Swappable engine mounts in BT-80?

[wscarvie]

Hi all,

I've got a scratchbuilt design in mind, using BT-80 tube. It's big and heavy (for a model rocket), so I'd like to fly it on either a D/E 24mm engine, or on a cluster of 4-5 18mm motors.

My question is, can two motor mounts be made such that they can be swapped out for each other in a BT-80 tube?

Seems to me that there are three problems:

1) Giving the engine mount something to push against in the rocket,
2) Making sure the engine mount won't eject itself, and
3) Making the whole thing strong enough to take these engine loads.

Suppose I built one of the engine mounts (let's say the cluster) into a BT-80 tube coupler. So, let's say 5 18mm standard engine mount tubes, centered in the tube coupler with custom-cut cardboard centering rings. If I used epoxy, I'll be the engine mount itself would be plenty strong. It's not significantly different from gluing the engine mount directly into the body tube, right?

If I glued a slice of BT-80 tube coupler into the rocket's body tube (like a really wide engine block), I would think that the engine mount would push nicely against this "engine block," solving problem 1.

But how to keep it from ejecting itself? I'd much rather use that ejection gas to, say, deploy the recovery device

Same approach would, I think, work for the 24mm engine mount. Solving problem 3 has be a bit stumped, though.

Any suggestions?

Thanks.

 

[vjp]

Everything you described sounds fine, just glue captive nuts
into the motor mount(s) and secure with short machine screws
through holes in the BT wall.

If you don't like screws showing on your BT, conceal them by
putting the nuts in the thrust ring, with corresponding holes in
the forward motor mount ring. In the rear motor mount ring,
drill holes large enough to insert your screwdriver into, and
use a magnetic screwdriver.

 

[wscarvie]

Hmm. Interesting. Assuming I put screws/nuts through the thin BT-80 wall, would they hold under ejection pressure?

 

[maddogdelta]

At a launch last week, I saw something like what you are describing.

If you start by building motor mounts out of BT-80 sized tube couplers.

The single engine couplers mounts will use the motor mount tube centered inside the tube coupler with standard centering rings. The centering rings and tube coupler are glued together, and a thrust ring glued inside the BT-80 tube.

The motor mount itself is friction fit into the body tube, with the thrust ring stopping it. A short piece of Masking tape should be enough to keep it stuck into the tube. The only thing that it has to withstand is the ejection charge, which is very small compared to the thrust of the motor. (tip on friction fit, wrap the tape around the motor/motor mount like a barber pole. The motor/motor mount screws right in, holds against ejection, and unwinds right out. It's pretty cool!)

Your only real limit is the number of tube couplers you have and stability issues when you put some big clusters together.

 

[rbeckey]

See this thread.
https://www.rocketryforum.com/showthread.php?s=&threadid=2043&highlight=crayon

29mm and 24mm clusters in BT 80 couplers. Tnuts with flat pieces of brass to hold in the motors mounts.

How about if you epoxy a piece of hard wood across the interior of the BT 80 coupler used as an engine block, somewhat above the end of the motor mount. Put a Tnut in it and use a long screw up through the motor mount to lock it into place.

 

[rkt2k1]

Will,

I've attached a picture of a swappable motor mount system used on an Estes MaxiForce that I found on the Internet. I kept it in case I ever wanted to try something similar. I hope it provides you some ideas!

 

[wscarvie]

Hi all,

Thanks to everyone for their advice. I've said it before and I'll say it again: this place is great.

I'm considering the options, and need to do some more sims to check the stability with both motor mounts. I think, however, that I'll end up using long screws passing down through the motor mount to secure it inside the body tube. I like the hardwood spar suggestion, as it will allow me access to the body tube above the spar for cleaning (the top is blocked with a big balsa transition).

Thanks everyone. I'll let you know what I come up with.

 

[saxophone]

If you opt for a 5 engine cluster, you could make the center
motor mount a 24mm motor mount and the outer 4 mounts
18 mm mounts. Then if you want to fly on a single E, just put
the E in the middle mount. You might want to fashion some plugs
for the other motor mounts to ensure good ejection pressure.

 

[bsexton]

Check out the Quick Change Motor Mount System at True Modeler's Rocket Kits (TMRK):

https://www.truemodeler.com/

Look under "Parts and Supplies". Even though the system is made for a 3.9" BT, it will give you some ideas. I had TMRK make a Primary Mount with a 3x18mm and a 1x24mm motor mount for a BT 80. They take a while, but it is worth the wait in my opinion. I modified an Estes Fat Boy with this system and you should see this baby go on 3 C6-5's!

 

[wscarvie]

bsexton:

Pretty cool quick-change arrangement. If my main body tube were larger than BT-80, using engine clips like they do would be a very workable solution. I wonder if using epoxy and kevlar fabric to simply glue the top 2/3 of three engine clips to the inside of the BT-80 would work? Hmm...

Thanks very much.

saxophone :

Not a bad idea (24mm + 4x18mm mount). The only thing this wouldn't allow me to do would be to clean out the space above the engine mount easily. After several flights I'm going to start getting a fair amount of buildup from ejection particles, etc.

But...worth considering for it's simplicity, for sure.

Thanks all.

 

[Rocketmaniac]

Originally posted by wscarvie
can two motor mounts be made such that they can be swapped out for each other in a BT-80 tube?

I like the idea and all of the suggestions..... I may copy you when I build my MaxTrax upgrade....... Many good ideas listed here.......

 

[bsexton]

Here is a picture of the "Quick Change" Motor Mount system that I got from TMRK. The primary mount will fit in a BT-80 and you can interchange the 1x24mm or the 3x18mm motor mount (the engine clips aren't installed on the 3x18mm).