Sometime around the early 1990’s, the people who get excited about such things got excited about “donuts on a rope” contrails, which many assumed were the results of pulse detonation engines powering hypersonic Aurora spyplanes. Of course it turned out that these contrails, if you actually traced them to their sources, were being left behind perfectly normal jetliners cruising at substantially subsonic speeds. I see them, or something very much like them, on a regular basis. Anyone can who lives anywhere near the normal jetliner traffic lanes. I saw this one a few days ago:
Crappy photo taken with my cell phone, but it gets the idea across. The aircraft producing it was traveling at a normal rate of speed for a jetliner… as is always the case when I see these things.
So my question is: what *is* producing these regular puffs in the contrail? They look like they come on the rough order of magnitude of once per second.