Stealth technology is a wing of military tactics, which encircles a range of technologies, mainly used by bomber aircraft to make them less glimpser (ideally invisible) to radar, infrared and other detection methods. It is a concept of camouflage i.e. illusion which tends to make an aircraft undetectable by the radar-based ground station.
The history of stealth technology seems quite fascinating. In 1956, American intelligence started to send Lockheed U2 spy planes in Russian territory for extensive photographs of soviet land. The plane flew above 80,000 ft. so that it would be out of range of anti-aircraft fire.
Later on, it became known to the American intelligence that the Russian radar was unable to detect the presence of Lockheed in their skies. After 4 years of on its job, the Russians shot the plane after they detected it with new radar equipment.
But the U2 didn’t receive a direct impact; a missile exploded nearby it, forcing the fragile plane to an uncomfortable dive, and hence be crashed. Successful mission of U2 gave a pathway to a confidential research, termed as stealth. The plane was gone invisible for so long because of its non-metallic structures which would absorb the radar waves rather than reflect back to base radar station. So by then, under the shield of secrecy, stealth technology development began to start.
So how this technology works???
The main aim of this technology is priority to aircraft or Warcraft bombers invisibleness. The success of this technology depends upon several modifications, structural and avionics changes in an aircraft. It comprises technologies, those achieve to reduce the aircraft detection, mainly by Radar Cross Section Reduction (RCRS), along with infrared signature reduction, acoustics, and other methods.
RCRS: It involves the modifications in shape or structure of an outer frame of an aircraft to make it undetectable. The main principle in radar detection is that the signals imparted by radar strikes the aircraft but gets absorbed or either reflected at a certain angle, such that the reflected waves do not succeed in returning back at the desired frequency to radar ground station or aerial radar station.
Stealth aircrafts have skin that is transparent to radar or highly absorbing, behind which there are structural components termed as reentrant triangles. Waves through the skin of aircraft get trapped in these triangles, reflecting continuously in a loop within it and hence losing energy. Energy lost waves have long wavelengths and hence they cannot penetrate back because of their low frequency. This was first practiced by the ‘Blackbird series’.
Corner reflectors, formed by the orthogonal metal plates such as corner section at empennage at intersection of vertical and horizontal stabilizer tails reflect the waves back to the radar station. So the stealth design modifies this configuration by tilting the tail sections to avoid corners (practiced by Lockheed F-117).
In addition to altering the tail, this design tends to merge engines within the wings or fuselage of aircraft. In the context of stealth extent aircraft, baffles are installed in air inlet, such that compressor blade motion are not detected by radar.
The leading edges of wings and tail edge planes, air intake doors and air refueling aperture are set at certain angles. This produces a ‘glitter effect’, which returns radar signal in a very determined and distinct direction, such that the signals are out of radar detectable radius. This was first practiced by F-22A Raptor. Engines and fuselage cutouts are severely reduced.
Another prior contributor to the stealth mode design is Radar Absorbent Materials (RAM). Current technologies use dielectric composites and metal fibrous elements made up of ferrite isotopes. RAM, also called as paints, absorbs the energy from radar signals and convert to heat rather than reflecting back.
Paint deposition makes pyramid like structures on reflecting out frame of the aircraft while the gaps between the pyramids are filled with ferrite based RAM. The maze of pyramidal structures deflects the initial signals and multiple collision of waves within the pyramids makes them lose energy in the form of heat. The most common material used is ‘Iron ball paint.’
Alongside with RCRS, an infrared signature reduction is also the main constituent in stealth mode design. The exhaust plume of gasses from engine contains a distinct heat signature, which is thus reduced by minimizing the exhaust cross-sectional area and optimization in a mixture of hot plumes of gasses with cool ambient air by the use of slit shape tail pipes rather than circular cross section.
Deliberate injection of cool ambient air into engine exhaust through a forced bypass of air boosts the IR signature reduction. (Used in Lockheed F-117 nighthawk). Somehow, the engine exhaust is channeled above the aircraft wing surface with proper thrust vectoring.
This prevents visual observation by an observer below and sometimes the fuels are used as coolant i.e. fuel is circulated through the exhaust pipe, fuel tank contributing as a heat sink which is thus cooled by air flowing over the wings.
Onboard radiation of any forms of energy (VHF, UHF, radio signals etc.) that can be detected is highly reduced in stealth mode design. So stealth design communication and navigation works on basis of using passive infrared signals, low light television sensor system to track the target (used by F-117) and LPI onboard radar to detect foreign aircrafts (used by F22 raptor).
Alongside with these technologies, other detectable categories like acoustics, vibrations, maneuvering etc. are highly made depressed with the use of respective false radar techniques.
In spite of the superiority of stealth mode designs, it has greater flaws too. The main limitation resides in banking of aircraft, where several out frames of aircraft and structural changes have the chances of reflecting a loom of radar signal back to ground radar station. The shape requirements directly proportionate in poor aerodynamic and instability of plane of which an example is F-117.
As per to conclude the article, stealth technology is a prior field of military under which several R&D projects are still in continuum for maximization of stealth design and achievement of technical invisibility of aircrafts.
Manish Koirala (069/Bachelors in Mechanical Engineering)
Final year elective subject : Aeronautics