Traveller-hot reactionless or cold?

[Fred Brackin]

The terminology refers to a difference between some reactionless drive systems that are silent and devoid of exhaust ("Cold") and others that emit fire or bright light or some other kind of radiation ("Hot").

I've seen ilustrations that shows the back of Trav ships with ports or something with something else shooting out of them and other illustations that show a kind of "nimbus" behind the ships.

Hot reactionless goes back to the Golden Age when Lensman ships had "drive flares even though they burned no fuel and Star Wars ships have glowing back ends. Cold reactionless is characteristic of UFOs and other vehicles with silent emissionless propulsion.

So,do the grognards out there have opinions or can they cite canon on this question?