Question about RocSim9 and cg/cp relation.

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May 15, 2009
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I’m sure this has been asked before, probably hundreds of times. In fact I know I have seen the answer here, but I can’t seem to find it now that I need it. I’m designing my next new scratch build after over a year vacation to building rockets and decided to use RocSim to help in design, instead of the old, fun, entertaining way for trail and error. I’m trying to find where on the model the cp should be in correlation to the cg. Right now they are almost on top of themselves however the rocket launches great in simulation. See attached. Thanks.

mmm.... calvin, I se you're new to rsim and also kinda new to this world....

a rocket can be "called" stable when CG is fw CP of about 2 calibers
rsim show you the stability in a simple way, it just tell you if your rocket is stable or marginally stable... or even unstable of course

in your case you're very marginally stable, close to unstable
i will never launch a rocket in this configuration:D

to ensure a good and safe flight you must add weight on the nose or move back the fins

you are sure your flight will go true if the distance of CG and CP is twice the diameter of the tube
of course shorter distance will work too, but the design of the rocket must be little different
lower stability margin can be used with no much worry in high drag rockets because the high drag base tend to pull the back preventing the rocket to rotate over the CG:rolleyes:

hope it helps

So your saying the cg and cp should be, in this case, separated by at least 2.6in to be “stable?" This helps a lot, as most of my designs I've drawn out would be considered "high drag" I like short and squat rockets. I experienced a similar situation with a rocket I built last year, I posted here actually under a different user name, I named the rocket Aquila. My first flight was disastrous. It leapt into the sky, somersaulted, did a nose plant into the earth and a few seconds later comically shot the body of the rocket up into the air and pulled the nose cone out of the earth. It didn’t damage any parts of the rocket or anyone. After that I downloaded RocSim demo and found I needed almost an ounce of weight in the nose. Next flight went perfect. So, these numbers and these so called cp and cg aren’t things made up by the rocket elders to scare little Rocketeers to practice safe rocketry after all. ;)

So in RocSim I’ve added a mass object of 1 oz to the nose and it now looks like so. If I am understanding you right this should be better.

So your saying the cg and cp should be, in this case, separated by at least 2.6in to be “stable?"
The body tube diameter is one caliber and is the typical minimum
distance the CG should be in front of the CP.

So in RocSim I’ve added a mass object of 1 oz to the nose and it now looks like so. If I am understanding you right this should be better.
RockSim shows the stability margin in calibers, not inches, so you
could probably reduce the nose weight a bit. (Your latest screenshot
shows a margin of 2.3- aim for 1 or more)...

BTW- that's a sweet lookin' fin design! Kinda reminds me of
the Blue Tube Phenix from Always Ready.
Rocksim accurately can determine your CP. However, the CG is determined by the weights of the components stored in the Rocksim files, which may not be their actual weights, and obviously the assembled rocket does not include weight of adhesive. The best - and most accurate - way to determine your TRUE CG is to build the rocket, pack it with recovery and the then balance test it. The point where it balances is the CG. Now, measure the distance from that point to the tip of the nosecone. As long as the difference between YOUR measured (and true) CG and the CP is equal to or greater than the diameter of the body tube (1 caliber as previously noted) you should be good. If it's less than 1 body tube diameter you'll need to add noseweight until the CG/CP relationship is that 1 caliber. I would also balance test the rocket the largest motor you intend to fly in the rocket when determining the CG. Determining the CG without the motor could be problematic because once the motor is loaded the CG will shift rearward and reduce the CG/CP distance and possibly make the rocket unstable in it's flying configuration.
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My experience is that the weights of the components in Rocsim are usually correct. The biggest problem I run into is things like glue, paint, and the dozens of little parts that are just inconvenient to include in the design screen. If you are building a sport rocket, then you just build the rocket and add nose weight until the actual CG is one caliber ahead of the CP as predicted by Rocsim. If you are looking for max performance, then you don't want to be adding nose weight and will want to develop your own database (based on experience) as to how much weight the paint, glue, etc., add and where to locate those additional masses.

The one caliber guideline is a generality and may not be appropriate for all rockets. At zero angle of attack, a circular cylinder gives no aerodynamic contribution to stability (or instability) and hence does not enter into the calculated CP. As the angle of attack increases, the cylinder (body tube) does begin to contribute and most often it is a destabilizing influence. The one caliber margin accounts for this and any inaccuracies in the calculation procedure. If your rocket is long and skinny, the impact of the body tube will be greater and you will need more than one caliber margin. If it is short and fat, it is entirely possible that the aerodynamic center of the body tube is behind the CG and is thus a stabilizing influence. In that case, you can get by with less than one caliber margin, but don't push it too far. You will find that the LOC Mini-Magg is such a rocket. People fly them successfully with less than one caliber of stability. I have actually seen one that had a large engine and was unstable at zero angle of attack and became stable as it yawed. It zig-zagged all over the sky. This is not a desirable situation.