A1 (Peppercorn) and A4 boiler dimensions

Which is why it is so difficult to design a really good one. Many designers (the vast majority) did not understand the nature of the machine they were dealing with and this in turn explains why it is so easy to achieve improvements. As yet there is no textbook that gathers together the full range of available information, data, research, invention etc. on and about the steam locomotive so that the interested reader would gain a full and balanced understanding of the subject. So Jimc, if you are having a bit of a struggle don't worry about it but remember that if designers didn't understand how can enthusiasts without some effort and struggle.

Cheers

(note to self: must write that book)

 

A couple of sources of good info on the shortcomings and potential for improvement :

The 5AT website which details proposals for a state of the art design

David Wardle's book Red Devil which documents his rebuild of an SAR 25NC

David is also the lead designer in the 5AT project.

 

I believe the 50sqft firebox of the A1's was something of an accident, resulting from their descent from the P2's via the A2/2's rather than being a totally 'clean sheet' design. It's questionable whether a new design would need a grate bigger than the A3's/A4's, because you are already at the limit for a normal fireman.

 

It is my understanding that the 50 sq ft grade area was considered beneficial with post war coal. It is considered that given equal quality coal that the output power of a locomotive is ultimately limited by the grate area. This could have been influenced by the fact that post Gresley the importance of the Kylchap exhaust doesn’t appear to be well appreciated. The A4 / A3 locos were having steaming issues and maybe the larger grate area was seen as a positive step. I have read that Gresley left instructions that all the A3 and A4 were to be fitted with Kylchap when the patent on the system expired, but Gresley died and it didn’t happen. As we now know when the A3s and A4s where all fitted with Kylchap in the late 50’s they steamed very well and if you look at published figures for dbhp/sq ft grate area the A4 is usually well ahead of anything else. So, as so often, Gresley Was Right.