PWDE is an acronym that stands for Pulse Wave Detonation Engine. In a Pulse Wave Detonation Engine, liquid methane or liquid hydrogen is ejected onto the fuselage, where the fuel mist is ignited, possibly by surface heating. The Pulse Detonation Engine works 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 molecules in the air are rapidly pushed aside near the nose of the aircraft, which in essence becomes a wall; like a sound barrier) is created in front of the aircraft. When the detonation takes place, the airplane's thrust wall is pushed forward. This all is repeated to propel the aircraft. This also produces the "donuts on a rope" contrail (above). Some people think this method is very strange because when you are traveling at speeds of Mach 5-8 extensive heating is a problem, so the last thing you would want to do is detonate an explosion outside the airplane and heat it up more.