In this paper, a simple but robust formation control scheme that can be applied to small unmanned aerial vehicles is proposed. The proposed formation control scheme is based on the virtual leader approach, where formation members control their position in relationship to a common virtual leader. The most distinguishing feature of the proposed scheme is that only information on the virtual leader is communicated between formation members. This feature gives the control scheme a extremely high robustness to communication failures and unit losses. Numerical simulation shows that the formation can be properly formed even if only 5% of the total communication succeeds. It is also shown that wingmen’s position can be estimated to some extent, using the information communicated.
The steady state of dipolar magnetic field expansion is examined by injecting a plasma jet from the center of the dipolar magnetic field (magnetic inflation). Using a two-dimensional hybrid particle-in-cell (PIC) code taking into account the finite Larmor radius effect, we examine the magnetic field inflation process when Argon (Ar) plasma is injected into a dipole magnetic field generated by a simple hoop coil; the plasma is injected within an angle of 30° in the polar direction. Compared to ideal magneto hydrodynamics (MHD) results, the results obtained using the hybrid PIC code are more accurate since the finite ion Larmor radius effect decreases the flow of magnetic flux with respect to the flow of the plasma jet.
Precise landing technology is one of the most important technologies for future lunar or planetary exploration missions. To achieve a precise landing, an advanced guidance scheme is necessary. This paper outlines a comparison of different solution methods for motion control equations utilized in guidance schemes for lunar descent, and proposes an advanced solution that allows a full depiction of descent vehicle motion from orbital states down to the final landing event. In the conventional solution methods, there exist some poor assumptions such as during descent, constant vertical gravitational acceleration is the only other force acting on the descent vehicle. This inadequate postulation limits the validity of the system solutions within a very low altitude terminal descent area; that is, close to the lunar surface. In this paper, an advanced descent solution is proposed where the centrifugal acceleration term is retained along with the gravitational acceleration term. It allows a complete representation of the descent module motion from orbital speed conditions down to the final landing state. Mathematical derivations of the new scheme are verified in terms of a conventional scheme, and comparative simulation results for a fully integrated solution, conventional schemes and a proposed advanced scheme are demonstrated to test the performance.
Different from calibrations previously conducted using the dynamic method, the GRACE-Level-1B non-conservative force data from the space-borne accelerometers between 1 January to 31 December, 2009, released by the Jet Propulsion Laboratory in the United States of America, is for the first time ever effectively calibrated using the priori Earth gravitational field model based on an improved energy conservation principle. The results are shown as follows: 1) The non-conservative force data from the space-borne accelerometers and attitude data from the star camera assembly are basically the same as the anticipated accuracies; 2) the systematic error of the raw non-conservative force data leads to a disturbed geopotential error linear drift of about 0.4 m2/s2 per day, however, the error is only 0.01 m2/s2 using the calibrated non-conservative force data, and a reasonable physical explanation is provided; 3) the advantages and disadvantages of calibrating non-conservative force data using the energy conservation principle and dynamic method are compared in detail; and 4) because the signal of the Earth’s gravitational field is sensitive to the systematic error of non-conservative force data from the space-borne accelerometers, the precise calibration of non-conservative force data is a key factor for the highly accurate recovery of the Earth gravitational field with high spatial resolution.
Analytical studies of the natural vibration characteristics and dynamic response for a simple model of an asymmetric flexible rolled-up solar array are presented. A natural vibration analysis of the solar array model including bending-torsion coupling due to geometric asymmetry provides natural frequencies and mode shapes. Unusual features of the lower mode frequencies and mode shapes are discussed. The dynamic response of the solar array induced by sudden radiation heating for a typical night-day orbital transition is formulated. Additionally, numerical calculations are conducted for the solar array of the Hubble Space Telescope (HST). Variations of the dynamic response with the axial preload forces of the solar array booms are examined, and the vibration response due to the radiation heating of the HST solar array is discussed.
The accurate evaluation of the ground reflection effect is of importance for acoustic measurement conducted during static-firing tests of rocket motors. The effectiveness of existing acoustic impedance models was examined by comparing experimental data collected from acoustically hard surfaces. Through these comparisons and the subsequent evaluation of the effect of meteorological conditions, it was confirmed that the existing impedance models are not suitable for the evaluation of long-distance propagation over a hard surface, which corresponds to the far-field conditions in the present static-firing tests of solid rocket motors. A new acoustic impedance model is proposed in this paper. Comparisons of the acoustic measurement data indicated that the new model is effective for both near- and far-field propagation of acoustic waves. The proposed model was applied to acoustic data collected during static-firing tests of solid rocket motors assuming distributed acoustic sources along the exhaust jet axis in order to remove the ground reflection effect.
Global positioning satellite/inertial navigation system (GPS/INS) integrated navigation systems are widely used for many military applications due to their complementary features. It is normally assumed that the military GPS receiver will be used in a signal-jamming environment. Various anti-jamming techniques have been studied from the point of hardware and/or software. The velocity aiding method is a well-known software-based anti-jamming technique used for GPS/INS integrated navigation systems. The dynamics of the GPS receiver can be measured by an INS and then fed into tracking loops to reduce the tracking loop bandwidth to promote jamming resistance. The Korean government has been developing a GPS adapter kit (GAK) that can change a conventional bomb into a high-precision GPS guided bomb (GGB). In this paper, we introduce a GPS/INS integrated navigation system in a GAK, a velocity aiding method applied to the navigation system, anti-jamming test procedures and the results for a GGB. The test results show that the GPS/INS integrated navigation system using velocity aiding method can provide enhanced anti-jamming capability by efficiently reducing the tracking loop bandwidth.
Cavitation is an inevitable phenomenon that occurs when improvements such as performance enhancement and weight reduction are made to the turbopump in liquid-propellant rocket engines. Unsteady cavitation may cause oscillations (cavitation instabilities) in the turbopump. Accurate prediction and efficient suppression of cavitation instabilities are important for designing turbopumps. We performed a numerical simulation of the unsteady cavitation in tandem cascades and compared the results with those obtained for a single-stage cascade. The type of cavitation instability could be controlled by changing the front- and rear-blade chord lengths. When the clearance gap between the front and rear blades was located near the cascade throat entrance, rotating-stall conditions could be easily achieved, even at high flow rates. Cavitation surge and super-synchronous and sub-synchronous rotating cavitations were suppressed when the clearance gap was located at 40% of the total chord length. When the clearance gap was located inside the cascade throat, cavitation reached a steady state at the σ value where the cavity length equaled the front-blade length; then, cavitation instabilities and unsteady cavitation were suppressed in the low-σ region. When the clearance gap was located at 80% of the total chord length, cavitation surge was completely suppressed, although rotating cavitation occurred over a larger region.
Controlling a space robot without actuators on the main body is an under-actuated control problem. As an approach to stabilization, various methods such as time-varying feedback controllers, discontinuous feedback controllers, center manifold-based methods, zero-dynamics methods, and sliding mode controllers have been proposed. However, with these methods, modeling errors and time delays have not been sufficiently considered. In order to obtain faster convergence time and compensate for modeling errors and time delays, an adaptive invariant manifold-based switching control method is proposed. In this paper, experiments are carried out to validate the proposed method, using the experimental setup of a planar two-link space robot. The experimental results show that the proposed method is capable of stabilizing the state variables in the required state, even if time delays and modeling errors exist in the system.
In this study, the high-speed impulsive (HSI) noise and aerodynamic performance of helicopter blades were improved using multi-objective design exploration (MODE) comprised of multi-objective shape optimization and data mining. To broaden the design space of the problem, geometry definitions with a high degree of freedom were adopted. As a result, remarkable improvements in both HSI noise and aerodynamic performance were achieved. For data mining, analysis of variance (ANOVA) and self-organizing map (SOM) were used to extract design knowledge of the helicopter blades. The results indicated that tip chord length and blade twist are important factors for HSI noise and aerodynamic performance, respectively. Based on the design knowledge obtained from data mining, an additional design variable (twist) was introduced. The solutions obtained from the shape optimization using variable twist showed better blade loading performance than that from shape optimization using fixed twist.