[roc-chat] Re: Stability of a rocket with strap-on boosters

On 8/10/2011 10:34 AM, Richard Dierking wrote:

I'm certainly no expert, and this is far beyond stuff I've done to this point, but I'll throw in my two cents...

First, make sure that you have the battery power for ignition (reliably) of NINE engines. Since you'll be doing six of those simultaneously, it would appear that redundancy is going to be very important, and almost as important for the three G's

It is my understanding that part of the stability added by the tube fins is the surface area provided by the INSIDE of the tube, with the commensurate air flow. Eliminating the flow-through eliminates a lot of the 'advantage'. My suggestion might be to put smaller fins on the strap-ons themselves, as since they will then be farther away from the center of the rocket, the moment arm of the smaller fins will be greater -- but that's just my seat-of-the-pants guess/comment.

David Erbas-White


I'm considering the size and shape of the 3 fins for my Delta II 7925-10L. It's a 4" core transition to 5" on top with 9 strap-ons, each approximately 1.5" dia (BT-60 tube). total height is 65". I don't want it to be a high-velocity rocket and I'll be using a J135 for the core, six F-30's strap-ons 0.5 sec after launch, shed those, then airstart three G's strap-ons then shed the last three strap-ons.

For the 3 fins, I'm using 3/16" thick abrasion resistant polycarbonate that by my estimate has a stiffness similar to 1/8" birch. The fins are through-the-wall and fiberglassed to the 54mm engine mount. I'm thinking to configure the fins as clipped deltas to avoid busting a tip during landing. Since the rocket has 9 strap-ons, seems to me this would add stability during launch (like tube fins). Am I wrong thinking this? I would like to keep the fins on the small size.

Please advise and thank you,
Richard Dierking


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