Steam speed records including City of Truro and Mallard

Maybe….but science don’t care much for the feelings of the room!

 

This possibly notorious sketcher of fictional GWR locomotives had a go at sketching up a post Churchward broad gauge locomotive and gave up! It speedily became apparent - in my opinion at least - that it would be an entirely blue sky job, and just about nothing from standard guage practice would be applicable. Some of the things that became apparent to me:

A 4 cylinder compound is almost a given with room for two huge cylinders between the frames and no more room for cylinders outside than standard gauge.
A 'wide' firebox is unnecessary if you have more than two feet extra width for a 'narrow' one.
Weight is a massive issue* (and this is where I really gave up). Yes, you can have a 4-6-0 with a huge boiler and firebox, with far more steaming capacity than any pacific, but what's it going to weigh? Potentially way more than any Pacific, let along a King. But if you try and put more wheels on what do they carry? There's a ferocious concentration of weight, but if you spread it out by say making the boiler longer and thinner you start losing the advantages of the large diameter boiler and...

You get the picture? I rapidly came to the conclusion that I don't have the knowledge or expertise to draw something credible. A Mallet or Garratt configuration might make sense, but that's even further from anything I might draw with even an ounce of credibility.

*that wasn't a deliberate pun, honest!

 

Certainly whenever I've seen train speeds measured using GPS online a disclaimer that GPS is not accurate is inevitable.

 

GPs is accurate enough to send missiles through chosen Baghdad windows. The problem appears (somewhat anecdotally) to be sloppy usage.

 

Oh well. If you ran into difficulty with your design attempt, the rest of us are unlikely to get far!

Some Pacifics were developed for 5ft 6in gauge, including a class of 4-cylinder compounds built by North British in 1929 for the Bengal Nagpur Rly. Cylinders were in the de-Glehn position - 16½in dia HP outside and 25in dia LP inside. Engine weight of 106t with axle-load 22t. The engines had drivers of the modest size of 6ft 2in and probably never ran close to GWR-like express speeds.

https://www.steamlocomotive.com/locobase.php?country=India&wheel=4-6-2&railroad=bn
http://sundar.altervista.org/M462BNR.htm
https://www.flickr.com/photos/124446949@N06/51554923034

The German Class 05 4-6-4s and Class 61 tanks were possibly unique in having 2.3m (7ft 6in) drivers. As far as I am aware, no other 6-coupled locomotives had drivers larger than 7ft, used on some American 4-6-4s.

 

OK, my 'O' level physics from 40 odd years ago is very rusty, but after doing a bit of googling to refresh my knowledge, I think the above might be slightly wrong, but I'm probably wrong as well!!!

Basically Hermod has based the calculation on a mile long section of track descending at 1:480, with a starting speed of 122.8mph (54.91ms-1) for 7.275 seconds, then accelerating to 125mph (55.878ms-1) over 14.5 seconds and staying at this for 7.275 seconds (i.e. 1/4 mile), for a total time of 29.05 seconds over the mile.

I agree with the kinetic energy calculations for the 54.91 and 55.878 ms-1 speeds (0.5 x m x v^2) and resulting difference between the two. However, I think the gravity assist (potential energy, mgh) hasn't been applied correctly for the 1/2 mile acceleration portion. Shouldn't this be subtracted from the required energy? If this is the case then:

m = 400tons x 1000, g = 9.8, h = 1/480 x 1609.3metres x 0.5 (the half mile portion)-> 1.676 metres = 6.57MJ
Therefore energy required is (21.5-6.57) 14.93MJ, which over 14.5s = 1026kW (excluding air and rolling resistances)

This is the energy required to accelerate the train to 125mph from 122.8mph, the power required to maintain the train at the speed prior and after acceleration is that which is required to overcome the air and rolling resistances etc. (minus any gravity assist). I don't know what this power is, but if the graph provided by Simon can be used, this equates to approx 375kW for air resistance at 125mph. So total power required to accelerate to 125mph is 1025kW + 375kW + the other resistances, so 1400kW + other resistances. Which is much less than the 2945kW that Hermod implies is required?

Presumably we could deduce the air and rolling resistances if the train was coasting with the regulator closed and no braking forces applied and we're able to determine the deceleration rate?

Anyone with a physics background able to verify/help further/expand on this?

 

Lovely .
Numbers .
The last fast mile and a quart of Mallard is more complicated than my hypothetical lump of 400 tons.
At MP91 gradient changed to zero and at MP90.5 to 1:240.
Change from one gradient to another on real rails are not instantaneus I think and mass of train stretches over 154 m.
Air/roll- resistance power for Mallard et al at 124.25 mph was more than 375kW.
See letter 1082 for an estimate

Mallard by Hermod posted Dec 5, 2023 at 10:03 AM

 

Though 89001 bogies are very similar if they need publicity a speed record is the way to go- doubt it quite has the power unless upgraded though.

 

The picture in letter 665 shows the paper drum drive and there is a worm and a worm wheel.
If the worm wheel is 0.2mm eccentric there will be a cyclic speed error of the magnitude discussed before.
If the two hexagonal nuts securing the worm is taken of,worm lifted and put on the twin brother shaft and secured with same nuts ,relation of paper feed and wagon travel is reversed.
I believe.
To do the same for the force measuring device is most likely more difficult and maybe not possible.
In letter 668 it is mentioned that car registered a drawbarhorsepower of 1800 at 116mph going down 1 to 200.
Only problem is that one source says 116mph happened at MP96 and another at MP94.
Part of these 1800 comes from gravity assist of locomotive so how much water and coal can there have been onboard at that point?

 

At the time, the murderous, genocidal Nazis had the record- so Gresley broke a locomotive to beat them. It was taken off the train well broken big end, so I've heard. But mission accomplished, giving the nation a feeling that Hitler could be beaten.