Build Thread: Totally Tubular with Swappable Motor Mounts

techrat

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Yes, I do realize there's a kit out there with that name, but I'm making my own tube-finned rocket, and for this venture, I'm leaving my comfort zone of a single, glued in motor mount and making the option for different sized motors as well as clusters. The rocket is already half built, but another user out in the forum asked for a build thread (I'll find his name later), so here's where I am.

The Rocket is a BT-70 airframe-based, using a BT-80 cut up for Fins. This is the left-over Tube from my "Drag Queen" rocket built a few months back. I am using BT-70 couplers to build the motor mounts, and inside the airframe is a "thrust ring" that the coupler pushes against as the motor fires. At ejection, I'm hoping the coupler has more friction than the nosecone and pops out the laundry, although I may hedge that bet with a little masking tape during flight.

Okay:
Image #1) The rocket as it stands currently, no paint yet, heck not even a shock cord yet. But you can see that it's Totally Tubular. I'm using paper drinking straws that act as butresses for the tube fins as well as "choose your own launch lug", additionally, paper straw is again used inside the tube fins to hold those steady.

Image #2) A 24mm single motor motor mount. I have cut about a 1/4" off each coupler to give me room for the plywood centering rings and the glue for the motor tube. The amount cut off each coupler gets used to build the thrust ring.

Image #3) The thrust ring inside the airframe. Built from leftovers of the couplers I cut down, and glued inside the airframe. I left it wide open in case I decide to build a 29mm motor mount.

Image #4) Showing the completed motor mount inside the airframe, with enough space sticking out that I can easily remove the motor mount and swap in a different one. That's where it sits against the thrust ring intentionally. It's parallel with the end of the tube fins.

Image #5) details my next motor mount build into a coupler. It will be dual 24mm, allowing me to run 2 D or 2 E motors if I so choose. There are also plans for a cluster of 3 18mm and a cluster of 4 18mm and possibly a 29mm.
 

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jqavins

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I have long loved the idea of exchangeable motor mounts assemblies. The motor combination possibilities are virtually endless. It looks like you could fit, for example, a central 18 mm surrounded by up to 6 or 8 13 mm. Or, imagine 2 Apogee E6s plus 2 Estes A10s.
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jqavins

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I got so caught up in complex clusters that I forgot the other thing I wanted to ask. Have you ever used tube fins with those cross beams before. The straws would have to affect the air stream through the tube, and I wonder it that diminishes its ability to move the CP. Or increase its ability, for that matter, but the universe is never that kind.
 

techrat

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Have you ever used tube fins with those cross beams before. The straws would have to affect the air stream through the tube, and I wonder it that diminishes its ability to move the CP. Or increase its ability, for that matter, but the universe is never that kind.
I've run straws down the sides of rockets before, but haven't yet flown anything with the spar across the tube fins, as I have done here on Totally Tubular. Makes me think I should finish up the rocket body and fly it as a test before I build too many motor mounts, but then again, I can re-use my motor mounts on any other BT-70 based rocket build. But, yes, good point. I suspect all the spars are going to do is add drag, it shouldn't affect stability.
 

jqavins

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I've run straws down the sides of rockets before, but haven't yet flown anything with the spar across the tube fins, as I have done here on Totally Tubular. Makes me think I should finish up the rocket body and fly it as a test before I build too many motor mounts, but then again, I can re-use my motor mounts on any other BT-70 based rocket build. But, yes, good point. I suspect all the spars are going to do is add drag, it shouldn't affect stability.
I expect you're right, it's just that I make a habit of seeing potential trouble points in order to check and eliminate them. It's probably nothing. I'm highly risk-averse.
 

Lee

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Back to your changeable motor mount coupler – I had the same thought. There is another post on TRF describing removeable baffles. It occurred to me – why not combine the concept – a modular system if you will. A fixed, glued in coupler towards the nose end of the body tube acts as a thrust ring to block the baffle module from moving forward. The motor mount module is inserted tight against the baffle and held in place with 2 set screws near the aft end. With this arrangement, I now have replaceable modules.



The First flight on a C11-3 was a success.



This flexibility will enable further experimenting – different baffle designs and motor size or clustering options. I am encouraged to continue tinkering.



Photos show the components

Modules painted with high heat paint for durability

Baffle – has screw eyes on both ends of the central dowel for shock cord attachment and easy retrieval with a piece of coat hanger wire with a bent hook on the end. Also makes the baffle reversable as baffle degrades from repeated ejection charge exposure.

I’m working on a second baffle which, instead of plates, will use some pot scrubber material or steel wool to capture ejection material.
 

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gdjsky01

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I built a Bertha with swappable mounts. 1x18, 2x18, 1x24.
Used a coupler for each and two plastic rivets to hold them in.
Worked great!
 

techrat

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Have given the rocket a temp paint-job, and will fly "Totally Tubular" tomorrow at CENJARS to check out the flight characteristics before I continue to build more motor mounts. D12-5 for the first flight. Will let you know.
 

techrat

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Good news everyone! Totally Tubular flew today and wow. Totally straight up. At apogee, it appeared to just hang in the air for a second, and then the ejection charge blew, the chute came out and it drifted with the wind. We had a breezy day but temps were mid-70's (in November, go figure!), everyone had a good time. The flight was so perfect that I'm thinking of a tube-fin rocket for my L1 attempt. Anyhow, all my guesswork paid off, and I can now build many motor mounts for this thing as I'm very likely to keep it until I die. If the rocket itself ends up in a tree, I'll just build another also with a BT-70 tube and keep flying the same motor mounts.
 

gdjsky01

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Anyhow, all my guesswork paid off, and I can now build many motor mounts for this thing as I'm very likely to keep it until I die. If the rocket itself ends up in a tree, I'll just build another also with a BT-70 tube and keep flying the same motor mounts.
Congrats on a great maiden! Many have done the L1 thing using tube fins. I gather they produce a lot of drag. There is a LOC kit that is L1 sized IIRC.
 

techrat

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Update: Just completed the second motor mount for Totally Tubular -- it's a dual 24mm capable of taking two D or E motors (or even C-11's, which I might try). Second photo shows the motor mount inserted into the rocket as a test fitting. The extended portions of the 24mm tubes do not interfere with the thrust ring inside the body tube, so we are good to go. Now I just need to wait for a calm day as whatever I put in this is going to make it go very high!
 

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Handeman

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I understand the straws along the tubes fins/BT for extra strength. Why use the straws inside the tube fins? Why not just a rectangular piece of balsa or thin ply? Should give you the same or more support to the tube with less drag. Just wondering.
 

techrat

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Keeps with the "Totally Tubular" theme?
Yes, you got it. I wanted something in there because the BT80 fins seemed a little wobbly, and I considered a strip of balsa, and then realized that wouldn't fit the way the rocket was coming along. so, it had to be more soda straw. Sure, it's more drag, but drag is OK in my book. I'm not going for altitude records, and I want to get the rocket back. It flies quite nice as-is, and if I want to have it go high, my double 24mm mount can take two E motors, which I guarantee will get me 1000 feet despite all the drag.
 
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