Bruce S. Levison has asked us to share this article with other RockSim users. It describes a method that he feels will help to simulate the effect of tube fins in RockSim. While the tubes are represented by flat plates, Bruce feels that the CP will be in the right location on the rocket. Please note: The user assumes all risk for the information obtained with this method.

This article contains links to download the RockSim designs. After downloading, place these files in the Design folder of RockSim Folder on your hard drive. These files can also be viewed using the demo version of the RockSim software.

Let's start with what is known about tube fin designs; this article by David Urbanek is from one of his postings on the rec.models.rockets newsgroup. It is a method to simulate tube fin designs on RockSim.

"Here's how I do tube fins in RockSim. Each fin is defined as a rectangular fin. The root and tip chords are the length of the tube fin. The length of the tube fin is figured this way: Pi * tube fin diameter Minus between 1/16" & 1/4" (depends on tube curvature) for the body tube attach point Minus between 1/16" & 1/4" for the attach point from one tube fin to the next Times 0.8. Make the thickness the wall diameter of the tube. The fin cross section is squared. The fin material is paper. It's worked three times. I always figured 1.5 caliber stability at the very least using this method. No unstable designs yet."

This brings us to the first file (rvsxpac1.rkt). It is a simulation for the Rocket Vision Six-Pack rocket that has six tubular fins that are cut at an angle at both their tops and bottoms. You can see a review the Six Pack model at: http://www.rocketvision.com

Note that RockSim determines the CP to be 11.760 inches from the tip of the nosecone on the rocket. The problems I have with this method are that the fins are sticking out too far from the body, making the design appear to be more stable than it actually is. Also the actual leading and trailing edges of the fin should be a curve, the straight edges will tend to overestimate the fin area and hence overestimate the rocket's stability. Keep in mind that a better or more conservative estimate places the CP further toward the nose of the rocket. For all these files I used the RockSim Equations, the argument is exactly the same for the regular Barrowman equations.

Now look at the next file (rvsxpac2.rkt). The Six-Pack rocket was simulated with two 6-fin units, for 12-fins total, that have half fin span of the the previous simulation.

RockSim determines the CP to be 11.699 inches from the tip of the nosecone on the rocket. Since the CP is farther forward along the air frame this is apparently a better, more conservative, estimate of the actual CP. The leading and trailing edges of these flat fins more closely estimate the curved surface that would be produced if you unrolled the tube fin split down its middle. This better model indicates less stability and thus gives a better estimate for the actual CP of the design. But the fins still don't have the right shape; they are still sticking out further from the body than they should be, probably making the design appear to be more stable than it actually is.

This brings us to the next file (rvsxpac3.rkt) which is a simple RockSim simulation of the rocket where the tube fins are treated as 6 flat fins with the same cross sectional area of the tube fins. RockSim calculates the CP at 10.993 inches from the tip of the nosecone on the rocket almost an 3/4 of an inch less stability margin than David's original tube fin method above! But this simulation uses only about one third of the fin surface area that is actually present, no wonder it simulates less stable! It should now be obvious that the actual CP of the Six-Pack rocket should be somewhere between the two values of 11.760 and 10.993 inches from the tip of the nosecone on the rocket.

Or at this point you could narrow the CP position down to between 11.699 and 10.993 inches from the tip of the rocket's nosecone.

Now take a look at the file (rvsxpac4.rkt). The Six-Pack rocket was simulated with two 6 fin units, for 12 fins total, using two identical 6 fin units that have the same flat fin design of the tube fin cross sectional area. RockSim calculates the CP at 11.465 inches from the tip of the nosecone on the rocket. The fins have the right shape and the simulation indicates more stability than when using just one set of six fins as above (CP at 10.993 inches). Note this is still more conservative simulation than David Urbanek's method (CP at 11.760 inches from the tip of the nosecone). Thus, one now would expect the actual CP of a tube fin design to be some where between 11.699 and 11.465 inches from the tip of the rocket's nosecone. But as above, this simulation takes into account less (about two thirds) fin area that should be present.

For a better estimate of the tube fin CP, I just added another set of six similarly shaped fins. Since the tube fin surface area equals pi times diameter times the length, I think the best way to simulate tube fins is with three flat fins that have the same cross sectional area. Since pi is 3.14..; the 5% lower area from the three (3.0) flat fins takes into account the contact points that the fins have between themselves and the body tube. Note: David Urbanek indicates he corrects for this in his method.

Now take a look at the file (rvsxpac5.rkt). The Six-Pack rocket was simulated with three 6 fin units, for 18 fins total, using three identical 6 fin units that have the same flat fin design with an area equivalent to the tube fin cross sectional area. RockSim calculates the CP at 11.634 inches from the tip of the nosecone on the rocket which is between 11.699 and 11.465 inches as expected. The fins have the right shape and the simulation indicates better stability than my two previous simulations which have a lesser fin area. Also note that the CP determined is slightly more conservative than either of the two approaches from David Urbanek's method where the fins don't have the right shape in the simulation!

I hope you followed this argument and all the issues I brought up. All said and done, a tube fin is probably best simulated by using three flat fins with the same shape and area as the cross sectional area of the tube fin. A six tube fin model would be simulated with 18 flat fins, a seven tube fin model would use 21 and etc.

I welcome any comments and criticism on this work.

There is a companion article by Bruce. It is about simulating Ring Tail Fins in RockSim. Read it now by clicking here.

Bruce S. Levison, NAR #69055

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