Looking at drawings of some challengers - D&H specifically.
It appears part of the firebox floor was blanked off, with the grate area itself only occupying the rearmost 143 inches of the otherwise 200+ inch long firebox. Was this just to simplify ash pan arrangement or for some other reason?

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The short wall in front of the grate is called a Gaines Wall with the front of the arch connected to it. The design dates back to the early 1900's when some non-articulated locomotives had the front of the firebox over the rear drivers. Their grates were flat and were behind the drivers. The area in front of the Gaines Wall was additional combustion space above the drivers. The design was carried over to some of the articulated locomotives with a flat grate and part of the firebox above the last one or two driver pairs. The D&H and the early UP Challengers had extra combustion space in front of the grates. The UP Big Boys and their later Challengers had grates that went all the way to front of the firebox. My guess is that they needed all the grate area that their firebox allowed. Since UP burned a lower btu western coal. The older Challenger had 108 sq. ft. grate area and the later design had 132 sq. ft.

Ya it is interesting that on the later 3980 series challengers and of course the 4000s the entire grate was used and an ash pan was built over the rear drivers.

Maybe they just calculated they didn't need that much grate area on the D&H/CRR challengers? Really there's little difference in size between the earlier D&H and later 3985 style locomotives. You'd think they'd want all the grate area possible like on the various USRA 2-8-8-2s or C&O H8 etc and not blank any off. Or maybe they just really didn't want to deal with an ash pan over the number 6 driver.

Maybe the most drastic use of a Gaines Wall was on the NP’s Yellowstone 2-8-8-4’s, which were built with 182 square feet of grate to burn sub-bituminous coal. After a few years in service, the NP shortened the grate by 2 feet with a Gaines wall, significantly reducing its area. This raised combustion temperatures and allowed more complete combustion of the coal, improving steaming despite the smaller grate area. Apparently they pushed the envelope a little too far with the original design.

Greetings:
Norfolk & Western No. 2050, USRA design 2-8-8-2 built by ALCO-Richmond has a Gaines Wall about four feet behind the combustion chamber.
Original equipment or an N&W modification?
J.David

Not sure on the USRA 2-8-8-2. Ive seen drawings for the y6 series which I'm nearly certain did not have it shown on their drawings. I was thinking the USRA also as per drawings did not have a wall like that.

I've seen arguments that a Gaines wall isn't a particularly effective method for solid-fuel firing, with a great deal of soot and dead 'sparks' accumulating to block the lower rows of tubes for very little actual 'thermodynamic' gain.

The Challenger setup was compromised right out of the gate by having relatively pathetic vertical circulation as well as effective radiant absorbing surface compared to a deep-firebox design. The chamber theoretically has much the same uptake surface as the deep box, but the flow patterns to it, and perhaps the gas temperatures at various points on the surface, won't be the same, even if there isn't severe carryover from the fuel type.

I personally suspect that much of the effectiveness of the later UP Challenger and Wahsatch designs is that the fines in that crappy subbituninous were easily levitated and burned much as oil firing would do, providing adequate luminous flame to the impaired radiant surface. It would be interesting to see if this could be documented for the Fetter(s) Challengers too.

If a 2-6-6-4/6 can be thought of as a 'Brerk-and-a-half' it would be logical to think of Challengers as the corresponding 'Northern-and-a-half' ... except that the steam generation can't cash the checks the 'faster' running gear would permit. Oil firing kinda-sorta solves this when correctly implemented (as I think it is with the Dickens-Barker burner) but when retaining solid fuel firing, the restriction of grate area to what provides clean firing without having to 'force' might be an interesting discussion.

interesting overmod, yet it must not have been that big of a hindrance to have a shallow box as all the 2-8-8-2s, all the 4-6-6-4s etc had them and I've not read anything about them performing poorly. even N&W's own Y series had the shallow style box.

Really the 2-6-6-6/2-6-6-4 deep box was very much the minority for articulated design.
I'd just think you'd want all the grate area possible like on the D&H challengers for max btu output.

You really can't shortchange the deep firebox designs. The N&W A and the Big Boy both had right at 6300 DBHP. The C&O H-8 tested at almost 7500 DBHP. Good WV coal helped a lot. The Big Boy and the later Challengers may have had the best firebox design for the subbituminous coal they had to burn. With the grate going the length of the firebox. There may have been length restrictions and problems with rear overhang of the cab on curves, that would have resulted from putting all the firebox behind the drivers.

The difference between deep and shallow fireboxes is only a relative thing, much like the difference between narrow and wide fireboxes.

Remember that grate area is just the first step in steam generation. The TOF of the evolved luminous carbon is then important, as is the surface area capable of absorbing the radiant heat quickly and the circulation on the waterside that precludes problems like DNB. The object in the box design is to make fire, couple the fire energy to water effectively to make steam, and then separate out the steam as expediently as possible.

This is where the deep-firebox designs right back to Woodard's 2-8-4 have proven their worth over 'wide-firebox' designs that perch entirely, slanted grate and legs and all, over the driver wheelbase (as in the various large coal-hauler 2-8-0s on D&H, RDG, L&NE, etc. as well as the shortest-sheelbase Y class articulateds. Keep in mind that the PRR Q2, faster than any Big Boy or Challenger, developed nearly 8000hp from only five driver axles, with a rigid wheelbase shorter than that of an ATSF 5001/5011 class 2-10-4.

I suspect that Cunningham circulators (and Snyder combustion-air preheaters) would go a long way toward improving the areas where shallow, wide fireboxes have their 'issues'.

Note that the greatest promise is not from a Mallet-type articulated at all; it's on Super-Garratts, where the grate can be almost at rail level, Ashaveyor-type screws can easily move ash from the pan to storage under the barrel, and the barrel length can easily accommodate the appropriate chamber length and 20'-odd optimal tube lengthin the cradle.

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The 2-8-4s had what would be considered a wide firebox. A decent bit wider than like a USRA 2-8-2 or something like a WM 734/Sou 630. The NW J is a quite wide firebox. Just so happens also deep and sat behind the drivers.

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The sentence was intended to be read as "wide firebox designs that are located above driver pairs" -- not wide fireboxes in general.

The Challenger is a wide firebox above driver pairs; many 4-6-0s have wide slanted-grate fireboxes above driver pairs; most 2-10-0s have wide fireboxes above driver pairs... sometimes with significant rear overhang.

On the other hand, while there were some locomotives with deep fireboxes over trailing wheels that were not full-width, that style of construction did not persist very far into the age of Schmidt superheating. There are locomotives with 'wider' grates than usual, both over drivers (as with the D&H, Reading, and L&HR humongo-2-8-0s) and behind them (as with a wide range of Reading power, including the rebuilt T1s) The distinction I am making is only wide over drivers vs. wide with significant drop above a trailing truck.

Where the fun starts is when you have a Garratt with a wide, deep firebox (and the special cradle design needed to accommodate that)


Kelly: the Fetter(s) Challengers were just as you note, a 4-12-2 with better guiding under the firebox and a 'hinge' in the middle. I got the impression that the Jabelmann Challengers went beyond three cylinders of power and a grate area that could be carried over a 2-wheel trailing truck, with the four-wheel lead truck retained for "pre-1928" ideas of high-speed guiding.

Either metaphor could certainly be applied to any class of Union Pacific Challenger. If I remember correctly, Kratville mentioned the "4-12-2 with more cylinders that bent in the middle" too,

OK, what is a Dickens-Barker oil burner? Something developed for the Big Boy?