Rbecky:
Can't see em.. that was the idea
they are .063 (1/16") clear Lexan. they are epoxy rivit mounted, useing 3/32" holes in both the crayon base and the fin. then a 1/4" fillet each side. She's flown about a doz. times without damage, recovery is a 36" nylon Hemi-chute sawn by my better 2/3rds. (Hey! Any woman who let's their hubby paint, epoxy, and all manor of other smelly things in the basement all these years and still make my chutes is way more than a better half.
) Back to the crayon. What size tube do you have? the 5D is a 4.5" dia. I also have a smaller 2.5" that flys well on 2D's but was under powered with one.
Eugenefl: This was the 5th flight of that perticular SR-71, I have since converted another ducting the outboards into the main body with 1/8" x1/4" rectanular brass tubes to eliminate the center Oops possibility. unfortunately this model suffered a cato on its second flight and was distroyed. 3 C6 motors Kicks butt in the SR. To answer you queston on additional nose weight. Oh yes! I found a good rule of thumb for this process. Add the total weight of any added components, rings, plugs, motor mounts, ducting materials, and a tiny bit for adhesives, then add one half of the mass of the largest motors you will be adding beyond the kit recommended motor. This process has worked out well for 2, 3 and 4 extra motors over the years, with NO surprises on launch day.
The Shuttle Stack is a very finicky configuration, Much care must be taken in allowing or the additional Offset thurst you are contemplating. She will likely loop into the ground if not counter balanced. George Gassaway flys a stack RC controlled, you will be hard pressed to convert the estes shuttle stack to a cluster.
Hope this helps