My main concerns would be :
( 1 ) Flutter / Structural Failure.
( 2 ) Turbulence, possibly rendering the lower fin set less effective.
( 3 ) Heat issues, from turbulence / air fraction.
Dave F.
Aeroheating is a known theoretical hand math computable value to a certain degree of verifiable accuracy. While scary and not negligible it is at least a easier known item.
Shocks take a CFD from a cad model by a flow expert or wind tunnel with schrelin (I’ve misspelled that) from an actual model at a actual real Mach number. A professor told me the FEA for structures can take experimental turbulence data. Guess what happens when you design a new rocket? You don’t simply have experimental turbulence data to compute structures accurately. These techniques are beyond most undergrad college engineering courses. You’re talking a masters student or a PhD dork knows. Until the technical limit they hit when they say they can’t solve it either.
There’s going to be tip vortices off those leading fins smacking the aft fin. That could lead to fin bending and deflection then failure.
The reality is it’s mostly unknown until flight tested in hobby rocketry. Flutter is slightly more predictable but the predictions have a larger error. Note I did not volunteer for structures on this project. Somebody really experienced engineer or hobby wise needs that role. I do not trust my level of expertise with that critiCal task.
For these technical reasons I dislike split fins on the R project. The smart thing was split fin solved a tooling constraint in manufacture. The bad downside is well theoretically it’s a scary math game getting harder to do until flight tested. I’m not an aero so take that advice with a grain of salt. I did do homework on a supersonic kit jet called BD-10 and it killed two test pilots because the engineers both mech and aero screwed up structures from an unknown turbulent cross flow in finite element analysis. They weren’t dumb. They assumed wrongfully when they couldn’t solve it. It broke up before the sound barrier: below the flutter prediction speed. Oh the horror of reality. It had twin tails like a superhornet. The tails sheared off.
At least unlike the plane nobody is physically riding in it, lol.
And unlike the Mach 5 missile this isn’t created in Arnold Engineering Developement Complex with national resources and millions in money, lol. This is a group of nerds doing it for fun on a very limited budget. Many theoretical issues are left unsolved by engineering standards. It’s a very large undertaking anyways even by hobby standards with very real technical challenges the same that aerospace companies face. The same physics and engineering techniques apply.
If somebodies done an O or something with split fins maybe I’ll shut up. The idea scares me for what we can’t solve with math by hand without a flight test. It’s one thing when it’s a H motor split fin. And you try it again after you break something. But this is an R. I just lump an R into a spacecraft category, lol. Don’t make an R project harder than it needs to be unless you truly have full confidence in your ability to design it or do it safely. An R project is very costly and more dangerous. It demands the utmost flight experience and respect by hobbyists. It demands teamwork.
To be honest it blows my mind what hobbyists have achieved. What other hobby lets you build a spacecraft motor? Test it? And fly it. Really wish an Aero would chime in on the split fins. They would have a better grasp of what the hell is happening in a concept level. Sorry if I got rambling way off.
