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  #11  
Old 10-03-2011, 06:19 PM
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bernomatic bernomatic is offline
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So... basically, go back to Levison's stuff?
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  #12  
Old 10-03-2011, 06:35 PM
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Quote:
Originally Posted by bernomatic
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

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  #13  
Old 10-03-2011, 08:45 PM
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Quote:
Originally Posted by bernomatic
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.


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.
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  #14  
Old 10-03-2011, 09:08 PM
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Quote:
Originally Posted by mwtoelle
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.
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  #15  
Old 10-04-2011, 12:13 AM
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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.
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  #16  
Old 10-04-2011, 05:24 AM
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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
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  #17  
Old 10-04-2011, 12:55 PM
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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 ), 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 ), 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.
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  #18  
Old 10-04-2011, 01:24 PM
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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.
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  #19  
Old 10-04-2011, 01:52 PM
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Quote:
Originally Posted by jharding58
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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  #20  
Old 10-04-2011, 02:16 PM
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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.
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