1,720,972 research outputs found
ON THE BELETSKY EQUATION
No full textThis paper was presented at the Beletsky Session of the 4th IAA Conference on University Satellite Missions and CubeSat Workshop held in Rome and dedicated to the memory of the great Russian mathematician and pioneer scientist of Astrodynamics. The paper is concentrated to the so called Beletsky equation, concerning the attitude motion of a satellite under gravity gradient torque. In fact Professor Beletsky dealt with many aspects of astrodynamics giving deep and useful contributions and establishing some theoretical basis for the development of the space activity in USSR, then it looks like disappointing that his name is so strictly linked to a simple equation such as(1 + e cos theta)theta '' - 2e sin theta ' + alpha sin theta = 4e sin thetadescribing the planar attitude motion of a satellite in elliptic orbit: The reason of the enormous success of the Beletsky equation is just in its simplicity and in the interesting characteristics of its phase space, where regular and periodic solutions are merged together with unstable and chaotic solutions. This is not unusual for a non linear differential equation, but in the Beletsky equation these characteristics can be understood in deep and the transition to chaos can be checked by various indices, so many researchers were attracted by the results that can be achieved from the abstract point of view of dynamical system theory while obtaining output of concrete interest in space applications. Most of the content presented in this paper is derived from my PhD thesis "Chaos in Astrodymanics" presented at the School of Aerospace Engineering of Rome in 1991 and developed under the supervision of Professor Filippo Graziani
Deployment strategies of a satellite constellation for polar ice monitoring
No full textThis research considers a constellation of 16 satellites equipped with SAR sensors and tailored to monitoring the polar ice evolution, with a suitable revisit time over the regions of interest. Satellite deployment includes three phases: (i) orbit injection, performed by the upper stage of the launch vehicle, (ii) orbit plane selection, and (iii) orbit phasing. This work is primarily focused on phase (ii). Carrier spacecraft are proposed as a valuable option to place the majority of satellites in their orbits. Two distinct strategies are proposed to complete this task. The first strategy is based on the use of chemical propulsion, combined with the perturbing action due to Earth oblateness. The second strategy considers the use of low-thrust electric propulsion, in conjunction with nonlinear orbit control. A comparison between these two approaches is drawn, in terms of deployment time and final mass ratio of the carrier. Orbit phasing concludes the constellation deployment, and is carried out by each satellite. A tradeoff is proven to exist between phasing time and propellant expenditure
Lunar formation flying invariant under zonal harmonic perturbations
No full textFormations flying of small satellites orbiting the Moon is currently considered a promising technology to provide communication and navigation services for both in-orbit and on-ground lunar activities. Their requirements set strict constraints on the altitude of the satellites, limiting it to medium and low lunar orbits, where the effects of perturbations associated to the non-uniform gravitational field of the Moon are not negligible. A major consequence of such perturbations is that any configuration designed based on the traditional Hill-Clohessy-Wiltshire model rapidly degrades, undermining the effectiveness of the formation or leading to potential collisions. To compensate for these effects, large reconfiguration maneuvers are necessary, but their extent is limited due to the little propellant budget characterizing small satellites. In this scenario, the design of formation configurations invariant under the zonal gravitational perturbation is of pivotal importance. We propose here a model that allows the compact characterization and simple design of relative trajectories for satellite formation flying that are invariant under zonal harmonic perturbations. The model is developed using a Hamiltonian formulation which includes all the zonal coefficients up to the desired degree. Canonical transformations are implemented (i) to absorb the first order terms in the perturbation and (ii) rearrange the Hamiltonian function of the linear problem as the sum of three terms, one associated to the saddle equilibrium and two associated to harmonic oscillators. In this new form, the complete set of bounded relative trajectories that are stable in the presence of gravitational perturbations can be identified by means of a single condition. Any orbit of this kind can than be constructed after applying the inverse of the canonical transformations, obtaining the position and velocity coordinates for the satellites of the formation. Compared to other solutions in the literature, the model proposed here does not require any truncation of the zonal harmonic terms or numerical refinement to provide initial states associated to Jn-invariant trajectories with desired geometric properties. The effectiveness of the model is verified by means of numerical analysis using the high-fidelity LP-165 orbit propagator integrated in the General Mission Analysis Tool by NASA, considering different lunar orbits and focusing on bounded relative trajectories. The analysis results in a negligible drift and a variation in the amplitude of the oscillations below 5% of their initial value over 1 month
An accurate modeling and performance of multistage launch vehicles for microsatellites via a firekwokk algorithm
No full textMultistage launch vehicles of reduced size, such as "Super Strypi" or "Sword", are currently investigated for the purpose of providing launch opportunities for microsatellites. This work proposes a general methodology for the accurate modeling and performance evaluation of launch vehicles dedicated to microsatellites. For illustrative purposes, the approach at hand is applied to the Scout rocket, a micro-launcher used in the past. Aerodynamics and propulsion are modeled with high fidelity through interpolation of available data. Unlike the original Scout, the terminal optimal ascent path is determined for the upper stage, using a firework algorithm in conjunction with the Euler-Lagrange equations and the Pontryagin minimum principle. Firework algorithms represent a recently-introduced heuristic technique, not requiring any starting guess and inspired by the firework explosions in the night sky. The numerically results prove that this methodology is easy-to-implement, robust, precise and computationally effective, although it uses an accurate aerodynamic and propulsive model
LUNISAT ORBIT MAINTENANCE AND LOW-THRUST MANEUVERS
No full textLunisat represents a next-generation microsatellite aimed at orbiting the Moon, and equipped with dispensers for the release of nanosatellites. This research is focused on orbital dynamics of the main microsatellite, and specifically addresses orbit maintenance and low-thrust maneuvers. Nonsingular equinoctial orbit elements are employed for orbit propagations, in conjunction with numerical averaging. This is a powerful technique for the numerical integration of the mean elements, and allows substantial computational improvements. Due to the masconian character of the lunar mass distribution, low altitude, near-circular lunar orbits are affected by a considerable number of harmonics of the Moon gravitational field. Thus, in the dynamical modeling a large number of harmonics are included, as well as the Earth and Sun perturbing influence as third bodies. Low altitude lunar satellites turn out to impact the Moon surface after a few weeks or months. In case of unsatisfactory lifetime, two simple orbit maintenance strategies are evaluated, together with the related propellant budget. Moreover, minimum-time low-thrust transfers for reducing the orbit altitude are investigated, either for nanosatellite release or for the conclusive phase of the Lunisat mission, which finally terminates with the impact on the Moon surface
Space traffic management: towards safe and unsegregated space transport operations
No full textProgress in spaceflight research has led to the introduction of various manned and unmanned reusable space vehicle concepts, opening up uncharted opportunities for the newborn space transport industry. For future space transport operations to be technically and commercially viable, it is critical that an acceptable level of safety is provided, requiring the development of novel mission planning and decision support tools that utilize advanced Communication, Navigation and Surveillance (CNS) technologies, and allowing a seamless integration of space operations in the current Air Traffic Management (ATM) network. A review of emerging platform operational concepts is conducted, highlighting both the challenges and the opportunities brought in by the integration with conventional atmospheric air transport. Common launch and re-entry planning methodologies are then discussed, where the physical and computational limitations of these approaches are identified and applicability to future commercial space transport operations is assessed. Attention is then turned to the on-orbit phase, where the unique hazards of the space environment are examined, followed by an overview to the necessary elements required for space object de-confliction and collision avoidance modelling. The regulatory framework evolutions required for spacecraft operations are then discussed, with a focus on space debris mitigation strategies and operational risk assessment. Within the atmospheric domain, possible extensions and alternatives to the conventional airspace segregation approaches are identified including promising Air Traffic Flow Management (ATFM) techniques to facilitate the integration of new-entrant platforms. Lastly, recent modelling approaches to meet on-orbit risk criteria are discussed and evolutionary requirements to improve current operational procedures are identified. These insights will inform future research on CNS/ATM and Avionics (CNS + A) systems and associated cyber-physical architectures for Space Traffic Management (STM)
Performance evaluation methodology for multistage launch vehicles with high-fidelity modeling
No full textMultistage launch vehicles of reduced size, such as ”Super Strypi” or ”Sword” are currently investigated for the purpose of providing launch opportunities for microsatellites. Currently, microsatellites are launched according to timing and orbit requirements of the main payload. The limited costs of microsatellites and their capability to be produced and ready for use in short time make them particularly suitable for ready-on-demand requests, such as facing an emergency. As a result, launch vehicles for the exclusive use of microsatellites would be very useful. This work considers the Scout rocket, a four-stage launch vehicle of reduced size used in the past. Its aerodynamics and propulsion are modeled with high fidelity, through interpolation of reliable, accurate available data. For the purpose of reducing the rocket complexity and size, as well as the launch cost per kg of payload, simplification of the rocket subsystems is advisable, and this includes also the guidance system and the related algorithm. In fact, open-loop guidance was actually employed during real Scout flights. In this research, open-loop guidance is investigated, under the assumption that the aerodynamic angle of attack is constant for each of the first three stages. Instead, for the upper stage the terminal optimal ascent path leading to orbit injection is determined through the use of a specific implementation of firework algorithm, in conjunction with the Euler-Lagrange equations and the Pontryagin minimum principle. Firework algorithms represent a recently-introduced heuristic technique inspired by the firework explosions in the night sky. The concept that underlies this method is relatively simple: a firework explodes in the search space of the unknown parameters, with amplitude and number of sparks determined dynamically. The succeeding iterations preserve the best sparks. The firework algorithm has several original features that can ensure satisfactory performance in parameter optimization problems, because both local search and global search are effectively performed through combination of various stochastic operators. With regard to the problem at hand, the unknown parameters are (i) the aerodynamic angles of attack of the first three stages, (ii) the coast time interval and (iii) the initial values of the adjoint variables conjugate to the upper stage dynamics. The numerical results unequivocally prove that the methodology at hand is rather robust, effective, and accurate, and definitely allows evaluating the performance attainable from multistage launch vehicles with accurate aerodynamic and propulsive modeling
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
