Flares

A flare is an IR (Infra-Red) countermeasure that is commonly used to protect target platforms against IR heat-seeking SAM (Surface-to-Air Missile) or AAM (Air-to-Air Missile) threats (see Figure 1).

It is not only employed to protect aircrafts, such as airplanes or helicopters, but also naval platforms from anti-ship threats.

As an expendable decoy, it works according to one of the following operating strategies:

• Seduction.
  Aim of the decoy is to disengage the IR tracking sensor of the threat by creating a “false target” that is more attractive for the missile seeker than the IR signature of the target platform itself.For reaching this goal, the IR radiation of the decoy must have radiant characteristics as much as possible similar to the platform signature, but a higher intensity.
• Distraction.
  Aim of the decoy is to be acquired by the threat tracking system before the seeker is able to acquire the platform target.
  For this reason, in order to be effective, one or more decoys are required to be launched before the target is acquired and tracked by the threat.

Figure 1: A C-130 Hercules deploying flares

• Dilution.
  Aim of this technique is to employ several decoys in order to force the threat seeker to attack all the potential targets in front of it.
  To reach this goal several decoys, with the same signature of the platform to be protected, are required to be ejected and persist simultaneously for long time.

A typical flare (Figure 2, left) is composed of a cartridge case, which protects the flare before the ejection and acts as launching tube, an ejector charge, which allows the ejection of the flare from the dispenser, a bore safety and initiator, which ignites the pyrotechnic pellet after the ejection and a closure cap, which seals the case.
The pellet has in charge the generation of the IR radiation by means of pyrotechnic or pyrophoric charges.

Figure 2: Components of a conventional flare (left) and of an aerodynamic flare (right)

The higher is the burning temperature, the higher is the energy radiated by the flare.
A small object (e.g. the decoy) with a higher temperature, or a higher emissivity, can radiate the same amount of energy of a larger body (e.g. the target platform) at a lower temperature.
However, objects with higher temperatures have a different spectral distribution: the higher is the burning temperature, the higher is the signature contribution at the lower wavelengths (Figure 3).
The misalignment between the decoy spectral distribution and the target could cause the decoy to be rejected by some threats that are able to recognize the anomaly associated to an unrealistic target high temperature.
Some other threats could also be able to recognize and reject the decoy for its shape or position with respect to the target.

Hence, several parameters need to be taken into account in order to properly evaluate the effectiveness of a flare:

• Peak Intensity.
  The decoy must radiate the sufficient amount of energy, within the wavelength region of interest, in order to be more attractive than the target and seduce the threat tracking system.
• Rise Time.
  After the ejection, the decoy must reach the required radiant intensity as early as possible, before it leaves the seeker FoV (Field of View).
  The high aerodynamic decelerations, to which the decoy is subjected immediately after the ejection, could cause the flare to reach its intensity nominal value when it is too far from the launching platform.
  However, the decoy cannot reach the peak radiant intensity too rapidly because some threats are able to distinguish a flare if its radiant intensity increases unrealistically fast.
• Spectral Characteristics.
  Most common flares radiate as a black body (or grey body).
  However, their spectral distribution differs from the target platform to be protected, in particular if the decoy has a higher temperature.
  As already stated, some threats are able to detect this deviation, so the spectral ratio must be kept under control during the combustion to be effective against this kind of threats.
• Function Time.
  In order to reduce the risk of target re-acquisition, the decoy must persist as long as possible.
  The minimum required time is evaluated by taking into account the time frame after which the target is no longer in the threat FoV.
• Ejection Velocity.
  A seductive decoy must arise where the threat is able to acquire it (inside its FoV), but, at the same time, generate a separation rate such as to ensure a sufficient miss distance for the safety of the target platform.
  However, this separation rate must be within the threat bearing and FoV limits, otherwise the missile will not be able to follow the decoy.
• Aerodynamic Characteristics.
  The separation between the decoy and the target platform is toughly affected by the aerodynamic stresses after the ejection. Therefore, aerodynamic parameters must be carefully taken into account, in particular those relevant to target separation and flight trajectories.
• Launch Technique.
  Besides the above-defined parameters that characterize the single decoy, also the launch technique plays a fundamental role to achieve the deception of the threat.
  In particular, the following parameters may affect the effectiveness of the technique: number and typology of ejected flares, ejection timing and direction of ejection.

Figure 3: Comparison of generic MTV flare and aircraft radiant intensity relative magnitude in SWIR (α band) and MWIR (β band)

During the years, the flare industry has undergone a continuous technological evolution that has led to the availability of several types of IR flares:

• MTV (Magnesium/Teflon/Viton) flares, the simplest, but at the same time the most common.
  They burn at very high temperature generating a strong radiant intensity.
  However, the spectral distribution shows a peak in the SWIR (Short-Wavelength IR) wavelength and a lower contribution in the MWIR (Mid-Wavelength IR) wavelength that can be detected by some threats.
• Spectral flares, having a spectral ratio more similar to the flying platform signature, unlike the MTVs that show a spectral distribution similar to a blackbody.
  This feature allows spectral flares to increase the effectiveness against advanced dual-colour threats that are able to discriminate false targets with unrealistic spectral distributions.
• Aerodynamic flares, unlike typical flares that rapidly separate from the platform due to the drag forces, are characterized by an aerodynamic profile and by deployable spring actuated fins that stabilize the flight trajectory of the decoy.
• Propelled flares, aerodynamic flares equipped with a small thruster in order to make the flight trajectory of the decoy even more similar to the flying platform ones.
• Pyrophoric flares that are based on the use of pyrophoric materials that ignite spontaneously in air.
  These decoys have an increased radiating area, compared to the classical flares, and emit a radiation similar to the classical jet engines.

Standard flares can be found in square calibres (1x1x8, 2x1x8, or 2×2.5×8 Inch) or cylindrical calibres (2.5, 1.5 or 1 Inch).