Pushing the Limits: Engineering Advanced RF Interconnects to Meet the Challenges of Hypersonic Missile Development

Originally published in Aerospace & Defense Technology

By Julianne Benson, Applications Engineer

The development of hypersonic missiles represents the most significant advancement of defense weaponry since the 1960s. However, they also pose unique challenges for both design and technology. The term “hypersonic” refers to any speed faster than five times the speed of sound, or above Mach 5. Modern hypersonic missile systems require extensive communications interconnects within a highly confined space. This space requirement creates a demand for solutions combining small form factor with reduced weight and rugged construction to withstand hand vibration and impact conditions from deployment to target.

Currently there are two types of hypersonic weapons. Hypersonic glide vehicles (HGVs), also known as boost-glide vehicles, typically launch from ballistic missiles and are released at a specific altitude, speed, and with the flight path tailored to a target without being powered. Hypersonic cruise missiles (HCMs) are powered all the way to their targets, flying at lower altitudes than HGVs and launched from rockets or jet aircraft. Power for HCMs comes from air-breathing scramjet engines, which have been in development since the 1950s and most successful since the 2000s.

RF technology is a key element to powering many advanced electronics interconnects in hypersonic missiles to ensure performance and reliability. However, designing a crucial RF interconnect system that will perform well and withstand hypersonic missiles’ extraordinary environmental and technical conditions requires unique, highly customized, coaxial cable and interconnect solutions to prevent failure.

Standard Phase and Temperature Requirements for Hypersonic Weapons

Meeting phase versus temperature requirements demands high-performance and phase-matched systems. A hypersonic missile can go through the top of the atmosphere, generating vast amounts of heat. As the cables move from cold to extremely hot temperatures, the phase matching between cables needs to track. To fulfill such an essential requirement and prevent failure, highly customized coaxial cable solutions are needed.

For RF systems that rely on highly accurate continuous transmission and reception of RF signals, phase is a crucial parameter for detection and measurement. RF signals must travel through coaxial  cables at consistent speeds regardless of environmental factors. Temperature variations degrade the electrical match between coaxial cable assemblies. This poor phase tracking over temperature can negatively impact system performance.

Polytetrafluoroethylene (PTFE) provides excellent flexibility and low loss, and traditionally has been the dielectric material of choice for many high-frequency cables. However, at +19 C PTFE exhibits a well-known deviation, commonly referred to as the “knee,” in its phase versus temperature characteristics due to a change in its crystalline state. This change in phase length can produce inaccuracies in systems relying on phase as a measurement parameter. Although slight, these inaccuracies can lead to much larger, more crucial issues; variations in the electrical behavior of RF/microwave coaxial cable assemblies can introduce amplitude and phase disparities that degrade the performance of a system overall.

An example system where this variation would be detrimental is in the electronically steered antenna used in many RF applications. These antennas employ an array of radiating elements to steer antenna beams rather than physically moving the antenna. Beam steering for transmission or reception is done by adjusting the phase of the individual antennas in the array, which are fed by high-frequency transmission lines. The accuracy of each individual element depends on the phase accuracy and stability of the coaxial cable assemblies. Therefore, cable assemblies must be optimized for phase performance to exhibit minimal changes in phase with temperature.

The Growing Challenge of High Temperature Requirements

Due to speeds exceeding Mach 5, hypersonic missiles will reach temperatures from 200 C to potentially exceeding 1,4000 C. These extreme temperatures create a unique material challenge, as the standard dielectric materials used in coaxial cables will melt under such conditions.

Hypersonic missile systems cannot tolerate phase variances, so cable designers must consider other materials that do not present the same shortcomings as PTFE. Completed high temperature developments at Times Microwave Systems have already led to a dedicated product line to address these unique challenges: Silicon Dioxide (SiO2). SiO2 offers semi-rigid assembly solutions up to 650 C.

Silicon Dioxide (SiO2) Cable Assemblies

Times Microwave Systems SiO2 cable was designed to meet the challenges of hypersonic missiles and other demanding environmental applications like spaceflight. Silicon dioxide is a well-known material in the microelectronics industry for its excellent insulating properties. Low loss, high-velocity silicon dioxide dielectrics can perform in extreme temperature ranges up to 1,000 C while maintaining excellent phase stability. In addition, SiO2 cable assemblies provide exceptionally low hysteresis; phase and loss return to the same values at given temperature even after extreme excursions.

The construction of SiO2 cable includes a solid oxygen-free copper center conductor, a silicon dioxide insulating dielectric, and a stainless-steel jacket with copper cladding to act as the outer conductor. Between the SiO2 dielectric and solid tube outer conductor, each cable assembly has an EMI shielding of better than 100 dB.

Current Connector Limitations

Silicon dioxide is hydroscopic, which requires the cable to be hermetically sealed at a lower leak rate than standard flexible assemblies. This is achieved through two critical design features. All SiO2 connectors utilize hermetic feedthroughs to seal the interface. A CTE mismatch between stainless steel and Corning 7070 glass generates hermeticity through creating a compressive seal at the interface. Laser welding is utilized to terminate the cable to connector. A 360-degree weld between the stainless-steel jacket and connector hermetically seal the termination. Use of the weld joint also eliminates standard high temperature solder reflow from occurring at approximately 230°C.

Current connector designs are limited to temperatures up to 650°C and are the overall limiting factor for high temperature RF assemblies. Two main failure modes are present in a standard high temperature connector. The first failure mode encountered is in the male-female mate present in the cable to connector junction and at male-female connector interfaces. Standard female contacts begin to soften when exposed to temperatures exceeding 250°C. This mechanical change can cause the female contact to open which results in intermittent continuity. Additionally, glass used in the hermetic seals present high temperature connectors reflows. This degradation of mechanical structure changes the line impedance and causes problems in high vibration environments due to the loss of DC continuity.

Ongoing Engineering Developments

Novel cable and interconnect solutions will be required to overcome the standard failure modes of existing high temperature product offerings. Times Microwave Systems has multiple developmental efforts currently undergoing prototyping that target these failure points with contactless terminations and the use of high temperature seals. These development efforts target two critical temperatures: 450°C for flexible assemblies and 1,000°C for semi-rigid assemblies.

Although SiO2 assemblies offer functionality up to 650°C, semi-rigid cables introduce challenges during integration. The semi-rigid nature of the SiO2 cables makes routing assemblies through confined spaces difficult and susceptible to damage. Flexible cable assemblies are preferred to eliminate these challenges. Flexible constructions utilizing a braided high-temperature RF dielectric are being developed as a silicon dioxide alternative. It has higher temperature resistance than conventional flexible dielectrics with a maximum operating temperature of 1,050°C. It is homogenous and has a dielectric material.

With a low CTE, it has good resistance to thermal shock. Its high mechanical strength also aligns well with standard coaxial cable manufacturing techniques. The overall cable construction will utilize a standard braided outer conductor in place of SiO2 cable’s stainless-steel jacketing to maintain flexibility. This dielectric construction is still hydroscopic and requires design characteristics to meet the same hermiticity level as silicon dioxide. FEP jacketing used on standard flexible assemblies will be replaced with a high temperature flexible sealant.

To overcome conventional silicon dioxide limitations, ceramic hybrid seals are being used in place of glass. An alternative termination method will be utilized to eliminate the need for spring hard materials. This marvel cable to connector termination eliminates standard male/female sockets with a new mating interface. Integration of these changes will allow temperatures of 450°C and 1,000°C respectively to be survived.

Takeaways

Hypersonic missile development is driving the need for products that survive harsher environments. Temperature requirements ranging up to 1,400°C exceed standard RF material limitations. Designing a crucial interconnect system that will perform well and withstand the extraordinary environmental and technical conditions of hypersonic missiles requires a supplier who understands these complexities. Times Microwave Systems has developed a wide range of emerging technologies that eliminate the gap between standard material limitations and hypersonic missile demands.