How do those lattice fins stabilise the upper stage? To me, it seems as though all they do is add drag.
For a model, the fins can be mounted one of two ways. They can be mounted on hinges like the blades of a heliroc, but the mounts have to be able to withstand flight under thrust. Or they can be fixed in the deployed position, in which case, if they're able to affect the CP of the second stage, they'll also adversely affect the CP of the combined first and second stage.
The second stage is basically just the nose cone. Can an ogive cone be made stable without fins by putting enough weight at the tip?
Don't get me wrong - I'd like to see this done! I'm just discussing what I see to be the problems with such a design, in the hope that someone can figure out how to solve them.
You're exactly right about the lattices adding drag. It's the differential drag that is the stabilizing force. The lattices only turn into fins when the rocket gets out of shape, and the part thats out in the wind stream causes more drag.
I did a quick and dirty design of the nosecone second stage without the fins. I'd use a BT20 MMT clustered to two BT20 tubes for recovery chutes. I'd plug the MMT and vent the ejection charge to the recovery tubes. Problem with this design is it's heavy. I needed 4oz of weight in the nose to get a .3 stability margin with a C5-3 with Barrowman and Rocksim calculations. The cut out method actually showed higher stability with a margin of almost 1.1. (Not sure which method handles cone stability best???)
The problem I haven't looked at yet is recovery of the booster. Staging would do it but not much room in the tail...
My guess is the lattice fins add base drag, which adds stability. Similar to why a spool rocket with only the bottom plate is stable. I have seen similar type fins on a military rocket (may have been solid). The effectiveness of the lattice could be related to things like the velocity, air density, etc. I looked in MIL-HDBK-762 which discusses such things but it only discusses similar solid fins that sit at low angles, more like 20 degrees vs. 90.