John,
Wow, you're making me actually feel "smart" for a change - I actually can answer some questions!!!
A front-mount pump is typically (not always) a smaller pump, driven off the front end of the drive shaft on a conventional (hood forward) truck chassis. The pump has its own, self-contained tranny, and, because it's not involved with the truck's transmission at all, offers the pump and roll possibility.
This is no small feature to a rural department fighting wild fires often. The literature touts "pump and roll, for fighting running grass fires!", and shows a fire fighter sitting on the front fender of the truck, with a booster line, spraying ahead of the truck.
The literature will also tout several other benefits of the front-mount pump, several of which I can vouch for.
1. Front-mounts offer excellent ability to draft water. Just nose the truck to the edge of a pond, lake, creek, pool, whatever . . . hook up the hard suctions and dump 'em in. The drive wheels stay on solid, dry ground. Simplified plumbing means far fewer opportunities for "air leaks", which prevent pump priming and drafting. With a mid-ship, you would typically have to "parallel park" against the water, which can isolate the engineer, provides less space between the truck and water, and is logistically less desireable.
2. Virtually always, a front-mount has fewer controls, and is far simpler to operate. Again, partly this is due to the elimination of the transmission interference issue, and also because these pumps are (were) rarely "two stage" pumps. A two stage pump has two impellers rather than one, and a more complicated case offering pumping in "volume" (both impellers pumping equal amount at equal, lower pressure), or "pressure" (one impeller pumps half-as-much water at one pressure, into the second impeller, which doubles the pressure, delivering half the volume.) For the engineer, this can add many complications to determine which is the better mode in which situation.
Not only that, but you can usually see all of the discharges on the front-mount, so there's no guessing about whether you're opening the captain's side discharge, the rear left, or rear right discharge, etc. etc.
3. Manufacturers claim better weight distribution, as well as leaving far more room for the water tank and/or equipment behind the cab.
NOW, a Mid-ship pump is typically mounted aft of the transmission, with a separate pump transmission which chooses between the pump and the rear differential. A mid-ship is engaged by pushing in the clutch, shifting the pump tranny from "Road" to "pump", shifting the truck into high gear (4th or 5th), and releasing the clutch. There's usually a locking mechanism to prevent the shifter from jumping out of gear.
Benefits:
1. Mid-ship mounts can be MUCH larger, since you can hang more weight between the wheels, than beyond the front axle. The drive-shaft arrangement provides the beefier power delivery required by a heavier pump pushing more water. This means greatly increased pumping capacity.
2. The larger pumps can offer the above-mentioned Series-Parallel (Pressure vs. Volume) capability, which allows a competent engineer to choose the best mode and lowest engine speed for a given pumping requirement.
3. The central location offers infinite plumbing options, including a deluge gun right above, and discharges anywhere you can point a finger.
4. Most would say the mid-ship pump is far better protected than the front-mount.
I've always said a front-mount is the best possible introduction to antique fire engines, because of their small, approachable size and absolute simple operation. Perfect for musters, too.
Mike Roache brings up a good point. Fire Engines have been built in every possible configuration. The airport crash trucks can flow tremendous amounts of water and foam while moving. But they cheat, by having a dedicated engine driving the pump only. And a mid-ship pump can be driven off of a PTO if desired, which would offer pump and roll, but PTOs are generally not sturdy enough to drive as large a pump as a traditional mid-ship arrangement.
And then there's American Fire Apparatus, which in the '60s and '70s mounted a full-size, mid-ship style pump, ahead of the engine, in a cab-forward chassis. I'm not sure how they squeezed the driver and officer into the cab . . .
I can't believe I actually wrote all this junk! Nobody could care this much about this stuff - I probably need to get out more . . .
