Minimum Diameter Build Comments

From TRF, a post from Senior Space Cadet.

His statements are in italic, my responses follow.

"By minimum diameter rocket I mean a rocket with a body tube the same size as the motor, such as an 18mm body tube with an 18mm motor.

My first suggestion, if you are considering it is, don't.  Your mother raised you better than that.

I'm nearing completion of my first one and I wish I hadn't gone down this road."

"I never built one before because the normal way of keeping the motor from flying out the back, when the ejection charge goes off, is to tape it in.  Seemed like a poor solution.  Still does."

Friction fit was never 100% reliable. Before masking tape was used, the oldest Model Missiles instructions suggested using rubber bands wrapped around the motor. It's like anything else, you learn through experience the proper amount of wraps before you end up crimping the body tube when pushing in the motor.

"In designing the rocket, you need to leave room for the tape, so you have to move the fins forward."  

As mentioned above, you only need about 1/4" clearance for the tape wrap around the body tube and extended engine.

"Then you have to move the motor back so you have enough surface for the tape to grip onto.  This moves the CG back.

Because you are using a smaller body tube and nose cone, which weigh less, you either have to use a longer body tube or add weight to the nose cone, which negates most of the advantages of using a minimum diameter in the first place."

The body doesn't have to be that long - Look back at classic models like the Estes Sky Hook. That model does have long, trailing fins that can get in the way of removing a friction fit engine.

"Then there is the problem of anchoring the shock chord.  Some glue it on the outside of the body.  Some use the old method of using a folded up piece of paper, but now you don't have room to get your finger in the body tube."  

Estes style Tri-Fold shock cord mounts aren't good in models smaller than a BT-50 anyway. Too small a tube and the chute or streamer ejection can be blocked by a thick, folded mount.

"Well, maybe a child or small woman could.  I wear XL gloves.  My solution was to tie a big knot on the end of some Kevlar chord and glue it under an engine block.  Seems pretty secure, but Murphy will have the last say."

Kevlar tied around the engine block is probably the best way to go in a small diameter model.

"I took some short cuts and made some mistakes in designing my rocket.  The result is, I had to put a bag of concrete in the nose cone."

I was going to use streamers, but with the weight in the nose cone, I'm thinking I better use a parachute, which now means I probably have more weight in the nose cone than I need.

We've all made design mistakes in our first scratch builds. A "Bag of concrete?" Even the old Estes Wac Corporal used two nose weights coming out to about 1/4 oz. How much weight did you use?

"Other than all that, it's going to be brilliant.  They will be talking about my fins, in reverent tones, for years to come.  It will be a thing of near beauty.  Well, I'm pretty sure you won't gag. I might have photos in 24 hours, or less, or I might wait till launch day, which might be a while."

You've got the right attitude about it being "Brilliant". Looking forward to seeing the pictures.