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A question about STABILITY....

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JSH56

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When I was young and built the good old Estes rockets (circa 1970's), I always diligently performed the swing test to determine how much weight to put on the nose. The original swing tests almost always resulted in a rocket cartwheeling and requiring adding weight to the nose. The way we always did it was to remove the screw eye, add a washer, and try again. Sometimes we'd end up with four or five washers and the rocket was just too heavy to get any real altitude. Not being educated in the CP and CG at the time, we just thought that was the way it was.

Now that I am getting back into the hobby with MPR, is the swing test really required for checking the stability of pre-kitted rockets? Aren't these designs stable by design? Some of the later Estes kits I've built require adding clay into the hollow nose cone, and each kit that includes this step usually have different amounts of clay included. I am assuming this is because the weight has been predetermined by the design itself.

So the general question is - Do kits today come stable by design or is the old string test still required? (Did I really need all those washers on the nose back in the day, or was it all a waste of time?) How would you add weight w/o ruining the max altitudes?

Thanks for the insight you might have....
 

blackbrandt

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The majority of kits (with a few exceptions) come pre-stabilized. The few exceptions tend to be scale models.

I have not ever done a swing test as the primary test. Here's what you do. Download openrocket from openrocket.sourceforge.net, and play around with it. You can build the rocket on the computer, and it will tell you the stability, weight, and simulate the flight for you. It's a must-have for any rocketeer. And the best part is, it's free.
 

Cabernut

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All kits should be stable by design - meaning they don't need nose weight but if they do, the kit will come with it(usually clay squares) and the instructions will tell you to add it.
 

JSH56

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Thanks, blackbrandt. I'll check it out.

BTW, I used to live in Cary. Moved to Florida in 2011. Miss the Triangle! Where do you fly your rockets up there?
 

blackbrandt

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I fly either with Tripoli East NC in Bayboro, or with Camden, SC (the club that used to fly in Orangeburg).

The Triangle is nice, but it's a 3 hour drive minimum to any HPR launch site. :(
 

Bat-mite

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Hello, and welcome to TRF!

First, let me warn you that you are asking a bunch of unstable people about stability! :wink:

Second, let me point you in the direction of OpenRocket, which is a rocket design tool, FREE, and pretty easy to use. Model your rocket and get stability numbers instantly.

Third, you don't need a swing test if you have a reasonable way of calculating the Center of Pressure of the rocket. You can balance the rocket on your finger or a piece of pipe or something to get the actual Center of Gravity. If the CG is at least on body tube width forward of the CP, you are golden.
 

rstaff3

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I only swing test rockets with oddball designs whose RockSim results may be questionable. Kits should be stable, but there is occasionally one that is not unless you add nose weight. I'd suggest Openrocket too. It should be both fun and instructive and you will know for sure that your rocket is ready to fly. As for nose weight, either lead shot or BBs embedded in epoxy is how I do it. Lead is more efficient but some fields prohibit its use.
 

JSH56

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Thanks all! I downloaded OpenRocket and it looks like it's going to be fun to use! :)
 

Oberon

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LPR kits, especially Estes kits, are designed to be stable (often overstable) as long as you use the recommended motors and build according to the instructions. If you "build heavy" or if you use bigger motors (and you should, 18mm Ds are a delight), you should verify that you didn't significantly change the CG location from the design. OpenRocket is great and pretty easy to use for simple designs, but more complex designs might need a swing.
 

dhbarr

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I only swing test rockets with oddball designs whose RockSim results may be questionable. Kits should be stable, but there is occasionally one that is not unless you add nose weight. I'd suggest Openrocket too. It should be both fun and instructive and you will know for sure that your rocket is ready to fly. As for nose weight, either lead shot or BBs embedded in epoxy is how I do it. Lead is more efficient but some fields prohibit its use.
I can specifically warn about the Estes Majestic. I've seen three of them skywrite built 95% stock... the 5% was my rail buttons.
 

OverTheTop

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All kits should be stable by design
Mostly, but not 100%, is my experience. Build techniques can vary wildly.

Run a simulator and you will have the answers you need :)

If you mark the CP position on the rocket when it is built you can do a quick stability check when you have it packed ready for flight, just by finding the balance point and comparing to the mark on the airframe. I do that for each flight, just as a check.

When you do a swing test on an 8" rocket will you please video it and post it here for our amusement :)
 

dr wogz

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the rule is that the CG is to be at least one caliber (diameter) ahead of the CP. The CG can move, depending on the motor and / or nose weight added. the CP is fixed, due to the fin design, placement, number of, etc.. Most 'larger kits' typically indicate the CP from either the nose or the tail. It is then up to you (the builder / flyer) to figure out if the CG (with motor installed) is in the correct location, and to add the appropriate amount of nose weight to get the desired balance / CG/CP relation.

The CP is harder to calculate, and there are a few methods. Simulating it is probably the easiest, but there is also the 'Rocket reviews' site that also notes the CG/CP locations for most reviewed rocket designs. There is a 'paper model' method to determine (roughly) where the CP will be. Trace the rocket profile (the shadow) onto a piece of cardboard and balance it a straight edge. This is the CP point. Lastly. you can enter in the rocket dimension into the 'Barrowman equation'. To get your CP.

As for the classic Estes designs, they leave this measurement out as they are designed to be stable or over stable right out of the bag, as others have mentioned. And this number / these numbers might be confusing to many.

https://www.apogeerockets.com/education/downloads/Newsletter238.pdf
 

shreadvector

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Swing testing is good, but if you do it wrong, you will almost always have results that trick you into thinking the rocket is not stable.

BEWARE:

1) The string can have twist in it, and when you swing the rocket, the rocket will "pull" outwards on the string and the string will try to un-twist, which results in a destabilizing force on the rocket. Use a braided cord, not a twisted 'twine' or string.

2) If the cord is not long enough, your rocket will not be flying into smooth air that appears to the rocket to be coming from straight ahead. The relative air will appear to be hitting the nose at a significant angle and this will result in unstable results until you add absurd amounts of nose weight. You want as long a cord as possible.

3) Starting the swing takes a smooth technique. If you just fling the rocket outward, it will be flailing around and you will get garbage results. You are trying to create the effect of a wind tunnel with the rocket on a string and moving smoothly. Try to turn around as fast as possible with your rocket held out in your outstretched arm and then release the cord gradually as you reach a good spinning speed. YES, you can get dizzy, but the results will be much better.

Practice, practice, practice.
 

cerving

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For almost all rockets of conventional design (3 or 4 fins at the back, nothing significant sticking up in front of the fins) the CP is usually about 1C or so ahead of the front fin root. So, if you CG is more than 2C ahead of the front fins, you're probably OK. Realizing that this is a seat of the pants non-computational method, but over the years I've found it to be pretty accurate. Adding little doodads to the back of the rocket (think Centuri/Semroc/eRockets Orion...) ususally doesn't affect this, because the extra weight (which would tend to move the CG backwards) is offset by the extra drag (which moves the CP backwards).
 
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