PHIL RUFFLES Hon FRAeS looks back at a 1966 paper given by Adrian Lombard, then Director of Engineering at Rolls-Royce, on how he saw the future development of propulsion systems.
To mark the Society’s Centenary in 1966 the Students and Graduates section asked some of the great names of aviation to predict what aeronautics would be like in 2016. A.A. (Adrian) Lombard Director of Engineering of Rolls-Royce at the time gave a presentation titled “Propulsion in the New Century”. We asked Philip Ruffles FRS FREng Hon FRAeSoc and a former Director of Engineering at Rolls-Royce plc to look at Lombard’s paper and see what the great man got right. Unfortunately Lombard died suddenly one year after giving his paper and was no longer able to influence events.
A A Lombard.
In considering Lombard’s paper it should be remembered that, at that time, in addition to producing civil and military aero engines and adaptations of them for the marine and energy markets, Rolls-Royce also produced motor cars, diesel engines, rocket engines and nuclear power plants for submarines. It was a very fertile time with three separate departments working on advanced projects, the Old Hall where a number of scientist were working on long term technologies one outcome being that the company entered the nuclear business, a non aero advanced projects group which was studying new applications for its products including gas turbines for trains and tanks and a preliminary design group looking at new engine concepts for civil and military aircraft and helicopters. The company acquired the Bristol Siddeley Engine Company on 7 October 1966 which added to its portfolio of products.
By its nature a lecture given to young graduates contemplating a career in aerospace should set out to stimulate their enthusiasm by proposing a number of radical ideas as well as more conventional developments. In any case predicting the future in terms of technological advancements is fraught with difficulty as often the external factors that affect the success or otherwise of a product are often not know or are given insufficient weighting in any analysis. For example the impact of the micro processor on all aspects of communications, vehicle control and stability systems, automated processes in engineering and manufacturing to name but a few; these could not have reasonably been forecast. Equally environmental concerns have had much greater impact on the development of civil aircraft than was envisaged in 1966. For example the adverse effect of exhaust emissions from all forms of heat engine on human health and the climate was grossly under estimated.
Lombard outlines the possible evolution for six different types of propulsion system as follows:
(a) A development of the positive displacement internal combustion engine, with pistons probably replaced by a purely rotary engine.
Mazda RX8 automotive rotary engine. (Atkins)
The internal combustion engine has advanced dramatically both in terms of reliability and performance in the last 50 years due to advances in combustion, replacement of the carburettor or low pressure diesel fuel injectors with electronically controlled direct high pressure direct injection systems, variable valve timing and turbo charging combined with advances in materials and manufacturing processes which have enabled engines to be much more affordable despite their increased complexity. However the rotary engine has not been able to penetrate this market to any great extent due mainly to difficulties in sealing between the rotor and stator thereby limiting its potential.
(b) Developments and derivatives of the gas turbine
The gas turbine is ideally suited to aircraft propulsion and it is here where the biggest advances have been made as I will cover later. This has also enabled the aero derivative gas turbine to be used for power generation, gas pumping and marine propulsion where its compactness and availability give it a big advantage. Large gas turbines have also been developed in combined cycle mode where the waste heat from the exhaust is recovered in a Rankine cycle with steam as the working fluid. This has replaced the conventional coal fired steam turbine for power generation.
(c) Engines using nuclear energy
Royal Navy nuclear submarine HMS Tireless. (US Navy Kevin Elliott)
The two applications where nuclear reactors have been deployed on a large scale are in power generation and naval propulsion for submarines and large surfaces ships .The reactor provides the heat to provide steam for a steam turbine operating on the Rankine cycle. It is possible to use a nuclear reactor as the heat source for a closed cycle gas turbine operating on the Brayton cycle and, while demonstrator programmes are underway, this has not yet emerged as a viable alternative. Concerns regarding safety and cost of ownership have precluded rapid development of nuclear technology. After a pause following accidents at Chernobyl and Three Mile Island, it is now re-emerging as a zero carbon emissions source of power generation. Consideration is also being given to much smaller units for power generation which would reduce the risks associated with raising the capital cost and potential accidents.
(d) Rocket engines
Launch of Space Shuttle Columbia on 12 April 1981. (NASA)
There have been enormous advances in rocket engines as propulsion systems for military delivery systems, for satellite launches, for space stations and taking people into space. The first time a rocket landed on the moon was in September 1959 followed by the Apollo 11 mission in July 1969. The NASA reusable low orbital space craft (Space shuttle) programme was a great success. The first launch was on April 12th 1981, 135 missions were completed including the building and support of the International Space Station and the last launch was on July 21st 2011.
NASA is now working on space craft able to go deeper into space and supporting commercial companies who are now developing rockets for satellite launching with the prospect of a reusable rocket now in sight.
(e) Fuel cells and electric motors
The fuel cell has great promise since it can convert fossil based fuels via a reformer into hydrogen for use in the fuel cell to generate electricity. Efficiencies of around 60% are possible as, unlike a heat engine, they are not restrained by the limitations of the Carnot cycle efficiency. There have been and remain many programmes mainly aimed at small scale power generation up to about 1MW and road transportation but as yet none have reached volume production.
There have been great advances in high speed permanent magnet electric motors and power electronics which make their application in small gas turbines for electrical generation a real possibility. Bladon Jets is developing a 12kw unit for powering telecom towers and other applications in under developed countries.
(f) Chemical engines
Apart from the fuel cell or the use of liquid air or nitrogen for energy storage which is then evaporated to drive a reciprocating engine, I am not aware of any developments along these lines but perhaps some other form of energy conversion will emerge in the next fifty years.
Propulsion systems for aeronautical applications
Lombard goes into some detail on the prospects for air and space travel
(a) Low speed 0-150mph short range
Harrier GR9. (Adrian Pingstone)
The paper proposes a whole number of options for vertical take-off and landing (VTOL) airborne vehicles for short distance travel so as to replace the motor car and the bus. This reflects the many studies that were underway in the 1960s using VTOL technology. However only the helicopter offers this capability today to any great extent. In the military field, however, the subsonic Harrier aircraft, powered by the Rolls-Royce vectored thrust Pegasus engine did survive from the many military projects that were undertaken in the 1960s. It has since been superseded by the supersonic Lockheed Martin F35 aircraft with a lift system designed and developed by Rolls-Royce. Yakovlev also developed the YAK 38 demonstrator and Yak 41 but this never entered production.
The only other novel aircraft in production is the Bell Boeing tilt rotor aircraft which is faster and has a longer range than a conventional helicopter.
(b) Higher subsonic speed, longer range
Sud Aviation Caravelle. (Pedro Aragao)
At the time the paper was written air transport was generally by turbo-propeller powered aircraft for short range operations with some new jet powered aircraft such as the Hawker Siddeley Trident, Sud Aviation Caravelle, BAC1-11 and Boeing B727 being used use for short and medium ranges. Long range operations were dominated the Boeing 707 and Douglas DC8 which initially could only fly from Europe to the East Coast of the United States although later versions could fly to the West Coast.
It is perhaps surprising that the paper focused so heavily on VTOL aircraft driven in part by concerns over airport capacity and on supersonic and hypersonic vehicles. Apart from Concorde, none of these projections have come to pass, so one might ask why?
Air traffic has grown steadily over the last 50 years initially at around 8% per annum between 1966 and 1976 reducing to around 5% more recently. The world is now a truly global village where people’s desire to travel for business and for pleasure remains high. Whilst communications have advanced dramatically they have served to enhance commerce and to encourage both leisure and business travel. In particular business travel has not subsided as some people predicted only a few years ago.
This demand has been satisfied by continued investment in aircraft and engines which are conceptually the same as those of 50 years ago apart from arguably the development of twin aisle wide body aircraft for passenger numbers above 200. Part of the growth in traffic has been accommodated by using larger aircraft whilst new airports have been built and existing ones have grown. Aircraft range has increased making it possible to fly anywhere in the world either directly or with only one stop or change of aircraft apart from a few very remote locations. The road and rail infrastructure around airports and airport designs have not kept pace so getting from home to the aircraft is both time consuming and frustrating when compared with time on board the aircraft.
(c) Low supersonic speed range
Tupolev Tu-144. (RIA Novosti archive Lev Polikashin
At the time that Lombard’s paper was written two supersonic aircraft were planned, the Russian Tupolev Tu144 which first flew in December 1968 and Concorde which flew two months later.
The Tu144 appeared at the Paris Air Show in 1973 where it crashed which delayed its further development but it finally entered service in 1977 nearly two years after Concorde. Another aircraft crashed in 1978 after which it was grounded as a passenger carrying aircraft but continued to be used for cargo and in support of the space programme until 1983.
Air France Concorde. (Alexander Jonsson)
Concorde was the only successful commercial supersonic airliner; it first flew in 1969 and entered service in 1976. 14 aircraft were operated by British Airways and Air France mainly on Trans Atlantic routes. It would carry up to 126 passengers and cruise of Mn2.04 (1354mph) and would halve the journey time from London to New York. Because of its high operating costs, it was used by only the extremely wealthy. It was eventually retired from service in 2003 after 27 years service.
Boeing won a contract in the USA to develop a more advanced supersonic transport aircraft than Concorde. It was planned to carry 250-300 passengers at a speed of Mach 3.0 and was a wide body design with six abreast seating. There were many concerns about the design, primarily the cost of development and production but also noise at take off, sonic boom and degradation of the ozone layer due to high levels of Nitrous oxides in the engine exhaust. In the event the programme was cancelled in 1971.
Aerion's proposed design for a supersonic business jet. (Aerion)
There have been more recent attempts primarily between England and France to launch a second generation supersonic transport with longer range and more passengers but the difficulties related to overcoming the environmental problems and acceptable economics have so far precluded its launch. Supersonic business jets have also been studied in some detail and remain the most likely route to passenger carrying supersonic flight. A programme exists between Aerion in the USA and Airbus to develop a jet carrying 8-12 passengers with a range of 4,750nm travelling just subsonic overland and at Mach 1.5 over water. It is planned to enter service in 2023.
(d) High supersonic speeds/Hypersonic speeds
SABRE rocket engine (Reaction Engines)
Rocket propulsion has been the primary means of realising speed in the region of Mach 5 and above either for military purposes or for space exploration. The US Space shuttle is the only returnable vehicle that has been successfully developed to date.
However work is underway with the European Space Agency (ESA) on a single stage to orbit vehicle using the SABRE (Synergistic Air Breathing Rocket Engine) under development by Reaction Engines. The SABRE engine draws in air from the atmosphere up to Mn 5.0 which burns the hydrogen fuel in the rocket combustion chamber. Above Mach 5.0 liquid oxygen replaces the air as the vehicle leaves the earth’s atmosphere until it is in orbit. This same concept could be used for hypersonic flight although all the concerns that have precluded supersonic flight will still apply, namely those related to economics and the environment.
Power plant development
Rolls-Royce carbon fibre fan blade. (Rolls-Royce)
Lombard’s projections for the evolution of subsonic power plants have turned out to be remarkably accurate particularly with respect to overall pressure ratio, turbine entry temperature and hence fuel consumption. New materials have contributed to this advance notably higher strength metallic’s including single crystal materials for turbine aerofoils. The use of composite materials has not been as dramatic as Lombard indicates, although there is a re-emergence of the use of carbon fibre composites in the fan blade, titanium metal matrix composites for rotating components and high temperature composites in the turbines initially applied to static components. Lombard championed the use of composite material in the 1960s and in particular the Hyfil (carbon fibre) fan blade which was unsuccessful primarily because of its vulnerability to bird strikes. This was replaced by a solid Titanium blade on the RB211 but was later succeeded by a hollow Titanium fan blade in 1984 which has been fitted to all Rolls-Royce civil engines until the present day. General Electric developed the carbon composite fan blade for the GE 90 which entered service in 1995 whilst Rolls-Royce has a carbon fibre fan blade under development.
Left: RB178-51 offered for the Boeing B747. Right: RB211-22B fitted to the Lockheed Tristar.
Now turning to propulsion Lombard shows a picture of the RB178-51 three shaft engine which was offered to Boeing to power the B747. The Rolls Royce RB211 engine was a scale of this design and entered service in 1972. Both designs are shown above. RB211 development continued for many years. A major re-design led to the Trent engine family, the first variant of which entered service in 1995. Six variants of the Trent engine have been developed, the latest being the Trent XWB shown below which powers the Airbus A 350 which entered service in 2015 whilst new variants of this basic three shaft engine concept are still being developed.
Rolls-Royce Trent XWB for the Airbus A350. (Rolls-Royce)
The RB211 was a significant advance over its predecessor the Conway where it offered a 25% reduction in fuel consumption, approximately twice the thrust and significantly lower noise levels. The latest Trent XWB engine which powers the Airbus A 350 aircraft offers similar levels of advance, namely a further 25% improvement in fuel consumption, over twice the thrust and a further significant reduction in noise levels. These achievements are broadly in line with the Projections made by Lombard in Figs 14 and 15 of his paper. Most importantly though, engine reliability is more than order of magnitude better than the early engines thus making twin engine long range operations possible.