A special motor adapter tube is needed to mount the HyperTEK hybrid motor into Coldfire. Some additional details about that are available here.   The drawing below shows a cutaway view that illustrates the design of the motor adapter tube.  An Aero Pack Inc threaded thrust ring is affixed to the aft end of the motor adapter tube.  The Coldfire tail cone screws onto this thrust ring.  There is also a flat aluminum thrust plate affixed about 1/3 the way up the adapter tube.  This thrust plate bolts to the aft centering ring in the booster section. 

The drawing at the left shows a cutaway view that illustrates how the HyperTEK fuel grain mounts into the adapter tube and how the adapter tube mounts into the booster motor tube.  It also shows the tail cone cross section.

The aluminum motor thrust plate was custom made by Front Panel Express.  Their free CAD software was used to design the plate including all the holes in it. It is a precision machined part that was produced  for a very reasonable price.

The two plywood centering rings needed for the adapter tube were cut from a sheet of 1/4" thick birch aircraft plywood. A circle cutter was used to cut the outside diameter dimension on these rings. This adjustable circle cutter is available from WoodCraft.

The inside diameter on the centering rings was cut with a 2-1/4" Forstner bit also available from WoodCraft.  The inside diameter dimension was then fine tuned by sanding on a drum sanding bit to get the final 2.28" that matches the adapter tube.

All three centering rings ready to be mounted on the adapter tube.

The surface of the adapter tube was sanded to remove the glassine layer so that epoxy would bond better.

The two piece tail cone assembly is made from anodized aluminum and sold by Aero Pack Inc.   The part number  is TRA5439PB.

After the centering rings were epoxied to the adapter tube it was time to put the tail cone thrust ring onto the end of the adapter tube.  The thrust ring was bonded to the tube using JB Weld as per the Aero Pack instructions.

Next some Kevlar cloth was added to reinforce the phenolic tube between the thrust ring and the thrust plate.  The Kevlar is 8 oz cloth available from Thermostatic Industries, Inc. (Part number 8K-60.)  The cloth was saturated with West System epoxy and then wrapped around the tube. It overlaps the Aero Pack thrust ring to help hold it on and to help transfer the thrust forces up to the aluminum thrust plate.

The thrust plate on the adapter tube has holes in it that align with threaded holes in the aft centering ring on the booster section of Coldfire.

Eight #6-32 x 1/2" machine screws hold the thrust plate in place.

End view of the adapter tube and thrust plate mounted in the booster.

The tail cone is simply screwed onto the aft end of Coldfire to give it a nice finished look that is also very functional.  It hides all of the mounting screws in the thrust plate.

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