Rocksim stability for asymmetric rockets?

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Marc_G

Marc_G

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Hi folks,

I'm working on a design that's not a standard 3/4FNC, but rather has fins and stuff somewhat asymmetrically placed. But as I was adding fins to the rocket, I found Rocksim saying it was stable way earlier than I would have thought. Even a single standard fin seems to be enough to make a rocket stable according to Rocksim 9. Check this out:

Asymmetric.jpg

View attachment Asymmetric Rocket.rkt

Just one fin on the rocket brings the stability to over 1. Adding a second fin 90 degrees brings it to over 2. But if I launched such a thing, I would expect the unbalanced forces to make it skywrite all over.

Is my understanding flawed, and I should just trust the sim, or am I missing something. Probably the latter!!

Please educate me so my design will stand a chance. (PS: the rocket I posted has little to do with the design I'm cooking up! :dark: )

Thanks!

Marc
 
Still researching this. I've read:

https://www.apogeerockets.com/education/downloads/newsletter105.pdf
and
https://www.apogeerockets.com/downloads/Newsletter220.pdf

I'm having trouble with generating the stability analysis, in that I haven't figured out how to create the margin versus radial angle view... I can bring up the radial angle view, but it just shows the back of the rocket with no stability lines overlaid. Any idea what I'm doing wrong?

Also I've tried the various Rocksim versus Barrowman stability calcs and they show similar (not identical) stability, even on a one fin rocket. I read the 220 article but need to take another pass through it.

Learning a lot but definitely interested in tips from others on asymmetric designs!

Marc
 
I haven't upgraded past version 7, but to get the radial stability analysis, I go to "Rocket" on the pulldown menus, then choose "Stability analysis". Sorry, I don't know where this has moved to in later versions.
 
It's still there, but it doesn't show me the actual analysis... just a back-view of the rocket. Check this out. Using the rocket pictured in my first post, I choose Rocket... Stability Analysis:

Stab1.jpg

The output looks like this:

Stab2.jpg

It's just the back end view of the rocket... no lines showing the stable/unstable stuff. Notice the name on the top of the plot is just Radial Stability... I think I need the Static Margin versus Radial Stability view. But no fussing around seems to get me there. Any suggestions? I've tried adjusting the speed, poking around in the plot style, to no avail.

Anybody can tell me what I'm doing wrong?

Thanks. This is driving me nuts.

Marc
 
That sounds and looks kind of goofy..Seems all RockSim is concerned about is that the CG/CP relationship is in the 'stable' orientation region.:confused2:
 
I emailed Apogee, but I'm pretty sure I'm just missing something simple. To be clear, I think this is user error, not a bug. Problem between monitor and keyboard.

But the documentation on this feature is a bit weak, and even after reading the relevant newsletter articles, I don't see what I'm doing wrong...
 
Just a slight update on this... this morning I got a very prompt response from Tim at Apogee (way to go Tim, for great customer service!). IT didn't quite answer all my questions so tonight (after hours) I sent him a follow up with some further questions. I'm sure he'll respond soon and I'll post what I get here, to help others with similar questions in the future.
 
Marc_G said:
... I'm sure he'll respond soon and I'll post what I get here, to help others with similar questions in the future.
Click to expand...

So Tim never responded?

I have exactly the same problem and at least as frustrated as you because I suspect it is NOT user error. I'm not getting any stability analysis on a design that I know is accurate and stable because it has been built and launched.
 
I just checked and it isn't working for me either. I can't get the radial graph to show nor is there any indication of stability for the 'stability analysis screen'. It used to work as I have used it numerous times. I have Version: 9.3.0f180 running on Windows 8.1.
 
rstaff3 said:
I just checked and it isn't working for me either. I can't get the radial graph to show nor is there any indication of stability for the 'stability analysis screen'. It used to work as I have used it numerous times. I have Version: 9.3.0f180 running on Windows 8.1.
Click to expand...

Same here on the version. So when was this feature render dysfunctional? One can't really say it was disabled because it can still be selected in the menu. Is there a way to contact Apogee over the weekend?
 
I don't remember the plot looking exactly like that, but it could have been my settings. It worked well after that point.
 
This page: https://www.apogeerockets.com/RockSim/Version_History

Describes:

For Version 9: "New stability overlay to see if your asymmetrical designs are stable in all axis."
For Version 8: A fix.. "The Rocket-Stability analysis screen now displays correctly."
For Version 7: "Radial stability graph."

,BUT no info about this functionality being REMOVED for the current version!
 
Hi folks,

I can't be much help because I haven't used the software much in the last two years, but here's the text I sent to Tim, and the reply he sent:

On June 27, 2013 I asked:

Hi Tim,

Thank you for your very prompt response!

I'm still having trouble, or "lack of understanding," so perhaps I can send some more information to help you guide me along. For clarity, I'm using Rocksim 9, and it is updated to the very latest according to the check for updates function (9.1.4f145). My platform is Win7 x64, quite up to date according to Windows update.

What I'm trying to achieve to help with my design is the plot showing the stability analysis similar to what was shown in the newsletter 105 (see file attached, stab-newsletter.jpg, which is just a capture from your newsletter with arrows showing the lines I'm talking about).

Using your helpful suggestion in your email, I went into my Edit... Preferences... 2D settings and indeed I saw the sections for Stability Graph overlay and Show Angle Grid were NOT checked. Ahah! I thought. So I checked them. I'm attaching a pic that is my actual settings as a screen capture (Marcs Settings.jpg) then went back to the 2D view. I now had a faint green circle around a grid (why so faint?), so I figured I was making progress. But the Rocket in question was a "2FNC" with fins about 90 degrees from each other. See attached asymmetric Wouldn't this have plenty of angles of view that were unstable? I also checked my actual rocket project, and got the same slightly green circle from all angles, despite expecting it to be unstable from certain views due to asymmetry.

Anyway, I checked the Rocket menu ... Stability Analysis and it is unchanged. See Stability Analysis.jpg. No grid or static margin plot there. Am I doing something wrong? I tried changing the Velocity, and nothing changed. Is there a reference I can consult to figure out how to use these features to their fullest?

So... I'm still missing something, probably something basic due to my "novice" nature with these features. I greatly appreciate your support, and thanks for making such a useful product.​



Later that day he replied:

Nope. I mentioned before that the feature changed. It didn't change for the better. Try doing a single fin. Two fins at 90 degrees will show stable.​

My interpretation at the time was that I wasn't going to be able to do the analysis the way I wanted to, and I moved on...

Hope this helps!
 
Marc_G said:
Later that day he replied:

Nope. I mentioned before that the feature changed. It didn't change for the better. Try doing a single fin. Two fins at 90 degrees will show stable.​
Click to expand...

Thanks for the followup! It does help some in that we now have conformation from Tim that functionality was changed, but what did he "mentioned before"?
 
rstaff3 said:
In other words its a bug...or a way to get people to get v10.
Click to expand...

Let's hope not, as removing a feature from the current version of a product just so folks will purchase the next version is actionable. :(

Dick, do you not use RockSim?
 
Yes I am a Rocksim guy. Unfortunately the last few versions I got as a download only so I can't investigate when this feature broke. I'd like to say it worked early in V9 but I can't say with certainty. I was just joking about the hook to buy an upgrade. It is more likely that the developer is off doing other things and maybe with Openrocket taking off the purchases of V9 were down enough that it doesn't pay to keep extending the product? I hope I'm wrong about that too.
 
I also see in the revision history that there is a newer version identified. Maybe that is the Mac version? My software sez I have the latest and greatest.
 
To answer the question about the discussion I had with him prior, here's the original exchange which preceded the one I posted above:

I submitted a help request via the Apogee website on June 19th:
Hi Apogee, I love Rocksim. I'm starting to work with some asymmetric rocket designs for the first time, and after reading your newsletter 105, went into the Rocket menu and chose Stability Analysis. However, all it did was show me a back-end view of the rocket. No static margin plot at all. I tried changing the velocity parameter... no change. I think somehow I need to enable a view of STatic Margin versus Radial Stability per the newsletter pictures, but I haven't figured out how to do this. I'm probably just missing something simple... probably not a bug. Could you please clue me in? Thanks Sincerely, Marc​

The next day he replied:

Hello Marc,

The look of the feature changed a bit since newsletter 105, but you can still get to it.

Go to the preferences menu, and from the 2D Drawing tab, you can turn it on. See attached image.

I hope that helps.​

He attached a picture of the 2D tab showing everything checked except for the "Enable antialias rendering" box. Essentially, by default, on the 2D view, the Base View decorations are unchecked. He was saying to turn them on by checking the boxes. I promptly did, but then it didn't really answer my question by providing a useful "stability from various angles" view in the way the newsletter said it would. That precipitated my next exchange (a couple posts above) where he essentially admitted the feature doesn't work anymore.

I reinstalled the software about 14 months ago when I redid my main computer, but haven't used the software significantly since then. I went into the 2D tab for the first time since reinstallation, and confirmed those "decorations" are off by default. But they don't work the way we expect anyway, so moot point. Sigh.

I have assumed Rocksim is in harvest/maintenance mode. I don't think it has been updated in a while, even a small update. I just checked to see if there were any updates waiting for me; none were there.

Marc
 
Well, I tried that and the results are mixed. In a quick test case, I went to two fins 180 degrees apart and the stability plot on the base view looked reasonable. However, when I went to two at 90 degrees, it still shows it's full stable, which is clearly wrong. This is a case where 100% broken may be better than half broken.
 
Thanks Marc, That explains where I expected this was going.

I love the folks at Apogee and RockSim as a whole, but like a misbehaving child I hate certain misbehaving facets of their personality. And for Apogee's that facet is support of Rocksim. As far as I know they are the sole vendor, so when they don't follow up with a resolution (they are already 0 for 1 for me) what's the amiable recourse? I suspect this is issue will make them 0 for 2.

I like your "harvest/maintenance mode" metaphor, but since this product appears to be used widely is Tim allowing it to "Die on the vine". This might make a good discussion for a new thread. What do you (or anyone else think)?
 
rstaff3 said:
Openrocket taking off the purchases of V9 were down enough that it doesn't pay to keep extending the product? I hope I'm wrong about that too.
Click to expand...

I hope you are too! If I recall correctly from my research there where many areas where OpenRocket is Deficient compared to RockSim. Does Openrocket do Stability Analysis similar to what RockSim used to do?
 
rstaff3 said:
Well, I tried that and the results are mixed. In a quick test case, I went to two fins 180 degrees apart and the stability plot on the base view looked reasonable. However, when I went to two at 90 degrees, it still shows it's full stable, which is clearly wrong. This is a case where 100% broken may be better than half broken.
Click to expand...

I'm not getting any plot at all could you post a screen shot?
 

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