The elegant - though highly lethal - de Havilland DH.108 "Swallow" was one of the more important research aircraft for British aviation during the 1940s and early 1950s. Not designed nor developed to any specific military fighter requirement, the aircraft actually existed researching supersonic flight and sweptback wings in regards to the de Havilland DH.106 "Comet" jet passenger airliner program. The Comet became the world's first jet airliner but its own issues limited its overall reach - allowing competitors to claim a large portion of the market share. The research-oriented DH.108 was, itself, a dangerous plane to fly for it claimed the lives of three test pilots in separate aircraft losses, forever staining the Swallow program. Despite the unfortunate setbacks, the DH.108 proved critical in advancing British understanding of swept-back wings, jet-powered flight, and supersonic speeds and control.
The Comet airliner was a program being championed by British authorities in the latter years of World War 2 in an effort provide local aero industry with commitments well after the fighting had ceased. The Comet would be a passenger-minded, turbojet-powered design utilizing a tailless configuration and an advanced sweptback monoplane wing assembly. The charge fell to the storied concern of de Havilland which used its DH.108 platform as the starting point for the finalized research aircraft. The dimensionally smaller, single-seat airframe would mimic many of the qualities to be found in the full-sized Comet and serve to test the various control conditions at supersonic speeds. The Air Ministry requested a pair of prototypes while the name of "Swallow" was only attached to the aircraft by the Ministry of Supply and never official adopted.
The de Havilland DH.108 research aircraft was formed from the body of the existing de Havilland DH.100 "Vampire" (F.Mk 1model), a jet-powered, single-seat fighter-bomber seeing service entry in 1946. The aircraft featured a centralized nacelle housing cockpit and powerplant with a monoplane wing arrangement held to either side of the fuselage. A twin-boom configuration was used to make up the tail section and this produced a twin vertical tail fin arrangement which attached via a single horizontal tailplane assembly between them. When adopted into service, the Vampire became Britain's second jet-powered fighter following the classic Gloster "Meteor".
The choice to modify an existing aircraft expedited the development process. The Vampire F.Mk 1 saw its twin tailbooms deleted which also meant that the vertical tail fins and stabilizer were lost. The cockpit nacelle and engine installation remained as in the original though the nacelle was lengthened some to accommodate a new single vertical tail fin mounted over the jetpipe. The wings were the true addition, specially-designed assemblies which featured 43-degree sweepback and installed in place of the original straight wings of the Vampire. The first aircraft in the Swallow line was known as "TG283" and recorded its first flight on May 15th, 1946 with Geoffrey de Havilland, Jr. - son of the famous company founder - at the controls. Power was from a de Havilland Goblin 2 engine of 3,100lb thrust.
Additional testing revealed a slew of issues under specific conditions, proving the aircraft a handful for even the most veteran of pilots. To help offset difficulties in ground running and landing, the undercarriage of the "Sea Vampire" - the navalized variant of the land-based Vampire - was installed. Its career in the air war short-lived for it was during a stall trial on May 1st, 1950 that the aircraft entered into a spin and was lost, killing the test pilot. During its time aloft, TG283 managed a maximum speed of approximately 350 miles per hour.
TG306 became the second aircraft of the triad and this version introduced retractable slats over the fixed, wooden Handley-Page slats featured on TG283. Power was served through a de Havilland Goblin 4 turbojet of 3,500lbs thrust. Its first flight came during June of 1946 and improved performance led to the aircraft attempting to break the World Air Speed record (held by the competing Gloster Meteor at 616mph). TG283 was met with a few adjustments for the attempt but a trial flight prior ended with the aircraft losing control once more, breaking up in mid-flight, and killing pilot Geoffrey de Havilland, Jr. on September, 27th, 1946. During its time in the air, TG283 recorded a maximum speed of 580 miles per hour - though this achieved in a dive.
The DH.108 program then introduced the final flying machine as VW120 - largely influenced by the TG306 model and its issues. First flight was on July 24th, 1947 and the design now boasted an ejection seat for the pilot and a revised cockpit and forward fuselage. Additional strengthening was added to the structure to help avoid the pitfalls of the previous mark in the series. Power-assisted wing surfaces and controls attempted to take some of the fatigue from the pilot. Power was now from a de Havilland Goblin 5 turbojet engine of 3,600lbs thrust. The aircraft went on to become the first British-designed aircraft to break the sound barrier, this accomplished during a steep dive on September 6th, 1948 and resulting in the official speed of Mach 1.02 being recorded. During the dive, the pilot lost control of the aircraft but was able to reclaim her.
It was during another flight on February 15th, 1950 that VW120 was lost when it disintegrated in-flight. Despite access to an ejection seat, the pilot was lost with the aircraft and such ended the tumultuous DH.108 research program. The aircraft proved just as lethal as it was beautiful by 1940s standards, showcasing clean lines and a certain simplicity about her. While the Dh.106 Comet airliner was eventually brought online, it too suffered a stained record with the in-flight disintegrations of three aircraft. The losses and publicity no doubt damaged sales and companies like Boeing were all to ready to take up the market share. Despite this, the Comet - introduced in 1952 - managed a long service life if only through the 114 total examples completed. These served with a handful of carriers with the final Comet retired in March of 1997. The Comet also made up the basis of the RAF's Hawker Siddeley "Nimrod" maritime patrol platform - which itself was not retired until 2011.
The finalized Comet form was completed with a conventional tail unit fitting a sole vertical tail fin and low-set horizontal planes. It did, however, sport sweptback wing surfaces all the main and tailplane units and its engine intakes were similarly placed as on the Swallow design, well-contoured at the wingroots to appear as part of the wing assembly itself.