7x18mm cluster motor mount for BT70 tubes.
Having built 2 clones of the Centuri Vulcan and a BT60 based upscale with a 29mm motor mount, I decided that BT70 was the next logical place to try and build a member of the Vulcan family.
In a bid to do something a bit different, though, I decided I wanted to use a cluster.
I finally settled on a 7x18mm mount and chose to try it with the motor mount produced by Fliskits.
The kit came with 7 motor tubes, 6 coupler tubes, a thrust ring and a set of vent blocks to form a "ring"
The first step in construction was to select one of the BT20 motor tubes and designate it as the central motor. The single provided thrust ring was then glued flush with the end using yellow glue.
The kit also came with 6 tube couplers. Each was marked at 1/4", the depth of the thust ring, and was inserted up to the mark in the remaining 6 motor tubes.
The six outer tubes were then separated into three pairs. Each pair was glued together along the length of the motor tube with yellow glue. This was done on a flat surface and butted up agains a steel rule to ensure straightness.
After the tube pairs had dried overnight, I began the process of mounting them to the central motor tube. This was simply a matter of smearing glue on the central mount at the approximate locations where the outer tubes would contact them and then pressing the outer tubes in place. I made sure that all tubes were in agreement in terms of fore and aft and glued the pairs on. After they were in place, I gave the assembly a wrap of masking tape to keep the sides tangent while the glue set up.
When the glue on the cluster mount had dried, I took off the tape and inspected the result. It seemed fine to me but I wish I had a piece of BT70 lying around.
The kit has no "centering ring" per se, instead it has 6 little pieces which, when arranged in a circle, suggest a ring with the innards cut out for the motor tubes. Each piece is fitted between a pair of the outer tubes and glued to the coupling tubes with the edges resting on the motor tubes below. They are intended to help block ejection gasses and, presumably, form an after bulkhead for the BT70 into which they will be inserted. To my delight, they fit just fine and were secured with yellow glue.
Construction Rating: 4 out of 5
As mentioned before, this mount was installed in my BT70 upscale of the Centuri Vulcan. It looked good hanging out the back end and its a shame that it had to sit so long before it finally saw some use. The day finally came, though, and the rocket was loaded with 7 of the longer burning Quest C6-3s. I also used the new Quest igniters and that made wiring the cluster easy.
Once everything was in place, the rocket was launched, all motors lit and it took off well. Unfortunately, it was underpowered and crashed to the ground after a short arcing flight. Ejection took place after it had hit the ground and it was obvious that there was no problem with gas leakage. It ejected just fine.
The motor mount was the only part of the rocket to survive the crash unscathed. It came through fine and was given to a young boy of my club to fabricate his own dream machine.
Flight Rating: 5 out of 5
I like the way Fliskits managed to squeeze 7 18mm motors into a BT70 tube. The mount went together easily enough and performed flawlessly. The crash had nothing to do with the mount and I would gladly buy another one if ever I need this combination.
I just with the rocket had been better.
Overall Rating: 5 out of 5