The Akron-class airships were a class of two rigid airships constructed for the US Navy in the early 1930s. Designed as scouting and reconnaissance platforms, they were intended to act as "eyes for the fleet", extending the range at which the US Navy's Scouting Force could operate to beyond the horizon. This capability was extended further through the use of the airships as airborne aircraft carriers, with each capable of carrying a small squadron of airplanes that could be used both to increase the airship's scouting range, and to provide self-defense for the airship against other airborne threats.
The two ships were built as a continuation of the US Navy's rigid airship program that had started just after World War I, and were used to further refine the tactics of the use of such machines in the fleet, predominantly over whether it was the airship that was the scout, with its air group only there for self-defense, or whether the airship was merely the mother ship and the airplanes were responsible for carrying out the long-range scouting mission.
Both ships had short careers in the US Navy, as each one crashed into the sea during routine flights less than two years after it was commissioned.
Background
The US Navy had been experimenting with rigid airships since shortly after the end of the First World War. In 1917, a German zeppelin, L 49, was forced down in France following a bombing raid over England, and was captured virtually undamaged. This led to the idea of the United States obtaining a pair of German airships as part of the reparations plan, but they were destroyed by their crews in 1919. As a substitute plan, it was agreed that Germany would build and pay for an airship to be turned over to the Americans, while the US would build one of its own. In July 1919, the US Navy placed an order with the Naval Aircraft Factory in Philadelphia for the components to build a new rigid airship, which would be assembled at Naval Air Station Lakehurst in New Jersey; initially designated as FA-1 (Fleet Airship Number 1), the ship was redesignated as . The plan for the Germans to construct an airship was modified when the Royal Navy cancelled its order for four rigid airships. The first ship of the class, R38, was already under construction, and so an agreement was reached in October 1919 to sell the incomplete airship to the United States, which gave it the designation ZR-2.
In 1921, ZR-2 was completed and undertook test flights from its construction site at Cardington, before travelling for further testing over the North Sea, being based at Howden. During a test flight in August, ZR-2 experienced a catastrophic structural failure and crashed into the Humber Estuary, killing all but five aboard. Despite this setback, the US Navy continued with its rigid airship program, starting construction of ZR-1 in June 1922. As a safety measure, it was decided that rather than using hydrogen as the lifting gas, which had been used on ZR-2, and which caused the fires following its crash, ZR-1 would be filled with helium. The scarcity of helium, and the expense of extracting it, meant that the airship used most of the world's reserve of the gas.Hayward (1978) p.64 ZR-1 was commissioned into the US Navy as USS Shenandoah in October 1923. At this time, another new rigid airship was under construction at the Luftschiffbau Zeppelin works in Germany - intended to replace the ships intended for reparations after the war, the ship was also used as a means of keeping zeppelin construction alive in Germany. Known initially by its construction number as LZ 126, it was appropriated by the US Navy as and commissioned as USS Los Angeles in November 1924. Owing to the scarcity of helium, upon its commissioning, Los Angeles used gas obtained from Shenandoah; the intention was to alternate use of the two airships until more of the gas could be obtained.
The use of Shenandoah and Los Angeles as platforms to evolve the tactics of airship use with the fleet led to the US Navy instituting a plan to procure a pair of new, purpose-built airships, which originated in a set of design studies undertaken by the Bureau of Aeronautics in 1924 as BuAer Design No. 60, intended as an improvement over the Shenandoah design. The loss of Shenandoah in a crash in Ohio in September 1925 did not interrupt this; indeed, the incident left the US Navy with only one rigid airship that, under the terms of her construction, was not permitted to take part in military operations. As a consequence, a pair of new airships was authorized in June 1926, with the Goodyear-Zeppelin Corporation winning the contract to build them in October 1928. To facilitate construction, the company built a brand-new construction and storage hangar, which came to be known as the Goodyear Airdock, in 1929. Upon completion of the building, work began on building the first of the new airships, which received the designations and .
Design
The two ships that became the Akron class were the first large rigid airships to be both designed and built in the US. Goodyear-Zeppelin was a joint venture between Goodyear and Luftschiffbau Zeppelin, with the sharing of German experts and ideas to train the employees of Goodyear in airship construction. As part of this collaboration, Luftschiffbau Zeppelin's Chief Stress Engineer, Karl Arnstein, went to the United States to work with Goodyear on new designs and techniques. This allowed Arnstein to develop ideas of airship design away from the more conservative methods employed by the German company's chief designer, Ludwig Dürr.
"Deep Rings" and triple keel Most traditional zeppelin designs were composed of a series of main rings, made from a single reinforced girder, with unreinforced rings, which provided shape but not structural strength, in the spaces in between. Arnstein's proposal for the two new ships was to have the main rings composed of a pair of rings, connected by supports that formed triangles all around the circumference of the ring. These "deep rings", made of duraluminum, were spaced further apart than the single rings used in zeppelins, and were believed to offer greater strength, for which the US Navy was prepared to accept that the framework was heavier than in similar German-produced airships. Similarly, rather than using a single structural keel along the underside of the hull, Arnstein's design had three, triangular shaped keels - one along the top of the airship, which was used to provide access to the valves of the ship's gas cells, and two more placed at 45 degree angles on each side of the bottom of the hull, which supported the engine compartments and crew spaces.
Engines and gas cells Unlike in previous airship designs, the new US ships, with their triple keels and using helium rather than hydrogen, were designed to have their engine compartments installed within the hull, rather than having external power cars, which had the benefits of easier access to the engines and reduced drag in flight. Each of the eight Maybach VL-2 12-cylinder gasoline-powered engines was capable of , with the propeller shafts attached being rotatable, allowing forward and reverse thrust, and downward thrust to assist in landing. The ships had installed a total of twelve gas cells made of gelatin-latex fabric, each containing of helium.
One compromise that had to be reached over the placement of the engines was that they were mounted in a straight line. Most airships of the period had their engines mounted at different heights along the length of the hull, which allowed each propeller to operate in relatively "clean" air that had not been displaced by the propeller in front. However, attempting to re-engineer the structure of the hull to try and stagger the placement of the engines was seen as both adding too much weight and being too complex.
Akron, as originally built, was fitted with two-bladed, fixed-pitch wooden propellers to each of her engines but, in June 1932, was modified to be fitted with new, three-bladed, variable pitch metal propellers. These new propellers were fitted to Macon as standard while it was under construction, increasing the ship's fuel efficiency. This, combined with design improvements that removed some hull protrusions (improving the aerodynamics) and a reduction of her overall dead weight by as much as , led to it achieving a speed of during speed trials in August 1933, more than 3 knots faster than the maximum speed requirement specified by the Navy, and 6 knots faster than Akron best recorded speed of .
Tail and stabilizers The design of the Akron class did away with the traditional cruciform tail and altered the shape and position of the stabilizers. The ship's stabilizers were originally designed to be secured on the hull at three main ring points, but this was changed by shortening them to have them secured at only two points, with the leading edge only attached to the intermediate frame rather than the load-bearing structure. This design change came about following an incident involving the German passenger airship Graf Zeppelin, which, when taking off in 1929, had almost struck a set of power lines with its lower fin, which could not be seen from the ship's control gondola. In order to make the lower fin visible, the design was changed to shorten the fin. This proved to be a design flaw that was a major contributory factor to the loss of Macon in 1935.
Water recovery When the decision was taken to use helium instead of hydrogen in 1922, Shenandoah was fitted with a set of condensers to allow the collection of water vapor from the engine exhausts to create ballast and manage the ship's buoyancy. In most airship designs this would have been accomplished by venting gas as fuel was burned, but because helium was so expensive to produce (approximately $55 per 1000 ft3 in 1923), and Shenandoah had required approximately to fill its gas cells, the decision was taken to not routinely vent the valuable gas, and instead collect water vapor. The Akron class required three times more gas to fill the cells, which made the collection of water more important, in spite of the increased availability of helium through improvements in production, transport and storage. The condensers appeared as black strips on the ship's envelope directly above each propeller.
Aircraft hangar One significant innovation of the Akron class airships was the inclusion in the hull of a hangar capable of accommodating up to five small airplanes for use as scouts and fighters. A trapeze-and-hoist apparatus was included on the airship for launching and recovering the parasite aircraft during flight. Each Akron class ship was built with four individual hangar bays; a fifth aircraft could be stored on the trapeze. In the USS Akron, however, structural girders obstructed the two rearmost hangars, meaning it was capable of accommodating no more than three aircraft when first commissioned. Plans were in place to modify the supports, but the ship was lost before the work could take place.
Each of the aircraft was fitted with a skyhook that latched to a horizontal spar on the trapeze. For launch, the aircraft were hooked on inside the hangar and lowered through a T-shaped opening in the hangar deck. Once below the airship, the pilot would start the engine before detaching the hook from the trapeze. The same process worked in reverse for in-flight recovery. The pilot would match the speed of the airship and carefully latch the skyhook to the trapeze. The aircraft could then be lifted back into the hangar. A second trapeze near the airship's tail allowed a second aircraft to "perch" while waiting for its turn to come aboard. To ease stability and lower lift requirements during the airship's launch, the normal practice was for the airship and its complement of parasite aircraft to depart separately and then to rendezvous once the airship was airborne.