The Haunebu series are experimental flying objects that look like UFO's or flying saucers. Rumors has it that the Germans made plans to manufacture these objects. Its unknown whether those rumors are based on fact. There are blueprints, photos--but it's not certain sure whether they are genuine or fake.
The best written reference is Henry Steven's "Hitler's Flying Saucers".
In the standard FHSW, the Haunebu was removed since v0.42. It has been located before on the FH Air test map. However, the Haunebu is still in the FHSW files so you can install it on your map. The vehicle is a big and fast flying object with incredible acceleration. It uses a plasma-like gun as its main armament.
Default vehicle armor takes advantage of very advanced technology that deflects any kind ammunition. It makes the Haunebu immune to almost any kind of enemy attack.