Jae-San Yoon

Measurement of the thermal expansion of space structures using fiber Bragg grating sensors and displacement measuring interferometers

A thermal deformation measurement system, composed of fiber Bragg grating (FBG) sensors for strain measurement and a displacement measuring interferometer (DMI) system for accurate specimen expansion data acquisition, was prepared and installed in a vacuum chamber where the temperature of the test specimen can be controlled to simulate space environments. The DMI system, which consists of two heterodyne interferometers, a laser head, electronics and a thermally stable specimen base made of fused silica, was used to validate the thermal expansions of the specimens measured by the FBG sensors. We measured the average coefficient of thermal expansion (CTE) of an Invar specimen, known as a thermally stable material, using both the FBG sensors and the DMI system in vacuum conditions from 20 °C to 40 °C. The CTE results of the Invar specimen were found to be 1.226 × 10−6 K−1 and 1.298 × 10−6 K−1 based on the FBG and DMI measurements, respectively. The present results show that it is possible to precisely measure the thermal deformation of a specimen or structure in space environments using FBG sensors.

Development of Integrated Simulation Tool for Jitter Analysis

Pointing stability of high precision observation satellites must satisfy the stringent requirements to perform at a designed level. As even a small vibrational disturbance can result in severe degradation of the optical performance, the effects of inorbit vibrational environment on the performance of optical payload must be predicted and analyzed in the design phase in order to ensure that the requirements imposed on the payload are fully met. In this paper, an integrated framework for the evaluation of the performance of optical payloads is developed. The developed simulation tool comprises of the reaction wheel induced disturbance model, state space model of a structure in modal form and Cassegrain reflector model. The performance degradation of the optical system due to jitter is expressed by using modulation transfer function (MTF) and image simulation. Moreover, vibration isolator model is also added to show the effectiveness of using a vibration isolator for the elimination of the effects of jitter in the acquisition of an image.

One-equation modeling and validation of dielectric barrier discharge plasma actuator thrust

Dielectric barrier discharge (DBD) plasma actuators with an asymmetric electrode configuration can generate a wall-bounded jet without mechanical moving parts, which require considerable modifications of existing aeronautical objects and which incur high maintenance costs. Despite this potential, one factor preventing the wider application of such actuators is the lack of a reliable actuator model. It is difficult to develop such a model because calculating the ion-electric field and fluid interaction consume a high amount calculation effort during the numerical analysis. Thus, the authors proposed a semi-empirical model which predicted the thrust of plasma actuators with a simple equation. It gave a numeric thrust value, and we implemented the value on a computational fluid dynamics (CFD) solver to describe the two-dimensional flow field induced by the actuator. However, the model had a narrow validation range, depending on the empirical formula, and it did not fully consider environment variables.This study presents an improved model by replacing the empirical formulae in the previous model with physical equations that take into account physical phenomena and environmental variables. During this process, additional operation parameters, such as pressure, temperature and ac waveforms, are newly taken to predict the thrust performance of the actuators with a wider range of existing parameters, the thickness of the dielectric barrier, the exposed electrode, the dielectric constant, the ac frequency and the voltage amplitude. Thrust prediction curves from the model are compared to those of earlier experimental results, showing that the average error is less than 5% for more than one hundred instances of data. As in the earlier work, the predicted thrust value is implemented on a CFD solver, and two-dimensional wall-jet velocity profiles induced by the actuator are compared to the previous experimental results.

Transverse Strain Effects on the Measurement of the Thermal Expansion of Composite Structure Using Surface-Mounted Fiber Bragg Grating Sensors

The effects of transverse strain on the wavelength shifts of surface-mounted fiber Bragg grating (FBG) sensors are investigated. A new FBG model including the concept of strain transmissibility coefficients along the longitudinal and transverse directions of the sensors is proposed. The finite element method is applied to obtain the strain transmissibility coefficients between the FBG sensor and surface of the base structure. The wavelength shifts of each FBG sensor are calculated with respect to the assumed thermal strain condition of the base structures; the estimated strains by the proposed FBG model are compared with the results from conventional FBG model. The numerical results show that the surface strains of the composite structure can be predicted well using the proposed FBG model, while the conventional FBG model results in a larger error when the transverse strain of the base structure is much larger than longitudinal strain of the base structure, as in composite structures under thermal loadings.

Effect of Dimensional Stability of Composites on Optical Performances of Space Telescopes

AbstractTo achieve high-resolution images from satellites, the dimensional-stability effect of telescope structure on optical performance should be considered in the operating environment. In this paper, the dimensional stability of composite telescope structure and optical performance degradation resulting from its dimensional instability have been evaluated under operating conditions. Dimensional-stability evaluation of the composite structure was achieved by a combination of precise deformation measurement and finite-element analysis of the composite-structure model. Thermal deformation of composites was measured by interferometer, and a brand-new dilatometer using an optical-scale sensor was developed for outgassing deformation measurement. The thermal/outgassing deformation of the telescope structure was interpreted as despace of the two-mirror Cassegrain telescope, and the optical-performance change resulting from despace was calculated in terms of the module transfer function (MTF) value. The MTF v...

Transverse strain effects on the thermal expansion measurement of composite structure using fiber Bragg grating sensors: Experimental validation

When transverse strain is much larger than the longitudinal one, conventional fiber Bragg grating measurement model presents significant errors in the longitudinal strain measurement. The present authors presented a fiber Bragg grating measurement model to consider transverse strain effects and numerically demonstrated the effectiveness of the model. This article experimentally validates the fiber Bragg grating measurement model by measuring thermal expansion of anisotropic composite specimens and an isotropic invar specimen. The longitudinal thermal expansion of the specimens is measured by using fiber Bragg grating sensors and compared with a reference measurement result using an optical interferometer. The proposed fiber Bragg grating model shows good agreement with the reference measurement for the longitudinal strain while large discrepancy (~22%) is observed according to the conventional one-dimensional fiber Bragg grating model.