by Dave Murray, Technical Fellow, Times Microwave Systems
Hypersonic missiles represent the most significant advancement in defense weaponry since the 1960s. However, they also present substantial challenges. The term “hypersonic” describes any speed faster than five times the speed of sound (above Mach 5). In fact, these missiles are so fast that, as they travel, their speed can alter the adjacent air molecules. Compared to ballistic missiles, which are fast but travel along a predictable trajectory, and cruise missiles, which are accurate but slower, hypersonic weapons combine speed and accuracy. In addition, they are low-flying and maneuverable, designed to be nimble enough that traditional missile defense systems cannot detect and react in time.
There are two types: hypersonic glide vehicles (HGVs), also known as boost-glide vehicles, are typically launched from a ballistic missile and released at a specific altitude, speed, and with the flight path tailored to reaching its target without being powered. A small propulsion system could be added to speed arrival at the target and provide directional control.
In contrast, hypersonic cruise missiles (HCMs) are powered all the way to their targets, flying high altitudes but lower than those of HGVs and launched from a rocket or jet aircraft. Power is delivered by air-breathing scramjet engines. Scramjets have been in development since the 1950s and they’ve proven to be extremely difficult to perfect, with the most successful results produced only since the 2000s. The trajectories of both types of weapons are shown in Figure 1.
RF technology is key to powering many advanced electronics applications in hypersonic missiles. However, designing a crucial RF interconnect system that will perform well and withstand the extraordinary environmental and technical conditions presented by hypersonic missiles requires unique, highly customized coaxial cable solutions to prevent failure.
For example, a hypersonic missile utilizes multiple antennas and sensors with phase-matched or time-matched cables. These elements must survive at speeds that can exceed Mach 5, at times topping 5,000 mph. At such speeds, the temperature on the surface and in the boundary layer of the missile may exceed 1,800° F (1000° C).
During its journey, hypersonic missile guidance will reach temperatures ranging from 200° C to as high as 1000° C. This creates a unique material challenge as extreme temperatures will melt the plastics and polymers typically used in coaxial cables.
High-performance dielectrics and phase-matched systems are essential to meet phase versus temperature requirements. A hypersonic missile will go through the top of the atmosphere, generating vast amounts of heat, like a space capsule during re-entry. As the cable moves from cold to very hot temperatures, the phase matching between cables needs to track.
Hence, phase is a crucial parameter for detection and measurement in RF systems that rely on high accuracy continuous transmission and reception of RF signals. RF signals must travel through coaxial cables at consistent speeds regardless of environmental factors. Temperature variations degrade the electrical match between coaxial cable assemblies. That small amount of degradation, known as phase tracking characteristic, can adversely affect system performance.
For example, the electronically steered antenna used in many RF applications employs an array of radiating elements to steer antenna beams rather than physically moving an antenna. Beam steering for transmission or reception is performed by adjusting the phase of the individual antenna elements in the array. High-frequency transmission lines feed each antenna array element. The accuracy of the signal phase presented to each array element depends on the phase accuracy and stability of the cable assemblies. Therefore, cable assemblies optimized for phase performance usually exhibit minimal changes in phase with temperature.
Modern missile systems also utilize extensive communications equipment packed within a highly confined space. This creates a demand for novel solutions that combine a small form factor with reduced weight and rugged construction to withstand the high impact and vibration conditions from deployment to target.
Polytetrafluoroethylene (PTFE) has traditionally been the dielectric material of choice for many high-frequency cables, as it provides excellent flexibility and low loss. 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 abrupt change in phase length at +19° C can generate inaccuracies in systems that use phase as a measurement parameter.
These inaccuracies, though slight, can lead to much larger, crucial issues; any variations in the electrical behavior of RF/microwave coaxial cable assemblies can introduce amplitude and phase variations that degrade the performance of a phased-array radar system overall.
Silicon Dioxide Coaxial Cables
Hypersonic missile systems cannot tolerate phase errors, so cable designers must consider other materials that do not present the same shortcomings as PTFE. Times Microwave Systems developed a proprietary silicon dioxide dielectric (SiO2) to address these unique challenges (Figure 2). Silicon dioxide is well known in the microelectronics industry for its excellent insulating properties.
The SiO2 construction starts with 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. Low loss, high-velocity silicon dioxide dielectrics excel in extreme environments as they provide excellent phase stability and can perform at extreme temperatures ranging from just above absolute zero to 1000° C. In addition, compared to other cable types, SiO2 cables provide exceptionally low hysteresis, with phase and loss returning to the same values at a given temperature even after extreme excursions.
The metal and silicon dioxide dielectric construction make the cable resist radiation naturally, up to 100 Mrads. All connectors for Times’ SiO2 cable assemblies employ a crack-free, fired glass seal to provide the optimum microwave performance. The stainless steel jacket is welded to the connectors with laser beam technology to create a hermetic seal.
Between the silicon dioxide dielectric and the tight tolerances of each cable assembly, SiO2 has an EMI shielding of better than 110 dB. The 50 ohm cables are available with outer diameters of 0.090, 0.141, and 0.270 in. with cutoff frequencies of 64, 36, and 18.5 GHz, respectively.