THE AIR FORCE IS WORKING ON A NEW TYPE OF POPULSON ENGINES.I HAVE READ ABOUT THIS NEW TYPE OF POPULSON, BUT FOR THE LIFE OF ME, I CAN'T REMEMBER WHAT IT IS CALLED ---SCRAM JET, RAM JET ?????FUEL TAKEN FROM ATMOSPHERE IN FLIGHT.WHEN RUNNING, IT MAKES A CONTRAIL THAT LOOKS LIKE DONUTS ON A ROPE OR SMOKE RINGS ON A STRING.THESE PLANES FLY VERY HIGH & VERY FAST !!