Electrons.

This is coolbert:

SR-71 stealth. To war with charged particles!

"Two giant electron guns were to be mounted on either side of the aircraft"

As seen at https://www.isegoria.net | May 7th, 2024.

"Area 51 by Annie Jacobsen The phenomenally low radar cross section on the Oxcart (proto-SR-71), Annie Jacobsen explains (in Area 51), had to be lowered even further"

Radar cross signature of the SR-71 indeed lowered:

"'Two giant electron guns were to be mounted on either side of the aircraft,' . . . the purpose of the guns would be 'to shoot out a twenty-five-foot-wide ion cloud of highly charged particles in front of the plane as it flew over denied territory.' That gaseous cloud . . . would further absorb radar waves coming up from radar tracking stations on the ground."

But according to Quora the idea had an overwhelming flaw:

"Testing on scale models of the Oxcart aircraft showed the theory would work. Testing the electron beam guns on the full scale Oxcart aircraft, the researchers soon discovered the radiation given off by the guns would kill the pilot. So more engineers worked on developing an x-ray shield the pilots could wear to protect them from the radiation. But the first test pilot to wear the shield said it was too cumbersome to allow the pilots to fly the aircraft. Project Kemper-Lacroix was abandoned."

At least to my knowledge the SR-71 reconnaissance warplane relied MORE on SPEED and flying at a HIGH-ALTITUDE for survival more than anything else.

SR-71 too as originally envisioned as a manned bomber aircraft. A role that never materialized.

coolbert.