Challenges

Innovation invariably introduces challenges, both to ways of thinking and to ways of doing. This was particularly true of Frank Whittle’s proposal of turbo-jet propulsion.

The feasibility of gas-turbines to drive aircraft occurred to several people in the late 1920s. Amongst those in England was Dr A.A.Griffith, a scientist with a growing reputation following submission of his report “Aerodynamic Theory of Turbine Design” (published in 1926). Another was Frank Whittle, a Royal Air Force pilot who, aged 22, introduced his idea of a compact gas turbine that used reaction from the exhaust to provide thrust. His superiors considered it of sufficient interest to obtain an expert opinion and turned to Dr Griffith at the RAE, who was working on axial-flow theory and plans for a gas turbine suitable for propeller-drive.
  1. Pilot Officer Frank Whittle
  2. Dr A.A.Griffith

His opinion of Whittle’s much simpler turbo-jet scheme was damning. That he found a mistake in Whittle’s calculations could not have helped, although in subsequently revising them Whittle found another error which largely cancelled out the first. Griffiths, however, seems to have underestimated the favourable effect of low intake temperatures at high altitude and of increased compressor efficiency due to ram-effect at high speed – his turbo-prop proposals would have operated lower and slower than Whittle was envisaging. He may also have failed to realised that less oxygen in the thinner air at higher altitudes would be offset by lower drag. The net effect of these factors is that turbo-jet aircraft operate more efficiently at high altitude than at low levels.

Another view held that the absence of propeller-driven airflow over the wings and control surfaces would lead to longer take-off and landing distances and less effective control surfaces at low speeds. In practice, this proved much less than some had anticipated – as was demonstrated by the E28/39 becoming airborne during its initial high-speed taxi-trials on the grass airfield at Brockworth.
  1. Flight Lieutenant Gerry Sayer
  2. Gloster E28/39, first prototype

Materials

The subsequent letter to Whittle from the Ministry also stated that gas-turbines were impractical since there were no suitable materials which could withstand the high temperatures and stresses involved, a view which was still substantially true (a matter of chickens and eggs) though based on an earlier (1920) report by the Air Ministry scientist, Dr W J Stern. He, in turn, had based this on the assumption of a bronze turbine rotor and cast iron combustion chamber. In the meantime, the need for high-temperature allows for such applications as internal-combustion-engine exhaust valves was already being tackled.

  1. W1
  2. W2B

In practice, the success of W1 did quickly spur research into metals specifically optimised to suit jet engines, with Inco’s Leonard Pfiel being credited with the development of the first of a series of nickel/chrome high-temperature and low creep alloys, Nimonic 80, which was first used in the W.2B. Ironically, this in turn led to the development of a related alloy, Nimonic 80A, specifically for exhaust valves. Soon there were a whole series of other improved nickel-based alloys in the Nimonic family that were and are widely used in later engines and other hostile environments.

Combustion

Essential to the success of Whittle’s proposal was the efficiency of fuel combustion within the chambers downstream of the compressor – he was aiming at an intensity of combustion not previously achieved. Originally injected, it was realised that vaporising the fuel might be more effective but, despite a long series of trials, stable, intense combustion proved difficult and held up progress. Isaac Lubbock, seconded from the Asiatic Petroleum Company, working with Shell engineers, came up with an atomising solution so successful that this ceased to be an obstacle to development.

One of the early combustion test rigs, using a barrel of “white spirit at the BTH facility at Ladywood

Centrifugal or Axial compressor?

Whittle filed the first patent for a turbo-jet on 16th January 1930, a few months after his interview with Griffith. Only as a means of inclusion, this showed a 2-stage axial compressor feeding a single-sided centrifugal. In reality, he intended a two-stage centrifugal and, by the time he embarked on building an engine, he had decided to employ only a single-stage double-sided unit.

Reproduction of drawings illustrating British Patent No. 347,206, filed 16th January 1930

With careful design, a centrifugal compressor on its own could achieve the required ratio. But Whittle chose a double-sided form to reduce the frontal area and the length of the rotor shaft. An axial compressor could also achieve this and with a smaller frontal area, but only in more tightly-controlled ideal conditions. The axial also had the advantage of lower blade-tip speeds. However, pressure increase per stage was low, requiring at least 8 stages with intermediate stator blades. It would be significantly less robust and aerodynamically much more sensitive to both intake conditions and downstream pressure changes, such as those induced by rapid throttle movements. These could lead to blade stalling and resulting surges, which could in turn produce catastrophic blade failures. The axial is also prone to icing (one of the greatest enemies of safe flight) whereas the centrifugal is ‘ice-resistant’.

A centrifugal compressor would be simpler and cheaper to produce. It would be more rugged and much more tolerant of the varying intake conditions and rapid throttle changes encountered by a flight-engine. It would however need to run at higher revolutions than an axial. But there was extensive experience of small centrifugal blowers developed as robust aero-engine superchargers. And there was little prior art on axials.
The same year that Whittle filed his patent (1930) also saw the Aeronautical Research Engine sub-committee recommend that all government-funded gas-turbine projects be discontinued. The work in question included that on axial compressors. When work did restart, at the Royal Aeronautical Establishment at Farnborough in 1936, it would be as a direct result of the formation of Power Jets – an independently-funded company.

Hans Joachim Pabst von Ohain

Meanwhile, in 1935, Dr Hans von Ohain, working as a graduate assistant at Göttingen University, attempted to patent his own proposal, which, unlike Whittle’s original patent of 1930, featured a single-sided centrifugal compressor back-to-back with a centripetal turbine. In later years, von Ohain explained that he knew nothing of Whittle’s patent at the time, but it can be safely assumed that the turbojet concept was alive and well in Germany by then. Both now set about building their engines; von Ohain with the help of Max Hahn and a team of engineers provided by Ernst Heinkel, and Whittle by contracting parts – mainly to the steam turbine maker British Thompson-Houston. Both first ran their engines in 1937, Whittle in April and von Ohain some 5 months later. Heinkel then designed and built an aircraft to test the Ohain engine. This achieved two brief (six-minute) flights – one in August and the other in November 1939. However, after much modification, the Ohain engine was abandoned in favour of other jet projects underway at the Heinkel Company.

In the same time-frame that Ohain was working at Heinkel A.G. (HAG), Dr Herbert Wagner was similarly working on a promising axial form of turbojet at Junkers Flugzeugwerke (JFA). The two projects were secret and unknown to each other. Subsequently, the Wagner project came over to Heinkel and became the He.S30 but was not given support by the German Air Ministry (RLM) who favoured the axial engines then under development at Junkers Motorenwerke and BMW.
Griffith and Hayne Constant now restarted work on axial-flow gas-turbines at the Royal Aircraft Establishment at Farnborough, with sufficient funds to both build and contract-out axial compressors, including to Metropolitan-Vickers. However Griffith then moved on to Rolls-Royce where he continued to work on a horrendously complicated and doomed-to-be-abandoned reverse-flow/contra-rotating turbo-prop. Metrovick, after two unsuccessful attempts to build an axial turbojet themselves, began development of the Hayne Constant-designed F2, running it in December 1941. This was the first non-German axial aero gas turbine. After 3 re-designs a small series of F2/4s were built in 1945. By this time, it had achieved 4,000lbs thrust. However, problems persisted and the company decided to use the experience for alternative developments.

Whittle’s W1 and Ohain’s He.S3B

In Germany, the government had placed contracts in 1937 with 3 engine manufacturers, later including Heinkel who had acquired a large part of a team from Junkers Aircraft, who had not received a contract. The four design teams had each decided to pursue axial compressor solutions – Heinkel now having both an axial and von Ohain’s centrifugal project. These decisions may have been partly influenced by the German Air Ministry (RLM) having carried-out research into axials at the time the British Air Ministry had stopped gas-turbine research. This left only Whittle and von Ohain developing centrifugal engines, although quite unknown to each other.

Manufacture

When, in mid 1939, Whittle successfully demonstrated to the government the re-build of his original engine, he and his small team at last gained government support. He had no production facility himself, so in April 1940 the Ministry allocated production of his newly-designed W2 engine to Rover, an independently-minded car manufacturer anxious to move into new territory. Rover found themselves gifted with new technology ‘FOC’. They became determined to extract as much advantage from the weak patent position as was possible. In this ambition they were supported by the Ministry of Aircraft Production and were effectively able to wrest control from Power Jets. As a result, nearly two years were lost in (mostly) unnecessary modifications. The opportunity to supply an operating engine for the proposed jet fighter to counter the German bomber offensive was lost.
Rolls-Royce took over the project in April 1943 and, from then on, development advanced at a fine pace. But the damage had been done. Whittle and his team had suffered excruciating disappointments during the time Rover was in control. The effect had a devastating influence on Whittle’s health. Under Rolls-Royce, the engine passed a 100-hour test at 1,600lbs thrust in May and powered the Gloster Meteor shortly after. The Metrovick F2 also began flight-tests. But by now the Ministry had decided to put the major effort into producing the W2B and another centrifugal engine based on Whittle’s layout but with straight-through combustion – the R-R Derwent. Meanwhile, the de Havilland H1 Goblin, designed by Frank Halford, had also reached the flight-stage by this time and powered the first Meteor to fly. This engine was intended for the de Havilland Vampire – a single-engine fighter.
In Germany, von Ohain continued to develop his engine until it was dropped in 1941 or 42. He was then transferred to manage the development of the Heinkel-Hirth 109-011 engine with a diagonal centrifugal compressor preceding a 3-stage axial – largely proposed by Helmut Schelp of the RLM. This was close to production when the war ended. The most promising of the axials, the Junkers Jumo 004 and BMW 003, ran into serious compressor and vibration problems and, although the Junkers Jumo 004 had reached initial production by the end of 1943, it only emerged in quantity from the supply chain in September, by which time it was being produced at the impressive rate of 1000/month. However, it continued to prove troublesome, having a time-between-overhaul of 10 hours and a scrap-life of 25 hours. This was partly due to inferior materials but compressor stall remained a major problem.
General Electric, supplied with the very first Whittle W1 engine when it became available in October 1941, had bench-run a GE-produced version (A-I/J31) in 6 months and had developed a series of designs to reach 4,000lbs thrust with the production I-40/J33 in 1945. An axial design, begun at the same time, the J35, reached production at the same thrust by September the following year. Rolls-Royce decided that the successor to the Derwent should be the RB41 Nene, another centrifugal. First run in October 1944, this was designed and built in 7 months (some say 5), produced 5,000lb thrust and powered a number of 2nd generation jets, with license production in Britain, USA, Russia (RD-45), Spain, Canada and Australia. Sir Stanley Hooker, Rolls-Royce Chief Designer, commented that their Avon with its axial compressor took as many years to reach success as the Nene had taken months. De Havilland also followed the Goblin with another centrifugal, the 5,000lb thrust Ghost.
  1. Rolls-Royce Derwent
  2. Rolls-Royce RB41 Nene
Therefore, by the middle of 1945, it could be said that there were 2 centrifugal (British) designs and 2 axial (German) designs of 2,000 lbs plus with a significant production and flight history. The centrifugal ones were simpler, more rugged and had a longer-life – the Welland had entered service in May 1944 with a TBO of 180 hours. Arguably they had also been developed in a shorter time with smaller design resources. Against this, the Allied blockade and bombing had seriously hindered German progress. Pilots who evaluated both British and German jets at this time had little doubt that the British centrifugal engines were more reliable and easier to handle than the German axial ones. There were also 3 second-generation engines, one American and 2 British, of 4-5,000lbs thrust (J33, Nene and Ghost) about to enter service at this time. All of these had centrifugal compressors and could trace their lineage to Whittle’s W1.
However, GE’s axial J35 was only months behind and, with the demand for ever more thrust, the long gestation period of the axials now began to pay off. The Avon and the Metrovik F2- derived Sapphire (which had by now been taken over by Armstrong Siddeley) both entered service in the late 40s with thrusts in the region of 7,000lbs. Both were highly successful and were produced under licence in the USA. At high thrusts the axial became universal. But what would be the choice now, with all that is known of both axial and centrifugal compressors, if one was designing an engine to produce around 2,000lbs thrust, as those early designers were? An example is the Rolls-Royce Williams FJ44-2, an engine of 2,300lbs thrust currently produced for business jets. A fan engine (an idea also first patented by Whittle) it has a 3-stage axial (including the fan) driving a centrifugal compressor. Without the fan, this corresponds to Whittle’s original turbo-jet patent of 1930. The axial/centrifugal arrangement (or a centrifugal on its own) is still the most common configuration in small engines – the blades of an axial becoming too small for the later stages.
Footnote:
  • In late 1943 Frank Whittle and his team at Power Jets embarked on the design of a high-bypass fan engine based on his ideas formulated c1932 and patented in 1936. This had a nine-stage axial compressor with a final centrifugal stage driven by a two-stage turbine and designated LR1 (Long Range 1). There is no doubt that the axial compressor incorporated the knowledge gained over many years by the RAE, one of whose leading scientists was on loan to the company. Regrettably, the project was cancelled by the government when 50% complete and it would be a further 10 years before the first production low-bypass turbo-fan, the Rolls-Royce Conway, would emerge. It would take GE to lead the way into the turbofan era with their TF39 (bypass ratio 8:1) that first ran in 1964.
  • Bibliography and further reading.
  • Genesis of the Jet, John Golley, Airlife Publishing.
  • World Encyclopedia of Aero Engines, Bill Gunston, pub. Patrick Stevens.
  • http://history.nasa.gov/SP-4306/ch7.htm#132