The first rigid airship was built by Count Zeppelin in 1900. In 1913 the Zeppelin Company had three commercial ships which operated successfully from Berlin as a base. When war was declared in 1914, the Zeppelin Company proceeded to build larger ships for the use of the German Navy, and by the close of the war had built about 143 ships. Since the Armistice, German development and operation have been so restricted by the Treaty of Versailles that nothing has been done except the construction and operation of the Bodensee, and the construction of the Nordstern, a passenger ship, and of the ZR-j for the United States Navy.
In 1917, the British, appreciating the value of rigid airships as fleet scouts, commenced building such ships, which construction culminated in the crossing of the Atlantic by the R-34 in the summer of 1919. Soon after this a wave of economy caused the British to close all airship stations and scatter the personnel.
The Italians had the misfortune to wreck both of the rigid airships which they obtained from Germany after the Armistice, and the French did not do much with theirs until last year.*
In this country, however, there were a few individuals who kept the art alive, and continued study and research. Past experience was analyzed, and four facts became apparent: first, that the German naval experience was, on the whole, highly successful; second, that the exclusive development of helium gas production in the United States was of fundamental advantage to us; third, that such a disaster as the R-38 wreck in England was preventable by care in design and testing; and, fourth, that Major Scott’s invention of the mooring mast would eliminate many difficulties in handling, and greatly increase the operating value of airships. The Navy Department, therefore, through the Bureau of Aeronautics, after a careful estimate of the situation, enlarged its program of research and decided to push the design and construction of the Shenandoah, our first rigid airship. The great shed at Lakehurst was completed with docking rails on the German system, while a mooring mast of Scott’s type was constructed for use in bad weather.
The design of the Shenandoah was based on a very complete survey and analysis of German successful practice so far as information was available, checked by comparison with unsuccessful practice when data were available, and finally checked * on an exhaustive theoretical investigation, involving original work in the theory of indeterminate structures, aerodynamics and naval architecture. Eventually, the design, so far as structural strength was concerned, was submitted for review to a special committee of engineers appointed by the National Advisory Committee for Aeronautics, and was approved by this committee.
The hull structure of the Shenandoah consists essentially of a framework of duralumin girders running transversely and longitudinally, with high tensile steel wires acting as diagonal braces in the quadrilateral panels formed by the girders. The rings formed by the transverse girders are sixteen and four-tenths feet apart, and the alternate rings are cross-wired with about eighty steel wires in each ring, giving transverse rigidity to the hull, and dividing it into twenty compartments, each of which contains a cell made of cotton cloth lined with goldbeater’s skin, and inflated with helium gas. The displacement when completely inflated is about the same as the water displacement of the Leviathan.
* Editor’s Note—This airship, the Dixwude, was recently lost during a severe storm over the Mediterranean
Along the bottom of the interior of the hull there is a corridor or keel of triangular section, apex upwards, about ten feet high by twelve feet wide at the midship section, and tapering somewhat toward the bow and stern. Along the center of this corridor is a narrow walkway, and on either side are the gasoline tanks and ballast bags suspended from a pair of longitudinal girders. Hammocks for the crew are also suspended from these girders.
Directional control, horizontally and vertically, is secured by four fins and rudders near the stern. At a little distance, the fins appear to be thin flat surfaces, but they are really triangular in cross section, and of such ample dimensions that the longitudinal walkway is carried in the lower fin.
The metal framework of the hull, fins and rudders is covered with cotton cloth, drawn tight to give a smooth exterior along which the air flows with the least possible friction. The proper tension in the outer cover is obtained by shrinking it with “dope” after it is laced to the girders. The silvery appearance of the airship is due to aluminum powder in the “dope” to provide a surface which reflects the sun’s rays as much as possible. The passage of sunlight to the interior of the ship is further prevented by black pigment on the inner surface of the cover. These precautions against the sun are taken to avoid an undesirable superheating of the gas, and to protect the gas cells against the deteriorating effect of sunlight.
The propelling plant consists of six Packard engines of special design, each with six cylinders in line, and capable of developing 300 horsepower, but ordinarily operated at less than half power. The engines are contained in separate cars distributed along the under side of the hull. Four of the engines drive geared-down propellers, and two have direct drive. Two of the geared propellers may be reversed through gears to assist in checking the headway when landing the ship.
The control car is incorporated with the forward power car. There is room in it for six to eight men, and it may well be compared to the bridge and chart room of a surface ship. Besides the wheels for operating the vertical and horizontal rudders, the controls for the gas valves and ballast bags, engine room telegraphs and the navigational instruments are here. At the rear of the control car there is a sound-proof radio compartment, equipped with powerful radio telephone and telegraph apparatus.
The metal of which the girders are constructed is an aluminum alloy known as duralumin, invented by a German named Wilm about twenty years ago. It contains about four per cent copper, five-tenths per cent magnesium and four-tenths per cent manganese. When properly heat treated it has the strength of mild steel, with only about one-third the weight. The manufacture of this material in America has been successfully accomplished by the Aluminum Company of America and the Baush Machine Tool Company, which are licensed by the Chemical Foundation under patents purchased from the Alien Property Custodian during the war. Credit is also due to the Bureau of Standards and the Naval Aircraft Factory in the development of the technique of the manufacture and heat treatment of duralumin.
Most of the girders are triangular in section, and are composed of longitudinal channels along the three corners, braced together by crossed, lattices formed of light, corrugated stampings, riveted to the channels, and to each other where they cross. A typical longitudinal girder is sixteen and four-tenths feet long between transverse frames, weighs only ten pounds, and is capable of sustaining a column load of 5,000 pounds, or a uniformly distributed transverse load of 2,000 pounds, without allowing for the extra strength given by the rigidity of the connections to the transverse frames.
The gas cells are of single ply cotton cloth made gas tight by goldbeater skins secured to the cloth by a rubber solution adhesive. The skins are obtained from the intestines of oxen, and nearly three-quarters of a million oxen contributed to the vast number of skins required on the Shenandoah. The manufacture of these gas cells was also an entirely new art in America, and was successfully accomplished by the Goodyear Tire and Rubber Company, following methods developed principally in England.
The design of the Shenandoah was based mainly on a German airship, the L-49, brought down intact in France in the autumn of 1917- Detailed drawings of the L-49 were made by the French Government, and forwarded to the United States Navy in 1918. The Shenandoah has one more cell, thirty-two and eight-tenths feet long, in the parallel middle body, than her German prototype, and the weight and strength of the framing are somewhat increased—especially near the bow, to provide for the airship lying to a mooring mast, an operating feature never practiced by the Germans. The power plant was also increased from five engines, of 240 horsepower each, to six of 300 horsepower each, although, as stated, these are supposed to run at about 200 horsepower. These changes and the use of helium have involved considerable loss of load capacity, and altitude attainable.
In spite of the reduced lift compared with the German airships, the Shenandoah can carry sufficient gasoline to maintain her normal speed of about sixty miles per hour fifty hours, or a cruising speed of forty miles per hour for 140 hours without refueling.
The girders and power cars were constructed by the Naval Aircraft Factory, Philadelphia, Pa., and the ship was assembled in the giant shed of the Naval Air Station, Lakehurst, N. J.
The general characteristics of the ship are as follows:
Length—680 feet.
Diameter—78.7 feet.
Height—96 feet.
Gas capacity—2,150,000 cubic feet.
Total lift (helium) about 130,000 pounds—(hydrogen) about 136,000 pounds.
Dead weight—about 74,000 pounds.
Speed—about sixty miles per hour.
Horsepower—six 200-300 H.P. engines, total 1,200-1,800 H.P.
Cruising radius without stopping—over 4,000 miles.
Crew—nine officers, twenty-two enlisted men.
The Shenandoah is filled with helium, which is one of the reasons we hope for success from our Navy rigids. Helium is found in certain of the natural gases occurring in a belt which extends from Texas north-eastward through Oklahoma, Kansas and Nebraska. It is one of the rare gases, and was discovered in the sun by means of the spectroscope in 1868, twenty-seven years before Sir William Ramsay discovered it on our globe. Up to 1917 there had not been produced more than two and eight-tenths cubic meters. Next to hydrogen, helium is the lightest known substance. Although it is about twice as heavy as hydrogen, it can impart to a balloon an ascensional force equal to ninety-two and six-tenths per cent of the force hydrogen would give it. This seems to be a paradox, but the explanation is very simple. The ascensional force of a gas is not measured directly by considering its specific weight, but by finding the difference between the weight of a certain volume of the gas and the weight of the same volume of air. Hydrogen and helium are both so light in comparison with air, that the difference between their respective ascensional forces becomes insignificant, when employed in balloons. If we consider, for example, a volume of twenty-eight cu. m. (1,000 cu. ft.) of hydrogen, it will be able to lift a weight of 34.038 kilograms (75 lb.), while a like volume of helium will lift 31.52 kilograms (70 lb.).
The helium project is a joint project of the Army and Navy, assisted by the Bureau of Mines. The helium plant is located at Fort Worth, Tex. The gas is supplied from the Petrolia wells, which are about 160 miles north of Fort Worth, and is piped into Fort Worth for domestic use. In the Linde process use is made of the Joule-Thompson principle, and the separation of the helium is based on the principle, and the fractional distillation of the gases by successive compression and expansion. The lowering of the temperature, by the expansion of the strongly compressed natural gas in passing through a small opening, is sufficient to cause the liquefaction of all the gases except helium, which is then quite readily separated from the others.
Helium has been found to be easier to retain in the gas cells than hydrogen, and the loss of helium by diffusion from the cells of the Shenandoah has been astonishingly small. Even though the helium does become contaminated by incoming air it is possible to pump out the impure helium and repurify it very much as was done with the original natural gas and thus use the helium over and over again.
We must be very jealous of this gas, as its supply is evidently very limited, and once it has escaped into the atmosphere it is gone forever. Consequently methods of conservation of the supply in the ground have been proposed, and are partially in Progress.
We must also conserve the gas in the operation of the ship.
This is rather difficult, because so far we have been accustomed to operating with hydrogen, which was comparatively inexpensive, and have not worried about its conservation.
When an airship rises the gas in the cells expands, and if the rise is great enough, the cell will be completely filled, and must relieved through safety valves. This gas discharge through safety valves is the normal way to keep the ship in equilibrium, or as nearly so as possible, so that it will not tend to rise or fall by an amount greater than can be counteracted by the use of the rudders. The burning out of the fuel tends to make the ship continually light, and there soon comes a time when it is impossible to avoid the loss of gas.
However, it has been happily discovered that, for every pound of gasoline burned in the engines, there is produced about one and one-third pounds of water vapor in the exhaust. If we could condense all of this water vapor as it was formed, we would actually increase the weight of the ship steadily as the fuel burned out. As this complete condensation would require very elaborate apparatus, we are content if we can get back just as much weight in water as we have burned out in gasoline. Apparatus for accomplishing this has already been developed. The Navy began the solution of this problem some five years ago, and actually succeeded in obtaining the desired rate of condensation. The apparatus, however, was much too heavy to be practical. In the meantime, the project had attracted the attention of the army engineers, and, with the assistance of the Bureau of Standards, they continued the work begun by the Navy Department, finally producing a very successful piece of apparatus with a weight which was practical for use on airships. In line with the spirit of co-operation which exists between the two services, complete information as to the apparatus and its results was given to the Navy Department, and we are now engaged in the modification and redesign of this apparatus to suit the Shenandoah. Before the beginning of next spring, it is expected that all of the engines on the ship will be equipped with water recovery apparatus, and from that time the loss of helium will be almost negligible.
The first airship to be inflated with helium was the U. S. Navy non-rigid C-7, which made a flight, while so inflated, from Hampton Roads to Washington and return in December, 1922. The second and only other ship to be filled with helium is the Shenandoah.
Mooring masts will be erected at different points so that airships can tie up to them and refuel with gasoline and oil, re-gas with helium (or hydrogen), take on other stores, ammunition, personnel, etc. They can also effect repairs while at a mast. In other words, if there is a mast at Panama the Shenandoah could fly down there, maneuver with the fleet, go to the mast, refuel, and continue her operations indefinitely. The R-33, the British rigid, lay at her mooring mast for five months, just as a ship will lie at a buoy.
The Bureau of Aeronautics has designed a mooring mast to be placed on board a ship. Such a mooring mast ship, equipped with helium and gasoline stowage, will greatly increase the value of rigid airships to the fleet by furnishing a mobile airship base with the fleet. It is perfectly practicable for an airship to stay at such a mast indefinitely, and they have also been successfully towed by vessels at sea. Such a ship will also be another step in carrying out the Navy Department’s policy of directing the main efforts of Naval Aviation toward aircraft with the fleet.
The mooring mast is a structural steel tower approximately 165 feet in height. At the extreme top of the mast is a revolving cup into which a cone on the bow of the ship is secured, and the ship thereby moored. The ship can swing to the wind, through an arc of 360 degrees, thus always heading into the wind.
Running up through the center of the mast are gasoline and oil lines for refueling, a water main for taking on water ballast, and a large main for helium or hydrogen. The mast at Lakehurst is also equipped with an elevator, by which personnel and material can be transferred, and food and other stores furnished, to the ship.
For normal flying the Shenandoah carries about four tons of water ballast, which can be let out when it is desired to lighten the ship for maneuvering.
The Navy is developing the rigid airship on the recommendation of the General Board, which policy has been approved by the Secretary of the Navy.
These ships are primarily for use as long distance and long endurance fleet scouts. The radius of the Shenandoah is over 4,000 miles. If a mooring mast has been put up, or some other means provided for refueling and re-gassing, she can go 4,000 miles more and keep this up indefinitely. As scouts, their radius of action, their ability to hover, their ability to climb quickly to great heights of over 20,000 feet, their ability to receive and transmit information, and protect submarines and other ships at long distances, make them indispensable to the Navy. Other uses are coastal patrol, long distance convoy work, anti-submarine operations, mine searching, bombing enemy fleets or bases, maintaining communications with detached expeditionary forces, providing supplies, and transferring personnel.
As helium cannot be ignited or exploded, and the ships are an excellent gun platform and are capable of carrying a large number of machine guns, one and two pounder automatics, and even three inch rifles, they are of considerable value as fighting units.
The following quotation is made from Admiral Jellicoe’s “Grand Fleet,” London, 1912, and certainly shows their value to the Germans at Jutland. Had they been more enterprising, they could have dropped tons of bombs on the Grand Fleet during the fleet action.
“From 8:24 a. m. onward, Zeppelins were frequently in sight from both the Battle Fleet and the Battle Cruiser Fleet, and were fired at, but they kept at too long a range for our fires to be effective. The Galatea sighted the first Airship at 8:24 a. m., and the second was seen by the Battle Fleet at 9:55 a. m.; at 10:00 a. m. the Harwich force reported being followed by a Zeppelin. This force was shadowed by airships during the whole period of daylight on the nineteenth. The trawler Ramexo had two Zeppelins in sight. It was evident that a very large force of airships was out. A total of at least ten were identified by our directional wireless stations, and they appeared to stretch right across the North Sea.”
Von Scheer also testifies in his official report to the valuable information received from Zeppelins at Jutland. It is obvious from a study of the battle that the Grand Fleet was playing a game of blind man’s buff, never certain of the position or composition of the enemy, and moreover, they ran a serious risk from submarines and mine fields whose positions they would have known if they had had aircraft. The German Fleet, due to their airships, followed the Grand Fleet’s movements throughout the day, and engaged when and where they wished, with a full knowledge of the enemy’s units.
Commercially rigid airships can carry a cargo of twenty-five tons 5,000 miles over land or water, or both, at a speed of over seventy miles an hour. They can navigate in all sorts of weather, and, if necessary, circumnavigate storms, even climbing over them, or hover and wait for fog to lift. It is undoubtedly possible to construct, at the present time, rigid airships having speeds of ninety miles an hour, and a cruising radius of 10,000 miles without refueling. Such ships would make the New York-London trip in thirty-four hours; the New York-San Francisco trip in thirty-one hours; the San Francisco-Honolulu trip in twenty-six hours; the Honolulu-Manila trip in forty-seven hours; the New York-Panama trip in twenty hours, and the New York-Buenos Aires trip in fifty hours.
The Navy, therefore, in keeping with its usual habit of interesting itself in the development of new and important scientific and mechanical inventions, expects to prove the great value of the rigid airship, both as a commercial carrier, and as an important aid to the defense of this country in time of war.