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Aerothermodynamics for Exploration Entry Systems
 

NASA Ames is at the forefront of aerothermodynamic analysis of entry systems, developing and applying aerothermal analysis models, tools, and processes to support strategic goals in space exploration.

Benefit
The Exploration Mission is an exciting and bold vision of discovery. In contrast to isolated ventures, the Exploration Mission calls for extended human presence in the solar system, starting at the Moon and striving toward Mars. To support this extended presence, repeated robotic and human travel to and from the Moon and Mars will be required. These transits will employ aeroassist maneuvers, such as direct entry or aerocapture at Earth and Mars. The science of entry physics and aerothermodynamics will be a critical discipline in engineering these operations.

Research Overview
Anticipating the technologies needed to enable the Exploration Mission, NASA Ames Research Center (ARC) is seeking to support development in shock layer radiation modeling, design environment process enhancement, uncertainty and risk analysis, traceability of ground-based testing to flight, and combined trajectory and entry vehicle shape optimization.

Transit to and from the Moon and Mars will have mission segments of atmospheric entry at super-orbital velocities at Earth and Mars. Furthermore, the size of the entry vehicle for human missions will be greater than the recent experience with robotic missions. The increase in both vehicle size and entry velocity shifts the dominant mode of entry heating from convective to radiative (50% of the Apollo peak heat rate was due to radiation generated behind the bow shock). As a result, shock layer radiation of high temperature air (Earth) and carbon-dioxide (Mars) will be critical processes to model and predict. Furthermore, it is likely that an ablative thermal protection system (TPS) will be needed to manage the high heat flux rates. ARC researchers have deep expertise and recent experience in all aspects of radiation modeling: identification of key reaction mechanisms; design and execution of ground-based shock tube experiments; chemical kinetic and internal energy partition model development; and flight prediction. Ames seeks to fill critical gaps in the knowledge of radiative environments in the Martian atmosphere, as well as the combined radiative-ablative flowfields.

The experiences of flight programs have motivated ARC personnel to improve the design process. The vehicle design process is evolving to rely more on computational simulation with select test data, than on an extensive testing campaign. The development of simulation processes that produce timely, adaptable, and reliable design environments is therefore important. Timeliness is promoted by focusing on all aspects of computational simulation, not just computer run time. Targeted for improvement are the long duration steps of geometry translation, grid generation, and quality assurance. Additionally, the objective is to develop processes that are efficient for many solutions, not just a single simulation. Adaptability is key in a concurrent design process where the entry shape and trajectory are developing daily. To keep the aerothermal database relevant and current, physics-based interpolation methods and database schemes are devised for leveraging existing data. Reliability is fostered through knowledge of the limitations of the physical and numerical models, ongoing comparison to ground and flight data, and a robust quality assurance process that implements best-practices.

Velocity

Future entry vehicles will not only be optimized for performance and cost, but also for minimum risk, so uncertainty and risk analysis will play an important role in the design process. ARC seeks to incorporate uncertainty and risk analysis principles into aerothermodynamic modeling. Several approaches are currently being pursued. The first approach is a recently developed Monte-Carlo analysis to identify key models and model parameters, for which entry heating results are most sensitive. These results provide the rationale for further investigation. The second approach is focused testing in hyperthermal ground test facilities (arc-jet, ballistic range, and shock tube) to reduce key uncertainties. Third is defining a consistent approach to connect environment uncertainties to TPS margins. The final aspect is injecting reliability principles into ground-based testing.

(Right) ARC supports shock layer radiation modeling.

Arc-jet facilities are employed to test and qualify TPS. In the arc-jet, gas phase enthalpies and model heating rates simulate flight. Since no ground-based facility reproduces the hyperthermal flight environment identically, a crucial step in evaluating the test data is to develop the relationship between the ground and flight environment. ARC seeks to continue development in arc-jet flow diagnostic techniques and analysis methods to support detailed ground-to-flight traceability. Current diagnostic methods include calorimetry, passive emission spectroscopy, and Laser-Induced Fluorescence.

To achieve the truly exceptional entry system that maximizes performance under minimum cost and risk, ARC seeks to develop and apply multi-disciplinary design optimization techniques for the entry segment. Recent approaches have followed two paths. The first path couples heating environment uncertainty with trajectory optimization to yield minimum risk approaches to TPS design. The second path leverages extensive experience in transonic vehicle design optimization (using objective functions based on aerodynamic force), into methods for shape optimization that employ objective functions based on aerothermodynamic heating. The first approach optimizes the trajectory for a fixed vehicle shape while the second optimizes the vehicle shape for a fixed trajectory. The ultimate goal is to combine both trajectory and entry shape optimization to minimize risk through robust performance.

Background
ARC is at the forefront of the aerothermodynamic analysis of entry systems. The mission is to develop and apply aerothermal analysis models, tools, and processes to support NASA strategic goals in space exploration. Personnel at ARC have performed aerothermodynamic analysis in support of Earth-to-orbit programs X-33, X-34, X-37, X-38, Columbia Accident Investigation, and Shuttle Return to Flight. For planetary entry, recent activities include Mars Pathfinder, Mars Exploration Rover, Mars Science Laboratory, Stardust, Titan Aerocapture, Neptune Aerocapture, and Venus direct entry.