Future space missions under NASA’s exploration vision require the development of new ablative TPS. Chemistry modeling using parallel software and high-end computing facilities at Ames Research Center provides the TPS developer a design tool that can expedite the design cycle.
Advanced thermal system technologies are needed for future exploration missions in order to reduce launch mass and volume and to ensure mission safety. While Ablative TPS has been used in previous lunar and interplanetary missions, new TPS materials tailored to specific missions will help improve performance and increase science payload. Because TPS is a single point-of-failure subsystem, the development of advanced ablative TPS is an important part in future space missions.
The Ames modeling tool, together with the supercomputers dedicated to support NASA’s missions, allow the developer to understand the underlying competitive chemical reaction mechanisms and fine tune the design choice. Together with ground testing, such design tool can reduce the design cycle time and support the development and qualification of new TPS materials.
The state-of-the-art high-end computing facilities at Ames and parallel chemistry software enable large scale simulations of the competitive chemical reactions occurring in an ablative TPS. These are first principles simulations that do not depend on experimental parameters. Such simulations enable the developer to fine tune a design parameter and furnish a less expensive tool and faster turn-around-time in the design cycle. Some highlights of the Ames modeling software are: Molecular dynamics (MD) simulation of pyrolysis and ablation: The large supercomputers at Ames allow MD simulations using millions of particles. The simulation can follow changes in chemical bonding at high temperature and, once the bonds are broken, the detachment of hydrocarbons or other pyrolyzed products into the gas phase.
Phase transition in char: Located at the outer layer of the TPS, char is the porous carbonaceous residue produced by pyrolysis. Under high temperature, char may undergo a phase transition into a graphite-like material. This will increase its density and change the ablation character, an undesirable property. Molecular dynamics simulations can be used to study the onset of the phase transition and means to minimize the probability of such phase transition to occur.
Catalytic surface reactions: Chemical species from the boundary layer can interact with the TPS surface and change the nature of the surface and its function. Simulation of gas-surface reactions can be studied using molecular dynamics simulations. Figure 2 presents an example of such study for SiF3 etching.
Improved interaction potential using quantum mechanics/molecular mechanics (QM/MM) tools: The validity of a simulation depends on the reliability of the interaction potential. Most MD simulation studies employ “canned” force fields which may not be appropriate for the particular system under study. Ames simulation software has the capability of using QM/MM tools to derive reliable interaction potentials for MD simulation, thus improving the accuracy of the simulation result.
Background The development of future exploration space vehicles, including the Crew Exploration Vehicle (CEV), is central to NASA’s exploration missions. These vehicles will encounter harsh entry/reentry conditions. Its aerocapture and atmospheric entry will require an advanced, ablative Thermal Protection System (TPS).
Right: Schematic diagram of aerocapture.
The function of an ablative TPS depends critically on its ablation and pyrolysis properties, and surface catalytic efficiency. However, the underlying mechanisms of these chemical reactions are not well understood. Thus much of the current design depends on a limited parameter space derived from experimental information. To fully explore the parameter space, modeling of the chemical mechanisms of ablation, pyrolysis, and catalytic reactions will be required. These reactions occur in a complex, high enthalpy environment. The viability or nonviability of a TPS candidate frequently depends on how these reactions compete with each other under the convective and radiative heating conditions during entry. Large scale chemistry modeling of these reactions, based on first principles, provides the designer a tool that can shorten the design cycle in the development of a new TPS.
Ground test facilities for testing TPS performance at the velocity and heating conditions of planetary entry are limited. Fine tuning a design parameter using ground test facilities tends to be expensive. On the other hand, large scale modeling and simulation can be used, together with ground testing, to provide a viable alternative during the design cycle.