Open Rocket and Nesting

[vahpr]

Hi guys,

Working on another Balls 2-stage, as usual the aft 3" or so of the (88mm motor/airframe) sustainer will nest into the transition/interstage of the 6" booster.

Previously, I've set the sustainer motor to overhang the aft end, so no problem adding the transition.

Couple problems with this however, it doesn't accurately place the CG since there's a carrier/nozzle I like to model separately, nor does the CP change with the motor overhang.

Is there a way to place the transition at the base of the fins? I'd like to experiment with moving the fin location and slotting the transition,

Thanks,
JerryO

 

[K'Tesh]

I'm not sure what you're trying to do... but I do have some tutorials on fins and transitions...

https://www.rocketryforum.com/threads/kteshs-openrocket-tutorials.62884/post-1595653



https://www.rocketryforum.com/threads/kteshs-openrocket-tutorials.62884/post-1595899

 

[vahpr]

Thanks for your quick reply!

I'd like to sim the actual nested rocket configuration - the transition begins either at the aft end of the fin roots or, in the case of a slotted transition, some ways up the fin planform. Although the image attached displays correct placement at the aft ends of the fin roots it doesn't accurately calculate cp or cg.

Can this be done so it accurately calculates cp/cg?

 

[H. Craig Miller]

Would you mind posting the .ork file for testing?

 

[vahpr]

Sure, modify as needed. This is the only way I know of to get the transition at the base of the fins, however the actual rocket is 3" longer at the base. The image in the 1st post is his design with a motor loaded but offset/overhanging 3".

 

[H. Craig Miller]

This is the only way I know of to get the transition at the base of the fins, however the actual rocket is 3" longer at the base.

It sounds as though this is what you are trying to achieve, although neither OpenRocket nor Rocksim 10 allow the nesting of a "body tube" inside of a transition.

​

 

[vahpr]

Thank you! So if I have this correct you used an "inner tube" same dia as the "body tube - that just didn't occur to me

 

[H. Craig Miller]

Correct. Good luck with your project.

 

[vahpr]

Well, sadly this doesn't address the problem because an inner tube doesn't affect the cp. I know RasAero handles this but I was hoping OR could as well..

 

[JoePfeiffer]

Well, sadly this doesn't address the problem because an inner tube doesn't affect the cp. I know RasAero handles this but I was hoping OR could as well..

Body tubes, inner or outer, don't affect CP in OR. They're really negligible.

 

[vahpr]

Hi Joe, Your comment got me thinking, for this situation it isn't the bt or inner tube by itself but the fin position relative to the bottom of the airframe/bt. I'd hoped that the presence of an inner tube aft of the fins (or an overhanging motor) might be considered the rocket base for fin position purposes but it doesn't.

For this sim it might be good enough to just let the transition hang off the back, even though in reality it's nested/sleeved to the base of the fins. This will affect the booster phase cp/cg but probably not enough to matter. The important part is the sustainer boost phase and stability at burnout.

I'll try to slot the transition this year so I can place the fins at the base and not up the airframe a ways which sorta renders this discussion moot. Good to have the option either way though! Thanks for looking at this.