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

Thank you David, and placing small fins on the individual strap-ons was 
something I hadn't considered.  Not a bad idea at all. Regarding the ignition 
and deployment of the strap-ons, I did kind of simplify what's planned for this 
post.  There are actually 6 timing events (not 4) that I'm planning.  The first 
6 strap-ons are ignited 3 at a time but very close in timing.  I'm planning for 
extra battery power (six 9 volt batteries).  Two of the batteries will be 
powering each timing event.  I did a lot of testing prior to the flight of my 
Delta X rocket with the 4 strap-ons, and I'm conducting additional testing 
prior to the Delta II.  Still, there are failure modes, and I realize that a 
failure for any of the 18 events (9 strap-ons with two events each) would 
result in the destruction of the rocket.  But, only one failure mode is really 
dangerous. I'm going to give some thought to adding fins to the strap-ons.   
Thanks again,RichardDate: Wed, 10 Aug 2011 11:12:21 -0700
From: derbas@xxxxxxxxxxxxxxxx
To: roc-chat@xxxxxxxxxxxxx
Subject: [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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