Exhaustive tests were made of the samples of Harbenite brought to Friedrichshafen by Jason Gridley. Plans were drawn, and by the time the shipment of the ore arrived everything was in readiness to commence immediate construction, which was carried on secretly. And six months later, when the O-220, as it was officially known, was ready to take the air, it was generally considered to be nothing more than a new design of the ordinary type of rigid airship, destined to be used as a common carrier upon one of the already numerous commercial airways of Europe.
The great cigar-shaped hull of the O-220 was 997 feet in length and 150 feet in diameter. The interior of the hull was divided into six large, air-tight compartments, three of which, running the full length of the ship, were above the medial line and three below. Inside the hull and running along each side of the ship, between the upper and lower vacuum tanks, were long corridors in which were located the engines, motors and pumps, in addition to supplies of gasoline and oil.
The internal location of the engine room was made possible by the elimination of fire risk, which is an ever-present source of danger in airships which depend for their lifting power upon hydrogen gas, as well as to the absolutely fireproof construction of the O-220; every part of which, with the exception of a few cabin fittings and furniture, was of Harbenite, this metal being used throughout except for certain bushings and bearings in motors, generators and propellers.
Connecting the port and starboard engine and fuel corridors were two transverse corridors, one forward and one aft, while bisecting these transverse corridors were two climbing shafts extending from the bottom of the ship to the top.
The upper end of the forward climbing shaft terminated in a small gun and observation cabin at the top of the ship, along which was a narrow walkingway extending from the forward cabin to a small turret near the tail of the ship, where provision had been made for fixing a machine gun.
The main cabin, running along the keel of the ship, was an integral part of the hull, and because of this entirely rigid construction, which eliminated the necessity for cabins suspended below the hull, the O-220 was equipped with landing gear in the form of six, large, heavily tired wheels projecting below the bottom of the main cabin. In the extreme stern of the keel cabin a small scout monoplane was carried in such a way that it could be lowered through the bottom of the ship and launched while the O-220 was in flight.
Eight air-cooled motors drove as many propellers, which were arranged in pairs upon either side of the ship and staggered in such a manner that the air from the forward propellers would not interfere with those behind.
The engines, developing 5600 horsepower, were capable of driving the ship at a speed of 105 miles per hour.
In the O-220 the ordinary axial wire, which passes the whole length of the ship through the center, consisted of a tubular shaft of Harbenite from which smaller tubular braces radiated, like the spokes of a wheel, to the tubular girders, to which the Harbenite plates of the outer envelope were welded.
Owing to the extreme lightness of Harbenite, the total weight of the ship was 75 tons, while the total lift of its vacuum tanks was 225 tons.