Apogee Quick Change motor mounts vs thru the wall fins

[William Feldmann]

https://www.apogeerockets.com/Build...-Adapters/Quick-Change-Removable-Motor-Mounts
I have plenty of experience with low power 1/4A thru C motors. I have build rockets in the past that used D and E motors. I am wanting to start building some larger rockets (not HPR, yet) and up my game to the reloadable 29mm case motors and just got my hands on a Super Big Bertha kit for cheap. I like the idea of being able to run it on lower power motors like D & E and having the capacity to run on 29mm. I have used 2 up clusters before, but not 3 or 4.

I saw Tim's video about this motor mount randomly while searching for a different video of his and like the idea, but then I realized that this will totally negate the thru the wall fins. Like period, you cannot use thru the wall fins with this motor mount, at least in a BT80. Maybe something with a 3 or 4 inch tube with a BT80 centered in it would allow through the wall fins and this at the same time?

Anyone use this thing or something like it and if so what did you do with the fins?

 

[sed6]

Slap a BT60 (1.6") in a BT80 (2.6") that gives you 1/2" on each fin internally for a fillet. Then get a 29mm tube or engine mount kit and put that in the BT60 to fly your larger motor. Get or make a removable D to F adapter for the small motors. That is how I'm making my 2.6" Estes Phoenix clone.

 

[Wrightme43]

If you build the aspire kit from Apogee, the adapter kit is included as part of the rocket, and you get a rocket. I am inexperienced, but it seems like a nice way to get what you want, and a cool new rocket for under 30 bucks.

 

[kuririn]

This system is similar to the twist lock system that was used in several Semroc SLS kits.
The Apogee fixed mount has to be smaller in diameter than the airframe in order to use TTW fins.
And it has to be glued into a fixed motor mount tube in order for the fin root edges to attach to something.
The Semroc Brighton had TTW fins glued to a fixed mother mount with different configurations of interchangeable mounts that slid into it.
https://www.rocketryforum.com/threads/semroc-sls-brighton-a-clone-build.160007/That reminds me, I need to get some paint on that build.
Laters.

 

[rcktnut]

Slap a BT60 (1.6") in a BT80 (2.6") that gives you 1/2" on each fin internally for a fillet. Then get a 29mm tube or engine mount kit and put that in the BT60 to fly your larger motor. Get or make a removable D to F adapter for the small motors. That is how I'm making my 2.6" Estes Phoenix clone.

I don't understand OP's post, the kit comes with a 29mm. motor mount, TTW fins, build stock and use a motor adapter.

Maybe something with a 3 or 4 inch tube with a BT80 centered in it would allow through the wall fins and this at the same time?

???????????? I thought you were talking about building the kit in your post.

 

[William Feldmann]

I don't understand OP's post, the kit comes with a 29mm. motor mount, TTW fins, build stock and use a motor adapter.

???????????? I thought you were talking about building the kit in your post.

I have a Super Big Bertha kit, it is BT80 with thru wall fins and a 29mm mount. I am interested in using the Apogee quick change motor mount (link in original post) That is only made in a BT80 size and will NOT allow thru the wall fins (since the mount comes all the way out and only the 'bulkhead' is fixed in place.

So, I was wondering if anyone had used that mount from Apogee, and if they had what they did with fins (in any kit or homebuild?
As a side question does anyone know of a similar quick change mount that fits in a smaller tube, like BT60?

The Semroc clone above is along the lines of what I was thinking, but that would still take quite a bit of work, and I don't own a laser.... Hey, honey...

 

[rcktnut]

I have a Super Big Bertha kit, it is BT80 with thru wall fins and a 29mm mount. I am interested in using the Apogee quick change motor mount (link in original post) That is only made in a BT80 size and will NOT allow thru the wall fins (since the mount comes all the way out and only the 'bulkhead' is fixed in place.

So, I was wondering if anyone had used that mount from Apogee, and if they had what they did with fins (in any kit or homebuild?
As a side question does anyone know of a similar quick change mount that fits in a smaller tube, like BT60?

The Semroc clone above is along the lines of what I was thinking, but that would still take quite a bit of work, and I don't own a laser.... Hey, honey...

I understood what you have, and I'm assuming now that you want the quick-change mounts because you are interested in the clustered ones, no need otherwise. Just trying to help you out. I have the original Super Big Bertha with the 24mm motor mount. I've flown it on F44's many times and it is holding together just fine. The fins are surface mounted with strakes.
You could cut your fin tabs off so they are flush with the inside of your BT and surface mount the fins. Make up some strakes and add them. I think they will hold up just fine. I think the strakes look great to begin with. Don't know if you are familiar with the original but that is how the kit was built with the strakes.

 

[kuririn]

You will notice that the Apogee Quick Draw which uses this modular motor mount system is also a BT80 airframe and does not have TTW fins. It's physically impossible. So the easiest way to implement this system to your SBB is to convert the fins to surface mount.
If you feel you need to reinforce the fin joints you can use gussets.
In fact the fins for the original SBB with 24mm mount were surface mounted with gussets.
http://www.spacemodeling.org/jimz/estes/est2165.pdfIf you want to use BOTH a modular motor mount system AND TTW fins then you will have to do some re-engineering.
Easiest way would be to glue in a mother mount, say BT 70 sized.
This would be two BT 70-80 centering rings glued on a BT 70 tube.
Then build modular mounts which would then nest into the mother mount.
Mount retention could be either two engine hooks like my Brighton clone, a threaded rod through the aft centering ring with a washer and nut for retention, or some type of Z clip system. The limited space between the BT-70 and BT-80 tubes might make the Z clip challenging.
And fins would be TTW to the BT 70 mother mount.
Looking at the BT-70 centering rings available at erockets, some modular mount possibilities are:
1 x 29
1 x 24
2 x 24
2 x 18 canted
5 x 18
https://www.erockets.biz/fits-bt-70-bth-70-tube-1/

 

[rcktnut]

kuririn, Thanks I changed my flush to surface mounts, was thinking about keeping the fin tabs flush on the inside of the BT. I think flush mounted fins would mean they would be totally inside the BT!!! As far as gussets or strakes I don't know, just so OP knows what we mean.

 

[William Feldmann]

I understood what you have, and I'm assuming now that you want the quick-change mounts because you are interested in the clustered ones, no need otherwise. Just trying to help you out. I have the original Super Big Bertha with the 24mm motor mount. I've flown it on F44's many times and it is holding together just fine. The fins are surface mounted with strakes.
You could cut your fin tabs off so they are flush with the inside of your BT and surface mount the fins. Make up some strakes and add them. I think they will hold up just fine. I think the strakes look great to begin with. Don't know if you are familiar with the original but that is how the kit was built with the strakes.

View attachment 600462

Those gussets or strakes are just another layer of balsa?
My fins are 3 pieces, a central fin with the tab and then two thin skins. I don't know what the original had. I would add the strakes to the outside and then probably sand the root edge rounded using the BT with sandpaper over it to get a good meeting surface.

 

[rcktnut]

The S/G's are balsa. The original fins are just solid balsa. Is the BT in your kit a typical Estes BT as far as thickness or is it a heavier BT? The only reason I suggested leaving that bit of fin tab on is to plug the fin slots to prevent any glue from getting inside the BT. If doing your way just need to be careful gluing fins on. I used wood glue to glue the fins and S/G's on then epoxy for the fillets. Wood glue has a tendency to bubble when used for fillets. This is the second SBB that I have. The first one flew so much that the motor mount burnt through right above the engine block. Never a problem what-so-ever with the fins. This one has a reinforced MMT.

 

[William Feldmann]

Yes this is standard weight BT80, my kit came with the original fins which are a balsa tab fin and then two thin balsa skins, but it also has a vanderburn kit that has plywood skeletonized tab fins and plywood rings (29mm) and then you would use the balsa skins from the kit. The tubes are NOT already slotted, which is part of the reason this hair-brained idea came to be. This project might get pushed off to be a winter build, so I can do more pondering on what I want to do with it.

 

[bad_idea]

Looking at the BT-70 centering rings available at erockets, some modular mount possibilities are:
1 x 29
1 x 24
2 x 24
2 x 18 canted
5 x 18
https://www.erockets.biz/fits-bt-70-bth-70-tube-1/

Doesn't require rings because it's a snug fit, but BT-70 will also take 7x18. I adopted and finished an incomplete 7x18 BT-70 scratchbuild I found in the DARS stash and prior to crashing it a year ago burned well over a hundred motors in it. I need to get off my duff and rebuild that one. It's a hoot to fly.

 

[shanejohnson2002]

I know it's an older thread, but this is a design challenge I've personally taken up recently so I thought I'd start by sharing here.

I currently have a couple of fin-tabbed modular MMT rockets I designed in OpenRocket that look promising. The trick seems to be fins with a loooooooong root, or a forward/aft fin setup that are connected with a single long tab under the body tube skin. Or, just take the engineering hit and ignore fin tabs completely, which works but is definitely the easy answer. And where's the fun in that?

I'll try to remember to post them here tonight. FWIW it *is* possible to do fin tabs and modular MMT cans, but you add some significant weight to the aft end. I also feel like I'm stretching the limits of 1/8" plywood I use for my designs, even if it *is* 'aircraft grade' and has proper fillets. I could have some g10 fins custom-cut or some 3d printed....but I have a laser cutter and plenty of wood.

One of my designs is a BT-70 copy of a Mach1 X-AGM Piercer, with a BT-60 MMT system. I would honestly consider that the bare minimum for 18mm, as it only allows a 3x18 cluster max. That said, I still designed a 2x18 and 1x18 for kicks, possibly for a D-impulse composite. It'll allow up to a 38mm though, which is pretty cool IMO.

BT-80 with a BT-70 MMT system would be a much better way to go IMO...but I like challenges.