From 2011:
...Dr. Abernethy, Pratt & Whitney states that he was working on a project for the “Navy” in 1957. Developing a remote site for scrubbing exhaust from the J-58 engine that would use “poison fuels”.
In 2007 the CIA released the OXCART document written by Kelly Johnson. He told of the use of cesium and that it was responsible for the existence of the Blackbirds.
...Ed Lovick’s more trustworthy Version of a A-12 design proposal review meeting late summer 1959
The exhausts were 60 inches in diameter so they returned large amounts of energy at all frequencies of interest and over large angles to the rear.
https://www.secretprojects.co.uk/threads/was-area-51-book-now-cesium-additives-in-jp-7.13095/
Very good post and thread, thank you.
Makes me wonder what how much more advanced the SR-72 must be, if, uh....if it exists.