Long Skinny Rockets

[pbernosky]

Hi.

I'm working on my L3 Cert - my rocket is a modified Madcow DX3 XL Fiberglass. Modified by adding a 12" extension to the fin can, and replaced the payload tube with 48" (3' longer total, rocket is now 11'4" tall).

OpenRocket is now saying stability of less than 0.351. I've added (in sim) 24oz of weight into the nose cone, which brings stability to 1.13 calibers. (Using an Aerotech M1297W motor).

Per Robert Galejs' paper, this may be way too low. (https://argoshpr.ch/joomla1/articles/pdf/sentinel39-galejs.pdf)

Should I consider this a long and thin rocket? Any advice on what target stability I should aim for? (no pub intended)

Thanks!

/p

 

[Exactimator]

Something's not right. That should be stable or close to stable stock without nose weight. What are you using to find the CP/CG?

 

[dhbarr]

Attach the sim here, there's something goofy.

 

[pbernosky]

fawkes.ork attached. Thanks in advance for your help!

/p

View attachment Fawkes.ork

 

[rharshberger]

When I open it up in OpenRocket it shows 1.13 calibers of stability with the M1297, still good as long as its not going Mach. It increased to 1.47 when I removed the CG overrides from the nose cone and the booster, were those input by you or was this Madcow's default RocSim file?

Edit: I withdraw my earlier statement, something seems odd and I am not sure what it is, IMO the stability should be higher than 1 but for some reason the CG seems to be in the wrong location for that long of rocket, and removing the nose weight did exactly as you said.

 

[pbernosky]

The overrides are based on actuals - measuring as it's been built. The Madcow file was all full of wrong, and of course I've changed quite a bit.

The Galejs paper warns against long and thin rockets - that stability needs to be waaaaay larger. But, not sure if this qualifies (though it sure seems like it when it's all assembled)


Ultimately want to fly an 6XL motor, stability goes down even farther.

So, still ok? Thanks!

/p

 

[Mugs914]

First thing I noticed was that the fins are listed as .375" (3/8") thick G-10. Madcow says they are 3/16". That takes 26.3 oz. off of the tail. Also, the tail cone is listed as being made from .079" aluminum that has been overridden to 5.61 oz. I'm assuming that is an AeroPac cone-type retainer. Is it really that heavy?

 

[Exactimator]

I see. It looks like the added length and tail cone move the CP forward a significant amount. I've never built or flown a rocket with this much extended length. I'm out of my realm, so I'm going to move aside and make room for those who know to help you.

Best of luck with your L3!

 

[MClark]

Run in a different program and see what you get.
I am a fan of the 10% of overall length stability, in this case about 14".

 

[pbernosky]

Overides are based on actual measurements, so they are spot-on. The retainer is a beast, "well made" so yes, actual weight.

 

[blackjack2564]

You need to check all your weights....rocket should not weight 22 lbs. you have 3/8th fins weighing 3.3 lbs.

I would think they really are 3/16 or 1/8. No need for payload over 24in. Go back and check/weigh everything or your entries in OR...something is amiss. It should be around 15lbs.

 

[OverTheTop]

It can be surprising how much CP can shift with the addition of a tailcone making the back end slippery! Does wonders for base drag though

 

[dhbarr]

You have two of the longer body tubes, and I'm pretty sure the override on the aft one is about 2x what it should be.

Fore: L 122cm , th 1/16in , wt 1166g , override 1298g
Aft : L 122cm , th 1/16in , wt 1166g , override 3005g

There's no way that aft tube has ~2.3x the density of the fore.

I think you probably weighed the whole fincan, but forgot to check "Override mass and CG of all subcomponents" in the sim. This would bring you up to about 2 calibers of stability and about mach 1.08.

HTH,
-dh.

 

[Kallahan11]

My Madcow DX3 xl came out to 27lbs dry and is stable without nose weight, I have a thrust plate and did tip to tip on it as well. The Madcow rocksim file is so bad your better off starting from scratch.

 

[pbernosky]

You have two of the longer body tubes, and I'm pretty sure the override on the aft one is about 2x what it should be.

Fore: L 122cm , th 1/16in , wt 1166g , override 1298g
Aft : L 122cm , th 1/16in , wt 1166g , override 3005g

There's no way that aft tube has ~2.3x the density of the fore.

I think you probably weighed the whole fincan, but forgot to check "Override mass and CG of all subcomponents" in the sim. This would bring you up to about 2 calibers of stability and about mach 1.08.

HTH,
-dh.

That was the problem! Whew! Thanks so much for figuring this out. Now stability at 2.3 calibers!

/p

 

[rharshberger]

That was the problem! Whew! Thanks so much for figuring this out. Now stability at 2.3 calibers!

/p

Much better.

 

[Nytrunner]

Glad you figured it out.
My IREC team used a modified DX3 xl for their first rocket on an M1297. It weighed ~25 lbs with motor and was quite overstable until the 10 lb payload went in just above the motor.

We hit 10,640 with 35lb pad weight, so your L3 is going to be pretty far up there.
Good luck!