R101 - Design and Development

Design and Development

The whole airship programme was under the direction of the Director of Airship Development (DAD), Group Captain Peregrine Fellowes, with Colmore acting as his deputy. Lieutenant-Colonel Richmond was appointed Director of Design: later he was credited as "Assistant Director of Airship Development (Technical)" with Squadron Leader Michael Rope as his assistant, and the Director for Flying and Training, responsible for all operational matters for both airships, was Major G.H. Scott, who had developed the design of the mooring masts that were to be built. It was based at the Royal Airship Factory at Cardington, Bedfordshire, which had been built by Shorts during the First World War and had been employed by the Admiralty to copy and improve on the latest German designs from captured rigid airships. It had been nationalised in 1919 but after the loss of the R38 (then in the process of being transferred to the US as ZR2) naval airship development was stopped and it had been placed on a care and maintenance basis.

R101 was to be built only after an extensive research and test programme was complete. This was carried out by the National Physical Laboratory (NPL). As part of this programme, the Air Ministry funded the costs of refurbishing and flying R33 in order to gather data about structural loads and the airflow around a large airship. These data were also made available to Vickers; both airships had the same elongated tear-drop shape, unlike previous airship designs. This shape had been found to produce the minimum amount of drag. Safety was a primary concern and this would have an important influence on the choice of engines.

An early decision had been made to construct the primary structure largely from stainless steel rather than lightweight alloys such as duralumin. The design of the primary structure was shared between Cardington and the aircraft manufacturer Boulton and Paul, who had extensive experience in the use of steel and had developed innovative techniques for forming steel strip into structural sections. Working to an outline design prepared with the help of data supplied by the NPL, the stress calculations were performed by Cardington. This information was then supplied to J. D. North and his team at Boulton and Paul, who designed the actual metalwork. The individual girders were fabricated by Boulton and Paul in Norwich and transported to Cardington where they were bolted together. This scheme for a prefabricated structure entailed demanding manufacturing tolerances and was entirely successful, as the ease with which R101 was eventually extended bears witness. Before any contracts for the metalwork were signed, an entire bay consisting of a pair of the 15-sided transverse ring frames and the connecting longitudinal girders was assembled at Cardington. After the assembly had passed loading tests, the individual girders were then tested to destruction. The structure of the airframe was innovative: the ring-shaped transverse frames of previous airships had been braced by radial wires meeting at a central hub, but no such bracing was used in R101, the frames being stiff enough in themselves. However, this resulted in the structure extending further into the envelope, thereby limiting the size of the gasbags.

The specifications drawn up in 1924 by the Committee for the Safety of Airships in 1924 had based weight estimates on the then existing rules for airframe strengths. However, the Air Ministry Inspectorate introduced a new set of rules for airship safety standards in late 1924 and compliance with these as-yet unformulated rules had been explicitly mentioned in the individual specifications for each airship. These new rules called for all lifting loads to be transmitted directly to the transverse frames rather than being taken via the longitudinal girders. The intention behind this ruling was to enable the stressing of the framework to be fully calculated, rather than relying on empirically accumulated data, as was contemporary practice at the Zeppelin design office. Apart from the implications for the airframe weight, one effect of these regulations was to force both teams to contrive a new system of harnessing the gasbags. The patented "parachute" gasbag harnessing, designed by Michael Rope, proved less than satisfactory, permitting the gas bags to surge unduly, particularly in rough weather. This caused the gasbags to chafe against the structure, causing holes in the fabric. Another effect was that both R100 and R101 employed a relatively small number of longitudinal girders, in order to simplify the stressing calculations.

R101 used pre-doped linen panels for much of its covering, rather than lacing the panels into place and then applying dope to shrink it. In order to reduce the area of unsupported fabric in the covering R101 alternated the main longitudinals with non-structural "reefing booms" mounted on kingposts which were adjustable using screw-jacks in order to tension the covering. The pre-doped fabric proved unsatisfactory from the start, with panels splitting because of humidity changes before the airship had even left its shed.

There were other innovative design features. Previously ballast containers had been made in the form of a leather "trousers", and one or other leg could be opened at the bottom by a cable-release from the control car. In R101, the extreme forward and aft ballast bags were of this type, and were locally operated, but the main ballast was held in tanks connected by pipes so that ballast could be transferred from one to another to alter the airship's trim using compressed air. The arrangement for ventilating the interior of the envelope, necessary both to prevent any buildup of escaped hydrogen and also to equalise pressure between the outside and inside, was also innovative. A series of flap-valves were situated at the nose and stern of the airship cover (those at the nose are clearly visible in photographs) to allow air to enter when the airship was descending, while a series of vents was arranged around the circumference amidships to allow air to exit during ascent.