Source or ideas for motor block for a 18mm motor

[KILTED COWBOY]

Building the Boyce Gemini-Titan and it uses two BT-20 tubes for the motor mounts.
I am having a heck of a time trying to find a source or how to make a motor block that will fit into the BT-20 tube. Everywhere I look is for complete kits or bulk supplies. Do not want to spend a lot for 2 little rings.
Was thinking of fabricating myself, need some ideas ideas I do not want to friction fit.
Thanks

 

[rharshberger]

Cut a ring off of a used Estes 18mm motor, it works great.

 

[CoachSteve]

cut 3 rings from the end of an 18MM tube
cut a slit in each one
fit them inside each other
fit in motor tube
Glue all in place
Rich's plan is easier though

 

[KILTED COWBOY]

Cut a ring off of a used Estes 18mm motor, it works great.

Great idea! Had not thought about that. DUH!

 

[samb]

The spent motor cases are easier to cut with a 1/2 inch dowel, sanded to fit, supporting the inside. A quarter inch dowel built up with masking tape works as well. Then some thin CA around the inside to keep the paper from unraveling. I've made a few that way.

 

[KILTED COWBOY]

The spent motor cases are easier to cut with a 1/2 inch dowel, sanded to fit, supporting the inside. A quarter inch dowel built up with masking tape works as well. Then some thin CA around the inside to keep the paper from unraveling. I've made a few that way.

Thanks Sam. Hope we can fly soon up at Gunter if it ever dries out.

 

[BABAR]

Easier and knock wood, have never had it fail. Engine casings are a pain to cut and heavy.

Cut a 1/4” length of your BT-20 motor mount tube (or ANY BT-20 tube.). Take a slice out of it maybe 1/8”, just enough so that it will roll up inside your BT-20 motor mount.

Use a complete 18 mm expended motor casing, insert it into the tail end of the mount to exactly the distance you want your REAL motor to be. Mark the CASING.

Use WHITE glue (less grabby than yellow, tends to “pucker” less), use a stick or if your mount is accessible from its forward end, spread glue over the inside surface as close as you can to where the mount’s final position will be.

Use your marked casing to smoothly push the 1/4” strip to the appropriate depth. With white glue, you don’t need to rush, but try to do it in one smooth motion and then IMMEDIATELY pull out the casing. If you try this with yellow glue, need to be in and out quick or it will grab your casing and destroy the mount.

Been doing this for over a decade with up to E motors without having it fail.

 

[BABAR]

What are your plans for motor retention? If you don’t use a motor hook, can you leave 1/2” of exposed BT-20 below the rocket aft end? If you can leave that much or more, paint the rocket as usual. Once paint is completely dry, take a piece of cellophane tape and wrap it around the outside 360 + degrees (a little wrap around is good, you don’t want to leave anything bare.

At launch, insert your motors, you should have at least 1/4” sticking out. Use either cello or masking tape, wrap a tight length 360+ around the exposed motor AND your cello taped exposed motor mount. Make it tight, and burnish it down hard.

Again, have done this with up to E motors without fail for a decade. If you use CELLO tape it is just about invisible on the pad (if that matters to you. If it doesn’t, just use masking tape.)

The cello tape you originally put on the motor mount isn’t to help keep the motor IN, it is to allow you post flight to take the last tape wrap OFF easily without peeling up the paint (or worse, delaminating your motor mount tube.)

 

[KILTED COWBOY]

What are your plans for motor retention? If you don’t use a motor hook, can you leave 1/2” of exposed BT-20 below the rocket aft end? If you can leave that much or more, paint the rocket as usual. Once paint is completely dry, take a piece of cellophane tape and wrap it around the outside 360 + degrees (a little wrap around is good, you don’t want to leave anything bare.

At launch, insert your motors, you should have at least 1/4” sticking out. Use either cello or masking tape, wrap a tight length 360+ around the exposed motor AND your cello taped exposed motor mount. Make it tight, and burnish it down hard.

Again, have done this with up to E motors without fail for a decade. If you use CELLO tape it is just about invisible on the pad (if that matters to you. If it doesn’t, just use masking tape.)

The cello tape you originally put on the motor mount isn’t to help keep the motor IN, it is to allow you post flight to take the last tape wrap OFF easily without peeling up the paint (or worse, delaminating your motor mount tube.)

This model uses the engine bells that screw on for retention. Pretty cool as it looks realistic when flying, so that idea is not an option.
Will try the BT-20 tube cut idea and the discarded engine casing idea discussed earlier and see which one works best.
Thanks for all the advice, y'all are great sources of knowledge

 

[Flash]

The following statement may be of interest to you. It was sent to me over a year ago when I was working on a 1/45 Gemini Titan with closer scale motors. The last paragraph should be of interest. You might would want to do some testing first if you want to consider two 18mm motors back to back. Would be a longer motor tube. Could use engine blocks when only using single motors. Just a idea, that’s all. I choose not to try it.

It was a rather short-lived "thing" in NAR competition in the 1976-79
> timeframe and was first demonstrated by Trip Barber. It allowed, for
> example, the use of a "B" class motor in a smaller diameter tube than
> "standard" 18mm B motors...obviously giving one an edge in competition. It
> was eventually prohibited from competition and quickly disappeared because
> it was both not a widely known modification and there was little need
> outside of competition. However, for serious scale modelers, competition or
> not, there is a desire in many cases to have the scale motors as true to
> scale as possible even with a model rocket motor housed inside.
>
> There were some interesting combinations of tandem motors (which, BTW, only
> works with "black powder" motors. For example, a standard 18mm motor fits
> inside a standard 24mm casing. This combination allowed use in most models
> designed for 24 mm motors without modification to accommodate the longer
> motor since the 18mm "front" motor would pass right through the engine block
> or forward engine hook tang. It also meant that you didn't have to have a
> slightly larger engine tube since there was no need to "overwrap" the joint
> with glass cloth. Of course, a smaller motor firing down through a larger
> motor reduced the sum of the impulse delivered by the two motors vs. if they
> were not "tandemed" but you still picked up performance nevertheless.
>
> Also, you didn't want to tandem a high thrust motor with a large nozzle
> above a lower thrust motor with a smaller nozzle. That would almost
> guarantee a rupture at the joint between the motors. Tests did show
> (surprisingly) that there was little performance lost by tandeming two
> identical engines. In other words, say, two 10n/sec C motors did in fact
> almost sum up to 20n/sec or basically a full D black powder motor in an 18mm
> casing (though it was twice as long).