Vacuum Test Chamber Research Articles

Numerical study on the influence of cell gas on the compression behavior of expanded polypropylene

AbstractExpanded polypropylene (EPP) bead foam mainly consists of entrapped gas within closed polymer cells. This numerical study presents a method to account for the influence of this entrapped gas on the compression behavior of EPP foam. The method developed combines a finite element (FE) model of the foam structure with a smoothed particle hydrodynamics model to simulate the effect of the cell gas. The foam structure is modeled using the open‐source software neper, the FE simulations are conducted using the explicit FE solver of LS‐Dyna. Numerically obtained stress–strain curves for the investigated foam materials, both with and without considering the cell gas, are compared with experimental data from tests using a specially designed vacuum test chamber. The comparison shows a good agreement between numerical and experimental results, indicating that entrapped cell gas increases the structural stiffness under compression. However, in load‐hold‐unload tests, the numerical model fails to accurately capture the stress relaxation behavior observed during the hold phase of the experiment. This study highlights the significant impact of cell gas on the compression behavior of EPP foam and the need for further refinement in simulation strategy to capture effects like the stress relaxation and multiaxial loading.

Research on welding simulation for vacuum chamber test specimens

The vacuum chamber is an important component of nuclear fusion devices, and the deformation control of the welding process is relatively strict. In this paper, the isothermal multi-level cyclic loading tests have been conducted to explore a nonlinear mixed hardening mode, which can be used as the material input for the welding simulation. Then, welding simulation has been performed for test specimens of the vacuum chamber, and the deformation of test specimens has been obtained under different welding paths. After the first welding simulation is completed, the clamp and limiting plates have been designed based on the deformation of the test specimens. The welding simulation has been performed again with the clamp and limiting plates, in order to evaluate the rationality of the welding process and the design for the clamp and limiting plates. Finally, according to the determined welding process and design for the clamp and limiting plates, welding tests have been conducted, and the welding deformation has been measured. It is found that the difference between the measurement values of the tests and the simulation results is small, which implies that the welding simulation is reasonable.

Cryogenic vacuum chamber testing of a conductively-cooled, high temperature superconducting rotor for a 1.4 MW electric machine for aeronautics applications

This paper presents the second ever demonstration of a superconducting rotor cooled only via conductive connection to a cryocooler, i.e., without pumped or solid cryogens. The full-scale rotor was operated statically in its intended environment – about 1e-3 torr vacuum with only conductive cooling through mechanical connections to a cryocooler contained on the rotor. Stable operation of the rotor in a representative magnetic environment was demonstrated up to its rated direct current (57.2 A) and the designed temperature limit for the superconductor (62 K). Additional electrical measurements increased confidence that the superconducting coils functioned as intended. A collection of steady state temperature distributions was measured at operating and non-operating conditions. In all but one case, the observed temperature gradient between the cold tip and superconducting coils satisfies the design but with little margin. Opportunities to reduce the gradient are identified.

Modeling the performance of multilayer insulation in cryogenic tanks undergoing external fire scenarios

Multilayer Insulation (MLI) is frequently used in vacuum conditions for the thermal insulation of cryogenic storage tanks. The severe consequences of the degradation of such materials in engulfing fire scenarios were recently evidenced by several large-scale experimental tests. In the present study, an innovative modelling approach was developed to assess the performance of heat transfer in polyester-based MLI materials for cryogenic applications under fire conditions. A specific layer-by-layer approach was integrated with an apparent kinetic thermal degradation model based on thermogravimetric analysis results. The modeling results provided a realistic simulation of the experimental data obtained by High-Temperature Thermal Vacuum Chamber tests reproducing fire exposure conditions. The model was then applied to assess the behavior of MLI systems for liquid hydrogen tanks in realistic fire scenarios. The results show that in intense fire scenarios degradation occurs rapidly, compromising the thermal insulation performances of the system within a few minutes.

A novel annular slit-type emitter developed for multi-jet electrospray propulsion

Electrospray thrusters employ ionization in the liquid phase to produce and propel streams of molecular ions or highly charged droplets at significant velocities. In this study, we developed a novel annular slit-type emitter for electrospray and investigated its operational modes under varying applied potentials in both open atmosphere and vacuum conditions. To assess the performance of the annular slit-type emitter in comparison to the conventional capillary-type emitter, benchtop electrospray experiments were conducted using water and glycerin as working fluids for both emitter types. The study examined the formation of the Taylor cone, cone-to-jet transition, stable jet, whipping jet, and multi-jet, along with their dependence on fluid viscosity and electric potential for both emitter designs. Clear distinctions in hydrodynamic mode, drop-to-cone mode, and cone-to-jet transition mode were observed between the two emitters. As the electric potential increased, the capillary-type emitter exhibited a whipping and pulsating water jet, while glycerin displayed a steady tilted jet. In contrast, the annular slit-type emitter demonstrated a pulsating water jet followed by a distinctive dripping mode at higher electric potentials, while glycerin formed multiple steady jets around the annular slit. Notably, the annular slit-type emitter, when subjected to an 18.5 kV potential, produced seven electrospray jets for glycerin, a phenomenon attributed to the novel design of the emitter and the viscosity of glycerin enabling the generation of multiple cone-jets at a specific electrostatic potential around the slit peripheral meniscus. Vacuum chamber tests of the annular-type emitter using liquid indium as an ion source at 1 × 10−5 Torr revealed an ion-current density of 0.3 mA/mm, resulting in a thrust of 290 μN.

Triangular fin array passively actuated by bimetallic coils for CubeSat thermal control

CubeSat thermal control can be especially demanding due to volume and size constraints, high power dissipation per unit surface area, and highly variable internal and external heat loads. As such, CubeSat developers must rely on thermal control systems that meet the specific requirements of their mission and environmental conditions. Deployable radiators enable satellites to increase their radiative surface area and to reveal highly emissive surfaces during times when increased heat rejection is desired. In colder conditions, when decreased heat loss is preferable, the radiators can be stowed. Passive thermal control systems are unpowered and can increase system reliability by not relying on control electronics while offering decreased weight and power requirements. In this work, we propose a deployable array of triangular radiator panels that are independently and passively actuated by bimetallic coils in response to changes in temperature. This system demonstrates the use of bimetallic coils to passively deploy and stow the radiator fins as well as the use of multiple CubeSat radiator fins with independent actuators to provide increased redundancy. An experimental prototype of the system design was subjected to vacuum chamber testing with panels held in a fixed, deployed position to obtain temperature data used to tune a thermal simulation model. The prototype was then placed in a cryogenically cooled vacuum chamber environment with the panels free to actuate passively in response to varied heating conditions. Both steady state and transient testing were performed. Through this testing and the use of the tuned thermal simulation, the experimental prototype is determined to have a turndown ratio (largest cooling power / smallest cooling power) of 5.4. Deployment of the radiator panels serves to reduce CubeSat body temperatures by 45 °C. Results show the efficacy of the bimetallic coil actuators and radiator fin array in providing passive, dynamic thermal control to small satellites. Recommendations are made to further enhance the performance of the system. Of these recommendations, the most critical is to improve the thermal conductive path from the CubeSat body to the bimetallic coils and to the triangular radiator panels across the deployable radiator hinge. Such changes provide the possibility of increasing the maximum heat rejection, responsiveness of the thermal control system, and turndown ratio to 9 or greater.

Mechanical performance evaluation and structural optimization of ceramic lined thin-walled vacuum chambers

Thin-walled vacuum chamber internally supported by ceramic spacers is a promising approach to provide sufficiently robust, ultra-high vacuum compatible and low gap vacuum devices for high energy, high current, heavy ions accelerators. The mechanical performance of the ring shaped ceramic spacers is a key factor in the structural design of these vacuum chambers. An extensive campaign was devoted to develop methodologies and equipment for evaluating the mechanical and structural properties of the ceramic spacers and to optimize the chamber design. To this aim, a force analysis on single ceramic rings was first conducted, and formulas for calculating the elastic modulus and bending strength of the ceramic rings were derived. The elastic modulus and critical position strength limits of alumina and zirconia ceramic rings were tested using a mechanical testing platform. Based on the test results of a single ceramic ring, a preliminary design and processing of ceramic lined thin-walled vacuum chamber test samples was finalized. A testing device capable to apply known water pressure values was then designed and built to test the chamber ability to withstand externally applied forces and measure the mechanical parameters of the ceramic spacers. Thanks to this, the acceptable stress limits of alumina and zirconia ceramic rings as the supporting components of the vacuum chamber were obtained. Knowing these values, through an iteration process, the whole design and properties of the ceramic lined thin-wall vacuum chamber was optimized. Finally, to reduce the impact of eddy current heat effect on ceramic strength and outgassing, a suitable cooling structure was designed and implemented.

A Coaxial Pulsed Plasma Thruster Model with Efficient Flyback Converter Approaches for Small Satellites

Pulsed plasma thrusters (PPT) have demonstrated enormous potential since the 1960s. One major shortcoming is their low thrust efficiency, typically <30%. Most of these losses are due to joule heating, while some can be attributed to poor efficiency of the power processing units (PPUs). We model PPTs to improve their efficiency, by exploring the use of power electronic topologies to enhance the power conversion efficiency from the DC source to the thruster head. Different control approaches are considered, starting off with the basic approach of a fixed frequency flyback converter. Then, the more advanced critical conduction mode (CrCM) flyback, as well as other optimized solutions using commercial off-the-shelf (COTS) components, are presented. Variations of these flyback converters are studied under different control regimes, such as zero voltage switching (ZVS), valley voltage switching (VVS), and hard switched, to enhance the performance and efficiency of the PPU. We compare the max voltage, charge time, and the overall power conversion efficiency for different operating regimes. Our analytical results show that a more dynamic control regime can result in fewer losses and enhanced performance, offering an improved power conversion efficiency for PPUs used with PPTs. An efficiency of 86% was achieved using the variable frequency approach. This work has narrowed the possible PPU options through analytical analysis and has therefore identified a strategic approach for future investigations. In addition, a new low-power coaxial micro-thruster model using equivalent circuit model elements is developed.This is referred to as the Carlow–Stuttgart model and has been validated against experimental data from vacuum chamber tests in Stuttgart’s Pulsed Plasma Laboratory. This work serves as a valuable precursor towards the implementation of highly optimized PPU designs for efficient PPT thrusters for the next PETRUS (pulsed electrothermal thruster for the University of Stuttgart) missions.

Minimizing the Ground Effect for Photophoretically Levitating Disks

Photophoretic levitation is a propulsion mechanism by which lightweight objects can be lifted and controlled through their interactions with light. Since photophoretic forces on macroscopic objects are usually maximized at low pressures, they may be tested in a vacuum chamber in close proximity to the chamber floor and walls. We report experimental evidence that the terrain under levitating microflyers, including the chamber floor or the launchpad from which the microflyer lifts off, can greatly increase the photophoretic lift forces relative to their free-space (midair) values. To characterize this so-called ``ground effect'' during vacuum-chamber tests, we introduce a miniature launchpad composed of three J-shaped (candy-cane-like) wires that minimize the microflyer's extraneous interactions with the underlying surfaces. We compare our J-shaped-wire launchpad with previously used wire-mesh launchpads for simple levitating Mylar-based disks with diameters of 2, 4, and 8 cm. Importantly, we discover that wire-mesh launchpads increase the photophoretic lift force by up to sixfold. A significant ground effect is also associated with the bottom of the vacuum chamber, particularly when the distance to the bottom surface is less than the diameter of the levitating disk. We provide guidelines to minimize the ground effect in vacuum-chamber experiments, which are necessary to test photophoretic microflyers intended for high-altitude exploration and surveillance on Earth or on Mars.

Vacuum and Hover Tests of a Dihedral–Anhedral Tip Composite Rotor

This paper presents test data from vacuum and hover tests of a 2.8-ft-radius dihedral–anhedral tip composite rotor. The paper describes the blades, their fabrication, properties, instrumentation, the test conditions, and the data acquired. The blades were Mach-scaled to a generic but representative modern rotor. Vacuum chamber tests measured rotating frequencies and strains. Hover tests measured performance, blade loads, pitch-link loads, and strains under steady and cyclic loading conditions. Three-dimensional finite element structural models were developed to ensure completeness and consistency of property definition. The three-dimensional analysis was also used for a preliminary assessment of the test data. The test data revealed that the dihedral–anhedral tip influences the torsional and higher frequencies of a rotor blade significantly. The oscillatory blade loads show patterns consistent with the vertical center-of-gravity offset introduced by the tip. The surface strains reveal interesting higher-harmonic patterns of loading particularly near the dihedral junction.

A New Algorithm for Determining the Noise Equivalent Delta Temperature of In-Orbit Microwave Radiometers

The noise equivalent delta temperature (NEDT) determines the radiometric resolution of a radiometer. Determining NEDT is indispensable for assessing the in-orbit radiometer performance and quantifying uncertainty propagation from radiance to climate data records. Agencies of EUMETSAT, UK MetOffice, and National Oceanic and Atmospheric Administration (NOAA) have developed their own algorithms for calculating and monitoring NEDT of in-orbit microwave radiometers. These algorithms are an essential means for monitoring NEDT and hardware health. While remarkable accomplishments have been made, there appears to be room for improvement. The investigation is needed for the NEDT underestimate found at channels like G-band that has pronounced <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1/f$ </tex-math></inline-formula> noise. Also, it is necessary to explain the inconsistency in calculated NEDTs of different algorithms. We have reviewed the theoretical basis for determining NEDT and developed an improved algorithm of clear physics and mathematics. We have developed methods for handling error sources that can result in either negative or positive biases with an overall underestimate of NEDT. We have done comparison and validation with the prelaunch thermal vacuum chamber (TVAC) test, in-orbit data, and simulation. The new algorithm significantly improves the estimate of NEDT, including the G-band and advances the understanding of algorithm structures and physical foundations. It can accurately monitor in-orbit NEDT and facilitate quantifying the associated uncertainty propagation through science products.

SU2 REAL GAS MODELS’ PERFORMANCE PREDICTIONS ON A COLD GAS THRUSTER

Cold gas propulsion systems are preferred, especially for the altitude and trajectory control for satellites, since the 1960s. Both depressurizing the propellant in the propellant tank throughout the mission and the expansion occurring in the divergent part of the thruster nozzle are the reasons for observing very low temperatures and pressure at the outlet section. We have decided to use an open-source compressible CFD tool, SU2, in order to predict the performance of the propulsion system in the early design phase. The scope of this study is the comparison of the results of the vacuum chamber performance tests and the outcomes of unsteady simulations with SU2 using different gas models, including ideal gas, van-der Waals, and Peng-Robinson models. Then, the accuracy and performance of these models are evaluated for the extremely low temperature and pressure conditions. Thruster performance tests have been conducted in the thermal vacuum chamber test campaign at the specially built testing facilities. In order to simulate nominal and low-temperature operation conditions, a propellant tank is thermally conditioned to the predetermined values, and performance test parameters are used as the input for the CFD simulations. The obtained results showed that the van der Waals gas model is the most appropriate gas model for both cases and provides the most realistic results in terms of performance parameters.

Effects of vacuum exposure on mechanical properties of thermoplastic materials

The present paper proposes the study of the behavior of three thermoplastic materials: acrylonitrile butadiene styrene (ABS), poly(lactic acid) (PLA), and polyethylene glycol terephthalate (PETG), processed by additive manufacturing type fused deposition modelling (FDM) when exposed to low vacuum. The experiment was composed of three moments consisting of tridimensional modeling and manufacturing of the specimens, drying process and vacuum exposure for 24 hours, according to American Society for Testing and Materials (ASTM) D6653/D6653M standards, and bending test for the determination of mechanical properties, based on ASTM D790 standards. The vacuum chamber tests exposed oscillations in the pressure indicating gases releasing from the specimen, but none of the samples showed visible deformations. Subjecting the materials exposed to low vacuum to bending tests and comparing them to the unexposed material, we observed a significant increase in the calculated modulus of elasticity and a change in the slope of graphic force versus deflection in all materials. This behavior demonstrates that it is possible to submit polymeric materials to vacuum, and low vacuum exposure can be a treatment for thermoplastic materials. In the future, a study using a spectrometer will be necessary to verify which gases are present during pressure oscillation in the chamber, thus making it possible to understand which factor has increased the mechanical properties of the materials. In sequence, experiments will be necessary to validate the vacuum exposure as a form of treatment of materials and to verify the possibility of applying thermoplastics commonly used in additive manufacturing for low-impact space applications.

Effects of vacuum exposure on mechanical properties of thermoplastic materials

The present paper proposes the study of the behavior of three thermoplastic materials: acrylonitrile butadiene styrene (ABS), poly(lactic acid) (PLA), and polyethylene glycol terephthalate (PETG), processed by additive manufacturing type fused deposition modelling (FDM) when exposed to low vacuum. The experiment was composed of three moments consisting of tridimensional modeling and manufacturing of the specimens, drying process and vacuum exposure for 24 hours, according to American Society for Testing and Materials (ASTM) D6653/D6653M standards, and bending test for the determination of mechanical properties, based on ASTM D790 standards. The vacuum chamber tests exposed oscillations in the pressure indicating gases releasing from the specimen, but none of the samples showed visible deformations. Subjecting the materials exposed to low vacuum to bending tests and comparing them to the unexposed material, we observed a significant increase in the calculated modulus of elasticity and a change in the slope of graphic force versus deflection in all materials. This behavior demonstrates that it is possible to submit polymeric materials to vacuum, and low vacuum exposure can be a treatment for thermoplastic materials. In the future, a study using a spectrometer will be necessary to verify which gases are present during pressure oscillation in the chamber, thus making it possible to understand which factor has increased the mechanical properties of the materials. In sequence, experiments will be necessary to validate the vacuum exposure as a form of treatment of materials and to verify the possibility of applying thermoplastics commonly used in additive manufacturing for low-impact space applications.

THERMAL DESIGN, ANALYSIS AND TEST VALIDATION OF TURKSAT-3USAT SATELLITE

When CubeSat projects are a useful means by which universities can engage their students in space-related activities. TURKSAT-3USAT is a three-unit amateur radio CubeSat jointly developed by the Space Systems Design and Test Laboratory and the Radio Frequency Electronics Laboratory of Istanbul Technical University (ITU), in collaboration with TURKSAT, A.S. company as well as the Turkish Amateur Technology Organization. It was launched on April 26, 2013 as a secondary payload on a CZ-2D rocket from China’s Jiuquan Space Center to an altitude of approximately 680 km. The mission of the satellite has two primary goals: (1) to voice communication at Low Earth Orbit (LEO) and (2) to educate students by providing hands-on experience. TURKSAT-3USAT was designed to sustain a circular, near sun-synchronous LEO, and has dimensions of 10 x 10 x 34 cm3. Within the course of this paper, TURKSAT-3USAT’s thermal control will be addressed. TURKSAT-3USAT’s thermal control model was developed using ThermXL and ESATAN-TMS software. Using this model, temperature distributions of the CubeSat when subjected to various experimental conditions of interest were computed. Using a thermal vacuum chamber (TVAC), thermal cycling and bake-out testing were carried out on the flight model to verify the thermal design performance and check the mathematical model. Based on thermal analysis results, the temperature of equipment was within the allowable temperature range except for the batteries that were between 42.56 oC and -20.31 oC. Heaters were used for the batteries in order to maintain the batteries’ temperature within the allowable temperature range.

Emissivities of vacuum compatible materials: towards minimising blackbody radiation shift uncertainty in optical atomic clocks at room temperatures

We report the results of measurements of the total hemispherical emissivity coefficients of the most popular vacuum compatible materials used in designs of optical atomic clocks. The experimental vacuum test chamber was designed to precisely quantify the heat transfer between the material’s samples and a calibrated plate that serves as a thermometer. The temperatures of vacuum chamber’s internal surfaces were measured and their heat balance equations were solved to designate their emissivities. The simplicity of the experimental set-up allows for analytical calculations of all necessary parameters without use of numerical finite elements methods (FEM). The reported results cover the temperature range between 0.0 and , that is crucial in optical clocks set-ups operating at room temperatures. The linear model of the emissivity change with the temperature is applied to the measured data to make them applicable in further calculations or FEM simulations.

A configurable ion source for validating spaceflight-based thermal plasma measurement systems.

Thermal ion instruments are valuable for analyzing Earth's upper atmosphere. Prelaunch vacuum chamber testing of such instruments with a thermal ion source is often used to validate the performance of such instruments in a relevant environment, which is a prerequisite for a NASA flight. In this paper, we describe a new ion source that can be used for such tests, compare its performance with simulation results, and present an example of the utility for the source for validating the performance of an instrument.

A Deep Learning Approach for Microwave and Millimeter-Wave Radiometer Calibration

Deep learning artificial neural network techniques can be applied for on-orbit calibration of microwave and millimeter-wave radiometer spaceborne instruments, including those for small satellites. The noise-wave model has been employed for noise characterization and validation of the proposed deep learning calibration technique for a synthetically generated Dicke-switching radiometer. The developed deep learning neural network radiometer calibrator produces high accuracy estimates of antenna temperatures from the measurements of radiometer output voltage and thermistor readings. Tests with noise-free and noisy samples of the developed model have shown that the proposed calibration method does not add any significant noise to the radiometer calibration. The performance of the proposed method does not degrade with increased nonlinearity for a radiometer, while nonlinearity is a challenging issue for conventional calibration techniques. The deep learning calibration model learns the radiometer noise characteristics from radiometer prelaunch measurements during thermal vacuum chamber testing. The neural network calibrator proposed in this paper has self-learning capability during the on-orbit operation of a radiometer that can be used to improve the performance of on-orbit calibration. The proposed technique is demonstrated by comparing the residual uncertainty of the deep learning calibration with the theoretical value. No numerical study is presented to compare the performance with conventional calibration techniques. The new method may be solely applied to calibrate the radiometer or applied along with conventional calibration techniques.

Background Pressure Effects on Ion Velocity Distributions in an SPT-100 Hall Thruster

Increased background pressure in vacuum chamber test facilities as compared to on-orbit operation has been shown to influence the operation of electric propulsion devices such as Hall thrusters. This study aims to elucidate the impact of pressure on the ionization and acceleration mechanisms in a stationary plasma thruster, model SPT-100 Hall thruster, using time-averaged and time-resolved laser-induced fluorescence velocimetry. The results are compared for the thruster operating at an applied 300 V (), with vacuum facility background pressures ranging from to . Time-averaged measurements reveal that, in general, an upstream shift in the position of the ionization and acceleration regions occurs as the facility pressure is increased above the nominal . Time-resolved measurements, implemented using a sample-hold scheme with resolution, emphasize that similar acceleration profiles are present within the Hall thruster discharge channel regardless of background pressure. Measurements taken at , where the facility background neutral density is similar to the neutral density emitted from the thruster, unexpectedly show increased ion acceleration over the next highest pressure condition at . These results indicate a not-yet well defined balance of the impacts of neutral ingestion, classical and turbulent electron transport on thruster operation, and that the ratio of the background to thruster neutral density is a more relevant benchmark than background pressure alone when evaluating Hall thruster operation.

Lithium-ion battery performance modeling based on the energy discharge level

In the present work, a new model for Li-ion battery performance is proposed. This model is based on the energy discharge level (in relation to full capacity), and was developed after testing carried out on the battery of the UPMSat-2 satellite. These thermal vacuum chamber tests showed extremely high energy efficiencies. The results from the proposed model reproduce the test results (discharging/charging processes) of a 17.2 A · h Li-ion battery.