The phenomenally low radar cross section on the Oxcart (proto-SR-71), Annie Jacobsen explains (in Area 51), had to be lowered even further:
In a hangar not far from the radar range, Edward Lovick got to work on a one-eighth-scale model of the Oxcart. In what became known as Project Kempster-Lacroix, Lovick designed a system straight out of Star Trek or James Bond. “Two giant electron guns were to be mounted on either side of the aircraft,” Lovick recalls. Remarkably, 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, Lovick determined, would further absorb radar waves coming up from radar tracking stations on the ground.
Using the small-scale model, the scientists were able to prove the scheme worked, which meant it was time to build a full-scale mock-up of Kempster-Lacroix. Testing the system out on a full-size aircraft, the scientists discovered that the radiation emitted by the electron guns would be too dangerous for the pilots. So a separate team of engineers designed an X-ray shield that the pilots could wear over their pressure suits while flying an Oxcart outfitted with Kempster-Lacroix. When one of the pilots made a test run, he determined that the thickness of the shield was far too cumbersome to wear while trying to fly an airplane at Mach 3. Then, while Lovick was working on a solution, the Air Force changed its mind. The Oxcart’s low observables were low enough, the Pentagon said. Project Kempster-Lacroix was abandoned.