Thermal expansion

[Random Flying Object]

I have searched and searched this forum for information about thermal expansion, primary in motor mount tubes. From what I have found filament wound fiberglass tube expands 18x10^-6 mm/mm/C, which at ~260C is enough to rip centering rings loose from the interior surface of a body tube or basically sheer the ring. This is not a good thing, especially if the motor tube you use is very long(60"), not to say that the entire tube will ever reach 260C. Does anyone have any insight?

 

[CarVac]

I have searched and searched this forum for information about thermal expansion, primary in motor mount tubes. From what I have found filament wound fiberglass tube expands 18x10^-6 mm/mm/C, which at ~260C is enough to rip centering rings loose from the interior surface of a body tube or basically sheer the ring. This is not a good thing, especially if the motor tube you use is very long(60"), not to say that the entire tube will ever reach 260C. Does anyone have any insight?

The motor itself should not go over 200 C, so the tube won't get that hot either. If it did, it would also start to lose strength, so it would basically cancel out.

I wouldn't worry about it, personally.

 

[Random Flying Object]

Does the 200C apply to larger motors(M and larger)?

Thanks for the info.

 

[daveyfire]

Yes, it's a requirement in NFPA 1125.

Looks like the standard may be updated to 220 ºC in the next update cycle.

 

[CarVac]

Yes, it's a requirement in NFPA 1125.

Looks like the standard may be updated to 220 ºC in the next update cycle.

I didn't realize that the standard was for the purpose of ensuring that the aluminum doesn't lose strength, rather than for the surrounding materials.

 

[AlexK]

It kinda makes sense on some level - that section of NFPA 1125 is only interested in making sure the commercially-available item performs as advertised (read: doesn't get so hot the aluminum yields and goes kaboom). It's up to the end-user to make sure it's used in an appropriate manner.

Since the MMT and the motor itself are in pretty poor thermal contact (even if it's friction fit, the motor is still inside a really good insulator), you're not going to have enough temperature change in the MMT to cause structural failure before it warms the epoxy joints holding it in place. Since epoxies lose strength really quickly (Aeropoxy ES6209 loses 75% of its shear strength going between 77 and 140 F, tensile losses will be similar), the MMT will lose its 'grip' on the centering rings and fins way before it can expand enough to break a stiff joint. The one place you commonly see high-temperature epoxy is not coincidentally also the only time you have good metal-to-metal contact: JB Weld to hold down an Aeropack retainer, which is in good (clamped) contact with the thrust ring on the motor.

 

[jimzcatz]

In all my years of highpower I have never ever heard of this issue. Even though its in NFPA 1125, is it really a concern? I use both FG and paper and never had a rocket come apart for this reason.

 

[Peter Olivola]

The most common form of non-CATO damage I've experienced from motor heat is separation of internal motor mount layers followed by ejection charge charring. At no time have I seen evidence of centering ring detachment. This isn't limited to HPR. It seems to be a function of the quality of tubing and assembly preparation. CA soaking can help but is not foolproof. Elimination of heat soaking, i.e., inability to cool the motor/mount after ejection would also seem to be a factor.

 

[Random Flying Object]

It kinda makes sense on some level - that section of NFPA 1125 is only interested in making sure the commercially-available item performs as advertised (read: doesn't get so hot the aluminum yields and goes kaboom). It's up to the end-user to make sure it's used in an appropriate manner.

Since the MMT and the motor itself are in pretty poor thermal contact (even if it's friction fit, the motor is still inside a really good insulator), you're not going to have enough temperature change in the MMT to cause structural failure before it warms the epoxy joints holding it in place. Since epoxies lose strength really quickly (Aeropoxy ES6209 loses 75% of its shear strength going between 77 and 140 F, tensile losses will be similar), the MMT will lose its 'grip' on the centering rings and fins way before it can expand enough to break a stiff joint. The one place you commonly see high-temperature epoxy is not coincidentally also the only time you have good metal-to-metal contact: JB Weld to hold down an Aeropack retainer, which is in good (clamped) contact with the thrust ring on the motor.

From what you are saying it looks as though thermal cycling could cause weakening of the centering ring joints over time, at least in an extreme case.

 

[GregGleason]

I don't know if it an issue or not.

But, I had a similar question a while back. Bottom line, I don't think it can hurt to have a small, say 1mm or thereabouts, vent hole.

The pressure would be a function of:

• how much enclosed volume
• how much the air is heated in the enclosed volume
• the change in altitude (delta of internal to ambient air pressure at apogee)

Sounds like a technical report in the making.

Greg

 

[Random Flying Object]

I have a scratch design that uses a set of fins to the very rear of the sustainer and another set of fins about 30" forward from the rear set. Both sets of fins are connected to a FB 5" x 60" body tube/MMT combination. Generally one would build as usual and attach the fins through the body tube to the MMT. I am worried that the MMT will heat up more than the surrounding body tube and start to crack the internal glueing sights of the forward fin set. And of course if everything is glued I have to way to inspect any of the internal joints preflight. This wouldn't be such an issue if the second set of fins were close to the rear set. With 30" separating major connecting points between two coaxial tubes operating at different temperatures I could be asking for some trouble. In the design I have centering rings placed forward and aft of each fin set, another centering ring about 45" up the MMT and then the MMT terminates on the aft radio bay(slip fit), which also doubles as the coupler to the remainder of the vehicle. I a basically worried about my forward fin set becoming loose after a few flights.

 

[Random Flying Object]

I don't know if it an issue or not.

But, I had a similar question a while back. Bottom line, I don't think it can hurt to have a small, say 1mm or thereabouts, vent hole.

The pressure would be a function of:


[*]how much enclosed volume
[*]how much the air is heated in the enclosed volume
[*]the change in altitude (delta of internal to ambient air pressure at apogee)


Sounds like a technical report in the making.

Greg

Agree with the vent(s) and I have incorporated venting between each centering ring. My centering rings look a bit like swiss cheese, all of my wiring harnesses run through carbon fiber conduits down the annular space between the body tube and the MMT.

I am more worried about the thermal expansion of the MMT relative to the the surrounding centering rings and body tube.

 

[GregGleason]

Agree with the vent(s) and I have incorporated venting between each centering ring. My centering rings look a bit like swiss cheese, all of my wiring harnesses run through carbon fiber conduits down the annular space between the body tube and the MMT.

I am more worried about the thermal expansion of the MMT relative to the the surrounding centering rings and body tube.

As I see it, this is also a heat transfer issue. How much of the MMT is going to get a thermal soak? Without running calcs or tests it's hard to know. For most motors, the burn time is short, therefore the heat transfer will be minimal. So most of the thermal transfer will occur after motor burn.

I'm thinking to get the answers you are looking for, you need to talk to a mechanical engineer who has a very good understanding of heat transfer and materials.

Greg

 

[AlexK]

From what you are saying it looks as though thermal cycling could cause weakening of the centering ring joints over time, at least in an extreme case.

I don't pretend to be an expert on the thermal-cycling fatigue properties of epoxy, so the best I can do is guess - not a proper Scientific Wild-**** Guess, just a regular WAG. Which I'm not going to do in this context :wink:

I have a scratch design that uses a set of fins to the very rear of the sustainer and another set of fins about 30" forward from the rear set. Both sets of fins are connected to a FB 5" x 60" body tube/MMT combination. Generally one would build as usual and attach the fins through the body tube to the MMT. I am worried that the MMT will heat up more than the surrounding body tube and start to crack the internal glueing sights of the forward fin set. And of course if everything is glued I have to way to inspect any of the internal joints preflight. This wouldn't be such an issue if the second set of fins were close to the rear set. With 30" separating major connecting points between two coaxial tubes operating at different temperatures I could be asking for some trouble. In the design I have centering rings placed forward and aft of each fin set, another centering ring about 45" up the MMT and then the MMT terminates on the aft radio bay(slip fit), which also doubles as the coupler to the remainder of the vehicle. I a basically worried about my forward fin set becoming loose after a few flights.

I doubt you'll ever see significant temperature at the forward end of the MMT. FWFG has a thermal conductivity somewhere south of bubkus (first number I found while Googling was 0.33 W/m-K). Unless the motor is the full length of that MMT, *and* it's a really long-burner, the front simply won't get hot to any significant degree (that's a thermal pun... I'll be here all night, folks! ) Since the forward end isn't going to heat up much, it's also not going to expand much. I'd be much more worried about the aft fins, but again, the epoxy is going to get rubbery-soft way before you expand enough to crack something. The fiberglass isn't going to stay stiff if it gets that hot, anyway. Unless you got some super-high-temp FWFG tubes, the adhesive holding the filaments together will soften and the fiberglass will get soft before the epoxy on the joints does (the tube is closer to the source of heat, after all).

 

[stantonjtroy]

I am more worried about the thermal expansion of the MMT relative to the the surrounding centering rings and body tube.

TCe will cause the tube to expand against the ring. Therefore the ring will get tighter. Not to worry though. the level of expansion (actual) is so small as to be negligable. Definately not enough to cause a failure. The greater risk reguarding temp is the epoxy. Most hobby grade epoxies arent Hi Temp. Even if one is used a proper post cure is typically needed to get full strength. Another thing to consider; The fiber winding (weave, filimant wound, Uni ect.) has little effect by TCe. The type of material (Glass, carbon, metal) is most greatly effected by Thermals; so much so that the type of epoxy suspender used is useally (but not always) not even factored in. I have some common TCe tables at work. I'll post some copies tomorrow.

 

[Random Flying Object]

TCe will cause the tube to expand against the ring. Therefore the ring will get tighter. Not to worry though. the level of expansion (actual) is so small as to be negligable. Definately not enough to cause a failure. The greater risk reguarding temp is the epoxy. Most hobby grade epoxies arent Hi Temp. Even if one is used a proper post cure is typically needed to get full strength. Another thing to consider; The fiber winding (weave, filimant wound, Uni ect.) has little effect by TCe. The type of material (Glass, carbon, metal) is most greatly effected by Thermals; so much so that the type of epoxy suspender used is useally (but not always) not even factored in. I have some common TCe tables at work. I'll post some copies tomorrow.

I am worried about the tube expanding along it's long axis, not really it's diameter. If you have some tables for filament wound tubing I would love to see them.

 

[AlexK]

I am worried about the tube expanding along it's long axis, not really it's diameter. If you have some tables for filament wound tubing I would love to see them.

Well if the entire MMT reached 200 degrees, it'd expand by 0.216". That's over the entire length, though, and it also assumes the ENTIRE tube reaches 200 C. Several things are going to limit how much expansion you actually get:

1: the tube isn't going to heat along the entire length. At least not uniformly. It's going to get hot at the motor end first, then the heat will spread up (VERY slowly, since the conductivity is so small). So you will really get a 'bloom' of heat near the motor and little to no effect closer to the nose for quite a while. The true maximum axial expansion, then, is not so easy to calculate - it has to involve integration, not a simple arithmetic calculation. It gets even worse when you consider that the temperature as a function of axial position is unknown and very hard to predict.

2: Thermal contact between the motor and MMT is really poor. Even though the motor gets super-hot, and it's highly-conducting aluminum, it's in not-so-good contact with a fiberglass tube. It's gonna transfer heat, but it's not going to be quick about it. You can't just assume that the MMT gets as hot as the motor casing.

3: Even if everything got as hot as the motor casing, you would experience material failure from the epoxy melting way before something expanded enough to break a joint. The tube is going to significantly soften somewhere around 100 C, at which point it's not rigid any more and thermal expansion is the least of your worries. (this changes, obviously, if your FWFG tube uses a high-heat adhesive)

4: Unless you use JB-Weld or something exotic (Cotronics?) the epoxy joining MMT to the centering rings will have almost no strength beyond 100C. So if the tube did expand as much as it possibly could, the epoxy at the centering ring joints would fail before the rings themselves do (the rings will, by virtue of distance, be cooler than the epoxy).

 

[stantonjtroy]

What Alex said :blush: . If you want an epoxy capable of higher heat loads I'd reccomend Hysol 9396 https://www.ellsworth.com/henkel-loctite-hysol-ea-9396-epoxy-adhesive-qt-kit/ . It's definately NOT cheap but it it good out to 350Deg F. Here's a smaller amount for a lower price: https://krayden.com/buy/manufacturers/henkel-products/hysol-ea-9396-50-gm-ez-packs.html FWIW