Kevlar Cord Attachment

[GL-P]

For my most recent rocket design, it will be very near to minimum diameter 24mm. The tube will be 25mm tube. There will be some room around the motor for a shock cord. There will be a boattail on the motor nozzle. I have this plan to glue some kevlar cord around the base of the nozzle and then glue on the boattail.

Any suggestions? The pic below will try to explain it better:

P.S. If it doesn't make sense what I'm trying to describe, just ask!

Thanks in advance!

 

[Mad Rocketeer]

I'll admit it. I looked at the picture but didn't quite grasp it. Looks like you're planning to put a 24mm motor in a 25mm rocket, allow the motor to hang out a bit from the end, and attach a boattail to the end of the motor to reduce the outer surface from the diameter of the motor to the diameter of the nozzle?

If so, seems like the boattail is going to get a lot of heat and fast-moving exhaust gas to deal with. You'd need to make the inside of the boattail solid too, as allowing the gas to expand within the tail cone and then be constricted again would cause a number of problems, including loss of thrust and a tendency to rip the boattail off.

More likely, I'm just misunderstanding the picture though. Tell us more.

 

[DynaSoar]

Originally posted by GL-P
For my most recent rocket design, it will be very near to minimum diameter 24mm. The tube will be 25mm tube. There will be some room around the motor for a shock cord. There will be a boattail on the motor nozzle. I have this plan to glue some kevlar cord around the base of the nozzle and then glue on the boattail.

Any suggestions? The pic below will try to explain it better:

P.S. If it doesn't make sense what I'm trying to describe, just ask!

Thanks in advance!

Gluing the cord to the motor nozzle?
And gluing a boat tail to the motor?
Won't this all make it difficult to replace the motor?

 

[GL-P]

I know it sounds kinda weird.

The solid boattail will only be glued to the motor, not the BT. It will act as a sort of thrust ring.

I'm doing this to allow me to have a fully integrated motor, boattail, shockcord unit so that I can replace the shock cord after each flight. This rocket is not expected to fly more than twice.

It will be a solid boattail of most likely basswood.

I'll admit it. I looked at the picture but didn't quite grasp it. Looks like you're planning to put a 24mm motor in a 25mm rocket, allow the motor to hang out a bit from the end, and attach a boattail to the end of the motor to reduce the outer surface from the diameter of the motor to the diameter of the nozzle?

You almost got it. It will have a kevlar cord running along side the motor to be the shock cord. That's why it's important to note that the tube is bigger than the motor.

I hope this pic is a little more descriptive.

Thanks guys!

Sorry!

 

[Stymye]

is that the actual scale shape of the boat tail ?

 

[GL-P]

is that the actual scale shape of the boat tail ?

No, this pics are not to scale.

Why?

 

[rstaff3]

I'm not sure what the question is. It sounds like you've decided on this unconventional arrangement and its not clear why, so it would be hard to say to do something different.

 

[Stymye]

mabey you could glue the cord to the inside tube wall (loc style), rather than wrap it around the motor where all the heat is.surely it would last 2 flights
I'm also unclear of the reasoning behind the arrangement

the boat tail in the diagram appears too squat to be really effective , thats why I asked about it,, I understand now you were just making a quick example drawing.

interesting idea

 

[GL-P]

Sorry, I've been unclear about my madness

I'm not sure what the question is. It sounds like you've decided on this unconventional arrangement and its not clear why, so it would be hard to say to do something different.

It's a small rocket so the G55 I'm using should really seperate it. I'm just worried about the shock cord attachment.

the boat tail in the diagram appears too squat to be really effective , thats why I asked about it,, I understand now you were just making a quick example drawing.

Yeah, it does look to be squat. remember, it's just the motor nozzle which is 0.25" high and 0.5" diameter.

I figure a squat boattail is better than none.

 

[DynaSoar]

Originally posted by GL-P
I know it sounds kinda weird.

The solid boattail will only be glued to the motor, not the BT. It will act as a sort of thrust ring.

That's a bad thing. If you do anything permanent to the motor that alters its function from the way it comes out of the bag, you're altering the motor. Adding a structural component (a thrust ring) is altering it from the original design.

#4 from the CAR HPR rules: "I will not alter a high power rocket motor, its parts, or its ingredients in any way."

It's it's cosmetic/aerodynamic only, and not a real structural component, it might be OK. Look around the house and see if you can't find a CAR certified RSO to consult on the matter. Betcha there's one handy.

 

[rstaff3]

This doesn't answer the question regarding CAR rules, but IIRC NAR ruled that thrust rings could be glued onto motors.

Also, this isn't bad in my book. Maybe not allowed, but not bad.

 

[GL-P]

#4 from the CAR HPR rules: "I will not alter a high power rocket motor, its parts, or its ingredients in any way."

Some inner voice was telling me that . . .

I should listen more

I know of a local CAR RSO who would know.

If I'm allowed to glue on a boattail, I would make absolutely sure not to clog the nozzle!

(Shivers!)

Thanks guys!

 

[Mad Rocketeer]

Madness? Hey! You encroaching on my territory?

Looks like the Kelar is being looped around the space behind the nozzle rather than anything solid. I think the way you'd have to do it is to loop the Kevlar around the motor body itself. The nozzle isn't really externally accessable for wrapping in disposable motors (and would probably be too hot to safely wrap that way, even for Kevlar, on any reloadable where the nozzle sticks out), and the space behind the nozzle is just space.

Like others have said, it would be easier to comment more helpfully if we had a fuller description of the challenge this proposed technique is designed to address.

Thinking outside the box is always in fashion.

 

[GL-P]

Yeah, just some crazy idea I had.

I basically am doing this because my mandrel is bigger than the motor. I have room around a 24mm motor. I was thinking of ways to glue the shockcord in securely yet be able to remove the motor even with a boattail. This rocket is too tiny for screws tho.

I would also act as positive retention instead of tape. These motors can get real hot (I flew a 24mm F like this one) and I was afraid of the tape gumming up.