Yet Another Question: Do external pods affect CP?

[Rocketbuilder]

Would adding an external pod to a rocket (just a small tube with a nosecone) change the location of the CP? If it does, I'll have to resize the fins on my 3-stager (haven't made them yet).
Seems like it would though.

[kjohnson]

To the air, it doesn't matter if it's a fin or a pod, if it has surface area and sticks out from the rocket it will have some effect on the CP.

They will move the CP back if the pods are towards the rear. If it's up front (like a camera pod strapped to the side) it will move the CP forward a little.

kj

[powderburner]

The above post by kjohnson is very correct, if it's on the exterior it will affect center of pressure

If you mount a pod on a short standoff fin (such as to get a better field of view from the pod, or a 'clean' air sample without rocket forebody effects) it will tend to have a greater aerodynamic effect. If you mount a pod right on the body tube and use big fat fillets to blend it into the rocket's main contour it will have much less aerodynamic effect (sometimes down to almost negligible).

Are you concerned about a specific design? Can you post a drawing or pic?

[Rocketbuilder]

Are you concerned about a specific design? Can you post a drawing or pic?

This one; my three-stage scratch build.
I added mass components in Openrocket the make sure the CG was correct, but forgot to account for the aerodynamic effects. I have completely built the sustainer, but I can change the sizes of the fins at the bottom to correct for the effect of the pods, as well as change the nose weight in the sustainer. Unfortunately, I have no way of knowing how much the pods effect the CP, so I can only add a good bit of nose weight to get an overstable rocket, which could weather-cock severely.

Any way to estimate how far the CP/CG need to move to make it stable? (It is stable without the pods)

BTW I know Rocksim V9 allows external pods, but so far I can't get it to run on my computer. If I could, though, would I be able to save my .ork file as a .rkt ?

[powderburner]

OK, in your first post you said '3-stager'

Looking at the image you posted (I assume it is roughly to scale) I can guess that the first and second stages are among those short sections near the aft end.

But that leaves me puzzling: is your third stage (1) aft end back near the boosters (and the forward fins and pods are near the middle of the third stage), or (2) is that an extra long second stage with extra fins? (and the forward fins and pods are at the aft end of the third stage)

If (1), then why do you have fins up there? (style? preference? scaled from an actual vehicle?) If you don't need them, get 'em outa there, they aren't going to help anything and could be detrimental to stability (at any staged condition).

If (2), then the pods are near the aft end of the third stage and the overall third stage stability should be just fine

Have you ever seen the Barrowman equations? They are available for free, online, in several places. Don't be afraid, they aren't that hard, you can do it.

Model each pod as a fin (fin semi-span = pod width, fin chord = pod length) and you should have a conservative answer. That is, if the Barrowman calculation shows your design is stable (remember to check all staging conditions) then I expect you will be a teeny bit extra stable. (Don't worry, not enough to cause catastrophic weathercocking) This is because representing each pod as a fin could be called a 'worst-case' approach; the pods will not be as aerodynamically effective at stabilizing/destabilizing as a fin of equivalent planform would be.

[luke strawwalker]

Agree with powderburners observations and recommendations above...

One other thought... you can do the "cardboard cutout method" which will give you a worst-case scenario... (CP moves forward with increasing angle of attack, and the cardboard cutout method, or center of lateral area, gives you the CP location at an angle of attack of 90 degrees, or flying SIDEWAYS!) Needless to say you *should* never even approach such an steep angle of attack (liftoff in a high wind, maybe, just before the rocket clears the rod) but other than that, no... so if it shows stable under the cardboard cutout method, then you should be good to go.

Later! OL JR

[Rocketbuilder]

Thanks for the help.
Powderburner, here is a picture showing the stage separation. (It's in the old color scheme, which I abandoned because I'm too cheap to buy the paint that I need but don't have. It also shows an older fin shape, but it still shows where it separates.)

[powderburner]

Pods at the rear of the third stage should not be an aerodynamic problem.

But now you have me curious about just exactly what you plan to put into those pods and how much weight you are adding to the aft end (of the third stage). You need to do a little homework here (obviously) to be safe.

If you need to add fin area to the first stage and do not want to tear up all your hard work to add bigger fins, remember you can just clean the fin tips down to bare wood and glue on tip plates. You can add a lot of fin area fast this way, it will be effective during the low-speed portion of the trajectory, and it will be 'left behind' at staging. The rocket will be moving faster, remaining fins on upper stages should be more aerodynamically effective, and the rocket should be on a steady flight path by then.

[kjohnson]

Any way to estimate how far the CP/CG need to move to make it stable? (It is stable without the pods)

JR is right, that for a quick method to approximate the CP change is to do a cardboard cut out.

You'll want to do one for each flight configuration (full stack, upper two stages, and thrid stage) with the pods in place. Where that balances is a very conservative estimate of CP.

kj

[Rocketbuilder]

Have you ever seen the Barrowman equations? They are available for free, online, in several places. Don't be afraid, they aren't that hard, you can do it.

Model each pod as a fin (fin semi-span = pod width, fin chord = pod length) and you should have a conservative answer.

Barrowman equations =

I did model the pods as fins, and tweaked everything so that it was a little over-stable in all configurations. As for what I'm putting in them, one of them holds the parachute for the second stage, a design suggested to me by another forum member. The other two just make it symmetrical.

I designed the entire rocket in OR long before I started building, so don't worry about it coming out unstable. A mass component was used to account for the weight of the parachute.