<div>Mission designs have been focused on using proven orbital maneuvers (i.e., propulsive maneuvers and gravity-assist) to deliver spacecraft to planetary destinations. Aeroassist maneuvers, despite their potential benefits, have not been given serious considerations due to the perceived risk and complexity. As entry technologies mature, aeroassist maneuvers need to be considered more extensively. Currently, there is no tool available that can perform rapid preliminary mission designs using aeroassist maneuvers. In this dissertation, integrated design methodologies for aerocapture and aerogravity-assist are developed, which can be readily converted to design tools that enable rapid mission concept formulations. </div><div> </div><div>The aerocapture design methodology is used to develop extensive design rules and relations for aerocapture missions to Titan, Venus, and Uranus, considering a wide range of vehicle parameters and interplanetary trajectories. These design rules and relations are intended as a convenient resource for mission designers and system engineers to evaluate the feasibility of aerocapture (e.g., effects of V-infinity on aerocapture missions) and the relevant design requirements, such as choices for vehicle characteristics and TPS materials. In addition, potentials for inclination change for Titan aerocapture are also quantified, presenting additional benefits of using aerocapture. Given the unusual orientation of Uranus, the changes in inclination and shift of line of apsides are also quantified for Uranus aerocapture. </div><div> </div><div>A novel design methodology is developed for Saturn system missions using nontraditional aerogravity-assist maneuvers at Titan. Compared with the existing literature, the novel methodology explores a comprehensive design space by integrating design considerations for interplanetary trajectories, atmospheric trajectories, arrival geometries at Titan, and vehicle designs. The methodology enables preliminary design trades and allows the mission designer to assess the feasibility of Titan aerogravity-assist and quickly develop requirements for trajectory designs and vehicle designs. The methodology also identifies potential Saturn and Titan arrival conditions. Results for an example Enceladus mission and Saturn system mission are presented, showing that a Saturn arrival V-infinity of 7 km/s renders Titan aerogravity-assist feasible for an Enceladus mission, while using the current entry technology. </div><div> </div><div>Bank modulation and drag modulation have been considered separately for aeroassist vehicles in the literature. The investigation combines bank modulation and drag modulation to improve the control authorities for aeroassist vehicles and such improvements are quantified using numerical simulations for a wide range of vehicle design configurations. The results show the potential of using a low-L/D vehicle for aerocapture at Uranus using combined bank and drag modulation. </div><div><br></div>

The development of a flexible, high-fidelity, generic simulation of transatmospheric and interplanetary motion is described. The simulation incorporates aerodynamic and gravitational force modelling implemented in a Cartesian reference co-ordinate set. Propagation of the motion of a vehicle is carried out in a "working" reference frame whose origin is determined by the current gravitational sphere of influence. A semi-analytic model of planetary motion propagates the motion of the nine planets and six major moons, allowing simulation at any point within the solar system. Expansion and improvement of the model is facilitated through the vector formulation of the problem. The use and applicability of the method of matched asymptotic expansions is examined as a means of producing high quality trajectory predictions quickly and easily. Ballistic launch and entry trajectories are considered incorporating a velocity dependent model for the aerodynamic drag coefficient. Using the derived relations direct launch is considered as a low-cost means of transporting acceleration insensitive payloads to a space station in low Earth orbit. In addition, it is shown that the high quality trajectory predictions may be obtained using a simple spreadsheet package. Analytic modelling is also used as the basis of a highly robust, computationally efficient, controller design for autonomous aerocapture in the context of the lunar return problem. The validity of this approach to lunar return is examined and found to be of considerable potential in both its robustness and the potential improvements in payload mass-fraction available through the substantial fuel savings over direct return to Earth or propulsive return to a space station. The study shows that, using the derived control, the aerocapture manoeuvre can be successfully performed with existing material and technological capabilities.

Aerocapture is a method for spacecraft orbital insertion that is currently being assessed for use in interplanetary missions. This method would use a low periapsis hyperbolic entry orbit to induce drag allowing the spacecraft to slow down without the use of a propulsion system. This is accomplished by using a ballute (balloon parachute), which is released after the appropriate change in velocity necessary to achieve the desired planetary orbit. Once released, the ballute could deploy a secondary mission vehicle. A MATLAB simulation was run to understand the environment a secondary payload would undergo, such as heating and deceleration, as well as to study the buoyancy due to the ballute. The stability of the spacecraft during entry is also discussed. The results showed that if the ballute can survive the aerocapture maneuver then it will be able to survive entry with a secondary payload. The deceleration from the separation of the primary and secondary payload will be large but it can be overcome. The stability of the vehicle is dependent on the location of the center of gravity. Buoyancy at Mars has little effect due to the low density of the atmosphere; at higher density atmospheres buoyancy does play a role in the payload descent. Results of the analysis show that a successful landing of a ballute with a secondary payload is possible.

This thesis aims to investigate the potential of a transfer orbit using successive aerocaptures at Saturn and Titan to establish a science orbit around Titan. Titan is an Earth-like moon with a dense atmosphere and organic compounds present. It has many similarities with Earth that are useful to study such as superrotation. Superrotation is when the atmosphere rotates faster than the body it surrounds. In order to study Titan, we need to establish an orbit around it. The Saturn system is distant from Earth, 8.5 Astronomical Units (AU) which makes it difficult to reach from a time and velocity point of view. We propose to use an aerocapture at Saturn to intercept Titan with lower relative velocity in order to perform an aerocapture at Titan. The analysis was performed in primarily MATLAB to simulate the orbits. The results of this showed that we can aerocapture a spacecraft at Saturn and arrive at Titan within roughly 4 to 8 km/s relative velocity regardless of the incoming hyperbolic excess velocity at the Saturn system. This can be improve upon by using intermediate transfer orbits, such as bi-elliptics, to arrive with even lower relative velocities to Titan of as low as 1 km/s. The drag acceleration experienced during the Saturn aerocapture had peak values of between 0.2 and 1.4 g's and acceleration over 50% of the peak is experienced between 6.8 and 8 minutes. This capture method has the potential to make Titan more easily accessible and allow for scientific study of a clear target for improving our understanding of Earth-like processes on other bodies in our solar system. / Master of Science / This thesis aims to investigate the potential of a transfer orbit using successive aerocaptures at Saturn and Titan to establish a science orbit around Titan. Aerocapturing is utilizing the atmosphere of a body to slow down a spacecraft. Titan is an Earth-like moon with a dense atmosphere and organic compounds present. It has many similarities with Earth that are useful to study such as superrotation. Superrotation is when the atmosphere of a body rotates faster than the body it surrounds. In order to study Titan, we need to establish an orbit around it. The Saturn system is distant from Earth, 8.5 Astronomical Units (AU) which makes it difficult to reach from a time and velocity point of view. It takes a large amount of time to get there so we attempt to get there faster by increasing velocity. This means we arrive at the Saturn system with a large amount of velocity that we need to counter-act in order to orbit. We propose to use an aerocapture at Saturn to intercept Titan with lower velocity in order to perform another aerocapture at Titan to slow into an orbit. The analysis was performed in primarily MATLAB to simulate the orbits. The results of this showed that we can aerocapture a spacecraft at Saturn and arrive at Titan within roughly 4 to 8 km/s regardless of the incoming velocity to the Saturn system. This can be improve upon by using intermediate transfer orbits, after capturing at Saturn, to arrive with even lower velocities at Titan of as low as 1 km/s. The drag acceleration experienced during the Saturn aerocapture had peak values of between 0.2 and 1.4 g's and acceleration over 50% of the peak is experienced between 6.8 and 8 minutes. This is relatively gentle for an aerocapture and means the spacecraft likely will not require significant structural support. This capture method has the potential to make Titan more easily accessible and allow for scientific study of a clear target for improving our understanding of Earth-like processes on other bodies in our solar system.

Approved for public release, distribution is unlimited / This thesis explores using a direct pseudospectral method for the solution of optimal control problems with mixed dynamics. An easy to use MATLAB optimization package known as DIDO is used to obtain the solutions. The modeling of both low thrust interplanetary trajectories as well as aerocapture trajectories is detailed and the solutions for low thrust minimum time and minimum fuel trajectories are explored with particular emphasis on verification of the optimality of the obtained solution. Optimal aerocpature trajectories are solved for rotating atmospheres over a range of arrival Vinfinities. Solutions are obtained using various performance indexes including minimum fuel, minimum heat load, and minimum total aerocapture mass. Finally, the problem formulation and solutions for the mixed dynamic problem of low thrust trajectories with a terminal aerocapture maneuver is addressed yielding new trajectories maximizing the total scientific mass at arrival. / Lieutenant, United States Navy

<p>Aerocapture has been envisioned as a potential orbit insertion technique for planetary destinations with an atmosphere. Despite not being flight proven technique, many studies found in the literature and recent mission proposals have employed aerocapture into their respective mission designs. The potential varying levels of trajectory dispersions experienced during atmospheric flight at each destination drives the need for robust and fuel-efficient guidance and control solutions. Existing guidance algorithms have relied on tracking precomputed reference trajectories, which are computed using significant simplifications to the flight mechanics, are not generally designed to be fuel-efficient, and require tedious performance gain tuning. When simulated with higher levels of uncertainty, the existing algorithms have been shown to produce large orbit insertion errors. Furthermore, existing flight control methodologies have been limited in scope to bank angle modulation. While some studies have introduced new methodologies, such as drag modulation and direct force control, they haven’t been tested at the same level of rigor as the existing methods. Advances in on-board computational power are allowing for modern guidance and control solutions, in the form of numerical predictor-corrector algorithms, to be realized. This dissertation presents an aerocapture guidance architecture based on a numerical predictor-corrector algorithm. Optimal control theory is utilized to formulate and numerically obtain fuel-minimizing flight control laws for lifting and ballistic vehicles. The unified control laws are integrated into a common guidance algorithm. The architecture is utilized to conduct Monte Carlo simulation studies of Discovery-class and SmallSat-class aerocapture missions at various planetary destinations.</p>

<p>  </p> <p>As space agencies look to conduct more scientific missions beyond Earth orbit, low-cost access to space becomes indispensable. Small satellites (smallsats) fulfill this need as they can be developed at a fraction of the cost of traditional large satellites. Consequently, smallsats are being envisioned for planetary science missions at several destinations including Mars. However, a significant challenge for interplanetary smallsats is performing fully-propulsive orbit insertion because modern smallsat propulsion technologies have limited total velocity change capabilities. At destinations with significant atmospheres, this challenge can be circumvented via <em>aerocapture</em>, a technique that uses a single atmospheric pass to convert a hyperbolic approach trajectory into a captured elliptical orbit. Aerocapture has been shown to enable significant propellant mass savings as compared to fully-propulsive orbit insertion, making it an attractive choice for smallsats. Performing aerocapture with smallsats requires a suitable vehicle design that satisfies the associated control requirements and volumetric constraints. To address this requirement, this dissertation proposes the <em>morphable entry system </em>(MES), a conceptual deployable entry vehicle that utilizes shape morphing to follow a desired atmospheric flight profile during aerocapture. The aerocapture performance of the MES at Mars is investigated using a six degree-of-freedom aerocapture simulation environment. The shape morphing strategy employed by the MES is shown to be feasible for targeting desired angle of attack and sideslip angle profiles that lead to successful orbit captures. Furthermore, the robustness of the MES to simulated day-of-flight uncertainties while employing angle of attack control is demonstrated through a Monte Carlo dispersion analysis. The major contributions of this research as well as areas of future work are described.</p>

Aerocapture offers a near propellantless and quick method of orbit insertion at atmosphere bearing planetary destinations. Compared to conventional propulsive insertion, the primary advantage of using aerocapture is the savings in propellant mass which could be used to accommodate more useful payload. To protect the spacecraft from the aerodynamic heating during the maneuver, the spacecraft must be enclosed in a protective aeroshell or deployable drag device which also provides aerodynamic control authority to target the desired conditions at atmospheric exit. For inner planets such as Mars and Venus, aerocapture offers a very attractive option for inserting small satellites or constellations into very low circular orbits such as those used for imaging or radar observations. The large amount of propellant required for orbit insertion at outer planets such as Uranus and Neptune severely limits the useful payload mass that can delivered to orbit as well as the achievable flight time. For outer planet missions, aerocapture opens up an entirely new class of short time of flight trajectories which are infeasible with propulsive insertion. A systems framework for rapid conceptual design of aerocapture missions considering the interdependencies between various elements such as interplanetary trajectory and vehicle control performance for aerocapture is presented. The framework provides a step-by-step procedure to formulate an aerocapture mission starting from a set of mission objectives. At the core of the framework is the ``aerocapture feasibility chart", a graphical method to visualize the various constraints arising from control authority requirement, peak deceleration, stagnation-point peak heat rate, and total heat load as a function of vehicle aerodynamic performance and interplanetary arrival conditions. Aerocapture feasibility charts have been compiled for all atmosphere-bearing Solar System destinations for both lift and drag modulation control techniques. The framework is illustrated by its application to conceptual design of a Venus small satellite mission and a Flagship-class Neptune mission using heritage blunt-body aeroshells. The framework is implemented in the Aerocapture Mission Analysis Tool (AMAT), a free and open-source Python package, to enable scientists and mission designers perform rapid conceptual design of aerocapture missions. AMAT can also be used for rapid Entry, Descent, and Landing (EDL) studies for atmospheric probes and landers at any atmosphere-bearing destination.