In the early 00's when on holiday, something made me look up at the clear blue sky and I saw what can best be described as a control which looked like button mushrooms laid top to bottom one after the other.

Fast forward to now and its likely I saw the contrail of an aircraft with a scramjet This is like what I saw but there was a gap between each button mushroom. https://files.abovetopsecret.com/images/member/a2712a44ce6c....

Some call it doughnuts on ropes which might be something called pulse detonation?