The CG (Center of Gravity) and CP (Center of Pressure) are very important fundamental design and flight parameters of any rocket, and have an important relationship to each other. The general relationship between the CG and CP is as follows: the center of pressure must be a minimum of 1 body diameter BEHIND the center of gravity on a rocket fully prepped for launch to ensure stability. Now well explain why it must be this way, and what you can do to make sure your rocket meets this requirement, whether its a kit or a scratchbuilt design.
You’re probably aware of the center of gravity (CG); this is the point on a rocket (or any object) where all the weight seems to be “centered”. The center of pressure (CP) is similar; it is the point on the rocket where the corrective force of the fins is “centered”. OK, now imagine just a simple stick, a piece of dowel 12 long. This is your rocket, with one end the nose end and the other the motor end. It weighs the same on the first 6 as it does on the rear 6; thats why the CG is exactly in the middle at 6. Thats what the CG means, where the weight (or, more correctly, mass) of the item is centered. Now, drive an imaginary nail into your stick rocket into the table its sitting on right at that CG point of 6. OK, now push on the front of the rocket, and it spins right at the nail, the CG. Push on the rear of the rocket, and it spins right at the nail, the CG. Thats a good way to think of the CGits the nail that your real rocket will turn around in flight.
Now, think about if you put fins on the back end of your stick (rocket). Though its not completely true, say that the CP is exactly where you stuck the fins on your rocket (its true enough for this example). If that stick was flying through the air front end first, and wind pushes on the rocket, the fins will push on the stick. The stick will rotate around the nail/CG. Since the fins are on the back, the rocket straightens back out and continues nose forward. Now remove your imaginary fins from the back of the rocket and put them only on the front near the nose cone end of your rocket. OK, now its flying again, and wind pushes on the fins; push on the fins of your imaginary rocket. What happens? The nose of the rocket turns around the nail/CG, and the front end flips over and turns into the rear end. This is bad! Of course, you want the front end of the rocket to stay the front! Thats the general reason why the CG must be ahead of the CP keeping the CP behind the CG makes sure the front of the rocket stays the front!
Many manufacturers specify the CP location on their kits. CP doesn’t vary in kit rockets, it’s controlled by the design of the kit. CP is affected by such things as the rockets body diameter, length of body, fin size, number of fins, and fin placement on the body tube. CP cant be changed in kits. However, you CAN vary CG, by adding more nose weight. That’s why the kit manufacturers specify CP, so the owner of the rocket can find their CG and compare it against the CP to see if they need to add some nose weight (to move the CG ahead of the CP) for stability.
The CG will be dependent upon how you build and use the rocket, which is why manufacturers usually don’t specify CG. Some people really use a bunch of epoxy, others don’t. It’s variable enough on how the person builds the rocket, but even more so as what type of payload you have (if any). You can imagine that given the same kit design, if someone were carrying some complex and heavy video electronics that the CG would be dramatically different from someone who flies no payload at all.
Once a kit is built and ready to fly (with payload), CG location is most dependent on the motors used. Of course a G motor is much lighter than an I motor, but a certain kit may be able to use Gs, Hs, and Is. The motor is obviously in the back of the rocket, and weight in the back of the rocket shifts the CG rearward, so the bigger (heavier) the motor, the worse the CG/CP relationship will be. Remember, CP doesnt changeany weight added to the rear of the rocket will move the CG back (bad), and weight added to the front of the rocket will move the CG forward (good).
Mark the CP on the rocket, then mark a point at least one body diameter ahead of the CP. Make sure you prep the rocket as it’ll be in flight (with the motor you intend to use installed, chute(s) installed, payload installed, etc.) before you do the CG/CP check. No need to add black powder to an altimeter or the delay area of a reload casing, or to put in igniters; theyre not heavy enough to matter. Now balance the rocket “teeter-totter style” on a piece of dowel, back of a chair, something like that. If the rocket balances level with the pivot point of the teeter-totter arrangement at or forward of the CG point you marked, youre good to go! If not, youll have to add nose weight until it does.
Up to two body diameters is usually even better, but dont go much over two diameters or the rocket will be overstable. By the way, this is whats meant by one-caliber or two-caliber stability. It comes from wartime artillery terminology, where the diameter of a gun is called the caliber, so one-caliber = one diameter, and so on. Don’t forget that if you use, say, a small H motor and set the CG, that if you then decide to use a big J motor, the J weighs more and you may have to add nose weight again to compensate. Thats why its usually recommended to set the CG/CP relationship with the largest motor you intend to use in the rocket.
If you dont have the biggest motor you intend to fly on hand, or if you havent reached a certification level where you can buy one, check out the weight of the propellant you intend to use and the reload casing from the manufacturer. Many times that information is available on the manufacturers web site. Simulate the weight of the casing and propellant in the motor mount tube. Use one or more rolls of coins, a baggie with dirt in it, whatever you can come up with that’s similar to the weight of the motor and casing.
Submitted by: Andrew Waddell