Of Liners and Grain Geometrys

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edwardw

Well-Known Member
Okay, I have a few ideas and was wondering what you all thought about them. First...

I have an odd shaped motor casing (1.75" ID) and needed to make some liners for it. I have a nice 1.5 mandrel. That means if I make the casting tube and liner from it both will have ~1/16" walls. More than enough for me. But also 1/16" walls are a lot of built up layers of gummed kraft paper. This is my attack plan... I'm going to do a three layer wrap of kraft paper. Gives me a nice solid tube. That doesn't equal 1/16" yet. So then I'm going to fiberglass that in 2 2oz layers. That gives my my 1/16" that I need. Now I have my casting tube acting as a mandrel for the liner. 1 2 oz and 1 6oz layer then would make up the liner for the motor.

I'm just wonder how well the fiberglass would hold up as a liner material. I know fiberglass by itself is very heat resistant, but the epoxy will burn - there are epoxy based propellants.

Next subject - grain geometry. I have been thinking about an easier way to 'throttle' a motor other than a finocyl grain. I'm thinking of something between a core burner (progressive) and BATES. It's a core burner that turns into a BATES part way through the burn. First, let's start off with our casting tube, full of propellant with a standard core in it. Next, we measure out how many BATES grains we want. We take this to the saw and score the casting tube approximately 1/2 the thickness of the 'web' of propellant. When you ignite the grain you get a core burner at the start - then when enough propellant burns it will burn through the web and begin to burn the ends of the grains just like a BATES would. I've been doing some Kn modeling and so far I get a steep startup curve then it it flattens quickly and the burn ends. Any ideas, thoughts or things I should watch out for?
The attached picture is to help with visualizing the geometry.

Edward

interesting idea although I think the pressuer might blow through the gaps, either way your burn still will be progressive because your grains will be real long

would the sudden change/jump in pressure be a problem?

it could be. I mean you are burning a progressive motor, but suddenly you break through and start burning at the ends of the grain. This would be the same thing as having a bates grain and opening the core way up. (increasing KN alot) This increased core size would increas pressure a ton and could very possible blow up your motor

Originally posted by edwardw
When you ignite the grain you get a core burner at the start - then when enough propellant burns it will burn through the web and begin to burn the ends of the grains just like a BATES would. I've been doing some Kn modeling and so far I get a steep startup curve then it it flattens quickly and the burn ends. Any ideas, thoughts or things I should watch out for?
The attached picture is to help with visualizing the geometry.

Edward

It's going to be hard to keep the exposed propellant surfaces in the slots from igniting prematurely. If it does, kaboom!

I'm not sure that I see the point anyway. This configuration will give you a lower initial thrust than a normal Bates geometry. Why would you want to start with low thrust?

- Jeff Taylor

I was just thinking of how such a grain would react. I was thinking it would be hard to keep it from lighting and toasting the liner. Just thinking out loud really. Talked to a friend and we wondered how it would react.

What does everyone think about the fiberglass liners?

Edward

it's my opinion that fiberglass liners are overkill...sure they'll work but they aren't nescessary unless you're making moon burners or D grains, where certain spots of the liner will be subject to heat for longer durations.

and not to mention more expensive and time consuming, and othewise a giant pain in the rear for something that is just going to burn up...

Well, the reason I wanted to fiberglass was because of time. It would take me longer to build up the layers of regular paper than to do three layers of paper then glass them. The cost in money isn't to much more. And, for the extra protection I thought I might give it a try.

Edward

Originally posted by DPatell
it's my opinion that fiberglass liners are overkill...sure they'll work but they aren't nescessary unless you're making moon burners or D grains, where certain spots of the liner will be subject to heat for longer durations.

and not to mention more expensive and time consuming, and othewise a giant pain in the rear for something that is just going to burn up...

I dont think so at all. In some cases FG liners can be used, some propellants, such as AMW's Blue Baboon, burn extremly hot, they use pure phenolic but a FG liner may work well.

I know someone who makes carbon fiber liners. Why does he take the time? Because it is actually the opposite of everything you just mentioned, it is less expensive, less time consuming, easy, and it doesnt burn up, you can reuse it.

Originally posted by Ryan S.
I know someone who makes carbon fiber liners. Why does he take the time? Because it is actually the opposite of everything you just mentioned, it is less expensive, less time consuming, easy, and it doesnt burn up, you can reuse it.

Carbon fiber liners, eh? Sounds interesting. Isn't it a bit of a hassle to clean it after flights, though?

Originally posted by Ryan S.
I dont think so at all. In some cases FG liners can be used, some propellants, such as AMW's Blue Baboon, burn extremly hot, they use pure phenolic but a FG liner may work well.

I know someone who makes carbon fiber liners. Why does he take the time? Because it is actually the opposite of everything you just mentioned, it is less expensive, less time consuming, easy, and it doesnt burn up, you can reuse it.

What does he use for a bonding agent (resin)?

Liners made from glass or carbon will be exactly as good as the resin used. Regular epoxy sux for this application.

-JT

Yeah Jeff I was thinking the same thing, I will need to ask him.

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