Challenge: Powering The Son Of Concorde

• Control emissions so the vital upper layers of the atmosphere aren’t damaged.

• Operate efficiently while generating about a third the noise of the Olympus Turbojet engines in the Concorde.

• Assist in minimising the sonic boom in overflight-what American publicists tried to sell as “the drum roll of progress” in the aborted American SST project of the 60s.

Appropriately this project is beginning to take shape in 1991, which the English-speaking world tends to regard as the jubilee of the jet engine. It marked the first flight of the Whittle powered Gloster test aircraft in the UK, even though Hans Ohain’s Heinke! 178 had flown two years earlier and Italian and Japanese aircraft (the Nikka project) flew about the same time.

Although Nazi Germany got jet powered fighters into action before the end of the war, the wrecked economy and Hitler’s misguided order that jets should power revenge bombers prevented them from having their full impact. However, it should be noted that the powerplants of the Messerschmitt Me 262 fighters incorporated axial flow engines, which Britain and the US were not to turn to until after the war.

Even in 1991, Sir Frank Whittle’s first centrifugal compressor experimental engine was enough to take the Gloster E28/39 experimental aircraft up to speeds of 370 miles an hour (almost 600km/h) on its first flights, while the second test-bed Gloster reached about 400mph (600km/h), both faster than the prop-driven Spitfires then in service. That first engine delivered thrust measured at 860Ib (about 390kg). By the first decade of the 21st century, engines delivering takeoff thrust of 100,000lb (about 45,500kg) may be available for the next generation of big commercial jets.

It’s still early to be putting figures on output of the engines for the HSCT, but a briefing by one of the key engineers in the US team working on the project, William S Clapper, of General Electric’s advanced technology operation, at the Aerospace Congress in Melbourne in May, indicated that thrust of 50-60,000lb (22,700-27,300kg) is being used as a basis.

Clapper’s GE team, together with their opposite numbers from Pratt and Whitney, are working on engine concepts for an HSCT to go into service after 2005 with the specifications of flying direct from Los Angeles to Sydney or Singapore to Hawaii or Tokyo to New York, reducing flight times by more than a half from the block schedules of today’s 7 47-400s.

Clapper, in a keynote address to the Melbourne congress, said the most significant difference between the HSCT and the Concorde was the need to improve economics by eight times. Fuel usage of the engine plays a significant part in these economies, designed to produce a 300 passenger, three class aircraft which will charge a premium of 10-15 per cent on subsonic fares when it goes into service.

The engines are also vital in two other key areas: emissions and airport noise. The engine also has an important role to play in the final problem, the supersonic boom. As supersonic flight will be ruled out for flying overland in most parts of the world, the engine has to be able to operate efficiently at subsonic as well as supersonic cruise.

Independently, both GE and P&W have been working on several types of variable cycle engines, which can be developed to meet the demanding requirements of the airframe designers, economists and environmental engineers. A variable cycle engine opens vanes on the casing to permit a fan-like airflow around the engine at subsonic speeds. These close so it becomes a turbojet in supersonic regimes.

The noise target is FAR 36 Stage Ill, which calls broadly for a third of the noise of the Concorde in approach, overflight and takeoff. “This has a major impact on the economics of the airplane,” Clapper said. “There is no fundamental scientific limitation on meeting the noise rule. Nevertheless, it is a challenging engineering job to develop the right combination of inlet, engine and exhaust nozzle in an integrated propulsion system to meet the regulation with minimum impact on aircraft TOGW (takeoff gross weight). Engine emissions impact the atmosphere. The magnitude of the impact is uncertain and highly dependent on the altitude and latitude at which engine emissions are occurring, the amount and chemical composition of emissions from a particular engine design and the aircraft fleet size and operation. In assessing possible atmospheric changes, the modeler needs to account for changes in the future atmosphere. The atmosphere is evolving with trace gases such as methane increasing substantially. Assessment of the impact of a HCST fleet is a major global atmospheric modelling challenge for the scientific community [which] NASA is addressing. ”

To this end GE has been looking at advanced combustor concepts capable of meeting the NASA goal of five grams of NOx per kilogram of fuel burned. These depend on burning either lean or rich, which raises a whole new set of engineering challenges.

“When operating lean, combustion is occurring near the stability limit, raising the operability issue of ‘blow-out”‘, Clapper said. “There is also the risk of premising duct flashback and autoignition. “When operating rich, the combustion liner cannot use conventional film cooling as the introduction of cooling air will alter the combustion process away from the rich condition. The combustion liner must therefore be convection cooled, requiring advanced high thermal conductivity and high temperature materials. The remainder of the fuel from the -rich combustion must be quickly quenched (in around 1 millisecond) to the lean condition to complete the combustion with minimum NOx formation.

“On exhaust nozzle design, HSCT economics is impacted by airport noise considerably more than engine emissions. The acoustic exhaust system may in all likelihood weigh as much as the engine. High specific thrust engines are the smallest and lightest in weight for supersonic cruise. However, these make the most noise because of their high jet velocities. Noise constraints are currently sizing the HSCT. Clapper described two methods of achieving a noise reduction of about 17dB:

• Breaking the jet flow into many smaller jets with high levels of ambient air entrapment and acoustically treated panels to absorb high-frequency noise.

• Reduce jet velocity via additional airflow through the engine, some ambient air entrapment and mean shear reduction to absorb high frequency noise.

“Our current studies indicate noise requirements can be met with . projected exhaust nozzle suppression technology and to the aircraft takeoff profile without oversizing the engine,” Clapper reported. ‘: A. dvancements in engine materials technology is also expected to have a major impact on the overall e?onomic viabil!ty of the HCST. A 1kg reduction of total propulsion system weight reduces the aircraft weight by 52kg for a four engine HSCT. But the benefits need to be weighted against the increased acquisition and maintenance costs [assessed] at the component and sub-component level and evaluated at the aircraft level to verify the ultimate economic value to the system.

“Comparing today’s technology with an HSCTpropulsion system designed for service m 2005, it is clear significant progress is being made. With the same payload, the T GOW of the HSCT is projected to be 32% lower [as a function of] improvements in specific fuel conumption, acoustic suppression technology and reduction of engine weight as a result of advanced materials.

“The HSCT, if it is to enter commercial service in 2005 cannot be a niche aircraft catering only to executives. To succeed, the HSCT must be a full fledged revenue producing member for the world’s commercial air fleet available to all sections of the travelling public at a nominal premium for speed. “