Stephen Paschall

Advances in POST2 End-to-End Descent and Landing Simulation for the ALHAT Project

Program to Optimize Simulated Trajectories II (POST2) is used as a basis for an end-to- end descent and landing trajectory simulation that is essential in determining design and integration capability and system performance of the lunar descent and landing system and environment models for the Autonomous Landing and Hazard Avoidance Technology (ALHAT) project. The POST2 simulation provides a six degree-of-freedom capability necessary to test, design and operate a descent and landing system for successful lunar landing. This paper presents advances in the development and model-implementation of the POST2 simulation, as well as preliminary system performance analysis, used for the testing and evaluation of ALHAT project system models.

A Self Contained Method for Safe & Precise Lunar Landing

The return of humans to the Moon will require increased capability beyond that of the previous Apollo missions. Longer stay times and a greater flexibility with regard to landing locations are among the many improvements planned. A descent and landing system that can land the vehicle more accurately than Apollo with a greater ability to detect and avoid hazards is essential to the development of a Lunar outpost, and also for increasing the number of potentially accessible Lunar sortie locations. This descent and landing system should allow landings in more challenging terrain and provide more flexibility with regard to mission timing and lighting considerations, while maintaining safety as the top priority. The lunar landing system under development by the ALHAT (autonomous landing and hazard avoidance technology) project is addressing this by providing terrain-relative navigation measurements to enhance global-scale precision, an onboard hazard detection system to select safe landing locations, and an autonomous GNC (guidance, navigation, and control) capability to process these measurements and safely direct the vehicle to a landing location. This landing system will enable safe and precise lunar landings without requiring lunar infrastructure in the form of navigation aids or a priori identified hazard-free landing locations. The safe landing capability provided by ALHAT uses onboard active sensing to detect hazards that are large enough to be a danger to the vehicle but too small to be detected from orbit a priori. Algorithms to interpret raw active sensor terrain data and generate hazard maps as well as identify safe sites and recalculate new trajectories to those sites are included as part of the ALHAT System. These improvements to descent and landing will help contribute to repeated safe and precise landings for a wide variety of terrain on the Moon.

The challenge of safe lunar landing

The Apollo lunar landings were both incredibly successful and challenging. As the world watched, each of the six landing missions faced potentially mission ending hazards within each of the landing sites while simultaneously dealing with diminishing fuel reserves and a unique landing environment. Hazards in the form of rocks, craters and slopes all were perilously close to each of the successfully landed missions and brought to light the incredible challenge each mission faced.

Hazard Detection Methods for Lunar Landing

The methods and experiences from the Apollo Program are fundamental building blocks for the development of lunar landing strategies for the Constellation Program. Each of the six lunar landing Apollo missions landed under near ideal lighting conditions. The astronauts visually performed terrain relative navigation while looking out of windows, and were greatly aided by external communication and well lit scenes. As the LM approached the landing site, the astronauts performed visual hazard detection and avoidance, also under near-ideal lighting conditions. The astronauts were looking out of the windows trying to the best of their ability to avoid rocks, slopes, and craters and find a safe landing location. NASA has expressed a desire for global lunar access for both crewed and robotic sortie lunar exploration missions [2] [3]. Early NASA architecture studies have identified the lunar poles as desirable locations for early lunar missions. These polar missions provide less than ideal lighting conditions that will significantly affect the way a crewed vehicle is to land at such locales. Consequently, a variety of hazard identification methods should be considered for use by the crew to ensure a high degree of safety. This paper discusses such identification methods applicable to the poorly lit polar lunar environment, better ensuring global access for the soon to be designed Lunar Lander Vehicle (LLV).

Apollo looking forward: Crew task challenges

During the Apollo landings, onboard astronauts, along with analysis and instructions from mission control, performed the majority of complex tasks beyond automated Guidance, Navigation, and Control (GN&C). The crew played a significant role in the landings and were critical to navigating to the landing site, selecting a safe landing aim point, and commanding the spacecraft via a hand controller. Thus the requirements and constraints for site selection, launch dates, and GN&C design were driven in large part by human capabilities. Each of the major tasks performed by the crew will be described to provide an understanding of the functions that must be performed by either automation or people (on or off-board) for the next generation lunar lander. Additionally, several of the Apollo missions faced significant issues in identifying the landing site and assessing the safety of that landing site. These missions serve as case studies for future landing challenges that must be overcome.

Small lunar lander/hopper performance analysis

The goal of this paper is to describe a first-order performance analysis of a lunar hopper 1,2. A hopper is a vehicle that has both landing and surface mobility capabilities on a single platform. Unlike rovers, which traverse the lunar surface while in contact with the ground, hopping reuses the landing propulsion system to lift back off again and “hop” over the lunar terrain. Hopping, as a form of surface mobility, is a novel concept. As such, analysis must be performed to assess how it would fit with an overall lunar landing system architecture. Two trajectory categories are investigated to perform this assessment: the ballistic hop, where the vehicle launches itself into a ballistic trajectory toward the destination, and the hover hop, in which the vehicle ascends and maintains a constant altitude as it travels toward its desired location. Initially, parametric studies of the ballistic and hover hop are carried out in order to make observations about the performance of each hop. Using this data, it is possible to investigate the fuel-optimal hop trajectory. The delta-V costs for the ballistic and hover hops are compared for hop distances between 500 meters and 5000 meters, and in this range it is found that the ballistic hop and hover traverse have comparable delta-V costs. For the entire hop maneuver, however, the hover hop will always be the more delta-V expensive option due to the ascent and descent phases. Nevertheless, this does not rule out the hover hop as a feasible option due to its operational advantages over the ballistic hop.

Approach phase ΔV considerations for lunar landing

The Autonomous Landing and Hazard Avoidance Technology (ALHAT) Project is studying the lunar landing descent phase from lunar orbit to the surface. In this paper, we give an overview of the timing and ΔV implications for key activities during the lunar landing approach phase. Timing and ΔV performance are evaluated while varying the approach phase design and key hazard detection parameters. Results show that there are significant system tradeoffs when considering ΔV, hazard detection schemes, and the time available for crew to select a safe point to land.

Development and deployment of a performance model for the prototype planetary exploration hopper

This paper describes the development of a detailed model of the TALARIS hopper and demonstrates its performance1,2. TALARIS (Terrestrial Artificial Lunar And Reduced gravIty Simulator) is a small prototype hopping vehicle currently being developed in collaboration between the Massachusetts Institute of Technology and Draper Laboratory. It will serve as an Earth-based testbed for guidance, navigation, and control (GNC) algorithms that will be used to explore lunar and other planetary surfaces remotely. As part of the overall design process, a three degree-of-freedom (3-DOF) performance model called HopperSim was created to simulate the vehicle and run various experiments. First, the design and details of the model are described. Then its capabilities are demonstrated in a study comparing the performance of the Earth-based TALARIS to its Moon-based equivalent. Through HopperSim it is shown that, in 3-DOF, TALARIS can closely emulate a lunar hopper with respect to relevant fight parameters. The model is seen to be a useful tool that can be matured to better predict important details of the performance characteristics of the TALARIS hopper and similar proposed vehicles.