BT-60 Interchangeable Motor Mount System

	After crashing my Big Bertha for the fourth time, I picked up a new one. And this time I wanted to modify it into an Estes Ranger, a popular ancestor of the Bertha which used a cluster of three 18mm motors, but I didn't want to give up the option of flying it with a single motor. What I came up with is a Better Big Bertha which uses an array of several interchangeable motor mounts including a 2x18mm, a 3x18mm and a 1x24mm. It can be flown on anything from a pair of A motors all the way up to E power and beyond. Download plans for the interchangeable motor mount system The system consists of one or more removeable motor mounts - three shown here - and the permanently installed retainer assembly which consists of a bulkhead with a length of all-thread.

	The bulkhead is mounted in a coupler and is ported to allow the ejection charges to pass through. The shock cord leader is also anchored to the forward side of the bulkhead assembly

The all-thread is anchored in the bulkhead. On the aft end - turned up in this pic - the nut is counter-sunk into the plywood so as to keep the bulkhead completely flush with the mating motor mounts. Some epoxy was applied to act as a thread locker.

The shock cord leader attaches to a screw eye anchored in the bulkhead. In the pic above, the tip of the screw eye has been ground off flush.

Thus far, I have built up four different motor mounts.

L-R: 3x18mm, 1x24mm, 2x18mm and 2x18mm+2x13mm.

The 2x18mm+2x13mm supersets the 2x18 essentially obsoleting the latter.

Possible future mounts could be a 1x29mm or a cluster of 13mm tubes.

In this blurry pic, the entire stack is assembled and oriented as would be installed in a rocket. The bulkheads at either end of the motor mount are drilled to slide over the all-thread. A nut aft secures the motor mount into rocket. (In the pic, the all-thread has not yet been trimmed to length.)

Appropriate gauges of all-thread depend on the size of the rockets and motors, but in this case, 2-56 all-thread was used.

In the pics below, the gussets are shown. In all three cases, the gussets straddle the opening where the all-thread slides through aft. In the event of an extra potent ejection charge, much stress can be placed on the aft bulkhead as the motor mount tries to eject itself aft, and the gussets guard against breaking the bulkhead.

In the blurry pic below, you can see two pieces of all-thread. The center one is part of the removeable 3x18 mount, and is used to retain the motors. The all-thread at the top retains the motor mount in the rocket.

In the case of hte 1x24mm mount, motor retention is achieved with a hook fashioned from piano wire and surface mounted to the tube. An S-bend was made in the wire, then the wire was bent to conform to the curvature of the tube. Here, a few wraps of 1/4" masking tape slathered with wood glue was used, but other materials, such as nylon mesh and wood glue or fiberglass cloth and epoxy, are better suited than tape. The key is that there is no forward tang on the hook protruding into the tube. That allows any length of motor from 70mm D's to the much longer F32's and G55's.

In the pic at right, the Better Bertha climbs on a combination of two B6's and two A10-PT's.

12-25-2006