391 research outputs found
The inside-out view on neutron-star magnetospheres
No full textWe construct hydromagnetic neutron star equilibria which allow for a non-zero electric current distribution in the exterior. The novelty of our models is that the neutron star's interior field is in equilibrium with its magnetosphere, thus bridging the gap between previous work in this area, which either solves for the interior assuming a vacuum exterior or solves for the magnetosphere without modelling the star itself. We consider only non-rotating stars in this work, so our solutions are most immediately applicable to slowly rotating systems such as magnetars. Nonetheless, we demonstrate that magnetospheres qualitatively resembling those expected for both magnetars and pulsars are possible within our framework. The ‘inside-out’ approach taken in this paper should be more generally applicable to rotating neutron stars, where the interior and exterior regions are again not independent but evolve together
Equilibria and oscillations of magnetised neutron stars
No full textWe investigate equilibrium configurations and oscillation spectra of neutron stars,modelled as rotating magnetised fluid bodies in Newtonian gravity. We also explorethe idea that these model neutron stars could have dynamics analogous to rigid-bodyfree precession.In axisymmetry, the equations of magnetohydrodynamics reduce to a purelytoroidal-field case and a mixed-field case (with a purely poloidal-field limit). Wesolve these equations using a nonlinear code which finds stationary rotating magnetisedstars by an iterative procedure. We find that despite the general nature ofour approach, the mixed-field configurations we produce are all dominated by theirpoloidal component. We calculate distortions induced both by magnetic fields andby rotation; our results suggest that the relationship between the magnetic energyand the induced ellipticity should be close to linear for all known neutron stars.We then investigate the oscillation spectra of neutron stars, using these stationaryconfigurations as a background on which to study perturbations. This is doneby evolving the perturbations numerically, making the Cowling approximation andspecialising to purely toroidal fields for simplicity. The results of the evolutionsshow a number of magnetically-restored Alfv´en modes. We find that in a rotatingstar pure inertial and pure Alfv´en modes are replaced by hybrid magneto-inertialmodes. We also show that magnetic fields appear to reduce the effect of the r-modeinstability.Finally, we look at precession-like dynamics in magnetised fluid stars, using bothanalytic and numerical methods. Whilst these studies are only preliminary, theyindicate deficiencies in previous research on this topic. We suggest ways in whichthe problem of magnetised-fluid precession could be better understood
Europa Lander Landing System: Study on Deployment, Landing and Preliminary Design
No full textThe Europa Lander is a mission by NASA to land scientific instruments on the Jovian Moon by 2030. The current design of the landing system envisions fixed landing legs with the lander being lowered down at very low velocities (≈ 0.1 m/s) by use of a Skycrane. This latter system not only adds mass, but also requires complicated moving parts and a high reliance on control systems to work properly. This thesis wants to propose an alternative design to the Europa Lander Landing System that uses sturdier legs that can withstand landing at higher velocities than originally envisioned. This eliminates the need to use a Skycrane system. In order to pack them in a more efficient way while coasting, the landing legs are of the deployable type.In order to track the deployment dynamics of the landing system a 2-dimensional Matlab deployment model based on double pendulum motion with damping is created. Furthermore, in order to prove the landability with a prescribed landing case, a landing model is created using FEA software Abaqus. The landing model is used to make sure that stresses, strains and G forces applied to the rest of the lander don’t exceed prescribed values as dictated by requirements. The implementation of both models is verified by comparing models found in literature with the ones made for the purpose of this thesis. Good correlation is shown with less than 10% of difference between literature results and results for this thesis.The result through the modelling found a specific geometry with parameters that include, but are not limited to: length of leg elements, joint damping and maximum deployment angles. The 2-dimensional geometry features output by the Matlab deployment model are fed into the Abaqus landing model in order to assess the landing performance with a single leg landing. The landing case has been chosen as a worst case scenario. 2 landing leg designs, with the same geometry, but one made of metallic parts and the other made of composite materials are compared in the landing model. In this model it was found that both designs pass the stress (or strain for the composite leg design) and G forcesrequirements when a shock absorber is attached to the leg and when they land on deformable Lunar like soil. On the other hand, the landing legs don’t pass G forces requirements when landing on stiff soil.Through the modelling, it was also possible to give preliminary design guidelines over the subsystems making the landing system. Therefore, system engineering practices have been used in order to initiate the design of some of the subsystems such as the landing damping, the leg joints and the locking mechanism to lock the legs at the desired angle and make sure they don’t retract. Simplifications have been made in order to model some of the landing and deployment aspects of the system for this thesis. Matters related to the adaptability and the detailed design of each of the subsystems have been left as future work.In the end, a comparison between the design developed in this thesis and the current Europa Lander Landing System design is made difficult by the lack of technical data on the latter related to mass. Nevertheless, a design that uses the decelerations of the engine module before touchdown is developed and the suggested material for the leg elements is CFRP. The design, though, didn’t pass mass fraction requirements when computing the mass of the extra subsystems. Even so, locations where potential gains through mass optimization can be performed for future studies have been pointed out.Aerospace Engineerin
The Mutational Landscape of Head and Neck Squamous Cell Carcinoma
No full text2012 August 09 Author ManuscriptHead and neck squamous cell carcinoma (HNSCC) is a common, morbid, and frequently lethal malignancy. To uncover its mutational spectrum, we analyzed whole-exome sequencing data from 74 tumor-normal pairs. The majority exhibited a mutational profile consistent with tobacco exposure; human papillomavirus was detectable by sequencing DNA from infected tumors. In addition to identifying previously known HNSCC genes (TP53, CDKN2A, PTEN, PIK3CA, and HRAS), our analysis revealed many genes not previously implicated in this malignancy. At least 30% of cases harbored mutations in genes that regulate squamous differentiation (for example, NOTCH1, IRF6, and TP63), implicating its dysregulation as a major driver of HNSCC carcinogenesis. More generally, the results indicate the ability of large-scale sequencing to reveal fundamental tumorigenic mechanisms
Descent, Touchdown and Repositioning of a hopping planetary lander on Enceladus
No full textThis report deals with the design of a complete guidance, navigation and control system for a planetary lander on Enceladus, for both descent and repositioning. A basic lander model with a main engine, attitude thrusters and a landing gear is presented, for which the equations of motion in different reference frames are derived, including the effects of third body perturbations and non-homogeneous gravity fields. The guidance system incorporates a gravity-turn guidance logic for a flat and a spherical moon model for the initial descent phase, a quadratic guidance logic for both pinpoint landing and repositioning, a velocity nullifying logic for the terminal guidance phase and a ballistic guidance logic for a more fuel-efficient repositioning. The control system consists of a linear quaternion controller combined with a pulse-width-pulse-frequency modulator. The navigation system is build around an Inertial Measurement Unit with an extended Kalman filter to fuse the attitude and position measurements. The on-board software incorporates a basic hazard avoidance system, which uses a simulated LIDAR scan of the target area to generate a hazard map. The retargeting is based on a reachability and fuel consumption analysis. The spacecraft is capable of fully autonomous landings. This guidance, navigation and control system is combined in a simulation software written in C++ to test the system performance in presence of various error sources. All program elements have been tested with results from literature. The Monte Carlo simulations of the full GNC system include variances of the lander's state vector and of the GNC configuration parameters. The success rate is 90% for the descent phase, and 98% for the repositioning phase. The mean landing precision is in the order of 1m, with velocity and pointing errors of 0.2 m/s and 1.4°, respectively. Stricter requirements on the lander's mass distribution, and the implementation of a feedback velocity nullifying guidance scheme can increase the mission success rate even further. The thesis work shows, that a landing mission to Enceladus is possible with current technology. The Enceladus Lander Simulator is comprehensive software package for descent and repositioning simulations, and its modularity allows extensions in the future.Astrodynamics and Space MissionsEarth Observation and Space SystemsAerospace Engineerin
Orpheus in America
No full textTranslated from original writings by Lander MacClintock in 1957, Orpheus in America: Offenbach's Diary of His Journey to the New World offers unique perspective and insight into the thoughts of notable German-French composer, Jacques Offenbach during his 1876 visit to the United States. The book is was originally self-described on its cover flap as follows: "The name of Offenbach, king of opéra bouffe, author of La Vie Parisienne and The Tales of Hoffman is synonymous with the gaiety and wit of the French Second Empire. Visiting America in 1876, Offenbach … was endlessly inquisitive about all he saw, from art and music to the New York Fire Department, from horse-cars to women's dress, from Niagara Falls to advertising. Offenbach found America and the Americans a source of unfailing entertainment, and the candid impressions which he recorded in his journal are fresh, droll, and full of charm.
Deforming prestacks : a box operadic approach
No full textAbstract: This thesis is devoted to the study of prestacks and their deformation theory through box operads. Prestacks are algebro-geometric objects which generalize presheaves of algebras and appear in Non-Commutative Algebraic Geometry as non-commutative deformations of schemes. The innovation we present in this thesis is that prestacks and their deformation theory are governed by an abstract calculus of stacking 2-dimensional rectangular boxes. The new algebraic gadgets governing this combinatorial theory we call box operads. We start out the thesis by developing in detail three deformation perspectives (elementary, moduli and Lie) for associative algebras and show that the three associated deformation functors are isomorphic. Further, we move from deforming associative algebras to deforming prestacks, passing by commutative algebras, schemes and presheaves along the way. In the second part of the thesis, we provide a combinatorial description of the symmetric coloured operad \u201cbox-op\u201d which encodes box operads, as stackings of rectangular boxes. In our first main result, we endow a suitable (graded, zero differential) totalisation of box-op with a morphism from the L-infinity operad. For a quiver V on a small category, we define an endomorphism box operad End(V) containing the Gerstenhaber-Schack object C_GS(V) as a L-infinity subalgebra of its totalisation Tot(End(V)). Further, we show that an element (m,f,c) of C_GS(V) satisfies the Maurer-Cartan equation precisely when A=(V,m,f,c) is a lax prestack. In the third part, we provide a box operad Lax encoding lax prestacks whose relations are far from quadratic. Indeed, their relations are cubic and quartic, and moreover inhomogeneous. We show that box operads constitute the correct framework to encode lax prestacks and resolve their relations up to homotopy. Our second main result is a Koszul duality theory for box operads, extending the duality for (nonsymmetric) operads. In this new theory, the classical restriction of being quadratic is replaced by the notion of being \u2018thin-quadratic\u2019, a condition referring to a particular class of \u2018thin\u2019 operations. We show that Lax is not Koszul by explicitly computing its Koszul complex. We then go on to remedy the situation by suitably restricting the Koszul dual box cooperad to obtain our third main result: we establish a minimal (in particular cofibrant) model Lax-infinity for the box operad Lax
A minimal model for prestacks via Koszul duality for box operads
No full textPrestacks are algebro-geometric objects whose defining relations are far from
quadratic. Indeed, they are cubic and quartic, and moreover inhomogeneous. We
show that box operads, a rectangular type of operads introduced in
arXiv:2305.20036, constitute the correct framework to encode them and resolve
their relations up to homotopy. Our first main result is a Koszul duality
theory for box operads, extending the duality for (nonsymmetric) operads. In
this new theory, the classical restriction of being quadratic is replaced by
the notion of being , a condition referring to a
particular class of ``thin'' operations. Our main case of interest is the box
operad encoding lax prestacks as algebras. We show that is not
Koszul by explicitly computing its Koszul complex. We then go on to remedy the
situation by suitably restricting the Koszul dual box cooperad
Lax^{\antishriek} to obtain our second main result: we establish a minimal
(in particular cofibrant) model for the box operad
encoding lax prestacks. This sheds new light on Markl's question on the
existence of a cofibrant model for the operad encoding presheaves of algebras
from arXiv:math/0103052. Indeed, we answer the parallel question with
presheaves viewed as prestacks, and prestacks considered as algebras over a box
operad, in the positive.Comment: 45 pages, lots of pictures; All comments are welcome
A Model-Based Systems Engineering approach to the conceptual design of an adaptable Small Solar System Body Lander
No full textStandardization is becoming more commonplace in space missions, aiming to increase reliability and drive down costs. Due to the relative similarities in operating environment between Small Solar System Bodies (SSSBs), a highly-adaptable lander may be able to service a majority of SSSBs with minor adaptations. The model-based systems engineering (MBSE) framework is implemented to enable agile development. The key enabling subsystems required for landing are defined, resulting in an impact dampening and a rebound suppression subsystem. Concepts for these subsystems are evaluated and traded-off. A number of astrodynamics simulations for the trajectory dynamics is made using Tudat for each SSSB, to find maximum deployment height and impact speed. The detailed subsystem design focuses on finding a suitable, highly-adaptable, commercial-off-the-shelf solution; with different variants of the final design allocated to each SSSB. Lastly, the scalability of the key enabling subsystems is analyzed. This results in the complete conceptual design of a highly-adaptable SSSB lander.Aerospace Engineerin
Studies of lunar habitats with application to the modeling and analysis of a deployable lunar lander
No full textStudied in this thesis is the response of a specific spring-mass damper design used for the deployment of a lunar base. Several variations of the geometric model are employed including a linear decoupled, linear coupled, nonlinear decoupled, and nonlinear coupled model. What has been shown is that these models can help inform design of a base and to predict the motion of the lander and provide useful information on the positions, velocities, and accelerations of different parts of the structure. Additionally, a large portion of this research work includes the analysis of the slider-crank mechanism which can also be a useful model for the deployment of lunar bases.M.S.Includes bibliographical reference
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