The Avro 720 was an in-development
British
British may refer to:
Peoples, culture, and language
* British people, nationals or natives of the United Kingdom, British Overseas Territories, and Crown Dependencies.
** Britishness, the British identity and common culture
* British English, ...
single-seat
interceptor
Interceptor may refer to:
Vehicles
* Interceptor aircraft (or simply "interceptor"), a type of point defense fighter aircraft designed specifically to intercept and destroy enemy aircraft
* Ford Crown Victoria Police Interceptor, a police car
* ...
of the 1950s. It was designed and being developed by
Avro in competition with the
Saunders-Roe
Saunders-Roe Limited, also known as Saro, was a British aero- and marine-engineering company based at Columbine Works, East Cowes, Isle of Wight.
History
The name was adopted in 1929 after Alliott Verdon Roe (see Avro) and John Lord took a c ...
-built
SR.53. While at least one prototype was partially-constructed, the order for the Avro 720, and quickly thereafter the project entirely, was terminated prior to any aircraft having been completed.
The Avro 720 was intended to have been a high-performance aircraft that would have utilised mixed propulsion to accomplish this, using a
rocket
A rocket (from it, rocchetto, , bobbin/spool) is a vehicle that uses jet propulsion to accelerate without using the surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entirely fr ...
engine to achieve quick acceleration and a high top speed while a more compact
jet engine would have been used during more mundane cruising flight. The termination of the aircraft had been partially due to its choice in powerplant, Avro having opted to utilise the in-development
Armstrong Siddeley Screamer rocket engine, which used
liquid oxygen
Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an app ...
as
oxidant
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
and
kerosene
Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regi ...
fuel; major questions were posed over the oxidant's practicality under operational situations.
Development
Origins
The
Second World War
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposi ...
had demonstrated the importance of
strategic bombing
Strategic bombing is a military strategy used in total war with the goal of defeating the enemy by destroying its morale, its economic ability to produce and transport materiel to the theatres of military operations, or both. It is a systematica ...
to modern warfare, and as the emerging
Cold War developed, the development of new and more effective air defences against large waves of hostile
bomber aircraft
A bomber is a military combat aircraft designed to attack ground and naval targets by dropping air-to-ground weaponry (such as bombs), launching torpedoes, or deploying air-launched cruise missiles. The first use of bombs dropped from an airc ...
armed with
nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...
s became a priority for many nations. During the war,
Nazi Germany
Nazi Germany (lit. "National Socialist State"), ' (lit. "Nazi State") for short; also ' (lit. "National Socialist Germany") (officially known as the German Reich from 1933 until 1943, and the Greater German Reich from 1943 to 1945) was ...
had extensively developed its own rocket-powered aircraft to augment its interception capabilities, in the final two years of the war, it had been able to deploy aircraft such as the
Messerschmitt Me 163
The Messerschmitt Me 163 Komet is a rocket-powered interceptor aircraft primarily designed and produced by the German aircraft manufacturer Messerschmitt. It is the only operational rocket-powered fighter aircraft in history as well as ...
(and design
Bachem Ba 349
The Bachem Ba 349 Natter ( en, Colubrid, grass-snake) was a World War II German point-defence rocket-powered interceptor, which was to be used in a very similar way to a manned surface-to-air missile. After a vertical take-off, which eliminate ...
) which, by using rocket propulsion, had been capable of unparalleled rates-of-climb, enabling them to (at least in theory) rapidly sortie to intercept enemy bombers before they reached their targets. As the performance of these aircraft had become increasingly well known to the Allies, RAF experts were eager to explore and understand the underlying technology.
[Wood 1975, p. 53.]
In the aftermath of the war, German rocket technology was studied extensively by various members of the former Allied nations. Britain had quickly opted to commence a programme to develop
liquid-propellant rockets with the aim of boosting aircraft during the take-off phase, known within the RAF as
rocket-assisted take-off gear (RATOG), and during the climb-to-altitude phase of flight.
In 1946, work began on a pair of new British-built
rocket motor
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordan ...
s, the
de Havilland Sprite
The de Havilland Sprite is a British rocket engine that was built by de Havilland in the early-1950s for use in RATO (Rocket-assisted take off) applications. A developed engine with slightly less thrust but a longer burn time was known as the ...
(5,000 lb thrust) and the
Armstrong Siddeley Snarler
The Armstrong Siddeley Snarler was a small rocket engine used for mixed-power experiments with an early turbojet engine. and was the first British liquid-fuelled rocket engine to fly.
Design and development
Unlike other British rocket engin ...
(2,000 lb thrust); these rocket motors made use of different propellants, the Sprite used a
high-test peroxide (HTP)
monopropellant
Monopropellants are propellants consisting of chemicals that release energy through exothermic chemical decomposition. The molecular bond energy of the monopropellant is released usually through use of a catalyst. This can be contrasted with bipro ...
while the Snarler harnessed a
methanol/water/
liquid oxygen
Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an app ...
mix. During the early 1950s, both engines proceeded to the flight testing phase; however, some of the demand for their role to provide fighters with increased performance was soon being met by the increasing prevalence of conventional jet engines being equipped with
reheat
An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and combat ...
instead.
In May 1951, faced with reports on the increasingly potential capability of, and thus the threat posed by, the growing
Soviet
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nation ...
strategic bomber fleet and that nation's newly developed
atomic weapons
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...
, the British
Air Ministry
The Air Ministry was a department of the Government of the United Kingdom with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964. It was under the political authority of the Secretary of State ...
proceeded to draft an
Operational Requirement
An Operational Requirement, commonly abbreviated OR, was a United Kingdom (UK) Air Ministry document setting out the required characteristics for a future (i.e., as-yet unbuilt) military aircraft or weapon system.
The numbered OR would describe ...
,
OR 301, which sought a rocket-powered interceptor that could attain an altitude of 60,000 ft (18,300 m) in just 2 minutes 30 seconds. Many of the performance requirement laid out by OR 301 was due to the anticipation of rapid increases in performance by the opposing Soviet aircraft; by the early 1960s, it was suspected that these bombers might well be capable of
supersonic speeds of up to
Mach 2 with a potential operational altitude as high as 80,000 ft.
[Wood 1975, pp. 53–54.] Avro's design team promptly set about designing an suitable interceptor that conformed with the specification's requirements, which would result in the Avro 720 proposal. In addition, various other proposals from a number of companies were received, including
Bristol
Bristol () is a city, ceremonial county and unitary authority in England. Situated on the River Avon, it is bordered by the ceremonial counties of Gloucestershire to the north and Somerset to the south. Bristol is the most populous city in ...
,
Blackburn
Blackburn () is an industrial town and the administrative centre of the Blackburn with Darwen borough in Lancashire, England. The town is north of the West Pennine Moors on the southern edge of the Ribble Valley, east of Preston and north-n ...
,
Shorts
Shorts are a garment worn over the pelvic area, circling the waist and splitting to cover the upper part of the legs, sometimes extending down to the knees but not covering the entire length of the leg. They are called "shorts" because they ...
and
Saunders-Roe
Saunders-Roe Limited, also known as Saro, was a British aero- and marine-engineering company based at Columbine Works, East Cowes, Isle of Wight.
History
The name was adopted in 1929 after Alliott Verdon Roe (see Avro) and John Lord took a c ...
.
Accordingly, the specification was aimed at providing a capable defence interceptor that could form part of the nation's measures to counter this threat would need to be capable of similar speeds and an exceptionally high rate of climb in order to reach high altitude bombers in time.
[Wood 1975, pp. 54–55.] Conceptually, the envisioned aircraft was to be operated in a similar fashion to that of the German Me 163, it would use its rocket engine to climb rapidly to meet and engage its target prior to gliding back to earth in a fuel-exhausted state before landing upon a retractable skid arrangement. According to aviation author Derek Wood, Saunders-Roe had quickly identified that the unpowered 'glide home' return flight could be a source of danger and expense, and thus approached the Air Ministry to discuss their concept for the adoption of a secondary jet engine, which would be used for the purposes of powering the interceptor's journey home. The Ministry was enthusiastic on this concept; in May 1951, all interested companies were asked to examine this arrangement.
[Wood 1975, p. 55.]
Revised specification and selection
If designed strictly to deliver the performance required by the original specification, the interceptor would have been forced to glide back to ground from heights of up to 100,000 ft (30,500 m), perform an unpowered landing many miles away, after which it would then have to be recovered and taken back to the airfield by ground vehicle. Revised specifications from the Ministry were issued to Avro and Saunders-Roe, under which the auxiliary
turbojet
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, an ...
engine had become an official and explicitly-stated requirement. The auxiliary engine was required to provide sufficient power to allow the interceptor to fly back to its base in the aftermath of a combat mission.
[Jones ''Aeroplane Monthly'' November 1994, pp. 32–33.][Mason 1992, p. 400.] On 12 December 1952, further refinement of the rocket interceptor concept had led to the release of the defined
Specification OR 337.
[London 2010, p. 31.] The changes contained within the defined specification mainly revolved around armament changes, such as the adoption of the
Blue Jay (as it was codenamed at the time)
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
-guided
air-to-air missile under development, which replaced the originally-envisaged retractable battery of 2-inch rockets.
[Wood 1975, p. 56.]
On 5 May 1953, an advisory design conference was hosted at the Ministry of Supply, which focused on the Saunders-Roe submission; three days later, a formal contract for the production of three prototypes was received by Saunders-Roe.
However, due to doubts within the RAF and the Ministry over the correct fuel/motor combination to select for the interceptor intended to meet the Specification, it was decided to issue a modified specification, which was later led to a development contract, for the competing Avro 720 aircraft as well. Of the six companies that decided to tender proposals, two were selected for development contracts: Avro with their Avro 720 and Saunders-Roe with the SR.53.
[Wood 1975, pp. 56–57.]
In response, a pair of prototypes were ordered by the Ministry.
In December 1956, it was reported that the Screamer engine, what had been intended to power the Avro 720, successfully completed flight clearance tests.
[''Flight'' 27 July 1956, p. 164.] Early development work on the Avro 720 was also proceeding smoothly; by 1956, the first prototype was claimed to have been virtually completed and that Avro viewed it as being capable of flying up to a year ahead of the rival SR.53.
[Wood 1975, pp. 60–61.] However, neither development programmes had managed to secure the backing of the
Royal Air Force
The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) an ...
(RAF), which had been showing signs of hesitancy and had apparent intentions to wait until after both aircraft flight evaluations had been conducted before the service would expressly make any determination on its preference.
[Wood 1975, p. 57.]
From September 1953 onwards, both the Avro 720 and the SR.53 projects were subject to increasing levels of scrutiny amid an overall requirement within the Ministry to implement cuts in order to reduce costs.
However, the Avro 720 was dealt an effective death blow when the Screamer rocket engine which was to power the interceptor was cancelled during late 1956. Reportedly, official concerns regarding the practicality of using liquid oxygen, which boils at -183 °C (90 K) and is a
fire hazard
Fire safety is the set of practices intended to reduce the destruction caused by fire. Fire safety measures include those that are intended to prevent the ignition of an uncontrolled fire and those that are used to limit the development and ef ...
, within an operational environment had heavily contributed to the Screamer's cancellation. The loss of the main engine in turn ultimately led to the termination of the Avro 720.
[London 2010, p. 30.][''Flight'' 24 May 1957, pp. 698–699.] One of the reasons for the Ministry's preferment of the SR.53, despite its development programme being comparatively behind and having suffered greater delays, was its rocket engine's use of hydrogen peroxide as an oxidiser had been viewed as less problematic than the Screamer's liquid oxygen oxidiser, and the Ministry did not want to support two separate rocket fuel programmes.
[Wood 1975, p. 61.]
At the time of cancellation, a single structural test airframe was partially complete. According to aviation author Barry Jones, photographs of the structural airframe, with the port wing fitted and the
serial number ''XD696'' painted on, have sometimes been mistakenly claimed to have been of the first prototype.
The Avro 720 had reportedly cost £1 million by the time of cancellation, while its Screamer powerplant cost a further £0.65 million.
[''Flight'' 17 August 1967, p. 262.]
Design
The Avro Type 720 was a small tailless
delta-wing
A delta wing is a wing shaped in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta (Δ).
Although long studied, it did not find significant applications until the Jet Age, when it proved suitab ...
ed aircraft. It was constructed of metal
honeycomb
A honeycomb is a mass of hexagonal prismatic wax cells built by honey bees in their nests to contain their larvae and stores of honey and pollen.
Beekeepers may remove the entire honeycomb to harvest honey. Honey bees consume about of honey ...
sandwich
A sandwich is a food typically consisting of vegetables, sliced cheese or meat, placed on or between slices of bread, or more generally any dish wherein bread serves as a container or wrapper for another food type. The sandwich began as a po ...
. The Avro 720's main power-plant was an 8,000 lbf (36 kN)
Armstrong Siddeley Screamer rocket engine, using
liquid oxygen
Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an app ...
as
oxidant
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
and
kerosene
Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regi ...
fuel. This differed from the competing
Saunders-Roe SR.53
The Saunders-Roe SR.53 was a British prototype interceptor aircraft of mixed jet and rocket propulsion developed for the Royal Air Force (RAF) by Saunders-Roe in the early 1950s. As envisaged, the SR.53 would have been used as an interceptor a ...
, which used a
de Havilland Spectre
The de Havilland Spectre is a rocket engine that was built by the de Havilland Engine Company in the 1950s. It was one element of the intended mixed power-plant for combination rocket-jet interceptor aircraft of the Royal Air Force, such as t ...
rocket engine which was fuelled by kerosene with
Hydrogen Peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
oxidiser. Both types were equipped with a single
Armstrong-Siddeley Viper
The Armstrong Siddeley Viper is a British turbojet engine developed and produced by Armstrong Siddeley and then by its successor companies Bristol Siddeley and Rolls-Royce Limited. It entered service in 1953 and remained in use with the Royal ...
as the auxiliary
turbojet
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, an ...
engine; on the Avro design, airflow for the Viper engine was provided via a small chin inlet installed under the aircraft's nose. Operationally, the Avro 720 was to have been armed with a pair of
de Havilland Firestreak
The de Havilland Firestreak is a British first-generation, passive infrared homing (heat seeking) air-to-air missile. It was developed by de Havilland Propellers (later Hawker Siddeley) in the early 1950s, entering service in 1957. It was the ...
Infrared homing air-to-air missiles, which could be carried upon under-wing pylons.
Specifications (Performance estimated)
See also
References
;Notes
;Bibliography
"Armstrong Siddeley Screamer" ''Flight'', No. 2478, Vol 70, 27 July 1956. pp. 160–164.
''Flight'', 17 August 1967, p. 262.
* Jackson, A. J. ''Avro Aircraft since 1908''. London:Putnam, 1990. .
* Jones, Barry. "Saro's Mixed-Power Saga". ''
Aeroplane Monthly
''Aeroplane'' (formerly ''Aeroplane Monthly'') is a British magazine devoted to aviation, with a focus on aviation history and preservation.
__TOC__
''The Aeroplane''
The weekly ''The Aeroplane'' launched in June 1911 under founding edito ...
'', November 1994. London:IPC. ISSN 0143-7240. pp. 32–39.
* Mason, Francis K. ''The British Fighter since 1912''. Annapolis, Maryland, USA:Naval Institute Press, 1992. .
"Mixed-Power Interceptor" ''
Flight
Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
'', 24 May 1957, pp. 697–700.
{{Avro aircraft
Cancelled military aircraft projects of the United Kingdom
720
__NOTOC__
Year 720 ( DCCXX) was a leap year starting on Monday (link will display the full calendar) of the Julian calendar. The denomination 720 for this year has been used since the early medieval period, when the Anno Domini calendar era ...
1950s British fighter aircraft
Mixed-power aircraft
Rocket-powered aircraft