anechoic
environment [8].
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Fig. 4. Free space method scheme [4]
4
SMART SKIN
New types of stealth materials are still under research. This kind of elastomeric material has
possible low-observable properties. However more research is required. A short characteristic of this
material is presented below.
A magnetorheological material (a smart skin) is a new development for morphing aircraft concepts.
In the primary phase of the morphing skin development, the magnetorheological material that would
make up the skin or face sheet was fabricated. Initially, a large number of rubber was tested for
viability with a carbonyl iron powder (CIP). Fig. 5 shows a view of a new smart material.
Elastomeric materials are ideal candidates for a smart skin. In morphing applications, where large
shape changes are expected, the design of a suitable skin is a huge challenge and a key issue. The skin
must be soft enough to allow shape changes, but stiff enough to withstand aerodynamic loads and
maintain the required shape/ profile. This requires thorough trade-off design studies between the
required loading scenario and the desired change in shape (one-dimensional or multi-dimensional).
Magnetic investigations of a magnetorheological smart skin were performed on the LAKESHORE
vibrating sample magnetometer (www.lakeshore.com). The range of a magnetic field was between 0
– 1600 [kA/m] (2 [T]). Presented results of experiments (Fig. 6) shows that magnetic properties of a
magnetorheological skin can be controlled.
Fig. 5. View of a smart skin.
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Fig. 6. Results of magnetic investigations of a smart skin.
Results obtained from experimental research carried out for the magnetorheological material
(smart skin) revealed that there is considerable influence of a magnetic field on properties of a smart
skin. Because these changes are entirely reversible, it is possible to use a magnetorheological material
as “intelligent” material - part of a morphing structure. Also, such results imply the need for tests of
the radar absorbing capability of the smart skin.
CONCLUSIONS
This publication covers the basic information about stealth. The knowledge presented in this paper
include technologies used to hide, or make units less visible for radars. Methods covered in this
publication are still in development, and the military has likely only revealed technologies that are
outdated to them. However, engineers have a wide field to develop new technologies thanks to their
research. The most rapid segments of stealth technology explored by civilian scientists are radar
absorbing materials. Nowadays, research is aimed at polymers, especially those designed in smart
technology, that can change their magnetic or other properties.
Results obtained from experimental research carried out for the magnetorheological material
(smart skin) revealed that there is considerable influence of a magnetic field on the properties of a
smart skin. Because these changes are entirely reversible, it is possible to use a magnetorheological
material as “intelligent” material - part of a morphing structure.
Abstract
Stealth technology continuous to grow in significance with the development of radars and an ever changing
battlefield. Nowadays, not only aircraft, but other types of units like helicopters, ships, land vehicles,
submarines and missiles are constructed with low-observable technology. This paper covers the most important
methods of making objects less visible and less-detectable. The paper will give a short description of radar,
infrared, visual, sonar (acoustic), and plasma. Radar absorbing materials, the most researched element of
stealth technology, still present enormous potential. Authors of the paper presented a magnetorheological
elastomer as a future material in the stealth technology.
Technologia stealth. Wczoraj, dzis i jutro
Streszczenie
Znaczenie technologii stealth ciągle rośnie wraz z rozwojem radarów i zmieniającym się polem bitwy.
W dzisiejszych czasach nie tylko samoloty, ale inne jednostki takie jak: śmigłowce, statki, pojazdy lądowe,
łodzie podwodne i rakiety są zbudowane z myślą o niskie wykrywalności. Praca ta obejmuje najważniejsze
sposoby wytwarzania obiektów mało wykrywalnych. W skrócie przedstawiono opis radaru, podczerwieni,
techniki wizualnej, akustycznej (sonar) i plazmy. Materiały pochłaniające fale radarowe, wymagają ciągłych
badań. Autorzy pracy przedstawiają elastomer magnetoreologiczny jako materiał w przyszłościowym
zastosowaniu w technologii stealth.
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REFERENCES
1. Ashish Z., Stealth Technology seminar report, Government Engineering College, Barton Hill,
Thiruvananthapuram, 2009.
2. B-2 Spirit - …, http://www.globalresearch.ca/the-usaf-b-2-spirit-stealth-bomber-an-instrument-ofpeace/24243, 18.07.2013.
3. Example of magnetic ferrite based RAM, http://www.f-22raptor.com/st_getstealthy.php,
18.07.2013.
4. Free space method scheme, http://www.ndt.net/article/wcndt00/papers/idn251/idn251.htm,
18.07.2013.
5. How AWCT Technology works, http://www.williamson-labs.com/ltoc/ship-stealth.htm,
18.07.2013.
6. Rudduck R.C., Yu C.L.,Circular waveguide method of measuring reflection properties of
absorber panels, IEEE Trans. Antennas Propag. AP-22: 251-256, 1974.
7. Saville P., Review of Radar Absorbing Materials, Defense R&D Canada – Atlantic, 2005.
8. Vinoy K.J., Jha R.M.,Trends in radar absorbing materials technology, Computational
Electromagnetics Lab, Aerospace Electronics & Systems Division, National Aerospace
Laboratories, Bangalore, 1995.
9. Ying L., Zhe W., Peilin H., Zhanhe L.,A New Method for Analyzing Integrated Stealth Ability of
Penetration Aircraft, Beijing, 2009.
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