Question on fineness ratio

[SDramstad]

When calculating the fineness ratio do you include the nosecone? Case in point. An 8 foot tall rocket with a 6 foot tall, 6 inch diameter body tube and a 2 foot nosecone. Is the fineness ratio 12 or is it 16?

 

[Steve Shannon]

When calculating the fineness ratio do you include the nosecone? Case in point. An 8 foot tall rocket with a 6 foot tall, 6 inch diameter body tube and a 2 foot nosecone. Is the fineness ratio 12 or is it 16?

16 - Yes, you include the nosecone, and the tailcones if there is one. Total Length divided by diameter or caliber.

 

[SDramstad]

Cool, Thanks Steve

 

[Steve Shannon]

Cool, Thanks Steve

It’s the length as in flight, not recovery.

 

[Glasspack]

Ok now I feel dumb....I have been into Model Rocketry for many years ………….. What is Fineness Ratio?
Is that a type error ?

 

[Glasspack]

Wow Love it I just got it from Google...…………….

Learn something NEW Every day !!

 

[Steve Shannon]

Wow Love it I just got it from Google...…………….

Learn something NEW Every day !!

I remember when it was first added it to the requirements for certification. I wondered why we didn’t just say L/D ratio. I still do.

 

[Glasspack]

My Level 2 model I'm working on has a Fineness Ratio of about 23:1

 

[AeroAggie]

I remember when it was first added it to the requirements for certification. I wondered why we didn’t just say L/D ratio. I still do.

L/D ratio means something different to airplane folks. I prefer fineness ratio in this context because it applies the same way to both rockets and airplanes.

 

[Alan15578]

I remember when it was first added it to the requirements for certification. I wondered why we didn’t just say L/D ratio. I still do.

I'm often amazed at the simple basic questions that come up in the HPR forum.

 

[Bat-mite]

I think the whole fineness requirement is there to rule out "oddrocs," etc. If the minimum acceptable is 4:1, that would be a 16" tall rocket that is 4" in diameter. Anything broader or shorter than that ratio would be pretty silly-looking.

 

[jd2cylman]

I'm often amazed at the simple basic questions that come up in the HPR forum.

Well, I'd never heard the term "fineness ratio" either. I had heard of L/D, I'm pretty sure...

 

[PSLimo]

I think the whole fineness requirement is there to rule out "oddrocs," etc. If the minimum acceptable is 4:1, that would be a 16" tall rocket that is 4" in diameter. Anything broader or shorter than that ratio would be pretty silly-looking.

I will agree with you on that. Here's what my "FAT MAN" looks like with a 1.75:1 ratio.

 

[Steve Shannon]

Well, I'd never heard the term "fineness ratio" either. I had heard of L/D, I'm pretty sure...

And that’s my point. Rules should be as intuitive as possible. New terms should be avoided unless they are clearly defined.

 

[Alan15578]

And that’s my point. Rules should be as intuitive as possible. New terms should be avoided unless they are clearly defined.

"fineness ratio" is a classic terminology, that is intuitive. Krushnick Effect, Bernoulli Lock, Landis Loop, etc. are not intuitive terminology. While an HPR flyer can indeed be a beginner, I would have thought that most HPR flyers would have learned the lingo from their MR experience. Of course one could always pose the question: I'm building a Mach 3 N powered rocket with a non constant diameter and a pito(sp) probe, how do I calculate the fineness ratio?

 

[Steve Shannon]

"fineness ratio" is a classic terminology, that is intuitive. Krushnick Effect, Bernoulli Lock, Landis Loop, etc. are not intuitive terminology. While an HPR flyer can indeed be a beginner, I would have thought that most HPR flyers would have learned the lingo from their MR experience. Of course one could always pose the question: I'm building a Mach 3 N powered rocket with a non constant diameter and a pito(sp) probe, how do I calculate the fineness ratio?

But at least as far as I remember that really wasn’t what was meant (although it probably should have been) when fineness ratio was introduced as part of the very rules. We were talking about L/D with the intent to limit the type of rockets used for certification. So, the term should be better defined in the rules. I had no idea it wasn’t the same as L/D.

 

[JLebow]

But at least as far as I remember that really wasn’t what was meant (although it probably should have been) when fineness ratio was introduced as part of the very rules. We were talking about L/D with the intent to limit the type of rockets used for certification. So, the term should be better defined in the rules. I had no idea it wasn’t the same as L/D.

Maybe (just a guess) L/D ratio was already being used to mean lift to drag ratio. Aspect ratio is another similar term that is slightly more intuitive than fineness ratio.

 

[AeroAggie]

...Of course one could always pose the question: I'm building a Mach 3 N powered rocket with a non constant diameter and a pito(sp) probe, how do I calculate the fineness ratio?

Ignore the pitot probe (assuming it has an insignificant diameter), and use an average diameter for the airframe. Post a pic if you're actually needing help.

In Aircraft terminology, Aspect Ratio is b^2/S, where b is the wing span and S is the wing area. Since wings often taper, this results in effectively a ratio of span to average chord length. I think the same could be applied to rockets and just use an average diameter.

 

[Steve Shannon]

Ignore the pitot probe (assuming it has an insignificant diameter), and use an average diameter for the airframe. Post a pic if you're actually needing help.

In Aircraft terminology, Aspect Ratio is b^2/S, where b is the wing span and S is the wing area. Since wings often taper, this results in effectively a ratio of span to average chord length. I think the same could be applied to rockets and just use an average diameter.

But is averaging the diameter the correct way to calculate the “fineness ratio”? The articles I have found since Alan15578 posed the question specifically says fineness ratio is total length divided by largest thickness (diameter for 3FNC rockets).

https://aviation_dictionary.enacademic.com/2794/fineness_ratio

After further considering the posts from Alan15578 and JLebow I feel that the term fineness ratio is correct in this context for the reasons they stated.

 

[Bat-mite]

This is a fine thread.

 

[TonyL]

Hey, what if we used terminology that was explicit? One can call it fineness ratio, but also define 'fineness ratio' as overall length/max diameter. A good requirement is unambiguous. Any easy way to realize that is to define terms.

br/

Tony

 

[Steve Shannon]

Hey, what if we used terminology that was explicit? One can call it fineness ratio, but also define 'fineness ratio' as overall length/max diameter. A good requirement is unambiguous. Any easy way to realize that is to define terms.

br/

Tony

Exactly!

 

[AeroAggie]

But is averaging the diameter the correct way to calculate the “fineness ratio”? The articles I have found since Alan15578 posed the question specifically says fineness ratio is total length divided by largest thickness (diameter for 3FNC rockets).
View attachment 417218
https://aviation_dictionary.enacademic.com/2794/fineness_ratio

After further considering the posts from Alan15578 and JLebow I feel that the term fineness ratio is correct in this context for the reasons they stated.

You’re right Steve. I got pulled away from the computer and posted mine before I could complete my example.

 

[Alan15578]

Ignore the pitot probe (assuming it has an insignificant diameter), and use an average diameter for the airframe. Post a pic if you're actually needing help.

In Aircraft terminology, Aspect Ratio is b^2/S, where b is the wing span and S is the wing area. Since wings often taper, this results in effectively a ratio of span to average chord length. I think the same could be applied to rockets and just use an average diameter.

The OP gave a discription that needed no picture. I was just providing a tongue in cheek example to make the question more worthy of posting in the HPR area. And yes, I know "tongue in cheek" is not a very intuitive terminology.

 

[Bat-mite]

A good thing to note is that your TAP/L3CC should be able to define what is meant by anything ambiguous in the certification instructions.

 

[SDramstad]

I think the whole fineness requirement is there to rule out "oddrocs," etc. If the minimum acceptable is 4:1, that would be a 16" tall rocket that is 4" in diameter. Anything broader or shorter than that ratio would be pretty silly-looking.

So I could take a 4 inch diameter 5.5 von Karman nose cone, stick in a motor mount and fins an it would qualify under Tripoli rules with a fineness ratio of 5.5.

 

[DaveW6DPS]

At the risk sounding critical of the Tripoli rule-makers:

I think any rules should be stated in terms that are understood by those making the rules. Then the rule should be documented in a way that the people who are expected to follow those rules can understand without a forum post and debate.

 

[dhbarr]

"fineness ratio" is a classic terminology, that is intuitive. Krushnick Effect, Bernoulli Lock, Landis Loop, etc. are not intuitive terminology. While an HPR flyer can indeed be a beginner, I would have thought that most HPR flyers would have learned the lingo from their MR experience. Of course one could always pose the question: I'm building a Mach 3 N powered rocket with a non constant diameter and a pito(sp) probe, how do I calculate the fineness ratio?

Can't find a picture of a Landis Loop anywhere on this forum, and I think right now may be the first time I ever heard of it.

 

[SDramstad]

If the fineness ratio is for the whole rocket then an "Odd Roc" could still sneak through like I mentioned above with a 5.5 Von Karmen nosecone and no airframe. What prompted my question is if the fineness ratio did not include the nose cone it would insure that odd rocs couldnt sneak in. With the nose cone included in the fineness ratio we could get "creative" with it.....

I'm often amazed at the simple basic questions that come up in the HPR forum.

Maybe the question isnt as simple as you think....

 

[Steve Shannon]

At the risk sounding critical of the Tripoli rule-makers:

I think any rules should be stated in terms that are understood by those making the rules. Then the rule should be documented in a way that the people who are expected to follow those rules can understand without a forum post and debate.

Dave,
I’ve gone on record in this thread saying that terms need to be defined whenever they are first used. But maybe you can help me out by being specific rather than simply critical. What rules are you talking about? I haven’t been able to find “fineness ratio” in the Tripoli rules, but maybe I’m just missing it. If I could find a problem, I’d suggest a change to the rule.