Mark Schoenenberger

Aerodynamic Challenges for the Mars Science Laboratory Entry, Descent and Landing

An overview of several important aerodynamics challenges new to the Mars Science Laboratory (MSL) entry vehicle are presented. The MSL entry capsule is a 70 degree sphere cone-based on the original Mars Viking entry capsule. Due to payload and landing accuracy requirements, MSL will be flying at the highest lift-to-drag ratio of any capsule sent to Mars (L/D = 0.24). The capsule will also be flying a guided entry, performing bank maneuvers, a first for Mars entry. The systems mechanical design and increased performance requirements require an expansion of the MSL flight envelope beyond those of historical missions. In certain areas, the experience gained by Viking and other recent Mars missions can no longer be claimed as heritage information. New analysis and testing is re1quired to ensure the safe flight of the MSL entry vehicle. The challenge topics include: hypersonic gas chemistry and laminar-versus-turbulent flow effects on trim angle, a general risk assessment of flying at greater angles-of-attack than Viking, quantifying the aerodynamic interactions induced by a new reaction control system and a risk assessment of recontact of a series of masses jettisoned prior to parachute deploy. An overview of the analysis and tests being conducted to understand and reduce risk in each of these areas is presented. The need for proper modeling and implementation of uncertainties for use in trajectory simulation has resulted in a revision of prior models and additional analysis for the MSL entry vehicle. The six degree-of-freedom uncertainty model and new analysis to quantify roll torque dispersions are presented.

Mars Exploration Rover Six-Degree-of-Freedom Entry Trajectory Analysis

The Mars Exploration Rover mission delivered the rovers Spirit and Opportunity to the surface of Mars using the same entry, descent, and landing scenario that was developed and successfully implemented by Mars Pathfinder. This investigation describes the premission trajectory analysis that was performed for the hypersonic portion of the Mars Exploration Rover entry up to parachute deployment. In this analysis, a six-degree-of-freedom trajectory simulation of the entry is performed to determine the entry characteristics of the capsules. In addition, a Monte Carlo dispersion analysis is also performed to statistically assess the robustness of the entry design to off-nominal conditions to ensure that all entry requirements are satisfied. The premission results show that the attitude at peak heating and parachute deployment are well within entry limits. In addition, the parachute deployment dynamic pressure and Mach number are also well within the design requirements.

Mars Entry Atmospheric Data System Trajectory Reconstruction Algorithms and Flight Results

The Mars Entry Atmospheric Data System is a part of the Mars Science Laboratory, Entry, Descent, and Landing Instrumentation project. These sensors are a system of seven pressure transducers linked to ports on the entry vehicle forebody to record the pressure distribution during atmospheric entry. These measured surface pressures are used to generate estimates of atmospheric quantities based on modeled surface pressure distributions. Specifically, angle of attack, angle of sideslip, dynamic pressure, Mach number, and freestream atmospheric properties are reconstructed from the measured pressures. Such data allows for the aerodynamics to become decoupled from the assumed atmospheric properties, allowing for enhanced trajectory reconstruction and performance analysis as well as an aerodynamic reconstruction, which has not been possible in past Mars entry reconstructions. This paper provides details of the data processing algorithms that are utilized for this purpose. The data processing algorithms include two approaches that have commonly been utilized in past planetary entry trajectory reconstruction, and a new approach for this application that makes use of the pressure measurements. The paper describes assessments of data quality and preprocessing, and results of the flight data reduction from atmospheric entry, which occurred on August 5th, 2012.

Aerodynamics for Mars Phoenix Entry Capsule

†* ‡ Pre-flight aerodynamics data for the Mars Phoenix entry capsule are presented. The aerodynamic coefficients were generated as a function of total angle-of-attack and either Knudsen number, velocity, or Mach number, depending on the flight regime. The database was constructed using continuum flowfield computations and data from the Mars Exploration Rover and Viking programs. Hypersonic and supersonic static coefficients were derived from Navier-Stokes solutions on a pre-flight design trajectory. High-altitude data (free-molecular and transitional regimes) and dynamic pitch damping characteristics were taken from Mars Exploration Rover analysis and testing. Transonic static coefficients from Viking wind tunnel tests were used for capsule aerodynamics under the parachute. Static instabilities were predicted at two points along the reference trajectory and were verified by reconstructed flight data. During the hypersonic instability, the capsule was predicted to trim at angles as high as 2.5 deg with an on-axis center-of-gravity. Trim angles were predicted for off-nominal pitching moment (4.2 deg peak) and a 5 mm off-axis center-ofgravity (4.8 deg peak). Finally, hypersonic static coefficient sensitivities to atmospheric density were predicted to be within uncertainty bounds.

Aerodynamic Interference Due to MSL Reaction Control System

An investigation of effectiveness of the reaction control system (RCS) of Mars Science Laboratory (MSL) entry capsule during atmospheric flight has been conducted. The reason for the investigation is that MSL is designed to fly a lifting actively guided entry with hypersonic bank maneuvers, therefore an understanding of RCS effectiveness is required. In the course of the study several jet configurations were evaluated using Langley Aerothermal Upwind Relaxation Algorithm (LAURA) code, Data Parallel Line Relaxation (DPLR) code, Fully Unstructured 3D (FUN3D) code and an Overset Grid Flowsolver (OVERFLOW) code. Computations indicated that some of the proposed configurations might induce aero-RCS interactions, sufficient to impede and even overwhelm the intended control torques. It was found that the maximum potential for aero-RCS interference exists around peak dynamic pressure along the trajectory. Present analysis largely relies on computational methods. Ground testing, flight data and computational analyses are required to fully understand the problem. At the time of this writing some experimental work spanning range of Mach number 2.5 through 4.5 has been completed and used to establish preliminary levels of confidence for computations. As a result of the present work a final RCS configuration has been designed such as to minimize aero-interference effects and it is a design baseline for MSL entry capsule.

Static Aerodynamics of the Mars Exploration Rover Entry Capsule

The static aerodynamics for the Mars Exploration Rover (MER) aeroshell are presented. This aerodynamic database was an integral part of the end-to-end simulation used in pre- entry analysis for determining the MER entry design requirements for development of the MER entry system, as well as targeting the MER landing sites. The database was constructed using the same approach used for Mars Pathfinder (MPF). However, the MER aerodynamic database is of much higher fidelity and tailored to the MER entry trajectories. This set of data includes direct simulation Monte Carlo calculations covering the transitional regime of the entry trajectory and computational fluid dynamics calculations describing the aerodynamics in the hypersonic and supersonic continuum regimes. An overview of the methodology used to generate the data is given along with comparisons to important features in the MPF aerodynamics and related heritage data. The MER and MPF comparison indicates that trajectory specific data is required to properly model the flight characteristics of a.blunt entry capsule at Mars.

Hypersonic and Supersonic Static Aerodynamics of Mars Science Laboratory Entry Vehicle

This paper describes the analysis of continuum static aerodynamics of Mars Science Laboratory (MSL) entry vehicle (EV). The method is derived from earlier work for Mars Exploration Rover (MER) and Mars Path Finder (MPF) and the appropriate additions are made in the areas where physics are different from what the prior entry systems would encounter. These additions include the considerations for the high angle of attack of MSL EV, ablation of the heatshield during entry, turbulent boundary layer, and other aspects relevant to the flight performance of MSL. Details of the work, the supporting data and conclusions of the investigation are presented.

Ballistic Range Testing of the Mars Exploration Rover Entry Capsule

Results from a 25 shot ballistic range test of the Mars Exploration Rover (MER) aeroshell are presented. The supersonic pitch damping properties of the MER capsule were characterized between Mach = 1.5 and Mach = 3.5 and total angles-of-attack from 0 degrees to greater than 25 degrees. Three capsule center-of-gravity positions were tested across this range of conditions, 0.27, 0.30 and 0.33 body diameters aft of the nose. Parameter identification results show that the capsule is dynamically unstable at low angles-of-attack across the Mach numbers tested, with instability increasing with lower speeds. This dynamic instability was seen to increase with aft center-of-gravity movement. The MER outer mold line was very similar to the successful Mars Pathfinder capsule with only minor modifications. Pathfinder relied on Viking forced oscillation data for preflight predictions. The pitch damping data calculated from this test program are shown to more accurately reproduce the measured Path finder flight data.

Survey of Blunt Body Dynamic Stability in Supersonic Flow

This survey presents a comprehensive investigation of blunt body dynamic stability. An examination of the experimental, analytical, and computational methods for predicting dynamic stability characteristics, along with the deficiencies accompanying each method is presented. The observed influence of vehicle and environmental parameters on the resulting dynamic response is discussed. Additionally, the proposed physical mechanisms that may govern this complex phenomenon are introduced. There exists a vast amount of literature and test data that is continually growing with each mission. Compiling the observations of dynamic behavior acquired from various test geometries, environments, and techniques, as well as the proposed explanations to the observed trends, sheds light on the validity of the proposed physical mechanisms. This in turn guides future efforts to improve the experimental and computational prediction techniques and further the fundamental understanding of blunt body dynamic stability.

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.