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Comet Too

I will describe these in the order in which I tackled them, I first got hooked on the notion of steam powered flight when the UK magazine Aeromodeller published details of David Parker's free flight version of Comet in 1967; it was hailed as 'The most remarkable model of 1967'. I had no lathe and no workshop at that time but twenty years later things were looking very much more favourable because in May 1989, details of David's latest version of Comet, by then Radio Controlled, were published as a short series in the UK magazine Model Engineer. It was probably another 10 years before I got my version Comet Too flying at our local flying field on the South Downs.

Comet is about 7 feet span and is a floater which flies at perhaps 20 mph (32 kph), I have had 20 minute glide times out of a two minute engine run, it needs some thermals of course which adds to the fun. The top picture on the right is 'Comet Too' on the verge of making her very first take off, below that she is making a flying debut at Middle Wallop UK. The photograph of Comet Too's boiler area taken a few years later shows all the evidence of burner heat and the occasional bump and knock from use and three fires.

H H Groves

This man is still remembered in UK for his remarkable series of Flash Steam powered model aircraft from around 1910 to the 1930's, having built and flown Comet I felt compelled to take it all a stage further and make a flash steam engine of one sort or another and I decided to make a copy of his 1936 engine published in the Model Engineer 9th. April 1936. This Engine is quite tiny being of 0.25" bore and 7/16" stroke (6 mm x 11 mm). It is built as designed with a sheet alluminium frame holding the engine, monotube boiler, pumps and lubrication system as a complete one piece assembly which makes it so much easier to remove from the aircraft when repairs and adjustments have to be made. This displays the fact that H H Groves anticipated problems with this very small intricate design, there were plenty of those! I felt I was mixing in very good company as I dealt with them one by one. An aircraft for this engine was designed by another very well known modeller at that time Mr. Richard Trevithick. At this time I found out that Richard Trevithick's son lived nearby and I went to visit him, only to learn that in his opinion that 1936 aircraft was never flown! A pity was that as by that time I replicated the engine with great success in that it went very well and I had decided to build the plane too! This was all a bit fraught in that the engine was too fragile for the load it had to drive and the plane as I had built it was too heavy. Apart from that all was well! That was about 8 years ago from today. I slunk back from that loss and went onto something new which I call Tiddler and this is my first and very successful 'own design' flash steam flying steam engine. That is where I still am in January 2007.


I was very disappointed with the short life of H. H. Groves' 1936 engine: after about 2 hours running the main bearings were worn out as was the piston valve the piston and the cross head, I may have over loaded it and in its new rebuilt form, when I run it I will do so with smaller loads and let it run at higher speeds.

As mentioned I decided to try to design and build an even smaller engine and monotube; the engine is 5 mm bore X 5 mm stroke, uniflow, the monotube is 7 feet (2.1 metres) of 2 mm overall diameter Stainless Steel tube; the bore of this tube is 1.6 mm. Because I had some, the crankcase is made of Titanium but steel would be just fine, the cylinder is Mild steel case hardened, the cylinder head is Titanium. The crank is a single piece turning from a car half shaft softened turned re-hardened tempered then finish lapped, it has two tiny ball races supporting it, the single poppet valve is turned from an old motorbike exhaust valve.

The connecting rod is of mild steel case hardened and the big end is a hand made, crammed roller race with rollers 0.6 mm diameter and 2 mm long. The rollers were made of piano wire lapped and vee grooved at 2 mm intervals in a softened condition hardened and tempered then broken off, I have a tiny 6 mm Boley Watch makers lathe on which these operations become far less irksome, leading to very rewarding results. The ends of the rollers were cleaned off after being broken off under a binocular microscope with the roller held in a tiny pin chuck, the big end has 13 rollers in it.

This is all a bit extreme but it is not so much more than is done to make tiny diesels like the 'Allbon Bambi' deliver a reasonable working life. I have to say that I once had one of those and Tiddlers design owes a little to that tiny gem of IC engine history. However the main inspiration and design for this engine came from Articles printed in the mid 1940's in 'Model Engineer' written by E.M.E. This man's real name was A. A. Sherwood and he built a series of steam powered and very tiny hydroplanes called Dot 1 through to Dot 4, the smallest of which was only 6" (150 mm) long with a twin cylinder engine of only 0.1" (3 mm) cylinder bore.

My engine is a uniflow, poppet valve version of Dot 4 with more compact water and oil delivery systems. Tiddler is not without problems and whilst its performance is excellent the oil system is at fault, as is the tiny water pump. I hope in the course of the next year or two I can put these points right and make reliable runs a matter of course, without a high level of engine reliability more work building the plane is a waste of effort. This engine is built on a lightweight frame that facilitates easy removal from the airframe, it is designed as a pusher and the engine will fly inverted. The boiler may well end up enclosed in its own perforated housing of very thin stainless steel shim forming an environment isolated from the slip stream to aid consistent boiler performance. This is a direct copy of H H Grove's practice, I cannot imagine a more simple, lighter and effective solution. Except of course to find a source of 0.002" (0.05 mm) thick titanium, where do I buy one square foot of 0.002" Titanium shim??

The output of this tiny machine is just staggering, the cubic capacity is about 0.13 cc and it drives a 7 x 5 inch prop at about 5,300 rpm on the bench. To establish an approximate idea of power I removed the propeller and fixed it to a 540 size motor and ran it up to 5,300 rpm at which speed it was consuming 28 watts. To allow for the motor inefficiencies I estimate that would be about 20 watts into the propeller. Now to get a specific power per litre of cylinder capacity I have to say that this engine has a capacity of 1000cc divided by 0.13=7,692 that is 1/7692 of a litre. |So the power per litre is 7692x20 watts. This equals 153,840 watts, now one horse power = 746 watts so 153,840 divided by 746 = 206 horse power per litre! That is going some for any IC engine and it is not a dream it is fact. The steam hydroplanes I have mentioned elsewhere in this site do much better! Bob Kirtley's engine develops at least 5 horse power from 13cc and that equates to 1000/13=77 and 77 x 5 = 385 horse power per litre!!!

So why are cars not steam powered? The explanation is very simple, first the boiler and burner are just dead weight, the fuel efficiency is one tenth that of an IC engine and the water consumption is a horror! In order to recover the water and reuse it needs a big condenser which means more weight plus pumps to drive the water through the condenser, etc. etc. The Beslar Brothers made the best ever effort in history and they only did that as a publicity stunt and were under no illusions about the real fuel consumption and weight penalties involved-----no passenger was flown in the Besler aeroplane which tells its own story!

NO, I regret it is a fact; a steam engine will never show a clean pair of heels to any modern vehicle but steam cars are much more fun!! So are steam models which is why I am here!


Skylark is a boat, not a bird or a plane! I bought Skylark in 2002 her main appeal lay in the fact that she is a flash steam powered vessel of a handy size and in fact the boiler was not that much bigger than some 'model' ones found in racing hydroplanes. It was a new challenge and another chapter in the exercise of learning more about Flash Steam and "Monotube Boilers". I prefer the title 'Monotube' as it is more direct and accurate; it is also the principle of the 'NEW' super economical gas fired domestic boilers we are being urged to buy in order to save the Planet. I got Skylark steaming again in 2003 and my involvement in that work forms part of this renewed and still expanding version of the "flysteam" web site. I will soon begin making a new core for the monotube and a rebuild of the feed pump so I will have a few more notes and pictures to add a few months hence.