Earlier this week I gave an updated version of my ‘Why Metrovicks‘ talk for my departmental postgraduate seminar series, and my past few weeks have been spent looking at sources to flesh out the basic story. So far I’ve looked mainly at the Engine Sub-Committee minutes, but I’ve also spent some time looking at Henry Tizard’s papers at the IWM, and hope to head back to the National Archives to go through the Air Ministry papers and see if I can find anything related to the Directorates of Scientific Research and Technical Development.
In going through the meeting minutes, I’ve come across some wild and wacky powerplant ideas that the ESC asked to consider in the 1930s, some of which I thought I’d share below. Most of these remained paper ideas, but they’re still of interest, as they were all thought at some point to offer a chance of being useful aero-engines. One of the problems in the history of technology is the ways in which the roads never taken are later written out of the story, making the choices taken retrospectively the obvious ones. This whiggish approach flattens the rich texture of history, making actors’ choices seem inexplicable and ‘wrong’ when they do not back the ultimate winners. I’m not going to go into the gas turbine discussions in any detail, which will be no doubt fully covered in future posts; this is about the also- and never-rans.
Interestingly in light of what I’ve said previously about the needs of air defence driving high-powered engine development, one of the earliest explicit statements about the need for very big engines was made in February 1935. The chairman noted that the ARC had referred the issue of powerplants for very large seaplanes to the sub-committee, and he asked them ‘to consider the possibility of producing engines of the order of 10,000 h.p., and to make suggestions as to lines of investigation that might facilitate their production. Further, in considering this problem the Sub-Committee should not confine itself to the internal combustion engine if other types of prime mover appeared to be more hopeful.’ This was an order of magnitude more than contemporary engines, and the sub-committee concluded that cylinder sizes could not be greatly increased without a loss in combustion efficiency and specific weight. Harry Ricardo suggested that a two-stroke might be suited to larger cylinders, as the gas pressure would counteract the inertia forces on the piston. W.S. Farren suggested that since six-throw crankshafts and a 9-cylinder radial engines were currently possible, it should be possible to combine these and build a six-row 54-cylinder liquid-cooled radial – with the caveat that there might be problems in manufacturing the crank-case, and ‘difficulties of a mechanical nature’ might arise!
Summing up, Tizard observed that ‘the general opinion seemed to be that the best method of producing the power was to use small cylinders and devise some method by which their combined power could be delivered to a single shaft. This might take 10 years and hence the question arose as to whether it might not be advisable to concentrate on the C.I. engine.’ Diesels are an interesting case of a road never taken; despite lots of support for them due to the possible advantages in fuel consumption, resistance to detonation (and – possibly – fire safety), they never took off for aviation use; partly because improvements in fuel chemistry meant that petrol engines could be developed to give more power, and partly perhaps because they were perceived to be too heavy, which meant that they were not developed as intensively as they might have been.
Tizard remarked that nobody had suggested using a lightweight turbine with a high-temperature working fluid, and the committee discussed some of the heat transfer reuquirements for such a system. Interestingly, the Velox boiler was mentioned as an example of a technology giving very high rates of heat transfer. Often cited as one of the progenitors of the gas turbine, the Velox boiler consisted of a burner supercharged by a turbine-driven axial compressor; although it was efficient enough to produce a small amount of shaft output, the main point was to achieve high rates of combustion in a limited space. A.A. Griffith was asked to prepare a note for the sub-committee on the practicality of such a system.
At the next meeting, in April 1935, Griffith stated that his preliminary calculations suggested that a condensing turbine would compare ‘very unfavourably’ with an internal combustion engine for aircraft purposes. One of the committee members then asked whether a swash-plate engine had been considered, but in the ensuing discussion Farren pointed out that the number of cylinders – and hence the power output – was limited in this type.
Perhaps more conventional – if only slightly- were Harry Ricardo’s plans for diesel two-strokes. Andrew Nahum’s excellent paper ‘Two-Stroke or Turbine’ examines these engines in detail, but these were to be ‘sprint’ engines of very high power/weight ratio, with the added advantage of running on 87 octane fuel, at a time when it was uncertain how much 100 octane would be available in wartime. The outcome was the Rolls-Royce Crecy, which never entered production. The final idea I want to mention in this post was a suggestion for a gas turbine that was to be driven by the hot gas output of a Pescara free-piston engine; the idea was that the compressor could be located in the fuselage of a large aircraft, and the hot gas could be piped to turbines in the wings providing power to propellers. Overly complex though this may sound, it was seriously considered by the ESC in late 1938, after it had committed to supporting the gas turbine projects underway at the RAE and at Power Jets. Clearly, even at this point, the ESC did not think that their advantages were so obvious as to rule out considering other options, and in my next posts I will look at what the RAE’s early gas turbine projects actually entailed.