Cooling the exhaust gas and closing the sound in the whirl makes it possible to create silent rockets and jet planes. When we are thinking of the possibility to make the craft that is hovering silently over the ground. We can simply make the rocket- or jet engine system where the jet is closed in the sonic whirl or the LRAD system will close the exhaust gas inside the pressure wall. In that case, the sound of the exhaust gas cannot get out from that sound wall. And that thing makes it possible that people cannot just hear any sound when they are outside that sound wall. The thing that explains why there is no burning effect in the grass. Would be that the exhaust gas will be cooled by using super cold water or gas like liquid nitrogen for cooling that gas. The thing is that by using this kind of system the craft can hover silently over the ground. This kind of virtual antigravitation system can be already in use. Closing sound in the sonic whirl or some kind of sound b...