Supersonic Airplanes

1 Aircraft Design

a) An iterative process that alternates between synthesis and analysis, the design of Aircraft is governed by conflicting requirements. Conventional configurations of modern jet aircraft date back to the 1950s and comprise: i. the Wing;  ii. the Empennage;  iii. the Powerplant;  iv. the Fuselage;  v. the Landing Gear.

The functional building blocks of systems architecture promote design abstraction where high-level partitioning, allocation of modules, or standards of interchange, are determined by multi-disciplinary participants while parametric variations of the baseline converge to a viable solution. In terms of systems the airplane may be delineated as follows:

• Aircraft structure: wings, empennage, fuselage, aerodynamics;
• Vehicle systems: propulsion, flight controls, fuel, hydraulics;
• Avionic systems: controls and displays, navigation, communications.

b) The calculated value of the speed of sound [Mach 1] is approximately 1'225 km/h. Although wind tunnel experiments and Compressible Flow Research were conducted in the 1920s/30s, the turbulence encountered at transonic speeds [Mach 0.8 - Mach 1.2] became a real issue with the advent of jet propulsion and rocket motors. At near-supersonic speeds, viscosity and thermal conduction cause the aircraft to produce a shock wave since the air in front will be abruptly compressed into a cone before expanding to its normal pressure behind the tail. These shock formations significantly reduce the aerodynamic efficiency [lift/drag ratio] of the aircraft. Supersonic configurations generally involve pointed noses, swept-back wings, slender fuselages, and powerplants with variable geometry.

2 The Bell X-1

The design and flight testing of research aircraft for the purpose of exploring transonic aerodynamics emerged in the 1940s. While the National Advisory Committee on Aeronautics (NACA) proposed the construction of a jet-powered research plane the Air Force encouraged the development of a rocket engine, resulting in the first generation X-1 by contractors Bell Aircraft and Reaction Motors. With structural design determined mainly by estimates, the X-1 would be 31 ft in length, have a wingspan of 28 ft, and weigh some 13'550 pounds. It had thin straight wings made from solid aluminium plate, and a liquid oxygen rocket with four chambers producing 6000 pounds of thrust.
The shape of the Fuselage was influenced by existing ballistics data. The cockpit contained a wheel, rather than a stick, and controls for a thrust selector, stabiliser, and emergency shutoff. Onboard instrumentation included propellant gauges, the altimeter, and a Machometer.
Since engine running time only amounted to a few minutes, the X-1 had to be air-launched from a converted B-29 at an altitude of 25'000 ft, with powered test flights commencing over a remote California airbase by late 1946. Each flight was carefully planned: the craft was fuelled at night and test instruments checked in the morning; the pilot would enter the X-1 (stowed in the bay of the B-29) at about 5'000 ft, then conduct the required experiments during the test flight before gliding back to earth. The sound barrier itself was officially broken in October 1947 at 42'000 ft altitude.

3 The Lockheed SR-71 Blackbird

Designed to replace the subsonic U-2, and based on the Lockheed A-12, this Strategic Reconnaissance aircraft was developed to gather intelligence at high altitude and supersonic speeds. The SR-71, which made its first flight in 1962, was configured to photograph the Earth's surface at Mach 3 from an altitude of 80'000 ft. In order to withstand the effects of extended supersonic flight, the delta-winged airplane required a titanium construction. The propulsion system included variable air intakes to permit the speed range and to prevent shockwaves from entering the engine. Designers expected the shape of the SR-71 and its near-black surface to generate a low radar profile (RCS) apart from radiating internal heat.

SPECIFICATIONS: Length: 32.7m;  Wingspan: 16.9m;  Wing area: 170m²;  Maximum Weight: 77'000kg; Powerplant: 2 x Pratt & Whitney J-58 turbojets with 34'000 lbs of thrust;  Cruising Speed: Mach 3.2;  Maximum rate of climb: 60m/s;  Range: 5926km;  Armament: None.

Engine & Afterburner
The powerplant of the SR-71 consists of two axial-flow turbojets with afterburners, developed by Pratt and Whitney in the late 1950s for sustained supersonic flight. Frequently modified, the J-58 engine has a nine-stage compressor-burner-turbine and a bleed bypass [six longitudinal pipes between the 4th compressor and the afterburner] to permit normal jet functionality up to Mach 2.0 and ramjet operation at greater speeds. The air inlets contain a large movable spike that begins to retract at Mach 1.6, thereby increasing the stream tube area so the engines will ingest the required core-flow- and bypass air. Until these inlets were regulated by an electronic system, the J-58 engine was prone to mid-air compressor unstarts.

Primary Flight Controls
The flight controls of the SR-71 consist of: i. the Cockpit Controls [Control Column and Rudder Pedals];
ii. four Elevons along the trailing edge of the delta wing;  iii. two Rudders on top of each engine nacelle.
Movement of cockpit controls is transmitted by linkages and mechanical input to a hydraulic servo actuation system, making the Control Surfaces fully power operated, while artificial force feedback is proportional to the degree of control deflection. The elevons are linked to the Control Column, the SAS, and the Autopilot, to permit longitudinal- and lateral control (with a mixer assembly combining pitch and roll inputs into a single command). Yaw control of the aircraft is provided by means of the rudder pedals.

Stability Augmentation System (SAS)
The autostabiliser system responds to small changes in aircraft attitude using gyroscopes that sense angular rates of rotation in each axis [roll, pitch, yaw]. An electrical signal is converted into hydraulic motion by computers and a transfer valve, allowing server assemblies to reposition relevant flight control surfaces and thus counteract the attitude change. The SAS pitch axis control system - with authority extending to 6.5 degrees [trailing edge] up and 2.5 degrees down - has three pitch rate gyro sensors and two servo channels to automate elevon deflection.

Map Projectors
A strip-film projector with a 9 x 9 inch display is used to access map films and mission data inside the RSO's cockpit. The projectors handle 35 mm film and are synchronised to aircraft groundspeed.

Environmental Control System
The Blackbird must overcome the temperatures and air pressures associated with Mach 3 flight. Insulating materials and refrigeration systems are used to regulate build-up from aerodynamic heating and the powerplant, while crewmembers operate in a sealed compartment (that accommodates both cockpits) apart from wearing full-pressure suits. The cockpit environment itself - maintained by pressurisation, ventilation, and conditioned air - includes the Life Support Systems of the pilot and his RSO.


PERFORMANCE: The SR-71 Blackbird was the world's fastest and highest-flying production aircraft during its service career. In 1974, the SR-71 made a trans-atlantic crossing in 1 hour, 54 minutes, and 56 seconds. It was never shot down - unlike its U-2 predecessor - but 12 out of 32 aircraft built were lost in accidents, resulting in one loss of life. The shape and colour of the airframe are examples of stealth technology, however the exhaust stream of the J-58s was still trackable by radar.
By the time the first SR-71s entered service in 1966/67, a number of reconnaissance satellites had been deployed. The SR-71 carrying the Pilot, a Reconnaissance Systems Officer (RSO), and a payload of camera equipment and Electronic Counter Measures, could fly irregular sorties over hostile areas and remain unreachable by interceptors or missiles. The J-58s had to be individually ignited using a start cart. A typical Blackbird sortie began with pre-flight checks, followed by take-off, a rendezvous with a KC-135Q tanker, and the climb to cruise altitude and Mach 3. The aircraft would then fly its reconnaissance mission, refuel one or more times, and return to base.

4 The Concorde

a) Feasibility studies for a Supersonic Transport (SST) were conducted in Europe during the 1950s. In the United Kingdom, the Supersonic Transport Aircraft Committee investigated cruising- and low-speed aerodynamics, jet engines, and airframes for a commercial plane capable of long-distance supersonic flight. French research had focused on a medium-range supersonic passenger aircraft possibly powered by the civil version of Pratt & Whitney's J-58 engine, the JT-11B3, to be manufactured under licence by SNECMA*. By the early 1960s a generic supersonic shape [the delta-wing and elongated fuselage] had emerged in the laboratories of both countries.

b) Inter-company talks on a joint SST project, leading to agreements in principle and the division of industrial responsibility, preceded the historic Anglo-French Agreement of 1962. The two governments therewith agreed on the shared design, development, and production of a supersonic airliner, involving four major firms: Bristol-Siddeley and SNECMA were to build the Olympus powerplant, and Aérospatiale [Sud-Aviation] and BAC (British Aircraft Corporation) were to collaborate on the airframe.
The undertaking included parts manufacture, sub-assemblies in BAC- and Aérospatiale factories, one general assembly line in each country, and flight testing of prototypes.
With six partners who were not subordinate to each other, a committee of civil servants from both countries to supervise, the key Concorde Management Board, and various specialist work groups, management by committee and design by consensus would prevail.
The structural design of the airframe was divided into clear areas of responsibility: the front fuselage and flight deck, the engine nacelles, the air intakes, the rear fuselage, and the fin and rudder, were produced by the British, while the mid fuselage, the wings and elevons, and the landing gear were made in France. The British team also designed the engine controls, the fuel system, the electrics, and the oxygen system; the French designed the flying controls of the Concorde, the hydraulics, the radio, the navigation systems, and the air-conditioning. Rather than combining aircraft parts and components into a shell structure with subsequent installation of systems, the Concorde assembly lines in Toulouse and Filton operated in a modular fashion using fully equipped structural components ready for final assembly.

* The existence of the Blackbirds [which were powered by the J-58] was classified until 1964.

c) In 1963 the makers of Concorde justified the unfavourable operating economics of their elegant fast plane [compared to contemporary wide-bodied subsonic aircraft] with an airline concept that would have integrated this first-class only service into a subsonic fleet with an increased proportion of economy-class seating, resulting in maximised economic benefits from both types of aircraft. When sales teams presented the first Concorde design to the airlines, the idea of a long-range supersonic airplane had been accepted - unlike its capacity of 90 seats. To make a larger fuselage and wing area possible, the twinspool Olympus turbojets required further development, leading to the 139-seat pre-production model of 1966/67.
Some 15 airlines were to take options on the Concorde between 1963 and 1967, including Pan American, Qantas, Japan Airlines, Lufthansa, and Air Canada.

d) The requirements of the Concorde differed markedly from those of high-speed military aircraft. The designers of this Mach 2 airliner needed to consider:

• Supersonic flight over trans-atlantic ranges;
• A cruise altitude of 60'000 ft;
• An airframe exposed to high temperatures;
• A cabin accommodating 100 passengers;
• Civil aviation safety standards;
• Commercial economic operation.

In supersonic flight, air passing the intake must decelerate to Mach 0.5 before entering the engine; nevertheless, the exiting jet stream must be faster than the speed of the aircraft if thrust is still to occur ... The powerplant of the Concorde had to meet the airflow requirements of the engine in subsonic phases of flight, the transonic regime, and supersonic cruise. In order to withstand very high operating temperatures, nickel-based alloys were used for the combustion chamber and turbine blades.

As the Concorde configuration evolved, several design compromises to achieve minimum drag at supersonic speeds, as well as acceptable stability at subsonic speeds, resulted in the cantilever low delta wing. Another design consideration was the shifting aerodynamic (or gravitational) centre of the airplane due to changes in velocity which could be regulated most efficiently by moving precise amounts of fuel forwards or backwards between multiple tanks.

SPECIFICATIONS: Length: 62.2m;  Wingspan: 22.5m  Powerplant: 4 x Rolls Royce / SNECMA Olympus 593 turbojet engines;  Cruising Speed: Mach 2.2;  Range: 6'580km.

The Wing
Research had indicated that delta wings were more suitable for a supersonic airliner than variable-geometry, with flight testing of the H.P.115 research plane providing early data on subsonic lift-drag ratios. For the Concorde a cruise speed of Mach 2.2 not only meant an acceptable compromise between supersonic and subsonic aerodynamics, but also the use of aluminium alloy for the airframe since thermal effects would not exceed 100ºC. The low set wing itself is of multispar torsion box construction and has control surfaces along the trailing edge.

Engine & Afterburner
The powerplant consists of three elements: i. the Air Inlet System, ii. the Engine, and iii. the Propelling Nozzle, all housed in a wing-mounted nacelle. The air intake ramps and spill door are variable in their geometry and hydraulically adjusted to ensure optimum airflow to the engine. Four Olympus 593 turbojet engines, controlled primarily by the pilot's throttle lever, each generate 169.5 kN of thrust. The variable nozzle geometry is controlled automatically over a range of speeds and altitudes, while the reheat system [afterburner] supplies extra thrust during take-off and transonic acceleration.

The Fuselage
The elongated fuselage of the Concorde atop the continuous structure of its delta wing is 62m long and breaks down into the Droop Snoot, Forward Fuselage, Centre Fuselage, and Rear Fuselage. It provides a pressurised volume for the flight deck and a passenger cabin with approximately 100 single-class seats.

AIR TESTING: The Concorde project presented a civilian leap into the unknown, an iterative process and development programme involving nearly 50'000 participants and spanning well over a decade during which the airplane was effectively designed three times. Air testing was conducted with prototypes and pre-production Concordes equipped with sophisticated performance instrumentation. Development flying included analyses of various failure scenarios, proving for instance that the aircraft could safely land with three engines only, or that flight controllability could be maintained if two engines were suddenly cut on the same side. Following two brief maiden flights in early 1969 of the French-assembled Concorde and the British prototype [as well as the first supersonic- and intercontinental test flights] the Concorde 002 made an endurance tour via the Middle East, the Far East, and Australia in 1972.

PERFORMANCE: Mach 2 performance with civil aviation safety, a payload of approximately 100 passengers, a cruising altitude without turbulence ... the airline flying the Concordes could offer its clientele scheduled intercontinental flights at twice the speed of sound, making the crossing between Paris and New York, for example, in less than four hours. Due to sonic boom, the aircraft would typically remain subsonic over populated areas before its climb to cruise altitude and Mach 2. "Technologically advanced and rather delicate," the Concorde represents a revolutionary design and technical success. Nevertheless, the airlines involved did not consider the operating economics of this prestige aircraft forgivable enough to convert their options into orders, and so only seven production Concordes each would be delivered to British Airways and Air France. -(see also, Airbus Industries)

5 Supersonic Transport Aircraft

a) While England and France were able to see the Concorde project through to production and commercial operation, a parallel American Supersonic Transport program would be cancelled in 1971. The next SST development phase - NASA's [1985-99] High-Speed Research program (HSR) - made its progress toward "an environmentally acceptable, economically viable, 300-passenger, 5'000 nautical mile, Mach 2.4 aircraft." In 1994 EADS [Aérospatiale], BAE Systems [British Aerospace], and Deutsche Aerospace AG (DASA) initiated the European Supersonic Research Program (ESRP) in order to investigate advanced materials, structures, aerodynamics, and propulsion concepts for a 250-seat Mach 2 reference configuration. Significant SST research has also been conducted in Russia and Japan. (3)

b) A report on breakthrough technologies for supersonic aircraft commissioned by NASA in the year 2000 identified the following representative configurations:

• A small business jet with a cruising speed of Mach 1.8 and a low sonic boom, permitting overland flight;
• An overland commercial transport [Mach 1.8 to 2.2] with a capacity of 100 to 200 passengers;
• A high-speed civil transport (HSCT) with a cruise speed of Mach 2.0 to 2.4 and a capacity of 300 passengers, used mainly for trans-oceanic services.

Sonic Boom
Sonic Boom intensity, a serious obstacle to overland supersonic commercial transport, is to be reduced using advanced airframe configurations capable of suppressing secondary shock waves.

Weight
Structural mass can be decreased with advanced lightweight alloys, for instance, or by replacing traditional glass cockpit and mechanical droop-nose arrangements with Synthetic Vision.

Noise
The nacelle cross-section of a supersonic airplane must be small in order to reduce drag at cruising speed, but this imposes a smaller mass flow rate - thus requiring a high jet velocity - which in turn produces a much louder engine. Solutions to control take-off noise include suppression devices and variable geometry.

6 Conclusion

To investigate the concept of supersonic flight and various engineering solutions, the Assignment was structured around three case studies - the X-1, the SR-71, and the Concorde - with a written presentation based on major aspects of aircraft design.

With increasing pilot experiences of transonic turbulence, fear of an approaching barrier of sound, and the development of jet propulsion in the late 1940s, the supersonic problem domain would require new technological solutions. Systematic flight experiments using research aircraft such as the X-1 generated reliable transonic data, enabling the design of the first supersonic fighter planes and contributing toward the development of high-performance machines like the A-12 and SR-71 Blackbirds. Supersonic Transport research in the UK and France converged by the early 1960s, resulting in the joint design, development, and production of a supersonic airliner. Technically successful, elegant, and safe, the Concordes made scheduled international flights at twice the speed of sound - yet their unfavourable operating economics, given a payload ratio of only 7 per cent, ultimately determined their small production run. The next generation supersonic civil transport will need to be environmentally and commercially acceptable to airlines and society in general - a formidable long-term design proposition requiring breakthrough technologies in areas related to sonic boom, propulsion, emissions, and noise suppression.

Concept, Text, Coding (c) Marcel Ritschel, Sydney 11.01.2006