The Cassini Mission, a NASA/ESA cooperative project which includes a deployment of probe into the atmosphere of Titan, is described, with particular attention given to the shock radiometer experiment planned for the Titan probe for the analysis of Titan's atmosphere. Results from a shock layer analysis are presented, demonstrating that the mole fractions of the major species (N2, CH4, and, possibly Ar) in the Titan atmosphere can be successfully determined by the Titan-probe radiometer, by measuring the intensity of the CN(violet) radiation emitted in the shock layer during the high velocity portion of the probe entry between 200 and 400 km altitude. It is shown that the sensitivity of the CN(violet) radiation makes it possible to determine the mole fractions of N2, CH4, and Ar to about 0.015, 0.003, and 0.01, respectively, i.e., much better than the present uncertainties in the composition of Titan atmosphere.

Titan atmospheric composition by hypervelocity shock-layer analysis

Flight in the atmosphere is examined from the viewpoint of including real gas phenomena in the flow field about a vehicle flying at hypervelocity. That is to say, the flow field is subject not only to compressible phenomena, but is dominated by energetic phenomena. There are several significant features of such a flow field. Spatially, its composition can vary by both chemical and elemental species. The equations which describe the flow field include equations of state and mass, species, elemental, and electric charge continuity; momentum; and energy equations. These are nonlinear, coupled, partial differential equations that have been reduced to a relatively compact set of equations in a self-consistent manner (which allows mass addition at the surface at a rate comparable to the free-stream mass flux). The equations and their inputs allow for transport of these quantities relative to the mass-average behavior of the flow field. Thus transport of mass by chemical, thermal, pressure, and forced diffusion; transport of momentum by viscosity; and transport of energy by conduction, chemical considerations, viscosity, and radiative transfer are included. The last of these complicate the set of equations by making the energy equations a partial integrodifferential equation. Each phenomenon is considered and represented mathematically by one or more developments. The coefficients which pertain are both thermodynamically and chemically dependent. Solutions of the equations are presented and discussed in considerable detail, with emphasis on severe energetic flow fields. Hypervelocity flight in low-density environments where gaseous reactions proceed at finite rates chemical nonequilibrium is considered, and some illustrations are presented. Finally, flight where the flow field may be out of equilibrium, both chemically and thermodynamically, is presented briefly.

https://ntrs.nasa.gov/api/citations/19890017746/downloads/19890017746.pdf

Hypervelocity atmospheric flight: Real gas flow fields

Hypersonic wake flows behind the Aeroassist Flight Experiment (AFE) geometry are analyzed using two Navier-Stokes flow solvers. Many of the AFE wake features observed in ballistic-range shadowgraphs are simulated using a simple, two-dimensional semicylinder geometry at moderate angles of attack. At free-stream conditions corresponding to a Hypersonic Free Flight Facility (HFFF) AFE experiment, the three-dimensional base flow for the AFE geometry is computed using an ideal-gas, Navier-Stokes solver. The computed results agree reasonably well with the shadowgraphs taken at the HFFF. An ideal-gas and a nonequilibrium Navier-Stokes solver have been coupled and applied to the complete flow around the AFE vehicle at the free-stream conditions corresponding to a nomial trajectory point. Limitations of the coupled ideal-gas and nonequilibrium solution are discussed. The nonequilibrium base flow solution is analyzed for the wake radiation and the radiation profiles along various lines of sight are compared. Finally, the wake unsteadiness is predicted using experimental correlations and the numerical solutions. An adaptive grid code, SAGE, has been used in all the simulations to enhance the solution accuracy. The grid adaptation is found to be necessary in obtaining base flow solutions with accurate flow features.

AFE base flow computations

The absolute specific intensity (W/cm2-μ-sr) spectrum (also known as the radiative heating rate spectrum), incident on the stagnation point of a blunt model placed in an arcjet wind tunnel, is presented. The test model was a flat disk, 15 cm in diameter, which produced an effective nose radius of about 60 cm. The test conditions in the arcjet arc column were 80% air, 20% argon, 1.02 atm pressure, 950 A, and 1800 V. The flow conditions in the test section were 0.51 atm and 7.8 km/sec, which correspond to flight in the Earth's atmosphere at about 76 km altitude. The total spectrum was recorded and calibrated from 110 to 900 nm. The intensity calibration is considered to be accurate to within about a factor of 2 from 130 to 900 nm. The tests were made in a separate experimental setup. The measured radiative heating rate in the vacuum-ultraviolet, from 130 to 200 nm, is about 10% of the total heating rate from 130 to 900 nm. This result is in qualitative agreement with an estimate of 25% for a larger vehicle (nose radius of 259 cm) flying in the Earth's atmosphere at 77.1 km altitude and 9.5 km/sec. These data show that the radiation and its spectral content, incident on a blunt model in an arcjet, are similar to those expected for flight in the Earth's atmosphere at similar conditions, except for much stronger NO radiation from 200 to 300 nm. The source of this strong NO radiation appears to be in the freestream ahead of the bowshock wave. The physical and chemical conditions in the freestream of an arcjet are far more complex than those in actual flight, due to the excitation and expansion processes. However, the results presented show that arcjet tests can be used to simulate the radiation environment during atmospheric entry, and can provide the data needed to advance the ability to calculate realistic radiative heating rates under non-equilibrium flight condition.

Measured specific intensity from 130 to 900 nm at the stagnation point of a model in an arcjet flow of 7.8 km/sec

The influence of spherical particles on the radiative properties of high-porosity fibrous composites is examined in this article. The radiative properties formulation is based on the independent scattering assumption, which utilizes the solution of Maxwell's equations for the interaction of electromagnetic waves with the respective types of particles. The extinction, absorption, and scattering coefficients, as well as the scattering phase function of the composites are formulated as the weighted average according to the fractional volume of the constituent particulates. The models are applied to predict the radiative properties of thermal insulation composites containing fibers and spheres that are made of zirconia and silica. Numerical results are presented to illustrate the influence of the respective types of particulates on the radiative properties of the composites.

Effective Radiative Properties of Fibrous Composites Containing Spherical Particles

A forthcoming NASA flight experiment is described that provides an opportunity to obtain a large base of radiometric data for high-altitude, high-velocity thermochemically nonequilibrated-flow conditions. As a preliminary to the design of a radiometer for this experiment, an approximate method for predicting both equilibrium and nonequilibrium radiative surface fluxes is described. Spectral results for one trajectory state, a velocity of 10 km/sec at an altitude of 85 km, are presented. These results are then used to develop some of the instrument parameters that will be needed for designing of the three genre of radiometers that are proposed for this experiment.

J. O. Arnold

Papers

Radiometer experiment for the aeroassist flight experiment