High Altitude Applications Research Articles

A 3-D Global FDTD Courant-limit Model of the Earth for Long-time-span and High-altitude Applications

A new global 3-D finite-difference time-domain (FDTD) model is introduced to simulate electromagnetic wave propagation around the Earth, including the lithosphere, oceans, atmosphere, and ionosphere regions. This model has several advantages over existing global models, which include grids that follow lines of latitude and longitude and geodesic grids comprised of hexagons and pentagons. The advantages of the new model include: (1) it may be run at the Courant-Friedrichs-Lewy (CFL) time step (as a result, it is termed the Courant-limit model); (2) subgrids may be added to specific regions of the model as needed in a straight-forward manner; and (3) the grid cells do not become infinitely larger as the grid is extended higher in altitude. As a result, this model is a better candidate than the others for investigating electromagnetic phenomena over long time spans of interest and for investigating atmosphere-ionosphere-magnetosphere coupling. The new model is first described and then validated by comparing results for extremely low frequency (ELF) propagation attenuation with corresponding analytical and measurement results reported in the literature.

On the low temperature fatigue crack growth behavior of AA7075-T651 in ultra-high vacuum environments

It is well-established that the fatigue crack growth behavior of aerospace aluminum alloys in high altitude applications is sensitive to both temperature and water vapor pressure. To isolate the role of temperature, fatigue crack growth experiments were performed on AA7075-T651 under ultra-high vacuum conditions (<6x10−6 Pa) at temperatures ranging from 23 °C to −65 °C. All da/dN–ΔK relationships exhibit the following: (1) a temperature-independent Paris regime at high ΔK, (2) a transition from the Paris regime to a sharply reduced da/dN, which occurs at higher ΔK as the temperature decreases, and (3) a coalescence in da/dN for all temperatures in the threshold region. Interestingly, the point of deviation from the Paris regime is found to correlate with a transition from a flat, transgranular fracture morphology to slip band cracking (SBC) across all conditions. The likely mechanisms governing the observed behaviors are discussed, focusing on the role of slip.

Design and development of an affordable and suitable rubber compounding for high altitude application up to −40 °C

The highest peak ever scaled by humans is Mount Everest at 8849 m. There is a constant decrease in pressure and a decrease in ambient temperature as one advance above sea level. Indian sub-continent has many places above 1500 m of spiritual importance. Variegated age group scale points of spiritual significance from children to the elderly. At such altitude there is a concern in breathing. Finding a hand-held portable breathing solution is therefore called for. This paper attempts to find an economical rubber-based polymer to be used as a material for a personal high-altitude lifesaving system up to −40 deg Celsius. Mechanical properties of different grades of rubber material with suitable fillers were investigated. Natural rubber with additives was compared with other rubber-based composites. The test samples were exposed to cryogenic conditions (-196 deg Celsius) by immersing in liquid Nitrogen for an appreciable time and were re-tested to exonerate the mechanical properties. Off all the tested samples, the best stiffness was exhibited by Butyl-Nitrile, which is 90% more than natural rubber. Similarly, after dipping in liquid Nitrogen, re-testing results confirm that the Butyl-Nitrile sample shows 95% higher stiffness than natural rubber. After being exposed to cryogenic conditions, the measured stiffness of samples is noted as more elevated than in normal conditions, attributed to its cross-linking properties. The cross-link bond of the butyl and nitrile rubber inhibits the motion inside it to increase the stiffness of the sample. The flow properties of samples were tested through a Mooney rheometer. The flow properties in cryogenic conditions were lower than in normal conditions. The article also delineates a design of the proposed breath assistance device and a wearable jacket.

A parametric design approach for multi-lobed hybrid airships

AbstractIn recent years, there has been an increasing interest in the research and development of hybrid airships for various applications. Airship design involves multiple design parameters from various disciplines that interact mutually. Existing design methodologies, however, are often limited to fixed shapes and geometry. This paper provides a comprehensive parametric design approach for the sizing of multi-lobed hybrid air vehicles for low- and high-altitude applications. The proposed design techniques are robust so that the designer has the freedom to change the number of lobes, the relative location of lobes, the envelope profile, and the optimiser for the design optimisation process. The outcomes of the proposed methodology are envelope volume, wetted surface area, length and span of the envelope, sizing and layout of the solar array, and sizing and layout of the fins. The modeling techniques highlighted in this paper are very efficient for the design and optimisation of multi-lobed airships in the conceptual design phase with a large design exploration space. The robustness of the shape generation algorithms is tested on some of the standard envelope profiles of airships. The effect of the shape and geometry of the multi-lobed envelope on added mass is demonstrated through the added mass estimation using Boundary Element Method.

Planar Common-Mode EMI Filter Design and Optimization for High-Altitude 100-kW SiC Inverter/Rectifier System

Silicon-carbide (SiC) devices are receiving popularity for high-power converter systems in aircraft due to many advantages over silicon counterparts. However, the electromagnetic interference (EMI) problems are more serious with the SiC devices operating at higher switching speeds and higher switching frequencies. The common-mode (CM) EMI filter design of the high-power SiC converter is especially challenging for high-altitude applications due to the harsher requirements of insulation and heat dissipation. The optimization of the parameters and physical design of a CM EMI filter in a 100-kW SiC inverter/rectifier system operating at 50 000 ft is conducted in this article to obtain the highest power density. The effect of the switching frequency on the EMI filter volume and the total power density of the converter system is analyzed. The lower breakdown voltage of air and the higher thermal resistance of natural convection make the conventional design of the CM choke infeasible at high altitudes. A printed circuit board (PCB)-based planar CM choke is designed with the consideration of partial discharge (PD) and heat dissipation at the altitude of 50 000 ft. The structure of the CM choke is optimized with embedded electric-field shielding plates, which controls the electric-field intensity in the air below 300 V/mm. The PCB windings improve the efficiency of heat dissipation at high altitudes and reduce the profile of the CM choke. A Pareto optimization is conducted to minimize the size of the choke and the volume of the final design is only 155 cm3. The experimental results show that the CM noise is reduced effectively with the CM EMI filter. PD is not observed even at the altitude of 50 000 ft and the thermal performance is excellent with a current of 190 A. With the optimization and the dedicated design of the CM EMI filter, the power density of the converter system is up to 33.3 kW/L.

Parameter selection and experimental study of AC line arrester with external series gap at high altitude area

In order to reduce the lightning flashover rate of transmission lines, it is necessary to install high-altitude line arresters in the vulnerable tower and phase. A series of key technical research, such as the structural design and parameter selection of the high-altitude AC line arrester, has been carried out, including the performance parameter research of the line arrester and the discharge voltage requirements of the line arrester series gap. The product design of line arrester is carried out, and the sample is developed. The sample of line arrester is tested to determine the product type, structure and parameter requirements of highaltitude AC line arrester. According to the special requirements of high-altitude application environment, the research on the discharge characteristics of line arrester with external series gap at high altitude is carried out. On the basis of the sample of developed high-altitude line arrester and the relevant performance test in low altitude area, the test items are completed in the Tibet high altitude test base at 4300m altitude. High-altitude line arrester is of great significance to ensure the safe operation of the line and reasonably coordinate the technology and economy of the project.

PDOP parameters improvement using multi-GNSS and signal re-transmission at lunar distances

Precise navigation for satellites of any kind is the key to future cost-effective Lunar missions. The usability of Global Navigation Satellite Systems (GNSS) for such high-altitude applications is still challenging due to reduced signal power levels, satellites visibility or signal processing complexity with all that resulting in a low position, velocity and time (PVT) accuracy, which is typically expressed by geometric dilution of precision (GDOP) parameters. This paper proposes a navigation method based on re-transmission combined with multi-GNSS concept, specifically signal reception from GPS, Galileo, Glonass, BeiDou and SBAS satellites’ antenna pattern main-lobes and side-lobes. Those signals are re-transmitted from the user satellite at the Lunar distance to the Earth, where all the signal processing is assumed to be performed. Based on the results from the tests of the of modelled scenario, the method presents an improvement of the position dilution of precision (PDOP, a subset of GDOP) parameters for the missions at the Lunar distance.

Detection Algorithm for Cellular Synchronization Signals in Airborne Applications

Cellular to air communication is critical for the booming aerial package delivery and transportation business. Detecting cellular signals in airborne applications is challenging because it requires receiving and processing waveforms that are subject to significantly more interference than those experienced in terrestrial settings. This paper highlights and tackles the complexity of 4G Long Term Evolution (LTE) signal synchronization in high altitude applications, e.g., cell access onboard an aircraft. Specifically, we design a novel cell detector that operates efficiently under high interference levels found in airborne applications, maintains a constant false alarm rate using an optimized threshold implementation for Zadoff Chu sequences, and monitors multiple towers with different time delays simultaneously. We validate our cell detector through simulation and experimentation. Lastly, the cell detector is used to estimate the interference in live waveforms taken from an aircraft at 2 to 2.5 km altitude and velocities of 200–400 km/h. Our cell detection model can be adapted to support 5G New Radio (NR) synchronization signals as NR deploys aerial support in the future. The threshold implementation to handle correlation spurs can be applied directly to other Zadoff Chu based signals such as random access signals found in both LTE and NR.

Nonlinear transport of rarefied Couette flows from low speed to high speed

The nonlinear transport properties and macroscopic flow features of rarefied plane Couette flows from low speed to high speed for a monatomic gas are investigated in detail using the direct simulation Monte Carlo (DSMC) method. The effective viscosity and thermal conductivity are directly computed from the DSMC results according to the linear constitutive relations. The detailed structure of the Knudsen layer (KL) and the functional dependence of the effective transport coefficients on local Knudsen numbers in the whole system are presented and compared with existing theoretical models. The results show that the effective viscosity and thermal conductivity distributions in the KL for different Mach number flows can be recast into the same profile (i.e., isothermal scaling function) in terms of a scaled wall distance η=∫0y1/λ(y)dy, though the local flow is nonisothermal. For all cases, the shear-stress Knudsen number distributions across the channel show a well opposite trend to the effective transport coefficient profiles. The functional dependence between them in the bulk region always coincides with the normal solution that is derived from the Boltzmann model equations for unbounded shear flows, while that in the KL for low-speed cases shows a large difference with the normal solution. As the Mach number increases, the DSMC data in the KL can also agree approximately with the normal solution at a large shear-stress Knudsen number. These results can be very useful for developing phenomenological models to describe a wall-bounded rarefied shear flow, showing a good prospect in both microflow and high-altitude applications.

Analysis of Insulation Structure Design for High Altitude UHV Reactor Bushing

The ultra high voltage reactor bushing is the important equipment to realize the electrical connection between the reactor and the valve tower. The high altitude application environment puts forward higher requirement for electrical structure design of reactor bushing. The paper mainly discusses the design of the internal and external insulation structure. The design and analysis of the tail of the bushing, the corona ring and the inner insulation of bushing have been done. The three dimensional electric field simulation model has been set up to calculate and analyze the insulation performance of the bushing. The results show that the elevation correction of the external ring and the flash-over voltage of the porcelain under high altitude should be done, and the calculation shows that the correction coefficient are 2.07 and 1.36 respectively, and the end of bushing should install with double type ring. The insulation distance of the hollow composite insulator is designed as 8500mm. The insulation distance of the bushing tail is designed as 1955mm. The maximum field strength of the corona ring is 1.251kV⋅mm−1, and maximum field strength on the surface of tail corona ring is 16.96kV⋅mm−1. The axial field strength of the bushing core near center conductor is higher than the flange side, and the maximum field strength is 0.55kV⋅mm−1. The results of the paper can provide the theoretical basis for the design of the insulation of the reactor transformer bushing used in the UHV transmission project.

Performance of 4H-SiC Bipolar Diodes as Temperature Sensor at Low Temperatures

In this paper we report on the performance of 4H-SiC bipolar diodes as temperature sensors far beyond 273K. The sensor is measured from 150K to 445K covering a temperature range of 295K. In this operating temperature range, the sensor characteristic VD-T is highly linear and it is dominated by the typical dependence of the p-i-n diode voltage on the temperature. The sensor sensitivity is -4.48mV/K for a diode current of 2nA with a maximum error of 4.3K across the full temperature range. Although 4H-SiC p-i-n are mainly focused on very high temperature applications, our analysis on the performance of bipolar diodes at low temperatures highlights its feasibility as temperature sensor for aerospace and high altitude applications where cryogenic temperatures are achieved.

Theoretical and experimental study on broadband terahertz atmospheric transmission characteristics**Project supported by the National Basic Research Program of China (Grant Nos. 2014CB339802 and 2015CB755403), the National Natural Science Foundation of China (Grant Nos. 61675146, 61275102, and 61271066), and the Science and Technology Support Program of Tianjin, China (Grant No. 14ZCZDGX00030).

Broadband terahertz (THz) atmospheric transmission characteristics from 0 to 8 THz are theoretically simulated based on a standard Van Vleck–Weisskopf line shape, considering 1696 water absorption lines and 298 oxygen absorption lines. The influences of humidity, temperature, and pressure on the THz atmospheric absorption are analyzed and experimentally verified with a Fourier transform infrared spectrometer (FTIR) system, showing good consistency. The investigation and evaluation on high-frequency atmospheric windows are good supplements to existing data in the low-frequency range and lay the foundation for aircraft-based high-altitude applications of THz communication and radar.

Design of FPGA-Based Missile-Borne Image Acquisition and Storage Recorder

The high-altitude missile and other special application occasions have requirements on image storage system, such as small size, high storage speed, low temperature resistance, etc. Commonly used image storage system in the market cannot meet such requirement. In the paper, real-time image storage system solutions on missile based on FPGA should be proposed. The system mainly consists of acquisition module and memory reading module. The whole system adopts FPGA as main control chip for mainly completing real-time decoding and acquisition on one path of PAL format video images, reading and writing of NandFlash chipset, erasure, bad block management and so on. The solution has passed various environmental tests with stable performance, large data storage capacity and easy expansion, which has been used in engineering practice.

Development of UV protective sheath for high performance fibers for high altitude applications

High performance fibers have distinguished properties such as high tensile strength, good thermal and chemical resistance, dimensional stability, lightweight, and high electrical conductivity. Due to these superior properties, high performance fibers made it to the scene of broad range of applications such as aerospace, automotive, windmill, fiber reinforced composites, high strength tethers, tendons for scientific balloon, tension structures, protective clothing, and marine. Examples of such fibers are Zylon®, Kevlar®, and Vectran®. However, the fibers lose their strength significantly upon exposure to Ultraviolet (UV) and visible light. In this research, UV protective films from extruded low density polyethylene (LDPE) loaded with different content of UV stabilizers (TiO2 nanoparticles and White PE CC®) were investigated. To assess the degree of UV blockage of each extruded protective film, their transmittance to UV and visible (UV-VIS) light was measured. Additionally, Zylon® braids were sheathed with the protective films and the strength of the braids and yarns raveled from braids was measured before and after UV exposure for different number of days. LDPE loaded with White PE CC® and 10 % TiO2 showed the least transmittance to UV-VIS and their yarns and braids exhibited highest strength retention after exposure to artificial UV. Strength retention of braids was higher than that of individual yarns due to weak link effect and braid structure assistant.

System Performance and Thermodynamic Cycle Analysis of Airbreathing Pulse Detonation Engines

A modular approach to the study of system performance and thermodynamic cycle efficiency of airbreathing pulse detonation engines (PDEs) is described. Each module represents a specific component of the engine, and its dynamic behavior is formulated using conservation laws in either one or two spatial dimensions. A framework is established for assessing quantitatively the influence of all known processes on engine dynamics. Various loss mechanisms limiting the PDE performance are identified. As a specific example, a supersonic PDE for high-altitude applications is studied comprehensively. The effects of chamber configuration and operating sequence on the engine propulsive efficiency are examined. The results demonstrate the existence of an optimum cycle frequency and valve close-up time for achieving maximum performance in terms of thrust and specific impulse. Furthermore, a choked convergent-divergent nozzle is required to render the PDE competitive with other airbreathing propulsion systems, such as gas-turbine and ramjet engines.

High-Altitude Considerations for Electrical Power Systems and Components

The design and application of electrical systems at elevations in excess of 1000 m (3000 ft) requires knowledge of the effects of atmospheric conditions on each particular component. Failure to understand adequately and include the effects of high altitude in the design and application of the equipment may result in its poor performance, premature aging, and/or failure. The relationship of relative air density and altitude is discussed, followed by the effects of altitude on electric power system components. Along with the discussion of the effects of high altitude on each component are suggestions or solutions to the high-altitude problem. Although the subject deals with high-altitude applications of equipment, the performance of equipment from sea level to 1000 m may be affected by the relative air density. Since the relative air density decreases at a rate of approximately one percent per 100 m above sea level, the operation of any piece of equipment which is dependent on the air density will be different at 3300 ft compared to sea level. This subject is discussed so that independent conclusions may be drawn.

Development of a Low NOx Lean Premixed Annular Combustor

An atmospheric test program was conducted to define a low NOx annular combustor concept suitable for a supersonic, high-altitude aircraft application. The lean premixed combustor, known as the Vortex Air Blast (VAB) concept, was tested as a 22.0 cm diameter model in the early development phases to arrive at basic design and performance criteria. Final demonstration testing was carried out on a full scale combustor of 0.66 m diameter. Variable geometry dilution ports were incorporated to allow operation of the combustor across the range of conditions between idle (Tin = 422 K, Tout = 917 K) and cruise (Tin = 833 K, Tout = 1778 K). Test results showed that the design could meet the program NOx goal of 1.0 g NO2/kg fuel at a reduced atmospheric cruise condition.

Terminal spacings for high-altitude and ultrahigh-temperature electronic transformer applications

Future electronic equipment mounted aboard high-speed military aircraft and guided missiles will have to withstand high altitudes, ultrahigh temperatures, and radiation. Present goals as established by the U. S. Air Force for electronic power transformers are 100,000 feet altitude, 500 C (degrees centigrade) ambient temperature, and intense nuclear radiation. With terminal spacings normally designed for air dielectric strengths at altitudes up to approximately 50,000 feet and temperatures up to 125 C, additional information is needed on breakdown voltages under these new conditions.