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bernomatic
10-03-2011, 01:28 AM
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, 01:35 AM
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, 02:21 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, 03:05 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, 05:37 AM
Have you tried the ring tailed design under Rocksim 9?

Brain
10-03-2011, 05:14 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, 05:33 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...

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, 05:48 PM
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/education/downloads/Newsletter253.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, 05:56 PM
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, 07:02 PM
...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 :)


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) (http://www.doug79.com/haleyscomet/). 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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bernomatic
10-03-2011, 07:19 PM
So... basically, go back to Levison's stuff?

Doug Sams
10-03-2011, 07:35 PM
So... basically, go back to Levison's stuff?Sorta. I use a bit different technique than his, being more conservative. For example, I treat a tube fin as being equal to two fins of the same silhouette rather than three as he does.

Anyway, as for my preferred modelling techniques, since I'm still driving Rocksim 7, that's how I gotta do it :)

Doug

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mwtoelle
10-03-2011, 09:45 PM
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/education/downloads/Newsletter253.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.

If I had the space, I would construct an analog computer, a wind tunnel, for these types of designs. The old V-2 will not work very well with the Barrowman equations (both simplified (i.e. Centuri's TIR -33) and the calculus-based original equations of 1966. I know the aerospace industry still uses wind tunnels to test models of new aerospace vehicles.

bernomatic
10-03-2011, 10:08 PM
If I had the space, I would construct an analog computer, a wind tunnel, for these types of designs. The old V-2 will not work very well with the Barrowman equations (both simplified (i.e. Centuri's TIR -33) and the calculus-based original equations of 1966. I know the aerospace industry still uses wind tunnels to test models of new aerospace vehicles.

The space isn't an issue here. My wife would, however, would be quite irate with me for spending more time with the "kiddie toys that grown men don't play with." Anyway, a swing test is far cheaper and except for large models which would require a larger wind tunnel anyways, far easier, if not always as accurate.

Mark II
10-04-2011, 01:13 AM
Prior to actual flight testing, wind tunnel testing is still the gold standard, isn't it? Don't get me wrong; simulators generally do an excellent job, especially when they are dealing with fairly conventional 3- or 4FNC designs. RockSim 8 nailed the flight of my Javelin XL for my L1 certification to the inch; I don't think that you could possibly get a more perfect correspondence between the simulation and the subsequent actual flight. But I understand that wind tunnels are still widely used, and there's probably a very good reason for that.

gpoehlein
10-04-2011, 06:24 AM
There has always been that conundrum about real-world testing vs simulation. Simulation is only as good as your math - and must be mistake free for your results to be valid. But the real problem with simulation is that you tend to get out what you expect to get out. On the other hand, real world testing, while not infallible (such as string testing with too long a rocket or too short a string) will give to real results whether they are what you expect or not.

It reminds me of the current battle over CG special effects vs real world models and pyrotechnics. These days, CG is cheaper and gives you just exactly what you expect it to give you. On the other hand, models and pyro can give rise to what one SFX model maker referred to as "happy little accidents" - unexpected results that look a lot cooler than what the artist expected/envisioned. As another (totally unrelated) example, if Fleming had been using computer modeling in his experiments, he never would have discovered penicillin - such results would simply not have been programmed in in the first place.

Bringing this back on-topic, a wind tunnel doesn't have to be large and elaborate to work and be useful for model rocketry. A simple one can be built using a honeycomb to BT-20 tubes, several large pieces of cardboard or foam-core and a portable fan (a box fan would work, a small vortex style fan would be even better). I know that Estes published the plans for a small wind tunnel in one of their tech publications - look for Classic Collection TR-TN or TR-5. TR-11 (Aerodynamic Drag of Model Rockets) also has a lot of useful info as well.

Greg

bernomatic
10-04-2011, 01:55 PM
The scope of the computer program in this reguard as I see it, is to get me somewhere in the neighborhood of a stable design as I envision the design. Then the design is built and tested (by wind tunnel or string test and/or actual launching :eek: ), and adjustments are made as necessary. In the old days, you had to have a feel for what would work before testing (or an unlimited bank account or lots of nose weights :chuckle: ), so I have no problem with the aid given by a design program.

However, if the design being created doesn't match what I envision and know to work (either by testing or by similar prior designs) I wonder if their is something I am missing or if their is a glitch in the program that needs to be worked out.

My biggest peeve is with the Wonks who believe just because the program allows certain design characteristics, that it is infallible in their application. I am not complaining about the RockSim program as much as I am about those who follow it blindly. To follow the analogy above, someone tells Dr. Fleming he can't use penicillin because the computer doesn't see it behaving that way.

jharding58
10-04-2011, 02:24 PM
I think that there may be a little more getting worked up than is needed over a piece of software that is after all designed for the hobbyist. Getting in to the more robust capabilities of RS-Pro I would expect a high order simulation of launch charactersitics and design considerations. With more esoteric creations it is a given that at some point you will reach the limitations of the software's abillity to accurately model. In the case of the Stovepipe for example, being a tractor design the CG cannot help but move aft during the flight reducing stability. I seriously doubt that Rocksim takes into consideration the effect of burn on motor mass (although I could certainly be wrong) and the translation aft of the CG. Again with the Stovepipe the CG and CP are so close coupled that stability would be difficult to accurately predict. Also the Reynolds numbers of components this small are very hard to model, although I am relatively sure that RS does not embark down that path.

Between the RockSim and Barrowman equations there is a high order of predictability in the ability to define a model, plot the characteristics of the proposed model in flight, and render a predictable confidence of stability and success in flight. I agree completely that there needs to be a balance in terms of reliance upon modelling and mingling of experience and instinct in the operations of any software (given that there is not a computer in the world which multiplies), but from the 99 percentile perspective RockSim provides a reasonable modelling modality without the effort and expense of producing and testing an artifact.

Doug Sams
10-04-2011, 02:52 PM
I seriously doubt that Rocksim takes into consideration the effect of burn on motor mass (although I could certainly be wrong) and the translation aft of the CG. Actually, it does. I had a conversation with Tim about that several years ago. The model is fairly simple - as I recall, the consumable motor mass is linearly depleted with burn time (versus a depletion rate which varies with thrust). But I think the model is that simple. I don't think it adjusts the CG of the motor (usually moving aft with the burn). This is more a concern with hybrids, but could apply to any motor.

In my version (v7), it doesn't allow you to calculate the CP of a booster section (to ensure it's unstable, for example). Instead, you have to delete the remainder of the rocket, a PITA.

On a related note, while it allows you to group parts together into modules, I couldn't make an entire stage into a group (altho I understand that in later versions that's possible, no?).

Anyway, it's a pretty good tool, but I got frustrated when, with the release of newer versions, I kept seeing some of the same old bugs. For example, last I checked, it still lists BT-80 as having an inside diameter of 2.588" (instead of 2.558").

The ultimate pisser for me was when I opened one of my better engine files in the Rocksim engine editor, then hit save. It reformatted the file deleting all the comments I had inside. This info was basically the source/history info for each of the motor curves therein. Man, was I PO'd! That's really bad form for it to remove comments from a source file.

Doug

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bernomatic
10-04-2011, 03:16 PM
I didn't mean for this to become a trash RockSim thread, by any means.

What I was looking for is...

1) to let users know I had this problem.

2) to see if anyone had a work around for the problem

3) To complain about a @#$%^&* RSO who wouldn't know if a design was inherently stable or not till it hit him on the head and prays at the mighty allknowing computer alter for wisdom and knowledge which he never receives.

If I tell you the design is stable and has been string and flight tested, should I be allowed to fly it or should I still need a RS file showing it viable?

If RS didn't have the ring fin ability, this probably would be a mute point.

Doug Sams
10-04-2011, 04:22 PM
If I tell you the design is stable and has been string and flight tested, should I be allowed to fly it or should I still need a RS file showing it viable?Yes, in my book. Furthermore, a good RSO should be able to say, "That looks about right", and let it fly.

Doug

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CPMcGraw
10-04-2011, 05:22 PM
I didn't mean for this to become a trash RockSim thread, by any means...

If I tell you the design is stable and has been string and flight tested, should I be allowed to fly it or should I still need a RS file showing it viable?...

FWIW, as one of the RockSim "wonks" here on this forum :D I'm not insulted. It's a bug in RS, plain and simple, that hits when we want to design ring fin and tube fin models. We just grind our teeth and crawl forward anyway; a good string-swing beats a RS calculation any day.

The best reason for using RS, aside from the reasonably-good-otherwise flight simulations, is the ability for other users to see how a design comes together, and not just how it looks assembled. The parts tree and part descriptions offer a lot of information that might otherwise be overlooked, especially if we designers remember to use it right. Since Apogee allows the program to be "demo" run for a while (30 days?), others can at least view the files we create. It's also a file format that can be used across all three computer platforms.

It's easier to work with than, say, trying to create individual TIF/PNG/JPG files for components and fitting them together into a single image. Been there, done that!

RandyT0001
10-08-2011, 10:13 PM
For the past few months I've been using RocSim for some preliminary work on a ring tail design for my L3 certification. I want to fly low without high mass and I had heard ringtail designs have high drag hence my starting point. My first design used a 7.7in airframe and 11in ringtail (7in AF LOC 11in Ringtail fin rocket V1) which RocSim said would be stable with 3+ calibers of stability using a CTI M1400. I had doubts about the stability and the calculated drag on the design. To test the the stability (and if stable to backtrack/zero in on the drag using an altimeter and RocSim) I built a 22% scaled down version (22% scale 7in AF 11in Ringtail V1) that used some BT-60 1.64"dia airframe tubing with a BT-80 2.6" tube for the ringtail. It was unstable.

My second design still uses a 7.7in airframe but I've increased the size of the ringtail to 16" in diameter (7in AF LOC 16in Sonotube Ringtail fin rocket V1). The stability margin decreases to slightly over two calibers but I had a better TLAR feeling as to the stability of the design. I still questioned the calculated drag on the design so I designed and built a scaled down version (7in AF LOC 16in Ringtail 35% scale). The stability margin is different but I was more interested in making some flights to determine the drag on the design. I had bought some G64's and G53's so decide to use them for test flights. On my first flight I packed the parachute too loose and it didn't deploy though the NC popped off ruining the aerodynamics enough for it to drop in a flat spin to the ground. From this one flight's altimeter data I used RocSim to backtrack the drag to about 1.05 instead of the calculated (approx.) 1.7 from RocSim. Unfortunately the ringtail got busted on when it hit the ground and I haven't had time to repair it these past few weeks in order to continue the tests. Hopefully over the winter months I will be able to repair it and finish the tests.

Tau Zero
10-09-2011, 05:18 PM
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 (CenturiGuy) might be able to pass along some comments, as this was an issue with his "Tau Zero" design.So... basically, go back to Levison's stuff?Bernard,

Yes.

Craig and I will be among the first to tell you that while RockSim is a great design tool, sometimes you have to jump through mindboggling hoops to get it to sim correctly (or display decals, or...[fill in the blanks]).

Sometime in the last year I tried to put two different sized sets of tube fins on the same rocket, and managed to "break" RockSim. [RockSim programmer] Paul Fossey asked, "What are you trying to do?" I replied, "I just want the tube fins to act the way they do in the real world."

So it's far from perfect, but it looks cool. But plenty of us "old guy" rocketeers slap together stuff That Looks About Right (TLAR), and it flies okay. (shrug)


As with everything else, your mileage may vary. :eek: :rolleyes: ;) :D

Not to mention your airspeed. :rolleyes:

Cheers,

Brain
10-11-2011, 07:10 PM
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.
But isn't that the idea? I am not a programmer by any stretch of the imagination ('cept for a little ActionScript and HTML) but if there is a 'design philosophy' behind the programming intentions (basic 3/4FNC) it currently doesn't rise to the level of creative design and implementation I've seen nowadays... but that sort of stuff may be difficult to add.

I had been dinking around with a free version for as long as it let me, but I don't have the financial wherewithal to get a real version (although I'd like to, I suppose).

jharding58
10-11-2011, 07:42 PM
Then there is the difference between RS and RS Pro...

bernomatic
10-12-2011, 01:49 AM
Then there is the difference between RS and RS Pro...

what? about $876.? :eek:

It was hard enough to justify the upgrade price of $41 to the wife.

I see the conversation now.

me: "dear :) I just bought a new rocket software program.

her: "that's nice. HOW MUCH DID IT COST? :mad: "

me: "it allows me to design my rockets with more accuracy and be able to guess where they're gonna land." :D

her: "HOW MUCH?"

me: "Only a mere thousand."

her: :eek: :eek:

me: (silence except for blood slowly seeping out of deep chest wound)

Brain
10-12-2011, 07:19 PM
what? about $876.? :eek:

It was hard enough to justify the upgrade price of $41 to the wife.

I see the conversation now.

me: "dear :) I just bought a new rocket software program.

her: "that's nice. HOW MUCH DID IT COST? :mad: "

me: "it allows me to design my rockets with more accuracy and be able to guess where they're gonna land." :D

her: "HOW MUCH?"

me: "Only a mere thousand."

her: :eek: :eek:

me: (silence except for blood slowly seeping out of deep chest wound)
Yes, this is exactly how it would go in my house.
The only difference being not having a DIVORCE emoticon to use here... :p

Tau Zero
10-18-2011, 12:10 AM
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.And that's *my* gripe, as well.

As Craig often says, welcome to the asylum. :rolleyes: ;) :D


"Gather inmates, while ye may..." :eek:
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