Quantcast

CG vs. CP

The Rocketry Forum

Help Support The Rocketry Forum:

TMJ

"Where the heck'd it go!"
Joined
Jun 22, 2016
Messages
245
Reaction score
0
Back when I flew Estes kits, and also scratch built my own rocket designs... I had a trick that I used to insure flight stability. I'd tie a string to the rocket, at the specified CG, then spin the rocket over my head while watching how the nose tracked within the arc of my circle. If the nose pointed 'outwards', I'd add weight to the nose and re-adjust my string to the new center of gravity, then swing the rocket again. If the nose pointed 'inwards', I new it was nose heavy and would reduce weight in the nose, or add weight to the tail. I'd keep swinging my rockets and making adjustments until its nose and tail both tracked in a perfect circle while being swung on my string... then I'd launch the rocket on its maiden flight. They always flew straight as a laser, however. The final CG was also typically located further towards the nose than the kits instructions stated it should be. The rockets were also always a bit heavier, due to added nose weight... but I preferred 100% stability over higher altitudes. That being said, I'm now building a rocket almost seven feet tall. I'll not be able to swing this thing around my head. Should I stick to the CG shown in my kit, or move it forward for better stability? If so, are there any general rules, 'guidelines', percentages, ect. that you experienced high power folks know about?
 

RocketFeller

Well-Known Member
Joined
Sep 15, 2009
Messages
1,187
Reaction score
72
With kits I would say build as designed. That is one of the advantages of a kit over a scratch-build, someone has already done the math for you.
 

DavidMcCann

Well-Known Member
Joined
May 15, 2016
Messages
2,656
Reaction score
166
Kits are fairly safe, but you still need to know the CP, and check the actual CG before launch. There's just too many motors and building styles out the to just trust that a kit will be ok.

Moving the CG too far forward can have adverse affects as well, so just loading up 5 pounds of shot in the nose isn't recommended. (coning, weather cocking, etc.)
You're looking for 1 body tube diameter or more....but not too much more. (just to keep it interesting, if you're short and fat you can go with less than half that ;) )

One thing you haven't mentioned (or likely decided yet) is what adhesive to use... It's not a bad opportunity to get used to using epoxy, but it adds weight to the rear of the rocket, and Titebond or other wood glue will be lighter, and make joints stronger than the wood and cardboard.

There's the cardboard trick- https://www.apogeerockets.com/education/downloads/Newsletter18.pdf


or, this is where getting into using Openrocket or rocksim helps.

CP is 61.6" from the nose tip.

 
Last edited:

Forever_Metal

JustAnotherBAR
Joined
May 21, 2016
Messages
493
Reaction score
37
Location
Madison, AL
TMJ,

Is this an upscale scratch build of a smaller kit rocket?

I had the same questions when I got into the HPR arena last year.

/Your 1st step was a good one; coming here and asking... TRF has MANY great fliers with endless knowledge.
/My advice is, if it's a kit you're building, the next place you should look is with the folks who manufactured it and get it as close as you can to the manufactures recommendations.
/Next, there is a wealth of knowledge with your local fliers and club reps, (it helps when they can see the bird in front of them), most if not 99% are willing to provide insights (get a few opinions), suggestions, or in a few cases outright build assistance.
/Another avenue, but by no means an end-all-be-all solution is to use simulation software. While I attended UAH last year :)headbang::clap:Go Chargers!:headbang::clap:), fellow students intro'd me to OpenRocket and RockSim. Both (there are a few more available) have +/-'s. Cheap answer (ie FREE)... OpenRocket, expensive answer... RockSim. I've had simulations of both be right on, and a couple have fallen flat. But they can get you close; a semi-educated answer if you will.
/Lastly, you could also take a look at a few sites that have the formulas needed to compute the CG/CP (I've heard them used to validate the sims); but be ready for math in all it's glory!!

IMO, the best way to start would be with the manufacturer and a local active rocketry club. Sim it in OpenRocket, then attend a local launch or meeting and intro yourself and your project (we like show-n-tells!), you'd be surprised how willing most are to help to keep the pointy end facing up!

Hope this rambling helps!

fm

https://www.apogeerockets.com/Rocket_Software/RockSim
http://openrocket.sourceforge.net/
http://www2.estesrockets.com/pdf/TIR-33_Center_of_Pressure.pdf
http://physics.gallaudet.edu/tools/rocketcop.html
http://www.aerorocket.com/cp.html
 

Tim51

Well-Known Member
Joined
Jul 4, 2015
Messages
919
Reaction score
194
Location
London, United Kingdom
Back when I flew Estes kits, and also scratch built my own rocket designs... I had a trick that I used to insure flight stability. I'd tie a string to the rocket, at the specified CG, then spin the rocket over my head while watching how the nose tracked within the arc of my circle. If the nose pointed 'outwards', I'd add weight to the nose and re-adjust my string to the new center of gravity, then swing the rocket again. If the nose pointed 'inwards', I new it was nose heavy and would reduce weight in the nose, or add weight to the tail. I'd keep swinging my rockets and making adjustments until its nose and tail both tracked in a perfect circle while being swung on my string... then I'd launch the rocket on its maiden flight. They always flew straight as a laser, however. The final CG was also typically located further towards the nose than the kits instructions stated it should be. The rockets were also always a bit heavier, due to added nose weight... but I preferred 100% stability over higher altitudes. That being said, I'm now building a rocket almost seven feet tall. I'll not be able to swing this thing around my head. Should I stick to the CG shown in my kit, or move it forward for better stability? If so, are there any general rules, 'guidelines', percentages, ect. that you experienced high power folks know about?
or, this is where getting into using Openrocket or rocksim helps.
+1. I too found it nerve wracking not being able to do a swing test. My advice is yes, follow the manufacturers' advice, but also use Open Rocket to build an accurate simulation that factors in the actual built weight of epoxy, paint etc, measuring and weighing as you go. OR also allows you to select from a wide range of motors and plot flight simulations. If you get into the habit of using a sim like OR, you'll find it useful in the future when you're thinking about other possible builds, using different motors etc, and all the data is there for filling out the flight card in the field. It's not hard to get the hang of, and again, there are plenty of people on TRF who are experts to help.
 

dingle

Well-Known Member
Joined
Dec 9, 2013
Messages
55
Reaction score
0
CP is the hard one, CG is no sweat.

CG you can start with what the kit specifies and once you get it fully loaded with motor to full flight status you can do the balance test using a string/cord (http://www.rockets4schools.org/images/Basic.Rocket.Stability.pdf). That will give you an accurate CG and you can recheck it against the stated CP and make sure you have enough stability.

IIRC the distance between the CP and CG should be from 1 to 2 times the diameter of the rocket. So if it's a 4" diameter rocket your CG and CP should ideally be at least 4 inches apart and no more than 8 inches apart. You can have greater than x2 but then you need to start worrying about weather-cocking due to the over-stability of the rocket.
 

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
Messages
10,884
Reaction score
1,627
Location
Maryland
Keep in mind that your CP is potentially changing during flight. The least amount of weathercocking will move your CP. Also, the CG begins changing as soon as the motor begins to burn. A stability sim in OR or Rocsim will show you the CP/CG ratio throughout the flight.

Transitioning into Mach speeds makes the CP leap forward, so an additional caliber of stability is necessary when expecting Mach speeds.
 

SpaceManMat

Space Nut
Joined
Dec 20, 2013
Messages
666
Reaction score
48
A swing test actually checks a rockets stability in a very tight turn. It's not likely to actually encounter such a turn in real flight so a swing test results in an overly stable rocket. IE it will be heavier than required
 

TopRamen

SA-5
Joined
Aug 9, 2013
Messages
9,955
Reaction score
83
A swing test actually checks a rockets stability in a very tight turn. It's not likely to actually encounter such a turn in real flight so a swing test results in an overly stable rocket. IE it will be heavier than required
That makes sense, and explains some of my earlier results back when I was a kid,(8-11ish) and started using transitions in Nike Apache style scratch builds.
I always seemed to over compensate for the second smaller set of fins, but my swing tests said fly it.
After a couple of ballistic arc learning experiences I went back to 3-4FNC and the occasional BBZ style split fin long body designs.
I had kits, but my Mom would also get me the builders specials from Estes with all the pieces parts.
 
Top