Numerical modeling of the CN spectral emission of the Stardust re-entry vehicle

Abstract

Re-entry vehicles designed for space exploration are usually equipped with thermal protection systems made of ablative material. In order to properly model and predict the aerothermal environment of the vehicle, it is imperative to account for the gases produced by ablation processes. In the case of charring ablators, where an inner resin is pyrolyzed at a relatively low temperature, the composition of the gas expelled into the boundary layer is complex and may lead to thermal chemical reactions that cannot be captured with simple flow chemistry models. In order to obtain better predictions, an appropriate gas flow chemistry model needs to be included in the CFD calculations. Although more arc-jet experimental data is becoming available for model comparison, very little flight data exists for comparison and validation. However, because of the observation mission campaign led by NASA, data is available for the re-entry of the Stardust sample return vehicle, which employed a heat shield constructed of phenolic impregnated carbon ablator (PICA). Using a recently developed chemistry model for ablating carbon-phenolic-in-air species, a CFD calculation of the Stardust re-entry at 71 km is presented. The result demonstrates the need to account for different species in the flow field than the ones composing the pyrolysis gas. It is also shown that the main heat flux reduction phenomenon is through mass diffusion, and not through translational-rotational conduction, as is the case at higher altitude. The flow field solutions are also used to generate nonequilibrium radiation spectra, which are compared to the experimental data obtained during Stardust re-entry by the Echelle instrument. The predicted emission from the CN lines compares quite well with the experimental results, demonstrating the validity of the current approach.

Microstructure and gas-surface interaction studies of a low-density carbon-bonded carbon fiber composite in atmospheric entry plasmas

“…Nitridation coefficients available in the literature differ by several orders of magnitude [47,48]. Although nitridation does not cause much recession of the surface, production of gaseous CN has a significant contribution to the overall radiative heat flux of fast reentry vehicles [49].…”

Superorbital re-entry shock layers: flight and laboratory comparisons

“…Martin, Farbar and Boyd's super-catalytic wall heat flux was also around 700 W · cm −2 at the stagnation point, which reduced to below 300 W · cm −2 when ablation was considered [69]. The difference between the results show the impact of catalycity, radiative equilibrium or ablation can have on the surface heat flux.…”

“…The flowfield did not equilibrate thermally or chemically, but T tr flattens for about half of the shock layer, between the shock front and the wall boundary layer. The peak in T tr for the 11 species air model is only 28 000 K and T ve barely reaches 10 000 K at 5 mm from the wall, which is lower than Martin, Farbar and Boyd [69] and Boyd, Trumble and Wright [68] The surface heat flux at 41 s after the 120 km re-entry start with the radiative equilibrium wall peaked at about 550 W · cm −2 , and surface temperature was predicted to peak at close to 3400 K, at an altitude of approximately 62 km. Gupta [71] and Olynick, Chen and Tauber [72] performed Stardust simulations at several theoretical trajectory points during early stages of the mission, to understand the behaviour of the capsule years prior to completion of its scientific objectives and return to Earth.…”

“…The N + 2 bandhead was not identifiable in numerical calculations. Martin, Farbar and Boyd predicted a heavily nonequilibrium shock layer at 71 km and a peak in T tr reaching towards 60 000 K [69], which is similar to Boyd, Trumble and Wright's modified chemistry case in DPLR, but the baseline chemistry case has a T tr peak of less than 35 000 K [68]. Liu et al also used DPLR and NEQAIR for their Stardust CFD flowfield and numerical spectra simulations, and at an altitude of 71 km, the atomic N lines came close to the IR flight data, while the atomic O lines in the same region were underpredicted by numerical methods [70].…”

“…Boyd and Jenniskens' resulting radiation calculations in NEQAIR, a NASA line-by-line radiation tool, were compared to the flight data at the two trajectory points and the spectra are much closer at 81 km for both flowfield methods than at 71 km [11]. Numerical spectra in the UV was calculated from a Stardust simulation at 71 km by Martin, Farbar and Boyd [69], which included ablative species generated by material response modelling of PICA, and overestimated the CN radiation by about a factor of 2. The N + 2 bandhead was not identifiable in numerical calculations.…”