High Aspect Ratio Airframe

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Frosty_Burrito

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Hello everybody. I've got a rocket I am working on with an aspect ratio of about 22.5 (length to diameter). At this point, I am not concerned, but I was wondering at what point I would start to encounter issues or have to take other considerations into account if I were to increase the length? Someone had mentioned that the stability calculations that OpenRocket runs may become inaccurate as the aspect ratio increases significantly, having something to do with the surface drag of the tubes. I've looked around and all I have found is information along the line of "if you're under 30 ish you're fine."

I don't (currently) plan on designing such a long rocket, but I am still interested in knowing what the impacts could be if I were to make a rocket with an AR of 30 or even 40.

Thanks!
 
Hello everybody. I've got a rocket I am working on with an aspect ratio of about 22.5 (length to diameter). At this point, I am not concerned, but I was wondering at what point I would start to encounter issues or have to take other considerations into account if I were to increase the length? Someone had mentioned that the stability calculations that OpenRocket runs may become inaccurate as the aspect ratio increases significantly, having something to do with the surface drag of the tubes. I've looked around and all I have found is information along the line of "if you're under 30 ish you're fine."

I don't (currently) plan on designing such a long rocket, but I am still interested in knowing what the impacts could be if I were to make a rocket with an AR of 30 or even 40.

Thanks!
It is called fineness ratio, not aspect ratio. Look for info on superrocs.
 
Thanks for the info everyone. Looking with search terms of "fineness" or "fineness ratio" gave me some good info but I still couldn't find anything specific about rockets or missiles with reported fineness ratios higher than about 25.

I've made and launched several rockets of 50:1 while researching horizontal spin recovery. Stability was never an issue for me.
Glad to hear that there were no stability problems. Anything else you had to worry about with such a high fineness ratio?
 
Because of lift generated by the body tube ("body lift") the CP will shift forward with angle of attack. The Barrowman equations don't account for this, and neither to RS and OR. That's the reason that stability margins less than 1 caliber are considered marginal; the CP shift will reduce that margin during flight. The effect is greater with longer airframes, which means that very long rockets need higher static margins. The Estes Mean Machine, for instance, has a static margin of about 10 calibers if memory serves, and it needs to. (See this paper, also reprinted in Peak of Flight #470.)

Another consideration is stiffness. I've seen Mean Machines flex during the boost, and those flights never go well.
 
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Thanks for the info everyone. Looking with search terms of "fineness" or "fineness ratio" gave me some good info but I still couldn't find anything specific about rockets or missiles with reported fineness ratios higher than about 25.


Glad to hear that there were no stability problems. Anything else you had to worry about with such a high fineness ratio?
- The tube just in front of the motor mount has a tendency to crumple.
- I never launched in winds above 8 mph, so weathercocking never became an issue.
- Transporting the rocket can be a pain in the ass. Look forward to taping the sections together.
 
Because of lift generated by the body tube ("body lift") the CP will shift forward with angle of attack. The Barrowman equations don't account for this, and neither to RS and OR. That's the reason that stability margins less than 1 caliber are considered marginal; the CP shift will reduce that margin during flight. The effect is greater with longer airframes, which means that very long rockets need higher static margins. The Estes Mean Machine, for instance, has a static margin of about 10 calibers if memory serves, and it needs to. (See this paper, also reprinted in Peak of Flight #470.)

Another consideration is stiffness. I've seen Mean Machines flex during the boost, and those flights never go well.

The paper has been extremely helpful, thank you.
 
Just another anecdote, I recently built and flew an ASP Tall Boy. Just over 10' tall and built of BT60s, it's got a fineness ratio of about 75:1. I highly recommend the kit for anybody interested in that sort of rocket. We got lucky and debuted it on a windless day for a couple great flights; there's a short video in the galleries thread.

One practical comment is that with a very long rocket, I would consider using a launch rail instead of a rod at a lower weight & motor class than I might otherwise. They present a lot of surface area to any wind encountered on the pad and act like a long lever. A rail's going to flex less and give better control over your launch angle, less likely to disturb the igniter leads, etc. etc..

I could also believe very long rockets might drift a fair bit on descent. Again, they're a big sail. Our Tall Boy was just floating up there.

Finally, I don't know any of the history or have experience with them, but I could believe the maximum lengths for NAR super-roc competitions in the upper categories are coming from some motivation about the feasibility and soundness of a rocket beyond those lengths for the likely diameters in each class. So those might reflect some reasonable upper bounds based on others' experience.
 
Finally, I don't know any of the history or have experience with them, but I could believe the maximum lengths for NAR super-roc competitions in the upper categories are coming from some motivation about the feasibility and soundness of a rocket beyond those lengths for the likely diameters in each class. So those might reflect some reasonable upper bounds based on others' experience.

Originaly, super-roc had no lenght limit. I flew a 21 foot A super-roc at NARAM 22, Trusswerk I. Immediatly after that, they put the lenght limits into the rules. The length limits were chosen so that design and qualified flights were still a challenge, and vendors would sell a lot of tubes. What the lenght limits did was make scoring all about altitude, or duration. If you had poor skills, a lenght between the min and max lenght might be best for you , but ideally you want to improve your skill to were the maximum lenght is best. There is no upper bound on length or finness ratio, it is simply whaterver the modeler decides is reasonable.
 
I've built a couple of long minimum diameter rockets that were about 25:1 L/D. I like to keep the margin between CP and CG at about 10-15% of overall length. This seems to work better than trying to define stability margin in calibers on very long rockets, probably works on short stubby rockets as well.
 
The Estes Mean Machine, for instance, has a static margin of about 10 calibers if memory serves, and it needs to.
I've never paid much attention to this kit before, but I bought one recently just to get the parts.

- Transporting the rocket can be a pain in the ass. Look forward to taping the sections together.
I found it interesting that the new Mean Machine kit comes with a plastic twist-lock coupler so you can carry in in 2 sections.
 
I've personally seen one of those twist lock couplers fail, and read hear about one or two others. I won't use it if I build one.
 
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