Avro Shackleton - Design and Development

Design and Development

The aircraft was designed by Roy Chadwick as the Avro Type 696. It was based on the Lincoln bomber and Tudor airliner, both derivatives of the successful wartime Lancaster heavy bomber, one of Chadwick's earlier designs which was the then current ASW aircraft. The design took the Lincoln's centre wing and tail, Tudor outer wings and landing gear and a new wider and deeper fuselage, powered by four Rolls Royce Merlin engines. It was initially referred to during development as the Lincoln ASR.3.

The design was accepted with Air Ministry specification R.5/46 written around it. The tail as adopted differed from the Lincoln. The Merlin engines were replaced with the larger more powerful and slower-revving Rolls-Royce Griffons with 13 ft (4 m)-diameter contra-rotating propellers, creating a distinctive engine noise and adding high-tone deafness to the hazards of the pilots due to their positioning in relation to the cockpit. The Griffons were necessary because of the greater weight and drag over the Lincoln and they provided equivalent power to the Merlins but at lower engine speed, which made for greater fuel efficiency for the long periods in the denser air at low altitudes that the Shackleton was intended for when hunting submarines – known as "loitering" in RAF parlance – possibly several hours at around 500 feet or lower. This also made for less stress and wear, and hence reliability problems, for the engines; if Merlins had been used they would have been needed to run at comparatively high power settings for hours at a time once a submarine contact had been detected. Using conventional propellers would have needed an increase in propeller diameter to absorb the increased power and torque of the Griffons, which was not possible due to limitations in undercarriage length and engine nacelle positioning of the Lincoln wing; the contra-rotating propellers gave greater blade area within the same overall propeller diameter.

When the Shackleton was being designed the "Battle of the Atlantic" was still being fought and all possible submarine targets (German U-boats) were diesel-electric powered types and had very limited underwater endurance, the time underwater being limited by both the air available for the crew to breathe, and the battery power remaining to drive the submarine's underwater electric motors. While submerged it was incapable of travelling any great distance away from where it was detected. Then any aircraft could call up friendly Convoy Escort surface ships who would then deal with a submerged target in the normal way, with depth charges aimed using their own ASDIC sets. Hence for the Shackleton endurance in terms of the length of time it could spend in the air – as opposed to all-out range – was of prime importance. Once a submarine had been detected it might be necessary for the aircraft to remain over the last sighted position of the submarine all day (or night), preventing it from surfacing and making away at its higher surface speed. All the time the submarine was prevented from surfacing the crew's breathable air was being exhausted, and the batteries were consuming power; eventually the submarine would be forced to come up for air. It could then be attacked by the aircraft itself, again, this time using its own air-dropped depth charges.

In the case of a submarine recharging its batteries and replenishing its air while submerged using a snorkel, this could be detected by the aircraft using ASV radar, and the submarine attacked as normal, with the added benefit that in visual conditions, location of the German snorkel mast made the submarine's underwater position obvious, aiding depth charge aiming.

The Avro design was ordered to Air Ministry specification R.5/46 as a replacement for the long range Liberator.

The first test flight of the prototype Shackleton GR.1, serial VW135, was on 9 March 1949 at the hands of Avro's Chief Test Pilot J.H. "Jimmy" Orrell. In the ASW role, the Shackleton carried both types of sonobuoy, Electronic warfare support measures, an Autolycus diesel fume detection system and for a short time an unreliable magnetic anomaly detector (MAD) system. Weapons were nine bombs, or three torpedoes or depth-charges, and two 20 mm cannon in a Bristol dorsal turret. The GR.1 was later re-designated "Maritime Reconnaisance Mark I" – MR.1.

The MR.2 was an improved design incorporating feedback from operations and is considered by aficionados to be the definitive type. The radome was moved from the nose to a ventral position and was retractable, to improve all-round coverage and minimise the risk of bird-strikes. The radar was upgraded to ASV Mk 13. Both the nose and tail sections were lengthened, the tailplanes were redesigned and the undercarriage was strengthened. The dorsal turret was initially retained, but was later removed from all aircraft after delivery. In order to keep pace with changing submarine threats, the Mk 2 force was progressively upgraded, with Phase I, II and III modifications introducing improved radar, weapons and other systems, as well as structural work to increase fatigue life.

The Avro Type 716 Shackleton MR.3 was another redesign in response to crew complaints. A new tricycle undercarriage was introduced, the fuselage was increased in all main dimensions and had new wings with better ailerons and tip tanks. The weapons capability was also upgraded to include homing torpedoes and Mk 101 Lulu nuclear depth bombs. As a sop to the crews on 15-hour flights, the sound deadening was improved and a proper galley and sleeping space were included. Due to these upgrades the take-off weight of the RAF's MR.3s had risen by over 30,000 lb (13,600 kg) (Ph. III) and assistance from Armstrong Siddeley Viper Mk.203 turbojets was needed on take-off (JATO). This extra strain took a toll on the airframe, and flight life of the RAF MR.3s was sufficiently reduced that they were outlived by the MR.2s. Due to the arms embargo against South Africa the SAAF's MR.3s never received these upgrades but were maintained independently by the SAAF.

The Avro Type 719 Shackleton IV, later known as the MR.4, was a projected variant using the extremely fuel efficient Napier Nomad compound engine. The Shackleton IV was cancelled in 1955.