So I have this 6" aluminum tube ...

I'm trying to add a composite fin can to the bottom of this tube. The first step was to roll a ring around the tube, and then add fins and tip-to-tip support. Guess what ... the ring won't come off. I can slide it about 3" but that's it. I think it would be possible to winch it off pretty easily, but I need a way to rig a harness on the ring so that I can pull on it. I also need to have the rigging so that it won't damage the ring as it comes off of the aluminum tube - the ring isn't very thick. Ideas appreciated.

Jim

freeze it?

leave it on and fill with propellant

The rocket gods are telling you something!

If the ring is composite and you could cool the aluminum tube enough, it might just come off.

If freezing in a standard freezer doesn't get it done I can chill it with liquid nitrogen but you would need to bring it to Houston, aluminum has a fairly large CTE compared to your composite.

Random Flying Object said:

If freezing in a standard freezer doesn't get it done I can chill it with liquid nitrogen but you would need to bring it to Houston, aluminum has a fairly large CTE compared to your composite.

Click to expand...

The tube is the motor, and I was wondering if expansion might be a problem going the other way (i.e., when it gets hot). Maybe the rocket gods are trying to tell me this isn't a good idea?

Jim

Obviously too big to fit in a freezer, and liquid nitrogen is scary stuff. Maybe try bagged ice, inside the tube?

JimJarvis50 said:

The tube is the motor, and I was wondering if expansion might be a problem going the other way (i.e., when it gets hot). Maybe the rocket gods are trying to tell me this isn't a good idea?

Jim

Click to expand...

Dry ice might be a safer alternative, pack the inside with dry ice then heat the outer section with a heat gun. Or heat fisrt and then pack with dry ice.

LN2 will do it right away. You could try CO2. A fire extinguishers upside down so you get a liquid cold feed will.

N2O also and most likely to hand with hybrid motors. Keeping it rocket related and all.

Overall something is saying no

Is there a grocery near by with a walk-in deep freeze that will let you chill it for a bit?

JimJarvis50 said:

The tube is the motor, and I was wondering if expansion might be a problem going the other way (i.e., when it gets hot). Maybe the rocket gods are trying to tell me this isn't a good idea?

Jim

Click to expand...

Jim, if you are worried about it that way I suppose a static test is the solution. I would do that before adding fins etc to make sure you do not have an expansion problem. What is the burn time of the motor projected to be?

A six inch motor as a booster for the rest of the three stage stack? Replace the wimpy N5800😀

markkoelsch said:

Jim, if you are worried about it that way I suppose a static test is the solution. I would do that before adding fins etc to make sure you do not have an expansion problem. What is the burn time of the motor projected to be?

A six inch motor as a booster for the rest of the three stage stack? Replace the wimpy N5800��

Click to expand...

Just a two stage. Could be a fairly long burn, we actually need to speed it up a bit. The fins just need to work once and don't have to survive landing.

I have a launch today, but I think dry ice is the thing to try.

Jim

John Coker has an article on his site about the two stage that used the Pegasus fin motors. I'm pretty sure he wound up using LN2.

Btw: Did you use a wrap of Mylar or did you lay it up directly on the motor?

Six inch to six inch? What epoxy are you using for the fin can? An engine like that could get pretty hot.

JimJarvis50 said:

The tube is the motor, and I was wondering if expansion might be a problem going the other way (i.e., when it gets hot). Maybe the rocket gods are trying to tell me this isn't a good idea?

Jim

Click to expand...

I am using the same sort of design on my new 5" tail section for Spikey 5. Problems will probably depend on burn time. Are you depending on a bond between the composite and metal motor tube? If you use dry ice make sure to crush it up and move it around, don't let it just sit in one little spot. If you use LN2 let it run the down the inside of the tube and slowly spin the tube. If you use Nitrous, about half way through the process your not going to care anyhow....

Just as I was getting ready to leave to buy dry ice, I decided to try a bag of wet ice. The ring pulled right off.

The flight will be 6" to 4". The fins on the 6" would replace a 25# aluminum fin can - can't fly that! - with a maximum speed around Mach 1.6. The fins just need to survive the motor burn (i.e., single use). The ring was made with aeropoxy and mylar around the motor case.

Thanks for the ideas (and any additional ideas/comments).

Jim

Yea!

Nice going!

Jim,

I'd recommend spiral wrapping some parchment paper first and then add your mylar over it. The parchment paper will aid in removal as it makes it slide.

Saved your self some money

ChrisAttebery said:

John Coker has an article on his site about the two stage that used the Pegasus fin motors. I'm pretty sure he wound up using LN2.

Click to expand...

I tried that (partly for the fun of it), but packing the tube with dry ice worked better. After the first part, it got much easier and successive tubes slid off without so much effort.

The rocket was my Nike-Asp and I built 5" ID tube on an aluminum mandrel.