Why so? Are tender locos' reversing gear designed differently from tank engines?
I don't know the ins and outs of it, but it was one of the reasons given to me for avoiding tender-first running when I was involved in Steam Tours with NR. I think it's to do with the Chimney updraft not drawing air as efficiently through the firebox, restricting air for combustion, because of the airflow over the top of the loco. The reversing gear may also had something to do with it.
In addition to the effects on draught through the chimney, I'd imagine aerodynamics have an effect as well. Smokeboxes are not very streamlined, but they must be better than the flat rectangle at the back of most tenders, and their area is smaller too. Similarly cab fronts were often angled in later designs, while when running tender first the side sheets of an open cab will tend to scoop passing air in, disrupting the flow.
Given how un-aerodynamic the wheels, valve gear, cab, and all the other stuff on the loco is, I'd be willing to bet aerodynamics have a negligible impact on loco performance, other than by their impact on the draught through the firebox. Especially given the Merchant Navys and West Country/Battle of Britains were rebuilt from Air-Smoothed casing to convetional boiler barrels, and many LMS streamlined locos later had the casing removed.
In theory a steam loco should be as fuel effective in forward as reverse (think of a Garrett type loco, where one engine is in forward and the other on in reverses). the updraft at the chimney is caused by the blast pipe (the exhaust from the cylinders) not by any air passing over the chimney. So as long as the dampers at the front and rear of the ash pan are of a similar size you will get the same air flow through the fire bed.
The primary purpose of smoke lifting and deflection measures in a design is to ensure forward visibility is maintained at all times for the crew. When running tender first, the chimney would be behind the cab so there should be no visibility problems from the exhaust, although the massive tender of an A4 does tend to get in the way instead! Perhaps a rear-facing camera might be concealed in the passageway of a Gresley corridor tender, looking out of the round porthole window and linked to a cab monitor display for working tender first.
In the old day it was called back blast normally caused by small bore tunnels and the front dampers been left open and the blower not been on. You also did not want to much exhaust steam as that caused its own problems.
Wasn’t it called Blowback as that’s how RAIB refer to it in thier reports ?
With a tender loco, it was not nice to have coal dust blowing in your face even after attempting to dampen it down regularly. On the western we had 22xx and Manor class locos with small tenders. I often had bits frozen that I never knew existed after running tender first in the wee small hours on a cold winter's night!
It was Southern Pacific - they eventually had a fleet of around 250, with the first ones built in 1910 and the last during WW2, big oil-burning Mallet-style articulated locos built by Baldwin. The problem they were designed to alleviate was operating trains through the 30+ miles of snow sheds and tunnels over Donner Pass (between Roseville and Truckee, California) - a steeply graded 80 mile climb for eastbound trains.
