The air/space-craft in my novel use a hybrid of ramjets and pulse detonation engines. The detonations are in the thousands per second. With each shockwave reinforced by the one preceding it and channeled out the exhuast port for forwrd thrust. The sound of the engines in an atmoshphere is an odd mix of a roar and that of a really loud huming bird. In an oxygen-containing atmosphere (15% oxygen or more) the oxygen is taken from the air. The intakes open or close depending on speed. In the supersonic to hypersonic realm they are almost closed. This is to compress the existing air. In space or on worlds without oxygen-containing atmosheres the intakes are completely closed and the engines use their own oxydants.
Of curse the big Assault Landing Craft/Assault Landers(tasked for carrying big things like the M35 tank into battle in numbers) use fusion. The atmosphere (or reaction mass) is taken in and then exposed directly to the fusion reactor's plasma and channeled out through thrust tunnels. Obviously everything for hundreds of meters around the ship is exposed to hurricane force winds so these require huge landing zones. The Assault landers are about the size of small warships (corvettes and smaller frigates which are in the 50 to 150 meter realm). Small warships like that are actually capable of landing and taking off although this consumes VAST amounts of reaction mass and need 'refueling' (deuterium fuel is only needed in small quantities) in orbit.