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-   -   Ring tail design (http://www.oldrocketforum.com/showthread.php?t=10023)

bernomatic 10-03-2011 12:28 AM

Ring tail design
 
I've been working on a design along the lines of the old flying stovepipe(?) and can't get rocksim to give me anything "stable".

I know the design works as I string tested it and have flown the design a half dozen times, but when I enter it in RockSim, it shows unstable.

:confused:

CPMcGraw 10-03-2011 12:35 AM

Quote:
Originally Posted by bernomatic
I've been working on a design along the lines of the old flying stovepipe(?) and can't get rocksim to give me anything "stable".

I know the design works as I string tested it and have flown the design a half dozen times, but when I enter it in RockSim, it shows unstable.

:confused:


Ring tails and RockSim have never quite gotten along together for several iterations of the program. RS shows a higher drag value, for one thing, and definitely doesn't calculate RTs the same as primary tubes. Jay Goemmer (CenturyGuy) might be able to pass along some comments, as this was an issue with his "Tau Zero" design.

bernomatic 10-03-2011 01:21 AM

I tried removing the ring and just using the fin supports and it comes out okay.

I'm not adverse to designing them the old fashioned way, but it seems like you need to have a RS file on it or people panic.

Mark II 10-03-2011 02:05 AM

People who are familiar with RockSim know that it cannot handle either tube fins or ring fins, yet many perfectly stable tube-finned and ring-finned rockets have been designed and built in the past 53 years. RS is a program that applies a set of equations to approximate the static and dynamic stability of a rocket design. Stability equations are approximations, not absolute truths. They have their limitations and problems and are still unable to accurately model the fluid dynamics of certain shapes. Anyone who views simulation results needs to understand that.

jharding58 10-03-2011 04:37 AM

1 Attachment(s)
Have you tried the ring tailed design under Rocksim 9?

Brain 10-03-2011 04:14 PM

Some sort of ring-tail or tube fin design has been around for a long time... it makes me wonder what the hold up for implementing these design variations has been with the programmers. I have designed and flown two variations on a 4ST/FNC ('Slanted Tube/Fin + Nose Cone') and they both perform very well! I'd love to be able to RockSim these designs...

jharding58 10-03-2011 04:33 PM

Quote:
Originally Posted by Brain
Some sort of ring-tail or tube fin design has been around for a long time... it makes me wonder what the hold up for implementing these design variations has been with the programmers. I have designed and flown two variations on a 4ST/FNC ('Slanted Tube/Fin + Nose Cone') and they both perform very well! I'd love to be able to RockSim these designs...


The design above is indeed stable and the pylon definition is pretty reasonable to specify. Also the Tau Zero has been modelled effectively. Which version and build are you using (118)? I would imagine that the changes adding pods in 9 should have some impact on the calculations - specifically drag components off the main tube.

bernomatic 10-03-2011 04:48 PM

Quote:
Originally Posted by jharding58
Have you tried the ring tailed design under Rocksim 9?


I looked at it, but could not find the factor which made the difference. My design is a minimum diameter, which to me would suggest that without the weight of the engine mount et al., I should have a better stability margin.

I am not unfamiliar with some of RockSim's quirks, http://www.apogeerockets.com/educat...wsletter253.pdf , and I think it is a great aid to designing and building model rockets. As I mentioned previously though, some think that if it's not stable in RockSim, it won't work and you shouldn't fly it. In other words, flight tests be ****ed, let's see the calcs!

While I don't adhere to this philosophy and was designing rockets long before there was a computer program to "do the calcs.", there are some in today's (younger) society who follow technology blindly and don't understand the age old concepts of trial and error and professional judgement.

Anyway, enough of my rambling, I was just wondering if there was anyone who may have come across this problem before and could offer a fix or go around.

bernomatic 10-03-2011 04:56 PM

Quote:
Originally Posted by jharding58
The design above is indeed stable and the pylon definition is pretty reasonable to specify. Also the Tau Zero has been modelled effectively. Which version and build are you using (118)? I would imagine that the changes adding pods in 9 should have some impact on the calculations - specifically drag components off the main tube.


Yes, version 9, build 118.

It's not that I can't get a ring tail to work, it's just one that I know works shouldn't according to RockSim.

Doug Sams 10-03-2011 06:02 PM

Quote:
Originally Posted by bernomatic
...there are some in today's (younger) society who follow technology blindly and don't understand the age old concepts of trial and error and professional judgement.
People who can't think outside the box have been around a long time :)


Quote:
Anyway, enough of my rambling, I was just wondering if there was anyone who may have come across this problem before and could offer a fix or go around.
I've done one scratch built ring tailed rocket (linky). One method of analysis I've used it to model the ring as four rectangular fins whose chord is the same as the ring tail's length and whose span is 1/4 the ring's circumference. This is in addition to the 3 or 4 supports. That is, your sim model will have two sets of fins - one set for the supports and one set for the subdivided ring.

In an actual analysis - way beyond my skills - none of my EE courses dealt with aerodynamics :) - there will be interfering turbulence between the supports and the ring which will create extra drag. And it will impact the effectiveness of the ring and fins. But I'm not sure that that is destabilizing - I think the added drag helps with stability. That is, the drag will improve stability while the turbulence reduces it for a net push.

Also, if the anular radius between the airframe and the ring is small, that will further impact the airflow and hence the stabilizing effects. So you want a ~1/2" or more of space between the airframe and the ring to ensure air is getting inside the ring.

Anyway, that's my two cents on ring fins :) I've pretty much used the same sort of approximation for tube fins as well.

Doug

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