Pulse Detonation Wave Engine
Essentially, liquid methane or liquid hydrogen is ejected onto the fuselage, where the fuel mist is ignited, possibly by surface heating. The Pulse Detonation Engine (PDE) operates by creating a liquid hydrogen detonation inside a specially designed chamber when the aircraft is traveling beyond the speed of sound. When traveling at such speeds, a thrust wall (the aircraft is traveling so fast that a molecules in the air are rapidly pushed aside near the nose of the aircraft which in essence becomes a wall)is created in the front of the aircraft. When the detonation takes place, the the aircraft's thrust wall is pushed forward. This all is repeated to propel the aircraft. From the ground, the jet stream looks like "rings on a rope".
Another reader thinks this method is very suspicious. He goes on "a serious problem with the SR-71 and other high-speed aircraft is excessive skin heating. The last thing you want is to add combustion at or near the surface."