PhLAME – Pyrotechnic Hybridized Layouts via Advanced Manufacturing of Energetics
Introduction
Military aircraft uses decoy flares to counter incoming threat missiles. Improvement in sensors and guiding mechanisms of missiles requires countermeasures to become more sophisticated to mislead them. One approach to enhance flare performance is through the introduction of multiple spectral characteristics in one device. Such functionally graded flare grains would have the ability to generate specific electromagnetic signatures in time and space.
This project was focused on the use of Additive Manufacturing for the fabrication of hybridized flare grains. The approach involved modification of currently used pyrotechnic compositions to allow their processing via AM as well as the design of smart nozzle geometries to allow fabrication of core-shell flare grains. The developed material/printer combination would allow for the fabrication of selectively tuned spectral signatures and inside-out burning characteristics that are not attainable by traditional pyrotechnic pellet production methods such as melt cast, melt cure, and powder pressing. The developed technology would allow for precise control of material layering and electromagnetic signature as a function of time and ultimately transform the performance of decoy flares.
Let’s talk about flares
Infrared and radar countermeasures such as flares and chaffs use pyrotechnic and pyrophoric compositions to generate heat, light, sound, gas, and smoke to counter and distract infrared and radar-guided missiles away from their targets. These flares are housed in aluminum-based cartridges with an electrical impulse ignition at the top of the flare. In particular, pyrotechnic flares work by briefly emitting a higher output of thermal energy (2000-2200 K) and light than the engine of the aircraft it was launched from. The generated thermal energy then causes the molecules to release photons in the form of mid-range infrared emissions (IR) as they fall from excited vibrational quantum states. This IR radiation is a form of invisible light in the maximum range of 1.5-1.3 µm and is what “heat-seeking” anti-aircraft missiles track. Simply put, the hotter the flare countermeasure burns, the greater its IR output will be, thus increasing the likelihood that a missile will be led astray from its initial aircraft target.
Pyrotechnic compositions are produced in various formats such as flares, flash powder, gunpowder, solid propellants, smoke compositions, delay compositions, etc. depending on the targeted application. The main components of the pyrotechnic composition are fuel and oxidizer.
Fuels are used in various formats such as metal powders, metal hydrides, non-metallic inorganics, and organic polymers and resins. Metallic fuels based on metal nanoparticles are most commonly used in defense countermeasures, with a larger surface area to volume ratio resulting in a faster reaction and a faster-burning composition. Fuels undergo non-detonative and self-sustaining exothermal chemical reactions to generate the necessary signatures for the IRCM and RCMs.
Oxidizers are also used in various formats with perchlorates, chlorates, nitrates, permanganates, chromates, and oxides being the most common. In certain cases, the lower the content of the oxidizer, the slower the burning, and hence, the larger the signature produced for defense countermeasures.
The format of the fuel and oxidizer used for the compositions dictates the rate of burning as well as the type and frequency of the signature produced.
Example compositions
Thermite composition based on magnesium/Teflon/Viton(a.k.a. MTV) is commonly used in pyrotechnic flares. A standard flare will contain 40-65% Mg, 30-55% Teflon® (Poly(tetrafluoroethylene) – (C2F4)n), and 5% Viton® (fluoroelastomer of FKM family – (CH2CF2)n(CF(CF3)CF2)n). In this case, Mg is a fuel, Teflon is a source of fluorides, and both Teflon and Viton act as binders. MTV flare grain is fabricated by milling all three components and combining them in an organic solvent to form a slurry. This slurry is stirred until a crumbling solid is obtained. The solid is then pressed into the correct shape of a pellet, which can be loaded into the flare cartridge. (Image on the left: MTV solids are pressed into a pellet (Source: Dr. Samuel Emery, NAVSEA)).
Traditional flare grain production
Classic methods to process energetic materials such as cast-cure, melt-cast, and powder pressing suffer from batch to batch variations, inhomogeneous products, and limited grain design options. Moreover, they do not allow the packing of several pyrotechnic compositions into a single flare grain. More specific drawbacks of individual production approaches are listed below:
Cast-Cure: ingredients settling, poor bonding between components, shrinkage and cracking;
Melt-Cast: irreversible growth, shrinkage, cracking;
Powder pressing: unsuitable for large munitions and munitions with unusual shapes;
To achieve the color-changing effect using various pyrotechnic compositions, fireworks manufacturers use inside-to-outside, layer-by-layer methods. These are made by hand, which is labor-intensive and layering is not accurate.
Processing pyrotechnic compositions via AM will enable controlled deposition of material in any shape and form factor. Moreover, AM allows for the deposition of multiple materials permitting fabrication of functionally-graded flare grain with selected spectral frequencies.
Project Goal: alter existing pyrotechnic compositions to make them compatible with additive manufacturing techniques for fabrication of functionally graded flare grain
Benefits: generation of specific electromagnetic signatures in time and space
Outcomes: advanced airborne expendable countermeasures
