Sung-Kwon Jo

Least squares frequency estimation in frequency-selective channels and its application to transmissions with antenna diversity

A new data-aided frequency estimator for frequency-selective fading channels is introduced. The proposed estimator is developed based on a least squares (LS) error criterion and can estimate frequency offsets without the need for channel information. Statistical analysis indicates that the resulting estimate is unbiased and tends to approach the Cramer-Rao lower bound (CRLB). Simulation shows that the proposed LS method is preferable to existing techniques in mobile communications. The application of the LS estimator to systems with transmitter antenna diversity is also considered. In particular, it is demonstrated that the LS method can be successfully applied to third-generation wireless communication systems.

Chugging Instability of H2O2 Monopropellant Thrusters with Reactor Aspect Ratio and Pressures

Among the three types of instabilities, the low-frequency instability (chugging instability) was experimentally investigated with respect to the chamber pressure and aspect ratio (L=D) of catalytic reactors in a monopropellant thruster. ThreeH2O2 thrusterswere used, and two parameters were found to be the dominant factors that generated a chugging instability of the order of several tens of hertz. Ahigh chamber pressure and lowL=D values (lowpressure drop across the catalyst bed) were preferable for reducing pressure oscillation inside the reaction chamber. In addition, it was found that these two parameters were not independent but coupled; therefore, the point where chugging instability occurred varied slightly depending on the interaction between these parameters.

Modified leaky LMS algorithm for channel estimation in DS-CDMA systems

A simple adaptive least mean square (LMS) type algorithm for channel estimation is developed based on certain modifications to finite-impulse response (FIR) Wiener filtering. The proposed algorithm is nearly blind since it does not require any training sequence or channel statistics, and it can be implemented using only noise variance knowledge. A condition guaranteeing the convergence of the algorithm and theoretical mean square error (MSE) values are also derived. Computer simulation results demonstrate that the proposed algorithm can yield a smaller MSE than existing techniques, and that its performance is close to that of optimal Wiener filtering.

Performance Characteristics of Hydrogen Peroxide/Kerosene Staged-Bipropellant Engine with Axial Fuel Injector

A1200N vacuum-thrust-class staged-bipropellant engine that uses decomposed hydrogen peroxide as an oxidizer and kerosene as a fuel was developed and tested with the aim of investigating an axial fuel injector integrated with a distributor. This fuel injector geometry, where fuel is injected into the turbulent flow of decomposed hydrogen peroxide, was tested to evaluate the influence of the designed injector on engine performance with respect to the equivalence ratio, the pattern of fuel injection orifices, and the characteristic length L . For characteristics such as autoignition and stable combustion, firing tests over awide range of equivalence ratios from 0.26 to 1.86were carried out. Autoignition was successfully achieved under all experimental conditions. The pressure rising time from monopropellant to bipropellant mode and the pressure fluctuation in the combustion chamber were approximately 100 ms and less than 1:1%, respectively. In the parametric study, the characteristic velocity c and its efficiency were influenced by the pattern of fuel injection, whereas varying the orifice diameter had no effect. The effect of L was also estimated, and the c efficiency was measured to be over 95% for all equivalence ratios at an L of 1.20 m.

Autoignition Tests by Injecting Kerosene into Vortex of Decomposed Hydrogen Peroxide

Using decomposed hydrogen peroxide as an oxidizer and kerosene as a fuel, a 1200 N vacuum thrust-class staged-bipropellant engine has been developed and tested with aim to investigate an axial fuel injector integrated a distributor. This fuel injector geometry that injects a fuel into turbulent flow of decomposed hydrogen peroxide was tested to evaluate influence of the designed injector on engine performance. For the characteristics such as autoignition and stable combustion, firing tests over a wide range of equivalence ratio from 0.26 to 1.61 were carried out. Autoignition was achieved in all experimental conditions and pressure variation in the combustion chamber was as low as ± 1%. The efficiency of characteristic velocity, C * , was measured at or over 100% in fuel-lean conditions and from 88% to 94% in fuel-rich conditions at the L * of 0.95 m.

Chugging Instability of H2O2 Monopropellant Thrusters with Catalyst Reactivity and Support Sizes

Concentrated hydrogen peroxide has been studied widely due to its green storable and high energy density characteristics. However, there are few experimental results regarding pressure instability with catalyst reactivity and support size in monopropellant thrusters. In the present study, hydrogen peroxide monopropellant thrusters with three different catalytic bed aspect ratios were tested with a MnO2-mixed PbO catalyst to investigate the effect on chugging instability and pressure drop across the catalyst bed. As a result, the chugging instability can be reduced by using a high reactivity catalyst and the pressure drop was considerably affected by catalytic support sizes.

Conceptual Design of a Small Lunar Return Vehicle

There are several reasons for lunar exploration; the contribution to the advancement in science and technology, the scientific information lying inside the Moon of our solar system and the origin of the Earth, and the existence of energy resources and water ice. Moon surface samples, therefore, are called for collection back to the Earth for in-depth examination. For this purpose, the conceptual design of a small lunar return vehicle was performed including the three phases of lunar circularization, the journey back to the Earth, and the atmospheric Earth entry. Direct entry like in an Apollo-style entry vehicle utilizing parachutes was applied and critical values such as flight-path angle and acceleration at Earth reentry phase was compared by changing the engine burning location during lunar circularization phase.

Least squares approach to data-aided frequency estimation in frequency-selective fading channels

A new data-aided frequency estimator for frequency-selective fading channels is introduced. This estimator is developed based on a least squares (LS) error criterion. The proposed method can estimate frequency offsets without the need for channel information. It is shown through simulation that the proposed LS method can be preferred to the existing techniques in mobile communications.

Preliminary Research of Regenerative Cooling Channel Design for Small Scale Bipropellant Thruster

ABSTRACT Applicability of regenerative cooling in 2,500 N-class biprop ellant thruster using hydrogen peroxide and kerosene was considered for improvement of performance and application in various missions. Calculation was performed by one dimensional approach using hydrogen peroxide as a coolant. The heat flux of thruster at nozzle throat was estimated at 18 ~ 20 MW/m 2 . Designed cooling channel width and height were 2.5 mm and 0.5 mm, respectively. Based on designed cooling channel configuration, flat plate model was manufactured and tested for estimation of pressure drop in cooling channel, and CFD analysis was compared with the test result. Th e maximum error between CFD analysis and experimental result was approximately 13% and average error was approximately 5%. 초 록 2,500 N급 과산화수소/케로신 이원 추력기의 성능 향상 및 다양한 미션에 적용하기 위하여 재생냉각의 적용가능성을 검토하였다. 1-D 계산을 통해 과산화수소를 냉각제로 하는 경우에 대한 계산을 수행하였다. 설계된 재생냉각 연소기의 노즐 목에서의 열 유속은 18~20 MW/m 2 로 예측되었으며, 그에 따른 유로의 너비는 2.5 mm 높이는 0.45 mm로 설계 되었다. 설계된 유로형상을 바탕으로 냉각 유로 내에서의 압력강하를 예측하기 위한 평판형 모델을 제작하여 실험을 진행하였고, 수치해석 결과와 비교를 수행하였다. 그 결과, 수치해석과 실험결과와의 최대 오차는 약 13%, 그리고 평균 오차는 약 5%로 계산되었다. Key Words: Regenerative Cooling(재생냉각), Bipropellant Thruster(이원추진제 추력기), Hydrogen Peroxide(과산화수소), Pressure Drop(압력 강하), Cooling Channel(냉각 유로)

Preliminary Flight Test of Hydrogen Peroxide Retro - Propulsion Module for Lander Demonstration

In this paper, we present a demonstration of a moon lander propulsion module that employs hydrogen peroxide as a m onopropellant. Our model consists of a pressuriz ed gas tank, propellant tank, pressure regulator, main thruster, valve driver , and valves for control of propellant flowrate. Both the p ressuriz ed gas tank (1 l) and the propellant tank (6.8 l) are made of aluminum liner and composite material to construct lightweight tanks with high strength. Nitrogen gas was used to pressuriz e the propellant and 90 wt% hydrogen peroxide was selected as the monopropellant. Our module uses a pressure regulating method to maintain constant pressure during propellant consumption. To reduce the total weight of the module , a honeycomb sandwich panel was also introduced . For control of a thrust force, two solenoid valves and a high -performance flow control valve were developed . The main thruster was designed to have a maximum force of 350 N . A MnO 2/Al 2O3 pellet was installed into the thrust chamber as a catalyst. For thrust development, a prototype thruster was designed and tested . A 1.3 -kg flight model ( FM ) thruster (including c atalysts ) was developed using the results from the prototype thruster , and all the components for the propulsion module in to a lander. The f inal mass of the lander was 14 kg (dry mass) and 23 kg (with propellant). In this paper, we present specific charact eristics of our lander model and test results for our thruster and lander.