Planar Velocity Measurements of Rarefied Hypersonic Flow over a Tether Using Laser-induced Iodine Fluorescence

An investigation is presented of Mach 12 flow over a wind tunnel model of a tether connecting an aerobraking spacecraft to an inflatable drag device. The study was undertaken to support numerical simulations employed in the design of a “ballute” (balloon-parachute) aerobraking device currently being developed by Ball Aerospace and NASA. The attachment point of the tether to the ballute is subject to extreme heating conditions due to flow impingement and wind tunnel measurements were desired to evaluate the numerical predictions. The wind tunnel model of the tether-ballute junction consisted of a flat plate with a sharp leading edge from which a circular rod extended. Measurements were conducted in a “cold” free jet wind tunnel facility, using nitrogen gas expanded from ambient temperature. Detailed velocity measurements were made from the Doppler shift of the iodine B-X absorption spectra produced by laser-induced fluorescence (LIF) of a trace amount of iodine seeded in the gas. A planar grid of local velocity with a resolution on the order of one mean-free-path in the flow free stream was obtained using a wide sheet-beam from a frequency-tunable multiwatt argon laser to excite LIF, and a low-noise liquid-nitrogen-cooled CCD camera to record multiple images.