Rocket Stability – CP data of OR vs Manufacturer Data vs RS Data

[Skyfall]

Dear all

Here now comes my first real post

Modifying a Sky Torpedo from 24 mm to 29 mm MM, I neared the topic of software-supported rocket design/modifications and its effects the first time.
Hence first trying to summarise my understanding so far, please correct/confirm.
Further down then the actual question that comes with it.

1. CP is a given attribute of a rocket and its exterior design. For our (LPR model rocketry) purposes we consider it as being static
2. Hence internal modifications to a model rocket kit will not impact CP (like my indicated modification of the MM)
3. By knowing CP of a kit, the CG should be designed to lay 1 to 2 calliper (body tube diameters) in front of the CP
4. This is to be achieved by (in most cases) adding nose weight

Trying to sum up: first one has to know the CP of a rocket, then the CG is to be positioned appropriately by adding weight.

This triggers the question, how to get to know the CP. This can be answered by the following two options:
a.) according to the designed rocket in OR/RS
b.) according to information of the kit manufacturer

Sometimes manufactureres provide the position of CG rather than of CP. In this cases the CG position is to be achieved (by adding nose weight), regardless of/not knowing the CP (purely relying on the provided CG to be correct for a stable rocket).

If all of the above is more or less correct, then I don’t get why the manufacturer data of the Sky Torpedo and the data according to the RS file of the vendor differ from each other.
According to the Apogee website (which is actually great and from which I really learned a lot!) the CP is located 22.12" (56.18 cm) from the tip of the nose cone.
Opening the provided RS file in OR, it states 51,4 cm.
(below related pics)

Is that a compatibility issue opening an RS file in OR?
Or (most likely) which part is it that I am missing?

Would be great if some can confirm/correct my general understanding on the approach to achieve a stable rocket and on my main question regarding differing CP data (RS file vs manufacturer data).

Thx

 

[Joekeyo]

The difference between the two is about 8%. I was told by an engineer that generally a 10% or more difference is considered significant. Both CPs are probably determined mathematically; I expect using different equations. Consider taking the average value: 53.79. I hope someone else can give you a better answer.

 

[smstachwick]

CP can be calculated in a number of different ways. The easiest way in the pre-digital age was to create a stiff paper cutout of the rocket and balance it on a ruler, the balance point being the CP. The idea was that the weight on either side of the balance point would roughly correspond to the aerodynamic forces. They do, so long as the rocket is flying sideways!

Rockets obviously don’t fly sideways, but this method is still useful because it illustrates the location of CP in a worse-case scenario. It will give you a rocket that is guaranteed to be stable in any flight regime you could reasonably expect, even though it may tend to weathercock a bit.

The other classic way is through a set of calculations based on the rocket’s dimensions developed by James S. Barrowman. His original equations were developed to calculate subsonic flight characteristics of sounding rockets and space launch vehicles. Model rocket builders typically use simplified versions.

The Barrowman method is far more accurate than the cardboard cutouts, but the equation turns my brain into mush. I, of course, am hopelessly out-of-practice with anything more advanced than basic arithmetic or perhaps some pre-algebra. If you’re a math shark, it should seem pretty basic.

The simplified Barrowman’s method as used by model rocketeers is still pretty conservative, though, and it assumes a pretty conventional design. Most V-2 kits will give readings of instability with the Barrowman method but will fly fine, due to the V-2’s unique design and some assumptions made in the simplified Barrowman method. Most sport models will match the calculated CP very closely, however.

RockSim and OpenRocket use Barrowman’s method but with a few more advanced elements included in the computations to better handle less conventional-looking airframes. RockSim even supports calculating stability using these simplified methods, last I checked.

My guess is that the manufacturer simply calculated CP differently. I think working with the OpenRocket value during pre-flight is your best bet, followed by acting on actual flight performance.

 

[kalsow]

AFAIK, OpenRocket uses the standard Barrowman equations. RockSim defaults to using "enhanced" equations. RockSim can use the standard equations. I'd bet that if you took that option it would agree quite well with OpenRocket.

"RockSim versus the Barrowman Equations":
https://www.apogeerockets.com/education/downloads/Newsletter238.pdf

 

[Skyfall]

AFAIK, OpenRocket uses the standard Barrowman equations. RockSim defaults to using "enhanced" equations. RockSim can use the standard equations. I'd bet that if you took that option it would agree quite well with OpenRocket.

"RockSim versus the Barrowman Equations":
https://www.apogeerockets.com/education/downloads/Newsletter238.pdf

That indeed sounds like a reasonable explanation on the difference, as OR is not just "opening" the RS file, but in fact "re-calculating" it.

Could someone having RockSim installed do me a favour and open the standard file as it is provided on the Apogee web page (and as added to this post) to check on the CP value as it turns out in RS?

 

[kalsow]

That indeed sounds like a reasonable explanation on the difference, as OR is not just "opening" the RS file, but in fact "re-calculating" it.

Could someone having RockSim installed do me a favour and open the standard file as it is provided on the Apogee web page (and as added to this post) to check on the CP value as it turns out in RS?

CP reported by RockSim10:
... cardboard cutout: 414.82 mm
... Barrowman: 517.81 mm
... RockSim enhanced: 561.91 mm

 

[Skyfall]

CP reported by RockSim10:
... cardboard cutout: 414.82 mm
... Barrowman: 517.81 mm
... RockSim enhanced: 561.91 mm

Great, thanks a lot - this pretty much explains the differences and indicates a general way forward (the truth might lay somewhere in the middle )

• The value according to vendor (Apogee) comes from (no surprise) RockSim enhanced
• The value of OR is according to Barrowman

Thanks to all!

 

[Bat-mite]

Great, thanks a lot - this pretty much explains the differences and indicates a general way forward (the truth might lay somewhere in the middle )

• The value according to vendor (Apogee) comes from (no surprise) RockSim enhanced
• The value of OR is according to Barrowman

Thanks to all!

If stability is questionable, never hurts to add a little nose weight.

 

[RocketRev]

Then there are the other factors that nobody has mentioned yet: builder practices and flier practices.

Some folks build rockets as close to the instructions as possible. Other folks tend to do some modifications. The CG of any built rocket can change a great deal depending on how the rocket is assembled and with what materials. Even how one person preps a rocket for a flight can change the CG from one flight to the next as well as from one flier to the next.

I've often heard that outboard motors can be plugged by the flier to eliminate the consequences of things like ejection charges in the motor itself. That's quite true, but I've seen fliers forget that adding epoxy to plug Estes motors used as outboards will change the CG of the rocket. I've seen this to devastating results. And I'm sure I'm not alone.

Then there comes differences in CP from differing building practices. It's been a while since I worked with RockSim and I've never worked with Open ROcket, but RS used to allow the user to change the "finish" on various rocket parts. The finish parameters can change the CP of a rocket design rather more than one might expect. I tend to fly my rockets with a pretty rough finish. I get whatever I can get with not much prep beyond scotchbrite and clean coton clothes followed by Krylon rattle cans. Other folks use automotive/professional spray guns and get finishes that are so smooth that I cant' imagine all the work they put into thier finish.

Going with internal fin filets rather than external fin filets change the CP as well as can so many things we do as we build and fly rockets.

Lastly is my personal bugaboo: people who don't actualy measure thier real finished ready to fly including motor weight and real finished CG and therefore never bother to input the actual data into either RS or OR. The sim programs that we use are amazingly accurate these days, but they are only as good as the information that is fed into them.

For good or ill, I happen to be both TAP and L3CC and I can't tell you how many times I've had L-3 cert wannabe's be flabbergasted by the actual weight of thier rocket as it sits on the scale at a rocket launch. I've heard far too many times, "But the sim says it only weighs.....," and you can fill in the blank. And they were trying to fly an L-3 cert rocket on a motor that turned out to be marginal for the weight of the rocket that they actually built rather than "ok" for the rocket that they simmed.

Be careful to imput the actual data into your sim programs, not what you think it is. As one political commentator is fond of saying, "facts don't care about your feelings." In rocketry it might come out as, "Your Results May Vary."

Brad the "Rocket Rev.," Wilson

 

[rharshberger]

Then there are the other factors that nobody has mentioned yet: builder practices and flier practices.

Some folks build rockets as close to the instructions as possible. Other folks tend to do some modifications. The CG of any built rocket can change a great deal depending on how the rocket is assembled and with what materials. Even how one person preps a rocket for a flight can change the CG from one flight to the next as well as from one flier to the next.

I've often heard that outboard motors can be plugged by the flier to eliminate the consequences of things like ejection charges in the motor itself. That's quite true, but I've seen fliers forget that adding epoxy to plug Estes motors used as outboards will change the CG of the rocket. I've seen this to devastating results. And I'm sure I'm not alone.

Then there comes differences in CP from differing building practices. It's been a while since I worked with RockSim and I've never worked with Open ROcket, but RS used to allow the user to change the "finish" on various rocket parts. The finish parameters can change the CP of a rocket design rather more than one might expect. I tend to fly my rockets with a pretty rough finish. I get whatever I can get with not much prep beyond scotchbrite and clean coton clothes followed by Krylon rattle cans. Other folks use automotive/professional spray guns and get finishes that are so smooth that I cant' imagine all the work they put into thier finish.

Going with internal fin filets rather than external fin filets change the CP as well as can so many things we do as we build and fly rockets.

Lastly is my personal bugaboo: people who don't actualy measure thier real finished ready to fly including motor weight and real finished CG and therefore never bother to input the actual data into either RS or OR. The sim programs that we use are amazingly accurate these days, but they are only as good as the information that is fed into them.

For good or ill, I happen to be both TAP and L3CC and I can't tell you how many times I've had L-3 cert wannabe's be flabbergasted by the actual weight of thier rocket as it sits on the scale at a rocket launch. I've heard far too many times, "But the sim says it only weighs.....," and you can fill in the blank. And they were trying to fly an L-3 cert rocket on a motor that turned out to be marginal for the weight of the rocket that they actually built rather than "ok" for the rocket that they simmed.

Be careful to imput the actual data into your sim programs, not what you think it is. As one political commentator is fond of saying, "facts don't care about your feelings." In rocketry it might come out as, "Your Results May Vary."

Brad the "Rocket Rev.," Wilson

The moral of the L3 cert story is...dont buy your motor before the fat rocket scales....

 

[Skyfall]

Thanks a lot for all the additional input - that just makes perfect sense

 

[RocketScientistAustralia]

You can always do the swing test easily on any rocket up to a few Kg weight. The swing test won't always say it's definitively unstable if it's close. But if it flies nose first in the swing test it will be definitively stable. ( unless you have a motor that is 2m long and extends past your CG and your stability DECREASES as it burns....... Hybrids......)
Norm

 

[Skyfall]

You can always do the swing test easily on any rocket up to a few Kg weight. The swing test won't always say it's definitively unstable if it's close. But if it flies nose first in the swing test it will be definitively stable. ( unless you have a motor that is 2m long and extends past your CG and your stability DECREASES as it burns....... Hybrids......)
Norm

Great, that’s a good one - and surely doable for the more ordinary LPR/MPR kit-stuff that I am doing!

 

[JimByrne]

I sure wish all the kit manufactures would list the CP---the rest is easy

 

[RocketScientistAustralia]

I sure wish all the kit manufactures would list the CP---the rest is easy

And mark it on the bodytube. And specify it from the rear of the body tube. Which is a lot easier to Measure than from the tip.

 

[Neo]

Great points made by @RocketRev, but I'm still gobsmacked by the findings that @kalsow shared. 6 inches of difference between the different methods for calculating CP, or even 1.7" between Barrowman (OpenRocket I presume?) & RockSim enhanced, is a pretty big deal on a rocket that is 2.6" diameter. Even the difference on the two closest measurement methods is over 0.6 caliber and over 6% of the total length, so where is the real point of aerodynamic center that the CG needs to be measured ahead of?

That was a great article linked to with Tim VanMilligan's explanation on this subject, but as he points out there is a lot of value in not carrying any extra weight on your rocket than is necessary. So if these two methods result in over 1/2 a caliber of difference in CP, then the nose weight that you do or do not need in order to have the CG be at least 1 caliber ahead of CP is significant.

I'm not building this particular rocket, I'm just using the information in this thread to illustrate the frustration I am finding on most of my rocket builds in determining the location of the CP so that I can plan for the CG ending up in a position that makes the rocket safe & stable. Whenever I bring this up at the launch field it seems like I am the only one who has put significant consideration into this subject and thus fly with a lot more nose weight in my rockets than many other rockets I see.

 

[smstachwick]

Great points made by @RocketRev, but I'm still gobsmacked by the findings that @kalsow shared. 6 inches of difference between the different methods for calculating CP, or even 1.7" between Barrowman (OpenRocket I presume?) & RockSim enhanced, is a pretty big deal on a rocket that is 2.6" diameter. Even the difference on the two closest measurement methods is over 0.6 caliber and over 6% of the total length, so where is the real point of aerodynamic center that the CG needs to be measured ahead of?

That was a great article linked to with Tim VanMilligan's explanation on this subject, but as he points out there is a lot of value in not carrying any extra weight on your rocket than is necessary. So if these two methods result in over 1/2 a caliber of difference in CP, then the nose weight that you do or do not need in order to have the CG be at least 1 caliber ahead of CP is significant.

I'm not building this particular rocket, I'm just using the information in this thread to illustrate the frustration I am finding on most of my rocket builds in determining the location of the CP so that I can plan for the CG ending up in a position that makes the rocket safe & stable. Whenever I bring this up at the launch field it seems like I am the only one who has put significant consideration into this subject and thus fly with a lot more nose weight in my rockets than many other rockets I see.

CP actually changes throughout flight, depending on the rocket’s angle of attack, velocity relative to the speed of sound, and other variables in airflow. Barrowman’s calculations just provide an approximation for extremely low angles of attack with conventional builds, RockSim is a bit more flexible in terms of accuracy in unconventional designs like the aforementioned V-2, and the cardboard cutout method gives you a worst-case scenario (the rocket has somehow turned completely perpendicular to the airflow and aerodynamic forces are now applied much more strongly across the body tube and other components without calculated Cn or Xn values).

 

[Frausing]

I will say, you need to do a swing test
Try to look at this youtube video's