# Rocksim and stability

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#### n27sb

##### N27SB
TRF Supporter
I have used Rocksim for many years but there is one issue I am not sure how to work around.
If I design a small rocket say 24mm x 20" long Rocksim will give me an acceptable stability margin with a certain size fin if the Barrowman option is on.
If I select the 2d paper cutout option it will be marginal or unstable.
In all instances a swing test and even real flight usually will be unstable. As the rocket size gets bigger in diameter this goes away.
For this reason I usually have to go up to a margin of 2.5 or more.
Any ideas.
The only response from Apogee was "Your fins look too small"

#### neil_w

##### Hunkered down and slowly going crazy
TRF Supporter
Can you post a file?

#### n27sb

##### N27SB
TRF Supporter
Can you post a file?
This example will probably not swing test and will be unstable even with a long rail.
Is it possible that really thin streamlined fins are producing less lift.

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#### chrocket

##### Member
It might be, that the span of the fins is to small. I remember that the fins span should be about 2 times the body tube diameter.
This because if the span of fins is to small most of the fins are in turbulences caused by the nosecone and bodytube.

#### neil_w

##### Hunkered down and slowly going crazy
TRF Supporter
Hmm, to be honest that rocket looks OK to me. OR shows CP at 15.726", what does Rocksim show? What motor?

Although those fins are on the small side, I wouldn't think of them as being too small to fly. They're at least as large, relative to the body, as the Patriot for example.

#### n27sb

##### N27SB
TRF Supporter
Hmm, to be honest that rocket looks OK to me. OR shows CP at 15.726", what does Rocksim show? What motor?

Although those fins are on the small side, I wouldn't think of them as being too small to fly. They're at least as large, relative to the body, as the Patriot for example.
Rocksim shows a CP 16.9 with Barrowman equation, 13.5 with 2D cutout equation.
CTI F30

#### neil_w

##### Hunkered down and slowly going crazy
TRF Supporter
I have no good answer. Looks like a stable rocket to me.

#### n27sb

##### N27SB
TRF Supporter
I agree, The fins on this are only about .015 thick and they are streamlined so at low velocities they may not generate much lift. They also are CF and a little thicker at the root. 3 weeks ago I flew this configuration and it spiraled out of control. I flew the exact configuration rocket as a sustainer and it was dead straight but it was going over 350 mph when it separated. I have had the same problem with other similar rockets in the past.

I have always suspected that there could be some vectored thrust on the 3 grain and longer CTI 24mm loads. I think they are a side burners. I usually find some offset ash and sludge around the nozzle. This may only occur on small rockets that you push hard.
Although the sustainer on my 2 stage was going fast at separation it also was using an Apogee E6 which is an end burn.

#### Bill S

##### Well-Known Member
Doesn't Barrowman give much more conservative stability numbers than using the Rocksim calculations? Maybe try the Rocksim numbers and see what you get.

#### n27sb

##### N27SB
TRF Supporter
Doesn't Barrowman give much more conservative stability numbers than using the Rocksim calculations? Maybe try the Rocksim numbers and see what you get.
Rocksim has 3 choices on stability in this order.
Rocksim
Barrowman
2D paper cut out

2D gives the most conservative
Barrowman next
Rocksim last

In other words, If 2D says you are good, you are way good or way overstable

Here are 3 screen shots changing only the method of calculating margin

Take a look at the method and the margin results.
I think that on this rocket Barrowman is marginal to unstable and 2D is too conservative.
The net result in real life with an F30-----Unstable

Last edited:

#### Jeff Lassahn

##### Well-Known Member
All of these methods are only rough approximations, so you can't trust them 100%.

In particular, Barrowman underestimates lift/drag from the body tube, and 2D area overestimates it. So for a design with a long body and small fins, Barrowman puts the CP too far back, and area maybe puts the CP too far forward depending on how close it really is to the center of the tube.

#### n27sb

##### N27SB
TRF Supporter
All of these methods are only rough approximations, so you can't trust them 100%.

In particular, Barrowman underestimates lift/drag from the body tube, and 2D area overestimates it. So for a design with a long body and small fins, Barrowman puts the CP too far back, and area maybe puts the CP too far forward depending on how close it really is to the center of the tube.
Thanks Jeff, That is what I am now guessing. I am not a big fan of spin testing but it points to a CP midway between the two.

#### Bill S

##### Well-Known Member
Is there some reason you aren't considering the Rocksim calculation method stability numbers?

#### n27sb

##### N27SB
TRF Supporter
Is there some reason you aren't considering the Rocksim calculation method stability numbers?
They place the CP even farther aft than the Barrowman.

I went back to my crash log and it looks like most of the failures on small high horse power launches were probably due to this issue.
I am planning another attempt but I will go bigger fins and swing test or car test. Not sure I trust RS on this anymore.

#### AfterBurners

##### Well-Known Member
Use RS for both settings you should be fine

#### lucanica

##### New Member
Just a couple of thoughts on this subject:
• Cardboard cutout: This method determines the center of pressure (CP) based on the silhouette of your model. If you project your rocket’s shadow on a wall and then “cutout” that shadow from a piece of cardboard, then the center of mass of that piece of cardboard will approximate the CP. This method assumes your rocket to be ascending in a perfectly “horizontal” position, i.e, with its airframe parallel to the earth surface. Of course, you don´t design rockets to fly like that, so this method predicts a “worst case” or “pessimistic” CP.
• Barrowman: Under several assumptions, James Barrowman derived a set of equations to obtain a “more realistic” estimation of a rocket’s CP. Since the very first assumption of his method was “The angle-of-attack is very near zero”, the method was intended to be used on, so to say, "well designed rockets".
• Rocksim: As far as I understand, this is not open source. Thus, I don’t know how these calculations are performed.
With that said, an “arrow-shaped” model like the one shared by n27sb (21” long by less than 1” in airframe diameter) may not fit Barrowman´s assumptions and, as a consequence, may produce misleading CP estimations under that method.

Just my $0.02 #### RobertH3 ##### No need to buy stands after a launch day! TRF Supporter How about individual mass of parts? You could go through and precision-weigh the unloaded rocket and compare it to calculated unloaded weight in Rocksim. If the weights do not match, then weigh each individual part and use "mass over-ride" on each part box and manually type it in. When I overbuilt a custom design, I found this to be an issue. If it is way-overestimating the nose cone weight, then the Cg may be in error. Since nose cone is checked as fiberglass, the software may be using a heavier mass. Cheers / Robert #### n27sb ##### N27SB TRF Supporter How about individual mass of parts? You could go through and precision-weigh the unloaded rocket and compare it to calculated unloaded weight in Rocksim. If the weights do not match, then weigh each individual part and use "mass over-ride" on each part box and manually type it in. When I overbuilt a custom design, I found this to be an issue. If it is way-overestimating the nose cone weight, then the Cg may be in error. Since nose cone is checked as fiberglass, the software may be using a heavier mass. Cheers / Robert I see your point, although the actual individual components that make up the CG may not be exactly as they are in the model, the end resulting balance point or measured CG is right on the money. My assumption was if the CG is actually where it belongs does it matter how you got there. #### n27sb ##### N27SB TRF Supporter Just a couple of thoughts on this subject: • Cardboard cutout: This method determines the center of pressure (CP) based on the silhouette of your model. If you project your rocket’s shadow on a wall and then “cutout” that shadow from a piece of cardboard, then the center of mass of that piece of cardboard will approximate the CP. This method assumes your rocket to be ascending in a perfectly “horizontal” position, i.e, with its airframe parallel to the earth surface. Of course, you don´t design rockets to fly like that, so this method predicts a “worst case” or “pessimistic” CP. • Barrowman: Under several assumptions, James Barrowman derived a set of equations to obtain a “more realistic” estimation of a rocket’s CP. Since the very first assumption of his method was “The angle-of-attack is very near zero”, the method was intended to be used on, so to say, "well designed rockets". • Rocksim: As far as I understand, this is not open source. Thus, I don’t know how these calculations are performed. With that said, an “arrow-shaped” model like the one shared by n27sb (21” long by less than 1” in airframe diameter) may not fit Barrowman´s assumptions and, as a consequence, may produce misleading CP estimations under that method. Just my$0.02
Thank you for your input. I believe you are correct. My goal is to come up with a method of how to compensate for this on these types of configurations.
A percentage of somewhere between 2D and Barrowman is most likely the answer.

#### n27sb

##### N27SB
TRF Supporter
I did some testing today, By adding weight to the nose and then a Wind Tunnel test I had the following results.
Both examples use the 2D paper cutout mode.

In this example the rocket tested unstable in the wind tunnel.

I this example it tested stable.

Conclusions,
With Rocksim and a long skinny rocket of these proportions the 2D method provides a stable design.
It depicts the CP very close to actual.

#### RobertH3

##### No need to buy stands after a launch day!
TRF Supporter
That is interesting. I did some min dia rockets back in the early 2000's and lost all of them But I manually calculated Barrowman for these, didn't know about the rebirth of rocketry (software/kits/etc.). My 24mm was a little on the short side and had a touch of wobble when it left the launch rod but proceeded to vanish.

Cheers / Robert

#### Alan15578

##### Well-Known Member
The cardboard cut out method is awful. It can be extremely conservative or extremely anti conservative, depending on the rocket. You only use it when you cant use something better. It's like:

Your Honor, we were not negligent. We know about the need for stability, We used the cardboard cut out test, but it proved catastrophic in this case. So we were merely incompetent, not negligent.