After few seconds, post liftoff(anywhere between 60s and 90s), the exhaust plume of PSLV turns into a bluish colour. This is quite evident in night launches than day launches(obviously because the brilliance of sky overwhelms the exhaust plume intensity).

Drop in atomospheric pressure must play a role but what exactly causes this cause to happen?

Why does this happen? Is it common for all solid motors?

Picture for reference from PSLV C28 launch: enter link description here

And a video reference of PSLV C44 is here. Time mark 1:30

  • $\begingroup$ Exhaust from a solid rocket will certainly have plenty of left-over carbon atoms and molecules. C-H and C2 molecules have strong blue (and green) emission lines though I can't guess why there would be a change from the bright white incandescence at lower altitudes. Next time bring a piece of plastic transmission diffraction grating and hold it over part of your lens, and you can see if there are distinct emission lines. $\endgroup$ – uhoh Jan 26 '19 at 13:08
  • $\begingroup$ This is blue engine exhaust, but it's different chemistry: What is the cause of the blue light from LH2/LOX rocket engines? $\endgroup$ – uhoh Jan 26 '19 at 13:10
  • $\begingroup$ Any recommendations for the wavelengths I have to look for in the narrow-band pass filters? I think that will also do a neat job right? $\endgroup$ – karthikeyan Jan 26 '19 at 13:12
  • $\begingroup$ Let me just do a bad guess. HTPB + Ammonium Perchlorate is the solid rocket fuel, which is supposed to burn with bright whitish/yellowish flame. Well could it be by any chance blue for the same reason why sky is blue? $\endgroup$ – Prakhar Jan 26 '19 at 14:15
  • $\begingroup$ @karthikeyan unless you have a super-fast filter wheel or a bunch of cameras each with a different filter, I think the filter is a bad idea. How can you know if you have a line? Use a blue filter, and everything will look blue. Get a grating. Even something like this or this (these are just examples) and experiment with flames or neon lights (if you can still find them) or various LEDs before your next launch. $\endgroup$ – uhoh Jan 26 '19 at 14:35

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