Steve,

The Ross drive was redeveloped by Tracy Crook of Real World Solutions (RWS). Tracy now uses the 6 planet planetary (Ford Unit) and is about the best you will get  weight wise and value for money.

 

There are other similar units - all have slightly different construction, however Tracy's is well proven. I will probably make my own, but only because of the tyranny of distance and associated costs.

George (down under).

 

Steve,

I went with the RD-1B PSRU from RWS.

 

T Mann