CG/CP relationship question

[Ouly]

I could use some expert advice on setting the CP/CG relationship for my model. The model is 44" long and the top 30" is 1.5" in dia. and the last 12" is 2.6" in dia. Rocksim seems to calculate the caliber based on 1.5". I have several motor mounts and plan to have 1.5 calibers of stability with the heaviest loadout but I don't know if I should base the caliber on 1.5" or 2.6". I don't want an unstable rocket on my hands when loaded for bear but at the same time I don't want a way overstable one when launched with less power. I'm not sure if I can swing test this thing either. Any comments would be welcome. Thanks.

 

[rstaff3]

I have also noticed that Rsim occasionally seems to use the 'wrong' diameter tube when there is a transition involved. I don't know whether this is right or wrong and have never really thought much of it. Since this seems to occur mostly when I'm modeling some weird shape with multiple transition sections, I generally just adjust the CP/CG to get the right caliber of stability based on the diameter I want. I use the CP/CG locations from Rsim but ignore the computed # of calibers. On weird designs I tend to be more conservative than on more standard 3/4FNC designs.

I bet you are OK if you go with 1.5 calibers based on the smaller tube since it is almost 1 caliber of the larger diameter.

 

[Electrode]

Ok,

I use Rock-Sim quite a bit.. And your question stumped me at first.. you really through me for a curve...

So I went to Rock-Sim and did some studies real quick... And from what you are saying.. Rock-Sim is only using the smaller dia. body tube for CP.. this is untrue... I just built your rocket in rocket sim.. changed the body tubes to the same size.. 1.5 and then 2.6.. and I got three diffrent CP's with all the componets the same just changeing the dia.


I would say you are safe with a 1.5 cal. stability.. I'd even be willing to take it down to 3/4 stability for some of your bigger motors, so you don't have to put soo much nose weight in it. I beilive you will be safe still. Just make sure you got the stabity there for the smaller and SLOWER motors.....

Good luck with it...

Tim Hansford
[email protected]
www.TimHansford.com

 

[rstaff3]

From what I've seen Rsim computes the CP and CG correctly, but uses the smaller diameter (the front diameter?) when computing the static margin. I have never paid enough attention to tell if this is consistent or not. In some extreme cases (ie weird designs) you should not necessarily believe the computed static margin but should look at the CP/CG relationship and figure the staic margin from those values.

 

[Ouly]

Thanks for the input guys.

Rock-Sim is only using the smaller dia. body tube for CP.. this is untrue...

Thanks for pointing this out Electrode. The static margin only appoximated the smaller diameter tube, so I was fooled. Something else that bothers me about stability is the effect of a transition on the airflow past the fins at higher airspeeds. My original design had the the transition furthur back towards the fins but I moved it forward out of concern for this. I know a transition itself adds a certain amount of stability to a rocket but wouldn't it begin to cancel the stabilizing action of the fins if it was located too close to them? It would be interesting to know how close is too close and at what speeds I need to worry about this. There must be information out there from windtunnel testing on this. I'll do a little more digging but could use a nudge in the right direction if anyone has one. Thanks again.

 

[Electrode]

Ok now your getting way beyond my mathimatical means..... you need a book that one of our club members has writ'n and published.. " A little Rocket Science" Check out his web page and email hime with your thoughts... Tell him Tim Hansford pointed you towards him and I'm sure He'll be glad to help with any really complicated calculations... I'm just an advanced Rock-Sim user...

https://hometown.aol.com/xcskier10/myhomepage/index.html


Tim Hansford
[email protected]
www.TimHansford.com
AOL IM: Electrode080472

 

[rstaff3]

Originally posted by Ouly
I know a transition itself adds a certain amount of stability to a rocket but wouldn't it begin to cancel the stabilizing action of the fins if it was located too close to them?

The transition will add more and more stabilizing effect as you move it to the rear. The limiting case is the 'cone fin' type rocket that only has a 'transition' with no rear tube and no real fins.

Maybe at very high speed the the transition would create a zone in which there is no airflow over the fins? If so would the transition itself be enough to stabilize the rocket? All interesting questions. Hope you get a better answer. (and let us know of course )

I searched thru MIL-HDBK-762 and only found this info. A "flare-type afterbody" appears to be a conical transition, or 'cone fin':

"Conical nose-cylinder configurations with flare-type afterbodies satisfy this requirement. Further, flare-type afterbodies have some advantages over other stabilizing devices that are suitable for a tube-launched, high acceleration, supersonic rocket. As noted earlier in par. 5-2, solid flares are more effective than fins as stabilizing devices at high supersonic Mach numbers."

There was no discussion of their effects on fins, but they seem to be more effective than fins over mach.

Still doesn't answer your question...