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In this 20-Nov-2018 Tweet from Rocket Labs XXX Peter Beck there is an image of the nine Rutherford engines of an Electron rocket burning. What is unusual is that the exhaust is luminescent but at the same time transparent.

Stage one is tested and ready to fly for our December ELaNa 19 mission for NASA! Electron is headed to the pad next week for checkouts.

Usually RP-1/LOX rocket exhaust at least appears to be opaque due to optical absorption of all the carbon-based soot and gas, although over-exposure may contribute to that visual effect as well.

I'm wondering if the image in that tweet (below) is just underexposed, or if a special filter has been used, or perhaps it's not using visible wavelengths and has been artificially colorized.

A quick analysis shows that the yellow color is not just a yellow tint added to a monochrome image, there is some variation within the image. The drop in overall intensity with decreasing wavelength (r g b) suggests the yellow could be the actual color of the exhaust.

enter image description here

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above: Cropped, zoomed, and sharpened.

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below: Similar, found in Facebook during image search:

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  • $\begingroup$ Compared to the F-1 engine, the Rutherford engine is tiny. Due to the very small diameter of the exhaust beam it is partially transparent, but the very large beam of a F-1 is not. Just as a thin layer of a colored liquid may be transparent and a thick layer is opaque. $\endgroup$ – Uwe Dec 17 '18 at 10:51
  • $\begingroup$ @Uwe your guess is quite plausible. It could be partially transparent, that would be most consistent with the photo. $\endgroup$ – uhoh Dec 17 '18 at 10:53
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    $\begingroup$ Some quick calculation suggests the thickness of the exhaust plume from a Rutherford is about 1/6 that of a Merlin 1D, 1/18 that of an F-1. $\endgroup$ – Russell Borogove Dec 17 '18 at 19:21
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    $\begingroup$ I think it’s a combination of small engine and underexposure. $\endgroup$ – Russell Borogove Dec 17 '18 at 19:31
  • $\begingroup$ @RussellBorogove I think you are right. Write it up? $\endgroup$ – uhoh Jan 25 at 0:10
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In this image of a Falcon 9 takeoff (CRS-14), you can see some transparency in the exhaust of the (much larger) Merlin 1D:

enter image description here

I can make out the rear half of the engine bells through the exhaust (this is clearer in the much larger original of the photo, so click to enlarge).

Now comparing to the Rocketlab photo, the structure of the exhaust is different, the F9 exhaust looks brighter overall, whereas on the Rocketlab photo there are peaks and troughs in the exhaust brightness. This would make it easier to see through the Rutherford exhaust. There may be a difference in mixture ratio between the engines, the exhaust dynamics look different with more pronounced shockwave effects in the Rutherford exhaust.

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  • $\begingroup$ Yep, there it is; well done! $\endgroup$ – uhoh Feb 3 at 13:06
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tl;dr: There's no reason to believe that RP-1/LOX exhaust is opaque just because we haven't notice details behind it in photographs we've seen. The exhaust column here is narrower than "popular kerolox rockets" and the shot is underexposed (as far as the overall scene) so it's no surprise that detail is seen. Merlin exhaust in space (sans atmosphere) is nearly transparent!

Until a better answer comes along, I'm going to concur with @Uwe's answer as comment 1

Compared to the F-1 engine, the Rutherford engine is tiny. Due to the very small diameter of the exhaust beam it is partially transparent, but the very large beam of a F-1 is not. Just as a thin layer of a colored liquid may be transparent and a thick layer is opaque.

and @RussellBorogove's answer-as-comments 1, 2.

Some quick calculation suggests the thickness of the exhaust plume from a Rutherford is about 1/6 that of a Merlin 1D, 1/18 that of an F-1.

I think it’s a combination of small engine and underexposure.

I'll quote a bit from this answer to the question Are rocket exhaust flames ever opaque?:

Bright is not the same as Opaque!

Transparency for a wide band of visible light (our eyes and our cameras) is not really reduced much by the atomic and molecular emission. It may be so bright that it's hard to notice the transmitted light, but that's not the same as opacity. Materials or regions of gas can be glowing AND nearly transparent at the same time.

Think about bioluminescent algae in water, or red-hot glass during glass blowing, or even Cherenkov radiation in water.

Anyway, here is a sequence of screenshots from the recent SpaceX launch of Thaicom-8 on YouTube. That's probably something between smoke and soot. LOX/Kerosene engines usually get the most thrust by running rich - more Fuel than the LOX can oxidize. That leaves a lot of carbon and hydrocarbons to absorb and scatter light. While these are not opaque - you can still see through them - they are genuinely darker due to light absorption and scattering.

GIF from SpaceX YouTube THAICOM 8 Hosted Webcast


From the question Is the visible light spectrum from “red-hot glass” at least close to Blackbody Radiation?

enter image description here

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