A special motor adapter
tube is needed to mount the
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
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
The inside diameter on the
centering rings was cut with a 2-1/4" Forstner bit also available from
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
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.
Threaded thrust ring in place
on the adapter tube.
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
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