Comparing The Challengers

[Pneudyne]

 

Kratville, in his book on the UP Challengers, said the following in respect of the six 'diverted' to the DRGW:

 

‘The Union Pacific applied in late 1942 for the 3975-3999 series, the engines being finally delivered in mid-1943.  The road desired more units but that is the most the W.P.B. calculated could be put through Alco in the light orders for other roads.

 

‘At the same time, the W.P.B. was assisting the Denver and Rio Grande Western gear up for the eventual Pacific Theatre operations transportation shift and added six additional Challengers to the U.P. order.  The D.&R.G.W. made it clear that it probably would not want the six units after the war particularly because of having to keep parts in stock for just six locomotives.  The Union Pacific did not offer to take them either since by this time Jabelmann was already laying out his own plans to dieselize the road right after the war.’

 

Thus, it looks as if it were predetermined that the DRGW would not keep these locomotives beyond the end of WWII.

 

 

Cheers,

[Pneudyne]

II Phrogs

How did the relatively poor quality coal used by the Northern Pacific impact their performance, and by extension did the conversion to oil for the SP&S locomotives give any advantage or boost to their performance? 

 

 

Nominally at least, one might reasonably expect that the much larger firebox volume and grate area of the NP 4-6-6-4, as well as other changes such as in the type of grate used, as compared with the UP original, was intended to compensate for the poorer quality coal used by the NP.  Whether in practice it  under- or overcompensated is unknown.  Possibly it did both, at different points in the power vs speed curve.

 

With oil firing, grate area is a meaningless parameter.  Firebox volume and firebox length appear to be more important.  Nonetheless, most oil fired locomotive fireboxes seem to have been designed with possible conversion to coal firing in mind, so that a notional grate area was typically quoted anyway.

 

Many of the UP Challengers were converted to oil firing.  None of the available information suggests that there was any differentiation between the oil-fired and coal-fired versions in terms of load schedules or expected end-to-end speed performance.  A reasonable inference is that in oil-fired form, the firebox and draughting details were chosen generally to match the coal-fired form in overall performance, notwithstanding the higher heating value of the fuel oil, say around 18 000 BTU/lb as compared with the 11 800 BTU/lb of the coal used by the UP.

 

Possibly the SPS oil-burning Challenger were setup to more-or-less match those on the NP.  The NP rosebud coal was said to have a hearing value in the range 6200 to 8000 BTU/lb ex mine, 10 000 BTU/lb when dried.  (Was it dried before use?)

 

A different view was expressed by LeMassena in his book ‘Articulated Steam Locomotives of North America’.  Therein he said: ‘It is little recognized that these SP&S articulateds were among the most powerful steam locomotives ever constructed, and they were able to deliver more power than those of the NP because the latter burned a low grade of coal instead of oil.’  One supposes that LeMassena had quantitative evidence to support that statement, and did not simply infer it from the relative fuel heating values.

 

An interesting case was the WP 2-8-8-2, built as an oil-burner, but said to have a notional grate area of 145 ft².  The firebox was 204⅛ inches long x 102¼ inches wide.  The same design was used as the basis for the DM&IR 2-8-8-4, which burned coal with a heating value of 13 500 BTU/lb.  Here the firebox was slightly longer, at 210⅛ inches, but the same width.  But the grate area, presumably chosen to suit the coal used, but also to allow the use of a Gaines wall, was 125 ft²; it did not occupy the whole firebox length.

 

 

 

Cheers,

[Pneudyne]

 

Kratville, in his book on the UP Challengers, said the following in respect of the six “diverted” to the DRGW:

 

‘The Union Pacific applied in late 1942 for the 3975-3999 series, the engines being finally delivered in mid-1943.  The road desired more units but that is the most the W.P.B. calculated could be put through Alco in the light orders for other roads.

 

‘At the same time, the W.P.B. was assisting the Denver and Rio Grande Western gear up for the eventual Pacific Theatre operations transportation shift and added six additional Challengers to the U.P. order.  The D.&R.G.W. made it clear that it probably would not want the six units after the war particularly because of having to keep parts in stock for just six locomotives.  The Union Pacific did not offer to take them either since by this time Jabelmann was already laying out his own plans to dieselize the road right after the war.’

 

Thus, it looks as if it was predetermined that the DRGW would not keep these locomotives beyond the end of WWII.

 

 

Cheers,

 

 

 

 

 

Returning to the DRGW six Alco Challengers, ‘Train Shed Cyclopedia (TSC) #45’ has a tabulation of steam locomotive orders and deliveries from 1939 onwards.  It shows that DRGW placed an order for six 4-6-6-4s from Baldwin in 1942 May, but that this was not approved by the WPB.  An alternative order for six Alco 4-6-6-4s was shown as placed in 1942 June, and approved by the WPB.  The UP order for 25 was shown as placed in 1942 February and approved by the WPB.

 

The UP “big” Challenger was a substantially new design, but it had slipped in before the WPB constraints were applied.  TSC #45 shows that the initial order for 20 was placed in 1941 May for 1942 delivery.

 

If one wanted to make a broad classification of the Challengers by basic design and builder, then the following, with arbitrary numbering, might work:

 

Alco type 1A                 1936                 UP, WP

Alco type 1B                 1940                 D&H, Clinchfield

Alco type 2A                 1936                 NP, SPS (earlier)

Alco type 2B                 1941                 NP, SPS (later)

Alco type 3                   1942                 UP, DRGW

 

Baldwin type 1              1938                 DRGW

Baldwin type 2              1940                 WM

 

 

Some of the Challengers were described in ‘Railway Age’ and ‘Railway Mechanical Engineer’ articles at the time of their appearance.  Known such articles are:

 

UP 'Small'	RA	1936   December 19	pp.900-903
UP 'Small'	RME	1937   January	pp.1-7
NP	RA	1937   March 06	pp.389-391
NP	RME	1937   April	pp.160-163
DRGW	RA	1938   July 09	pp.42-44,70
DRGW	RME	1938   September	pp.323-329
D&H	RA	1940   August 10	pp.207-218
D&H	RME	1940   September	pp.337-344
WM	RA	1941   January 25	pp.209-215
WM	RME	1941   February	pp.45-52
UP 'Big'	RA	1942   October 03	pp.516-519
UP 'Big'	RME	1942   October	pp.413-417

 

As I think is well-known, both journals are available at the Internet Archive, RME under its later name of ‘Railway Locomotives and Cars’.

 

https://archive.org/details/pub_railway-age?sort=-date&and%5B%5D=year%3A%221937%22

 

https://archive.org/details/pub_railway-locomotives-and-cars

 

 

 

Cheers,

[Conductor_Carl]

Bringing this thread back a little bit, does anyone have the unit costs of these? I've seen some numbers thrown around for NP Z-6's (185,000 ish) the UP CSA-1's (130,000 ish) and the last UP Challengers (225,000 ish) but apart from the Z-6 I dont really have faith in these numbers. Would be interesting to see how costs stack up. 

[II Phrogs]

Pneudyne

 

Maybe look at the 4-6-6-4 vs. 2-6-6-4 comparison this way:

 

Start with the UP “big” Challenger of 1942 as a baseline.  Then design a 2-6-6-4 (or 2-6-6-6 if needs be) within the following constraints:

 

1.     The same driving axle load (67 500 lb)

2.     Lateral railhead forces (during curving and arising from restraint of yaw oscillation) no higher at any speed, recalculating the 4-6-6-4 for any lateral control improvements developed for the 2-6-6-4 case that could also be applied to the 4-6-6-4.

3.     No perceptible difference in whole locomotive and front engine unit stability at any speed,

4.     The same factor of adhesion.

 

What then would be the likely advantages conferred by the 2-6-6-4 (or 2-6-6-6), realizable in daily service?

 

(I don’t know the answer, by the way.)

 

 

 

Cheers,

 

I thought this was an interesting set of questions and wanted to revisit it to see if anyone with more knowledge on the subject has any answers? I feel that discussing and finding answers to specific questions such as these challenges (hehe) people to push beyond their natural biases and overcome the usual "well this locomotive is clearly best since it ran in the same geographical location that I happen to be from"! 

[Overmod]

There are a number of references, none yet quite rigorous, which point out the superiority of a proper deep firebox with circulators over something with a wide grate 'over the drivers'.  This is further enhanced if active circulation is provided in the water legs (a la Cunningham circulator, which draws from a downcoming region in the convection section of the boiler, and uses a jet pump to distribute it through nozzles in the outer wrapper above the mud ring).

The deep firebox implies greater mass on the trailing truck, both from the additional metal structure and weight of water.  Note that on C&O, which had dramatically high axle-load capacity, the Alleghenies have a six-wheel trailing truck.

Rear-end stability on a deep-firebox engine of suitable capacity is almost incomparably better than a Challenger.  Look at the Bissel formula that keeps the truck wheelbase 'normal to the railhead' in curves, then extend the truck out so rear bearing and steering forces are as far outboard and to the rear of the chassis as possible, and angle the restoring-force devices (usually rockers or segments of gears) to match the swing radius at the rear.

In a pinch, you could use the dodge that was introduced in the 'intermediate' Berkshire trailing truck frame, when a long frame pivoted at the original 'articulation' point was used as a Delta-style trailer.  This was treated dynamically as a long 2-wheel Delta trailer, with the leading axle only weight-bearing -- it could float laterally on a pair of hardened-steel rollers independently of truck-frame angularity.  Any of the subsequent schemes of lateral-motion compliance could be used on such an axle if desired.

The only American engines with a deep firebox and divided drive that used a four-wheel lead truck were the PRR Qs, and those did not have the 'compound-pendulum' guiding concerns of a Mallet-style chassis.  All the six- and eight-coupled simple articulateds with deep fireboxes used some form of two-wheel Bissel lead truck, and in at least one situation (the B&O EM-1s in M&E service) this caused a service-speed reduction -- I do not know whether this was actually based in engineering analysis or just advisory/CTA.  The primary reason for the two-wheel truck is simple: it reduces the overall length of the engine at the front, rather than by siamesing the firebox over the drivers at the rear; a secondary reason is that all four mains can be the same dimensions for balance without needing to resort to an extended piston rod and crosshead guides a la PRR T1.

We need not look further than the N&W A (and the late Y-class) for a 'correct' two-wheel high-speed lead truck.  It could be argued that LS&MS had some careful design on their 2-6-2s (which were supposed to be among the world's fastest engines when introduced), and that New York Central kludging them into rather inferior Pacifics after the disaster with Wilgus' "1-D-1"s having to have four-wheel engine trucks shoehorned under them was 'unnecessary surgery' from an actual dynamic performance point of view -- but I do not know for sure either way.

Certainly the A was good for 70mph as designed, and I strongly suspect the last 5 (with the lightweight rods) had some 'breathing' done on their lead trucks for better curve entry and mitigation of leading-driver-pair flange forces (the mass of the chassis being considerably less than that of a conventional locomotive with the same truck and driver spacing).

You run into the same overweight problem with any real high-speed articulated just as Lima did with the Allegheny/Blue Ridge types: the weight is in the high-capacity streamlined steam and exhaust piping arrangement, and this would apply to a proper Challenger.  We note that for high-horsepower high-speed designs, nothing more than six-coupled engines are used -- there is no particular reason why you couldn't make a ginormous 2-8-8-x with 76" drivers except that it would be too long, relatively heavy, and possess a diabetic water rate requiring auxiliary tenders to get even mediocre range between water fills.  Any single-unit steam locomotive, reciprocating or turbine, hits that limit somewhere around 8000dbhp, and of course many hit it before.  This, for example, is why we all laughed so hard at the railfan version of Chapelon's Big Boy 'improvement regimen' that would have it producing "10000hp" or greater -- even with proper Chapelon compounding and American detail design for high thrust, you'd have no fun running it at that power.

For fun, start with the rough dimensions of a N&W A (or Seaboard R-2) and give it a double-Belpaire chamber.  This would give you up to 76" potential driver diameter to play with, but I wouldn't go above 72" or so with modern dynamic balancing and thin-section rods.  Use the Allegheny piping dimensions, with some reasonable boiler pressure that doesn't cause ridiculous maintenance requirements (this would likely still be somewhat south of 300psi, and certainly no higher!) and carefully design your chests and valve size to optimize high-speed flow.  (You will definitely want reversible compression control on the forward engine, and that indicates you'd likely want it on all four cylinders).  With proper Snyder combustion-air preheating and Cunningham circulation, this might keep the required firebox+chamber dimensions in line with what a four-wheel truck could bear, and the corresponding grate area might make the engine more flexible to fire at less than full load. 

[II Phrogs]

Forgive my ignorance but would a deep firebox as described be just as useful when using the lesser quality coal utilized by many of the roads that employed Challengers? I was under the impression that the shallow firebox was necessary to increase volume to make up for the lower-energy coal being used, and as such were a necessary compromise.

For the sake of discussion how would a N&W A run using UP Sub-Bituminous coal, or in an even more extreme case Northern Pacific Rosebud coal? Conversely, how would a wide, shallow firebox Challenger run using the high BTU coal used on the N&W?

[Overmod]

Keep in mind that the effective tube/flue length in any modern boiler is in the range of 20 - 24'.  Anything longer than that is a waste, even if used in a Chapelon or Porta sectional arrangement.

The issue with burning subbituminous 'correctly' is that the fuel is extremely friable, and breaks up and levitates in the combustion plume.  You will see ignorant railfans saying something like "40% of the coal never hits the grates" and indeed it doesn't, but it burns similar to carbureted oil, with some of the same luminous-flame advantage that oil has.

In order to get this effect with a typical 'deep' firebox, you would need a method of firing either capable of placing fuel far down under the arch, or feeding at the throat as many methods of oil firing do to get the longest plume.  In practice that would be more difficult than the methods used on Challengers.

There are a couple of presumptions: that the lavish amount of fuel to produce equivalent mass flow of steam is available, and that the net cost of all the fuel is lower than bituminous or oil.  A more important consideration (since 1970, nearly a decade after it ceased to matter on UP) is the extraordinarily dirty exhaust from that method of firing using nothing more complicated than a regular stoker arrangement for control.  Careful tinkering with the secondary-air arrangements and providing effective combustion-air preheat might go a long way toward addressing this stuff.

You wouldn't fire an A on subbituminous or any other low-rank coal; it wasn't designed for that.  It might be said that the Allegheny could be run on lower-quality coal -- but I suspect it, too, would not do well.  That might be different if you arranged to feed the fuel from the throat, something that was the subject of a great deal of development in the late Thirties (and that, as discussed in a couple of other threads, might have been one of the great epic failures of steam technology in practice...)

Of course, there is a better answer: the Garratt.  That gives you unhindered access to a grate that can be as low and wide as your loading gage permits, combined with a barrel diameter uncompromised by driver or frame height and adequate room for multipass Franco-Crosti economization, effective air preheat, etc.  A couple of 2-6-6-2 engines and you have everything a large engine could want for...