Hi John,
I don't recall ever saying the fins were solid. If I did I apologize, I know better! The fins on the Smoke rounds were hollow cast magnesium, according to the Atlantic Research Corporation "catalog" of Nike fin configurations (PDF attached below).
The fin in the pic is not the same fin type as typically used on the Smoke rounds. The diamond type were called "Standard Nike fins" by ARC and are very typical for the Nike when used for a booster stage. The one in the pic is the first stage of a Nike/Apache combination. The Standard fin was made in three versions, Lightweight Standard, Standard and Heavy Duty Standard. All of the Standard fins were, as you pointed out, constructed of an inner structure with aluminum or magnesium skin. There were also various means of mounting for the Standard fins, and different shrouds and sleeves.
The Smoke rounds typically use what the catalog calls a "modified diamond" type that were used when a lower drag configuration was required. They were made of cast magnesium (hollow) and also used a different shroud configuration than the standard Nike fin. They were available in several sizes and the catalog even talks about custom sizes of this fin being available. These are the ones I was trying to model. I thought about using the cardstock methd, but chickened out because I had ideas about flying this model on mid-power motors and wanted to make sure the fins were up to it. From what you are saying it sounds like I could have gone heavy cardstock and been okay. I'll have try it next time!
Below is the ARC fin catalog in PDF form. I thought it was really interesting. There are lots of drawings and pics that give an idea of the variety of tail feathers that were used on the various types of Nike rockets. The one disappointment for me was that the military fin types (Ajax and Herc) are not included.
The problem with this info is that, if you are like me and get all OCD when it comes to scale stuff, you now have all this data on the various fin sets but still have to figure out which type was used on the rocket your wanting to model! The airfoil shape of the modified diamond used on the Smokes is a give away, but it is hard to tell with the standard fins.
I especially enjoy your work and look forward to seeing what you are working on, John. The X-7 was one of my particular favorites! how did it fly?
Thanks for posting!
Mike