09-16-2010, 07:34 PM

I will be starting my Kill Bill project. It will be a 5.5" split fin design that will be designed to fly to about 10K feet on a M.

One thing I learned that may be useful is that Rocksim (version 7) does not calculate CP correctly for split fin designs using either Rocksim or Barrowman equations. Probably the problem is in the Barrowman equation.

The way to prove is to create a rocket with two sets of SQUARE fins that are split. Move the forward set of fins so that they are just touching or overlap the rear set. Note the CP location and/or margin of stability. In my rocket design the CP was 89" Barrowman, 92" Rocksim.

Now delete the rear set of fins and elongate the front fin to a rectangle that has the same shape and dimension of the set of split fins before. In my case the CP changed to 80 Barrowman, 85 Rocksim. The CP should have been identical to the split fin model because the fin geometry was exactly the same.

The difference between these calculation was almost 2 calipers in the Barrowman case and 1.5 calipers in the Rocksim case. That is a lot of discrepancy especially if you are designing for a 1-2 caliper stability margin.

If any has newer versions of Rocksim I'd be interested if it has the same problems with split fin designs. If so, I would trust Rocksim or Barrowman equations for modeling CP in split fin designs.

One thing I learned that may be useful is that Rocksim (version 7) does not calculate CP correctly for split fin designs using either Rocksim or Barrowman equations. Probably the problem is in the Barrowman equation.

The way to prove is to create a rocket with two sets of SQUARE fins that are split. Move the forward set of fins so that they are just touching or overlap the rear set. Note the CP location and/or margin of stability. In my rocket design the CP was 89" Barrowman, 92" Rocksim.

Now delete the rear set of fins and elongate the front fin to a rectangle that has the same shape and dimension of the set of split fins before. In my case the CP changed to 80 Barrowman, 85 Rocksim. The CP should have been identical to the split fin model because the fin geometry was exactly the same.

The difference between these calculation was almost 2 calipers in the Barrowman case and 1.5 calipers in the Rocksim case. That is a lot of discrepancy especially if you are designing for a 1-2 caliper stability margin.

If any has newer versions of Rocksim I'd be interested if it has the same problems with split fin designs. If so, I would trust Rocksim or Barrowman equations for modeling CP in split fin designs.

John Derimiggio