Technical and strategic analysis of the differences between active and passive stealth on recent fighter aircraft.

The concept of stealth

Stealth is a tactical function designed to delay the detection of a stealth aircraft by enemy sensors. It does not make an aircraft invisible, but reduces its observable signatures: radar, infrared, acoustic, and electromagnetic. Stealth has been a requirement in the design of modern fighter aircraft since the 1980s, starting with programs such as the F-117 Nighthawk.

Two main technical approaches coexist: passive stealth and active stealth. The first consists of reducing the reflection of energy emitted by enemy systems (radar, thermal sensors). It relies on specific aerodynamic shapes, absorbent materials, and internal equipment. The second acts directly on the signature perceived in real time, using electronic means or dynamic countermeasures.

The equivalent radar signature (Radar Cross Section, RCS) of a standard aircraft such as a Su-30 can exceed 10 m², while that of an F-22 Raptor is estimated at less than 0.001 m². This gain is mainly due to passive techniques. However, in the face of modern multi-band sensors, this passive reduction is no longer always sufficient. Active stealth therefore provides means of interference, electronic absorption, or adaptive jamming.

In a networked warfare environment saturated with radars, satellites, and data links, geometric or thermal reduction alone is no longer sufficient. It is now necessary to actively jam, deceive, and manage one’s signature. This requires greater technical sophistication, but also higher operational, logistical and strategic costs.

Passive stealth based on restrictive physical principles

Passive stealth is based on three fundamental principles: reducing radar reflection, limiting infrared signatures, and avoiding uncontrolled electromagnetic emissions.

The most important factor is the shape of the aircraft. The angles of the surfaces are designed to reflect radar waves away from their source. This requires compromises in the airframe design: angular fuselage, slanted tail fins, and concealed air intakes. Aircraft such as the F-117 and B-2 Spirit illustrate a geometry based solely on radar deflection, at the expense of maneuverability.

Radar Absorbent Material (RAM) complements the shape. These complex coatings absorb some of the radar energy, reducing the overall signature. The coating on the F-35, for example, incorporates several composite layers with conductive fillers made from special polymers. These coatings are difficult to maintain. A damaged surface increases the RCS. The cost of a square meter of RAM often exceeds $8,000.

The highly reflective air intakes and compressors are concealed by S-shaped grilles and diffusers. The nozzles are often flattened and extended by cooling structures to limit the infrared signature.

Finally, electromagnetic behavior is controlled. Stealth aircraft cut their active radar emissions in passive mode. Data links are routed through directional channels or via discreet relays such as relay drones.

But this passive approach has its limits. It works in certain radar bands (X, C, S), but becomes less effective against low-frequency radars such as VHF radars. The latter perceive the entire airframe, without relying on local reflection. In addition, a passive stealth aircraft remains visible if its temperature is high or if it is flying at high altitude in clear skies.

Active stealth that actively manipulates enemy perception

Active stealth involves altering the way the aircraft is perceived in real time. It relies on several technologies: electronic jamming, active radar cancellation, infrared countermeasures, and even electromagnetic camouflage.

The most advanced form is active radar signal cancellation. The principle is to capture the incoming radar wave, calculate the expected return, and then broadcast an opposite signal, partially annihilating the radar echo. This system, tested on some American experimental drones, requires very detailed knowledge of the frequency, phase, and origin of the transmission. It is only effective on targets that are limited in space and spectrum.

Another application is signature decoys. Certain drones or missiles (such as the MALD-J) simulate the radar or thermal signature of a stealth aircraft, forcing the enemy to engage a fictitious target. The F-35 can coordinate several drones to create a “ghost cloud,” dissipating enemy surveillance.

Onboard jammers (such as SPECTRA on the Rafale or AN/ASQ-239 on the F-35) operate adaptively. They analyze enemy emissions and generate targeted jamming. This can be noise, directional jamming, or phase modulation, rendering the signal unreadable.

Active stealth can also extend to the infrared domain. Dynamic engine cooling, variable geometry deflectors, or IR extinguishing systems (as on the Su-57) reduce thermal contrast. Some programs, notably American and Israeli, are also exploring variable refractive index materials capable of diffusing heat in wavelengths that are difficult to detect.

However, these active systems require significant energy management, increase the electronic load, and pose detection risks. Their effectiveness depends on the quality of onboard intelligence, timing, and coordination with other sensors.

A strategic trade-off between cost, effectiveness, and doctrine

The choice between passive stealth and active stealth is not based solely on technical considerations. It reflects the doctrine of each air power.

The United States has historically invested heavily in passive stealth. The F-22 Raptor, F-35 Lightning II, and B-21 Raider prioritize signature suppression from the design stage. This allows for deep penetration missions without turning on active systems, reducing the risk of detection from the outset.

This choice comes at a cost. An F-35A costs around $90 million, a significant portion of which is related to materials and airframe processing. The MCO (maintenance in operational condition) of a passive stealth aircraft is more complex: each flight requires precise inspections and even refurbishment of the coatings.

Russia, on the other hand, combines partial passive stealth with active countermeasures. The Su-57 Felon has a semi-stealth airframe (RCS estimated at around 0.1 m²), but incorporates a wide range of jammers, active decoys, and electronic warfare relays. China is taking a hybrid approach: the J-20 relies on a stealth airframe, but compensates for its passive shortcomings with advanced electronic systems.

Active stealth offers operational flexibility. It allows the level of discretion to be adapted to the threat and deceives the enemy without grounding the aircraft for maintenance. However, it cannot replace a discreet design, especially against multi-band radars or long-range passive sensors.

In the long term, sixth-generation aircraft will incorporate dynamic signature management. They will be able to alternate between passive reduction, controlled emission, and active jamming, depending on the mission phase. Stealth will no longer be a fixed characteristic, but a variable flight parameter.

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