Friction Fitting Motors

[lakeroadster]

Are there any tricks or tips to friction fitting motors for LPR / MPR?

I installed the D12 motor in the sustainer of my Ahpla today. I used regular masking tape and wrapped it around the motor casing until it was an interference fit into the cardboard BT-50H motor tube.

I actually used a pair of pliers to finalize getting the motor into position.

This rocket has 3.6 ounces of nose cone / ballast weight. So it's going to take a lot of force to push it off the rocket. I'm trying to avoid ejecting the motor and ending up with a lawn dart.

Thanks in advance for your thoughts and comments!

 

[waltr]

Is there a thrust ring? If so then that keeps the motor from moving forward.
The friction fit then is only to keep the ejection charge pressure from ejecting the motor. If you needed plyers to seat the motor then that should be more than tight enough as long as the nose cone fit is looser and wadding is loose.

 

[BigMacDaddy]

When I have a heavy nose I rethink the ejection charge and imagine it only needs to be strong enough to push the body off the nose (instead of the nose off the body).

I cannot see the inside of that booster but it is not minimum diameter so any chance you can incorporate a motor hook?

EDIT: Although it sounds like you do NOT need to worry given how tight you made things.

 

[NateB]

In addition to masking tape on the motor itself to make a snug fit in the motor mount, I often run a strip of foil tape along the outside of the body tube and the motor that sticks out. I then crimp the edge of the tape along the rear end of the motor making sure the nozzle is clear. Of course, the tape can damage the paint so you might be more reluctant to use this on your designs and finishes than I am.

 

[lakeroadster]

When I have a heavy nose I rethink the ejection charge and imagine it only needs to be strong enough to push the body off the nose (instead of the nose off the body).

Total rocket weight is 14.1 ounces. 3.6 ounces of nose cone - 10.5 ounces for the rest of the rocket.

I cannot see the inside of that booster but it is not minimum diameter so any chance you can incorporate a motor hook?

Sure. Complicated due to having a booster.

 

[lakeroadster]

In addition to masking tape on the motor itself to make a snug fit in the motor mount, I often run a strip of foil tape along the outside of the body tube and the motor that sticks out. I then crimp the edge of the tape along the rear end of the motor making sure the nozzle is clear. Of course, the tape can damage the paint so you might be more reluctant to use this on your designs and finishes than I am.

No room for external tape.... due to the booster.

 

[lakeroadster]

Is there a thrust ring? If so then that keeps the motor from moving forward.
The friction fit then is only to keep the ejection charge pressure from ejecting the motor. If you needed plyers to seat the motor then that should be more than tight enough as long as the nose cone fit is looser and wadding is loose.

Yes, there is a thrust ring.

 

[samb]

I've had the best luck with a spiral wrap starting at the aft end forward. Add a wrap around the forward end if needed.





I agree with NateB, it looks like you have room to add a wrap around the motor overlapping the motor tube. Belt AND suspenders.

 

[Mugs914]

Interesting technique Samb...

I friction fit almost everything in LPR. The way I do it is to just wrap a couple of layers of 1" masking tape around the nozzle end of the motor. The tape is wrapped around the motor even with the end of the casing, and small bits of tape can be added or removed until you get a good, snug fit. It's really easy to get the fit you need that way and removal is easy too; worst case, grab the casing with pliers, give a very slight twist and the casing is free.

Can't remember ever spitting a motor with this method...

Something like this...

 

[lakeroadster]

I've had the best luck with a spiral wrap starting at the aft end forward. Add a wrap around the forward end if needed.

View attachment 529471 View attachment 529472



I agree with NateB, it looks like you have room to add a wrap around the motor overlapping the motor tube. Belt AND suspenders.

View attachment 529474

No room. The booster has an internal BT-50 that slides over the sustainer motor.

 

[waltr]

No room. The booster has an internal BT-50 that slides over the sustainer motor.

Yep, that is how I do 2nd stage coupling.
This also makes a motor hook not work. Friction fitting of sustainer motor is the only way but this works well.

 

[lakeroadster]

Yep, that is how I do 2nd stage coupling.
This also makes a motor hook not work. Friction fitting of sustainer motor is the only way but this works well.

I actually have a motor hook on sustainer of my boosted Rocket Sled Rocket. I cut a slot in the booster motor tube, then sliced and glued another BT-50 over the slotted BT-50.

Then I glued one of the booster fins were the O.D. gap was.

 

[samb]

Interesting technique Samb...

I friction fit almost everything in LPR. The way I do it is to just wrap a couple of layers of 1" masking tape around the nozzle end of the motor. The tape is wrapped around the motor even with the end of the casing, and small bits of tape can be added or removed until you get a good, snug fit. It's really easy to get the fit you need that way and removal is easy too; worst case, grab the casing with pliers, give a very slight twist and the casing is free.

Can't remember ever spitting a motor with this method...

Something like this...

View attachment 529483

I’m sure I learned it from one of my rocket buddies at the DARS club. If I remember I’ll give them credit. All the old kit instructions had concentric wraps. Diff’rent strokes.

 

[samb]

No room. The booster has an internal BT-50 that slides over the sustainer motor.

Ok got it. I would guess a short lifespan for the top of that part. It’ll take a lot of heat from second stage ignition.

 

[Daddyisabar]

Motor hooks hah! I don't need no stinking motor hooks! Got a whole zip lock bag full of unused steel. The ole RSO found my lack of positive motor retention... disturbing. Cheap, sticky masking tape is your friend. The old world art of friction fitting is dying away with all the phancy new gear. Sometimes friction fitting, especially on staged models or silly oddrocs, is the only way to go.

Place one layer of tape firmly on motor surface in the pattern you choose, either straight or spiral. If you quiet your mind you may be able to hear the midichlorians telling you which technique to use. Insert, remove, NO ROLLING ON THE FRONT EDGE OF THE TAPE! . Then build up tape layers as needed until nice and tight. Feel the Force between the tape, the motor tube, the motor, the rocket. If done the night before always check before flight, especially if there are large temperature changes.

I once saw a young lad starting towards the RSO table with a 5xD12 cluster rocket. All friction fit with highly questionable igniter wiring. I stopped him before he got to the line. Oh the horror. I took him, his parents and school group back to my car. After an hour of refit and lots of tape and superglue, it was out to the pad for a five motor hookup. It was the last flight of the day and turned out to be a near perfect flight of his first model rocket. His next one was his Jr. Level one.

 

[teepot]

I'm going to use aluminumized tape to hold the motors in on my MD rockets.

 

[heada]

Wouldn't the aluminum tape be fairly low friction surface and so it becomes interference fit and not friction fit?

 

[lakeroadster]

Ok got it. I would guess a short lifespan for the top of that part. It’ll take a lot of heat from second stage ignition.

Isn't that true for every BP booster?

 

[samb]

Isn't that true for every BP booster?

Yes, boosters in general live a hard life. I think a motor tube extended from the booster to engage the sustainer motor like that will be especially vulnerable. I'm envisioning what you describe as a kind of a stuffer tube to ensure second stage ignition. Not sure it's needed for that purpose based the length of the booster. My 2 cents. Some after action pictures will tell the story.

 

[hcmbanjo]

Some interesting info about pulling out a used friction fit engine:
https://modelrocketbuilding.blogspot.com/2013/09/friction-fitting-engines.html

 

[Daddyisabar]

Some interesting info about pulling out a used friction fit engine:
https://modelrocketbuilding.blogspot.com/2013/09/friction-fitting-engines.html

Most of the time this is true. However, I had some weird things happen with tape adhesive melting, super easy to pull out hot, nearly impossible when cooled. Summer, Winter, hot or cold motor, humidity or being soaked a bit in the lake. All can have huge impacts on friction fit motor removal.

 

[teepot]

Wouldn't the aluminum tape be fairly low friction surface and so it becomes interference fit and not friction fit?

I'll be using the tape on the outside. Tape the motor to the BT.

 

[heada]

I'll be using the tape on the outside. Tape the motor to the BT.

Ah, ok. I think of that more of tape retention vs friction fit. I think most people use both when they can.

 

[lakeroadster]

Update: Flew the Ahpla and the friction fit motors worked great. Thanks for the advice fellas.

Yes, boosters in general live a hard life. I think a motor tube extended from the booster to engage the sustainer motor like that will be especially vulnerable. I'm envisioning what you describe as a kind of a stuffer tube to ensure second stage ignition. Not sure it's needed for that purpose based the length of the booster. My 2 cents. Some after action pictures will tell the story.

Booster recovered undamaged. Heck, it was hardly even sooty.

Follow the little white speck... that's the booster tumbling. Click through the photo's... it's like and old timey movie.

 

[mh9162013]

Most of the time this is true. However, I had some weird things happen with tape adhesive melting, super easy to pull out hot, nearly impossible when cooled. Summer, Winter, hot or cold motor, humidity or being soaked a bit in the lake. All can have huge impacts on friction fit motor removal.

Sounds like the best strategy is to see if you can pull out the used engine as soon as it lands. If you're having trouble, let it cool and see if that helps. If it doesn't, at least you know you did everything you could to get the engine it when you had the chance.

 

[Handeman]

In my experience, if you have to use a pliers to get a friction fit motor in place, it is WAY TOO TIGHT! If you have to press really hard to get it in, it's too tight. You should be able to put it in and pull it out without tools. I was always amazed has how little actual friction was really need.

 

[mh9162013]

In my experience, if you have to use a pliers to get a friction fit motor in place, it is WAY TOO TIGHT! If you have to press really hard to get it in, it's too tight. You should be able to put it in and pull it out without tools. I was always amazed has how little actual friction was really need.

I've never read or heard anyone saying that to use friction fit with your engines, you need to use pliers or other tools to get the engine in the rocket.

 

[lakeroadster]

Sounds like the best strategy is to see if you can pull out the used engine as soon as it lands. If you're having trouble, let it cool and see if that helps. If it doesn't, at least you know you did everything you could to get the engine it when you had the chance.

That's good advice.

When I recovered Ahpla, the first thing I did was pull out the D12 BP motor. When I installed it, I had to use pliers it was so tight. But after the flight, while the motor and motor tube were still warm, the motor pulled right out with my bare hands.

In my experience, if you have to use a pliers to get a friction fit motor in place, it is WAY TOO TIGHT! If you have to press really hard to get it in, it's too tight. You should be able to put it in and pull it out without tools. I was always amazed has how little actual friction was really need.

What kind of rockets? The standard kit style LPR / MPR rockets have lightweight nose cones. The rockets I'm building are oddrocs that have very heavy nose cones/ballast. The heavier the nose cone the more force it takes to eject it. And the reaction to that force is trying to push out the motor. I usually use metal motor retainers made of steel. But couldn't in this case due to the booster.

 

[Daddyisabar]

Sounds like the best strategy is to see if you can pull out the used engine as soon as it lands. If you're having trouble, let it cool and see if that helps. If it doesn't, at least you know you did everything you could to get the engine it when you had the chance.

In total Desperation there is always the freezer!

 

[mh9162013]

In total Desperation there is always the freezer!

You mean freezing the engine before putting it in your rocket?