Features of Afterbody Radiative Heating for Earth Entry

Abstract

Radiative heating is identified as a major contributor to afterbody heating for Earth entry capsules at velocities above 10 km/s. Because of rate-limited electron-ion recombination processes, a large fraction of the electronically-excited N and O atoms produced in the high temperature/pressure forebody remain as they expand into the afterbody region, which results in significant afterbody radiation. Large radiative heating sensitivities to electron-impact ionization rates and escape factors are identified. Ablation products from a forebody ablator are shown to increase the afterbody radiation by as much as 40%. The tangent-slab radiation transport approach is shown to over-predict the radiative flux by as much as 40% in the afterbody, therefore making the more computationally expensive ray-tracing approach necessary for accurate radiative flux predictions. For the Stardust entry, the afterbody radiation is predicted to be nearly twice as large as the convective heating during the peak heating phase of the trajectory. Comparisons between simulations and the Stardust Echelle observation measurements, which are shown to be dominated by afterbody emission, indicate agreement within 20% for various N and O lines. Similarly, calorimeter measurements from the Fire II experiment are identified as a source of validation data for afterbody radiation. For the afterbody calorimeter measurement closest to the forebody, which experiences the largest afterbody radiative heating component, the convective heating alone is shown to under-predict the measurement, even for the fullycatalytic assumption. Agreement with the measurements is improved with the addition of afterbody radiation. These comparisons with Stardust and Fire II measurements provide validation that the significant afterbody radiation values proposed in this work are legitimate.

Characterization of a Reentry Plasma Wind-Tunnel Flow with Vacuum-Ultraviolet to Near-Infrared Spectroscopy

“…Furthermore, the error bars are used to obtain an error estimate of the electronic excitation temperature by using the two least favorable possibilities to connect the measured data. These boundaries also indicate that an extrapolation to low-lying energy levels, such as the ground state, results in large errors [21,63,64]. As the difference in energy of the excited states is small, the sensitivity of this method with respect to electronic excitation temperature is small, i.e., a large uncertainty in electronic excitation temperature exists [65].…”

“…The limiting criterion (i.e., how much self-absorption is required that a line peak emits according to a blackbody) is exp−n l σ λ Δz < 0.1 (13) with the absorption cross section σ λ and a characteristic length Δz [21]. The characteristic length needed to fulfill this criterion is a fraction of a millimeter for strong lines, e.g., lines of the multiplets at 149.…”

“…Therefore, these lines are assumed to be blackbody limited, and thus a blackbody temperature can be assigned to the measurement results. For the general case of a non-Boltzmann distributed atom, the blackbody limit of a single transition is L λ;Limit 2 hc 2 λ 5 n u n B l n l n B u exp − hc λkT exc (14) with the deviations from the Boltzmann distribution n u ∕n B u and n B l ∕n l for the upper and lower levels, respectively [21].…”

“…Nitrogen transitions in the VUV region feature three different lower levels: the ground state and two low-lying metastable states. Lower states of oxygen transitions in the VUV region also feature the ground state and two low-lying metastable states [21,42]. The multiplets considered for evaluation are listed in Tables 4 and 5.…”

“…The only flight data available to date belong to the flight investigation of the reentry environment II reentry mission where the influence of VUV radiation was assessed [20,21]. Furthermore, VUV radiation has been investigated extensively in impulse facilities [22][23][24][25][26].…”