Motor liner

the model rocket show
SilicdyneTRF
Advertise with us
madcow rocketry
The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
N

north boy

Well-Known Member
Joined
Feb 27, 2012
Messages
242
Reaction score
0
Hi everyone, question about the liner that you insert into the motor housing. I was wondering what its full purpose in life was. Does its roll cover more than just an insulator or is that its primary function ???

Does it actually increase the strength of the motor ??

What would happen if you disregarded the liner and just reloaded the motor with propellant that was wrapped in home made insulator ??

For example if you rolled it in 3 rolls of A4 Printer paper and it was a snug fit that way.

Im referring to research motors like gorilla type motors not altering shop bought reloads.
 
The liner is indeed an insulator, and the insulation required depends on the propellant and grain geometry.

This is why you see different liner materials for different reloads.

-Kevin
 
troj said:
The liner is indeed an insulator, and the insulation required depends on the propellant and grain geometry.

This is why you see different liner materials for different reloads.

-Kevin

Thanks Kevin, Ive been searching on here for a bit trolling the propulsion section and couldn't find anything and soon as I posted found a thread Bobreach and yourself had responded in. Thanks
Click to expand...
 
It gets into research motor stuff, but it isn't propellant, so...

The inside of a rocket motor, when burning, is like a high pressure high temperature super torch. You do not want bare metal exposed to this for any appreciable time. Aluminum, such as in our motor cases, melts at around 660 degrees C and will lose its temper at a much lower temperature than that. When it loses its temper, it will be perhaps a third as strong. Even just being a few hundred degrees F above room temperature costs about half the strength in use. So you want to keep the temperature rise of the case to only a couple hundred degrees F - that is unless you want to possibly bulge the case / CATO, or have it as single use. Thicker case walls help because they have more thermal mass. That is one of the reasons why larger research motors often have fairly thick walls.

A smoke grain is like a little torch sitting in there for a fair number of additional seconds. So the addition of a smake grain means the liner has to work for a longer period of time.

Some propellant formulations are harder on liners than others. They burn at different temperatures and different rates. The oxidizer balance may be different. For instance, in a hybrid design, it may be possible for the liner to be exposed to concentrated hot oxidizer. In that case, chemical reactivity comes into play. This is a reason that hybrids try to use the fuel as the primary liner.

Liners are a pretty integral part of designing a motor.

Gerald
 
G_T said:
It gets into research motor stuff, but it isn't propellant, so...

The inside of a rocket motor, when burning, is like a high pressure high temperature super torch. You do not want bare metal exposed to this for any appreciable time. Aluminum, such as in our motor cases, melts at around 660 degrees C and will lose its temper at a much lower temperature than that. When it loses its temper, it will be perhaps a third as strong. Even just being a few hundred degrees F above room temperature costs about half the strength in use. So you want to keep the temperature rise of the case to only a couple hundred degrees F - that is unless you want to possibly bulge the case / CATO, or have it as single use. Thicker case walls help because they have more thermal mass. That is one of the reasons why larger research motors often have fairly thick walls.

A smoke grain is like a little torch sitting in there for a fair number of additional seconds. So the addition of a smake grain means the liner has to work for a longer period of time.

Some propellant formulations are harder on liners than others. They burn at different temperatures and different rates. The oxidizer balance may be different. For instance, in a hybrid design, it may be possible for the liner to be exposed to concentrated hot oxidizer. In that case, chemical reactivity comes into play. This is a reason that hybrids try to use the fuel as the primary liner.

Liners are a pretty integral part of designing a motor.

Gerald
Click to expand...

Thanks for the post Gerald
 
Research motors have thick walls for one of two reasons:

a. They are a convenient size that works with other parts.
b. They are made of inferior material that needs to be thick to withstand reasonable pressure.

Thickening the case is no a very good way to combat thermal issues - a properly designed liner is far superior from a mass and heat transfer perspective.

Heat transfer is driven by pressure and temperature.

Properly designed solids will use propellant to mitigate heat transfer to the case. They act as a liner for some or most of the burn.
 
Agreed, however, I still think it is one of the reasons it is done. Not the best reason perhaps, but, have you sourced and priced good liners?

My current EX motor crop is leaning towards cases where I can get cheap usable liners. But not those lousy cardboard burn-thru specials, nor the spiral wound insufficient phenolic content burn-thru specials. I'm always happy to hear about better options for liners! Anyway that's why I'm playing with 88mm and 115mm. Affordable available marginally acceptible liners.

Heck, I toasted a medium quality phenolic liner to shreds with a little dinky four grain dual thrust 38mm static test. It was a moment from tossing the whole thing out the nozzle when it ran out of fuel. Large area burn thru on 76mm L static test with spiral phenolic and glued grains. Etc.

The GOOD phenolic liners are hard to find and harder to afford. They can cost more than my fuel.

If I'm aiming for a high performance flight somewhere with the waiver for it, then I'll happily use a minimalistic case designed specifically for the load, and be willing to consider it single use. Hopefully someday I get to go somewhere with a high waiver! That would be fun! But where I fly in the east, case weight isn't a minus as it helps keep the rocket down lower.

Besides, take that 6061 or 7075 case and heat it a few hundred degrees (by 475F anyway - higher than what we are supposed to be achieving by the rules and regs) and all its superior strength is pretty much gone. There goes the margin for using a thin walled case that could otherwise take the pressure and hold the snap rings or other retention. How hot does the case get with liner burn-thru and a long burn full diameter smoke grain?

Gerald

PS - BTW, bates grains do not allow completely using fuel as an insulator. Star, finocyl, some others, sure.
 
Liners are mostly insulators as posted, some are ablators. Some motors even use 2 liners, plus a casting tube.

There are people who make thier own liners..

I had a motor that took an abnormaly long ammount of time to start. So the ends burned longer than anticipated.
(glad my liner did its job.. insulating, and ablating)

PIC 018.jpgPIC 042.jpgPIC 044.jpg
 
daveyfire said:
Quick, someone call Frank and tell him this won't work.

<img src="https://www.rocketryforum.com/attachment.php?attachmentid=93692"/><img src="https://www.rocketryforum.com/attachment.php?attachmentid=93693"/>
Click to expand...

I think it's safe to say that Frank knew a few things more about making motors than most people who do so
 
No disagreement on Frank, but since you brought it up, would you kindly explain how the fuel in bates grains act as complete case insulators??? I'm thinking your point is that a case can survive it in at least some instances. That is not a counterpoint to what I posted. However, you might convince people to skip the liners, and is that really the point you want to be making?

Thank you,
Gerald
 
G_T said:
No disagreement on Frank, but since you brought it up, would you kindly explain how the fuel in bates grains act as complete case insulators??? I'm thinking your point is that a case can survive it in at least some instances. That is not a counterpoint to what I posted. However, you might convince people to skip the liners, and is that really the point you want to be making?
Click to expand...

I think his point is that Yes, it is possible to make motors without liners.

But, at the same time, I think you really need to know what you're doing to attempt it.

-Kevin
 
Almost all high performance professional motors have steel motor casings with case bonded propellant. The casing is lined with HTPB or PBAN rubber without an oxidizer and the propellant is cast in place and bonds to the rubber liner. Rubber is a thermal insulaor to begin with, and will pyrolize and form a graphitic char which is an even better high temperature insulator. This prevent the casing from being heated and destroyed by the blow torch hot gas inside the motor casing.

Bob.
 
G_T said:
No disagreement on Frank, but since you brought it up, would you kindly explain how the fuel in bates grains act as complete case insulators???
Click to expand...
Kevin's got it. The exception that proves the rule, maybe.

(It's propellant, BTW. Not fuel. Unless we're talking hybrids or biprops.)
 
bobkrech said:
Almost all high performance professional motors have steel motor casings with case bonded propellant. The casing is lined with HTPB or PBAN rubber without an oxidizer and the propellant is cast in place and bonds to the rubber liner. Rubber is a thermal insulaor to begin with, and will pyrolize and form a graphitic char which is an even better high temperature insulator. This prevent the casing from being heated and destroyed by the blow torch hot gas inside the motor casing.

Bob.
Click to expand...


Just to be clear, the HTPB (there are other things in there) liner is not acting as the primary insulation in most cases which can be made of other materials than steel.

G_T

6061 is the material I was referring to as inferior actually. 7075 is a different story but good luck finding a case made out of it. I was trying to point out that mitigating heat transfer to the casing was the primary objective of a properly designed liner, not that mechanical properties didn't change with temperature. And yes, I have sourced, priced, and purchased various types of liners...and I understand the limitations of the east coast, I've flown there my whole life.
 
"Most" case-bonded motors are lined with aramide filled EDPM bonded to their steel cases.
We (TRA) only get to use 6061-T6 .... no steel, no 7075...
 
FredA said:
"Most" case-bonded motors are lined with aramide filled EDPM bonded to their steel cases.
We (TRA) only get to use 6061-T6 .... no steel, no 7075...
Click to expand...

I dont think the Temper/treatment is regulated by TRA... Just the alloy.

Also, we get to use composite cases FG, and Carbon... (edit..)
 
troj said:
I think his point is that Yes, it is possible to make motors without liners.

But, at the same time, I think you really need to know what you're doing to attempt it.

-Kevin
Click to expand...


So basically, line your motor as an insulator if your in the slightest not sure. If you don't line it then the first burn might be ok as a single use motor but the heat will undermine the integrity and strength of the casing.

Correct me if I'm wrong.....

The 38mm Gorilla motor that I used was a 320n motor and I used a cardboard tube as a liner and it was totally burnt out after the first burn of the motor. There was no discolouring of the casing or any signs of problems with the case after and it was a nice 1.5 second burn of Sugar propellent. The second burn was a cato and the case was totally destroyed beyond recognition. It sounds like the liner was my problem plus a grain had 2 cracks in it when I loaded it. If someone could confirm what I suspect then I'd be grateful. Bad liner + 2 cracks in grain= BAD IDEA :confused:
 
>> The casing is lined with HTPB or PBAN rubber without an oxidizer
>> and the propellant is cast in place and bonds to the rubber liner.

EPDM is cheap and works well in amateur motors using this design.

Another alternative is to use beefy casting tubes, forget the liner, put O-rings in between the grain segments, and place the whole grain/O-ring system under a fair amount of compressive stress.
 
north boy said:
Bad liner + 2 cracks in grain= BAD IDEA :confused:
Click to expand...

I'll join the chorus here - you could have had a perfect liner, and it would have still failed. Cracks in grain = bad idea, regardless of other factors.
 
"I dont think the Temper/treatment is regulated by TRA... Just the alloy."

You are right - NFPA, not TRA, spec's that 6061-T6 is the only metal approved.
But since TRA follows NFPA....6061-T6 is it....
 
rockets4kids said:
Does NFPA (and by corollary NAR and Tripoli) really not allow 2024 and 7075 alloy motors?
Click to expand...

manufacturers and Certification process for the motors are bound by NFPA by our agreement.

While the choice for an amature rocket motor, is not bound by nfpa. Tripoli's research code only restricts particular materials from motor making. This means it its not specificaly exlcuded, its is allowed.
your mileage will vary in respect to local club law. Motors made by amatures, and flown under the Tripoli RSC, are not held to NFPA motor manufacturing code.

where i fly most of the time 2024 is thought to be too brittle and thus frangible, so i wont waste my time using it.
 

Similar threads

watheyak
Pro98 Nozzle Not Fully Seated in Liner
Replies
8
Views
557
watheyak
watheyak
prfesser
Sold Green Monkey Aerospace, 18mm research motor set, never used, $275 OBO
Replies
5
Views
691
prfesser
prfesser
c0c0m0ke
3D Printing Exploring 3D printing, questions
Replies
7
Views
202
Bravo52
Bravo52
prfesser
Sold 76 mm 2-grain Loki motor case w/nozzle, $245 OBO (read description!)
Replies
0
Views
530
prfesser
prfesser
L
What grease do you use for assembly?
2
Replies
35
Views
4K
High Desert Rocketry
High Desert Rocketry

Latest posts

Back
Top