We had some conversations off line and I thought I'd share the results:
Original question from mci2929:
Is there any special formula or template I can use to make a "collar" reducing the my body tude from a BT80 to say a BT50? In Stine's Handbook of Model Rocketry there is a formula that only a mathematician can figure out...laugh! Any help you can offer would be appreciated.
The answer from Carl Tulanko:
OK, here is a fairly easy method of building a paper reducer, which can be used as is or as a template for a stiffer cardboard reducer. Note that the BT50 tube should be permanently glued into the BT80 using at least two centering rings since the reducer is for show and not physical stress.
We start by rolling a paper cone. One end of the cone is the same diameter as our LARGER body tube. Make it a bit larger so there is some overhang over the BT that can be trimmed.
Now, if the base of the cone is 2.5" and 2" up from the base we need it to be 1.5", we can figure out what the length of the cone should be when rolled.
Since it takes 2" of cone length to decrease the diameter by 1" (2.5" - 1" = target of 1.5"), then it will take 5" of cone length to reach 0, or the tip.
Here's the formula: Larger Diameter x Reducer Length = Cone Total Length. Once the cone is made, make two bulkheads out of cardboard at the inside diameter of the larger tube, in this case, 2.5" take a small stiff STRAIGHT wire rod that is longer than the cone. Push the rod through the tip of the cone and place a Bulkhead you just made on the end. Insert this assembly into the larger body tube until the cone base touches the top edge of the body tube. Put the other bulkhead on top and push it down to hold the cone base centered in place with the larger body tube. Draw your outside line around the cone base and trim evenly. Remove the cone from the tube, and measure your reducer length from the previously drawn base line. Mark in small increments around the forward cone area, then connect the dots. You now have a Transition cone or a template for one made of heavier cardboard stock.
Follow up from Carl:
I forgot to mention...instead of drawing dots for the smaller diameter, you could make two more bulkheads the diameter of the smaller BT and repeat the assembly procedure. This will give you a perfectly drawn line for the reducer top and bottom. Also, mark the template vertically at the top and bottom on BOTH the inside and outside sheets. When you unfold the cone, draw vertical lines from these marks, cut out the reducer leaving some
overlap for gluing, and you have a perfect reducer or template.
Once overlap is glued(your reducer is assembled), it can be slid over the smaller tube and cemented directy to the smaller tube. Use an extra centering ring just above the top of the larger body tube so the bottom of the reducer sits over it and can be cemented to this centering ring. If you use cardboard stock, you can use thin CyA to stiffen the reducer so it can hold it's shape when handled.
Thanks Carl and mci2929. That's helpful stuff.
Original question from mci2929:
Is there any special formula or template I can use to make a "collar" reducing the my body tude from a BT80 to say a BT50? In Stine's Handbook of Model Rocketry there is a formula that only a mathematician can figure out...laugh! Any help you can offer would be appreciated.
The answer from Carl Tulanko:
OK, here is a fairly easy method of building a paper reducer, which can be used as is or as a template for a stiffer cardboard reducer. Note that the BT50 tube should be permanently glued into the BT80 using at least two centering rings since the reducer is for show and not physical stress.
We start by rolling a paper cone. One end of the cone is the same diameter as our LARGER body tube. Make it a bit larger so there is some overhang over the BT that can be trimmed.
Now, if the base of the cone is 2.5" and 2" up from the base we need it to be 1.5", we can figure out what the length of the cone should be when rolled.
Since it takes 2" of cone length to decrease the diameter by 1" (2.5" - 1" = target of 1.5"), then it will take 5" of cone length to reach 0, or the tip.
Here's the formula: Larger Diameter x Reducer Length = Cone Total Length. Once the cone is made, make two bulkheads out of cardboard at the inside diameter of the larger tube, in this case, 2.5" take a small stiff STRAIGHT wire rod that is longer than the cone. Push the rod through the tip of the cone and place a Bulkhead you just made on the end. Insert this assembly into the larger body tube until the cone base touches the top edge of the body tube. Put the other bulkhead on top and push it down to hold the cone base centered in place with the larger body tube. Draw your outside line around the cone base and trim evenly. Remove the cone from the tube, and measure your reducer length from the previously drawn base line. Mark in small increments around the forward cone area, then connect the dots. You now have a Transition cone or a template for one made of heavier cardboard stock.
Follow up from Carl:
I forgot to mention...instead of drawing dots for the smaller diameter, you could make two more bulkheads the diameter of the smaller BT and repeat the assembly procedure. This will give you a perfectly drawn line for the reducer top and bottom. Also, mark the template vertically at the top and bottom on BOTH the inside and outside sheets. When you unfold the cone, draw vertical lines from these marks, cut out the reducer leaving some
overlap for gluing, and you have a perfect reducer or template.
Once overlap is glued(your reducer is assembled), it can be slid over the smaller tube and cemented directy to the smaller tube. Use an extra centering ring just above the top of the larger body tube so the bottom of the reducer sits over it and can be cemented to this centering ring. If you use cardboard stock, you can use thin CyA to stiffen the reducer so it can hold it's shape when handled.
Thanks Carl and mci2929. That's helpful stuff.
