Current Issue : October - December Volume : 2012 Issue Number : 4 Articles : 6 Articles
Ammonium-nitrate-(AN-) based composite propellants prepared with a hydroxyl-terminated polybutadiene (HTPB)/polytetrahydrofuran\r\n(PTHF) blend binder have unique thermal decomposition characteristics. In this study, the burning characteristics of\r\nAN/HTPB/PTHF propellants are investigated. The specific impulse and adiabatic flame temperature of an AN-based propellant\r\ntheoretically increases with an increase in the proportion of PTHF in the HTPB/PTHF blend. With an AN/HTPB propellant, a\r\nsolid residue is left on the burning surface of the propellant, and the shape of this residue is similar to that of the propellant.\r\nOn the other hand, an AN/HTPB/PTHF propellant does not leave a solid residue. The burning rates of the AN/HTPB/PTHF\r\npropellant are not markedly different from those of the AN/HTPB propellant because some of the liquefied HTPB/PTHF binder\r\ncover the burning surface and impede decomposition and combustion. The burning rates of an AN/HTPB/PTHF propellant with\r\na burning catalyst are higher than those of an AN/HTPB propellant supplemented with a catalyst. The beneficial effect of the blend\r\nbinder on the burning characteristics is clarified upon the addition of a catalyst. The catalyst suppresses the negative influence of\r\nthe liquefied binder that covers the burning surface. Thus, HTPB/PTHF blend binders are useful in improving the performance of\r\nAN-based propellants....
An experimental and numerical analysis of the response of laminated composite plates under high-velocity impact loads of soft\r\nbody gelatine projectiles (artificial birds) is presented. The plates are exposed to tensile and compressive preloads before impact in\r\norder to cover realistic loading conditions of representative aeronautic structures under foreign object impact. The modelling\r\nmethodology for the composite material, delamination interfaces, impact projectile, and preload using the commercial finite\r\nelement code Abaqus are presented in detail. Finally, the influence of prestress and of different delamination modelling approaches\r\non the impact response is discussed and a comparison to experimental test data is given. Tensile and compressive preloading was\r\nfound to have an influence on the damage pattern. Although this general behaviour could be predicted well by the simulations,\r\nfurther numerical challenges for improved bird strike simulation accuracy are highlighted....
During atmospheric reentry, radiative heating is one of the most important component of the total heat flux. In this paper, we\r\ninvestigate how the thermal radiation coming from the postshock region interacts with the spacecraft structure. A model that\r\ntakes into account the radiation reflected by the surface is developed and implemented in a solid solver. A partitioned algorithm\r\nperforms the coupling between the fluid and the solid thermal fields. Numerical simulation of a hollow cone head and a deployed\r\nflap region shows the effects of the radiative cooling and the significance of the surface radiation....
The analysis of the optimal strategies for the deployment of a spacecraft into a highly elliptic orbit is carried out by means of\r\nan indirect optimization procedure, which is based on the theory of optimal control. The orbit peculiarities require that several\r\nperturbations are taken into account: an 8 Ã?â?? 8 model of the Earth potential is adopted and gravitational perturbations fromMoon\r\nand Sun together with solar radiation pressure are considered. A procedure to guarantee convergence and define the optimal\r\nswitching structure is outlined. Results concerning missions with up to 4.5 revolutions around the Earth are given, and significant\r\nfeatures of this kind of deployment are highlighted....
A model for the mechanical dynamics of a wind turbine is developed, which is the composition of three physical mechanisms:\r\nflexion, torsion, and rotational dynamics. A first contribution is the identification of the essential physical parameters that provide\r\na time-scale separation of these three mechanisms. Under the assumption of singular perturbations the time-scale separation\r\nallows to work with a reduced model of order one. This reduction has been essential for the control of this system allowing to\r\ncontrol designers to take into account only the reduced-order model. A second contribution consists in employing a measurement\r\nof the fore-aft nacelle acceleration with the reduced model, together with a Kalman filter to estimate the flexible DOFs of the system\r\n(tower and average blade deflection). The successful approach is tested on high-order nonlinear aeroelastic simulator (FAST)....
The coplanar orbit transfer problem has been an important topic in astrodynamics since the beginning of the space era. Though\r\nmany approximate solutions for power-limited orbit transfer problem have been developed, most of them rely on simplifications\r\nof the dynamics of the problem. This paper proposes a new approximation method called primer vector approximation method\r\nto solve the classic power-limited orbit transfer problem. This method makes no simplification on the dynamics, but instead\r\napproximates the optimal primer-vector function.With this method, this paper derives two approximate solutions for the powerlimited\r\norbit transfer problem. Numerical tests show the robustness and accuracy of the approximations....
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