Bradley A. Steinfeldt

Guidance, Navigation, and Control System Performance Trades for Mars Pinpoint Landing

Landing site selection is a compromise between safety concerns associated with the site’s terrain and scientific interest. Therefore, technologies enabling pinpoint landing performance (sub-100-m accuracies) on the surface of Mars are of interest to increase the number of accessible sites for in situ research, as well as allow placement of vehicles nearby prepositioned assets. A survey of the performance of guidance, navigation, and control technologies that could allow pinpoint landing to occur at Mars was performed. This assessment has shown that negligible propellant mass fraction benefits are seen for reducing the three-sigma position dispersion at the end of the hypersonic guidance phase (parachute deployment) below approximately 3 km. Four different propulsive terminal descent guidancealgorithms were examined. Of these four, a near propellant-optimal analytic guidance law showed promisefortheconceptualdesignofpinpointlandingvehicles.Theexistenceofapropellantoptimumwithregardto theinitiationtimeofthepropulsiveterminaldescentwasshowntoexistforvarious flightconditions.Subsonicguided parachutes were shown to provide marginal performance benefits, due to the timeline associated with descent through the thin Mars atmosphere. This investigation also demonstrates that navigation is a limiting technology for Mars pinpoint landing, with landed performance being largely driven by navigation sensor and map tie accuracy.

High Mass Mars Entry, Descent, and Landing Architecture Assessment

As the nation sets its sight on returning humans to the Moon and going onward to Mars, landing high mass payloads ( 2 t) on the Mars surface becomes a critical technological need. Viking heritage technologies (e.g., 70 sphere-cone aeroshell, SLA-561V thermal protection system, and supersonic disk-gap-band parachutes) that have been the mainstay of the United States’ robotic Mars exploration program do not provide sucient capability to land such large payload masses. In this investigation, a parametric study of the Mars entry, descent, and landing design space has been conducted. Entry velocity, entry vehicle conguration, entry vehicle mass, and the approach to supersonic deceleration were varied. Particular focus is given to the entry vehicle shape and the supersonic deceleration technology trades. Slender bodied vehicles with a lift-to-drag ratio (L=D) of 0.68 are examined alongside blunt bodies with L=D = 0.30. Results demonstrated that while the increased L=D of a slender entry conguration allows for more favorable terminal descent staging conditions, the greater structural eciencies of blunt body systems along with the reduced acreage required for the thermal protection system aords an inherently lighter vehicle. The supersonic deceleration technology trade focuses on inatable aerodynamic decelerators (IAD) and supersonic retropropulsion, as supersonic parachute systems are shown to be excessively large for further consideration. While entry masses (the total mass at the top of the Mars atmosphere) between 20 and 100 t are considered, a maximum payload capability of 37.3 t results. Of particular note, as entry mass increases, the gain in payload mass diminishes. It is shown that blunt body vehicles provide sucient vertical L=D to decelerate all entry masses considered through the Mars atmosphere with adequate staging conditions for the propulsive terminal descent. A payload mass fraction penalty of approximately 0.3 exists for the use of slender bodied vehicles. Another observation of this investigation is that the increased aerothermal and aerodynamic loads induced from a direct entry trajectory (velocity 6.75 km/s) reduce the payload mass fraction by approximately 15% compared to entry from orbital velocity ( 4 km/s). It should be noted that while both IADs and supersonic retropropulsion were evaluated for each of the entry masses, congurations, and velocities, the IAD proved to be more mass-ecient in all instances. The sensitivity of these results to modeling assumptions was also examined. The payload mass of slender body vehicles was observed to be approximately four times more sensitive to modeling assumptions and uncertainty than blunt bodies.

Guidance, Navigation, and Control Technology System Trades for Mars Pinpoint Landing

Landing site selection is a compromise between safety concerns associated with the site’s terrain and scientific interest. Therefore, technologies enabling pinpoint landing (sub-100 m accuracies) on the surface of Mars are of interest to increase the number of accessible sites for in-situ research as well as allow placement of vehicles nearby prepositioned assets. A survey of various guidance, navigation, and control technologies that could allow pinpoint landing to occur at Mars has shown that negligible propellant mass fraction benefits are seen for reducing the three-sigma position dispersion at parachute deployment below approximately 3 km. Four different propulsive terminal descent guidance algorithms were analyzed with varying applicability to flight. Of these four, a near propellant optimal, analytic guidance law showed promise for the conceptual design of pinpoint landing vehicles. The existence of a propellant optimum with regards to the initiation time of the propulsive terminal descent was shown to exist for various flight conditions. In addition, subsonic guided parachutes are shown to provide marginal performance benefits due to the timeline associated with Martian entries, and a low computational-cost, yet near fuel optimal propulsive terminal descent algorithm is identified. This investigation also demonstrates that navigation is a limiting technology for Mars pinpoint landing, with overall landed performance being largely driven by navigation sensor and map tie accuracy.

Smart Divert: A New Mars Robotic Entry, Descent, and Landing Architecture

This study investigates the performance and feasibility of a new entry, descent, and landing architecture onMars, termed Smart Divert, for landing in one of a number of small safe zones surrounded by hazardous terrain. Smart Divert consists of a ballistic entry followed by supersonic parachute deployment. After parachute release, the vehicle diverts to one ofmany predefined, fuel-optimal safe zone sites. The Smart Divert concept does not require hypersonic guidance or real-time terrain recognition. Instead, it relies on a priori orbital observations to identify small, multiple safe zones within a larger hazardous region and additional terminal descent propellant to land at the fuel-optimal safe zone.Before launch,mission designers could trade thenumber and size of the safe zones as part of the landing site selection process.Reasonable propellantmass fractions of 0.3 canbe achievedby initiating the divert at 5 kmaltitude, providing a 10 km horizontal divert capability. The number of safe zones is shown to be a function of landing ellipse size. Assuming Mars Science Laboratory state-of-the-art interplanetary navigation, four safe zone sites, randomly placed throughout the landing ellipse to simulate unknowndestinations of futuremissions, require a propellantmass fraction less than 0.3 for 97% of the cases analyzed. The unconstrained optimal arrangement of four safe zone sites within the same landing ellipse reduced the required propellant mass fraction from 0.3 to 0.22. The propellant mass fraction may be further reduced as the number of safe zone sites is increased. An example scenario using rock count data for the Phoenix landing site region demonstrates that Smart Divert can be implemented to land in previously unreachable terrain for a propellant mass fraction of 0.2.

Using Dynamical Systems Concepts in Multidisciplinary Design

A general multidisciplinary design problem features coupling and feedback between contributing analyses. This feedback may lead to convergence issues requiring significant iteration to obtain a feasible design. This work casts the multidisciplinary design problem as a dynamical system to leverage the benefits of dynamical systems theory in a new domain. Three areas from dynamical system theory are chosen for investigation: stability analysis, optimal control, and estimation theory. Stability analysis is used to investigate the existence of a solution to the design problem. Optimal control techniques allow the requirements associated with the design to be incorporated into the system and allow for constraints that are functions of both the contributing analysis outputs and input values to be handled simultaneously. Finally, estimation methods are employed to evaluate the robustness of the multidisciplinary design. These three dynamical system techniques are then combined in a methodology for the rapid robu...

Design Convergence Using Stability Concepts from Dynamical Systems Theory

The inherent iteration required in the multidisciplinary design problem allows the design problem to cast as a dynamical system. The iteration in design is a resultant of the two rootnding problems. The rst root-nding problem is in seeking out candidate designs while the second is in optimizing the candidate designs. Viewing the root-nding schema as a dynamical system allows the application of established techniques from dynamical systems theory to design. Stability theory is one of the techniques that is enabled by viewing multidisciplinary design as a dynamical system. Stability theory is capable of providing information on whether or not a design will converge for a given iteration scheme, starting values for the iteration that will lead to convergence, as well as information regarding how fast a design will converge. Following the theoretical development, each of these concepts is demonstrated on sample problems showing the benet of the application of stability theory in the design realm.

Analytically-derived Aerodynamic Force and Moment Coecients of Resident Space Objects in Free-Molecular Flow

acting on a general body in free-molecular regime to derive aerodynamic force and moment expressions. The analytical aerodynamics prediction method is described and relations have been developed for the sphere, cylinder, panel, and rectangular prism. The NASA-developed Direct Simulation Monte Carlo Analysis Code is used to validate the analytical expressions and it is shown that expressions are accurate within 0.38%. These generalized analytic expressions in terms of angle of attack, sideslip angle, freestream conditions, wall temperature, and accommodation coecients allow near-instantaneous computation of the rareed aerodynamics and enables space situation awareness analysis.

Utilizing Dynamical Systems Concepts in Multidisciplinary Design

A general multidisciplinary design problem features coupling and feedback between contributing analyses. This feedback may lead to convergence issues requiring significant iteration in order to obtain a feasible design. This work provides a description for casting the multidisciplinary design problem as a dynamical system in order to overcome some of the challenges associated with traditional multidisciplinary design and leverage the benefits of dynamical systems theory in a new domain. Three areas from dynamical system theory are chosen for investigation: stability analysis, optimal control, and estimation theory. Stability analysis is used to investigate the existence of a solution to the design problem. Optimal control techniques allow the requirements associated with the design to be incorporated into the system and allow for constraints that are functions of both the contributing analysis outputs and input values to be handled simultaneously. Finally, estimation methods are employed to obtain an evaluation of the robustness of the multidisciplinary design. These three dynamical system techniques are then combined in a complete methodology for the rapid robust design of a linear multidisciplinary design. The developed robust design methodology allows for uncertainties both within the models as well as the parameters of the multidisciplinary problem. The performance of the developed technique is demonstrated through a linear and nonlinear example problem.

Smart Divert: A New Entry, Descent, and Landing Architecture

To date, Mars robotic landing site selection has been a compromise between scientific interest and safety. Due to the rather large landed footprint major axis lengths of the Viking, Pathfinder, Mars Exploration Rovers, and Phoenix missions, these landed ellipses have been placed in vast, relatively flat areas to ensure a high probability of landing success. Scientists are interested in exploring more geologically interesting areas that may contain landing hazards, including sloping terrain, craters, and rocks. Smart Divert is a new entry, descent, and landing architecture that could allow robotic missions to safely land in hazardous terrain without the requirement of hypersonic guidance. Smart Divert consists of a ballistic entry followed by supersonic parachute deployment. After parachute release, the vehicle diverts to one of many predefined, fuel-optimal safe zones. Smart Divert performance and entry design is discussed and is followed by a discussion of Smart Divert for random terrain. An initial assessment of optimal landing site arrangement is performed and an example of the usefulness of Smart Divert is performed for actual Mars terrain using Phoenix landing site rock count data.

Rapid Robust Design of a Deployable System for Boost-Glide Vehicles

Deployable devices have the potential to reduce or eliminate physical constraints placed on vehicle design while enhancing the aerodynamics characteristics of the system. This investigation looks at augmenting an existing boost-glide system with a deployable device to increase the system’s range or accuracy by varying design parameters. Two dierent congurations are considered, one which has a single-delta shape and one with a doubledelta. A rapid robust design methodology that views the multidisciplinary design problem as a dynamical system is implemented to robustly design the deployable. This methodology allows concepts from dynamical system theory to be used in order to broaden the computational tools available to the MDO problem. In addition to the physical parameters of the deployable device, the impact of the guidance algorithm is also considered. The product of this investigation is a family of designs which compare favorably to those obtained through traditional Monte Carlo methods and are achievable in less than 5% of the computational time. The obtained deployable designs have the capability to enhance the baseline boost-glide system’s 1 range by 50% and improve the 1 accuracy by an order of magnitude. It is seen that the single-delta conguration