Concepts and Approaches for Mars Exploration 6154.pdf

NEXT-GENERATION ENTRY/DESCENT/LANDING SYSTEM FOR MARS LANDERS. S. W. Thurman1 1Jet Propulsion Laboratory, California Institute of Technology, M. S. 264-440, 4800 Oak Grove Dr., Pasadena, CA 91109-8099. (E-mail:sam.w.thurman@jpl.nasa.gov).

Introduction: Many important scientific objec- The touchdown event itself is made robust as pos- tives for Mars exploration require the ability to land sible to any residual terrain hazards. Figure 1 shows safely at select sites. The “first-generation” entry, de- one example of a robust landing approach; a pallet- scent, and landing (EDL) systems used in previous type structure augmented with webbed shock struts to missions imposed limitations on target site selection help prevent tip-over. This scheme and other alterna- due to the delivery accuracy achievable and those sys- tives are discussed further by Rivellini.1 tems’ inability to recognize and avoid hazardous ter- The architecture of this system is structured not rain. This abstract outlines key capabilities of a pro- only to incorporate current sensor technology and posed second-generation EDL system, currently under guidance/navigation logic, but also to readily accom- development by a consortium of NASA centers, in- modate future capabilities as warranted. Examples of dustry, and academic institutions. potential future additions include the capability to per- EDL System Description: An illustration of a rep- form onboard radio navigation via orbiting spacecraft resentative system concept is provided in Fig. 1 below. or surface beacons, and guided parachute descent for The entry capsule pictured is being designed for both “pinpoint” delivery to a designated target site. direct entry, as has been done in the recent Mars Path- EDL Sequence of Events: The key events occur- finder and Mars Polar Lander missions, or delivery ring during entry, descent, and landing are illustrated into the atmosphere from orbit, if it is desired to carry in Fig. 2. This figure also provides approximate values the spacecraft into orbit prior to landing. Hence, carrier of the altitude, velocity, and timing of each event for a vehicle options range from a cruise stage to an orbiter representative direct entry mission. spacecraft with a mission of its own. Approach Phase (not shown). Prior to entry the spacecraft must be guided to the target entry corridor.

TWO-STAGE The spacecraft’s own propulsion system and guidance BICONIC BACKSHELL PARACHUTE system are capable of doing so, or the entry capsule CANISTER may be augmented with an external propulsion system

ENTRY RCS if desired, controlled by the onboard guidance system. THRUSTERS Entry/Atmospheric Deceleration Phase. Once the (4´ ) LARGE ROVER (STOWED) spacecraft begins to encounter the atmosphere, its en- try guidance logic is activated. The guidance system computes bank angle commands to steer the capsule’s HEATSHIELD LANDING PALLET w/ lift vector such that the correct parachute deploy con- SHOCK STRUTS TERMINAL DESCENT ditions will be achieved at a desired position relative to RADAR ANTENNA/ ENGINES (15´ ) LIDAR MIRROR ASSEMBLY the target landing site. This guidance scheme is a de- rivative of the Apollo entry guidance approach,2 and has been tested extensively in a high fidelity simula- 3 Figure 1: Entry Capsule Cutaway View tion environment for use at Mars. Parachute Descent Phase. Deployment of the su- The entry capsule is designed to accommodate po- personic parachute is triggered by the entry guidance tentially large (600-1000 kg) payloads while providing logic at approximately Mach 2.2. This parachute is a aeromaneuvering capability for closed-loop guidance derivative of the Mars Pathfinder mortar-deployed to within ±3 km (3s ) or better of a designated target parachute, and serves as a drogue parachute in this site. A biconic backshell is used to obtain high volu- EDL system, decelerating the spacecraft quickly to metric efficiency in payload packaging (a large rover is subsonic speeds. Once the vehicle reaches Mach 0.8, shown in Fig. 1 as an example). A two-stage parachute the backshell and supersonic parachute are jettisoned system is employed, enabling deceleration of very (eliminating mass that is no longer needed), and a large spacecraft while allowing time for terminal much larger (up to 30 m) subsonic main parachute is sensing and hazard avoidance during terminal descent. deployed. This parachute is designed to quickly bring Both radar and lidar sensors are used for local terrain- even large vehicles to low (40-50 m/s) terminal ve- relative navigation to identify safe landing sites to the locities that provide sufficient time for terminal sens- spacecraft’s guidance system. ing prior to powered descent. Concepts and Approaches for Mars Exploration 6154.pdf

NEXT-GENERATION EDL SYSTEM FOR MARS LANDERS: S. W. Thurman

surrounding the guidance system’s current projected landing site. The lidar elevation maps are used within the guidance system to identify any potential hazards near the projected site, and to redesignate the target site to a safer location if necessary. Powered Descent Phase. Once the navigation sys- tem and hazard identification logic have designated a safe, and reachable, local target site, the lander’s guid- ance system computes an appropriate time to separate from the subsonic parachute and begin powered de- scent. This computation establishes a trajectory that will reach the designated target site while maximizing the amount of available performance margin. The radar and lidar sensors, along with the hazard detection and retargeting logic, continue to operate during powered descent, scrutinizing the target site and the surrounding area as the effective resolution of the lidar-generated terrain maps improves, redesignating the target site as needed. The guidance system periodi- cally computes a new reference trajectory leading to the current target site, using a set of algorithms derived from the powered descent guidance logic for the Apollo Lunar Module.4 Touchdown. Powered descent concludes with thrust termination approximately 1 m above the surface, re- sulting in velocity components at touchdown of ap- proximately 3 m/s (vertical) and a tolerance of ±0.5 m/s (horizontal), well within the capabilities of the landing/arrest approaches under consideration. Development Plan: Prototype development and test activities for new system components have already been initiated, including a prototype lidar/hazard de- tection system, subsonic parachute, and aerodynamic implements for hypersonic maneuvering. Acknowledgements: The author would like to rec- ognize the contributions of EDL team members at NASA’s Ames Research Center, Moffet Field, CA, Johnson Space Center, Houston, TX, and Langley Re- search Center, Hampton, VA, along with the Naval Air Warfare Center at China Lake, CA. The work de- scribed in this abstract was carried out, in part, at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aero- nautics and Space Administration. References: [1] Rivellini, T. P., Oritz, G. M. and Steltzner, A. D. (2000) LPI Workshop, Houston, TX. [2] Figure 2: EDL Sequence of Events Carman, G. L., Ives, D. G. and Geller, D. K. (1998) AIAA During parachute descent terrain-relative naviga- Atmospheric Flight Mechanics Conf., Boston, MA, No. 98- tion is initiated. The landing radar acquires the surface 4570. [3] Striepe, S. A., Queen, E. M., Powell, R. W., Braun, at an altitude of 3700 m, allowing the onboard naviga- R. D., Cheatwood, F. N., Aguirre, J. T., Sachila, L. A. and tion system to accurately determine the spacecraft’s Lyons, D. T. (1998) AIAA Guidance, Navigation, and Con- surface-relative altitude and velocity. In the 1500 to trol Conf., Boston, MA, No. 98-4569. [4] Klumpp, A. R. 1000 m range a scanning lidar begins periodically gen- (1974) Automatica, 10, 133-146. erating local elevation maps of the surface, in the area