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In watching this video:

of the recent Orbital ATK’s Cygnus launch, I noticed that when the 2nd stage engine fires (after the ullage engine finishes firing), that there does not appear to be any exhaust plume or visual distortion of the 1st stage fading away in the distance, even though the visual goes right through the 2nd stage exhaust plume.

I would have expected to see at least a minimal visual distortion of parts the image due to the heat of the exhaust bending light in the exhaust area due to a different coeffecient of refraction due to high exhaust temperatures, but the image does not appear to be distorted across the whole field of view of the camera. Does light not get bent (in absence of an atmosphere) despite an exhaust? I would have expected to see at least some heat effects looking through the nozzle. For comparison, the recent SpaceX launch after 1st stage separation (

) does show some exhaust presence around the nozzle, but still not a visual distortion looking through the exhaust plume. Just curious as to why the exhaust plume doesn't visually distort light going through the plume.

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The visual distortion you see due to heat is due to the dependence of the refractive index of air on its temperature, combined with turbulent mixing which causes the temperature to vary chaotically in space. For a second stage fired largely in a vacuum, that turbulent mixing is going to be nearly nonexistent, and the variation of optical density within the exhaust plume is going to be far smaller in comparison, to the point that distortion will be nearly invisible.

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    $\begingroup$ +1 I was really surprised at the following; 21000 kg of propellant, 840 seconds gives only 25 kg/s. With an effective exhaust velocity of 4400 m/s, that's only 6 milligrams per linear meter of exhaust! Assume a cross-section of a square meter, that gives a density of only about $10^{-5}$ atmospheres, rapidly dropping as soon as it starts to expand. There's practically no matter there to do any refracting! $\endgroup$ – uhoh Apr 20 '17 at 18:01

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