On the dynamics, navigation and control of a spacecraft formation of solar concentrators in the proximity of an asteroid

This purpose of this dissertation is to ascertain whether solar sublimation is a viable method for the deflection of a Near Earth Asteroid. From a research view point, the methods and analysis are applicable to proximal motion around a celestial body, in particular one with a non-Keplerian or irregular orbit as in the case here with the orbit being constantly altered by the deflection action and subject to perturbations, such as solar radiation pressure. Two concepts, and the corresponding dynamics and control, are presented based on previous trade-off and optimisation studies. The first uses a paraboloidic reflector to concentrate the solar radiation onto a solar-pumped laser, which is then directed onto a specific spot on the NEO by a small directional mirror. The spacecraft orbits are designed to fly in formation with the asteroid around the Sun, and are based on the orbital element differences. The formation orbits were optimised based on a number of single and multiple objective functions. A feedback control law is presented for the orbital maintenance required to counteract the solar radiation pressure (due primarily to the large surface area of the primary reflector), and the third-body effects due to the gravitational field of the asteroid. The second option takes advantage of the balance between the gravity attraction of the NEO and solar pressure acting on the collector. The mirror focuses the light directly onto the asteroid surface, controlling the beam by adjusting the focal point of the primary reflector. By altering the shape of the mirror surface, both the focal point and the vector of the solar radiation pressure can be manipulated. An interesting navigation strategy is proposed based on the attitude measurements, the inertial position of each spacecraft, the intersatellite position and velocity measurements, and a 2D image from a rotating onboard camera. The navigational data is used for both the orbital control of the spacecraft and for the beam pointing. The results of simulations of a hypothetical deflection mission of the asteroid Apophis are presented for the dynamics, control, attitude and navigation, accounting for solar radiation pressure, the gravity field of the asteroid, and the deviation of the NEO orbit. The results show that both concepts provide the required deflection with a feasible mass at launch, solving most of the issues related to the solar sublimation method. One of the critical aspects of this deflection concept is properly placing the concentrators in the proximity of the asteroid in order to avoid the plume impingement and the occultation from the asteroid itself. Issues regarding the contamination of the mirrors are addressed and compared with the simulated deflections predicted considering no contamination. Lastly, initial system mass budgets are presented

Deployed payload analysis for a single stage to orbit spaceplane

The growth of the small satellite market requires an affordable and flexible launcher to satisfy the demand for insertion to a varied range of specific orbits. This paper presents the application and results of a tool created for the preliminary analysis of a reusable lifting body vehicle to address the demand of small to medium sized payloads. Trajectory optimisation is performed on the ascent mission of the launcher from a nominal cruise altitude to the release of the payload into different orbits with different mission and vehicle objectives and constraints. Two vehicle configurations are studied and applied to test cases of insertion to 200 km circular orbits at different inclinations. Further analysis on the performances obtained at different altitudes is conducted to study the effect on maximum payload mass. The advantages and limitations of the single stage to orbit approach are discussed and ways to overcome the latter are further analysed with the introduction of a small upper stage or tug. This additional engine can perform small manoeuvres to correct the flight path for orbital insertion, which expands the type of orbits that can be reached, and is integrated in the optimisation routine. The overall performance of the different launch systems are optimised for each mission, and compared against each other looking at the total payload mass and required mass fraction to reach different orbits

Spaceplane trajectory optimisation with evolutionary-based initialisation

In this paper, an evolutionary-based initialisation method is proposed based on Adaptive Inflationary Differential Evolution algorithm, which is used in conjunction with a deterministic local optimisation algorithm to efficiently identify clusters of optimal solutions. The approach is applied to an ascent trajectory for a single stage to orbit spaceplane, employing a rocket-based combine cycle propulsion system. The problem is decomposed first into flight phases, based on user defined criteria such as a propulsion cycle change translating to different mathematical system models, and subsequently transcribed into a multi-shooting NLP problem. Examining the results based on 10 independent runs of the approach, it can be seen that in all cases the method converges to clusters of feasible solutions. In 40% of the cases, the AIDEA-based initialisation found a better solution compared to a heuristic approach using constant control for each phase with a single shooting transcription (representing an expert user). The problem was run using randomly generated control laws, only 2/20 cases converged, both times with a less optimal solution compared to the baseline heuristic approach and AIDEA

Comparison of the emissions of current expendable launch vehicles and future spaceplanes

This paper compares the environmental impact of two types of launch vehicles, an expendable vertical launcher (Delta IV) and a conceptual SSTO spaceplane. A realistic trajectory for the spaceplane is generated using a multiple-shooting trajectory optimisation method, which integrates physical models and generates an optimal control law minimising the fuel consumption and the emissions of the flight. These were compared with the emissions from a standard Delta IV trajectory. The launch was to a 200 km circular LEO at 27.5° inclination. The chemical investigated is H2O, which contributes to the depletion of the ozone layer in the stratosphere. The study shows that for the ascent trajectory the spaceplane produces a total of 5.0143 x 105 kg of H2O, compared with 2.24 x 105 kg for the Delta IV. The spaceplane has a peak production altitude in the sensitive lower stratosphere, compared to the much lower peak production altitude of the Delta IV

Optimisation of ascent and descent trajectories for lifting body space access vehicles

One of the forerunners for future space access vehicles is the spaceplane, a lifting body vehicle capable of powered horizontal take-off and landing. Employing strategies from multidisciplinary design optimisation, this paper outlines the approaches and models used towards developing an integrated design platform to assess the preliminary design and performance of a spaceplane. The trajectory and control is optimised, based on different mission objectives and constraints, for the ascent and descent mission segments of a conceptual single stage to orbit vehicle, to a circular low Earth orbits from different take-off and landing sites. A modular approach is employed, dividing the mission into phases based on model discontinuities, changes in the operating environment or vehicle operation, mission objectives or constraints. The problem is reformulated by direct transcription using multiple shooting into a constrained NLP problem, and solved by a combination of genetic algorithms for a global search, and SQP plus interior point methods for local refinement with hard constraints

Multi-objective optimal control of ascent trajectories for launch vehicles

This paper presents a novel approach to the solution of multi-objective optimal control problems. The proposed solution strategy is based on the integration of the Direct Finite Elements Transcription method, to transcribe dynamics and objectives, with a memetic strategy called Multi Agent Collaborative Search (MACS). The original multi-objective optimal control problem is reformulated as a bi-level nonlinear programming problem. In the outer level, handled by MACS, trial control vectors are generated and passed to the inner level, which enforces the solution feasibility. Solutions are then returned to the outer level to evaluate the feasibility of the corresponding objective functions, adding a penalty value in the case of infeasibility. An optional single level refinement is added to improve the ability of the scheme to converge to the Pareto front. The capabilities of the proposed approach will be demonstrated on the multi-objective optimisation of ascent trajectories of launch vehicles

Direct solution of multi-objective optimal control problems applied to spaceplane mission design

This paper presents a novel approach to the solution of multi-phase multi-objective optimal control problems. The proposed solution strategy is based on the transcription of the optimal control problem with Finite Elements in Time and the solution of the resulting Multi-Objective Non-Linear Programming (MONLP) problem with a memetic strategy that extends the Multi Agent Collaborative Search algorithm. The MONLP problem is reformulated as two non-linear programming problems: a bi-level and a single level problem. The bi-level formulation is used to globally explore the search space and generate a well spread set of non-dominated decision vectors while the single level formulation is used to locally converge to Pareto efficient solutions. Within the bi-level formulation, the outer level selects trial decision vectors that satisfy an improvement condition based on Chebyshev weighted norm, while the inner level restores the feasibility of the trial vectors generated by the outer level. The single level refinement implements a Pascoletti-Serafini scalarisation of the MONLP problem to optimise the objectives while satisfying the constraints. The approach is applied to the solution of three test cases of increasing complexity: an atmospheric re-entry problem, an ascent and abort trajectory scenario and a three-objective system and trajectory optimisation problem for spaceplanes

Multi-objective optimisation under uncertainty with unscented temporal finite elements

This paper presents a novel method for multi-objective optimisation under uncertainty developed to study a range of mission trade-offs, and the impact of uncertainties on the evaluation of launch system mission designs. A memetic multi-objective optimisation algorithm, named MODHOC, which combines the Direct Finite Elements in Time transcription method with Multi Agent Collaborative Search, is extended to account for model uncertainties. An Unscented Transformation is used to capture the first two statistical moments of the quantities of interest. A quantification model of the uncertainty was developed for the atmospheric model parameters. An optimisation under uncertainty was run for the design of descent trajectories for a spaceplane-based two-stage launch system

Multi-objective optimal control of re-entry and abort scenarios

This paper presents a novel approach to the solution of multi-phase multi-objective optimal control problems. The proposed solution strategy is based on the integration of the Direct Finite Elements Transcription (DFET) method, to transcribe dynamics and objectives, with a memetic strategy called Multi Agent Collaborative Search (MACS). The original multi-objective optimal control problem is reformulated as two non-linear programming problems: a bi-level and a single level one. In the bi-level problem the outer level, handled byMACS, generates trial control vectors that are then passed to the inner level, which enforces the feasibility of the solution. Feasible control vectors are then returned to the outer level to evaluate the corresponding objective functions. A single level refinement is then run to improve local convergence to the Pareto front. The paper introduces also a novel parameterisation of the controls, using Bernstein polynomials, in the context of the DFET transcription method. The approach is first tested on a known atmospheric re-entry problem and then applied to the analysis of ascent and abort trajectories for a space plane

Design of a Formation of Solar Pumped Lasers for Asteroid Deflection

This paper presents the design of a multi-spacecraft system for the deflection of asteroids. Each spacecraft is equipped with a fibre laser and a solar concentrator. The laser induces the sublimation of a portion of the surface of the asteroid. The jet of gas and debris thrusts the asteroid off its natural course. The main idea is to have a swarm of spacecraft flying in the proximity of the asteroid with all the spacecraft beaming to the same location to achieve the required deflection thrust. The paper presents the design of the formation orbits and the multi-objective optimization of the swarm in order to minimize the total mass in space and maximize the deflection of the asteroid. The paper demonstrates how significant deflections can be obtained with relatively small sized, easy-to-control spacecraft.Comment: Advances in Space Research, 201