Bruce and Rocksim Gurus...will this fly?

[geof]

Will this fly with a G64? It's not a perfect rocksim file, but I'm just a beginner. I think I have implemented the fat rocket adjustment, and the Levison ring and tube fin approximations. The tube fins have a bevel cut in the shape shown in the phony CP fins. All the phony stuff is nearly transparent.

I haven't yet figured out how to manage chute stuffing/ejection. Also, the body tube materials are just stand-ins. One other detail: the portion of the ring that passes through the tube fins won't really be there, but I couldn't make it disappear in rocksim.

Geof

View attachment gnat.rkt.rkt

 

[geof]

Here's a jpg to get an idea of the plan.

 

[BobCox]

So, just to be clear, what you are building has three beveled tube fins, with a ring fin bridging between them?

1) I think you need to triple the number of virtual fins to simulate the airflow on the tube fins. Two of the virtual fins have the same beveled shape as the real beveled tube. The third set has a narrower chord, equal to the shortest part of the bevel. This will improve your apparent stability margin.

2) After adding these extra virtual fins, the model appears to be nicely stable, with a margin of 1.34 when loaded. However, a large portion of that margin is provided by the "Levison Fat Rocket adjustment cone" hanging off the back. Without that, the margin is only 0.75 calibers.

Overall, I'm a bit leary of its stability. The Levison approximations work pretty well for small angles of attack, but I'm not sure they still apply when the AOA gets too high. If that is true, you might have a rocket that flies okay on some flights but goes unstable if it sees an ill-timed gust of wind.

This definitely warrants a good swing test before flying.

 

[geof]

Thanks, Bob. Yes, you understood the design correctly. I had thought that one finset of triple thickness was about the same as 3 finsets of standard thickness when approximating the tube fins. I'll take a look at your file and see how you fixed it up. I appreciate your help.

Geof

 

[BobCox]

Originally posted by geof
I had thought that one finset of triple thickness was about the same as 3 finsets of standard thickness when approximating the tube fins.

Having a single flat fin with triple thickness will have the same frontal area as the tube, but the exposed area will only be 1/3 what it should be. The exposed area is the dominant contributor to the restoring force of a fin.

A flat fin has an area that is S * C, where S= the semi-span and C is the average chord. A tube fin has an exposed area of pi * D * L, where D is the diameter and L is the length. Since pi is approximately 3, it is possible to approximate a tube fin with by using three flat fins whose semi-span matches the tube diameter, and whose chord matches the tube length.

If the flat fin thickness matches the tubing thickness, then the frontal area will also match. The frontal area is one parameter that feeds into fin drag calculation. The exposed area also contributes to the drag of the fin, along with the root chord.

I didn't notice that you had tripled the thickness of the virtual tube fins. You will need to change them to normal thickness.

 

[teflonrocketry1]

Originally posted by geof
Will this fly with a G64? It's not a perfect rocksim file, but I'm just a beginner. I think I have implemented the fat rocket adjustment, and the Levison ring and tube fin approximations. The tube fins have a bevel cut in the shape shown in the phony CP fins. All the phony stuff is nearly transparent.

I haven't yet figured out how to manage chute stuffing/ejection. Also, the body tube materials are just stand-ins. One other detail: the portion of the ring that passes through the tube fins won't really be there, but I couldn't make it disappear in rocksim.

Geof

Geof,

Your simulation is correct! I now recommend this method for tube fins; the CP of a tube fin is best simulated as one flat fin with an area equivalent to the lateral cross-sectional area of the tube fin. For a CD estimate I make the flat fin three times the thickness of the tube fin.
Here is what I came up with using my most advanced method for tube fins with angle cut ends. Notice the calculated CP is only .015" futher aft than the technique that you used (using the RockSim equation for the 2D drawings). A lot more work for not much of a difference!
7 ounces of nose weight seems like quite a bit more than you need for this design unless you are going to use motors larger than a G. I suggest using about 4 ounces of nose weight to give a stability margin of 1.08 calibers.

Bruce S. Levison, NAR #69055