Center of Pressure

I was looking at several things, and I was just wondering what Center of Pressure really means? Is this an area where most likely a body tube could fold under too much acceleration? Or the body tube collapse from external pressure, or explode from internal pressure? I'm using RockSim, that info will probably help.
I was thinking that with an Engine most likely used in a perticular rocket, it would be good to have the top Centering ring for the engine mount located at this point. Dealing with LPR here, D/E's of the single use black power type.
I have never incountered any problems, but from a design stand point I am interested in what this CP really means to the rocket and what needs to be done to over come any problems that may accur from it's location in relavence to the structural integrity of the body tube.

The center of pressure is the point where all of the aerodynamic forces meet.

From microgravity.grc.nasa.gov

"As a model rocket flies through the air, aerodynamic forces act on all parts of the rocket. In the same way that the weight of all the rocket components acts through the center of gravity cg, the aerodynamic forces act through a single point called the center of pressure cp. How do you determine the location of the center of pressure?"

RockSim puts a marker at the CP point, I'm assuming it is close as can be given that I've choosen components in the data base.
So does that mean there is external pressure bearing down on the body tube at this point from all angles? Seeing 3 large cranes buckle in my construction career, I'd have to say if that is true, then that is NOT where you would want a centering ring. Due to the fact that reinforcement makes it a stress point and you would need it flex, not bare strees at a point it should be able to flex.

think of cp as cg, but to air pushing past the rocket, instead of gravity pulling on it. Cp is the point where the effect of the fins, and the drag of the components above the Cp "zero" out. Not where the maximum pressure is on the air frame.

i dont believe there is extra stress or forces at the cp - it helped me to look at the cardboard cutout method of figuring the cp and then comparing that to what the cg really is. Have you read about the cardboard cutout method? if not you make a cut out of the profile of your rocket on card board and then balance that on a straightedge - where that balances is the cp - think about wind blowing against the side of your rocket - the fins obviously would be affected the most and the body some - now if you were to figure out a point where all these forces balance at that is the cp - kinda like cg but instead of weights it is aerodynamic forces. your rocket wants to turn with the cg as the point around which it turns - the cp is where the averaged out force is applied but I dont think there are extra forces there usually so it doesnt need to be reinforced internally. Ideally you want your internal structures as far forward as possible to keep cg forward of cp

I hope this makes sense to you - I know everybody thinks in different ways and it took me a day or 3 to make sense of cp

I think of it as the point at which the aerodynamic forces above and below are equal. The cardboard cutout method is approximate as it is basically showing the point at which the area ahead and behind the CP are equal. However the rounded body tubes behave somewhat differently than a flat plate.

The CG is the point where the gravitational forces above and below are equal.

By having the CG ahead of the CP by some margin means as the rocket tries to move away from vertical there will be a restoring force due to the mass that will try to keep the tail end behind the nose.

If the CG and CP were at the same point the rocket would basically want to tumble, same as an airplane would tumble if the CG were too far back.

Frank

Ok, now I understand what is exactly the CP is. Now, in RockSim, the CP needs to be above the CG! If the margin is less than 1.0 you get a failed launch. PC needs to be above the CG, or as stated, the rocket is unstable.
So now that I understand what the heck CP is, I see that is in no way to effect design besides what I already know. But, I thank you all for letting me understand what that point is actually for. So in the light of things, it should have no bearing on where I put a centering ring other than to keep the rocket stable, which is one of, if not thee most important thing.
I just didn't know what Pressure ment as a design feature other than making the rocket stable. Now I do, and I than you Kindly!
OFR (Old Fart Rocketry) as my 8 year old son has boldly named me.... Poot

The center of pressure is the place on the rocket where all of the aerodynamic forces that are acting on it during flight balance out. The mass of all of the rocket's components all balance out at the center of gravity (center of mass). Similarly, as it is flying through the air, the various effects of air moving around and across the rocket are collectively averaged at a point called the center of pressure.

https://www.grc.nasa.gov/WWW/k-12/airplane/cp.html
https://microgravity.grc.nasa.gov/education/rocket/rktcp.html

Woody's Workshop said:

Ok, now I understand what is exactly the CP is. Now, in RockSim, the CP needs to be above the CG! If the margin is less than 1.0 you get a failed launch. PC needs to be above the CG, or as stated, the rocket is unstable.

Click to expand...

It's actually the other way around. The CG should be ahead of the CP for stable flight.

Woody's Workshop said:

Ok, now I understand what is exactly the CP is. Now, in RockSim, the CP needs to be above the CG! If the margin is less than 1.0 you get a failed launch. PC needs to be above the CG, or as stated, the rocket is unstable.

Click to expand...

Nope -- it's the other way around. The center of gravity is the balance point around which the rocket will rotate. If aerodynamics forces are able to come together at a point forward of this spot, they will push on the front end of the rocket (the nose) and cause it to rotate around the CG. Cartwheeling flights are less than optimal. Since this usually happens while the rocket is under thrust, it will cause the flight to be unstable and unpredictable. If these same forces come together at a point aft of the CG, the rocket will tend to keep its nose pointing in the direction of travel and fly in a straight, predictable trajectory. This is good. If the CP and CG are real close together or at the same location, then theoretically under ideal conditions the rocket will fly straight, but in reality it will take very little push to make it go unstable. If the CP is well aft of the CG (at least 1 x the airframe diameter aft is the usual rule of thumb), then the fins will be able to counteract nearly any destabilizing forces and keep the rocket going straight.

Woody's Workshop said:

I was looking at several things, and I was just wondering what Center of Pressure really means? Is this an area where most likely a body tube could fold under too much acceleration? Or the body tube collapse from external pressure, or explode from internal pressure? I'm using RockSim, that info will probably help.
I was thinking that with an Engine most likely used in a perticular rocket, it would be good to have the top Centering ring for the engine mount located at this point. Dealing with LPR here, D/E's of the single use black power type.
I have never incountered any problems, but from a design stand point I am interested in what this CP really means to the rocket and what needs to be done to over come any problems that may accur from it's location in relavence to the structural integrity of the body tube.

Click to expand...

Center of Pressure is a focal point through which all the aerodynamic forces upon the rocket theoretically center around and balance. Just like how the Center of Gravity is the point at which all the weight and their arrangment inside the rocket all center around and balance. Finding CG is easy-- tie a loop of string and put it around the rocket, hang it horizontally, and move the loop forward a bit or back a bit until it balances horizontally. Finding CP is a LOT harder, because CP is a "moving target" and depends greatly on shape, angle of attack, airspeed, etc. (Well, CG actually moves around too a bit, as propellant is expended in the motor CG shifts forward because the rear of the rocket is getting lighter, opposite of "real" rockets using liquid propellants where the CG actually shifts aft as the level of propellants in the tanks gets lower and lower).

If a rocket is mounted on "pivots" at the CP, and exposed to an airstream, it should not turn or otherwise respond to the airflow, because the airstream's effects to the parts of the rocket in front of the pivot are exactly balanced by those behind the pivot point... THEORETICALLY. As I said, CP is a moving target, and depends greatly on shape, speed, and angle of attack.

Shape because different shapes create different aero effects-- some shapes generate lift, and sometimes even shapes that don't 'ordinarily' generate lift (like a cylinder) can if the airflow or motions are just right (which is where it gets REALLY complicated. Of course fin shape is the first thing we think of with 'shape' and different shapes and profiles have different effects, but the biggest factor relating to fins is their position on the rocket and their size in their effects upon CP. Putting fins further back moves the CP back, moving them forward moves CP forward. Making fins bigger moves CP back, making them smaller moves CP forward.

Speed plays a big factor is exactly where the CP is located. At low speed, fins create less 'corrective force' than they do at high speeds-- in fact at zero their corrective force is zero, which is why we use launch rods. Generally speaking, about 30 mph is the "rule of thumb" where adequate corrective forces are generated by the fins to stabilize the rocket. (This varies with launch conditions, wind, motor choice, rocket weight, fin size, number, and shape, etc.) As speed increases, the CP tends to shift further back. As speed decreases, CP tends to move forward. I say "tends" because other aero effects can come into play, like turbulence and mach effects which can really move the CP around in unexpected ways, as well as excessive angle of attack leading to fin 'stalling'.

Angle of attack is also quite important. Here's an example... say your launching a rocket in a 15 mph wind. Sitting on the pad, the wind is actually pushing against the fins from directly to one side-- IE the fins are at a 90 degree angle of attack to the airflow (wind). As the rocket takes off, when it hits precisely the same speed as the wind speed, the angle of attack is 45 degrees, because the wind is still moving past the rocket 90 degrees to it's travel path, but the rocket is moving forward at the same velocity, so the APPARENT "wind direction" that the fins experience is 45 degrees. As the rocket picks up speed, this "apparent wind direction" becomes less and less of an angle of attack from the direction of flight. Why is this important?? Well, it explains a lot about how rockets behave on windy days for one-- heavy rockets that lift off more slowly, or rockets with smaller motors, are more effected by wind for this reason. Rockets with bigger fins and/or higher stability margins respond the same way, unless they're boosted on powerful motors that really rip them off the pad, or if they're very lightweight.

What does this tell us about the rocket's stability margin?? Well, at zero angle of attack, the CP should be as far aft as it can go for a given rocket's size, shape, and speed. As the angle of attack increases above zero, the CP begins to move forward. The CP will continue to move forward as far as it possibly can for a given rocket size/shape/speed, when the rocket is exactly 90 degrees to the oncoming airflow. Essentially the rocket would be "flying sideways" (which of course they don't want to do). Why is this important?? Well, it establishes a handy method to determine the "worst case scenario" CP for one... If you drew an exact copy of the rocket's profile onto a sheet of cardboard and cut it out, and balanced it on the edge of a ruler, you could easily see where this "worst case scenario" CP is located. This is called the "cardboard cutout method". Notice, the CP is NOT actually at this location-- this is the WORST CASE, or as far forward as the CP is practically capable of being... in flight the CP will ACTUALLY be at SOME POINT BEHIND this "worst case" CP point... Why is this helpful?? Well, if you know the worst case cardboard cutout CP, and balance the rocket so it's CG is about a body diameter IN FRONT of this point, then you can be 99% sure that the rocket will be stable in flight, since the "REAL" CP will be behind this point. This is how most rockets stability was determined before Rocksim, the Barrowman equations, and other math-intensive ways to locate CP. Rocksim STILL has the option of using the "cardboard cutout method" to determine the CP point on the drop down menu... switch between methods and see what I mean on a rocksim design sometime.

The cardboard cutout method is actually "the center of lateral area" and not the CP, but it serves the same purpose, because the center of lateral area and the forward-shifted CP at a 90 degree angle of attack worst case scenario is at the same point. This is what the rocket "looks like" to the wind... To ACTUALLY calculate CP, requires mathematics, and quite a bit of it. The Barrowman equations simplify it (through the use of certain "assumptions" to simplify the math) enough for practical use by the masses (unless your math-handicapped like me LOL) Rocksim and other simulation programs has made it very easy, and use "more advanced" methods and assumptions to calculate CP for various shapes (the Barrowman equations were very limited in fin size, type, shape, body tube shape, arrangement of the fins to the body, etc.-- "Weird" shapes like the space shuttle or the average Shrox rocket (like the Quest Stilletto) simply wouldn't work with the Barrowman equations, or rather the equations wouldn't work with those rockets). Of course the "assumptions" used in the equations and their estimation of actual aero-effects is also actually an ESTIMATION of the actual CP location, and, of course the CP location changes every moment of the flight as speed, angle of attack, and the surrounding airflow changes.

Hope this helps you 'get your head around' CP... it's really a fascinating concept...

Later! OL JR

Man...did you fill my head with stuff I DIDN"T NEED to know to design rockets.
No lifes are on the line here, but I understand what you are saying because of the way you described it. I never got past simple math, which I'm glad becasue I would not be here if I did. I'd be too busy figuring out equasions with attintion to detail as I am.
But roll it up and throw it at me a ball, I get the concept as you have done.
It all comes down to my original question. Do I need to reinforce the rocket design at the CP point? NO. For one is changes. For two it was not as I had imagined so there is no need. Three is that you need to make sure your design with withstand what you want to put in it as per motor, slow, fast, or whatever.
Before I get a headache from reading too much, I think I will end this as "Well explained in laymans terms as best it can be". And I thank you all very kindly for your imput.
I think I will give it a rest and go build something simple in the woodshop!

Well, it's not rocket science...








:duck:

It's not rocket science?
I guess it's gardening!

The analogy of a weather vane might be helpful.

Imagine the CG is the axis on which the vane rotates. The CP needs to be behind the axis so that the pointer faces the wind. The wind has more effect on the area behind the axis, so it rotates the vane until the wind force and rotational force are in equilibrium.

If the CP and CG were at the same point, the vane wouldn't move much (if at all).

As an aside, I wonder if a rocket built of solid foam (so a solid 3D model, think uniform density within the volume) of a rocket would have a CG and CP at coincident points? Just thinking out loud. Someone know the answer?

Greg

Greg, I think the answer is that depending on the fin mass, maybe. Think of the flat plate, it basically has the approximate CP and actual CG at the same spot because it balances there. Your 3-d version would be similar, however the rear end has a vertical component of the fin mass(assuming 4 fins).that isn't modeled in a flat plate(top view) method.

Frank

GregGleason said:

The analogy of a weather vane might be helpful.

Imagine the CG is the axis on which the vane rotates. The CP needs to be behind the axis so that the pointer faces the wind. The wind has more effect on the area behind the axis, so it rotates the vane until the wind force and rotational force are in equilibrium.

If the CP and CG were at the same point, the vane wouldn't move much (if at all).

As an aside, I wonder if a rocket built of solid foam (so a solid 3D model, think uniform density within the volume) of a rocket would have a CG and CP at coincident points? Just thinking out loud. Someone know the answer?

Greg

Click to expand...

For some reason I was thinking about boattails the other day. I decided that a boat tail has a negative affect on CP. for instance when the rocket going sideways the airflow "sees" the entire projected area of the boattail, but when the rocket is traveling at just a few degrees away from forward, the airflow does not see the boattail.

I also have an interest in cars, and I was reading a story about a team that picked a Firebird to use to set a speed record at Bonneville. Traction is an issue so they put a lot of weight in the trunk, then in their speed runs they crashed. I thought about it a little bit and realized that by adding weight they moved the CG of the car BEHIND the CP, causing it to be aerodynamically unstable.

Wow this is a fun discussion and everyone seems right --just explaining it differently. Let me throw me 2 cents in. First with the car--putting wieght in the rear is a novel way getting initial traction but no good for high speed stuff. Mainly because you have to continue to accelerate that weight as a penalty and also nose lift is exagerated leading in some cases to aerodynamic lifting and hence a crash. Not so much a CP/CG effect as a poorly-aerodynamically ballanced car. Anyway, the weather vane analogy is a good one and using the cardboard cutout balanced on a rule is a good basic tool and does work for finding CP which is basically1/2 the total area of the ship static. Since most of my rockets are assemetric--I use graph paper sketches of the top and side. I find the equal amount of squares--front to back-then overlay them and find the average between the two. Not really hard and it gives you a chance to sort of design on the run. It gives me a chance to finesse some designs. Just a thought---ok you can have my soap box now !