Material marvels: Concorde

Materials World magazine
,
1 Jun 2017

Simon Frost looks at the engineering behind the definitive supersonic jet airliner.  

1969 The processor for the Apollo Guidance Computer, which helped steer man to the moon, was roughly 60,000 times slower than those found in today’s smartphones. 1969, when the tallest structure in the world was the Empire State Building – you could double its height today and it would still fall short of the UAE’s Burj Khalifa. This was the year when a three-letter message sent electronically from one laboratory in California to another was enough to crash the ARPANET system, an early predecessor to the internet.

How improbable to think, then, that this was the year of the first test flights for Concorde – the supersonic passenger plane that would travel at more than twice the speed of the fastest in service today. Brian Trubshaw CBE, the first British pilot to fly Concorde, perhaps put it most succinctly in the opening lines of his autobiography – ‘It is not unreasonable to look upon Concorde as a miracle.’

Beginnings

The Supersonic Transport Aircraft Committee, UK, was established in 1956 under the leadership of Welsh aeronautical engineer Morien Morgan, and delivered a report in 1959 that led the Government to commission feasibility studies on supersonic transport from the Bristol Aeroplane Company and Hawker Siddeley. 

Bristol Aeroplane Company, which became the British Aircraft Corporation (BAC) in 1960 following a merger with three other British aircraft manufacturers, was the Government’s choice for the next stage of design. BAC delivered the initial concept for a four-engine, 100-passenger supersonic plane called BAC223 in 1961. 

Meanwhile, the UK Government was beginning negotiations to enter the European Economic Community (EEC), and keen to establish European partnerships. French President Charles de Gaulle craved for Europe to rival the USA’s postwar dominance in air transport, with the (subsonic) Boeing 707 and Douglas DC-8 jet airliners being produced in their hundreds. So France signed a partnership treaty with the UK to develop the supersonic plane together – France’s Sud Aviation was already working on designs for its own supersonic plane. 

The UK Government hoped this would help to gain it entry to the single market, but it would not be until 1973 that the UK was accepted to the EEC, and the interim years were politically challenging for the Concorde project – the UK Prime Minister Harold Wilson even tried to break free of the partnership in 1964 based on concerns over cost, but backed down when France threatened an international lawsuit. One of the 20th Century’s great Anglo-French engineering projects was underway.

Olympus

Based on the Olympus engines previously developed for the Vulcan and BAC TSR-2 bombers, and developed chiefly by Bristol Siddeley Engines, UK, which was purchased by Rolls-Royce two years into its development in 1964, the Olympus 593 engine was at the heart of Concorde’s legendary power plant. 

Jenny Tye, Concorde Operations Manager at Brooklands Museum, UK, explained to Materials World, ‘It is important to note that it was not the Olympus 593 alone that led to Concorde’s thrust. The “power plant” combined the efforts of the engine, intake system, reheats and exhaust nozzles. Various different manufacturers had to work closely together to develop the power plant system, including BAC, Rolls-Royce and Snecma, France.’

The four Olympus 593 engines that powered Concorde were twin spool turbojets. Two independent compressors, made of titanium, were each linked to their own turbine. ‘Being made of titanium meant that they were light but strong enough to withstand possible foreign objects that could be ingested into the engine,’ Tye said. 

Further along the engine, to cope with much higher temperatures, nickel-based alloys were used. David Ford was a student apprentice with Bristol Siddeley Engines and Rolls-Royce during the 1960s and spent time working at the Precision Foundry at the Filton plant. ‘This foundry was based on the investment process in which the cast part was formed from a wax pattern that had been “invested” with a zircon/alumino-silicate shell mould,’ he told Materials World. ‘The Olympus engine was of an advanced design for
the time – a two-spool turbojet – and the very hot turbine required the aerofoils – rotor vanes and guide vanes – to be internally cooled.’

Cooling air was fed from the compressor, but this required innovative manufacturing technology and canny materials choices to create internal passages in the aerofoils. The materials for each component required high performance by a range of measures.

‘The turbine rotor blades were in three stages – one to drive the high-pressure compressor and two for the low-pressure compressor,’ Ford said. ‘The materials properties required were creep, tensile, high cycle fatigue, impact resistance, oxidation/corrosion and coatability (aluminising). Since these parts were cast, they also had to be resistant to hot cracking, microporosity and mould reaction.’ The rotor blades were cast from the nickel alloy IN100, containing 0.16% carbon, 10% chromium, 15% cobalt, 3% molybdenum, 5.5% aluminium, 4.7% titanium, 0.04% zirconium and 0.015% boron. ‘It was the best high-temperature material available and had good castability. Compared with other superalloys, it also had a relatively low density of 7.75g/cc, which was advantageous for the engine weight and disc design,’ Ford said.

Cooling off

Hollow quartz tubes were used in the high-pressure blades to create radial passages in the aerofoil. The tubes were then removed from the casting by cycling pressurised concentrated potassium hydroxide in autoclaves. ‘Retaining the position of the tubes to create 18 radial passages was a major challenge, as quartz is liable to distort at casting temperatures – 1,500°C – or suffer devitrification and breakage,’ Jones noted. The low-pressure blades were cast solid, with internal radial cooling holes created using shaped tube electrolytic machining drilling, allowing high metal removal rates without transferring thermal or mechanical stresses to the part.

Tye notes that the Olympus engine was particularly remarkable due to the high compression ratio that was required, using both high and low pressure compressors running in series. ‘To put it into context, when air entered the engine (having been slowed down by the air intake system of ramps producing shockwaves), it would be approximately -60°C,’ she said. ‘However, due to compression, the air temperature would be approximately 130°C when it reached the face of the engine. When the air left the compressor it would be at about 550°C.  The Olympus 593 demonstrated a thrust of over 40,000lbs!’ 

For Tye, Concorde’s most impressive engineering aspect is the air intake system integrated into the engine’s nacelles. ‘While at the University of Surrey, UK, studying Aerospace Engineering, I was offered the opportunity to help with the voluntary project to restore Concorde G-BBDG. It was an honour to be involved, and have such a unique, hands-on experience with this iconic aircraft,’ she told Materials World. ‘My final year dissertation was a research project about the intake design. The principle of slowing down the air with a series of ramps to generate shockwaves, so that the engine could accept airflow in order to produce thrust is absolutely mind blowing. And to think that it was designed some 65 years ago is even more impressive,’ she said.

Lighten the fuselage 

Materials for Concorde’s structure required low weight, high tensile strength and proof stress that would be retained throughout prolonged exposure to high and low temperatures, creep strength, fatigue endurance and superior crack propagation behaviour. 

‘Many different materials were considered for the fuselage, but the main limiting factor was the high temperatures, or rather, the contrast in temperatures that the material would endure,’ said Tye. ‘At subsonic speeds, the aircraft fuselage would endure lows of -35°C, but at Mach 2 the temperature (due to the kinetic heating effect) raised to 127°C at the nose.’ 

Titanium was considered, but its expense and difficult fabrication techniques made it a less attractive option than aluminium alloys. British Corrosion Journal summarised in 1972 some of the aluminium alloys that were tested, ‘notably: L73 (duralumin), 2024-T81 (duralumin with high magnesium content), and RR58 (Y-alloy family, ie containing nickel). While in most respects these three did not differ greatly from each other, in fatigue tests on notched specimens at 120°C, RR58 was notably superior.’  

Tye added that ‘RR58, containing approximately 6% copper, had been used previously in the manufacture of pistons for aircraft engines, so suppliers were able to assure the designers that it could be produced in various forms and to specific sizes.’ RR58 had already been used, for example, for the forge impellers fitted to the Whittle jet engine in the Gloster Meteor MK1 fighter. The material was supplied by High Duty Alloys, UK. 

To protect against corrosion, the sheet material required cladding on each side with an anodic metallic coating to a thickness of 5% of the gauge. ‘The most effective protection was given by an aluminium-1% zinc cladding. Another refinement adopted in the treatment of RR58 has been the application of cold compression before ageing at 200°C to enhance the resistance to stress-corrosion,’ British Corrosion Journal stated. In areas where cold compression could not be applied, such as in die forgings, artificial ageing was carried out at 215°C, producing some over-ageing but improving the material’s resistance to stress corrosion. 

‘To have such a thin skin that could withstand such phenomenal temperatures and pressure differences is key to her design,’ Tye said. Brooklands Museum runs tours of a retired Concorde, including a virtual flight simulator experience. ‘Part of our aircraft in the rear cabin is exposed down to her fuselage skin. You can tap it and see just how thin it really is. To think that that's all between you and the edge of space is pretty special.’

Speed and control

Among Concorde’s most iconic attributes were its graceful, slender, ogival delta wings. While most plane wings featured numerous flaps, leading edge slats and other moveable components for control and trim, these would cause unacceptable drag on a machine designed to reach twice the speed of sound. But how can you take off and land without these components to provide control? 

Concorde’s wings featured just six honeycomb-structured elevons at the rear of the wings – a portmanteau of the traditional elevators and ailerons it replaced to control the plane’s pitch and roll – but these alone would not provide the necessary control. It was the mathematically envisaged shape of the wings that made control across a wide range of speeds possible.  

Like the fuselage, the wings were constructed chiefly from RR58, and were constructed in a torsion box design. The ‘ogival’ shape of the delta refers to the ogee – the meeting of two opposite arcs, as is often seen in routed mouldings or architectural arches. 

Swept wings create vortices at their wing tips, but the delta wing creates larger, slower moving vortices that creep forward along the leading edge as its angle of attack increases, gradually enveloping the entire upper surface of the wing and increasing lift and, therefore, control. Concorde was able to manoeuvre across a range of speeds simply by adjusting its angle of attack, much like a bird. 

Concorde’s other distinctive feature was its long, pointed nose cone, which also played a key role in landing. Formed of resin-bonded glass fibre, as metals would interfere with the signals detecting storm clouds, the hydraulically powered nose cone could be moved independently to an angle of 12.5° on landing, allowing the pilot to see the approaching runway. The interior, too, was lined with resin-impregnated glass fibre. 

The 27 club

Operated by British Airways and Air France, Concorde went into operation in 1976, following 5,000 hours of flight testing. Seating 100 passengers, it was a unique flying experience catering to the privileged or business traveller. The most popular route for the British carrier, first run in 1977, was from London Heathrow to New York JFK in just three hours and 20 minutes – a flight that currently takes around eight hours. 

Cruising at Mach 2.02, passengers experienced heights of 60,000ft, a vantage point from which they could see the curvature of the Earth below and dark skies above at midday. In 1985, Phil Collins performed at both London and Philadelphia Live Aid concerts on the same day thanks to Concorde. ‘The thing about Concorde is that it was so cherished by so many people. It was the pride of the nation, and because she turned out rather graceful, she appealed to both the artistic and the technical,’ Tye said. 

But by the turn of the millennium, sales for Concorde flights were waning, while the fleet’s maintenance was becoming more expensive and low-cost carriers were beginning to change the market. A series of unfortunate events pushed Concorde into retirement.

On 25 July 2000, a flight departing from Paris ran over a piece of titanium that had fallen from another aircraft onto the runway, bursting the tyre, fragments of which punctured the fuel tank causing it to ignite, killing all 109 onboard and four on the ground. These were the only fatalities on Concorde, which had an excellent safety record, but passenger numbers fell further, and further still following the 9/11 attacks in 2001, the outbreak of SARS in 2002 and the early 2000s recession. 

Concorde was withdrawn from service in 2003, after 27 years. It was not the only supersonic jet airliner – the Russian Tupolev Tu-144 flew passengers from 1977–78 and cargo until 1983 – but certainly the most successful. There is still no supersonic passenger jet in service, although several companies have presented plans in various stages of development for a successor. 

‘I am very optimistic that another supersonic passenger aircraft will exist within the next 20 years, maybe even sooner,’ Tye said. ‘We had a visit from an American supersonic project recently, Boom, to use the original Concorde Simulator that was used to train all British Airways flight crew, which we have restored to be operational. Boom were able to use the 1970s technology to simulate various scenarios in supersonic flight to gather knowledge on how aircraft behave at such high speeds. I was so proud to feel that we could help this supersonic project progress.’ Boom hopes to fly its prototype aircraft in 2018, half a century after Concorde’s first test flight.