cg/cp ?? from new guy..

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Jun 24, 2012
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yes. I am curently working on the comenche-3 and would like to check it for a very stable flite, when i have it completed. how do i check the ,cp/cg. and not just for the comenche,but for scratch building to. when do you have to add weight to the nose or tail. thanks for any help.
There's a bunch of ways to get that data. Here's what some folks do:

Buy Rocsim. It's a program that does everything but walk out and retrieve the rocket. Pricey, but really cool. SpaceCad is another one. Not as pricey.

Download a free copy of WinRoc. I have not ever used it, but they say it does stability calculations.

Order the Barrowman Method packet from the NAR and the Handbook of Model Rocketry by G. Harry Stine (you will want a copy of it anyway), and work out the center of pressure by hand. I did this once, it's tricky, but by all means doable if you can deal with a little algebra and have a calculator that does square roots. The NAR website has lots of packets of how to stuff for rockets. I think they may sell The Handbook there too, which will tell you what you need to know about adding weight to adjust the center of gravity. You gotta buy The Handbook.

And finnally, hit up your friends and people on the Forum that have the aforementioned software! I have done this, and it works until you have annoyed someone so badly with your project they don't answer emails anymore!:D

I hope this stuff helps. If you need help tracking down any books or info, send me an email.
I forgot to mention this. If you are looking for the CP of an existing rocket kit, check out Essenence's Model Rocketry Reviews. At Really cool site. It has a CP library I think.
thanks justin, I will check out nar, and get hand book, i am back into rocketrey with a lay off time since the mid 80's, yes it's bin along time!! back then we dident worrey adout cp/cg.. and all the auther abbrevations out there, all we wanted to do was get are rockets back, and if we dident or it came apart,well we would rebuild. it was alot of fun then, and i look forward to it being fun again. i now want to learn all i can.. but above all have fun.. thanks..
do your self a favor.. unless you are planning on modding it, ie making it fly with motors bigger than recommended, glassing fins/body, etc, don't worry about cp/cg on estes kits. they give you a recommended motors list for a reason, and i have rarely seen an estes kit not perform well (their V2 and Saturn V are the only two i know of) based on those recommendations.

just my thoughts. not trying to rain on your parade, just trying to help prevent a potential headache. :)

ya chris, i am glad you mentioned that, being that the comanche is a kit it sould be stable " with recomended engins " so i plan to use it to see how good my math is, and yes i do plan on building my own creations, fiderglass,bigger moters, so i need to know what i can do and what i souldent do, to make as least misstakes as posible, thanks, any info will be great.
If you have a few minutes, google "Barrowman" and you will come up with several websites where you can get the equations free. By all means, do go ahead and hunt down a copy of Stine's Handbook of Model Rocketry, but you can get started right now at calculating your center of pressure.
Besides the Barrowman equations, you can also use the cardboard cutout method. Draw the profile of your rocket (with fins) and cut it out. You can use paper but it's kind of saggy/droopy, most guys use cardboard because of the next step. Balance the cut-out across a straightedge and you will have located the longitudinal c.p. (Note that this method will not give you exactly the same c.p. as the Barrowman eqns.)
To find the c.g. of your rocket, balance it on a 'knife' edge in flight condition (make sure you include wadding, and the MOTOR). You don't have to actually use a knife, that is just an expression----you can use the edge of a sheet of balsa or anything else fairly thin without actually having a sharp or cutting edge to damage the rocket.
Come on back with any more questions, someone here will help you!
A update. I just received my hand book by g. harry stine, and i must say it is worth every penny. this book has it all. also i downloaded winrock for cg/cp measures. but i need to ask a dume ?? what is say 14.77 or 11.68, i am asuming 14 is inches whats 77 ?? 11 is inches so what would 68 be, is it mm, cm,?? how do you come you with this figure?? THANKS
Im guessing it means exactly what it says - IE 14.77 means 14.77 means 14 and 77/100.
I too have Winroc (cause I'm to cheap to buy Rocsim) and it has done a decent job of calculating CP for me. The number that I think you are asking about is the distance from the nose end of the rocket that marks the spot of CP. Ex. 11.77 means the CP is 11.77 inches from the front of the rocket. Just measure down approx. 11 3/4" from the top of the nose cone. That's the spot.

Since I'm suprised no one else has said it yet, WE NEED PICTURES! Everyone loves to see pics on this forum of whatever your doing.

Good luck,

Another CP calculator is VCP:
It's free and easy to use.

The CP should be aft of the CG; the distance between them should be about 1-2 times the body diameter. (Less for short, fat rockets like the Fat Boy; more for long rockets like the Mean Machine.) If the CP is not far enough back, you can either move the CP by adding bigger fins or move the CG by adding nose weight. Adding bigger fins is easier if you're designing the rocket on paper or computer before actually sticking pieces together. :) Adding nose weight is easier if you're correcting a rocket which has already been built, or if you're building a scale model and don't want to spoil its appearance with oversize fins.

Generally the only time you want to add tail weight is by fitting a bigger motor. :D
ok based on what texasck1 says, if you have a no# 14.77 inches. just measure down 14 3/4 of an inch. based on that .25 would be 1/4 inch, .50 1/2 inch .75 3/4 inch. right?? ok at first i figured that would be fine,but when a no# comes up like 12.21 or 13.52, what do you do round it off to the nearest no#?? that dose not seam at all very percise, and what started this hole thing was ,i whent to one of the cp, libareys, and i figured i would see if could work the math out to find the cp of any rocket. first problem was that every rocket listed had at least 2 diferent measurements for any given rocket, and even from the same programe, rocsim, winroc, and outhers. how could this be?? I thout to my self ., so i grabed one of my rockets.opened winroc. and started to measure. to see if i could come close to any body elses answer for that particular rocket. I used estes, chrom dome. well when i got done, there was a inch diference. between mine and there's. now are you following so far?? if you are rounding off this numbers say, .22 is .25 witch would be 1/4 of an inch. by the time your done you could be 1 to 2 inches longer or shorter then you should be. right or wrong?? and every body wrights there fig. like that. it has to be more percise then that. thanks
just one more thing to add, should i convert inches to mm, or cm to be more acturate, insted of rounding off numders?? thanks.
Well a few hundredths of an inch really is negligible. You should only be rounded the final answer so the errors shouldn't stack. If it really bothers you though you can pick up some machinists scales, calipers etc. They will measure to hundredths and thousands of inches. They are also kinda pricey. Or you could just go to metric. Much easier to work with and metric rulers are easy to find too.
Conversion from one measurement system to another does not really affect the level of accuracy that you maintain through your calcs.

What you need to do is: note how accurate your 'worst' input data is---this will limit the accuracy of your output answers.

You are perfectly free to carry your calculations to a hundred decimal places, but you need to realize that the only truly accurate part of your number might be the first three or four digits.
In the rockets that I have built, I have always rounded off to whatever measurement would move the CP toward the nose. This is the safest way to round the CP, the worst that could happen is your slightly overstable. If you round toward the tail your could end up without the CG far enough forward. It's easier to remove some clay if it's overstable than rebuilt a wreck if it's under! Metric might be the way to go if your concerned with being too far off.

I have a machinist scale from a "previous life" when I worked as a machinist and it has a conversion chart on the back from decimal to fraction. I use it for just about everything. It's small and easy to use including as a guide for cutting with an Exacto. You can pick one up at a tool shop or machinist supply store for about $2.

Again if you err towards the nose you will be just fine.

Good Luck,

I agree with someone who posted earlier, that VCP is a good free program to use for CP calculations. There's also the free demo version of RockSim, but it doesn't let you save anything and can't read files produced by the current full blown version of RockSim.

William, in your last post, I'm not sure if you're talking about the location of your rocket's center of gravity (CG) or center of pressure (CP). The CG is wherever you measure it with the rocket ready to launch with motor, wadding, parachute, etc., and you can adjust the CG by adding weight (often in the form of clay) to the inside of your rocket's nose cone. The CP is hopefully where one of the programs says it is. There might be more than one location shown because RockSim can use 2 or 3 different methods to do the calculation. Either the Barrowman or RockSim method is going to give you a number that's probably quite close to the actual location.

If you're using a regular 12" ruler to measure, each 1/16th of an inch is equal to 0.0625 inches. So if you have a CP that's supposedly at 14.77 inches, then it's at 14 whole inches and 0.77 / 0.0625 = roughly 12/16ths of an inch.

You don't have to actually measure the location of the CP on your rocket unless you want to, all that really matters is that your measured CG is at least one body diameter in front of the calculated CP.

Does this make sense or help at all?