AFTER A TRANSATLANTIC FLIGHT the German Zeppelin airship Hindenburg is shown moored at Lakehurst, New Jersey. In the background is the U.S. Naval Airship Los Angeles at her mooring mast. An accident on May 6, 1937, caused the destruction of the Hindenburg at Lakehurst at the end of a flight from Germany. Launched in 1936, she had a maximum gas capacity of over 7,000,000 cubic feet, an overall length of 803 feet and a maximum diameter of 135 feet.
THE three types of airship, rigid, semi-rigid and non-rigid, are the outcome of the solutions by airship pioneers of various early difficulties. One of the chief obstacles that had to be overcome, for instance, was the problem of attaching a car to a streamlined gasbag, making the car a reasonable length yet adding as little as possible to weight and resistance.
An envelope of streamline form must be three times the length of the car it has to carry. The weight of the car is concentrated below the centre of lift of the balloon, whereas the lift of the gas is distributed along its entire length. Thus the ends have a strong tendency to rise and the centre portion, loaded beyond its capacity, tends to sag.
To overcome this difficulty, the French designers, Paul and Pierre Lebaudy, built an airship with a framework slung underneath the envelope and running nearly the whole length. To this framework they attached the car. Thus the semi-rigid type of airship was evolved. The Lebaudy airship was not a success, for the framework offered great head resistance and made her a slow ship. Additional trouble was experienced because of the extremely high bending movement to which the structure was subjected. The Italian semi-rigid airship has been more successful. In this the framework is slung close up under the envelope and is entirely covered in. The car being attached close to the framework, resistance is reduced to a minimum.
The semi-rigid, like the non-rigid, is a pressure airship - that is, the shape of the balloon is maintained by the pressure of the gas. In addition to its metal keel, the semi-rigid airship has a metal framework at the nose (to prevent it from being forced in by air pressure) and also at the tail, where the control surfaces are.
This variety of airship, which has been developed chiefly by the Italians, lies between the rigid and the non-rigid types; but, with a cubic capacity of 700,000 to 800,000 cubic feet, it has not the range and carrying capacity of the one, nor the handiness of the other. Russia, however, under the direction of the Italian airship pilot, General Nobile, has built a fleet of five or six of this type for commercial and military purposes, but no reliable information is available as to their performance.
With the failure of the semi-rigid Lebaudy type, the French next introduced the Astra-Torres design. This design has two strong items in its favour. The points at which the rigging is attached to the envelope, instead of being below the centre line of the envelope, are considerably above it, so that the overall height of the airship can be reduced by a corresponding amount. Again, the greater part of the rigging is enclosed and this accounts in a large measure for the considerable increase in speed obtained with this type of airship.
The Astra-Torres design was used extensively by Great Britain during the war of 1914-18 and a number of airships of this type are still in service with the French Navy.
The Astra-Torres envelope has a trilobe, or trefoil cross-section. Four ballonets, or air compartments, are situated in the envelope, two in each of the lower lobes. Air from the slipstream of the propellers is conveyed to the ballonets through the airscoops, thence by means of fabric air ducts running parallel to the longitudinal centre line of the envelope. Transverse ducts also connect each pair of ballonets. Four air valves and two gas valves are used. The gas valves are placed well aft, one to each lobe and fitted on either side of the rudder plane. No top valve is fitted.
The internal rigging girdles and lacing girdles are attached to the whole length of the top ridge and these support the top and side curtains. The object of these curtains is to make the envelope keep its shape, not to divide the bag into separate gas compart-ments. The rigging girdle consists of a number of fabric scallops through which many strands of hemp are led towards the bottom of the ridge, where they are drawn together and secured to a rigging sector. The main external cables are attached to these sectors. The nose of the envelope is stiffened to prevent it from blowing in. This, a British contribution to airship development, is effected by canes fitted in fabric pockets round the nose and meeting at a point in front of the nose. A metal cap is fitted over the canes and the whole is covered by a fabric nosecap.
The car has a small compartment in the bows, for the steering coxswain. Then comes the control platform, which is the highest part of the car, and here are the usual controls and instruments. Leading down from this position is a narrow passage running fore and aft through the centre of the car. Off this passage are the radio and navigating rooms, the auxiliary blower and engineer’s station, with instruments and controls. The two engines are outrigged and repairs can be carried out in flight. In the after part of the car are the water ballast bags, petrol tanks and stern cockpit.
France and Germany had for many years been developing the airship before the British Government entered the field in 1907. Colonel J. E. Capper, R.E., then head of the Balloon Factory, built the first Government airship. Named the Nulli Secundus, she had a 50 horse-power Antoinette engine driving two metal-bladed propellers, and on her trials in September 1907 she attained a speed of sixteen miles an hour.
In May 1909 appeared the Baby, having a capacity of only 20,000 cubic feet, and two engines of 12 horse-power This airship was rebuilt with a volume of 33,000 cubic feet and was given a 30 horse-power Green engine. She took the air in 1910, and was renamed the Beta. Two years later her power was still further increased to 35 horse-power.
RELEASING BALLAST as she nears the ground at Pulham, Norfolk. The Norge, which was completed in 1924, was the first semi-rigid airship of the N type designed by General Umberto Nobile. In 1926 she was acquired for Amundsen’s expedition to the North Pole, which was reached on May 12 of that year.
Airship development and experiment in the hands of the Army was carried on with successive non-rigid ships, each larger and showing certain improvements on its predecessor. The Beta was followed by the Gamma I, which was rebuilt and named Gamma II in 1912. This was the first craft to be fitted with swivel propellers, which gave better control in landing and in taking off. She was also one of the first aircraft to carry radio equipment.
While Continental airship pioneers tried to solve the problem of car suspension by internal rigging, the British adhered to outside attachment, and a compromise between the height of the ship and the length of the frame of the car was aimed at. The climax was reached in the airship Delta (1912), in which the car was of a minimum length and no extension of the frame existed. The next airship, the Eta, built in 1913, gave a system of car rigging for non-rigid types that remained the standard practice until some time after the war of 1914-18. Although those early British non-rigid airships laid the foundation of present-day knowledge, perhaps the SS (Sea Scout) type, or “Blimps”, may be regarded as the true forerunner of the non-rigid airship of today. SS 1 was originally designed and built as an experiment for coastal patrol in 1915.
The envelope was of only 20,500 cubic feet, with a total length of 146 ft 6-in and a maximum diameter of 27 ft. 9-in. Later airships of this type had envelopes of between 60,000 and 70,000 cubic feet capacity.
The envelopes then, as today, were composed of rubber-proofed fabric, two fabrics being used with rubber interposed between them and with rubber on the inner or gas surface as well. To make the airships gastight and as impervious as possible to the action of weather and sun, various coatings of “dope” were applied, followed by one of aluminium varnish.
RIDING AT HER MOORING MAST at Cardington, Bedfordshire, the British rigid airship R 101. This airship marked several departures from previous practice. Her five engines were Beardmore Tornado heavy-oil engines working on the compression-ignition system. They proved heavier than had been estimated and an additional gas compartment was incorporated to increase the lift, giving a total capacity of 5,500,000 cubic feet and a final length of 777 feet. The R 101 was wrecked in France on October 5, 1930.
A ripping panel was fitted on the top of the envelope near the nose. This ripping panel had a length of 14 ft 5-in and a breadth of 8-in. The part which had to be torn away overlapped the edge of the opening on either side, the overlap being sewn and taped on the envelope and forming a seam as strong and as gastight as any other part of the envelope. At the forward end the two strips, which overlapped the opening by about 3 feet, were loose and had a toggle inserted at the end to which the ripping cord was attached. This cord was led aft from the ripping panel to a pulley fixed in the centre and over the car, from which it passed round the side of the envelope, emerging through a gland immediately below the pulley.
It was in the SS 1 that nose stiffeners, as applied to the French Astra-Torres type, and a standard feature of today, were first used.
The rigging of the car was of the type first used in the airship Eta. On either side were six main suspensions of 25-cwt stranded steel cable, called C suspension. Each of these six branched into two halves known as B bridles, which in turn were supported at either end by A bridles. To attach the ends of the A bridles to the envelope “Eta patches” were used. These consisted of metal D-shaped fittings round which rigging was spliced. A number of webbing bands passed through this patch in fanlike shape, which made them convenient for being stuck on to the envelope. This system of rigging assured equal distribution of the car’s weight over the greater part of the envelope.
Four handling guys for manoeuvring the airship on the ground were provided under the bow and stern. A group of “Eta patches” placed close together formed the point of attachment for two guys, the forward of these groups forming the anchor point. A bridle of 25-cwt steel cable was attached here, and connected to the fore part of the skids of the car. The junction of this bridle, with two cables from the skids, formed the mooring point at which the main trail rope was attached. This consisted of 2-in Manila rope, 120 feet long. Coiled up, it was fixed to the side of the car, from where it was dropped by releasing gear.
For steering and stabilizing purposes, four fins and rudders were set exactly radial to the envelope in the SS 1. In later airships of this type the two lower fins and rudders were abandoned, and a single vertical fin and rudder were fitted centrally under the envelope. These control surfaces have remained the standard design for this type of airship.
For the first forty or more of these ships that were built, the fuselage of a BE 2c aeroplane, fitted with a 70-horsepower Renault engine, was slung under the envelope and the car. For two years this type of craft patrolled the British coasts in all weathers.
THE INTERNAL STRUCTURE of the R 101 consisted of fifteen main longitudinal members with as many intermediate longitudinals between them. The main transverse frames, of which there were also fifteen, were of deep triangular section. Steel tubing was extensively used.
By this time the airship service had been more or less organized and the SS Zero type was designed.
In this type the car was shaped like a boat and gave comfortable seating to a radio operator forward, a pilot amidships and a gunner aft. In shape the SS Zero type was as nearly streamlined as practicable; it had a keel and ribs of wood, with curved longitudinal members, the strut-ends being housed in steel sockets. The whole frame was braced with piano wire set diagonally between the struts.
The car was floored from end to end and the sides were enclosed with three-ply wood covered with fabric. The engine was a 75 horse-power water-cooled Rolls-Royce, mounted in bearers above the level of the car at the after end. It drove a four-bladed propeller. The car was suspended from an envelope of 70,000 cubic feet capacity, and the system of rigging was as before. The petrol was carried in aluminium tanks attached by fabric slings to the axis of the envelope and the engine was fed by flexible pipes to a two-way cock and thence to the carburettors. This type of airship had a speed of forty-five miles an hour, with a theoretical endurance of seventeen hours. This figure was largely exceeded in practice, for the SSZ 39 remained in the air for fifty hours fifty-five minutes, and several others carried out patrols of over twenty-four hours. The Zero was one of the successes of the war of 1914-18 and was entirely a British product.
To make the SS Zero type less liable to loss from engine failure and to increase its speed and endurance, a twin-engined SS was designed in 1918. The capacity of the new type was 100,000 cubic feet, with a length of 164 ft 6-in, and a greatest diameter of 32 feet. The car was built to carry five - two pilots, radio-operator, coxswain and engineer. Two 75 horse-power Rolls-Royce engines were disposed on gantries on the port and starboard sides.
THE LARGEST NON-RIGID AIRSHIP IN AMERICA at the time of her building, in 1933, was the TC 13. She has a capacity of 360,000 cubic feet, a length of 233 feet and a diameter of 54 feet. Her design incorporated several novel features, the car being flush with the envelope. A similar airship, the TC 14, was built in 1935.
The largest British non-rigid airship appeared in 1916. The type was known as the N.S. (North Sea) type and the Astra-Torres envelope was used, with the capacity increased to 360,000 cubic feet. A completely enclosed car, 35 feet long, accommodated a crew of ten. Two 240 horse-power engines were carried in separate cars aft of the main car, approached from a wooden gangway supported by wires. The N.S. airship was 262 feet long and had a maximum speed of nearly sixty miles an hour. The endurance of this type was remarkable. In 1919 the NS 11 made a continuous cruise of 101 hours, during which she is stated to have travelled 4,000 miles.
Simultaneously during the war of 1914-18 non-rigid and semi-rigid types were being developed in Germany, France and Italy. In Germany the work of Major von Parseval was somewhat overshadowed by the success of the Zeppelin rigid airships and considerably handicapped by the lack of material sufficiently strong for pressure-type envelopes.
In Italy the engineer Forlanini had designed a number of airships before the war. In the early twenties two memorable semi-rigid airships were produced. The first was the Roma, designed for transatlantic service. She was 410 feet long and had a capacity of 1,200,000 cubic feet. Unfortunately she was destroyed by fire in 1922, after having been acquired by the United States Army.
Two years later appeared the Norge, the first of the N type designed by General Umberto Nobile. The envelope had a capacity of 665,000 cubic feet and consisted of ten gas compartments. The nose was built in the form of a rigid cap supported from the keel. Valuable experience for airship practice was gained when the Norge was acquired for Amundsen’s expedition to the North Pole in 1926. On the return journey from the Pole to Nome, Alaska, ice formation caused damage and necessitated a forced landing. The landing was successfully accomplished, a member of the crew having previously left the airship by parachute to direct ground operations.
Further development of non-rigid airship types has been carried out in the United States. In 1925 was launched the Pilgrim, the first of a fleet of Goodyear-Zeppelin non-rigid airships. The Pilgrim was taken out of commission in 1931, but in 1938 there were still five of the fleet in commission - named Puritan, Volunteer, Reliance, Enterprise and Resolute. These vessels have a capacity of 112,000 cubic feet and are each 140 feet long. By July 1, 1937, this fleet had flown more than two and a half million miles and had carried more than 250,000 passengers.
A larger non-rigid airship, the TC 13, was built in 1933 for the US Army Air Corps. With a capacity of 360,000 cubic feet, the TC 13 was at the time the largest non-rigid airship in America, and her design incorporated several novel features. She was built with a length of 233 feet and a diameter of 54 feet.
The envelope is similar to that of the British non-rigid type, complete with nose stiffeners, ballonets and the like. The rigging is contained within the envelope, and the car, over 40 feet long, brought flush with the envelope. The bottom of the car, similarly to the British Zero, is shaped like a boat to allow alighting on the water.
The airship is powered by two 375 horse-power geared air-cooled engines, mounted on outriggers extending from the sides of the car. Walkways are provided on these outriggers to give access to the engines for inspection or minor adjustment during flight.
Propellers 12 feet in diameter are fitted and the engines are geared in such a way that the propellers turn at a comparatively slow speed. This, with the special mufflers on the engines, gives a quiet power plant, a feature which for tactical reasons was given special attention during the designing of this ship.
AN ENVELOPE OF TRILOBE CROSS-SECTION was a distinctive feature of the Astra-Torres type of airship built in France before and during the war of 1914-18. The trilobe or trefoil section appeared to be formed by two balloons side by side with another resting above them. The system of internal rigging helped to reduce resistance. The Astra-Torres design was extensively used by Great Britain as well as France.
Besides the two main engines, three small marine type outboard engines are installed in the oar. One of these is for driving an air blower to supply air to the ballonets in the envelope. Another is to drive an auxiliary radio generator in the car. The third small engine is used on the windlass for raising and lowering the sub-cloud observation car at a rate of 200 feet a minute.
This sub-cloud car is built of wood, streamlined and painted to harmonize with the colours to be expected in a dark, cloudy sky. The object is to make the car virtually invisible in these conditions. The car is lowered from the airship by a flexible cable. A telephone line is provided for the observer to communicate with the crew of the airship and even to direct its manoeuvres when advisable.
Another feature of special interest is the fuel and water ballast tank installation. The fuel tanks can be filled with water ballast to compensate for the fuel consumed and so maintain the airship in equilibrium at all times. The water is obtained by apparatus dropped into the ocean while the airship is flying at a reduced speed. To ensure that no water enters the fuel system, special fuel and water segregators are provided. A similar airship, the TC 14, was built in 1935, but two years later the US Army Air Corps airships were handed over to the US Navy.
Previous experiment in the United States had led to the introduction of the metal-clad airship of the semi-rigid type. The first metal-clad airship was produced in Germany in 1895, but its importance lay in the fact that it originated the rigid type.
In 1922 a group of men in Detroit, Michigan, gathered together to see whether it would be possible to build an airship entirely of metal. Research went on for seven years and a sum of £30,000 was expended. Original specifications called for a hull covering of plain duralumin 0·008-in thick. It was the first time that sheet of this thickness had been used, and its resistance to corrosion was not definitely known.
Six months’ exposure revealed that a serious deterioration from inter-crystalline attack had occurred. It was evident that this was not a safe material with which to proceed, but the conclusion was that failure was due to the thickness of the sheet. A new metal, “Alclad”, having a thickness of 0·0095-in was produced, and this proved to have a superior resistance to inter-crystalline corrosion. This metal is plain duralumin coated on either side with pure aluminium.
It was decided that the joining of the thin sheets of “Alclad” could best be done by riveting. An automatic riveting machine was developed on a new principle. In its operation three strands of wire are fed like thread into the machine and three rows of rivets are “sewn” simultaneously. The machine shears off the wire rivet length. The tiny wire sections are punched through the two sheets of metal and the revolving cams head up the rivets. With this machine two men are able to accomplish as much work in a given time as 128 men working by hand. It inserts and completes 135 rivets a minute, and successfully drove about 3,500,000 rivets of 0·035-in diameter into the hull of the metal airship.
THIS ALL-METAL AIRSHIP, the ZMC 2, with a capacity of 200,000 cubic feet, was built by the Metalclad Airship Corporation at Detroit, Michigan, in 1929. Seven years of research were necessary before construction. The envelope is formed of sheets of “Alclad”, 0·0095-in thick, riveted and “sewn” together. This airship is not officially considered to be semi-rigid, even though she has internal frames.
The internal structure of this airship, officially known as the ZMC 2, is composed of twenty-four equally-spaced longitudinals and twelve circular frames which are riveted to the outer cover of “Alclad”. Five of the transverse frames are of built-up heavy girders, braced with wiring in their own planes. These frames carry all concentrated loads. The rest of the structure is built of light sections designed primarily to maintain the shape of the hull and to support the outer cover when the ship is deflated. The ZMC 2 is a semi-rigid of the most modern type, with a gas capacity of 200,000 cubic feet.
The car is attached to the two main frames near the maximum diameter of the hull, and is 24 feet long, being 6 ft 6-in wide at the maximum section. The structure consists of transverse frames and several longitudinals braced by diagonals, and covered with 0·014-in corrugated “Alclad”. Two 220-horsepower Wright Whirlwind engines are fixed to the car by outriggers. In the bow of the car are the pilot’s navigational and radio equipment, and in the stern are the fuel and ballast tanks. Two ballonets made of two-ply rubberized fabric are placed inside the hull fore and aft of the car.
The system for the supply of air to the ballonets is that of the non-rigid. The airship is equipped with eight fins equally placed round the circumference, four of them acting as vertical, and the other four as horizontal surfaces. This arrangement allows a higher aspect ratio (ratio of span to chord) than is customary, thus assisting efficiency.
The ZMC 2 was completed in 1929 and has been in continuous service at the Lakehurst Naval Air Station for experimental and training purposes. She has a maximum speed of about seventy miles an hour. In 1935 she was deflated for examination and overhaul. Microscopic examination of the hull for surface corrosion showed no pits deeper than 0·0003-in.
German Origin of Rigid Type
The rigid type of airship originated in Germany. In 1895 David Schwartz built an airship with a capacity of 130,000 cubic feet and 156 feet long. A 12 horse power Daimler engine worked twin aluminium propellers, one on either side. The trials of this airship took place in November 1897, but, encountering a 16 miles-an-hour headwind, she could make no progress. She was damaged on landing and was later destroyed. This airship had an outer covering of thin aluminium sheeting and was the first metal-clad airship.
The credit for the initial development of the rigid airship, however, lies with Count Ferdinand von Zeppelin. It was on July 2, 1900, that his first airship was launched from Lake Constance, Germany. The LZ 1 was 418 feet long and had a diameter of 38 ft 3-in. Her volume was 339,000 cubic feet. Shaped like a pencil, the airship had an aluminium framework of sixteen loops, these being connected and kept rigid by wire stays running longitudinally and diagonally. Transverse aluminium frames divided the structure into seventeen gas compartments, each ballonet being made of waterproof cotton cloth. A similar fabric was used for the outer cover. Along the whole length of the airship a triangular latticework keel of aluminium was arranged, which not only acted as a passageway from end to end but also helped to strengthen the structure. At points about 140 feet from either end, spaces in the keel were made for two 21-feet aluminium cars, in each of which was a 16 horse-power motor. Each of these motors operated two four-bladed high-velocity propellers, one on either side of the car.
Horizontal control was effected by planes placed underneath at both ends. For vertical control there was in the keel a movable weight which could be moved fore or aft as required. The weight was later augmented by a rudder fitted under the bow.
The trial of the first airship was of short duration. She travelled three and a half miles, proved to be perfectly stable and navigable, and attained a speed of seventeen miles an hour.
VALUABLE RESEARCH WORK was carried out in the twenties by the rigid airship R 33, based on Pulham, Norfolk. A sister airship to the R 34, which made a historic double crossing of the Atlantic in 1919, the R 33 was 643 feet long, with a maximum diameter of 79 feet Her capacity was ,960,003 cubic feet and her maximum speed about sixty-two miles an hour.
Improved Zeppelin airships followed one after the other and on March 4, 1912, the Viktoria Luise, Germany’s first commercial airship, carried twenty-three passengers a distance of 200 miles in seven and a half hours. The first British rigid airship was built for the Admiralty in 1911. Known as Naval Airship No. 1 (or R 1), she was referred to unofficially as the Mayfly. She was 500 feet long and the metal used was duralumin. The structural details followed Zeppelin design.
R 1 was badly damaged in 1911, and activity with rigid types in Great Britain was only spasmodic until HM Airship R 9 was commissioned during the war of 1914-18. The main structure formed a seventeen-sided polygon, 526 feet long.
Eight more rigid airships were built before the R 33 and the R 34 appeared in 1918. History was made when the R 34 made a double crossing of the Atlantic Ocean in 1919. The airship left East Fortune, Scotland, on July 2, and landed at Mineola, Long Island, after a voyage of 108 hours 12 minutes. The return voyage was made in 75 hours 3 minutes a few days later. Weather conditions were not favourable. The advantage of the rigid type was proved when the R 33, which was being used for research, was torn from her mooring mast at Pulham, Norfolk, in April 1925, but succeeded in returning to her base after having drifted for thirty hours in a damaged condition. Unfortunately the R 38 broke in two on August 24, 1921, while over the mouth of the Humber; this disaster caused a temporary abandonment of airship building. The airships R 100 and R101 were built in 1929 and they represented a great advance in British design. The R101 was fitted with five heavy-oil engines of the Beardmore Tornado type. The eventual capacity was 5,500,000 cubic feet and the length 777 feet.
The R 100 was similar in size and shape, but followed Zeppelin practice more closely in structural design. She was driven by six Rolls-Royce Condor III B engines, each of 670 horse-power. In 1930 she flew to Canada and back.
Safety of Helium
The R101, however, met with disaster at Beauvais, France, and this caused a curtailment of the official airship programme. The
R 100 was kept in commission, but for reasons of economy was sold and broken up in 1931. In America rigid airship design followed Zeppelin practice closely. The first State airship was the Shenandoah (ZR 1), completed in 1923. Like the R 33, she successfully regained her base, at Lakehurst, N.J., after having been torn from her moorings by a gale. In October 1924 the Shenandoah cruised a distance of 9,000 miles round the borders of the United States. Severe and exceptional stresses caused by a violent squall near Marietta, Ohio, on September 3, 1925, were responsible for structural failure and the Shenandoah broke into three parts. The forward portion of the broken airship acted as a free balloon, and made a safe landing, twelve miles away, with the commander and twenty-seven of the crew.
The Los Angeles, built at the Zeppelin works as the ZR 3 for the US Government in 1924, achieved a remarkable record for performance. Before being taken out of commission in 1932 she had been in the air for 6,430 hours, for 2,080 of which she had been moored. She had covered about 200,000 miles.
Two important modifications appeared with the airships Akron and Macon. First, the use of three internal keels gave additional structural strength and, secondly, the use of helium permitted the engines to be mounted in the hull without risk of fire. The development of rigid airships has been everywhere influenced by the success of the Zeppelin Company. One of the most remarkable examples of Zeppelin construction is the Graf Zeppelin (LZ 127). Completed in 1928, she has made many historic flights, including a voyage round the world in 1929. Her structure consists of twenty-eight longitudinal girders, with main transverse frames (28-sided polygons) spaced 50 feet apart. The Hindenburg, launched in 1936, with her regular transatlantic flights, continued the fine tradition established by the Zeppelin airships, and the accident which caused her destruction in 1937 did not deter the Zeppelin Company from its programme of new building.
ONE OF THE EARLIEST BRITISH AIRSHIPS was the Beta, which appeared in 1910. She was a reconstruction of the Baby of the previous year, with the capacity increased to 33,000 cubic feet. In 1912 she was again rebuilt, and was followed by many other types of non-rigid airship.