Engine options

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.

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11 Responses to “Engine options”

  1. Ross Says:

    Jakob if defence is driving engine development it might be worht having a look at the papers of the Air Fighting Committee and see if anything is mentioned in there.

  2. Jakob Says:

    Thanks for that Ross; I’ll have a look when I’m next down there.

  3. Ross Says:

    Jakob the references are:

    Minutes of Meetings – AIR 5/1126

    Papers – AIR 2/1651

    It appears the key scientist on the Committee is Professor Melvill Jones.

  4. James Says:

    Fascinating post, and a good highlight of the other power routes thought about. I’d add steam aero engines – Adler’s Eole and the Basler Steam engined aircraft were tried, and worked to differing (limited) extents.

    “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;”
    But they were used in aviation! Not only that, but they were perceived to offer greater fire safety than standard petrol engines for a time. The Germans undertook a good deal of diesel engine development and a number of their aircraft in the 1930s were diesel powered. (The Bv 138, Do 18 and 26, and notably the Ju 86, including the high-altitude Ju 86P.) Notably also, the diesel engine was very popular in airship design, particularly given their heavy fuel adding safety to the operation of airships. The Wikipedia page gives a somewhat disorganised overview of the ramifications of the aircraft diesel.

    http://en.wikipedia.org/wiki/Aircraft_diesel_engine

    Certainly they were of limited utility and so far have proven to lack a long term critical advantage to establish any widespread use, but they certainly ‘took of’ and landed successfully, in military and airline service.

    Sometimes they are overlooked in English language publications as they can be referred to as ‘heavy oil’ engines.

    Regards,

  5. Jakob Says:

    James: My ‘didn’t take off’ was meant metaphorically (with bonus unintended punnery!) rather than literally, but I can see that I was rather unclear! I was aware of the Junkers engines (built under license in the UK by Napiers), but they weren’t hugely successful to the best of my knowledge.

    The interesting thing about the Ricardo diesel designs put to the ESC is they were to take advantage of the CI engine’s resistance to detonation to raise compression ratio and engine boost, and so develop high power on low-octane fuel at an acceptable weight, rather than aiming for low fuel consumption. Once very high power became of the essence, one of the Ricardo test units was converted to petrol operation, and promised extremely high power/weight ratios; it was taken up by Rolls-Royce and developed to become the Crecy. The Nahum paper on the two-stroke isn’t entirely clear on this, but from what I’ve found in the Tizard correspondence, initially at least the petrol two-stroke test units were run on 87 octane, and still produced high power; in the pre-war years this was of interest because of the uncertainty about 100-octane supplies.

  6. Urban Garlic Says:

    This is great stuff. Not immediately useful to you, but in the late 70s and early 80s (and probably today, too) another good place for “roads not taken” was Popular Science magazine. They had an automotive focus, and it was there that I first heard of swash-plate engines, and read numerous articles about how they, or Stirling engines, or two-strokes, or Wankel rotaries, were going to save us all.

    I’m not a historian by any stretch, but I’m enormously pleased that projects like yours exist, and that the roads not taken are also not forgotten.

  7. James Says:

    Perhaps aero diesels weren’t ‘hugely successful’, but I’d suggest that production and service (both military and civilian) is a reasonable criterion for ‘successful’; and if one is concentrating on designs that didn’t even get through testing, one needs to be a bit more precise about machinery that did achieve production. Certainly I’d expect to be picked up by a reader or an examiner if I ignored those that flew in my writing.

    Fascinating area!

  8. Jakob Says:

    James: A fair point, and I should be careful not to conflate the high-powered diesels that didn’t achieve production with the more conventional ones that did. A quick google suggests that Junkers built about 900 Jumo diesels, which is not bad by any standard. Perhaps it would be better to say that their success was limited, judged against the predictions being made at the time. Whenever the diesel is mentioned in the ESC minutes, there is a clear sense that with development it could become the dominant powerplant type, at least for civil aviation and bombers.

  9. James Says:

    I’d agree with that. There’s an attraction of ‘fancy but failed’ over ‘dull but limited’ in these cases, but we do have to tip the hat to the limited journeyman. Junkers gets a poor deal, IMHO, given their range of contributions to sucessful aero-innovation is rarely gathered togerther and recognised.

    You’ve got me thinking of a blog post on the number of methods of aircraft power.

    I’ve got a good list started…

  10. Chris Williams Says:

    Your starter for ten . . . Did anything come of that proposal in the early 1920s to fit the Bristol Tramp with a steam turbine? Was there continual RAE interest in this option throught the 1930s – Or was Griffiths’ conclusion in 1935 that the condenser requirements too steep result of re-opening the question for the first time in some years?

    I always favoured the JATO + steam turbine WIG machine as the best road not taken, but that’s owing to my love of baroque technology. It would have made a lousy interceptor, I’ll grant you.

  11. Jakob Says:

    Hi Chris! I missed the email telling me this was stuck in the moderation queue, sorry. Any future comments should now go through automatically. Paxo wouldn’t be impressed…

    I haven’t seen anything else about steam turbines in the RAE stuff I’ve looked at; from the comments in the Engine Sub-Committee minutes, it seems to have been the first time the question had been looked at in a good few years, and was perhaps prompted by reports from the US that GE and the Lakes Co. were considering building one.

    I’m with you on the dieselpunk; a WIG with a Velox boiler and steam turbine would be suitably baroque… You’ve read Charlie Stross’s ‘Missile Gap’ I assume?

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