mounting an engine

[MotorCityChris]

So, I'm almost embarrassed to ask this question. I was once heavily into building and flying model rockets. But life and the collection of responsibilities got in the way. I'm attempting to make a comeback and I've got an annoying nagging question.

Is it considered poor form or bad practice to mount an engine directly into a body tube and use an engine hook on the outside of said body tube?

Doing a scratch build with a BT-20 tube as the body. Due to the size and number of fins, I do not want to use the 13mm Estes motors, but using an 18mm won't allow me the ability to use a proper engine mount.

If I can simply use the body tube as an engine mount, any best practices for attaching the engine hook securely to the body?

Like I said, I know it's a terribly simple question that I'm sure 25 years ago I would have scoffed at. Appreciate any feedback.

 

[neil_w]

That’s called a “minimum diameter” or MD rocket. It’s totally fine. It will possibly tend to go very high, depending on the rest of the design.

For retention, you can put on an external hook, or just wrap the motor with tape to create a friction fit.

 

[MotorCityChris]

That’s called a “minimum diameter” or MD rocket. It’s totally fine. It will possibly tend to go very high, depending on the rest of the design.

For retention, you can put on an external hook, or just wrap the motor with tape to create a friction fit.

Thank you! I knew I came to the right place.

 

[GlenP]

the vintage Centuri Bandito puts a notch in the cardstock fins to go over the mylar ring used for the engine hook retainer:

https://www.spacemodeling.org/jimz/ka-7.htm

 

[Rex R]

iirc the Estes 'Hi-Flyer' has an exterior engine hook as well.
Rex

 

[Charles_McG]

So did the original Photon Disruptor.

 

[OverTheTop]

You can also leave the motor about 1/4" out the back and a single wrap of tape will hold it onto the tube. Some of the rocket kits I have seen use this method. Perhaps coat the tube with a bit of cyanoacrylate adhesive (superglue) to toughen it up a little and prevent it being damaged a little on each flight.

 

[MotorCityChris]

You can also leave the motor about 1/4" out the back and a single wrap of tape will hold it onto the tube. Some of the rocket kits I have seen use this method. Perhaps coat the tube with a bit of cyanoacrylate adhesive (superglue) to toughen it up a little and prevent it being damaged a little on each flight.

Using the CA to "toughen" up the interior is a great call out. Thanks!

Everybody who provided examples of other rockets that used an engine hook on the body tube.... Thank you. It'll help guide me.

In this type of instance, the "easiest" way to secure the hook is using a retainer ring. Aside from requiring a cutout on the fin, it also would appear to add an element of drag during flight. has anybody ever used thread or fishing line wrapped around the tube affixed with CA to secure the hook? Or is there a "low profile" ring I should be seeking out?

Again, appreciate the sage advice!

 

[heada]

I've used several wraps of a kevlar thread on the outside of the BT to hold an external motor hook in place. I also drilled very small holes near the root of the fins and threaded that kevlar thread through the holes to help hold the fins on since they were surface mount and not through the wall. It worked but the thin mylar sleeve that I've seen Estes use on some of their mounts would have been a better solution for the motor hook.

If you don't want to notch the fins to support a sleeve, then forego the hook entirely and go with friction fit with a wrap of tape on the aft end. It works just as well as a hook and doesn't have the drawbacks of finding a way to secure the hook.

 

[kuririn]

Using the CA to "toughen" up the interior is a great call out. Thanks!

Everybody who provided examples of other rockets that used an engine hook on the body tube.... Thank you. It'll help guide me.

In this type of instance, the "easiest" way to secure the hook is using a retainer ring. Aside from requiring a cutout on the fin, it also would appear to add an element of drag during flight. has anybody ever used thread or fishing line wrapped around the tube affixed with CA to secure the hook? Or is there a "low profile" ring I should be seeking out?

Again, appreciate the sage advice!

The mylar ring is pretty thin, not much drag. In the old days (60's) there were some designs that used a strip of gauze and white glue to secure the engine hook.The Astron Sprite is an example. The modern day variant from Semroc/erockets uses a Tyvek strip.

 

[neil_w]

On at least one occasion I just epoxied the hook to the motor mount. For a neater exterior finish, you could layer a piece of cardstock on top, fit between the fins. Actually, if you use the cardstock you could just glue it on with Titebond.

 

[VAXman]

Booster of my venerable Estes Nighthawk did what you are askin about. A piece of paper over the engine hook holds it in place.

 

[hcmbanjo]

You can also leave the motor about 1/4" out the back and a single wrap of tape will hold it onto the tube. Some of the rocket kits I have seen use this method. Perhaps coat the tube with a bit of cyanoacrylate adhesive (superglue) to toughen it up a little and prevent it being damaged a little on each flight.

This is probably the easiest and cleanest looking option. 1/2" wide masking tape (or sticky vinyl strip) is wrapped
over the end of the body tube and the exposed back 1/4" of the engine.
Your fins should be glued 1/4" above the rear end of the tube to allow room for the tape wrap.
Any damage can happen from when the tape is pulled off after a flight, that's where the coat of super glue comes into play.
Tubes can peel if the tape isn't carefully pulled off.

 

[JStarStar]

I've used cutout sections of body tube to glue over and secure external motor hooks for minimum diameter rockets.

But usually I install the interior thrust ring (engine block in Estesese) to have the base end of the motor protrude about 1/4 inch below the bottom of the body tube.

Then, depending on fin clearance, you either use a ring of masking tape, or 3-4 tape tabs, to secure the motor in the body tube.

I avoid at all costs the "friction fit" method of wrapping a turn or two
of tape around the motor casing in order to make the fit in the body tube tight enough to avoid ejection at burnout. In
my experience this leads to a crumpled or crunched body tube a high percentage of the time when trying to extract the motor from a too-tight friction fit.

Or, alternatively, depending on the makeup of your masking tape, if it's coated with a rubber or plastic-based outer layer, that layer can melt or scorch due to motor heat, effectively cementing the motor in there for good.

 

[Sooner Boomer]

One other thing you can do is to run the engine hook along side a fin. Depending on the design of the rocket, you may not be able to fillit the entire length of the fin if you do this, leaving the last 1/2" or so free so the end can flex.