CG/CP Stability - nose weight with larger motors?

[JSW]

Fairly new to HP rocketry. Prepping an Apogee Zephyr for it's 2nd flight.

Trying to confirm my understanding of the stability calc.

(BTW, while typing this question up,... I researched quite a bit,.. and may have answered my own question. Still, would appreciate a sanity check.)

Per NAR guidelines, CG needs to be at least 1/2 tube diameter forward of the CP (1) with an new motor, and (2) with a depleted motor.

For my Apogee Zephyr:

• No motor: OpenRocket without any motor calculates CG 6" forward of CP = 1.5x tube diameter. Actual CG matches OpenRocket.
• H283: For the 1st flight on an H283 motor, the actual CG was 3.5" forward of the CP = 0.875x tube diameter.
• I140: For my upcoming 2nd flight, OpenRocket calculates CG 2" forward of the CP = 0.5x tube diameter.
• J825: Just for fun, I ran OpenRocket with the largest motor that Apogee recommends: J825. Calculated CG is 1.5" forward of CP = 0.375x tube diameter.

Stability for the H and I motors is ok since they are at least 0.5x the tube diameter.

But for the J motor, the stability is 0.375x the tube diameter. Does that mean with the J motor I would need to add nose weight to get the CG 1/2" higher in the rocket?

Thanks in advance for any feedback.

(BTW, the reason I even started looking at this was because I was wanting to mount a camera on the rocket and was trying to do an OpenRocket model to confirm where best to locate the mount. I presume best to put it right around the CP so that it has negligible impact?)

 

[rharshberger]

Is that 1.375 cal of stability or .375 cal of stability? Anything over 1 is considered stable up until transonic

 

[dr wogz]

Typically, you want 1-2 calibers for stability.

The CP tends to be fixed*, while the CG shifts; with different motors, with electronics, even in flight as the motor burns.

Easiest method is to mark the rocket with it's CP (and this should be stated in the instructions somewhere). Then make a mark one caliber ahead of this. This is your minimum CG location. load up the rocket as per your intended flight. Balance at this point. If it balances, or is Nose heavy, you are good to go! If not, add nose weight until you can balance at or ahead of this minimum 'CG' mark. The RSO should be asking you to point out these two points, and prove your rocket is stable. (I do when I sit at that table: the sim on paper is pretty, but prove to me the actual rocket matches the predicted sim)

We typically will add nose weight for the heaviest motor we intend to fly. This may be an I motor, or it may be a K.. (Motor mount & cert level usually indicate the largest 'acceptable / expected' motor) it's also not uncommon for us to add a bit of ballast at the field to get the CG/CP right.. Some have even devised a method making 'adjustable nose weight', usually a threaded rod in the nose cone where washers can be added or removed for balance.

Put your camera at or ahead of the CG.

Min exit velocity is another concern. Make sure your motor is powerful enough to get you up to speed for when you leave the launch rail. Remember the I140 is less punchy than the H238..


* The CP is a very small range, not an exact point. It will shift as the speeds increase, the air gets heavier / lighter, etc.. OR can give you a CP chart so you can see how it shifts in flight.. You can also work out the CP by doing the 'Barrowman' equation.
http://www.rocketmime.com/rockets/Barrowman.html

 

[JSW]

Typically, you want 1-2 calibers for stability.

The CP tends to be fixed*, while the CG shifts; with different motors, with electronics, even in flight as the motor burns.

Easiest method is to mark the rocket with it's CP (and this should be stated in the instructions somewhere). Then make a mark one caliber ahead of this. This is your minimum CG location. load up the rocket as per your intended flight. Balance at this point. If it balances, or is Nose heavy, you are good to go! If not, add nose weight until you can balance at or ahead of this minimum 'CG' mark. The RSO should be asking you to point out these two points, and prove your rocket is stable. (I do when I sit at that table: the sim on paper is pretty, but prove to me the actual rocket matches the predicted sim)

We typically will add nose weight for the heaviest motor we intend to fly. This may be an I motor, or it may be a K.. (Motor mount & cert level usually indicate the largest 'acceptable / expected' motor) it's also not uncommon for us to add a bit of ballast at the field to get the CG/CP right.. Some have even devised a method making 'adjustable nose weight', usually a threaded rod in the nose cone where washers can be added or removed for balance.

Put your camera at or ahead of the CG.

Min exit velocity is another concern. Make sure your motor is powerful enough to get you up to speed for when you leave the launch rail. Remember the I140 is less punchy than the H238..


* The CP is a very small range, not an exact point. It will shift as the speeds increase, the air gets heavier / lighter, etc.. OR can give you a CP chart so you can see how it shifts in flight.. You can also work out the CP by doing the 'Barrowman' equation.
http://www.rocketmime.com/rockets/Barrowman.html

Thanks. This all makes sense.

I like the idea of marking the CP and min CG location. I’ll do that for sure.

 

[UhClem]

The CP is a very small range, not an exact point.

The CP is a function of angle of attack. It will be at its worst, usually, just as the rocket exits the rod/rail.

The 1 caliber rule of thumb was useful when we didn't have tools that could plot the CP shift during flight. Now that those tools are widely available, they should be used instead. Run a simulation with the lowest thrust motor you plan to use with the worst wind settings you expect. Then plot the CP. The more it moves the more margin you want to the CG.

 

[manixFan]

dr wogz has excellent info in his post. To add just a bit more, you can weigh components as you build/add them and use the over ride function to note where the actual GC is. For example, if you use a lot of epoxy and filler on your fins, that will shift the actual CG backwards, same with a motor retainer. If you put your camera in the nose cone, make sure you weigh it and add it to the sim as a mass object that is accurately placed. All of the will help you better predict your CG once your rocket is finished. But of course, you'll still want to check it once it's all assembled for flight with the motor installed.

I have enough rockets that I have several nosecones in 54mm and 3" that have different amounts of nose weight so I can pick the one I need based on the motor. I also have nosecones with threaded rods that allow adjustable weight - but those use a fiberglass nosecone with a metal threaded tip.

Good luck and enjoy the journey,


Tony

 

[dr wogz]

Thanks. This all makes sense.

I like the idea of marking the CP and min CG location. I’ll do that for sure.

all HPR rockets are supposed to have their CP/CG locations marked.. Do the CP at the very least.

 

[dhbarr]

Remember that long motors sticking past the CG can bring -more- stability. Often not worth worrying about, but it does mean sometimes, e.g., a long 6g 29mm could be more stable than a similarly-performing short 2g 38mm.