Basically, a dirigible is a tapered tube, with formers and stringers to make the shape.
The bits and pieces are soft balsa, braced internally to create a truss structure that can be cantilevered from the aft bulkhead. The internal bracing transfers loads for flying, support, and dimensional conformity.
Gram for gram, the aluminized mylar skin is the heaviest portion of the airframe weight, accounting for 1/3 the total weight. It is supported by the formers and stringers. Curiously, mylar by itself is porous to Helium, the .0002 inch (yes, two ten thousandths) thick aluminum deposition forms the Helium barrier. Since this barrier is not 100% some leakage will occur despite all the "holes" closed. None the less, when the airframe is filled and under a slightly positive pressure, it is quite robust. The rupture modulus is high. A paper clip shot at the mylar from more than 10 feet will usually bounce off.
Although large, it is extremely light. A 13 foot cylinder 26 inches in diameter weighs less than three ounces before covering. and less than a pound when covered.
The cruciform tail is fully cantilevered from the fuselage and contains a controllable rudder. I later went to single sheet horizontal stabilizers and cut the 17 gram weight (each surface) down to 7 grams. For comparison, a US penny weighs 2.5 grams, a quarter weighs 5.6 grams. I occasionally run without the top fin to reduce the liklihood of snagging ceiling obstructions. When you are 50 feet underneath it, obviously you can't see the top fin.
Batteries, speed controllers, receiver, and a servo are housed in the main gondola, an additional servo is found in the tail area. Four geared electric motors drive 3.5 inch balsa propellers. The nacelles are hung from the fuselage on articulated struts. Due to the flimsy nature of the struts I run dynamically balanced propellers of my own design.
Mission equipment is taped to the skin with scotch tape. This is normally affixed to the underside and about mid station. Using a mini board camera and downlink, the DG-650 has proven to be a very stable surveillance platform