The Battle of the Atlantic was a crucial element of the Second World War, in which Britain sought to protect its shipping from the German U-boat threat. The development of increasingly capable diesel-electric submarines had been rapid, in particular the elimination of oxygen restrictions that had previously limited underwater endurance via the use of a snorkel to eliminate the need for surfacing when recharging a vessel's batteries. Aircraft that had once been highly effective submarine-killers had very quickly become incapable in the face of these advances. In addition, lend-leased aircraft such as the Consolidated B-24 Liberator had been returned following the end of hostilities. Several Avro Lancasters had undergone rapid conversion - designated as Maritime Reconnaissance Mk 3 (MR3) - as a stop-gap measure for maritime search and rescue and general reconnaissance duties; however, RAF Coastal Command had diminished to only a third of its size immediately prior to the Second World War.
In the emerging climate of the Cold War and the potential requirement to guard the North Atlantic from an anticipated rapid expansion of the Soviet Navy's submarine force, a new aerial platform to perform the anti-submarine mission was required. Work had begun on the requirement for a new maritime patrol aircraft in 1944, at which point there had been an emphasis for long range platforms for Far East operations; however, with the early end of the war in the Pacific, the requirement was refined considerably. In late 1945, the Air Staff had expressed interest in a conversion of the Avro Lincoln as general reconnaissance and air/sea rescue aircraft; they formalised their requirements for such an aircraft under Air Ministry specification R.5/46. Avro's Chief Designer Roy Chadwick initially led the effort to build an aircraft to this requirement, designated as the Avro Type 696.
The Type 696 was a significant development upon the Lincoln. Elements of the Avro Tudor airliner were also reused in the design; Lincoln and Tudor had been derivatives of the successful wartime Avro Lancaster bomber. Crucially, the new aircraft was to be capable of a 3,000 nautical mile range while carrying up to 6,000 lb of weapons and equipment. In addition to featuring a large amount of electronic equipment, the Type 696 had a much improved crew environment over other aircraft types to allow them to be more effective during the lengthy mission times anticipated. At one stage during development, the Type 696 was referred to as the Lincoln ASR.3 before this was discarded in favour of the Shackleton name.
The first test flight of the prototype Shackleton GR.1, serial VW135, was made on 9 March 1949 from the manufacturer's airfield at Woodford, Cheshire in the hands of Avro's Chief Test Pilot J.H. "Jimmy" Orrell. The GR.1 was later re-designated "Maritime Reconnaissance Mark I" (MR 1). The prototype differed from subsequent production Shackletons in a number of areas; it featured a number of turrets and was equipped for air-to-air refuelling using the looped-line method. These did not feature on production aircraft due to judgments of ineffectiveness or performance difficulties incurred. However, the performance of the prototype had been such that, in addition to the go-ahead for the MR1's production, a specification for improved variant was issued in December 1949, before the first production Shackleton had even flown. By 1951, the MR1 had become officially considered as an interim type due to several shortcomings.
The MR 2 was an improved version of the Shackleton, featuring numerous refinements that had been proposed for the MR1. The radar was upgraded to ASV Mk 13, and the radome relocated from the aircraft's nose to a ventral position aft of the bomb bay, the radome was retractable and could only be fully extended with the bomb bay doors open. It had improved all-round radar coverage and minimised the risk of bird-strikes. Both the nose and tail section were lengthened, the tailplane was redesigned, the undercarriage was strengthened and twin-retractable tail wheels were fitted. The dorsal turret was initially retained, but was later removed from all aircraft after delivery. The prototype, VW 126, was modified as an aerodynamic prototype at the end of 1950 and first flew with the MR 2 modification on 19 July 1951.
VW 126 was tested at Boscombe Down in August 1951, particular attention was paid to changes made to improve its ground handling, like the addition of toe-brakes and a lockable-rudder system. One production Mk 1 aircraft was modified on the line at Woodford with the Mk 2 changes and first flew on 17 June 1952. After trials were successful, it was decided to complete the last ten aircraft being built under the Mk 1 contract to MR 2 standard and further orders were placed for new aircraft. 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. Production of the MR 2 ended in May 1954.
The Type 716 Shackleton MR 3 was another redesign in response to crew feedback and observations. 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. To facilitate 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 so 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 Type 719 Shackleton IV, later known as the MR 4, was a projected variant intended to meet a Canadian requirement for a long range patrol aircraft. The MR 4 would have been a practically new aircraft, sharing only the nose, cockpit, and outer wings with earlier variants; it would have also been powered by the Napier Nomad compound engine. The Shackleton IV was cancelled in 1955.
In 1967, ten MR 2s were modified as training aircraft to replace the T 4 in-service with the Maritime Operational Training Unit; known as T 2s, the crew rest areas were replaced by additional radar equipment and the original radar fittings removed.
The Shackleton was a purpose-built aircraft for the maritime patrol role; however, the legacy of Avro's preceding aircraft is present in many aspects of the overall design. The centre section of the Shackleton's wing originates from the Lincoln, while the outer wing and undercarriage were sourced from the Tudor outer wings; at one stage during development, the tail plane had closely resembled the Lincoln's, but were enlarged and changed soon after. An entirely new fuselage was adopted, being wider and deeper to provide a large space in which to accommodate the crew, their equipment, and a large bomb-bay. Later variants of the Shackleton were substantially redesigned, adopting a new nose-wheel undercarriage, redesigned wings and centre-section, and a larger fuel capacity for more range.
Avro 696 Shackleton prototypes
Three prototype Type 696s were ordered in May 1947 to meet specification R 5/46:
Avro 696 Shackleton Mk.1
- Shackleton MR.Mk.1
- Shackleton MR.Mk.1A
- Shackleton T.4
Avro 696 Shackleton Mk.2
- Shackleton MR.Mk.2 Phase 1 or MR.Mk.2C
- Shackleton MR.Mk.2 Phase 2
- Shackleton MR.Mk.2 Phase 3
Avro 716 Shackleton Mk.3
The aircraft underwent several phased modifications:
- Shackleton MR.Mk.3 Phase 1
- Shackleton MR.Mk.3 Phase 2
- Shackleton MR.Mk.3 Phase 3
- Avro 717 and 719 Shackleton MR.Mk.4
- Shackleton MR.Mk.5