Zachary R. Putnam

Entry System Options for Human Return from the Moon and Mars

Earth entry system options for human return missions from the Moon and Mars were analyzed and compared to identify trends among the configurations and trajectory options and to facilitate informed decision making at the exploration architecture level. Entry system options included ballistic, lifting capsule, biconic, and lifting body configurations with direct entry and aerocapture trajectories. For each configuration and trajectory option, the thermal environment, deceleration environment, crossrange and downrange performance, and entry corridor were assessed. In addition, the feasibility of a common vehicle for lunar and Mars return was investigated. The results show that a low lift-to-drag ratio (L/D = 0.3) vehicle provides sufficient performance for both lunar and Mars return missions while providing the following benefits: excellent packaging efficiency, low structural and TPS mass fraction, ease of launch vehicle integration, and system elegance and simplicity. Numerous configuration options exist that achieve this L/D.

PredGuid entry guidance for Orion return from low Earth orbit

When returning from low Earth orbit, Orion will perform a lifting atmospheric entry with precision landing using the PredGuid entry guidance algorithm.12 The PredGuid algorithm is designed to guide Orion to the desired landing site while accounting for vehicle and environment uncertainties and day of flight dispersions during atmospheric entry. The low Earth orbit mode of the PredGuid entry guidance algorithm consists of three phases: the Initial Roll Phase which maintains proper entry attitude and steers the vehicle to proper transition conditions for the Final Phase; the Final Phase, a terminal point guidance algorithm that uses a stored reference trajectory to steer out range error and achieve precision landing; and the Terminal Phase, which seeks to null the remaining crossrange error through a simple proportional steering law. Simulation results indicate that the PredGuid algorithm provides precision landing capability to Orion as well as significant robustness to day-of-flight uncertainties and dispersions.

Feasibility of Guided Entry for a Crewed Lifting Body Without Angle-of-Attack Control

The feasibility of flying a crewed lifting body, such as the HL-20, during entry from low Earth orbit without the steady-state body flap deflections required for angle-of-attack control was evaluated. This entry strategy mitigates the severity of the aerothermal environment on the vehicle’s body flaps and reserves control power for transient steering maneuvers. A real-time numeric predictor–corrector entry guidance algorithm was developed to accommodate the range of vehicle trim angle-of-attack profiles possible in the absence of angle-of-attack control. Results show that it is feasible to steer the vehicle from low Earth orbit to a desired target with real-time guidance while satisfying a reasonable suite of trajectory constraints, including limits on peak heat rate, peak sensed deceleration, and integrated heat load. Uncertainty analyses confirm this result and show that the vehicle maintains significant performance robustness to expected day-of-flight uncertainties. Additionally, parametric scans over ...

Guided Entry Performance of Low Ballistic Coefficient Vehicles at Mars

Current Mars entry, descent, and landing technology is near its performance limit and may be unable to land payloads on the surface that exceed 1 metric ton. One option for increasing landed payload mass capability is decreasing the entry vehicle’s hypersonic ballistic coefficient. A lower ballistic coefficient vehicle decelerates higher in the atmosphere, providing the additional timeline and altitude margin necessary to land more massive payloads. This study analyzed the guided entry performance of several low ballistic coefficient vehicle concepts on Mars. A terminal point controller guidance algorithm, based on the Apollo Final Phase algorithm, was used to provide precision targeting capability. Terminal accuracy, peak deceleration, peak heat rate, and integrated heat load were assessed and compared with a traditional Mars entry vehicle concept to determine the effects of lowering the vehicle ballistic coefficient on entry performance. Results indicate that, while terminal accuracy degrades slightly w...

Trajectory options for human mars missions

This paper explores trajectory options for the huma n exploration of Mars, with an emphasis on conjunction-class missions. Conjunction-class missions are characterized by short in-space durations with long surface stays, a s opposed to the long in-space durations and short surface stays characteristic of oppositio n-class missions. Earth-Mars and Mars- Earth trajectories are presented across a series of mission opportunities and transfer times in order to explore the space of possible crew and cargo transfer trajectories. In the specific instance of crew transfer from Earth to Mars, the p otential for aborting the mission without capture into Mars orbit is also of interest. As suc h two additional classes of trajectories are considered: free-return trajectories, where the tra jectory would return the crew to Earth after a fixed period of time; and propulsive-abort trajectories, where the propulsive capability of the transfer vehicle is used to modif y the trajectory during a Mars swing-by. The propulsive requirements of a trajectory, due to their associated impact on spacecraft mass, are clearly of interest in assessing trajecto ries for human Mars missions. Beyond the propulsive requirements, trajectory selection can h ave a significant impact on the entry velocity and therefore the aeroassist system requir ements. The paper suggests potential constraints for entry velocities at Earth and Mars. Based upon Mars entry velocity, the 2- year period free-return abort trajectory is shown t o be less desirable than previously considered for many mission opportunities.

A conceptual design environment for technology selection and performance optimization for torpedoes

Presented at the 9th Multi-Disciplinary Analysis and Optimization Symposium in Atlanta, GA, September 4-6, 2002.

Improving Lunar Return Entry Footprints Using Enhanced Skip Trajectory Guidance

The impending development of NASAs Crew Exploration Vehicle (CEV) will require a new entry guidance algorithm that provides sufficient performance to meet all requirements. This study examined the effects on entry footprints of enhancing the skip trajectory entry guidance used in the Apollo program. The skip trajectory entry guidance was modified to include a numerical predictor-corrector phase during atmospheric skip portion of the entry trajectory. Four degree-of-freedom simulation was used to determine the footprint of the entry vehicle for the baseline Apollo entry guidance and predictor-corrector enhanced guidance with both high and low lofting at several lunar return entry conditions. The results show that the predictor-corrector guidance modification significantly improves the entry footprint of the CEV for the lunar return mission. The performance provided by the enhanced algorithm is likely to meet the entry range requirements for the CEV.

Drag modulation flight control for aerocapture

Hypersonic deployable aerodynamic devices, both rigid and inflatable, have the potential to enable a broad spectrum of next-generation aeroassist missions by mitigating shape and size constraints on aeroassist vehicles and providing an in-flight reconfiguration capability. Such a capability provides new options for flight control during atmospheric flight, such as drag modulation. Drag modulation is an attractive flight control option for future aerocapture missions because it requires only minimal additional system complexity for vehicles with deployable aerodynamic devices, in contrast to more conventional lift-modulation steering methods. This study expands upon previous aerocapture drag modulation studies by extending the analysis of single-event jettison systems to Earth and Mars. A single-event jettison guidance algorithm was developed and used to evaluate the feasibility of real-time targeting of apoapsis altitude during flight. Results indicate that sufficiently large ballistic coefficient ratios provide adequate aerodynamic and guided corridors for future aerocapture missions. While the preliminary guidance algorithm demonstrates only modest insertion accuracy, this level of accuracy may be tolerable for certain missions.

Extension and enhancement of the Allen-Eggers analytical ballistic entry trajectory solution

The closed-form analytical solution to the equations of motion for ballistic entry developed by Allen and Eggers is presented using modern nomenclature and is extended and enhanced through several new developments. A method of choosing an appropriate constant flight-path angle is identified. Analytical limits are proposed that bound the domain of applicability of the approximation. Closed-form expressions for range and time are developed that are consistent with the assumptions in the Allen–Eggers approximation. Collectively, the improvements address key weaknesses in the original approximate solution. Assessment of the accuracy of the approximate solution relative to the planar equations of motion shows that the extended and enhanced Allen–Eggers solution provides good accuracy across a wide range of ballistic coefficients at Earth with initial flight-path angles steeper than about −7  deg. In some instances, the expression developed for range-to-go may be accurate enough for use in onboard real-time gui...

Advances in Inertial Guidance Technology for Aerospace Systems

The origin, evolution, and outlook of guidance as a path and trajectory manager for aerospace systems is addressed. A survey of theoretical developments in the field is presented demonstrating the advances in guidance system functionality built upon inertial navigation technology. Open-loop and closed-loop approaches for short-range systems, long-range systems and entry systems are described for both civilian and military applications. Over time, guidance system development has transitioned from passive and open-loop systems to active, closed-loop systems. Significant advances in onboard processing power have improved guidance system capabilities, shifting the algorithmic computational burden to onboard systems and setting the stage for autonomous aerospace systems. Seminal advances in aerospace guidance are described, highlighting the advancements in guidance and resulting performance improvements in aerospace systems.