USS Akron (hull number ZRS-4) was a helium-filled rigid airship of the U.S. Navy, the lead ship of her class, which operated between September 1931 and April 1933. She was the world's first purpose-built flying aircraft carrier, carrying F9C Sparrowhawk fighter planes, which could be launched and recovered while in flight. With an overall length of , Akron and her sister ship the USS were among the largest flying objects ever built. Although LZ 129 Hindenburg and LZ 130 Graf Zeppelin II were some longer and slightly more voluminous, the two German airships were filled with hydrogen, so the two US Navy craft still hold the world record for the largest helium-filled airships.
The USS Akron was destroyed in a thunderstorm off the coast of New Jersey on the morning of 4 April 1933, killing 73 of the 76 crewmen and passengers. The accident involved the greatest loss of life in any airship crash, and was the deadliest aviation disaster of any kind prior to World War II.
Technical description
The airship's skeleton was built of the new lightweight alloy duralumin 17-SRT.Smith (1965). p. 181 The frame introduced several novel features compared with traditional Zeppelin designs. Rather than being single-girder diamond trusses with radial wire bracing, the main rings of Akron were self-supporting deep frames: triangular Warren trusses "curled" around to form a ring. Though much heavier than conventional rings, the deep rings promised to be much stronger, a significant attraction to the navy after the in-flight breakup of the earlier conventional airships R38/ZR-2 and ZR-1 Shenandoah.Smith (1965). p. 187 The inherent strength of these frames allowed the chief designer, Karl Arnstein, to dispense with the internal cruciform structure used by Zeppelin to support the fins of their ships. Instead, the fins of Akron were cantilevered:, mounted entirely externally to the main structure.Smith (1965). pp. 161 & 189 Graf Zeppelin, Graf Zeppelin II, and Hindenburg used a supplementary axial keel along the hull centerline. However, the Akron used three keels, one running along the top of the hull and one each side, 45° up from the lower centerline. Each keel provided a walkway running almost the entire length of the ship. The electric and telephone wiring, control cables, 110 fuel tanks, 44 water ballast bags, eight engine rooms, engines, transmissions, and water-recovery devices were placed along the lower keels. The inert gas helium was used instead of flammable hydrogen, which improved streamlining by allowing the engines to be safely placed inside the hull. A generator room, with two Westinghouse direct-current generators powered by a 30 hp internal combustion engine, was forward of the No. 7 engine room.
The main rings were spaced at and between each pair were three intermediate rings of lighter construction. In keeping with conventional practice, "station numbers" on the airship were measured in meters from zero at the rudder post, positive forward and negative aft. Thus, the tip of the tail was at station −23.75 and the nose mooring spindle was at station 210.75. Each ring frame formed a polygon with 36 corners, and these (and their associated longitudinal girders) were numbered from 1 (at the bottom center) to 18 (at the top center), port and starboard.Smith (1965). p. 191 Thus, a position on the hull could be referred to, for example, as "6 port at station 102.5" (the number-one engine room).
While Germany, France, and Britain used goldbeater's skin to gas-proof their gasbags, Akron used Goodyear Tire and Rubber's rubberized cotton, heavier but much cheaper and more durable. Half the gas cells used an experimental cotton-based fabric impregnated with a gelatin-latex compound. This was more expensive than the rubberized cotton, but lighter than goldbeater's skin. It was so successful that all the gasbags of Macon were made from it.Smith (1965). p. 196 The 12 gas cells were numbered 0 to XI, using Roman numerals and starting from the tail.Smith (1965). pp. 182 & 191 While the "air" volume of the hull was , the total volume of the gas cells at 100% fill was . At a normal 95% fill with helium of standard purity, the of gas would yield a gross lift of . Given a structure deadweight of ,Smith (1965). pp. 181 & 183 this gives a useful lift of available for fuel, lubricants, ballast, crew, supplies, and military load (including the skyhook airplanes)
Eight Maybach VL II gasoline engines were mounted inside the hull.Hook 1976. p. 47 Each engine turned a two-bladed, diameter, fixed pitch, wooden propeller via a driveshaft and bevel gearing, which allowed the propeller to swivel from the vertical plane to the horizontal.Summit Memory. U.S.S. Akron – Propeller. Retrieved 2008-07-22 With the engines' ability to reverse, this allowed thrust to be applied forward, aft, up, or down.Smith (1965). p. 193 From photographs, the four propellers on each side apparently were contrarotating, so it would appear that the designers were aware that running the propellers in the air disturbed by the one ahead was not ideal. While the external engine pods of other airships allowed the thrust lines to be staggered, placing all four engine rooms on each side of the ship along the lower keel resulted in Akrons propellers all being in line. This proved problematic in service, as it induced considerable vibration, which was especially noticeable in the emergency control position in the lower fin. By 1933, Akron had two of her propellers replaced by more advanced, ground-adjustable, three-bladed, metal propellers.Smith (1965). p. 75 These promised a performance increase and were adopted as standard for Macon.
The outer cover was of cotton cloth, treated with four coats of clear and two coats of aluminum-pigmented cellulose dope. The total area of the skin was and it weighed, after doping, .Smith (1965). p. 182
The prominent, dark, vertical bands on the hull were condensers of the system designed to recover water from the engines' exhaust for buoyancy compensation. In-flight fuel consumption continuously reduces an airship's weight and changes in the temperature of the lifting gas can do the same. Normally, expensive helium has to be released to compensate, so any way of avoiding this is desirable. In theory, a water-recovery system such as this can produce a unit by weight of ballast water for every unit of fuel burned, though this is unlikely to be achieved in practice.
Akron could carry up to of gasoline () in 110 separate tanks, which were distributed along the lower keels to preserve the ship's trim, giving her a normal range of at cruising speed.Smith (1965). pp. 180–183 Theoretical maximum ballast water capacity was in 44 bags, again distributed along her length, though normal ballast load at unmasting was . Maximum ballast was never an option, because a full fuel and ballast load would have left only lifting capacity for aircraft, crew, and supplies, and each fully loaded F9C fighter alone weighed .
The heart of the ship, and her sole reason for existing, was the airplane hangar and trapeze system. Aft of the control car, in bay VII, between frames 125 and 141.25, was a compartment large enough to accommodate up to five F9C Sparrowhawk airplanes. Two structural girders, though, partially obstructed Akrons aftmost hangar bays, limiting its capacity to three airplanes (one in each forward corner of the hangar and one on the trapeze). A modification to remove this design flaw was pending at the time of the ship's loss.Smith (1965). p. 67
The F9C was not the ideal choice, being designed as a "conventional" carrier-borne fighter. It was heavily built to withstand carrier landings, downward visibility was not very good, and it initially lacked an effective radio, but the primary role of Akrons airplanes was long-range naval scouting. What was actually needed was a stable, fast, lightweight scouting airplane with a long range,Smith (1965). pp. 27 & 201 but none existed capable of fitting between the structural members and into the airship's hangar, as the F9C could.
The trapeze was lowered through the T-shaped door in the bottom of the ship and into the slipstream, with an airplane attached to the crossbar by the skyhook above its top wing, with its pilot on board and its engine running. The pilot tripped the hook, and the airplane fell away from the ship. On his return, he positioned himself beneath the trapeze and climbed up until he could fly his skyhook onto the crossbar, at which point it automatically latched shut. Now, with the engine idling, the trapeze and airplane were raised into the hangar, the pilot cutting his engine as he passed through the door. Once inside, the airplane was transferred from the trapeze to a trolley, running on an overhead monorail system by which it could be shunted into one of the four corners of the hangar to be refueled and rearmed. Having a single trapeze raised two problems; it limited the rate at which airplanes could be launched and recovered, and any fault in the trapeze would leave any airborne scouts with nowhere to land. The solution was a second, fixed trapeze permanently rigged further aft along the bottom of the ship at station 102.5 and known as the "perch". By 1933, a perch was fitted and in use. Three more perches were planned (at stations 57.5, 80.0, and 147.5), but these were never fitted.
Akron revived an idea used, and eventually rejected, by the German Navy Zeppelins during World War I, the Spähkorb or "spy basket".Smith (1965). p. 55 The "angel basket" or "subcloud observation car" allowed the airship to remain hidden in a cloud layer, while still observing the enemy below. The small car, rather like an airplane fuselage without wings, could be lowered on a 1000-foot-long cable. The observer on board communicated with the ship by telephone. In practice, the device was unstable, almost looping over the airship during its only test flight.
During the design stage, in 1929, the Navy requested an alteration to the fins. Seeing the bottom of the lower fin from the control car was considered desirable. Charles E. Rosendahl had witnessed, from the control room, Graf Zeppelin almost snagging her fin on high-tension power lines during her heavy take-off into an unsuspected but very marked temperature inversion from Mines Field, Los Angeles, at the start of the last leg of her round-the-world flight earlier that year.Rosendahl (1932). pp. 194 et al The design change would also allow direct vision between the main control car and the emergency control position in the lower fin. The control car was moved aft and all the fins were shortened and deepened. The leading-edge root of the fins no longer coincided with a main (deep) ring, and instead, the foremost attachment was now to an intermediate ring at frame 28.75. This achieved the required visibility, improved low-speed controllability, due to the increased span of the control surfaces, and simplified stress calculations, by reducing the number of fin attachment points. The designers and the Navy's inspectors, led by the very experienced Charles P. Burgess, were entirely satisfied with the revised stress calculations. However, this alteration has been the subject of much criticism as an "inherent defect" in the design, and is often alleged to have been a major factor in the loss of Akrons sister ship Macon.Smith (1965). p. 197 Construction for both ships amounted to $8,800,000 (in 1931 dollars, approximately $150,000,000 when adjusted for inflation in 2026) with the Akron accounting $5,538,400 of the total.
Construction and commissioning