Lev D.R.,Rafael Laboratories
Journal of Physics: Conference Series | Year: 2017
In this paper we review plasma propulsion development approach which focuses on innovation. We then bring the example of the state of Israel in general, and Rafael in particular, and show how it has adopted an innovative approach to develop a low power Hall thruster and a low current cathode. To present one special test-case of innovation we elaborate upon the development process of a heaterless hollow cathode that was developed at Rafael. In particular, by presenting the cathode characterization and wear test results we demonstrate that the heaterless cathode developed has a sufficiently wide operational range and may operate continuously for 1,500 hours without any measurable degradation in performance. © Published under licence by IOP Publishing Ltd.
Partom Y.,Rafael Laboratories
AIP Conference Proceedings | Year: 2017
It is widely accepted that shock initiation and detonation of heterogeneous explosives comes about by a two-step process known as ignition and growth. In the first step a shock sweeping through an explosive cell (control volume) creates hot spots that become ignition sites. In the second step, deflagration waves (or burn waves) propagate out of those hot spots and transform the reactant in the cell into reaction products. The macroscopic (or average) reaction rate of the reactant in the cell depends on the speed of those deflagration waves and on the average distance between neighboring hot spots. Here we simulate the propagation of deflagration waves out of hot spots on the mesoscale in axial symmetry using a 2D hydrocode, to which we add heat conduction and bulk reaction. The propagation speed of the deflagration waves may depend on both pressure and temperature. It depends on pressure for quasistatic loading near ambient temperature, and on temperature at high temperatures resulting from shock loading. From the simulation we obtain deflagration fronts emanating out of the hot spots. For 8 to 13 GPa shocks, the emanating fronts propagate as deflagration waves to consume the explosive between hot spots. For higher shock levels deflagration waves may interact with the sweeping shock to become detonation waves on the mesoscale. From the simulation results we extract average deflagration wave speeds.
Rusnak I.,Rafael Laboratories
57th Israel Annual Conference on Aerospace Sciences, IACAS 2017 | Year: 2017
The Mean Squares Error criterion is used to derive the optimal estimator for continuous nonlinear systems with prescribed-guaranteed level of time-varying convergence rate on finite time interval. The solution is explicit and gives recursive formula of the optimal estimator. It is shown by simulations that the time-varying convergence rate on finite time interval approach enables to improve in Pareto sense the speed of convergence (transient period) and the steady state behavior simultaneously.
Partom Y.,Rafael Laboratories
AIP Conference Proceedings | Year: 2017
It is well known that the shear modulus (G) and the yield stress (Y) of metals increase with pressure (P) and decrease with temperature (T). Steinberg, in his popular compendium of dynamic material properties, assumes for Y/Y0(P,T)=G/G0(P,T) linear relations based on derivatives determined experimentally at ambient conditions. But recent tests with high pressure dynamic loadings of certain metals obtained results that generally deviate from Steinberg's predictions. Here we use a different approach to estimate G/G0(P,T). As a first approximation we let G/G0=K/K0, where K is the isentropic bulk modulus. With this assumption we compute the longitudinal sound speed of tantalum along its principal Hugoniot and compare to recent measurements. There is a very slight disagreement, which we can correct by assuming (second approximation) that Poisson ratio decreases slightly with pressure and increases slightly with temperature. As K=ρc2 is always available in a hydrocode run from the equation of state, so are therefore also G/G0 and Y/Y0.
Luttwak G.,Rafael Laboratories |
Falcovitz J.,Hebrew University of Jerusalem
Computers and Fluids | Year: 2013
A recently formulated frame-invariant monotonicity criterion and slope limiter for vectors was applied to the Staggered Mesh Godunov-SMG/Q scheme for Lagrangian and ALE hydrodynamics. The VIP (vector-space polygon or polyhedron) was shown to be a natural extension of monotonicity constraints from scalar to vector variables. Taking notice of the fact that gradients of scalars are vectors, we now seek to use this new concept to devise better, and perhaps truly multidimensional, slope limiters for scalar variables. The proposed scheme constitutes a generalization of a 1D monotonic-averaging limiter for scalars, to a VIP type limiter for (vector) gradients of scalars in 2D or 3D. Test cases computed by the SMG/ALE scheme, using the new VIP limiter for gradients, are presented. As we can see from them, the new method, while being robust in strong shock computations, can better preserve gradients of density under advection. © 2012 Elsevier Ltd.
Rusnak I.,Rafael Laboratories
Journal of Guidance, Control, and Dynamics | Year: 2010
In preliminary analysis of guided systems, it is required to assess the miss distance performance from some small set of parameters. This paper presents analytical formulas of bounds on the achievable rms miss by a radar-guided missile against a variety of target maneuvers. These formulas use a set of core parameters that affect the miss distance; thus, they can be used for synthesis and analysis of the performance of radar-guided tactical missiles. The bound is derived, subject to the assumption that the missile guidance law and estimator are fully matched to the missile dynamics, the target maneuver, and the glint noise. The glint is the dominant noise source, the missile applies frequency agility, and there is no missile acceleration limit. No system can achieve smaller rms miss distance than the one presented, subject to the stated assumptions. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Marcus S.W.,Rafael Laboratories
IEEE Transactions on Antennas and Propagation | Year: 2013
The optical transformation is generally used to deform free space in order to obtain novel effects on waves. Such effects can be realized by replacing the deformed region by a 'metamaterial' characterized by ε and μ tensors that are determined by the transformation. A new type of transformation is proposed which not only deforms a free space region, but transforms a non-trivially shaped conducting surface that bounds this region as well. The deformations are designed to map the original region into a planar slab of metamaterial, and to map the conducting surface into a plate which bounds this slab. Since the fields outside the deformed region-which include the fields scattered by the original conducting surface-are unaffected by the transformation, they can, in effect, be produced by scattering from the planar metamaterial-coated plate created from the transformation. This substitution of a non-trivially shaped conducting surface by a planar metamaterial-coated plate is applied to (a) a face of a wedge to produce reflection from the planar metamaterial-coated plate that is not in the specular direction, and (b) a parabolic reflector antenna to design a planar system which produces the same radiation pattern as the reflector. Both devices are numerically validated. © 2013 IEEE.
Sason I.,Technion - Israel Institute of Technology |
Shuval B.,Rafael Laboratories
IEEE Transactions on Information Theory | Year: 2011
A design of robust error-correcting codes that achieve reliable communication over various channels is of great theoretical and practical interest. Such codes are termed universal. This paper considers the universality of low-density parity-check (LDPC) code ensembles over families of memoryless binary-input output-symmetric (MBIOS) channels. Universality is considered both under belief-propagation (BP) and maximum-likelihood (ML) decoding. For the BP decoding case, we derive a density-evolution-based analytical method for designing LDPC code ensembles that are universal over various families of MBIOS channels. We also derive a necessary condition for universality of LDPC code ensembles under BP decoding; this condition is used to provide bounds on the universally achievable fraction of capacity. These results enable us to provide conditions for reliable/unreliable communications under BP decoding that are based on the Bhattacharyya parameter of the channel. For the ML decoding case, we prove that properly selected regular LDPC code ensembles are universally capacity-achieving for the set of equi-capacity MBIOS channels and extend this result to punctured regular LDPC code ensembles. © 2011 IEEE.
Djerassi S.,Rafael Laboratories
Multibody System Dynamics | Year: 2012
This paper deals with one-point collision with friction in three-dimensional, simple non-holonomic multibody systems. With Keller's idea regarding the normal impulse as an independent variable during collision, and with Coulomb's friction law, the system equations of motion reduce to five, coupled, nonlinear, first order differential equations. These equations have a singular point if sticking is reached, and their solution is 'navigated' through this singularity in a way leading to either sticking or sliding renewal in a uniquely defined direction. Here, two solutions are presented in connection with Newton's, Poisson's and Stronge's classical collision hypotheses. One is based on numerical integration of the five equations. The other, significantly faster, replaces the integration by a recursive summation. In connection with a two-sled collision problem, close agreement between the two solutions is obtained with a few summation steps. © Springer Science+Business Media B.V. 2011.
Bar D.E.,Rafael Laboratories |
Raboy S.,Rafael Laboratories
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing | Year: 2013
A novel approach to automatic detection of moving vehicles from a single satellite WorldView-2 imagery is presented. The technique is based on the time gap between three sensor band groups: panchromatic, multispectral, and four new additional multispectral bands. The entire process is automatic and includes movement estimation followed by moving object spectral restoration and construction of spatially built object profiles to estimate the movement direction and velocity. The approach neither relies on external information like road data or site models, nor is limited to vehicle type. The performance of the new approach is demonstrated via detection of several vehicle types on WorldView-2 satellite imagery of the San Francisco area. © 2008-2012 IEEE.