714 research outputs found
Lunar Ascent and Orbit Injection via Neighboring Optimal Guidance and Constrained Attitude Control
Future human or robotic missions to the Moon will require efficient ascent path and accurate orbit injection maneuvers, because the dynamical conditions at injection affect the subsequent phases of spaceflight. This research focuses on the original combination of two techniques applied to lunar ascent modules, i.e., (1) the recently introduced variable-time-domain neighboring optimal guidance (VTD-NOG), and (2) a constrained proportional-derivative (CPD) attitude control algorithm. VTD-NOG belongs to the class of feedback implicit guidance approaches aimed at finding the corrective control actions capable of maintaining the spacecraft sufficiently close to the reference trajectory. CPD pursues the desired attitude using thrust vector control while constraining the rate of the thrust deflection angle. The numerical results unequivocally demonstrate that the joint use of VTD-NOG and CPD represents an accurate and effective methodology for guidance and control of lunar ascent path and orbit injection in the presence of nonnominal flight conditions
Design Methodology and Performance Evaluation of New Generation Sounding Rockets
Sounding rockets are currently deployed for the purpose of providing experimental data of the upper atmosphere, as well as for microgravity experiments. This work provides a methodology in order to design, model, and evaluate the performance of new sounding rockets. A general configuration composed of a rocket with four canards and four tail wings is sized and optimized, assuming different payload masses and microgravity durations. The aerodynamic forces are modeled with high fidelity using the interpolation of available data. Three different guidance algorithms are used for the trajectory integration: constant attitude, near radial, and sun-pointing. The sun-pointing guidance is used to obtain the best microgravity performance while maintaining a specified attitude with respect to the sun, allowing for experiments which are temperature sensitive. Near radial guidance has instead the main purpose of reaching high altitudes, thus maximizing the microgravity duration. The results prove that the methodology at hand is straightforward to implement and capable of providing satisfactory performance in term of microgravity duration
Lunar ascent and orbit injection via locally-flat near-optimal guidance and nonlinear reduced-attitude control
This work deals with an explicit guidance and control architecture for autonomous lunar ascent and orbit injection, i.e., the locally-flat near-optimal guidance, accompanied by nonlinear reduced-attitude control. This is a new explicit guidance scheme, based on the local projection of the position and velocity variables, in conjunction with the real-time solution of the associated minimum-time problem. A recently-introduced quaternion-based reduced-attitude control algorithm, which enjoys quasi-global stability properties, is employed to drive the longitudinal axis of the ascent vehicle toward the desired direction. Actuation, based on thrust vectoring, is modeled as well. Extensive Monte Carlo simulations prove the effectiveness of the guidance, control, and actuation architecture proposed in this study for precise lunar orbit insertion, in the presence of nonnominal flight conditions
Simone Assemani da Vienna a Trieste
The essay deals with nineteen hitherto unpublished letters by Simone Assemani
to the apostolic nuncio in Vienna, Giuseppe Garampi (1725-1792): they all
stem from 1783, the first of two years Assemani spent in Trieste as an employee of the Belletti-Zaccar company, which ran the trade between the Christian
world of Northern Europe and the Turkish Levant (Egypt). In these texts, Assemani appears as a translator and interpreter from and into Arabic, as well as
the man in charge of the relationships with Cairo's bankers both on land and
for the maritime trade. The tone of the letters is rather atypical if compared to
Assemani's later correspondence: we glean crucial evidence to better understand the moral and material heritage of the Assemani family, who had given
a decisive thrust to Oriental studies in Rome from the beginning of the ISth
century through 17S0. We also get to know important elements concerning
the trading activities in Trieste's outport in 17S3, as well as Simone's lifelong
political and diplomatic activity in the frame of Propaganda Fide, a totally
unexplored field to date
Biomimetic emulsions reveal the effect of homeostatic pressure on cell-cell adhesion
Cell-cell contacts in tissues are continuously subject to mechanical forces
due to homeostatic pressure and active cytoskeleton dynamics. While much is
known about the molecular pathways of adhesion, the role of mechanics is less
well understood. To isolate the role of pressure we present a dense packing of
functionalized emulsion droplets in which surface interactions are tuned to
mimic those of real cells. By visualizing the microstructure in 3D we find that
a threshold compression force is necessary to overcome electrostatic repulsion
and surface elasticity and establish protein-mediated adhesion. Varying the
droplet interaction potential maps out a phase diagram for adhesion as a
function of force and salt concentration. Remarkably, fitting the data with our
theoretical model predicts binder concentrations in the adhesion areas that are
similar to those found in real cells. Moreover, we quantify the adhesion size
dependence on the applied force and thus reveal adhesion strengthening with
increasing homeostatic pressure even in the absence of active cellular
processes. This biomimetic approach reveals the physical origin of
pressure-sensitive adhesion and its strength across cell-cell junctions.Comment: 20 pages, 5 figure
Evidence for marginal stability in emulsions
We report the first measurements of the effect of pressure on vibrational
modes in emulsions, which serve as a model for soft frictionless spheres at
zero temperature. As a function of the applied pressure, we find that the
density of states D(omega) exhibits a low-frequency cutoff omega*, which scales
linearly with the number of extra contacts per particle dz. Moreover, for
omega<omega*, D(omega)~ omega^2/omega*^2; a quadratic behavior whose prefactor
is larger than what is expected from Debye theory. This surprising result
agrees with recent theoretical findings. Finally, the degree of localization of
the softest low frequency modes increases with compression, as shown by the
participation ratio as well as their spatial configurations. Overall, our
observations show that emulsions are marginally stable and display
non-plane-wave modes up to vanishing frequencies
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