Eric M. Queen

Entry, Descent, and Landing Performance of the Mars Phoenix Lander

On May 25, 2008, the Mars Phoenix Lander successfully landed on the northern arctic plains of Mars. An overview of a preliminary reconstruction analysis performed on each en-try, descent, and landing phase to assess the performance of Phoenix as it descended is pre-sented and a comparison to pre-entry predictions is provided. The landing occurred 21 km further downrange than the predicted landing location. Analysis of the flight data revealed that the primary cause of Phoenix’s downrange landing was a higher trim total angle of at-tack during the hypersonic phase of the entry, which resulted in Phoenix flying a slightly lift-ing trajectory. The cause of this higher trim attitude is not known at this time. Parachute deployment was 6.4 s later than prediction. This later deployment time was within the varia-tions expected and is consistent with a lifting trajectory. The parachute deployment and in-flation process occurred as expected with no anomalies identified. The subsequent parachute descent and powered terminal landing also behaved as expected. A preliminary reconstruc-tion of the landing day atmospheric density profile was found to be lower than the best apri-ori prediction, ranging from a few percent less to a maximum of 8%. A comparison of the flight reconstructed trajectory parameters shows that the actual Phoenix entry, descent, and landing was close to pre-entry predictions. This reconstruction investigation is currently on-going and the results to date are in the process of being refined.

Parachute Aerodynamics from Video Data

A new data analysis technique for the identification of static and dynamic aerodynamic stability coefficients from wind tunnel test video data is presented. This new technique was applied to video data obtained during a parachute wind tunnel test program conducted in support of the Mars Exploration Rover Mission. Total angle-of-attack data obtained from video images were used to determine the static pitching moment curve of the parachute. During the original wind tunnel test program the static pitching moment curve had been determined by forcing the parachute to a specific total angle-of -attack and measuring the forces generated. It is shown with the new technique that this parachute, when free to rotate, trims at an angle-of-attack two degrees lower than was measured during the forced-angle tests. An attempt was also made to extract pitch damping information from the video data. Results suggest that the parachute is dynamically unstable at the static trim point and tends to become dynamically stable away from the trim point. These trends are in agreement with limit-cycle-like behavior observed in the video. However, the chaotic motion of the parachute produced results with large uncertainty bands.

Internal Moving Mass Actuator Control for Mars Entry Guidance

An internal moving mass actuator control system is proposed for angle-of-attack and sideslip-based Mars precision entry guidance. Internal moving mass actuators provide vehicle control moments without direct interaction with the external flowfield. This enables trajectory control without destabilization concerns associated with aerodynamic flap ablation and flowfield interaction for both flap and thruster control systems. Angle-of-attack and sideslip control is proposed as an alternative to bank angle guidance for decoupling range and crossrange control for improved precision entry guidance. A new variant of the Apollo Earth return terminal guidance algorithm is presented and used to provide closed-loop attitude commands. A linear-quadratic controller with integral error action is synthesized and tuned for a Mars Phoenix-sized entry vehicle with two internal moving mass actuators for guidance command tracking. Nonlinear simulation of the controller for the eight-degree-of-freedom system demonstrates preci...

The aerodynamics of axisymmetric blunt bodies flying at angle of attack

The Mars Science Laboratory entry capsule is used as an example to demonstrate how a blunt body of revolution must be treated as asymmetric in some respects when flying at a non-zero trim angle of attack. A brief description of the axisymmetric moment equations are provided before solving a system of equations describing the lateral-directional moment equations for a blunt body trimming at an angle of attack. Simplifying assumptions are made which allow the solution to the equations to be rearranged to relate the roll and yaw stability with sideslip angle to the frequency of oscillation of the vehicle body rates. The equations show that for a blunt body the roll and yaw rates are in phase and proportional to each other. The ratio of the rates is determined by the static stability coefficients and mass properties about those axes. A trajectory simulation is used to validate the static yaw stability parameter identification equation and a simple method of identifying the oscillation frequency from the body rates. The approach is shown to successfully extract the modeled yaw stability coefficient along a simulated Mars entry. Mars Science Laboratory flight data results are presented from earlier work which indicate that results from both the validation case and flight data are in agreement with preflight predictions. A brief discussion of the dynamic stability is also provided. Trimming at a nonzero angle suggests that the typical axisymmetric models of the dynamic stability coefficients should be modified. However, further experimental or computational work must be done to separate damping due to body rates and wind relative rates before the correct lifting formulation would affect simulation results.

Mars Phoenix Entry, Descent, and Landing Simulation Design and Modeling Analysis

The 2007 Mars Phoenix Lander was launched in August of 2007 on a ten month cruise to reach the northern plains of Mars in May 2008. Its mission continues NASA’s pursuit to find evidence of water on Mars. Phoenix carries upon it a slew of science instruments to study soil and ice samples from the northern region of the planet, an area previously undiscovered by robotic landers. In order for these science instruments to be useful, it was necessary for Phoenix to perform a safe entry, descent, and landing (EDL) onto the surface of Mars. The EDL design was defined through simulation and analysis of the various phases of the descent. An overview of the simulation and various models developed to characterize the EDL performance is provided. Monte Carlo statistical analysis was performed to assess the performance and robustness of the Phoenix EDL system and are presented in this paper. Using these simulation and modelling tools throughout the design and into the operations phase, the Mars Phoenix EDL was a success on May 25, 2008.

Entry, Descent, and Landing Operations Analysis for the Mars Phoenix Lander

The Mars Phoenix lander was launched August 4, 2007 and remained in cruise for ten months before landing in the northern plains of Mars in May 2008. The one-month Entry, Descent, and Landing (EDL) operations phase prior to entry consisted of daily analyses, meetings, and decisions necessary to determine if trajectory correction maneuvers and environmental parameter updates to the spacecraft were required. An overview of the Phoenix EDL trajectory simulation and analysis that was performed during the EDL approach and operations phase is described in detail. The evolution of the Monte Carlo statistics and footprint ellipse during the final approach phase is also provided. The EDL operations effort accurately delivered the Phoenix lander to the desired landing region on May 25, 2008.

Multi-Body Modeling and Simulation for the Mars Phoenix Lander Entry, Descent and Landing

A multi-body flight simulation for the Phoenix Mars Lander has been developed that includes high fidelity six degree-of-freedom rigid-body models for the parachute and lander system. The simulation provides attitude and rate history predictions of all bodies throughout the flight, as well as loads on each of the connecting lines. In so doing, a realistic behavior of the descending parachute/lander system dynamics can be simulated that allows assessment of the Phoenix descent performance and identification of potential sensitivities for landing. This simulation provides a complete end-to-end capability of modeling the entire entry, descent, and landing sequence for the mission. Time histories of the parachute and lander aerodynamic angles are presented. The response of the lander system to various wind models and wind shears is shown to be acceptable. Monte Carlo simulation results are also presented.