Center of Pressure

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90+% of people use a computer simulator of some kind. (RockSim, OpenRocket, RasAero, VCP, etc.) There are also the older, manual ways to do it described in several books and the internet at large. Those methods hold true regardless of physical scale (low power vs high power) but most of those also don't account for very high speed (trans-sonic or higher) To do it manually, look up and study the Barrowman equations. There are several references on the internet for them. If you're planning on high speed, it is highly recommended that you use one of the computer simulation programs. Some of them are free as well.
 
Before learning open rocket, I used to trace the silhouette on cardboard and cut it out, then find the center of gravity on that piece, and mark that as center of pressure.
 
crossfire said:
How does finding CG tell you where CP is ?
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since the CP is based on the shape of the rocket, it works out that if you find the CG of the 2D shape of the rocket, it hits pretty near the CP. But it is not a true measurement.
 
Wow...didn't expect such a great response. Thanks to everyone who pitched in with an idea. Have had some issues with RockSim, but I'll give the other suggestions a shot. Liked the cardboard cut-out idea; however, one of my builds is 14 feet tall and the other is 12 feet
 
Yo, Bat-mite...can you elaborate a little on your reply? Seems as though this might just work. . .and I sincerely appreciate your reply.
 
SHH said:
Yo, Bat-mite...can you elaborate a little on your reply? Seems as though this might just work. . .and I sincerely appreciate your reply.
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Cutout-based approach is an approximation at best. You're much better off using a sim program. Either Rocksim or OpenRocket will do the job just fine.
 
SHH said:
Yo, Bat-mite...can you elaborate a little on your reply? Seems as though this might just work. . .and I sincerely appreciate your reply.
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I was just elaborating on the cardboard cutout method. It should be noted that for most standard shaped 3FNC rockets, the CP is just forward of the tops of the fins.
 
SHH said:
Wow...didn't expect such a great response. Thanks to everyone who pitched in with an idea. Have had some issues with RockSim, but I'll give the other suggestions a shot. Liked the cardboard cut-out idea; however, one of my builds is 14 feet tall and the other is 12 feet
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CP scales proportionally.
 
neil_w said:
Center of gravity of the silhouette cutout yields a crude approximation of CP. It was more commonly used before programs such as OR and Rocksim were available.
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FWIW the cutout method gives what's called the lateral center of pressure. That is, where on the rocket the force of air would be equal if the rocket was flying at right angles to the vertical...which it usually doesn't...though I've seen otherwise.:(

Since rockets don't usually fly that way, the cutout method is rather conservative. That is, if the cutout method gives a caliber of stability, the rocket is probably good. OTOH Barrowman calculations or Rocsim's method are a bit closer to reality. They both assume that the rocket is flying more-or-less straight up. When it tilts even a bit from the vertical, the fins now stick out from the vertical, and they provide a significant restoring force to make Mr. Rocket fly good and true. :)

I don't know how Barrowman/Rocksim's calculations are done, those are out of my league. I just have a very basic understanding of what they do.,

Best -- Terry
 
The problem with the cardboard cutout method is it vastly overestimates the effect of the body tube on CP. At low angles of attack the effect of the body tube on CP is negligible, while it has a big impact on the cutout. Result is its estimate is 'way forward of the actual CP. Any of the other methods suggested in this thread are vastly preferable.
 
rklapp said:
That's a lovely dotmatrix report.
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Thank you, but you should see the lovely plastic binder that my teammate added. That may have been my 24 pin printer, but it was certainly done with a word processor on my C64 computer. Lovely for '88. Still, it is a little disgusting that what could have been a half page technical note in a newsletter, gets ballooned into 13 pages with all the requirements of an NAR R&D report. And totally aside, I'd like a 9 pin dot matrix font for modern systems.

This is essentially and extension to the Barroman Equations to ease the rigid body limitation. I'd be more interested in questions about the content or use, than the format of the report. Also, if you are only interested in computed results, instead of a basic understanding, you could give flexroc a try.
 
rklapp said:
My snarky remark was in response to you not answering my question. Good day to you.
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Sorry, I may have picked the wrong message to reply to. I don't use RS. However, I am familiar with stability, CP, Barrowman Equations, and rocket dynamics, including long rockets. I was simply posting a relevant report that some readers may not have seen.

Happy Trails
 
SHH said:
Wow...didn't expect such a great response. Thanks to everyone who pitched in with an idea. Have had some issues with RockSim, but I'll give the other suggestions a shot. Liked the cardboard cut-out idea; however, one of my builds is 14 feet tall and the other is 12 feet
Click to expand...
as stated above the card board cut out method can be scaled. Make a cut out of the same shape in inches instead of feet, might not be as accurate, for example.

The balance point of the card board cut out is essentially the centroid of the area, where if you have an assumed constant pressure, like the aerodynamic force averaged out when at 90-deg angle of attack, that is where the net aerodynamic force will act. If the actual CG is in front of that estimated CP, then you ensure the rocket would weathervane in the correct direction into the wind, or nose first into the direction of flight. The actual CP location varies with the angle of attack, and you usually start with a near zero in a no-wind take off. The 90-deg CP location will probably be more forwards or closer to the CG, and thus more conservative, as compared to the 0-deg launch configuration, which the Barrowman Equations (there might be a spreadsheet version -UPDATE: see post #8 above- of those if you can not run RockSim or OpenRocket) is based on.

What is the danger of being too conservative? You might be led to believe you should add too much nose weight to acheive a higher stability margin than is actually required, which can slow down your speed at the end of the rod, which is another consideration in launch stability. If you are not fast enough at the end of the rod then the fins are not aerodynamically effective and you may have a drastic change in orientation when the rocket leaves the pad under thrust. You can use another rule of thumb for that as to peak thrust during the initial fraction of a second while you are on the rod, from a thrust curve plot, and the total weight of the rocket, so you have a certain ratio of Thrust to Weight, in terms of G's to make sure you have sufficient acceleration on the rod, or you might have to use a longer rod or rails, etc.
 
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GlenP said:
as stated above the card board cut out method can be scaled. Make a cut out of the same shape in inches instead of feet, might not be as accurate, for example.

The balance point of the card board cut out is essentially the centroid of the area, where if you have an assumed constant pressure, like the aerodynamic force averaged out when at 90-deg angle of attack, that is where the net aerodynamic force will act. If the actual CG is in front of that estimated CP, then you ensure the rocket would weathervane in the correct direction into the wind, or nose first into the direction of flight. The actual CP location varies with the angle of attack, and you usually start with a near zero in a no-wind take off. The 90-deg CP location will probably be more forwards or closer to the CG, and thus more conservative, as compared to the 0-deg launch configuration, which the Barrowman Equations (there might be a spreadsheet version of those if you can not run RockSim or OpenRocket) is based on.

What is the danger of being too conservative? You might be led to believe you should add too much nose weight to acheive a higher stability margin than is actually required, which can slow down your speed at the end of the rod, which is another consideration in launch stability. If you are not fast enough at the end of the rod then the fins are not aerodynamically effective and you may have a drastic change in orientation when the rocket leaves the pad under thrust. You can use another rule of thumb for that as to peak thrust during the initial fraction of a second while you are on the rod, from a thrust curve plot, and the total weight of the rocket, so you have a certain ratio of Thrust to Weight, in terms of G's to make sure you have sufficient acceleration on the rod, or you might have to use a longer rod or rails, etc.
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That’s what happened to my TVM when it turned into a cruise missile. The added NC weight fixed the instability but not the orientation.

 
samb said:
Here’s a spreadsheet tool that implements the Barrowman calcs:
https://www.rocketmime.com/rockets/Barrowman.htmlRead all about it:https://www.nar.org/wp-content/uploads/2016/01/barrowman_cp_extended_edition.pdfMr Estes wrote about cardboard cut-outs before Barrowman:https://www.ninfinger.org/rockets/EstesTR1.pdf
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Thanks for posting those, that PDF includes some great pictures and example problems. I noticed some of his aerodynamic text book references from the 1950's are still used in some college courses today (Shapiro), well they were still used back in the 1980s at least.

This is one of my favorite pictures, well I might have modified it a little...but you get the idea.

Screen Shot 2020-10-01 at 6.09.27 PM.png
 
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JoePfeiffer said:
The problem with the cardboard cutout method is it vastly overestimates the effect of the body tube on CP. At low angles of attack the effect of the body tube on CP is negligible, while it has a big impact on the cutout. Result is its estimate is 'way forward of the actual CP. Any of the other methods suggested in this thread are vastly preferable.
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Hmmm....Not defending the cutout method, but this statement doesn't really make sense. On a cutout, the 'body tube' is rather neutral. Even if the body tube was pencil thin, it still has to just balance the point between no aerodynamic pressure ( say at that end of the nose) and the most aerodynamic pressures (at the fins). I see no reason for it to vastly overestimate or be way over the actual cp. In a perfect world sim on a simple, basic rocket shape, it should be very, very close.
 

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