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‘Fire Fly’ class - Research






One thing leads to another and what, for me, started as a small project to build the interesting-looking Posting Carriage from the early years of Brunel’s Broad Gauge railway, for the GWR, rapidly extended to include a Luggage Truck and Horse Box.


All these vehicles appeared in some of the beautiful lithographs by J.C Bourne, published in 1846. In particular, his illustration of Bristol Temple Meads Station, shows an engine of Gooch’s ‘Fire-Fly’ class heading a train including vehicles that I have now modelled.



extract from a Lithograph by J.C.Bourne, 1846


Thus, the next step for me was clearly to add a model of this type of engine, in order to create a complete train from the 1840’s period, when railways were on the threshold of transforming the mobility of goods and people throughout Britain.


Background Research


Before starting to create a model, I needed to learn a lot more about the ‘Fire-Fly’ class of engines.  These became famous as the designs by the young Daniel Gooch that transformed the fortunes of the GWR.  The first engines that had been delivered in accordance with Brunel’s specifications had mostly been complete failures, although some became useful after extensive re-building.


Gooch saved the day when he managed to procure the engine ‘North Star’,  which Stephenson’s had to hand from a failed overseas contract.  This engine and its subsequent companions inspired Gooch to create his own design, incorporating new principles that paved the way for ‘mass production’.  He supplied drawings and templates to several different manufacturers, insisting that they conformed to his detailed specifications, so that replacement parts would be common to the whole class.


It was a period of massive development in workshop practice, with engineers like Maudslay and Whitworth introducing methods that allowed accurate machining of components, including such key items as screw threads, to standardised dimensions. A the same time, new metallurgical processes were producing hardened surfaces that resisted wear and tear, so allowing reliable operation of machinery over long periods.


It was these overall design features, rather than individual mechanical innovations that were to make the ‘Fire Fly’ class such successful engines. Brunel’s adoption of the Broad Gauge helped, in that it enabled all the working parts to be laid out in a spacious and easily-accessible manner.


Two reference books in particular proved very useful to me, in finding more details about the ‘Fire Fly’ class: (i) Part Two of the RCTS series about ‘The Locomotives of the Great Western Railway’ provides dates and dimensions of all the engines, with information about their service lives, (ii) a more recent publication by Rev. Canon Brian Arman in Part Two of his survey of ‘The Broad Gauge Engines of the GWR’ contains a wealth of drawings and photographs of the class, throughout its period of use until 1878.


For a contemporary source, Wishaw’s ‘Railways of Great Britain and Ireland’, 2nd ed. published in 1842 contains the following drawing:




There is another contemporary description in David Joy’s diary (he later designed the ‘Jenny Lind’ engine) from his early working days in 1842, with Fenton, Murray and Jackson: “… and then got at the interest in the engines. These were Great Western Railway passenger— engines, 16 in. by 20 in. cylinder; 7 ft. wheel. They were a very handsome looking engine, with bright brass dome, and wheel splashers—old fork and gab motion—and I fitted one of these forks, having learnt to file and chip and [he adds] to mash my knuckles with the hammer.



Similarities and Differences


In spite of Gooch’s tightly controlled specifications, there were many differences in detail between the products from the different manufacturers, which affected the overall appearance of their engines. Thus, as so often happens in railway modelling, it is necessary to look at individual engines in order to create an accurate model.


One very obvious difference lay in the shape of the outer firebox cladding (although the internal dimensions were standardised) Some manufactures used a ‘Gothic’ shape with prominent ‘arches’ on each side of the firebox, while others adopted the ‘Haycock’ shape, with a domed steam space above a rectangular lower cladding.


Another obvious difference lay in the position of the manhole cover, provided to allow cleaning of the interior of the boiler.  In most cases, these covers were on top of the cylindrical boiler but, in those engines built by Fenton, Murray, and Jackson, the cover was on the front face of the outer firebox. This distinguishing feature is clearly visible in the engine shown in the Bourne lithograph, above.


Rather surprisingly, these Fenton Murray and Jackson engines also had an overall wheelbase that was 2” (50 mm) longer than other engines. This alteration must surely have been approved by Gooch but the reason is unclear.  Gooch is, however, reported to have said that these engines were the best of the class.


As with most engines, variations soon began to creep in, with various re-builds. Some changes were major, such as conversion to tank engines, while others related mainly to boiler fittings, such as removal of the separate safety valves, originally mounted towards the front of the boilers in tall cylindrical brass casings.


When first built, all the engines used the ‘gab gear’ to operate the valves, which was based on that used by Stephenson in ‘North Star’.


‘Gab’ valve gear


By the mid 1840’s the advantages of variable valve gear were being recognised, to allow ‘expansive’ working of the steam, which reduced coke consumption. The Gooch brothers devised their own version of expansion gear and this was applied retrospectively to existing engines, over a period of several years, with 26 engines having been converted by 1849.  Another improvement to the ‘Fire Fly’s was lengthening the piston stroke from 18” to 20”, which seems to have started in around 1844, some of the later engines being delivered new with the longer cylinders. It seems possible that this modification was applied to existing engines at the same time as the replacement of the valve gear.


Gooch Expansion Gear


In later years, round-top fireboxes were substituted for the ‘haycock’ design and the boilers were extended by about two feet, so that these engines came to resemble the proportions of much later locomotive designs, with cabs and other ‘modern’ features making their appearance!


So, what to choose? I decided to be guided by the Bourne engravings and also by a very clear early photograph of ‘Argus’, which was a Fenton Murray and Jackson engine, as shown below:



Fire-Fly class engine ‘Argus’



My 3-D Model of the Firebox


My main challenge in designing a model of this engine lay in the shape of the ‘Haycock’ firebox. I puzzled over this for some time, drawing arches in ‘Fusion 360’ and attempting to fill in the panels between them. It possibly can be done in this way but I didn’t find out how, so I eventually settled on a different method, as follows:


I first drew the outline of the base and added an arc quadrant in the vertical plane as shown in ‘Step 1’ below. I then extruded the arc to form a half-arch, as shown in ‘Step 2’ below. Next, I drew a triangle in the horizontal plane below the arch and used the ‘extrude tool’ in ‘cut’ mode, to produce one segment of the ‘Haycock’ top of the firebox:


Steps in Creating 3D ‘Haycock’ Firebox


After that, it was simply a case of copying the first segment and moving and rotating it, to form the remaining three segments. Once all the parts were in position, I used the ‘Combine’ tool to join the segments of the firebox into a single body. As a finishing touch, I used the ‘Fillet’ tool, to round off the corners.



3-D design for ‘Haycock’ Firebox


Measurements of various drawings showed that the ‘Haycock’ tops on the prototypes were a little higher than indicated by my result, based on circular arcs, but this can easily be adjusted in the ‘Cura’ software, immediately before printing, by a change to the vertical scale.


Next Steps


Constructing the firebox top was the main difficulty I had expected to encounter, when designing my 3D-model of a ‘Fire Fly’ class engine.  I am now reasonably confident that I can continue with the design of the remaining parts of the engine, by using the same ‘hybrid’ techniques that I have used previously for my various existing designs, for both the Broad and Standard gauges.


Brass tube for the boiler is now on order from Cornwall Model Boats.



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Now this is really exciting! By coincidence I’ve just modelled some very similar sandwich frames for my 1841 Rennie ‘Satellite’ built for the London & Brighton Railway. Looking forward to seeing your project develop.


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Thank you 5&9.  Those frames do look extremely similar to what I shall be tackling!  I've not worked it out yet but I'll probably use brass for the inner frames and 3D-print the cosmetic outers but I've not got that far in the planning yet.  My brass tube for the boiler arrived in today's post, so I need to make a start soon :)

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Lengthening the stroke of the cylinders would also have involved forging a new crank axle.

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The journey continues! Thanks for taking us along, Mike :good:


I enjoyed the bakground info on the class. I don't have all the RCTS volumes, and was wondering when and why Gooch abandoned the Haycock firebox for his own roundtopped version, e.g. on the Pyracmon class.

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12 hours ago, Compound2632 said:

Lengthening the stroke of the cylinders would also have involved forging a new crank axle.

Agreed - but it certainly was done.  There was much trial and error in those early days and they must have found some significant advantage in the lengthened stroke.  In later life, the remaining engines were virtually unrecognisable as 'Fire Fly's.


6 hours ago, Mikkel said:

The journey continues! Thanks for taking us along, Mike :good:


I enjoyed the bakground info on the class. I don't have all the RCTS volumes, and was wondering when and why Gooch abandoned the Haycock firebox for his own roundtopped version, e.g. on the Pyracmon class.

Once you start a little research, it's amazing how a complex story usually emerges!  Even these 'standard' locomotives show a considerable amount of variation.  For anyone interested in early Broad Gauge engines, the Brian Arman series of books contains a wonderful collection of drawings and photos.  i believe that Part Three is in the pipeline.


Those 'haycock' tops must have been expensive to produce and, once the round top had been found to offer sufficient steam space, I expect the change was for economic reasons.  The various series of 'Standard Goods' are an interesting example of 'continuous improvement'


Perhaps, one day, I'll make a set of models to show the line of development - but don't hold my feet to the fire on that idea :)

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