Operational Considerations for High-Reliability Interconnects in Military and Aerospace Applications
In the recent past, military acquisition has shifted from being locked into large long-term contracts to developing complex systems in discrete increments that can be further optimized in future design cycles. These shorter design cycles allow for equipment to be deployed more rapidly, thereby mitigating the risk of subsystems going obsolete as increasing proportions of budget dollars go towards operation and support (O&S). This transition to evolutionary acquisition (EA) leaves a major opportunity for vendors to develop commercial off-the-shelf (COTS) components that are military-compliant.
Typically, commercial-grade devices have a wide range of quality across manufacturers as the standards (or lack thereof) vary from organization to organization. Since reliability is essential for mission-critical components, it is important for companies to apply standards that are relevant to their product. This remains true for the respective interconnect between components in military systems.
Oftentimes, cables are not a significant consideration in the design process as they are far less complex than the systems they connect to. However, greater consideration may be warranted because a failing or degraded cable can cause significant loss in signal integrity and/or failures that are time consuming to troubleshoot and repair. This is especially true in aerospace applications where there are miles of cable routing in a single aircraft.
It is therefore critical to ensure that a cable assembly can tolerate the various electrical, mechanical, and environmental conditions that it may be exposed to. This involves a rigorous process of standard electrical tests, burn-in and additional relevant testing before being deemed as field deployable. RF interconnect such as coaxial cables have several primary standards that help to ensure cable performance regardless of environmental factors; these standards include MIL-DTL-17, MIL-PRF-39012, and MIL-STD-348. A more thorough listing of standards can be seen in Table 1.
The combination of standards such as these allow for reliable integration of interconnect in most military systems and mitigating the risk of failure. The MIL-DTL-17 and MIL-PRF-39012 general specifications for cables and connectors provide an outline for coax construction and testing while the J-STD and SAE standards list assembly related standards.
Lot traceability is another important requirement for any components deployed in the military. Lot traceability can be defined as the ability for a vendor to track the materials used in a product back to their respective sources. This type of tracking must be implemented from the receiving stage to the shipping stage, and all other stages in between. Likewise, data capture of the relevant lot information is important and often best accomplished with regimented uploads to an ERP system. Figure 1 shows a sample flowchart of incoming material flow process. The assembly and test processing flow for a coaxial cable brings many more steps in this process where logging connector/soldering/heat shrink installation and verification are key for every step in cable construction.
Materials and Assembly of High Reliability Cables
By utilizing qualified (QPL'd) cable and connector materials in combination with other industry standards and process certifications, such as J-STD-001, design and manufacturing defects can be reduced, thereby making the coaxial cable assemblies better suited for the high-reliability applications where the cost-of-failure is high. While not every high-performance cable and connector are DOD qualified, it is common for these other components to at least certify to the most applicable paragraphs of MIL-DTL-17 or MIL-PRF-39012.
The J-STD standard, for instance, relates to the types of solder and methods to use when attaching center pins, outer conductors, and shielding of the coaxial cable to the connectors. The SAE International standards listed in Table 1, allow for consistency in tools, parts, and labeling between coax assemblies. The conformance testing listed in MIL-DTL-17 and MIL-PRF-39012 ensures that a cable and connectors can operate in various conditions and is therefore field deployable. Table 2 covers some of the test parameters that are listed in MIL-DTL-17 and, depending upon the military application of choice, some of these parameters are more relevant than others.
Relevant Conformance Tests for Various Military Applications
Coaxial assemblies are necessary for the connection of high frequency signals. Military applications can range from S-band Interference Friend or Foe (IFF) to millimeter-wave imaging.
While the performing frequency band of a coax generally changes its dimensions, the environment has a significant impact upon the materials used in its construction. Some natural and induced environmental factors are shown in Figure 2, an image taken from the MIL-STD-810 standard for environmental engineering considerations of various military systems. It can be readily seen that factors such as cable size, weight, power handling, and ability to withstand certain hazardous conditions become paramount to the functionality of an entire system. The choice of materials for the dielectric, inner and outer conductors, and jacketing material can then become serious considerations.
Aerospace Cable Considerations
Cable installations in virtually any aerospace system including interference friend or foe (IFF) transponders and ground moving target indicators (GMTI) equipment will experience both high and low temperatures as well as humidity. At 30,000 feet, atmospheric temperatures can range from -40°F to -70°F. Aircraft travel globally and must also therefore function in desert and arctic conditions. Moreover, heat conduction from a powerful front bulkhead and engines on the aircraft's metal shell can reradiate heat for further contribution to high temperatures. Cables are protected from much of this stress within a sealed aircraft that contains various heat exchanging avionics but must still have a wide operating temperature that can range from -55°C up to 165°C.
Cable contamination can occur through accidental spillage, leakage, or spray or during installation and routine maintenance with poor handling. These contaminants include fuel, turbine fuel, hydraulic fluid, lubricants, and coolant. Typically, jacketing exposed to these type of contaminants will cause migration and dispersion of the plasticizers in the material leading to melting, swelling, or cracking. This continual exposure alongside ambient mechanical stressors, the cable shielding can be exposed and corrode or a lack of conformity can occur in the cross sectional dimensions of the coax, thereby degrading performance or causing a failure. High-reliability cables must then be able to resist these contaminants through the use of employing specific plasticizers or by using materials that are inherently more robust.
Mechanical vibration will almost certainly be applied to cabling in aircraft during flight. This can occur during normal operation or from gunfire vibration. Mechanical shock can be induced through heavy landing or sharp aircraft maneuvers. This can cause connectors to un-mate or become dislodged. It is therefore valuable to assess connectors with vibration and shock tests as specified in MIL-PRF-39012 to ensure that there are no electrical interruptions exceeding 1-microsecond during test and no damage to the connector has occurred.
There is also a mechanically induced signal generation that occurs during vibration between the shield and dielectric material as shown in Figure 3. This tribo-electric induced noise can cause additive noise on sensors for in-flight testing or other highly sensitive equipment. It is important that this noise stay below a certain threshold for the proper performance of these systems.
Precautions must also be considered for the risk of fire where cables are often a source of toxic emission, or worse, flame spread. This is especially true for enclosed spaces such as aircraft, train cars, ground-based vehicles, or indoor facilities. Parameters such as flammability, flame propagation halogen content, and toxic/smoke index are all relevant in ensuring that no toxic fumes are dispersed when the cable comes into contact with heat. Fluorinated Ethylene Propylene (FEP), for instance, has a high resistance to flames as well as chemicals and could be ideal in these applications. The design considerations for aerospace high-reliability cabling has many overlaps with other military applications as well.
Ground-based Cable Considerations
Ground-based military applications such as Land Mobile Radios (LMR), Unmanned Ground Vehicles (UGV) and ground-based radar (e.g.: phased array radar) experience both high and low temperature extremes as well as humid conditions. Mechanical vibration and shock can be induced in mobile vehicles and train transports. Enclosed spaces will require that any toxic fumes released by the cabling be minimal. And a similar type of fluid contamination can occur with UGVs causing the need for robust jacketing material. Land vehicles may also require a level of crush and abrasion resistance to prevent the coax from breaking during vehicle run over. This can be accomplished through the use of armored cables or a suitable elastomer jacket.
Ground-based radar systems will often have high transmit powers and, therefore, parameters such as dielectric withstanding voltage, corona extinction voltage, and insulation resistance become very valuable as they assess the quality of the dielectric as the insulation also becomes key in transferring heat away from the high-power carrying conductors. For instance, partial discharges in high-voltage electrical wiring generates noise that can be conducted to connected low-level circuits that produce a corona of ozone, light, acid, and ultimately the deterioration of di-electrics. This is also true in high-voltage avionics as circuits are densely packed. As an additional note, the rate of deterioration of the dielectric due to partial discharges is proportional to the operating frequency, so higher frequency radar systems will especially require the use of insulating materials with a high dielectric strength.
What to Look For
The interconnect between mission-critical equipment in military applications must be highly robust to withstand the myriad of environmental, electrical, and mechanical stressors that it can endure. The specific construction and testing of a coax can vary greatly depending on the component materials specified and the process standardization applied during manufacture. Measures should therefore be taken against any potentially damaging situation that could cause failures. Cables that abide by military standards offer a level of reliability that would not be found in many commercial-grade assemblies.
This article was written by Dan Birch, Product Manager, Pasternack (Irvine, CA). For more information, visit here .
- Moir, Ian, and Allan G. Seabridge. Design and Development of Aircraft Systems. John Wiley & Sons, 2013.
- Defense Technical Information Center report.
- Defense Technical Information Center report.