It does have an extra bank of cylinders, that would amount to 20-25% more parts?
True
More compact the X-24/Vulture is not. The width of the Asso 750 was 1060 mm on the widest point, due to the stroke of 170 mm. Vulture was 910 mm, with small stroke - 140 mm? 3B Merlin would be closer to Vulture, not just because of modest stroke (152 mm), but because the outboard banks will be set 80 deg apart vs. Vulture's 90 deg.
Perhaps, it depends on the layout, Outer banks 40 degrees from center bank for compactness (80 degrees total spread) or outer banks 80 degrees from center bank (160 degrees total spread) ? with the 40 degree spread you have a lot less volume to fit intake and exhaust. Getting a good firing order may be a bit trickier than with the wider spread.
20mm of height isn't worth worrying about.
continued: They also have Centaurus ready, the Griffon will soon produce 2500 HP etc. What I'm after is a 1700-1800 HP engine for early war, 2000 HP mid-war, up to 2500 HP for late war, that would use some existing engine parts, hence cutting on risk time to develop.
trouble is the parts you are saving are actually the easy parts. The parts you need new are the hard ones. Once RR drops the ramp heads and goes back to the scaled Kestrel heads the cylinder head and valve problems are about over.
connecting rods for Lion.
With a 60 degree V-12 the cylinders on each crank pin fire about 300 and 420 degrees of crankshaft revolution apart, but one piston follows the other by 60 degrees. One cylinder is firing every 60 degrees of crankshaft rotation. This is the firing order for a Merlin--- 1A-6B-4A-3B-2A-5B-6A-1B-3A-4B-5A-2B
With an X-24 you have a cylinder firing every 30 degrees and have the cylinders on crank pin giving two firing pulses per crankshaft revolution but not evenly, you do have an even movement of the reciprocating parts. When upper right cylinder is at TDC the bottom left cylinder is at BDC and the other two cylinders are at mid travel, one going up and one gong down. Opposing pistons will fire at at 180 and 540 degrees from each other.
With the 3 bank if you have the 40 degree bank (80 total) you can have all three pistons ascending or descending at the same time although briefly. I haven't tried figuring out the firing order (it makes my head hurt
edit: with a 40 degree spread you can have the cylinders on one crank pin fire 40 degrees apart and then 640 degrees of crank rotation before they fire gain or left hand cylinder fires (assuming clockwise rotation facing away) , center skips, right had fires 80 degrees later, middle fires 320 degrees after that and then another 320 degrees before left cylinder fires again, repeat. Or right hand cylinder fires, 280 degrees later left hand cylinder fires with middle cylinder firing 400 degrees after that but then the right hand cylinder fires the 2nd time just 40 degrees after the center cylinder.
in theory you can have one cylinder firing every 40 degrees of crankshaft rotation, getting it to work with the cylinder bank angles gets a bit tricky.
Now
IF you built a 18 cylinder Y engine with 120 degrees between each bank figuring out a firing order and piston movement would be a lot easier although it would be one wacking
BIG engine
This is the challenge/hard part. Getting the crankshaft to live at high RPM with the higher out of balance forces and and trying to squeeze as even a firing order out of it as you can given the limitations of piston movement. Torsional vibration can be a real problem here. the twisting (or loading) of the crankshaft along it's length as the cylinders fire. It gets worse with higher pressure (boost) in the cylinders.
Strangely this was an advantage for the Napair Lion over the contemporary V-12s. For a 12 cylinder engine of about equal power the shorter crankshaft of the Lion had less problems and could be lighter than a V-12 crankshaft and crank case. Now if you were trying to build a 3 bank 18 to compete with a V-16 or V-18 (?) you may have the same shorter crankshaft advantage but if your crankshaft is as long (or close) to the V-12 the advantage doesn't exist. Each blow to the crankshaft is a bit smaller than a big V-12 but weight of the reciprocating parts thrashing around may cancel that out.
This is were you will spend your development time, crankshaft, crankcase and con rod design and development (the same things that killed the Vulture) plus supercharger and reduction gear ( needed for any 1500-2000hp engine at the time). As I said before the ONLY stuff you are saving is the easy stuff.
