GRAHAM WARWICK/WASHINGTON DC
The 1950s are often regarded as the Golden Years of aircraft design. But imaginative concepts did not always prove practical
The path of aviation progress over the past 100 years has not been smooth or straight. There have been many detours and dead-ends on the way from the Wright Flyer to today's aircraft. There are fewer blind alleys these days because the industry can ill-afford to make mistakes, but in the heady post-war years, anything seemed possible.
The 1950s were the heyday for heroic failures. Fuelled by the technical advances of the Second World War and steered by the political insecurities of the new world order, the aircraft industry acknowledged few boundaries to its capabilities. Unfortunately, back then, the speed of technological advance was outpaced only by the rapidity with which military thinking could change, and many a bold concept was proved solid only to see the requirement evaporate.
Ideas came thick and fast in the post-war years as turbojets and turboprops opened up new possibilities: tiny parasite fighters, vertical take-off tailsitters, supersonic seaplanes and high-altitude bomber interceptors. there was no shortage of creativity.
Long-range bombers proved their worth in the Second World War, and when the Convair B-36 Peacemaker entered service in 1947, the US Air Force believed it had the ultimate intercontinental weapon. But defending the six-engined behemoth from the emerging threat of Russian jet fighters posed a problem - no US fighter had legs long enough to escort the 12,000km (6,500nm)-range bomber through enemy airspace.
One solution was for the bomber to carry its own protection, and fledgling McDonnell Aircraft proposed a parasite fighter that could fit in one of the B-36's three bomb bays. The XF-85 Goblin was the smallest jet fighter ever built, less than 4.6m (15ft) long with a 6.4m-span folding wing and a 3,000lb-thrust (13kN) Westinghouse XJ34 turbojet. If attacked by enemy aircraft, the bomber would lower the fighter on a trapeze and release it. After combat, the fighter would hook on to the trapeze and be lifted back into the bomb bay.
Two XF-85s were built, but neither ever flew from a B-36. Instead flight tests used a modified Boeing B-29, nicknamed Monstro after the whale that swallowed Pinocchio. The tests were not an overwhelming success. On its first free flight in August 1948, the Goblin successfully separated from the launch aircraft, but encountered severe buffeting when reattachment was attempted. The trapeze broke the canopy, and the pilot had to make a skid landing.
Because of turbulence encountered as the XF-85 approached the trapeze, only three flights ended in successful hook-ups. The other four ended in emergency landings. The programme was discontinued in late 1949, but the US Air Force had not given up on parasite fighters.
After cancellation of the XF-85, the Fighter Conveyor (Ficon) programme investigated the potential of carrying a Republic F-84 in the bomb bay of a B-36 for long-range reconnaissance missions. Flight tests were conducted between 1952 and 1954 using first a straight-wing F-84E Thunderjet then a swept-wing RF-84F Thunderstreak. Despite continued difficulty reattaching the fighter to the trapeze, a handful of B-36s modified to carry the RF-84K Thunderflash reconnaissance aircraft entered service in 1955.
In 1956, Project Tom-Tom explored the alternative of attaching RF-84Fs to the wingtips of a B-36. A jaw capture mechanism on the fighter's wingtip engaged a coupling on the bomber's wingtip. The effective increase in span reduced drag and offset the extra weight, but flying through the wingtip vortices to hook up was difficult and pilots had to sit in the fighter cockpits for the duration of a mission.
Successful development of air-to-air refuelling killed the parasite fighter, but the US Air Force did look at one other way to extend fighter range: supersonic turboprops. McDonnell fitted one of its XF-88 Voodoo penetration fighter prototypes with a 1,975kW (2,650shp) Allison XT38 turboprop in addition to its two J34 turbojets. The XF-88B first flew in March 1953 and went on to reach Mach 1.12 on propeller power, albeit in a dive.
Ungodly noise
Republic's XF-84H had a 4,360kW Allison XT40 turboprop driving a supersonic propeller that made a noise so unpleasant that it earned the aircraft the nickname Thunderscreech. Two prototypes made 12 test flights between July 1955 and October 1956 - 11 ended in emergency landings. The XF-84H never approached supersonic speed. Poor performance and high maintenance meant it never became operational.
The US Navy, meanwhile, had its own use for turboprops. The Japanese kamikaze attacks of the Second World War had highlighted the vulnerability of aircraft carriers, so the concept emerged of vertical take-off and landing (VTOL) fighters that could operate from any ship in a convoy. In 1951, Convair and Lockheed were awarded contracts to build experimental tailsitter VTOL fighters, nicknamed Pogo, using Allison's powerful (but ultimately unreliable) YT40.
Turboprops were still in their infancy then, and the US Navy gave the only powerplant rated for both vertical and horizontal flight to Convair for its XFY-1. The company began tethered flight tests inside a massive airship hangar, but the turbulence generated by the 4.8m-diameter counter-rotating propellers made it dangerous work. The first free hover was accomplished in August 1954, and in November the aircraft completed its first transition from a vertical take-off to horizontal flight and back to a vertical landing.
Lockheed, meanwhile, had fitted its XFV-1 with an ungainly temporary undercarriage and began flight-testing in December 1953 with conventional take-offs and landings. The aircraft made several successful in-air transitions between horizontal and vertical flight, but was never able to take off and land vertically because of the engine. Tests with both aircraft showed the concept worked, but it required enormous skill to land vertically through the turbulent propeller wash. Meanwhile, the US Navy was losing interest and work was discontinued in 1956.
Tiny tailsitter
But that was not the end of the story for the tailsitter. In 1953, Ryan was awarded a US Air Force contract to build an experimental jet-powered VTOL aircraft, the X-13 Vertijet, that could be launched and recovered vertically using a special trailer. Only 10,000lb thrust was available from the Rolls-Royce Avon turbojet, so the X-13 was small - 7.3m long with a 6.4m-span delta wing. Fitted with temporary landing gear, conventional flight tests began in December 1995. Tailsitting tests began in 1956 and then, in April 1957, the X-13 took off vertically from the raised trailer bed, transitioned to horizontal flight and back, then descended to the trailer for a vertical landing.
But the US Air Force had no requirement for a VTOL fighter, and development was discontinued. Over in France, meanwhile, Snecma was working on a tailsitter powered by its Atar turbojet. The final step was the C450 Coléopètre, an 8m-long aircraft with a 3.2m-diameter annular wing enclosing a 7,700lb-thrust Atar 101E. Hover control was provided by vanes in the nozzle, with small fins providing control in forward flight. After several successful free hovers, the aircraft crashed attempting its first transition to horizontal flight. The pilot ejected, but was injured, and work on the concept was halted.
When the US Navy's post-war plans to build a "super carrier" for strategic bombers and supersonic fighters were rejected, there was a brief flurry of interest in seaplane fighters and bombers. The UK's Saunders-Roe had begun work on a turbojet-powered flying-boat fighter during the Second War War, but the SR/A.1 did not fly until July 1947, by which time interest had evaporated. Around 1950, the company tried and failed to secure funding for its P.121 project, a high-subsonic seaplane using retractable hydro-skis for take-off and landing.
Convair, meanwhile, was awarded a contract in 1951 to build the F2Y Sea Dart supersonic interceptor seaplane. This was a delta-wing fighter powered by two 6,100lb-thrust Westinghouse XJ46 turbojets, and with a pair of retractable hydro-skis. The XF2Y-1 prototype first flew in April 1953, and it was quickly discovered that the skis caused serious vibration - the rougher the water, the more severe it was. This led to a redesign to a single large ski, which ultimately allowed the aircraft to operate in wave heights up to 3m.
The Sea Dart eventually exceeded Mach 1 in a shallow dive, but the aircraft was designed before area ruling was available and was underpowered. An F2Y-2 version with single hydro-ski, area-ruled fuselage and an afterburning, 15,000lb-thrust Pratt & Whitney J75 was planned, but the US Navy lost interest and cancelled the programme in 1956.
A similar fate befell the Martin P6M SeaMaster jet-powered flying boat. This was part of the Seaplane Striking Force concept - the US Navy's answer to the US Air Force's domination of the strategic nuclear strike mission with its land-based long-large bombers. The concept called for a small number of forward-deployed seaplanes able to perform strike, reconnaissance and mining missions, supported by tenders or submarines for refuelling and rearming.
The SeaMaster was designed to fly 3,200km and have a M0.9 dash speed at low altitude. The aircraft was powered by four 13,000lb-thrust Allison J71 afterburning turbojets mounted above the swept wing. The XP6M-1 prototype first flew in July 1955, and although the first and second aircraft crashed, the test programme was successful. Costs were escalating, however, and the US Navy kept cutting back numbers until, in August 1959, the SeaMaster was cancelled. It was to be the last aircraft produced by Martin.
The transition from propellers to jets in the late 1940s and early 1950s was traumatic for the aircraft industry. Early turbojets were weak and thirsty, but promised speed, forcing aircraft designers to find ways round their anaemic thrust and excessive consumption. This led to several mixed-power concepts, some more successful than others, but all of which proved to be just a passing phase.
The US Air Force's Republic XF-91 Thunderceptor was an experimental high-speed, high-altitude interceptor, intended to meet the threat posed by Soviet long-range bombers. The aircraft had a number of unusual design features beyond its combination of a 6,700lb-thrust General Electric J47 afterburning turbojet with a 6,000lb-thrust Reaction Motors rocket. The XF-91 had an inverse-taper wing to eliminate the risk of tip stall, which was a problem with early swept wings. The tandem-wheel main gear retracted into the wingtips, which were wider and deeper than the roots. The incidence of the wing could be varied in flight, from high angle-of-attack for take-off and landing to low for high-speed flight.
Looking remarkably advanced for its day, the XF-91 first flew in May 1949, breaking the sound barrier in level flight by December 1952. The Thunderceptor went on to reach M1.71. Two examples were built and there were several revisions to the configuration during testing, including a radar nose and V-tail, but in the end the aircraft did not make it into production, in part because it could only carry enough fuel for a 25min flight.
In 1949, French aircraft manufacturer SNCASO selected combined rocket-jet power for a quick-reaction interceptor, the SO.9000 Trident. This aircraft was unusual in having two 800lb-thrust Turbomeca Marboré II turbojets mounted on the tips of a thin, straight wing, and a 9,900lb-thrust SEPR rocket motor mounted in the tail. The Trident first flew in March 1953 under jet power, making its first rocket flight in September 1954.
The Marborés were woefully underpowered, and a Trident crashed when the rocket shut down on take-off, so the turbojets were replaced with 1,640lb-thrust Armstrong-Siddeley Vipers built under licence by Dassault. The aircraft went supersonic in April 1955, eventually reaching M1.55. The improved SO.9050 Trident II followed, first flying in July 1955 and setting a western European speed record of M1.93 in January 1957. The low jet-only thrust was still a problem, and resulted in the Trident IISE with two 2,400lb-thrust Turbomeca Gabizo afterburning turbojets, which in July 1958 became the first French aircraft to exceed Mach 2. But all this was to no avail, and the production Trident III was cancelled in favour of the Dassault Mirage III. And the rest, as they say, is history.
The UK's experience with mixed power followed a similar path beginning in 1952, when Avro and Saunders-Roe were awarded contracts to build prototype high-altitude interceptors. The Avro 720 was later cancelled, and the rival SR.53 did not fly until 1957, which proved to be an inauspicious year for the British aircraft industry. The SR.53 was a lightweight fighter, powered by a 1,640lb-thrust Viper 8 turbojet and an 8,000lb-thrust de Havilland Spectre rocket motor.
Paper cuts
By the time the SR.53 flew in May 1957, Saunders-Roe was already developing a much larger, more capable mixed-power fighter, the SR.177, for the Royal Air Force, Royal Navy and German air force. But an infamous 1957 defence white paper decreed that the English Electric Lightning would be the RAF's last manned fighter, killing the SR.177. The SR.53 continued flying into 1958, eventually going supersonic, but the second prototype was destroyed in a crash and the programme ended.
Although it had abandoned mixed power, US Air Force interest in rockets resurfaced in the 1950s amid studies of concepts to free fighters from their dependence on vulnerable runways. Under the Zero-length Launch/Mat Landing (ZELMAL) programme, a massive rocket was strapped to an F-84G Thunderjet to boost the fighter off a launch pad. The aircraft was intended to land, gear up, on a huge inflatable mat, using an arresting cable to stop. Tests began in December 1953. Take-offs went well, but because of leaks a mat landing was not attempted until June 1954. It was a disaster - the aircraft was written off and the pilot injured. Two more mat landings were performed, less destructively, but the concept did not work.
The idea was not abandoned, however, and re-emerged in 1957 as the Zero-length Launch (ZEL) programme without the mat landing. The concept was to launch a nuclear-armed strike aircraft from a mobile trailer and have the pilot bail out over friendly territory after completing the mission. A North American F-100 Super Sabre was fitted with a 130,000lb-thrust Rocketdyne booster, which could accelerate the aircraft to 245kt and 400ft in 45. Sixteen test flights were conducted in 1958, with one crash when the booster did not separate. One hundred Super Sabres were modified for zero-length launch.
Doubts about the advisability of driving nuclear-armed fighters around Europe on trailers eventually killed the concept. In the early 1960s, the German air force sponsored zero-length launch tests with the Lockheed F-104G, but abandoned the idea in favour of the ultimately fruitless development of vertical take-off and landing fighters. But that is another story.
Source: Flight International
