Protecting missiles from EMI: real-world rocket science

This article originally appeared in the June'24 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

With most global conflicts unlikely to de-escalate any time soon, the missile defence market is only set to strengthen in the years ahead. According to Revenue Impact firm MarketsandMarkets, the rocket and missile market is expected to grow from $57.7 billion in 2023 to $77.4 billion by 2028, an impressive 34% rise in just five years. In Europe, Mordor Intelligence estimates the market size as $3.79 billion in 2024, potentially reaching $4.83 billion by 2029, a not dissimilar 27% increase.

In September 2023 the European Union (EU) agreed to allocate €500 million for the support of investment projects worth up to €1.4 billion, thereby incentivising the ramp-up of ammunition and missiles production in the EU. The UK is also providing support: in December 2023 the Defence Secretary announced that hundreds of British-made air defence missiles are on their way to conflicts to protect civilians and infrastructure.

Capability levels rocket

Among the major factors that require careful consideration in the ramp-up of production is missile design. The basic premise of missile design has seen a great deal of evolution in recent years, with more electronics required for flight systems and positional accuracy. Indeed, the complexity of missile electronics and on-board intelligence has increased significantly in all launch mechanisms: from air, sea, and land.

Missile electronics serve a number of applications, including control systems for stats and flaps, on-board navigation systems, search and tracking units, and gyro units that provide stability and orientation control. Some missiles even use data links to communicate with ground-based or airborne control systems, allowing for remote guidance and control. 

While these capabilities are impressive, they are subject to attack from a number of invisible forces. Primary among them is EMI: interference in an electrical path or circuit caused by an outside source such as radar, or even hostile systems. Today, electronic warfare is a battle strategy, where the threat of radar jamming, electronic deception, and electromagnetic pulse (EMP) attacks is prevalent. These factors are driving the need for greater protection from EMI, as any failure in this regard can potentially prove catastrophic.

Bridging the gap

EMI shielding solutions typically take the form of conductive gasketing materials to plug the gaps in mechanical assemblies and keep EMI at bay. Missiles have several metal interfaces, as well as many covers and access panels, which all require shielding. Any unprotected potential pathways for EMI could compromise the effectiveness of the missile.

Conductive gasketing materials are generally metal-impregnated elastomers (silicone or fluorosilicone), such as the Parker Chomerics CHO-SEAL family of products. These elastomers look and behave like rubber for easy application and conformability, but around 75% of their content is actually plated metal particles that provide electrical  conductivity.

Electrically conductive elastomer gaskets are typically available as moulded sheets or bespoke parts, allowing for greater component complexity and detail, or extruded into strips and offered either as cord stock at length or spliced (vulcanised) to form a continuous seal. Typical missile applications for conductive elastomers include access panels, hatch caps and radar.

Conductive paint

Further popular options that often serve as a complementary solution to conductive elastomers are electrically conductive coatings and urethane or epoxy-based paints, which adhere well to challenging substrates. These coatings, such as the Parker Chomerics CHO-SHIELD product family, again feature plated precious metal particles, making them ideal for providing a conductive path at mating edges.

Such solutions are perfect for the structural panels and flanges of missiles, where they also provide significant protection against galvanic corrosion: these paints withstand extreme temperature fluctuations, high humidity, and salt fog.

In terms of market differentiators when selecting a conductive elastomer or paint, always check for actual accreditation to military specifications such as MIL-DTL-83528 or MIL-C-22750. Some products state that are ‘made in accordance with’ military specifications, but this differs to accreditation. Accreditation requires frequent rigorous tests to ensure compliance and suitability for demanding applications such as missiles.  

Radio silence

Aside from EMI, other invisible ‘enemies’ of missiles include RFI (radio frequency interference), essentially unwanted electromagnetic signals that disrupt the reception of radio signals. Typical solutions centre on elastomer-based absorber materials, which serve to focus the wave. 

A good example is the Parker Chomerics CHO-MUTE product range, which comprise of a silicone elastomer matrix with ferrous filler material to provide RF absorption performance for missiles over a broadband frequency range, including very high frequencies. These materials also minimise cavity-to-cavity cross-coupling and microwave cavity resonances. They are generally available as sheet stock and offer easy cutting.

As a point of note, the EMI/RFI gasketing solutions available for missiles are equally suitable for associated equipment, such as launchers and ground systems.

A grounded approach

Electrical grounding is also relevant. Grounding is a complex subject, yet there is no hiding from its vital role in protecting sensitive, high-performing missiles. Proper grounding avoids circuit faults, which is essential in these critical systems. 

Depending on the design requirements, suitable gaskets can take the form of conductive foams/fabric-over-foams, conductive elastomers or metallic fingerstock. The use of these solutions for grounding takes the current produced by EMI to a safe place.

Of course, by design, many grounding products also function as EMI shielding devices and are thus meant to serve as a conductive interface between shields and grounded surfaces.

Stay cool

Another issue is heat, which can compromise the efficiency of electronics and shorten the operating life of components. New heat management materials are essential to ensure sensitive electronic components remain within their operating temperature range limits.

A typical solution is a gap pad: a soft and easily conformable material that provides a thermal interface between heatsinks and electronic devices, accommodating uneven surfaces, air gaps and rough surface textures. A prime example is the Parker Chomerics THERM-A-GAP series of thermally conductive gap filler pads and gels, which come in various carrier and liner configurations for increased operating performance.

Partner of choice

When it comes to identifying a suitable technology partner for EMI/RFI, electrical grounding and thermal issues in any type of missile, design engineers should consider their options carefully. Technology partners able to support from design concept to aftermarket, while also helping customers to streamline module manufacture and assembly, offer an obvious advantage. An expansive catalogue of high-quality products and in-house manufacturing accredited to military standards are further differentiators that help ensure the complete protection of battle-ready, mission-critical missile systems.