Alan Arnold Griffith (b 13 June 1893 - 13 Oct 1963) was a British engineer, who, among many other contributions, was one of the first to develop a strong theoretical basis for the jet engine. He followed this up with work on his design at the steam turbine company of Metropolitan-Vickers, but developer was so lacksidasical that Frank Whittle's efforts at Power Jets was first to deliver a working design.

Griffith took a first in mechanical engineering, followed by a Master’s Degree and a Doctorate from Liverpool University. In 1915 he was accepted by the Royal Aircraft Factory as a trainee, before joining the Physics and Instrument Department the following year in what was now been renamed as the Royal Aircraft Establishment (RAE).

Some of Griffith's earlier works remain in widespread use today. In 1917 he and G.I.Taylor suggested the use of soap films as a way of studying stress problems. Using this method a soap bubble is stretched out between several strings representing the edges of the object under study, and the coloration of the film shows the patterns of stress. This method, and similar ones, were used well into the 1990s when computer power became generally available that could do the same experiment numerically.

Griffith is more famous for a theortetical study on the nature of stess and failure in metals. At the time it was generally taken that the strength of a material was E/10, where E was the Young's modulus for that material. However it was well known that those materials would often fail at 1000 times less than this predicted value. Griffith discovered that there were many microscopic cracks in every material, and hypothesized that these cracks lowered the overall strength of the material. This was because any void in a solid concentrates strain, a fact already well known to machinists at the time. This concentration would allow the strain to reach E/10 at the head of the crack long before it would seem to for the material as a whole.

From this work Griffith formulated his own theory of brittle fracture, using elastic strain energy concepts. His theory described the behavior of crack propagation of an elliptical nature by considering the energy involved. The equation basically states that when a crack is able to propagate enough to fracture a material, that the gain in the surface energy is equal to the loss of strain energy, and is considered to be the primary equation to describe brittle fracture. Because the strain energy released is directly proportional to the square of the crack length, it is only when the crack is relatively short that its energy requirement for propagation exceeds the strain energy available to it. Beyond the critical Griffith crack length, the crack becomes dangerous.

The work, published in 1920, resulted in sweeping changes in many industries. Suddenly the "hardening" of materials due to processes such as cold-rolling were no longer mysterious. Aircraft designs immediately understood why their designs failed unless they were built much stronger than it seemed they should, and soon turned to polishing their metals in order to remove cracks. The result was a series of particularily beautiful designs in the 1930s, such as the Boeing 247. This work was later generalized by G. R. Irwin , in the 1950s, applying it to almost all materials, not just rigid ones.

In 1926 he published a seminal paper, An Aerodynamic Theory of Turbine Design. He demonstrated that existing turbines were designed incorrectly and the blades were "flying stalled", proposing a modern airfoil shape for the blades that would dramatically improve performance. The paper went on to describe an engine using an axial compressor and two-stage turbine, the first stage driving the compressor, the second a power-take-off shaft that would be used to power a propeller. In modern terms he described a turboprop engine. As a result of the paper, the Aeronautical Research Committee supported a small-scale experiment with a single-stage axial compressor and single-stage axial turbine. Work was completed in 1928 with a working testbed design, but for some reason at that point work stopped.

At about this time Frank Whittle wrote his thesis on turbine engines, using a centrifugal compressor and single-stage turbine, the leftover power in the exhaust being used to power the aircraft directly. Whittle sent his paper to the Air Ministry in 1930, who passed it on to Griffith for comment. After pointing out an error in Whittle's calculations, he stated that the large frontal size of the compressor would be it unacceptable for aircraft use, and that the exhaust itself would provide little thrust. The Air Ministry replied to Whittle saying they were not interested in the design. Whittle was crestfallen, but was convinced by Johnny Johnson to persue the idea anyway. Luckily for all involved, Whittle patented his design in 1930.

After his own work, Griffith moved to became the principal scientific officer in charge of the new Air Ministry Laboratory in South Kensington. There he evolved the contraflow gas turbine design, which used two sets of compressor disks rotating in opposite directions, one "inside" the other. This is as opposed to the more normal design in which the compressors blow the air against a stator, essentially a non-moving compressor disk. The effect on compression effeciency was noticable, but so was the effect on complexity of the engine. In 1931 he returned to the RAE to take charge of engine research, but it was not until 1938, when he became head of the Engine Department, that work on developing an axial-flow engine actually started. Joined by Hayne Constant, they started work on Griffith's original design, working with steam turbine manufacturer Metropolitian-Vickers (Metrovick). After a short period Whittle's work at Power Jets started to make major progress and Griffith was forced to re-evaluate his stance on using the jet directly for propulsion.

A quick redesign in early 1940 resulted in the Metrovick F.2 Beryl, which ran for the first time later that year. The F.2 was ready for flight tests in 1943 with a thrust of 2,150 lbf, and flew as replacement engines on a Gloster Meteor, the F.2/40 in November. The smaller engine resulted in a design that looked considerably more like the Me 262, and had improved performance. Nevertheless the engine was considered too complex, and not put into production.

Griffith's original rejection of Whittle's concepts has long been commented on. It certainly set back development of the jet engine in England by several years. His motivations have long been the topic of curiosity, with many people suggesting that his endless quest for perfectionism was the main reason he didn't like Whittle's "ugly" little engine.