Short and Fat Rocket Stability

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Tronman

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I know that the 1+ caliber rule doesn't apply to short, fat rockets, but how does one determine if it's stable? Is there a formula? Can it be done in Rocksim? String testing is a little difficult with a short 8 pound rocket.
 
I know that the 1+ caliber rule doesn't apply to short, fat rockets, but how does one determine if it's stable? Is there a formula? Can it be done in Rocksim? String testing is a little difficult with a short 8 pound rocket.

Hang it from a string in the breeze, or use a decent size fan.

I have a rocket which is ~.7 calibers stable according to simulation programs and it flies fine: it's in the foreground here.

attachment.php
 
Hang it from a string in the breeze, or use a decent size fan.

I have a rocket which is ~.7 calibers stable according to simulation programs and it flies fine: it's in the foreground here.

I think I'm going to need a pretty heavy duty string for an 8-pounder :wink:
 
I know that the 1+ caliber rule doesn't apply to short, fat rockets, but how does one determine if it's stable? Is there a formula? Can it be done in Rocksim? String testing is a little difficult with a short 8 pound rocket.

The 1 caliber rule of thumb is just a simple way to capture all of the complicated bits of stability. The rocket doesn't have to have 1 caliber of static stability so long as the CG is always ahead of the CP. The problem is that both are mobile. The CG moves a little bit as the motor burns or the rocket contents shift under acceleration but these are usually so minor that they can be ignored. The CP on the other hand can move a lot. On long thin rockets it can move much more than 1 caliber while on short fat rockets it moves less. With Rocksim you can plot the CP location (use the Rocksim method) during flight. The biggest shift will be as the rocket comes off the rail in high winds. Check using the highest winds and lowest thrust motors you expect.
 
Open Rocket also allows you to calculate CG and CP at different speeds. I was able to gain a 1 cal. static stability by putting 12 oz. of weight in the nose of a Wildman Gizmo, in addition to the DD cannon I had there as well. Any software that factors in aft drag should be able to be effective in simming a short fact rocket, whose stability is gained through aft drag. In the case of my Gizmo, under weather conditions similar to our launch site and a launch configuration into the wind that we normally take, the difference in altitude between OR simulated and Stratologger reported was only 4%. That is an acceptable margin of error.

Open Rocket Configuration.jpg
 
Open Rocket also allows you to calculate CG and CP at different speeds. I was able to gain a 1 cal. static stability by putting 12 oz. of weight in the nose of a Wildman Gizmo, in addition to the DD cannon I had there as well. Any software that factors in aft drag should be able to be effective in simming a short fact rocket, whose stability is gained through aft drag. In the case of my Gizmo, under weather conditions similar to our launch site and a launch configuration into the wind that we normally take, the difference in altitude between OR simulated and Stratologger reported was only 4%. That is an acceptable margin of error.

View attachment 120610

Base drag in drag calculations is completely separate from base drag in stability calculations.
 
@ CarVac, I stand corrected. However it is the base drag that increases stability. That's why pyramid and saucer rockets don't need fins.
 
The 1 caliber rule of thumb is just a simple way to capture all of the complicated bits of stability. The rocket doesn't have to have 1 caliber of static stability so long as the CG is always ahead of the CP. The problem is that both are mobile. The CG moves a little bit as the motor burns or the rocket contents shift under acceleration but these are usually so minor that they can be ignored. The CP on the other hand can move a lot. On long thin rockets it can move much more than 1 caliber while on short fat rockets it moves less. With Rocksim you can plot the CP location (use the Rocksim method) during flight. The biggest shift will be as the rocket comes off the rail in high winds. Check using the highest winds and lowest thrust motors you expect.

I plan on using high thrust motors only and avoiding launching in high winds. My concern is that Rocksim shows a static CG only slightly ahead if the CP, so I guess I better chart the in flight movement.
 
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