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

Abstract

The absolute specific intensity (W/cm 2-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.