[roc-chat] Re: Stability: Time to Use the Mojo
- From: Richard Dierking <redierking@xxxxxxxxxxx>
- To: <roc-chat@xxxxxxxxxxxxx>
- Date: Thu, 11 Aug 2011 15:36:58 +0000
Not at all; very good question. I considered this. I wanted to keep the fin
number down to a minimum because the strap-ons are ejected (with pyro bolts)
and I don't want them hitting the fins. The first six strap-ons that are
ejected are on opposite sides of each fin. This will leave the last three
airstarted strap-ons centered between the three fins and should help to provide
fin area for the correcting force during the last part of powered flight.
Richard Date: Wed, 10 Aug 2011 21:01:58 -0700
Subject: [roc-chat] Re: Stability: Time to Use the Mojo
From: rubr.duky@xxxxxxxxx
To: roc-chat@xxxxxxxxxxxxx
Silly question... Why not six?
On Wed, Aug 10, 2011 at 20:46, Richard Dierking <redierking@xxxxxxxxxxx> wrote:
Thank you! After reading the first couple of replies I was beating my head
against my desk and saying "why did I make the fins pink and blue!" and "why
did I ask those guys!" I did appreciate the humor.
If I showed the rocket core without the strap-ons (I'll use different colored
paper next time), you might say the blue fins are large enough. But, the
strap-ons hide the size. For one side, the pink fin is 120 cm^2 and the blue
fin is 90 cm^2. So, the pink fin has about 25% more surface area but only
sticks-out 3/4" more. 4" Goblin fins have about 85 cm^2 per side There are 4
fins on a Goblin and I usually build with 4 fins. I can't do that with a 9
strap-on configuration.
As usual, as the mass of the solid engines is consumed, the CG will move
forward. But, the strap-ons will shed during the flight and change the CP and
keep in mind that each of the core fins are between strap-ons that will be
released during powered flight. This really complicates things.
I thought about adding fins to the strap-ons but this creates some additional
problems like re-contact after deployment and the possible roll force on the
attachment points.
(one hour later...)
I'm going to use the pink-sized fins.
Thank you, and I'm going to blame all of you if the rocket's overstable or I
become unstable. :)
Richard
> Subject: [roc-chat] Re: Stability: Time to Use the Mojo
> From: allen.farrington@xxxxxx
> Date: Wed, 10 Aug 2011 13:49:29 -0700
> To: roc-chat@xxxxxxxxxxxxx
>
> Pink, definitely. You know, how big should they be if the pods had no CP
> effect, but still have the mass? I think that they look small.
>
> Allen
> --------------------------------------------------------------------
> Allen H. Farrington
> 818-653-2284
> web: http://www.allenfarrington.org
>
> On Aug 10, 2011, at 12:58 PM, Richard Dierking wrote:
>
> > When you're working on something like this Delta II you start to develop
> > mind roll. I realized that I need to make a decision on the fins before I
> > become a really old man.
> >
> > So, please look at the two photos showing a blue and pink fin. There will
> > be 3 fins and they will be transparent and the rocket's about 6' tall. As
> > Barney would say, "time to use our imagination..." Please provide your
> > vote.
> >
> > Thank you. I'm going off to do something simple like work.
> >
> > Richard
> > <bluefin.jpg><pinkfin.jpg>
>
>
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