MMT Shrinkage From Heat?

Kruegon · Sep 12, 2015

I've been flying HPR recently. Mostly HPR cased Gs and baby Hs. I've had a few rockets where the MMT seems to have constricted post flight. First flight the casings goes in without issue. By the second or third, the case feels very snug (but not hard) going in.

Is it possible that the higher power motors are generating enough heat to alter the MMT? If so, which would reduce it better? Remove the motor case ASAP after a flight? Or allow the case to cool inside the MMT before removing?

g zilla · Sep 12, 2015

Probably the motor expanding from the heat unless you have a plastic MMT

tbonerocketeer · Sep 12, 2015

Kruegon said:
I've been flying HPR recently. Mostly HPR cased Gs and baby Hs. I've had a few rockets where the MMT seems to have constricted post flight. First flight the casings goes in without issue. By the second or third, the case feels very snug (but not hard) going in.

Is it possible that the higher power motors are generating enough heat to alter the MMT? If so, which would reduce it better? Remove the motor case ASAP after a flight? Or allow the case to cool inside the MMT before removing?

Paper tubing?

jrkennedy2 · Sep 12, 2015

Possibly the motor casing expanding a bit and maybe a little ejection charge residue on the inside of the MMT? Does it keep you from putting in a new motor? Maybe get a 32mm bottle brush (or threrabouts) and give the inside of the MMT a quick "dusting" after a flight just to know any residue loose.

Handeman · Sep 12, 2015

Motor ejection? I suspect it has more to do with BP residue getting into the MMT. Not much will get past the case, but it doesn't sound like you are getting a lot more resistance. Try a damp rag or baby wipe through the MMT, that should tell you pretty quick if residue is causing the issue.

pyrobob · Sep 12, 2015

Thermal expansion of a cylinder will actually increase the diameters. My bet is in the above post, that BP residue is causing the increased friction. I know a few flyers that have occasional problems with the stickers on certain motors binding up upon removal. Perhaps that's another contributing factor? Although if you're flying reloads it wouldn't matter.

astrowolf67 · Sep 12, 2015

Are the fin cans foamed? If so, the foam could be expanding under the heat. Other than that, ejection charge residue. If you have a dremel with flex shaft, these make quick work of cleaning the residue out of your mmt. You can tape the end of the flex shaft to a wood dowel for added reach into the mmt.

https://www.dremel.com/en-us/Accessories/Pages/ProductDetail.aspx?pid=511E

https://www.dremel.com/en-us/Accessories/Pages/SubCategories.aspx?catid=2043&catname=Flapwheels

gerbs4me · Sep 13, 2015

I'm going to say is from bp residue as well.

JDcluster · Sep 14, 2015

Are you using tape to friction fit these motors?
The tape might be binding up in between the motor and the MMT.

JD

blackjack2564 · Sep 15, 2015

Kruegon said:
Remove the motor case ASAP after a flight? Or allow the case to cool inside the MMT before removing?

Remove right after flight, if problem is not solved then:
There are other issues such as grit in tube from multiple flights , charring from motor eject, any number of things. If paper tubes, could be fuzzing up, grabbing motor case. Bit of ca wiped around, then sanded smooth helps that.

When asking a question about a problem..........more info you supply..... will alleviate a plethora of unwanted answers. :wink:

Bat-mite · Sep 15, 2015

Do you lube the casing before sliding it into the MMT? A little white grease solves a lot of problems.

Handeman · Sep 16, 2015

Kruegon said:
I've been flying HPR recently. Mostly HPR cased Gs and baby Hs. I've had a few rockets where the MMT seems to have constricted post flight. First flight the casings goes in without issue. By the second or third, the case feels very snug (but not hard) going in.

Is it possible that the higher power motors are generating enough heat to alter the MMT? If so, which would reduce it better? Remove the motor case ASAP after a flight? Or allow the case to cool inside the MMT before removing?

Reading the replies and re-reading the post, are you friction fitting the motor case? Are you using motor retention and can't pull the motor out after the flight? Did either condition persist long after the launch?

As others have said, we need more info to zero in on the problem.

Kruegon · Sep 29, 2015

No they held in by retainer.

The MMTs are paper.

Using DD so no motor eject BP should be in the MMT.

I've never greased a case on the outside. Seems like that would be bad for paper MMTs.

No the can is not foamed.

tbonerocketeer · Sep 29, 2015

Kruegon said:
No they held in by retainer.

The MMTs are paper.

Using DD so no motor eject BP should be in the MMT.

I've never greased a case on the outside. Seems like that would be bad for paper MMTs.

No the can is not foamed.

Swelling from humidity....

dixontj93060 · Sep 29, 2015

tbonerocketeer said:
Swelling from humidity....

+1. How are the rockets being stored?

Kruegon · Sep 29, 2015

In my spare bedroom. Low humidity - around 50% relative. That's VERY low for where I live. It's currently 91% humidity outside. And it didn't start until I started using the 29mm HPR cases. Didn't do it using the hobby line case. The ones that have only flown on hobby line cases haven't had this effect.

rharshberger · Sep 30, 2015

Humidity is probably the culprit here, as BP residue and BP are both hygroscopic (as is the paper to some degree), if the MMT is open to any part of the ejection process then residue can and will find its way into the MMT, not much but some. You might try sealing and hardening the inside of your MMT with CA then using a dowel with some cloth and a piece of sandpaper to open it back up to the correct diameter for the motor.

Kruegon · Sep 30, 2015

I may have to try that. I've been trying to figure out a way to cause a humidity expansion then plug the MMT with something 1-2mm larger and then allow to dry out. Short of actually spraying the MMT with water, I've found no viable way to achieve this once the rocket is built.

The rag and sandpaper sounds possible. Sand it out, ca the inside, lightly re-sand and test fit. May look at trying that tonight. Thanks for the input.

AlphaHybrids · Oct 2, 2015

Please don't put grease on the casings and into paper motor mounts.

MMT Shrinkage From Heat?

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