Concorde - Rear End Gas?
 

Strange one this. A few years back, while observing a Concorde take-off, just before the take-off roll commenced, I saw a white/grey mist being vented from the rear of the tail.

Note, it was a fairly cold morning.

Question is, was I seeing things? As I've never seen it since!

might have been fuel from the rear trim tank vent line you can see in this Picture

Volume - I think you are correct. Next question, why did it vent? Presumably just some excess?

I'm sorry this has nothing to do with your original question, but I don't think it warrants its own spot so to say! I witnessed an Air India 747 seconds from touching down at LHR 10 odd years ago, with the APU ablaze! Now, that's NOT normal!! But, could anyone tell me what sort of thing would spark that off so close to landing? (I'm not sure how long it had been flaming, but it was quite a big flame nonetheless!)