View Full Version : find CP of odd rockets
GiachiG aka Typico
7th June 2009, 09:10 PM
hi, i can't find anything on the net about this argoment...
i remember an apogee newsletter that talked about it, but i can't find the newsletter anymore
the shape of the object i have to make stable is similar to a drop, i remember that round(spherical) objects are stable because of the drag generated by motion, but in my case??
i need this information for my next project, an upscale of a strange & cool object(can't say more)
thanks for any answer
7th June 2009, 10:52 PM
You could try the cardboard cut-out method. Trace the shape onto cardboard, cut out and find the balance point of the shape. I don't how reliable for your design, but might give you a good starting point.
7th June 2009, 11:00 PM
If you have a giant tear-drop shape you are still going to need enough fin area in the back to move the CP aft, or enough ballast in the nose to move the CG forward, or both. If your design has unconventional shapes, or even “normal” shapes but oriented in unusual ways, you are not going to find a cook-book method (like Barrowman) that applies. If you have portions of the design (especially fins) immersed in turbulent flow that is produced by fore-bodies (or other fins) that is also going to complicate things.
Estes produced several kits with Star-Wars themes that were made stable by attaching the desired object/shape to a long body tube. This serves as a boom or arm to help hold nose ballast at a greater distance out the front of the rocket, making it possible to use less ballast weight. You could possibly do something similar, whether using a body tube or using a rod (1/4 inch wooden dowel rod?) to hold a small nose pod with ballast, hidden a bit (or at least, made less visually insulting) by painting it black.
A safe bet is always to use tons of nose ballast but that has obvious performance penalties. Some people use oversize tail fins and (sort of) hide them by using clear plastic sheet. Other people use a base plate as a drag-producing device (which can also be hidden slightly by making them from clear plastic sheet.
Before you invest a lot of time and work in a shape that cannot be made stable, you might do a “boilerplate” version with a rough representation of the size, shape, and weight. See if that version flies OK before you put a lot of energy into building a deluxe version.
I think you will find that CFD tools will not work reliably on unconventional shapes either (at least, for most of the “simplified” CFD methods available for use with rocketry). Part of the premise for being able to use computer procedures to model airflow is that the airflow is assumed to be generally smooth and “well behaved” and that the surface of your rocket is smooth and has gentle changes in contour. It is fairly easy to overwhelm the CFD internal mathematics if you are trying to analyze too many features, or odd features, or zones of stalled/separated/severely turbulent airflow near your rocket. Confidence in analysis results degrades accordingly.
You could build a scale model and test it in a wind tunnel. That is going to be way more trouble than it’s worth, plus afterward you will still be faced with the problem of scaling the results back up to full scale (unless you test at full size and full q).
Short answer: there isn’t an easy way to determine this. Trial and error works as well as anything else. A boilerplate test vehicle, launched away from crowds, is likely to be your easiest approach.
GiachiG aka Typico
8th June 2009, 06:38 PM
thanks both for answering so fast
cutout can't work for this project ;)
so the boilerplate is the only way i can actually use to do tests
i will make a downscale copy of it to test it in general, something around 50cm, but i can't be sure it would work on a 3mt tall version
the boilerplate is also a solution.. but it gonna cost alot, maybe a "bigger than first try copy" can be the way
the thing i can't do is to put a rod or tube with ballast weight, the object i choose to upscale is a real piece of art, and i can't ruin the visual by adding stuff, that's unacceptable ;)
and also, i don't want to put fins on it... i will just make it stable by adding weight.
i tryed to ask for a tule of thumb for this problem to start drawing the structure with a 3d modeling software
obviously, without any idea of what i could need to make it fly straight, i can't draw a good plan.
this time i have no problems of time... this project is for 2011, next year i will take a pause to make my wallet breathe a little :D:D
8th June 2009, 08:51 PM
From everything I'm given to understand, CP is the same no matter how large or small the rocket is. Thus, if you build an Alpha that is EXACTLY twice the size of the original, the CP will be in exactly the same place, proportionally, on the larger version as the smaller. Thus, if you build a small scale version of your design and can make it stable, just make sure the larger version has it's CG in the same place as the smaller.
8th June 2009, 09:21 PM
CP = 14/19*(length of the rolling pin) + 7/12*(# of ping-pong balls) + 1/(total surface area of plastic parts)*(angle of the most canted motor).
9th June 2009, 12:28 AM
I don't know if this will be of any help to you, but Art Applewhite did some wind tunnel testing of 24mm and 38mm versions of his Delta Saucer with various motors, and he presents diagrams of the results on his website here (http://www.artapplewhite.com/cp.html). The interesting thing is that the CP is behind the model in each case.
GiachiG aka Typico
9th June 2009, 12:07 PM
but Art Applewhite did some wind tunnel testing.... The interesting thing is that the CP is behind the model in each case.
that's what i'm talking about...
in some cases, CP is behind the model
i remember that in those cases, when drawing it, you have to add a cone in the back of the model to find aproximately where CP is...
the dimensions of this cone are in proportion to the diameter and lenght of the model(something like that)
it's the same case of drag plates...
where is the CP on a spool rocket??? does anyone have a Rsim file of one of it??
i can take interesting data from it ;) ;)
what is that for?? ping-pong balls...
can you explain me better your equation??
yes, CP remain in the same position, but if you don't know where is it, when building a biger version you cannot change the position of CG.. and also, in the case you put a bigger motor in a marginally stable rocket, you can't be sure of how ballast you might add to keep it stable or even if you need to add nose weight
and to end, in bigger version forces are higher, and things that don't make difference in a small version(drag generated by fin thikness for example) can kill big boys
thanks for the help, bring new data if you have some!!! i will write an article when i have enough data
9th June 2009, 02:31 PM
I do it the poor mans way: The CP is never going to be further than your biggest motor. I put the casing of the largest motor I intend to use and then figure from there at least 1 body tube diameter ahead of that will the CG.
For example: An Aerotech 38/480 casing is 9 1/2" long. If I'm putting this in a rocket that is 4" in dia, my CP will be 9 1/2" from the rear, and the CG will be a minimum of 4" beyond that.
I use my finger as a balancing point, and then add whatever nose weight is necessary. Now for your "mystery" design, this may or may not work. But the principle is the same. This method does not work on short stubby rockets, i.e. if your planning on making a Big Daddy take a 54mm motor.
9th June 2009, 04:53 PM
If your talking about the upscaled bottle that your building. when I did mine in Rocsim I found out that Rocsim has problems with a flat tipped nose cone, it put my CP below the bottom of the rocket. I redid it as a very short round nose cone & it helped. I also did it as a cardboard cutout & did it with math. All three had the CP with in about 1/2" of each other.
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