Having gone right back to 1804 with Trevithick’s locomotives, I decided to start moving forward again - to Stephenson’s famous ‘Rocket’, which was to put passenger-carrying railways firmly on the map.

 

When I built my Trevithick model, I wanted to put it alongside a model of ‘Rocket’ to illustrate the progress made over 25 years but, although I know I have a 4 mm scale model built from an Airfix kit, ‘somewhere’, I couldn’t find it!

 

I did find however that there is a 3D printable model of ‘Rocket’ on the same Printables website, where I had found the Trevithick 3D model files. Again, the ‘Rocket’ model was designed for a larger scale than my usual 4 mm/foot but, in this case, the scaling factor needed was not so great.

 

The model I downloaded was by Václav Krmela, who has made his .stl files available under the Creative Commons (4.0 International License) Attribution-NonCommercial   His model was designed for 1:32 scale, so needed reduction by 42% to print at 4 mm/foot.

 

Most of the parts printed satisfactorily at the smaller scale but, as with the Trevithick model, I had to thicken the walls of the boiler and chimney to make them sufficient robust. I particularly liked the way in which the design incorporated location tabs, which made it very easy to assemble the printed parts in their correct orientations. For example. I show the basic chassis which has tabs to locate with both the firebox and boiler:

 

Components of Václav Krmela’s model, showing helpful tabs

 

These tabs worked perfectly well on my reduced-size prints and are a feature I must aim to include in my own future models.

 

A feature I didn’t like was that the cross-heads on both sides of the engine were fixed in the same position, so didn’t allow for ‘quartering’ of the driving wheels. When I imported the relevant .stl files into ‘Fusion 360’, however, and converted the mesh bodies into ‘base features’, I found that they split into two parts – the piston body and the piston rod and slide bar – so I was able to re-position the cross heads to appropriate positions along the slide bars. This was unexpected and a feature that must have been ‘lost’ when the .stl files were created!  I have found that the ability to import .stl meshes and convert them to ‘base features’ in ‘Fusion 360’ is very useful, as it allows modifications to be made to downloaded model files.

 

 

Bodies created in Fusion 360 from original .stl file

 

To check the overall assembly, I brought all the parts together within ‘Fusion 360’, as shown below.

 

 

Vaclav Krmela’s mesh model, assembled in ‘Fusion 360’

 

So now, all I had to do was print the parts. I printed some of the smaller parts in groups, including a set of wheels and a set of various small pipes, rods, and stays. The latter group was on the borderline of what is possible at my chosen scale and I made several attempts before I managed to print a reasonably ‘clean’ set. These tiny parts only took 1 minute to print but adhesion to the print bed was rather ‘hit and miss’ until I tried applying some ‘Pritt Stick’, which increased the success rate considerably – here’s a set, as printed, warts and all:

 

3D printed pipes, rods, and stays, before fettling

 

I attached these fragile parts by applying a touch of a 200° soldering iron tip  while holding the ends in place with fine tweezers.

 

It has been an interesting print job although, looking at the assembled model, the pipework in particular is rudimentary, when compared with the full-size replica. A very significant omission is the pair of pipes carrying exhaust steam from the cylinders to the base of the chimney – this use of the exhaust blast to ‘draw’ the fire was an important factor in the success of Rocket at the Rainhill trials.

 

Full-size Rocket replica at Tyseley, 25 June 2011

 

I felt I had to rectify this omission, so I made 3D prints of the two upper pipes and the pipe at the bottom of the firebox. Again I used ‘Pritt Stick’ to ensure that these tiny parts didn’t wander on the printer bed.

 

My additional 3D-printed pipework

 

Printing this model has served my purpose in providing a comparison to the Trevithick model, so demonstrating the progress made over the intervening years. The appearance of the ‘Rocket’ model does suffer, though, from the ‘Tri-ang’-like profile to the wheels!

 

 

3D prints of the downloaded Trevithick and Rocket models

 

Whereas Trevithick conceived his engine as a ‘general purpose’ machine, ‘Rocket’ was built to meet the very specific requirements laid down for the Rainhill Trials. In particular, ‘Rocket’ was designed to haul light loads at high speed and Stephenson realised that, for this task, adhesion weight would not be a critical factor.

 

Nevertheless ‘Rocket’ needed improvement for general service use, so there were further rapid developments by the Stephenson company. The next step was the ‘Planet’ class 2-2-0, which introduced the use of a pair of inside cylinders under the smokebox, an enduring feature of British locomotives. ‘Planet’ was followed by the ‘Patentee’ in 1833, which established the 2-2-2 as the archetypal British express engine and could be described as the first ‘modern’ locomotive.

 

A detailed description of Stephenson’s ‘Patentee’ Locomotive Engine was published by John Weale in 1838 “by the liberality of Robert Stephenson, Esq., having been written under his direction and revision by Mr W.P. Marshall”

 

Stephenson’s ‘Patentee’

 

My model of the broad gauge ‘missing link’ is an example of this type of engine, which set the pattern for express engines until increasing size and weight dictated the use of more carrying wheels.

 

Mike