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I've seen multiple sources attribute the clear flames to the use of hypergolic fuels for propulsion, but I have been unable to find any explanation for why hypergolic fuels burn clearly.

(Image Source: Spaceflight Now)

Brian Tompsett - 汤莱恩
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user13503
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    I was wondering if you actually mean "why not the red-brown we associate with Nitrogen Tetroxide", for which the point is that colour is the colour of room temperature non-combusting vapour, of which I think a fair bit is dissociating into N02. Anyway, after a bit of thought I suspect you are there already.This article is interesting in that it provides a few more photos http://www.clavius.org/techengine.html. By the way, I haven't posted this as an answer as, sticking to your question as to "why" exactly, I know no more than you. – Puffin Feb 21 '16 at 20:14
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    @Puffin: Nitrogen dioxide is the red-brown we associate with "nitrogen tetroxide" (which, in fact, is almost entirely dissociated into nitrogen dioxide at greater-than-cryogenic temperatures); dinitrogen tetroxide itself is colourless. – Vikki Jun 23 '20 at 00:23

2 Answers2

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Flame color should not be directly related to hypergolic...ity(?). Rather, it's dependent on the chemical properties of the particular fuel and oxidizer, and their products.

Wikipedia says:

The Titan II also used storable propellants: Aerozine 50, which is a 1:1 mixture of hydrazine and unsymmetrical dimethylhydrazine (UDMH), and dinitrogen tetroxide.

As to why those chemicals burn with a colorless flame, I can tell you that it has to do with electron configurations and excitation states, but for any more detail, you might want to ask on the chemistry site.

Jacktose
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I read in the book Ignition by John D. Clark about the liquid rocket fuel history in a footnote on page 105 that the Titan II flame is distinct in being nearly invisible because of the N2O4 and 50:50 propellants. The author writes that the LOX RP-1 mix has free carbon which produces a luminous flame.

Elsewhere in the book is mentioned that a stated goal of the propellant designer is to avoid carbon in the exhaust as it provides little to no thrust being a solid so it may be that all the carbon in the hypergolic mix is burnt to CO and becomes a colourless gas while providing thrust.

The Titan I used LOX and RP-1 so would have shown a bright orange exhaust.

There is a picture of a Titan with carbon containing exhaust plumes down on a Popular Science page but this may be a Titan I even though it is captioned a Titan II or it may have been retouched to add colour where there was none (for artistic reasons). Compare with the image in this question and on the Titan II wikipedia page.

KalleMP
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    The first picture of a Titan with a bright yellow exhaust flame on that Popsci page is a Titan I (and so captioned) -- the upper stage necks down the body diameter starting about halfway up the vehicle. The others are all Titan II -- the first and second stages are the same diameter all the way up to the payload fairing -- but the brightness and coloration of the plumes varies with lighting, film exposure, and film processing. – Russell Borogove Oct 16 '18 at 21:24
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    Highly recommend Ignition. It has been reprinted and is available on Amazon. – Mike Miller Oct 16 '18 at 21:25
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    How do rocket propellant combinations rank in terms of “brightness”? could use more answers; if you have anything to share there from Ignition that would be great, thanks! – uhoh Dec 26 '18 at 05:41