Ethiraj Venkatapathy

Computational Aerothermodynamic Design Issues for Hypersonic Vehicles

A brief review of the evolutionary progress in computational aerothermodynamics is presented. The current status of computational aerothermodynamics is then discussed, with emphasis on its capabilities and limitations for contributions to the design process of hypersonic vehicles. Some topics to be highlighted include: (1) aerodynamic coefficient predictions with emphasis on high temperature gas effects; (2) surface heating and temperature predictions for thermal protection system (TPS) design in a high temperature, thermochemical nonequilibrium environment; (3) methods for extracting and extending computational fluid dynamic (CFD) solutions for efficient utilization by all members of a multidisciplinary design team; (4) physical models; (5) validation process and error estimation; and (6) gridding and solution generation strategies. Recent experiences in the design of X-33 will be featured. Computational aerothermodynamic contributions to Mars Path finder, METEOR, and Stardust (Comet Sample return) will also provide context for this discussion. Some of the barriers that currently limit computational aerothermodynamics to a predominantly reactive mode in the design process will also be discussed, with the goal of providing focus for future research.

Current grid-generation strategies and future requirements in hypersonic vehicle design, analysis and testing

Abstract Recent advances in computational power enable computational fluid dynamic modeling of increasingly complex configurations. A review of grid-generation methodologies implemented in support of the computational work performed for the X-38 (Experimental Crew Return Vehicle) and X-33 hypersonic vehicles are presented. In strategizing topological constructs and blocking structures, the factors considered are geometric configuration, optimal grid size, numerical algorithms, accuracy requirements, physics of the problem at hand, computational expense and available computer hardware. Also addressed are grid-refinement strategies, the effect of wall spacing and convergence. The significance of grids is demonstrated through a comparison of computational and experimental results of the aeroheating environment experienced by the X-38 vehicle. Special topics on grid-generation strategies are also addressed to model control surface deflections and material mapping.

Adaptive Deployable Entry and Placement Technology (ADEPT): A Feasibility Study for Human Missions to Mars

The present paper describes an innovative, semi-rigid, mechanically deployable hypersonic decelerator system for human missions to Mars. The approach taken in the present work builds upon previous architecture studies performed at NASA, and uses those findings as the foundation to perform analysis and trade studies. The broad objectives of the present work are: (i) to assess the viability of the concept for a heavy mass (landed mass ≈40 mT) Mars mission through system architecture studies; (ii) to contrast it with system studies previously performed by NASA; and (iii) to make the case for a Transformable Entry System Technology. The mechanically deployable concept at the heart of the proposed transformable architecture is akin to an umbrella, which in a stowed configuration meets launch requirements by conforming to the payload envelope in the launch shroud, and when deployed in earth orbit forms a large aerosurface designed to provide the necessary aerodynamic forces upon entry into the Martian atmosphere. The aerosurface is a thin skin draped over high-strength ribs; the thin skin or fabric with flexible material serves as the thermal protection system, and the ribs serve as the structure. A four-bar linkage mechanism allows for a reorientation of the aerosurface during aerocapture or during the entry and descent phases of atmospheric flight, thus providing a capability to navigate and control the vehicle and make possible precision landing. The actuators and mechanisms that are used to deploy the aerosurface are multi-functional in that they also allow for reorienting the

Application of a solution adaptive grid scheme to complex three-dimensional flows

A new three-dimensional adaptive grid code (SAGE) based on the algebraic, solution-adaptive scheme of Nakahashi and Deiwert has been developed and applied to a variety of problems. This report describes its application to a range of complex three-dimensional, supersonic, and hypersonic flows. Examples discussed are a tandem-slot fuel injector, the hypersonic forebody of the Aeroassist Flight Experiment, the three-dimensional base flow behind the Aeroassist Flight Experiment, the supersonic flow around a three-dimensional swept ramp, and a generic, hypersonic, three-dimensional nozzle-plume flow

Small Probe Reentry Investigation for TPS Engineering (SPRITE)

Small Probe Reentry Investigation for TPS Engineering (SPRITE) is a novel concept for a comparatively low-cost means of certifying thermal protection system (TPS) materials for spaceflight. By developing a fully instrumented small-scale test platform that can be tested both in arc-jet facilities and in flight, SPRITE promises to improve the ground-to-flight traceability of TPS qualification programs by implementing the NASA “test-like-you-fly” policy. This paper discusses the design and manufacture of the proof-of-concept SPRITE test articles, development and capabilities of the SPRITE flight-like data acquisition system, and fidelity of the design tools used in the effort. With the successful completion of this stage of the SPRITE project, the next test article to be built will have a hemispherical backshell as originally designed for flight stability. Unlike the initial proof-of-concept probes, TPS for the next test article will be scaled according to current margins policies for arc-jet testing at relevant flight-like aerothermal environments. At the conclusion of the ground test campaign, SPRITE probes will be ready for flight testing and, ultimately, acceptance into future mission planning where they will contribute to more affordable access to space for both large and small science payloads.

Single expansion ramp nozzle simulations

The single-expansion-ramp-nozzle (SERN) experiment underway at NASA Ames Research Center simulates the National Aerospace Plane propulsive jet-plume flow. Recently, limited experimental data has become available from an experiment with a generic nozzle/afterbody model in a hypersonic wind tunnel. The present paper presents full three-dimensional solutions obtained with the implicit Navier-Stokes solver, FL3D, for the baseline model and a version of the model with side extensions. Analysis of the computed flow clearly shows the complex 3-D nature of the flow, critical flow features, and the effect of side extensions on the plume flow development. Flow schematics appropriate for the conditions tested are presented for the baseline model and the model with side extensions. The computed results show excellent agreement with experimental shadowgraph and with surface pressure measurements. The computed and experimental surface oil-flows show the same features but may be improved by appropriate turbulence modeling.

A Web-based Analysis System for Planetary Entry Vehicle Design

An integrated analysis environment for designing planetary entry vehicles has been developed. The system uses a Web-based graphical user interface, so that members of a geographically dispersed design team, with heterogeneous hardware, can readily access analysis modules located at other sites. The analyses and their implementation are briefly described. A sample application illustrates the value of this system, and indicates where further development effort would provide significant return.

Comparison of Nonequilibrium Solution Algorithms Applied to Chemically Stiff Hypersonic Flows

Three solution algorithms, explicit under-relaxation, point implicit, and lower-upper symmetric Gauss-Seidel, are used to compute nonequilibrium flow around the Apollo 4 return capsule at the 62-km altitude point in its descent trajectory. By varying the Mach number, the efficiency and robustness of the solution algorithms were tested for different levels of chemical stiffness. The performance of the solution algorithms degraded as the Mach number and stiffness of the flow increased. At Mach 15 and 30, the lower-upper symmetric Gauss-Seidel method produces an eight order of magnitude drop in the energy residual in one-third to one-half the Cray C-90 computer time as compared to the point implicit and explicit under-relaxation methods. The explicit under-relaxation algorithm experienced convergence difficulties at Mach 30 and above. At Mach 40 the performance of the lower-upper symmetric Gauss-Seidel algorithm deteriorates to the point that it is out performed by the point implicit method. The effects of the viscous terms are investigated. Grid dependency questions are explored.

Hypersonic single expansion ramp nozzle simulations

The single-expansion ramp-nozzle experiment underway at NASA Ames Research Center simulates the National Aerospace Plane propulsive jet-plume flow. Recently, limited experimental data have become available from an experiment with a generic nozzle/afterbody model in a hypersonic wind tunnel. The present paper presents full three-dimensional solutions obtained with the implicit Navier-Stokes solver, FL3D, for the baseline model and a version of the model with side extensions. Analysis of the computed flow clearly shows the complex three-dimensional nature of the flow, critical flow features, and the effect of side extensions on the plume flow development. Schematics of the flowfield appropriate for the conditions tested are presented for the baseline model and the model with side extensions. The computed results show excellent agreement with experimental shadowgraph and with surface pressure measurements. The computed and experimental surface oil flows show the same features but may be improved by appropriate turbulence modeling.

Venus In Situ Explorer Mission design using a mechanically deployed aerodynamic decelerator

The Venus In Situ Explorer (VISE) Mission addresses the highest priority science questions within the Venus community outlined in the National Research Councils Decadal Survey. The heritage Venus atmospheric entry system architecture, a 45° sphere-cone rigid aeroshell with a carbon phenolic thermal protection system, may no longer be the preferred entry system architecture compared to other viable alternatives being explored at NASA. A mechanically-deployed aerodynamic decelerator, known as the Adaptive Deployable Entry and Placement Technology (ADEPT), is an entry system alternative that can provide key operational benefits and risk reduction compared to a rigid aeroshell. This paper describes a mission feasibility study performed with the objectives of identifying potential adverse interactions with other mission elements and establishing requirements on decelerator performance. Feasibility is assessed through a launch-to-landing mission design study where the Venus Intrepid Tessera Lander (VITaL), a VISE science payload designed to inform the Decadal Survey results, is repackaged from a rigid aeroshell into the ADEPT decelerator. It is shown that ADEPT reduces the deceleration load on VITaL by an order of magnitude relative to a rigid aeroshell. The more benign entry environment opens up the VISE mission design environment for increased science return, reduced risk, and reduced cost. The ADEPT-VITAL mission concept of operations is presented and details of the entry vehicle structures and mechanisms are given. Finally, entry aerothermal analysis is presented that defines the operational requirements for a revolutionary structural-TPS material employed by ADEPT: three-dimensionally woven carbon cloth. Ongoing work to mitigate key risks identified in this feasibility study is presented.