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How did the external Firebox on Stephenson's Rocket work?

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Simon Robinson 422/02/2017 01:00:38
56 forum posts

Might seem a bit of a dim question but I've looked at loads of pictures and cutaways of the external firebox on Rocket and can't figure where the steam from the water jacket around the firebox goes to. There is a pipe on either side towards the bottom of the firebox water jacket and if it fed up into the main boiler, how would it get into the boiler against much greater boiler pressure? I assume both the firebox water jacket and the boiler would be at different pressures.

Secondly with the pipe exiting the firebox water jacket below the steam line, how does steam get into the main boiler?

Paul Lousick22/02/2017 06:36:56
1408 forum posts
540 photos

I don't think that the firebox had a water jacket, only firetubes to transfer the hot gasses thru the boiler to the chimney. Briggs type boilers are a similar design with no water jacket. The sides of the firebox get very hot and this type of boiler are not as efficient as modern water jacket type.


Adrian Johnstone22/02/2017 08:30:22
21 forum posts

Simon - the authoritative reference on Rocket is

'The Engineering and History of Rocket - a survey report' by Michael R Bailey and John P Glithero, an NRM book from 2000.

Rocket evolved over its lifetime, and this book is an extremely detailed look at the currently existing machine which was stripped down for the process, and an analysis of previous authors' attempts to reconstruct the original design along with Rastrick's contemporary notes. It's a fascinating read.

As built for Rainhill, Rocket did have a water jacket firebox, but the front and back plates were dry. Essentially there was an arch of water that went up and over the fire. Authorities differ on whether the external box was rectilinear or arched; but there is agreement that the inner firebox was arched, and that the two sides of the arch were copper. Rastrick says that the water gap was 3 inches; some believe that to be the external dimension which would have given an inner gap of 2 1/2 inches. The wrought iron backplate was lined with firebricks for insulation (and to protect the driver from heat, presumably).

This saddle has not survived. Probably around 1830, a wrought iron water jacket backplate was fitted with a 3 inch gap, stayed every five inches. This part survives.

The low down pipes you mention that went from the boiler to the firebox were water pipes, not steam pipes. It is a mistake to assume that the water in the firebox would be at a different pressure to that in the boiler. Even were the pipes to be so narrow as to cause a noticable pressure drop (they're 2 1/2 inches in diameter, by the way) the primary energy source is in the firebox, not the boiler, so the pressure would be higher in the firebox. So, you should assume that the water in the boiler and that in the firebox are directly connected and free to circulate. (There is some evidence to show that Rocket might have been part of later experiments to promote water circulation, and thus thermodynamic efficiency, by the way.)

There are also smaller steam connecting pipes from the top of the firebox arch to the top of the boiler; in side views these are often obscured by the cylinders, and since the arch and its pipework are missing on the current remains of Rocket they sometimes get left out of models.

The regulator was bolted directly to the back of the boiler tube plate. Unlike on a modern engine, the top of the firebox was below the top of the boiler, so there was space for the fitings. Originally steam was taken from the boiler directly, or perhaps via a small internal up-pipe. In its original form, Rocket had a serious priming problem (that is, water getting into the steam feed and from there into the cylinders) so, probably in Novenber 1830, a dome was fitted with an extended riser that went up into it.

Apologies for the length of this; I'm fascinated by early-stage technologies.


Edited By Adrian Johnstone on 22/02/2017 08:30:57

Edited By Adrian Johnstone on 22/02/2017 08:32:55

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