Keith_Sketchley

It’s contrails at very low altitude.

Will vary with aircraft type as newer engines produce less soot.

(Probably no original KC-135s and B-52s still flying, smoky machines. nor 747s with water injection for takeoff.
USAF et all hung onto the turbojets for a pitifully long time, I saw one on display at the Abbotsford Air Show decades ago.
Both types still in operation with turbofans of 707B vintage, but some KC-135s re-engined with CFM-56 engines as in B737-300, and now the KC-46A replacement based on B767 design is sort of in service. And USAF is thinking about re-engining B-52s with a one-for-one replacement engine, having been too cheap to accept lease of B757 engines from Pratt and Rolls. (Which would require new pylon and supporting structure to change from eight to four engines. The change would have eliminated need for refueling logistics for strike missions deep into enemy territory.)

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Keith_Sketchley

B747s moved on from water injection as engine makers finally came up with higher thrust engines like the JT9D-7Q.

Water injection is tough on engines.

Factoid: On one demo of increased takeoff weight, a Boeing flight test became 6,000 pounds lighter during the takeoff roll, due consumption of water.

Keith_Sketchley

Error: the linked article incorrectly refers to particles in the exhaust as ‘metallic’ when in fact they are carbon.

(Best not be metallic, that would mean the engine is wearing rapidly.)

Well, it is WaPo, quality as poor as Jeff Bezos’ other operation - Amacrick makes mistakes but is very difficult to contact to fix problems.