The Blimps Anti-Submarine Airships with Engines from Bristol

The ruthlessness of this new form of warfare was confirmed in May when the liner Lusitania was torpedoed and sunk by submarine U-20; 1,198 people died, nearly all civilians. By the end of the war almost 5,000 ships would have gone to the bottom.

One immediate response of the First Sea Lord, Admiral ‘Jackie’ Fisher, had been to call for a new type of small airship to patrol coastal waters. It should be simple as possible with a crew of two, capable of carrying wireless and a few small bombs, with a speed of 50 mph and a patrol endurance of 8 hours.  A prototype comprising the wingless fuselage of a BE.2c aeroplane suspended from the envelope of a small pre-war airship was flying within a month and met the basic requirements. Fisher saw it demonstrated and exclaimed, ‘Now I must have Forty!’

In fact, by the end of the war 158 of these ‘Submarine Scout’ or SS airships had been built. They were ‘non-rigids’.  Unlike Zeppelins, their hydrogen-filled envelopes had no internal framework. A tube behind the propeller took some of the slipstream to inflate two small bags or ‘ballonets’ inside the envelope, pressurising the hydrogen sufficiently to maintain the airship’s shape. Simple flap valves stopped the air escaping when the engine was idling but after a while the pressure would drop and the envelope go limp. Which is probably why they were always called ‘blimps’. 

The most numerous and most successful of the blimps was the SS Zero (SSZ) type. There were 77 of them and all were powered by engines built by Brazil Straker in Lodge Causeway, Fishponds. These Hawk engines were designed by Rolls-Royce in Derby but demand for their big Eagle engine meant they had no capacity to build the 200 Hawks on order. Brazil Straker was the only firm R-R would approve to build engines carrying their name. They would go on to build most of the R-R Falcon V-12s used in the famous Bristol Fighter.

The Hawk was almost ‘half a Falcon’ with six individual cylinders, each with a water-jacket of thin welded steel.  Nominally rated at 75 horsepower they actually produced over 90 hp at 1500 rev/min. They proved to be extremely reliable.  One pilot called it ‘The sweetest engine ever run – it only stops when switched off or out of petrol.’ This reliability reflected both Henry Royce’s meticulous design and the quality control of Roy Fedden at Fishponds. In 1920 Fedden would take his own designs to found the Engine Department of the Bristol Aeroplane Company, now part of Rolls-Royce.

The SSZ blimps were three-seaters. The pilot was in the middle of the ‘car’, sitting behind the observer who was also responsible for the wireless and the Lewis machine gun. The engineer sat directly in front of the rear-mounted engine which drove a pusher propeller. The car was sometimes called the gondola and it was rather like an open boat, wooden framed, skinned with 8-ply covered with aluminium. This allowed the airship to sit on the water – if the sea was calm.  The gondola was suspended by multiple ropes attached to the skin of the streamlined envelope which was over 143 feet long (43.7m). This was made from layers of rubberised fabric painted with four coats of dope and a top coat of aluminium varnish.

The envelope’s nominal volume was 70,000 cubic feet (1982 cu.m) so the hydrogen gas gave a gross lifting force of 2.2 tonnes. The complete airship weighed about 1.6 tonnes so with crew and fuel there was not much to spare for weapons. Besides the machine gun they normally carried just eight 16 lb (7 Kg) or two 65 lb (30 kg) bombs.  It is hardly surprising that there is no confirmed case of a U-boat being sunk by these, though the larger bombs carried by Coastal class airships may have had some success.  But the real purpose of the blimps was not to attack the submarines; it was to spot them and call surface vessels to the scene by wireless. Their Type 52 transmitters had a range of up to 60 miles, using Morse code. On seeing a blimp a U-boat would normally submerge and at this time their underwater speed and endurance was very limited. In the clear waters of the Mediterranean the airships could sometimes track a submarine cruising under water but in UK coastal waters this was rarely possible. However, they could often spot the wake of a raised periscope.

There were a dozen airship stations around the coasts of Britain stretching from Aberdeen south to Mullion in Cornwall.  These in turn had sub-stations where airships could land and perhaps operate from temporarily. Facilities at the sub-stations varied from simple tie-down points in a field, manned by a small crew with a few hydrogen cylinders, as at Bude, to sites with a canvas-covered hangar as at Bentra in Northern Ireland. These multiple stations meant that by the end of 1917 airships could escort convoys from beyond the Scilly Isles up the Channel, or up the Irish Sea, and far out across the North Sea.

The blimps’ pilots were generally young junior officers. Tom Elmhirst, for example, was 19 and had just been promoted from Midshipman to sub-Lieutenant. (He would end his career as Air Marshal Sir Thomas.)  Sorties might vary from two hours escorting the regular ferry from Larne to Stranraer to long days patrolling the open sea in the cramped gondola, buffeted by the continuous 45 mph slipstream, eyes straining to glimpse a distant wake.  The average duration of patrols was some 8 hours but there were several instances exceeding 24 hours and the record, set by SSZ 39, was 50 hours and 55 minutes.

The peak of the U-boats’ success was in April 1917 when they sank 880,000 tons of shipping. They were seriously threatening to starve Britain out.  The adoption of convoys in May and the increasing availability of aircraft and the SSZ blimps during 1917 transformed the situation. Losses in April 1918 were less than a third those of the previous year. In 1918, it is said, convoys with an air escort did not lose a single ship while across all oceans almost half the U-boat fleet (69 from 142) was sunk.

In 1919 George Whale wrote, in the style of the age, that: ‘Although small, the (SS) Zero airship has been one of the successes of the War and we can claim proudly that she is entirely a British product.’ Hopefully it’s not too jingoistic to feel some local pride in Bristol’s contribution to their success.