Avro 720
| Industry | Aerospace |
|---|---|
| Founded | 1910 |
| Headquarters | United Kingdom |
| Products | Aircraft |
The Avro 720 was a British single-seat interceptor developed during the 1950s. Designed by Avro, it was intended to compete against the Saunders-Roe SR.53, produced by Saunders-Roe. Although construction of at least one prototype had begun, the project was halted and cancelled before any complete aircraft were finished.
The aircraft was planned as a high-speed interceptor employing mixed propulsion: a rocket engine for rapid acceleration and high velocity, complemented by a smaller jet engine intended for standard cruising flight. The cancellation partly stemmed from the selection of its powerplant, the then experimental Armstrong Siddeley Screamer rocket engine, which operated on liquid oxygen as the oxidiser and kerosene as fuel. Concerns arose regarding the operational practicality of using liquid oxygen in service conditions.
Origins
The significance of strategic bombing in modern warfare became evident during the Second World War. As tensions escalated into the Cold War, many countries prioritised developing advanced air defence systems capable of countering large formations of hostile bomber aircraft equipped with nuclear armaments. In the latter years of WWII, Nazi Germany had made considerable progress with rocket-powered interceptors such as the Messerschmitt Me 163 and designs like the Bachem Ba 349. These rocket-propelled fighters demonstrated exceptional climb rates, theoretically enabling them to rapidly intercept enemy bombers before they reached critical targets. Allied experts, aware of these capabilities, sought to investigate and understand this technology in depth.
Following the war, German rocket propulsion knowledge was extensively examined by former Allied nations. The United Kingdom promptly initiated programmes to develop liquid-propellant rocket engines aimed at enhancing aircraft performance during takeoff and climb phases, known within the RAF as rocket-assisted take-off gear (RATOG). In 1946, British efforts produced two new rocket motors: the de Havilland Sprite, delivering 5,000 pounds of thrust, and the Armstrong Siddeley Snarler, providing 2,000 pounds of thrust. These engines utilised different propellant combinations; the Sprite employed a high-test peroxide (HTP) monopropellant system, while the Snarler used a mixture of methanol, water, and liquid oxygen. Both engines entered flight testing in the early 1950s; however, their demand diminished somewhat as conventional jet engines with afterburners increasingly fulfilled performance enhancement roles.
In May 1951, in response to intelligence regarding the expanding Soviet strategic bomber fleet and their development of atomic weapons, the British Air Ministry issued an Operational Requirement, designated OR 301. This specification called for a rocket-powered interceptor capable of reaching an altitude of 60,000 feet (18,300 meters) within two minutes and thirty seconds. The performance criteria reflected concerns about anticipated advancements in Soviet bomber capabilities, which were expected to reach speeds up to Mach 2 and operational ceilings near 80,000 feet by the early 1960s. Avro promptly began designing an interceptor to meet these requirements—the Avro 720—while other companies such as Bristol, Blackburn, Shorts, and Saunders-Roe also submitted proposals.
The specification aimed to deliver an interceptor with exceptional speed and climb rate to effectively counter high-altitude Soviet bombers. The operational concept resembled that of Germany’s Me 163: employing rocket propulsion for rapid ascent to engage targets swiftly before gliding back to land once fuel was depleted, typically on a retractable skid. Aviation historian Derek Wood notes that Saunders-Roe identified risks and costs associated with an unpowered glide return flight and proposed adding a secondary jet engine to power the interceptor’s return journey. The Air Ministry supported this idea enthusiastically; consequently, in May 1951 all interested manufacturers were invited to explore designs incorporating dual propulsion systems.
Revised specification and selection
Had the interceptor been developed solely to meet the initial performance criteria, it would have been compelled to glide back from altitudes reaching 100,000 ft (30,500 m), execute an unpowered touchdown several miles distant, and subsequently require recovery and transport back to base by ground vehicle. The Ministry later issued updated specifications to Avro and Saunders-Roe, formally mandating the inclusion of an auxiliary turbojet engine. This engine was essential to provide adequate thrust for the interceptor to return autonomously to its airfield following a combat sortie.[1][2] On 12 December 1952, further evolution of the rocket interceptor concept culminated in Specification OR 337 being issued.[3] The principal modifications within this specification involved changes to armament, notably replacing the initially planned retractable battery of 2-inch rockets with the Blue Jay—the codename at that time for an infrared-guided air-to-air missile under development.[4][5]
An advisory design meeting focusing on Saunders-Roe’s proposal was convened by the Ministry of Supply on 5 May 1953; three days later, Saunders-Roe received a formal contract for three prototype aircraft.[6] However, uncertainty within both the RAF and Ministry regarding the optimal fuel and propulsion system for the interceptor led to a decision to issue a revised specification. This adjustment paved the way for awarding a development contract to Avro for their competing Avro 720 design as well. Among six companies submitting proposals, only Avro with their Avro 720 and Saunders-Roe with the SR.53 were granted development contracts.[3][6]
Consequently, the Ministry placed an order for two prototypes.[1] By December 1956, reports indicated that the Screamer engine intended for powering the Avro 720 had successfully passed flight clearance testing.[7] Progress on the Avro 720 was reportedly smooth; by that year, its first prototype was nearly complete and considered capable of achieving flight up to a year earlier than its SR.53 competitor.[8] Nonetheless, neither project had yet secured firm endorsement from the Royal Air Force (RAF), which appeared hesitant and inclined to postpone any definitive preference until after flight evaluations of both aircraft had been conducted.[9]
Starting in September 1953, both Avro’s and Saunders-Roe’s projects encountered intensified scrutiny amid broader Ministry efforts to reduce expenditures.[10] The Avro 720 program suffered a critical setback when its Screamer rocket engine was cancelled late in 1956. This cancellation was largely influenced by official apprehensions over handling liquid oxygen—a substance boiling at −183 °C (90 K) and posing significant fire risks—in operational settings. The loss of its primary propulsion system ultimately led to termination of the Avro 720 project.[10][2][11] One factor favoring continued support for the SR.53 despite its slower progress and delays was its rocket engine’s use of hydrogen peroxide as oxidizer—considered less problematic than liquid oxygen—and the Ministry’s reluctance to maintain two separate rocket fuel programs.[10][12]
At cancellation, only one structural test airframe had reached partial completion. Aviation historian Barry Jones notes that images showing this airframe—with port wing attached and bearing serial number XD696—have sometimes been incorrectly identified as depicting the first prototype.[1] By termination point, expenditure on the Avro 720 reportedly amounted to £1 million, with an additional £0.65 million spent on its Screamer engine.[13]
Design
The Avro Type 720 featured a compact tailless configuration with a delta wing design. Its structure incorporated metal honeycomb sandwich composites for enhanced strength and lightness. At its core, the aircraft was powered by an Armstrong Siddeley Screamer rocket engine delivering 8,000 lbf (36 kN) of thrust, utilizing liquid oxygen as the oxidizer and kerosene as fuel. This contrasted with the rival Saunders-Roe SR.53, which employed a de Havilland Spectre rocket engine fueled by kerosene combined with hydrogen peroxide as the oxidant. Both models included a single Armstrong-Siddeley Viper turbojet engine producing 1,750 lbf, serving as an auxiliary power source; in the Avro design, this turbojet received airflow through a modest chin inlet positioned beneath the nose. The intended armament for the Avro 720 consisted of two de Havilland Firestreak infrared-guided air-to-air missiles, which were to be mounted on pylons beneath the wings.
Specifications (Performance estimated)
See also
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References
- Notes
- ↑ 1.0 1.1 1.2 Jones Aeroplane Monthly November 1994, pp. 32–33.
- ↑ 2.0 2.1 Mason 1992, p. 400.
- ↑ 3.0 3.1 London 2010, p. 31.
- ↑ Template:Usurped
- ↑ Wood 1975, p. 56.
- ↑ 6.0 6.1 Wood 1975, pp. 56–57.
- ↑ Flight 27 July 1956, p. 164.
- ↑ Wood 1975, pp. 60–61.
- ↑ Wood 1975, p. 57.
- ↑ 10.0 10.1 10.2 London 2010, p. 30.
- ↑ Flight 24 May 1957, pp. 698–699.
- ↑ Wood 1975, p. 61.
- ↑ Flight 17 August 1967, p. 262.
- Bibliography
- "Armstrong Siddeley Screamer". Flight, No. 2478, Vol 70, 27 July 1956. pp. 160–164.
- "Cancelled Projects:The list Up-Dated". Flight, 17 August 1967, p. 262.
- Jackson, A. J. Avro Aircraft since 1908. London: Putnam, 1990. ISBN 0-85177-834-8.
- Jones, Barry. "Saro's Mixed-Power Saga". Aeroplane Monthly, 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. ISBN 1-55750-082-7.
- "Mixed-Power Interceptor". Flight, 24 May 1957, pp. 697–700.