Andrzej Jeziorski/MUNICH
The Heinkel He 162 Salamander was a product of desperation. A wooden airframed jet fighter designed in 1944 as the tide of war was clearly turning against Germany, the aircraft - also known as the Volksjäger (people's fighter) - was intended to be mass-produced and then flown by teenage Hitler Youth pilots who had hardly any powered flying experience.
The aircraft flew adequately in the hands of experienced aviators, but stability problems made it a nightmare for the kind of green cannon fodder that Hitler had in mind. By the end of the war, some 275 Volksjäger had been built, and 500 were still on the production line.
One of the many remarkable aspects of the design, which went from mockup to prototype in 90 days, was its single dorsally mounted jet engine:a BMW 003E-2, unusual in that it had an axial compressor instead of the radial unit more common in those days. The engine also powered the Arado Ar 234 Blitz bomber and variants of the Messerschmitt Me 262.
After the war, BMW's aerospace activities were focused on licensed production of the Rolls-Royce Tyne. Meanwhile, the 003's designer, Hermann Östrich, was snapped up by France's Snecma, and worked on the Atar engines which powered early Dassault fighters.
Günter Kappler, research and development director of today's BMW Rolls-Royce, takes pleasure in reeling off the history of BMW's aero-engine manufacturing endeavours, if only to remind the world that the Bavarian company best known today for powerful luxury cars stems from an aviation tradition - even though this tradition endured a lengthy hiatus after the sale of BMW's ailing aero-engines business to MAN in 1965. Later, MAN teamed up with Daimler-Benz to form MTU - now BMW R-R's arch-rival in Germany.
BMW originally started out in 1917 manufacturing aircraft engines, as symbolised by its famous logo:a stylised propeller spinning against a blue and white sky. Now, the symbol is once again associated with aviation, since the formation of the joint venture company with R-R in July 1990, split 50.5/49.5% between the German and UK sides, leaving it technically a German company.
Towards the end of the 1980s, studies showed that a clear market requirement was emerging for a new generation of engines in the 14,000-23,000lb (62-102.3kN) thrust class. Existing engines in this sector were mainly developments of 1960s technology, adapted from military powerplants. The market was increasingly focusing on low fuel consumption, emissions and noise levels, and high reliability and maintainability, which were already being achieved by the turbofan powerplants of medium and long range commercial aircraft.
NO ENTREPRENEURIAL RISK
"When we were formed, Rolls would not take the entrepreneurial risk of developing a Tay competitor," says Kappler, who was hired by BMW from his professorial post at the Munich Technical University to help with the market study and propose a solution. R-R recognised the need for a new engine, but was at the time deeply ensconced in its Trent development programme to meet the propulsion requirements of the Airbus A330 and Boeing 777.
Thus, it was decided that BMW would take on the entrepreneurial risk of such a programme, while R-R would offer its technical expertise in the field. BMW R-R was provided with full access to R-R civil aerospace technology, allowing its new product, named the BR700, to be based on the most up to date, in-service technology.
While R-R's experience with the Tay was certainly drawn on, and most of the leading engineers on the programme at the start came from the UK partner company, BMW R-R stresses that it has developed its own technology - partly funded by the German Government's aerospace research support programme - for the BR710 and its larger BR715 stablemate.
The goal from the start was to base the engine on derivative technology, minimising the programme risks and development and production costs and cutting down the time to market, while still allowing growth potential around a common core.
The development of an engine core - high pressure (HP) compressor and turbine, and combustion chambers - accounts for some 40% of the total engineering costs associated with a new engine, says BMW R-R. Deciding on a family of engines based on a common core therefore spreads the recovery of non-recurring design costs over several market segments. The common core also offers the cost benefits associated with larger scale production, while thrust ranges can be targeted by the addition of a tailored low pressure (LP) system to the engine.
COMMON CORE CONCEPT
Kappler and his team at the University had come up with the common core concept, primarily aiming for regional jet programmes around the 100-seat class. The target programmes at the time were such as the 90- to 115-seat Sino-German MPC 75, then under study by MBB and China's CATIC, and the 137-seat Fokker 130 stretch of the Fokker 100.
The basis of the engine family was going to be the BR715, originally covering a thrust range of 62-80kN. Larger aircraft were to be catered for by the BR720 (80-98kN), while business jets would be covered by the BR710 (35.6-17.8kN). The MPC 75 and other programmes in its size class never got off the ground, however, and the priorities and thrust ranges shifted over time.
Now the basis of BMWR-R's product palette is the BR710, which powers the Gulfstream V and Bombardier Global Express business jets, and has been selected to power variants of the Tupolev Tu-334 regional jet, as well as the Royal Air Force's replacement maritime patrol aircraft, the British Aerospace Nimrod 2000. Larger aircraft requiring a thrust class from 80-102kN are now covered by the BR715, which has been selected as the sole powerplant for Boeing's 717 regional jet, formerly the McDonnell Douglas MD-95.
BMWR-R remains contractually prevented by R-R from developing an engine with a higher thrust than that of the BR715, although in tests the 715 has been run up to 111kN, according to the manufacturer.
In designing the BR700 core, BMW R-R's engineers recognised that it would need to provide corporate aircraft with good top-of-climb thrust, allowing them to reach their high initial cruising altitudes - about 41,000ft (12,500m) - quickly, and a specific fuel consumption (SFC) 10% lower than that of the Tay, allowing the aircraft to operate over longer ranges (6,500nm was considered a typical target). Regional jets, on the other hand, cruise lower (33,000ft) which allows the aircraft to have a smaller, higher-loaded wing, which in turn requires more take-off thrust for operation from small regional airports. Here, the low SFC pays off in terms of direct operating costs (DOC).
Once the thrust requirements had been identified, a core configuration was selected with a ten-stage axial flow HP compressor, an annular combustor and a two-stage turbine, resulting in a core pressure ratio of 17. For corporate jet applications (BR710), the LP system would comprise a fan driven by a two-stage LP turbine, while for regional jets (BR715), a larger fan - plus a two-stage booster compressor - would be added, driven by a three-stage LP turbine.
Examination of the effect of bypass ratio on climb thrust and SFC led to the conclusion that, for corporate aircraft, a bypass ratio of about 4:1 is the best compromise between fuel efficiency and low noise (characteristic of high-bypass engines)on the one hand, and time-to-climb on the other. A 112cm-diameter fan was found to meet the thrust requirement for corporate aircraft, but a 122cm fan was chosen instead for lower noise, allowing night-time operations at noise-sensitive airports. This allows the engine to run "derated" for take-off, but makes it heavier.
For the regional jet application, a 142cm-diameter fan was chosen to meet take-off and climb thrust requirements. One other possible core configuration would have been a seven or eight stage HP compressor driven by a single stage turbine.This would have limited the core pressure ratio to 12 or 13 (Pratt & Whitney is now achieving a pressure ratio of 11 with a six stage HP compressor in its PW6000), which would have increased SFC and been poor for high altitude operations. "Those crazy business jets will fly at 52-56,000ft, and an engine can operate at this altitude only if its core pressure ratio is very high," says Kappler.
High altitude performance becomes even more critical when one considers that both BR710-powered corporate jets - the Bombardier Global Express and the Gulfstream V - are being offered for the UK Airborne Stand-Off Radar (ASTOR) high altitude military surveillance requirement.
The core's 10-stage axial flow compressor is derived both mechanically and aerodynamically from Rolls' HP compressor for the International Aero Engines V2500, achieving single stage pressure ratios of some 1.31, according to Kappler. The basic rotor construction is the same as the V2500, with a six-stage titanium front drum bolted to a four stage nickel based rear drum. The blades in the first three stages are attached at the root with axial dovetail fixings, while the latter stages feature lower-cost circumferential dovetail fixings.
FADEC CONTROL
Controlled by full authority digital engine control (FADEC) units, variable inlet guide vanes and three rows of variable stator vanes pivot in bushes in the front compressor casing, while the inner ends of the variable vanes carry a shroud seal assembly eliminating cantilever vibration modes.
The front compressor stages are encased in a longitudinally split steel casing, while the aft stages, where consistent blade tip clearance is critical, are enclosed in a casing of complete rings suspended in a forged steel outer casing. Compressor air is bled from the stage five and eight inner rings into the bypass duct for engine handling purposes, or to the aircraft for anti-icing and cabin conditioning. Further bleeds are extracted for engine seal pressurisation and cooling.
The starting point for the combustor design was the R-R Trent Phase 5 combustor. The BR700 retains its predecessor's general annular, axial flow arrangement, based on a scaled combustion concept, and has 20 fuel injectors. Inside the combustor, there is one lean burn primary zone, followed by a quick quench dilution system to cut down nitrous oxide and other polluting emissions. BMW R-R says that the combustor offers NOx emissions "significantly below ICAO [International Civil Aviation Organisation] Annex 16 regulations".
Cooling air is admitted into the combustion chamber through laser drilled holes in machined, lipless Z-rings. The cooling of the combustion chamber barrels is supplemented by external forced convection, with air en route to the HP turbine nozzles directed over the outside of the combustion chamber.
FUTURE TWO STAGE COMBUSTOR
The engine has been designed to accommodate a two stage combustor in future, if and when anticipated tightening of emissions regulations comes about early in the coming decade. BMW R-R has been developing such a combustor as part of the Government-sponsored Engine 3E research programme for next generation powerplants, and is to install and run it in a BR700 core in January. Kappler says, however, that the company will not push a greener engine onto the market until the requirement is there. The new combustion chamber will involve a substantial cost increase and new FADEC software will have to be installed.
The high compressor pressure ratio dictates the need for a two stage HP turbine, but this means that the stage loading is relatively light, and allows high blading efficiency and affordable materials and construction techniques. According to BMW R-R, the turbine work split and blading aerodynamics are derived from R-R's HP turbine for the multinational RTM322 engine, enhanced by the use of the latest computational fluid dynamics techniques.
All the turbine blading is investment cast and internally cooled using compressor air. The rotor blades are manufactured as directionally solidified castings, to improve high-temperature performance, held in the turbine discs by multi-lobe serrations. The turbine cooling and sealing arrangements have been derived in part from later variants of R-R's RB211 family of civil engines.
The turbine discs - which endure some of the most arduous conditions in the engine - are cast and wrought from nickel based Udimet 720 Li alloy, which offers superior properties at high temperatures. This production technique is significantly cheaper than production in the same material by powder metallurgy.
The low blade speed allows the first stage rotor to carry an integral tip shroud, for reduced blade tip inefficiencies. A shroudless design was selected for the second stage, to reduce blade stresses and cost.
The LP system in the BR710 comprises a 24-blade, 122cm-diameter fan, driven by a two-stage turbine. The fan has solid titanium wide-chord blades held by curved inclined roots in a twin diaphragm titanium disc, inside a casing with an aluminium isogrid/Kevlar wrap containment system. Aluminium outlet guide vanes and engine stators respectively diffuse and straighten the airflow into the bypass duct and engine core.
The LP turbine has been optimised for high altitudes, and features laminar flow blading designed for high Mach numbers and low Reynolds numbers. Outlet guide vanes are used to remove the high whirl component exiting the second stage rotor, to moderate the stage loading.
Mechanically, the LP turbine is conventional. It has two forged nickel alloy discs bolted together at the rim, carrying cast shrouded blades fixed with multilobe serrations. The steel of the turbine casing was selected for its containment properties and low thermal expansion. Power is transmitted to the fan through a co-rotating drive shaft via the basic structure of the core.
Noise levels have been kept to a minimum by the use where possible of noise suppressing materials, combined with optimised numbers of blades and blade spacing. As a result , says BMW R-R, both the BR 710 and the BR715 fall well below US Federal Aviation Regulation 36 Stage 3 noise requirements under all conditions. Maintainability has been ensured with a modular design which allows easy access for inspection and maintenance work, helped by a large number of nacelle and bypass duct access panels.
The BR700 core engine engine was launched in March 1991, with the BR710 launch coming in August the following year, just before Gulfstream became the engine's launch customer, closely followed by Bombardier. By September 1996, the engine had won certification from both the European Joint Airworthiness Authorities and the US Federal Aviation Administration.
To date, the BR710 has logged about 10,000 flying hours and nearly 130 engines have been delivered to Gulfstream and Bombardier. Twelve Gulfstream Vs are now in service, and Bombardier is due to complete the Global Express flight test programme in June, with aircraft certification scheduled for August.
The engine has also been selected for Russia's 102-seat Tupolev Tu-334-120, and BMW R-R has agreed to provide two engines for flight testing on the prototype. The programme has remained grounded, however, because of a lack of the necessary funds to fly the first aircraft with its Ukrainian Progress D-436T engines, let alone to integrate the BR710. Kappler says, however, that BMW R-R has been lobbying German politicians to lend Tupolev the necessary DM350-450 million ($194.4-250 million) to kick start the test programme, and has succeeded in eliciting some interest from Bonn.
He adds that he believes that D-436T-powered flights will begin this year, but the BR710 may not fly on the aircraft until 1999. If the programme does get under way, Kappler says that the market for such an aircraft in the CIS is substantial, amounting to some 450 aircraft. Later plans for a 126-seat variant suggest additional prospects for the BR715.
MARINISED VERSION
Work is now under way to marinise the BR710 for its first military application in the BAe Nimrod 2000 patrol and anti submarine warfare aircraft. Kappler says the marinised engine looks set to "turn and burn" sometime in August, and the overall contract is worth DM300 million to the engine manufacturer.
The first issue which had to be tackled in this engine was the Nimrod's peculiar engine mountings, embedded deep in the wing root. Other than that, protective measures have to be taken against salt water corrosion in the engine before it can be exposed to maritime conditions.
Kappler admits that the Nimrod programme is running "about two months" behind schedule , but believes a recovery programme is still possible to meet the delivery schedule.
Source: Flight International