“I am thoroughly convinced from my observance of the naval lessons of this war that in the future rigid airships will be part of the fleet of every first-rate naval power. Delay of one year now in the development of the art will serve to keep the United States in an unfavorable position in comparison with those of the other great powers for some years.”
Admiral William L. Sims, Commander, US Naval Forces Europe, writing in April, 1919
Given the outstanding performance of non-rigids to that date, why did Sims specify rigid airships? Part of the reason would have been the envelope seaming technology of the day, as well as what was foreseen in 1919. Another was the inherit advantages of the rigid over the pressure ship:
- No deadweight air system – no ducts, fans, valves and ballonets to rupture
- Redundant cells offer damage-survivable compartmentation
- Lower pressure gas containment, fewer leaks
- Structural framework more compatible with pinpoint loads
Fact was, then as now, there is a practical limit to the size of a pressure airship, owing to the fabric and joining technologies available to constructors. The rigid airship’s overall size is constrained only by the volume of the available construction shed. Because of the “square-cube” phenomenon, rigid airships can be sized more for their intended missions rather than be limited by fabric technology. Likewise, the square-cube “law” limits the size of heavier-than-air craft until each generation’s structural weight / strength breakthroughs.
MIT’s Edward Warner, who called for calm in the hysteria surrounding the loss of the US Army’s semi-rigid ROMA just months after the ZR-2 tragedy, explained the square-cube law as applied to displacement vessels: “The lighter-than-air craft has inherent advantages, when enormous sizes are in view, which can hardly be counterbalanced by any technical skill that may be applied to the improvement of the heavier-than-air types. The most evident of these advantages is that the lift of an airship, depending on the volume of the gas contained in the envelope, goes up in proportion to the cube of the dimensions, while the lift of the airplane, dependent on the area of the wings, varies only in proportion to the square of the dimensions.”
On this page we reproduce a slightly annotated version of Pennoyer’s history, written before the delivery of ZR-1 or ZR-3. It should be noted that, while helium was at that time already published in National Geographic magazine, and therefore no longer a state secret, there is no mention of helium or its associated hype. This is important, because this report was written after the crash of ZR-2, whose fatal accident is detailed:
RIGID AIRSHIPS IN THE UNITED STATES NAVY
By LIEUTENANT R. G. PENNOYER, U. S. N.
Reproduced from PROCEEDINGS of the UNITED STATES NAVAL INSTITUTE
Vol. No. 48, No. 4 APRIL, 1922
Since rigid airships undoubtedly will in the near future be attached to our fleet, it is believed that information concerning their general characteristics and possible sphere of usefulness to the fleet will be of interest to the service at large.
The first practical rigid airship was constructed by Count Von Zeppelin as early as the year 1900. This remarkable man was a German military observer with the Union forces during our own Civil War, and it will be remembered that it was during this war that a free balloon was released for the purpose of obtaining information for the first time in the history of [American] warfare. This was observed by Count Von Zeppelin, who realized how wonderful it would be if propelling machinery could be installed in a balloon so that it could be directed where desired. A few years later he watched with interest the development of the non-rigid and semi-rigid types of airships in France and became convinced that a rigid type was not only possible, but if successful would be a distinct improvement over the others. Accordingly Count Zeppelin employed two German engineers to assist him in the design of a rigid airship. His first ship was flown from Lake Constance during the summer of 1900 and although a speed of only seventeen miles per hour was attained, the maneuverability was considered excellent and the ship a success. This ship was later destroyed by a storm (sic), no housing facilities having been provided (sic), but it was the large and rather healthy child which was the prototype of all that have since been brought forth in Germany and elsewhere. The Zeppelin, as these ships were popularly called by the German people after the designer, went through many vicissitudes and disasters due to lack of proper facilities for handling and housing and to lack of experience.
At the outbreak of war in August, 1914, out of the twenty-six airships which had been built by the Germans fourteen had been destroyed from one cause or another, so that only twelve ships were available for military purposes. The size and power of these ships were considerably increased over the earlier ships, several having a capacity of nearly one million cubic feet and with almost a thousand horsepower, and a speed of fifty-five miles per hour. In the early stages of the war, the Germans used these ships for raids on the English coast and industrial centers, and since the British had little or no defense to offer against them the moral and material effect was, contrary to general belief, exceedingly great, especially that of the raids directed against the industrial cities of the Midlands. The British Secret Service was excellent, and in almost all cases obtained advance warning of an intended raid, whereupon all blast furnaces would be closed down and fires banked so as not to give their position away at night to the enemy raiders. This closing down of blast furnaces in a city like Sheffield, which was turning out tremendous quantities of war materials, necessarily disrupted the organization and cut down the production of munitions which were sorely needed by the British army in the early part of the war. As the defences of these cities were perfected, these raids became less effective and were finally abandoned by the Germans, but not before a great many of these airships had been lost.
Rigid airship construction was not started in Great Britain until 1911, and nothing of any real importance was evolved until the Admiralty decided on a design similar to the German L-33, which was brought down in England almost intact during the year 1916. Orders for two of this class were placed in November, 1916, and were known as the R-33 class. The famous R-34, which crossed the Atlantic and recrossed in July, 1919, was the second of this class. These two ships were not completed until May, 1919.
During the war the United States -Navy Department saw the value of the rigid airship for naval use and the General Board recommended that this type of craft should be provided for the navy. In due course Congress appropriated funds for two ships, one to be built in the United States and the other to be purchased abroad. In the month of October, 1919, the Navy Department instructed the Commander, U. S. Naval Forces Operating in European Waters, to enter into negotiations with the British Air Ministry for the purchase of the rigid airship R-38, which ship represented the latest design in airship development in Great Britain. These negotiations were completed to the mutual advantage of both parties. At the same time arrangements were made for the training of a U. S. naval crew for the ship. The trials of the R-38, renamed the ZR-2, were conducted during the past summer, and it was during her last trial flight on August 24, 1921, that disaster overtook this airship, and the whole civilized world was stunned by the terrible catastrophe which took place near the city of Hull, England, resulting in the loss of forty-five of the forty-nine persons on board.
Many times I have been asked to state the cause of this accident and in passing it might be of interest to cover this point. The ZR-2 was the largest airship that has ever been constructed and several distinct departures were made in the design. She was roughly of 300,000 cubic feet greater capacity than the L-72, the last and largest German rigid to be completed, and-to obtain this volume the diameter was increased over that of the L-72. During the construction of the ship various tests were given to ascertain if the structure would have the required strength, and on some of these tests it was found necessary to strengthen the hull structure. On the third trial flight of this airship held on the seventeenth and eighteenth of July, when attempting to make full speed on four engines, the hull structure started to fracture in several places. The engines were immediately stopped and further damage averted. She was then flown slowly back to her base and landed without further incident. A conference was held immediately after this flight, in which it was decided to strengthen the ship throughout the under side of the parallel body ; that is, from frame four forward to frame nine aft. (The ZR-2 had fifteen main frames.) It was on the flight following the completion of this strengthening that the disaster occurred. Various and sundry tests were to be carried out on this flight, the last, test to be a rapid movement of the rudders in both directions. A full speed trial lasting about fifteen minutes had been conducted successfully, in which the ship attained a speed of over sixty knots. Shortly afterward, rudder trials were started with the ship’s speed about fifty-four knots. During these tests the ship’s structure failed between frames nine and ten (the strengthening was only carried back as far as frame nine) and completely separated into two parts. The gasoline leads were severed and a fire started in the after end of the forward portion almost at the moment of fracture. An explosion followed shortly after, causing the collapse of this portion, and as it struck the water a second explosion occurred. The after portion descended slowly into the Humber River without catching on fire, and four of the five survivors were in this part. From all the evidence it appears that this airship was weak structurally, and that the rapid movements of the rudders caused a strain which the structure could not stand, and the ship fractured aft of the portion which had been strengthened.
The construction of a rigid airship presents some very difficult problems in design, and some of the calculations involved are almost impossible of solution in the present state of the art. As with most design work, most calculations are based on past experience, and the experience is limited. We must remember that rigid airships are very modern indeed, and the ZR-2 was the largest rigid airship that had ever before been attempted. In many instances the designers were entering entirely new fields. In this connection naval constructors have been designing ships since the time of Noah, but even with the thousands of years of experience behind us, ships that sail the seas sometimes prove to be structurally weak. Compare this experience with the very limited experience designers have had with rigid airships, and one marvels they have gone so far.
A GERMAN RIGID FOR THE UNITED STATES
In December, 1918, after the signing of the Armistice, a commission of aeronautical experts visited Germany and found a total of seven rigid airships intact in their hangars. These were subsequently assigned to the Allies as follows: two to England; two to France; two to Italy ; and one to Japan. In considering the allocation of these German rigid airships, the Supreme Council made a decision in September,. 1919, that France and Great Britain should have a choice of the two best airships left in Germany, the choice to be exercised in the order named. (France chose the L-72 and Great Britain the L-71, which were subsequently delivered.) The remaining rigid airships were to be distributed to the United States, Italy, Great Britain, France, Japan, and Belgium, each power to exercise one choice in alternate rotation. In view of the fact that the United States did not ratify the Treaty of Versailles, she did not exercise her right of choice. When the United States signed a separate treaty of peace with Germany, she reserved all the rights that might otherwise have accrued to her if she had been a signatory power at the Treaty of Versailles. By virtue of these reservations, the United States claimed the right of delivery of a rigid airship by Germany to the United States. On the sixteenth of December, 1921, at the 157th meeting of the Conference of Ambassadors, this right of the United States was acknowledged and it was further agreed that the United States should have constructed in Germany at Friedrichshafen on Lake Constance a dirigible of approximately the L-70 type (about 2,400,000 cubic feet capacity). The president Of the Inter-Allied Aeronautical Commission of Control was accordingly instructed to take the necessary steps to see that the construction of this ship was started immediately. It was also agreed by the United States that this ship would be devoted to purely civil purposes, and in view of this it is contemplated that this ship will incorporate a passenger car in the design, somewhat similar to the passenger cars which have been included in former commercial rigid airships. The Joint Army and Navy Board agreed that the acquisition of a German airship should be left to the navy and accordingly the navy was designated as the government department to handle all matters in connection with the acquisition of this ship.
Work in connection with the construction of this airship is progressing and naval officers have been detailed to the works of the Zeppelin Company as inspectors. In view of the fact that the Germans were required to deliver all other airships to the Allied countries, the same requirement is to be exacted in this instance and this rigid dirigible delivered at the Naval Air Station, Lakehurst, N. J., where the navy has completed the construction of the world’s largest airship hangar. Some one has said that “The proof of the pudding is in the eating.” In this case the proof of the ship is in the delivery. Some may ask the question, “What does the navy want with a commercial type airship?” It is of course acknowledged that the Germans probably know more about the design, construction, and operation of rigid airships than any other people, and the experience to be gained by our design staff and operating personnel is invaluable, and this experience can be gained just as well with this type as with a purely military ship. Also, following the destruction of the ZR-2, the American public has to a great extent lost faith in the rigid airship, and they need to see such ships flying about carrying passengers in order to regain their lost faith, so that the navy may not be handicapped by lack of funds for their future development.
As previously stated, the total gas capacity is estimated at 2,400,000 cubic feet. In common parlance, the gas capacity of an airship is the total volume of gas contained in the gas bags when full, the ship being at sea level and with normal atmospheric conditions. The largest previous ships to be commissioned were the German L-70 class, which had a gas capacity of about 2,400,000 cubic feet and were designed and constructed for the purpose of bombing New York. These ships were completed just prior to the Armistice, and in accordance with the terms of the peace treaty have since been surrendered to Great Britain and France. The total lift of this latest airship is estimated at about seventy-two and eight-tenths tons. The “total lift” is the difference between the weight of air displaced and the weight of the gas used (hydrogen), the gas bags being full. Part of this lift or buoyancy is required to lift the ship or, as it is technically expressed, the “fixed weights.” These include the hull, gas bags, car, and machinery weights, gasoline tanks, etc.; in other words, the fixed weights necessarily carried in flight, but which cannot be moved. In this case the fixed weights are estimated at thirty-five and four-tenths tons. The difference between the ship’s total lift and the fixed weights gives the lift disposable for crew, passengers, fuel, water ballast, food, bombs, ammunition, etc., and is called the “disposable lift.” This works out at approximately thirty-seven and four-tenths tons for a passenger-carrying ship of the above gas capacity. Mere figures are of little value for purposes of illustration. However, to visualize the dimensions of this huge craft, compare its length overall of 743 feet with the overall length of our largest battleship afloat (a little over boo feet) ; nor does the overall width of approximately ninety feet compare unfavorably with the Maryland, which has only a slightly greater beam. In this way, it is possible to picture these air monsters.
The hull of a modern Zeppelin consists of transverse frames or rings, made of duralumin, rigidly connected to longitudinal members of the same material, the rectangular panels formed between longitudinals being braced diagonally by what are known as diagonal wires, the whole forming a built-up tube, over which an outer cover of fabric (somewhat similar to airplane fabric) is secured. This structure is tapered at the ends to give a streamline form. An internal keel or corridor is fitted of triangular or trapezoidal shape in cross section, the base of which forms the bottom side of the ship, and runs practically the whole length. In this keel are fitted the gasoline and oil tanks, water ballast bags, crew’s space, and other accessories. A walking way is fitted along the bottom of the keel on the middle line, to allow passage between different parts of the ship. This walking way is somewhat narrow (about eight inches wide) and if in flight a person should accidentally step off, there is nothing between him and the great open spaces, except the outer cover of fabric. A line is strung along the top of the keel which is grasped when walking along the corridor, and until one becomes accustomed to it, this is grasped rather anxiously.
The transverse frames or rings are made rigid by fitting across them diaphragms of wire. These diaphragms also act as bulkheads separating the adjacent gas bags. These transverse frames are of polygonal shape, each side of the polygon forming a side of the ship. In the L-70 class there are sixteen of these main transverse frames, which form with their wiring the longitudinal boundaries of the gas bag sections. There are therefore fifteen gas bags in this type ship. The gas bags are made of cotton fabric, lined with goldbeater’s skin to make them tight. Automatic gas valves are fitted to each gas bag to permit escape of gas when necessary. Cowls are fitted along the top of the ship to permit escape of this gas and also to assist ventilation of the keel and crew’s quarters. A control cabin of duralumin structure is built into the under side of the hull in the fore part. This cabin corresponds to the bridge of a ship, and contains the elevator and rudder controls, engine room telegraphs, telephone, navigating instruments, charts, tables, radio cabin, instruments, and signaling devices, besides various minor controls. Normally while in flight the following persons are on duty in the control cabin: captain, navigator, quartermaster at the wheel (rudder controls), height coxswain (elevator controls), radio operator (in radio cabin), and when coming in to the landing field, one man standing by engine room telegraphs, and a signalman standing by for visual signals. This airship will have engine cars slung from the hull of the ship, by means of wires ropes and struts, each car containing one Zeppelin Maybach engine, with necessary instruments and controls. Each engine develops three hundred brake horsepower at 1,400 r. p. m. Access to these engine cars is obtained by gangways or ladders from the cross passages of the keel. One set of wing engines are fitted with reversing gear, so as to make maneuvering easier when coming in to the landing field.
A military airship of the above gas capacity would have a maximum speed of sixty knots and carry sufficient fuel to give her a radius of action of about 4,500 nautical miles at this speed. Of course no ship can be expected to cruise at full speed continuously. However, at a cruising speed of forty-five knots, a radius of action well over 7,000 nautical miles can be expected.’ Hence, with favorable weather conditions, such a ship would be able to cruise from any port, on the Pacific Coast of the United States to Honolulu, and thence to Manila, without once being required to land and refuel. Truly a wonderful range, but it is even more wonderful when we consider that such a flight could be made in seven days. Most of us are familiar with the double crossing of the Atlantic by the British rigid airship R-34 during the month of July, 1919. This ship had a gas capacity of approximately 2,000,000 cubic feet and her power plant consisted of five 270 horse-power Sunbeam “Maori” engines. She shoved off on her “great adventure,” from the airship base at East Fortune, near Edinburgh, Scotland, at 1 :48 A. M. G. M. T., July 2, and, after encountering adverse winds and weather, finally landed at Mineola, Long Island, at 2 P. M. G. M. T., July 6, with only two hours’ fuel supply remaining on board. The time taken to cover the 3,100 nautical miles was 108 hours. The return trip was started at 3:54 A. M. G. M. T., on July 10, and ended three days, three hours, and three minutes later at Pulham, Norfolk, England. Probably the most remarkable flight ever ,accomplished by an airship was made during the war by a German Zeppelin. This flight was made from Jamboli in Bulgaria to German East Africa and return. The Zeppelin was of the naval L-59 class, and had the same gas capacity and many characteristics in common with the later L-7o class. The ship left Jamboli at 8:35 A. M. on November 21, 1917, with a crew of twenty-two men, and carried medical supplies, munitions, and special instructions for General Von Leltow, commander of the German forces in East Africa. She crossed over the Mediterranean, flew over Cairo and Khartoum, being seen at both of these places, and followed the Nile River to Lake Victoria Nyanza, where she received radio messages from the high-powered station at Nauen, Germany, to return to Bulgaria, since the German forces in East Africa had been captured. She accordingly turned back and arrived safely at Jamboli at 7: A. M., November 25, having completed a round trip non-stop flight of about 4,300 miles in the remarkable time of ninety-five hours. This represents an average speed of forty-five miles an hour.
Officers of experience in the navy often ask the questions: “What use are rigid airships ?” “What do we want with them?” These questions have been answered by no less an authority than Admiral Lord Jellicoe, commander-in-chief of the British grand fleet at the battle of Jutland. In his book, The Naval Crisis, he makes the assertion that one rigid airship, for purposes of naval scouting, is worth two scout cruisers. If anything, this is an understatement of their value, since in this capacity they are undoubtedly without a serious rival. One need only consider their range, speed and endurance to realize this. Think of the advantage of having a scout capable of ranging about over the sea with the fleet and ready when needed to proceed in search of information at a speed of sixty knots. Suppose adverse weather conditions are encountered and the airship has to buck a thirty-knot wind. It is still capable of making a speed of thirty knots over the sea. Even under these adverse conditions, the airship still maintains her superiority over the sea scout, for what surface craft can run at full speed into a thirty-knot wind? A stiff wind has very little effect on an airship, except in so far as it affects its progress. The steadiness of an airship in a squall has been remarked upon by the late Air Commodore Maitland, C.M.G.,D.S.O.,A.F.C., R.A.F., in his book, The Log of H. M. A. R-34, an excerpt from which reads as follows:
“7:10 P. M. Struck by a fierce squall. Heavy rain. Ship remarkably steady considering intensity of the squall. The rain is driving through the roof of the fore car in many places, and there is a thin film of water over the chart table. The wind is roaring to such an extent that we have to shout to make ourselves heard. Am struck by the steadiness of the ship in this squall, which is a very severe one. Beyond a gradual and very slow pitching, causing us to hold on and snaking everything slip about very considerably, we feel no inconvenience and not the slightest symptoms of seasickness. The sea, on the other hand, when we last saw it, was very rough and I, for one, being the worst possible sailor, would certainly be feeling horribly ill.”
In addition to the speed superiority, the visibility from an airship is much greater than from a surface craft. As a scout, the airship also has the advantage of being able to obtain information with respect to an enemy’s main body without the necessity of fighting an engagement with the enemy’s advance force. It is also possible, by hovering about in the clouds, to obtain such information without being seen. Even if discovered and enemy fighting craft are sent out to drive the ship away, the information will already be obtained and it is a simple matter to climb and escape into the clouds. It is not possible for heavier-than-air craft to climb as rapidly as a rigid airship. In addition, fighting aircraft may be carried by sacrificing the number of bombs carried by the airship. While it is admitted that the Germans employed their Zeppelins in raiding expeditions for some time after an efficient defence had been developed against them, it is known that even with the small bombs in use and with the poorly constructed and inaccurate bomb sight employed in the first part of the war, from 1914 to 1916, a great deal of damage was done by them, far more than was admitted at the time, which is proved by statistics published since the war. Consider then the effectiveness of a fleet of rigid airships, similar to the ZR-2, each capable of carrying a number of high explosive bombs weighing a ton each, when directed against the navy yards, arsenals, and advance bases, particularly at the outbreak of, and following closely on the heels of a declaration of war. Unless these places were adequately defended (and few of them are) the effect would be paralyzing. For convoy they are particularly useful due to their superior vision, speed, and range of action, and for the same reason they are very effective for anti-submarine patrol. During the late war the British used these ships effectively for convoy and anti-submarine patrol duties.
In time of peace these ships will have a multiplicity of uses for the fleet. For example, during target or torpedo practice, the airship could hover directly over the target so that photographs could be taken which would form a permanent and accurate record of the practice. Also the control car of a rigid airship would be an excellent position from which to view the fleet at maneuvers, or during the latter stages of a war game. Any one who has ever flown appreciates the detail with which everything below is shown, and the different moves of the opposing forces could be observed much the same as the chess player observes the moves of his antagonist on the chess board. On account of their speed and endurance, these,. ships will undoubtedly be most useful for rapid communication with our outlying possessions of Hawaii, Guam, and the Philippines, and tend to knit them more closely into the defense of the Pacific.
Although an airship has little to fear from any except the strongest winds when she is in the air, great difficulty has been experienced in handling her on the ground and even under normal conditions a handling party of from two to four hundred men is necessary. That is the reason there were no Zeppelins present with the German high sea fleet until the second day of the battle of Jutland. Although the wind was not particularly high, it was sufficient to prevent the ships being taken from their hangars. However, the second day of the battle a few Zeppelins were taken out of their sheds, and these assisted the scattered units of the German fleet to regain their base successfully. It was therefore very essential for the future of airships that some method be evolved whereby they would be as independent of their sheds as a transatlantic liner is of a dry dock. This has been successfully accomplished by the mooring mast. The mooring mast consists of a steel structure about 150 feet in height, and the airship is secured, by a bow mooring gear, to a special attachment at the top of this mast. To moor, the airship approaches the landing field as for a normal landing. Previous to this a special wire cable, wound on a drum and operated from a winch at the foot of the mast, is carried up the mast through the mooring attachment and veered out to the ground in a direction opposite to that from which the wind is blowing, a distance of about 600 feet from the foot of the mast, the end of the cable being marked by a red flag. The ship is headed into the wind and brought directly over this flag, the ship having an altitude of about 500 feet, and at this instant the airship’s mooring cable is released, the end being weighted, and when it strikes the ground, a man stationed there for this purpose, quickly connects the ends of the two cables together, by means of a special coupling. The ship’s engines are then stopped and she is permitted to rise slowly until she rides from the cable at an altitude of about 1,200 feet, the weight of the cable tending to gradually stop the ascent of the ship. The winch is. now started and the cable reeled in until the nose of the ship is hauled to the top of the mast and properly secured. The mooring attachment at the top of the mast is mounted on a ball bearing so that it may revolve freely and thus permit the airship to swing head to the wind, at the same time holding the ship rigidly. Fuel, gas, and water lines are carried up the mast to the upper platform, where they may be connected to the ship’s systems, and the ship refueled or gassed without the slightest difficulty. Airships have been moored out to a mast for months on end without the slightest trouble, and have been released from the mast in winds of about fifty miles per hour, and moored to the mast in winds of slightly less force. The most remarkable part of the whole operation is the fact that only one man is needed to unmoor and only six men needed to moor the largest ship. When ready to unmoor, the ship is held to the mast by means of a stopper, and it is only necessary to knock off the link with a hammer to release the ship entirely from the mast. With equipment of this sort at all fleet bases, rigid airships will be able to operate directly with the fleet at all times. [END]
The Naval Airship Association employed a media production company to video tape an introduction by Rear Admiral Carl Seiberlich, a former blimp pilot and strong LTA advocate, and recut VADM C. E. Rosendahl’s 1946 Rigid Airship History film.