Ye Olde Rocket Forum - View Single Post - Estes Ranger and Big Bertha

Quote:

Originally Posted by vulcanitebill

I was trying to reconcile bits of my memory and I think I jumped to incorrect conclusions. I just took some measurements and that changes my conclusions.

I thought I remembered hand rolling a paper tube to use as a motor mount. The motor tube in the rocket is thinner than normal Estes tubes, but smoother on the inside than I would have expected myself to make at age 14 or so. The ID of this tube is 0.945" and as I recall it fits the FSI engine about like an Estes BT20 fits their 18mm engines, or maybe a bit larger. That should be a good gauge of what the FSI engines required.

I knew that the other rocket that I still have used a tube somewhere in between BT20 and BT50, and I had forgotten about BT40 until I read this thread. This rocket has centering rings at the back end and a balsa adapter glued into the front end so I can't measure the ID. The OD measures about 0.905".

So obviously my FSI engine mount doesn't use a BT40.

The Quest D5 motors are 20mm in diameter (0.787"), FSI's A4, B3, C4, D4 and E5 motors were 21mm in diameter (0.827"). Their E60, F7 and F100 motors were 27mm in diameter (1.063") and their A6, B6 and C6 motors were 18mm (0.709") in diameter. When I was thinking about the possibility of using BT-40 for motor tube, I was extrapolating from the fact that I use it for the 20mm Quest D5 motors. But I was incorrect because the FSI motors wouldn't have fit. The best fit in existing tubing would have been in Centuri ST-8, which had an ID of 0.865" and an OD of 0.908". This was a commonly-available tube and is probably what you used. I never had any FSI motors so I have no experience with adapting them to fit into a motor mount.

FSI themselves used RT-8 as a motor tube for their 21mm motors. This had an ID of 0.885" and an OD of 0.921". Throughout most of their catalogs, the company incorrectly labeled RT-8 as "0.903" tubing." I have no idea how they derived that number, but it doesn't match any dimension of RT-8.

Did you mean to say that 0.945" was the outside diameter of your old tube? If so, it matches the OD of Semroc Series 085 tubing. It would have been a perfect fit inside 24mm motor mount tubing, such as Estes BT-50.