Um I think I'm lost. I'm think that the Impeller Ring and Nodes (the drive) are 2 diffrent things. The Ring holds the Nodes right?
What about doing it this way.
Have you concidered using SJ Propulsion system (forget the name. Bigger ship = more engines needed for same movment). IMO this would work well concidering that, while all ships in HH are suppose to reach the same top speed, smaller ships are able to accelrate faster than bigger ships (its in the tech bible).
So propulsion would works like this.
Each ship has to have an Alpha and Beta Impeller Ring (required) that gives you a standard +1 movment point. The size could very depending on the size of the ship or not. I think thats what your getting at with the Small Impeller Node Ring, Imperller Node Ring and Larg Impeller Node Ring, right? If so you could simply name them; DD Impeller Rings, BC Impeller Rings, DN Impeller Rings and so on. While I don't know how to stop someone from using a DD ring on a DN you can make a minaiml size requirement so it can not fit on a smalle ship (which might be a good reason to just have them all the same size, but where the fun in that
). You could also think about having a Impeller Drive reasurch field which is seperate from the normal propulsion field.
Next the Nodes, which if I understand it right, will be like (and work like) normal SE4 engines, just change the name. You could either keep the same size for all the node regardless of the ship size or not. But I don't remember their being any diffrent sizes for DD nodes or DN nodes. I could be wrong, but if i remember correctly how fast a ship was able to accelrate related to not how big its nodes where but how they where
placed on the Impeller Ring (you could only space them so close together, but you only had so much hull space to work with). Any of that make sense? My guess is that you would want Alpha and Beta Nodes in which case you might be able to have a requirment of equal number of Alpha and Beta Nodes on the ship. (But the only thing diffrent between Alpha and Beta Nodes is the name)
Thier would have to balance with price/size and how fast you want the ships to move. But I think it could work.