Pods, airscoops, and stability

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tdn

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Yesterday I flew my first rocket that I had designed on my own. The flight was uber-fantastic. My girlfriend was jumping up and down with excitement. So was I!

So it's time to move on to another design. I want to keep it pretty simple, but add just a smidge more to the design. I'd like to add pods (either open air or with little nose cones) to either the body tube or the tips of the fins.

The problem is, I don't know how that will affect stability. RockSim (at least version 5.0) doesn't handle that.

Any suggestions?

(FWIW, I'm not algebraphobic, but calculus is pure anethema to me.)
 

rstaff3

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You need to check out the Rsim newsletters on this subject. Member teflonrocketry has published several article showing how to model tube fins (open pods) and closed pods. The latter requires a text edit of the XML file but is easy to do.

OOPS just saw the 5.0 reference. Tube fins can be done in ver 5 but the side pods required 7.x
 

powderburner

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Other than adding a small amount of weight toward the back end of your design, adding pods on fin tips should not degrade the stability of your rocket. It might actually help it.

There is a phenomenon called a tip vortex that occurs at and around the outboard edge of your fin. (Tip vortices are more commonly discussed in connection to aircraft wing tips, but the same idea applies to rocket fins.) When your rocket is deflected from the ideal flight path, it is travelling one way but pointing another (momentarily). That is when your fins go to work. They are effectively turned to an angle of attack, measured from the plane of the fin to the path of the rocket's flight. In this condition the fins act as little wings to try to pull your rocket's back end back in line with the flight path. When they generate lift, they also create tip vortices.

This vortex is easy to understand when you think about the (relatively) higher air pressure on the 'bottom' side of the fin/wing and the (relatively) lower air pressure on the 'upper' side. At the tip, these pressures try to equalize by introducing a swirl, or vortex, around the outer edge of the fin from the high pressure side to the low pressure side.

The energy that is spent in the process of creating this vortex comes from your rocket. The energy is lost in the form of an increase in the aerodynamic drag. So, we really don't want any more vorticity than we have to have.

Fin tip plates (and end pods) act as a barrier to the creation of tip vortices. They act to stop (or greatly reduce) the tip swirl and therefore reduce the drag. In stopping the tip vortex, these end plates have the effect of making the fin more aerodynamically effective. Your rocket should actually become slightly more stable with the same size fins.

If you are using tip pods, some part of the pod will also contribute to the effective fin area. It is easy to visualize a fin-with-tip-pod at some severe angle of attack, and to imagine that the flow over the outboard half of the pod would just roll on across that outer surface. The airflow that strikes the inboard half of the pod, however, tries to move slightly inboard but is sort of 'trapped' by the fin; the inboard half of the pod effectively acts as fin area.

Did any of that make sense?
 

tdn

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Originally posted by powderburner
Did any of that make sense?
Yes, actually, it did. The question now is whether to leave the pods as open tubes or to cap them with little nose cones. And if the latter, whether to close them off at the bottom with little fake nozzles. I would seem, from your description, that leaving them open would make more sense, but probably not by very much.
 

powderburner

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My notes were written assuming the tip pods would be closed or capped.
If they are left open, you will have even more effective fin area (and a bit more drag).
As far as making the pods open or closed, maybe at this point you should begin to include some practical considerations. If they have NC and a base plug, the pods will be a lot more damage resistant if they get banged up at touchdown?
 

tdn

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Originally posted by powderburner
My notes were written assuming the tip pods would be closed or capped.
If they are left open, you will have even more effective fin area (and a bit more drag).
As far as making the pods open or closed, maybe at this point you should begin to include some practical considerations. If they have NC and a base plug, the pods will be a lot more damage resistant if they get banged up at touchdown?
Good point.
 

powderburner

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Welcome to TRF! You have already figured out that you can get help here on many subjects, so come on back anytime.
Give us a little more info on your background and we will be able to figure out better how to help you.
And don't forget to post pictures, they like LOTS of pictures here (I think this TRF crowd maybe migrated here directly from reading comic books?)
 

tdn

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Thanks!

As for pictures, I only have 2 cameras right now -- An Estes AstroCam and an underwater camera (for snorkel diving). Once my girlfriend figures out how to use her digital camera, I may start posting some rocket pics.

I'll flesh out my background when I get a chance.
 

teflonrocketry1

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RockSim version 7 is definately the best for side pod, tube fin and ring tail fin simulations. Do you have a RockSIm version 5 file for your design? If so post it or PM me the file, I will do my best to simulate the side pods on it.

My Side Pod Simulation article from Apogee's Peak of Flight Newsletter Issue #119 (which also updates ring tail and tube fins simulation techniques) is at:

https://www.apogeerockets.com/education/downloads/Newsletter119.pdf

Bruce S. Levison, NAR #69055
 

Micromeister

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As powder said Welcome to TRF.
I'd also suggest a copy of the handbook of model rocketry. I'm sure you'll find one in the library or you can buy a current copy through NARTS. at nar.org. but any edition you can find will give you a load of info to help with your design work. We'll help every way we can but ya just can't beat first hand knowledge, and reference material.
 

tdn

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OK, I justed posted some profile information.

Thanks for the suggestions on the books and software. I'll look into both as soon as I have some money.
 

Micromeister

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tdn:
that is precisely why I mentions the library! most every public library I've visited has had a least one editison of G. Harry Stine book the handbook of model rocketry. for basic model rocket information it's a must have. You can buy a current edition later for now your looking for data:D

Here a pic of a downscaled Micro-maxx powered version of an OLD centuri ESS Raven model. It has wing/fin tip open cardstock pods, Cardstock side intakes, high T type tail and a very nice bottom scoop. If this model has the stuff your looking for, the plans are on Jim-z web site. it makes a great clone. all of parts are cardboard and cardstock, except the nosecone;)
hope this helps in your quest.
 

tdn

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Originally posted by Micromeister
that is precisely why I mentions the library!
Meh. I don't use libraries. If I want a book, I buy it. This explains why a) my bookcase is bulging at the seems, and b) my wallet isn't.

I just ordered it from Amazon.
 

Micromeister

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Originally posted by tdn
This explains why a) my bookcase is bulging at the seems, and b) my wallet isn't.

I just ordered it from Amazon. [/B]
Oh yea!
I do indeed know and understand:D
 

rokitflite

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OK John,
Its official. That micro Raven is the coolest thing you have done so far. Just thought you ought to know. I will be in Vegas with Quest this week and I will tell Bill about it!
 

tdn

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OK, so I got Stine's book, and I got RockSim 7.0. What now? If there's something in Stine's book, then I haven't gotten to that part yet. And I tried with RockSim, but I couldn't find any way to do it.
 

rokitflite

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Or you could just do it the way countless others have which is to put something together, swing test it and if its stable fly it! I'll bet none of Estes coolest designs went through a computer program before they made their way into the hands of thousands of rocketeers.
 

tdn

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Originally posted by rokitflite
Or you could just do it the way countless others have which is to put something together, swing test it and if its stable fly it! I'll bet none of Estes coolest designs went through a computer program before they made their way into the hands of thousands of rocketeers.
Sure, probably what I'll end up doing. Still, it would be cool to figure out exactly how it will perform before I actually launch it. I especially want to know how high it's going to go.
 

JRThro

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Originally posted by rokitflite
I'll bet none of Estes coolest designs went through a computer program before they made their way into the hands of thousands of rocketeers.
Or if they did, it probably wasn't anything more advanced than RASP. (and that's RASP without the small "w" in front!)
 

Justin

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I did a scratch build with side pods. They were closed. I'll try to find the thread on it for you.

All I did was keep them close to the body (I think Barrowman says if the thingy sticking out of the body is under like 10 degrees angle of attack it won't effect stuff much) and added a stuffer tube and some details on the nose to move the CG forward to compensate for the weight of the pods. It worked great on multiple flights under different power levels.
 

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