Modified motors

[thrownthetowel]

At what point is a motor classed as modified. Putting masking tape on the casing to make a better fit isn't. But what about welding or epoxying bits to the out side of a casing? It is a modification, but not to the actual motor. So is it legal or what? Or is it classed as Ex?

TTT

 

[MarkABrown]

I am by no means a lawyer but, my personal opinion considers the permanency of the modification. In your example, I would consider welding a permanent modification and therefore would make the casing EX from that point forward. Epoxying something onto the case could/should be able to be removed, so, I wouldn't consider it altering the case. The RSO would have the final say.

 

[eugenefl]

Originally posted by MarkABrown
I am by no means a lawyer but, my personal opinion considers the permanency of the modification. In your example, I would consider welding a permanent modification and therefore would make the casing EX from that point forward. Epoxying something onto the case could/should be able to be removed, so, I wouldn't consider it altering the case. The RSO would have the final say.

I also lay down my "not a lawyer" disclaimer here, but since this is kind of a grey area matter I'll pitch my best opinion.

I would say "not an EX modification." The purpose of epoxying or welding something to the exterior of the case, relative to the example of masking tape for retention purposes, would lead me to believe that the modification is done so for retention purposes and not for the purpose of achieving an effect from the actual propellant grain. I would think that so long as the forward and aft ends are not interfered with or modified in any way to alter the flow of thrust or ejection, or make for an unsafe situation, that the physical modification (not alteration) would not make it any different.

If I recall correctly, I remember reading somewhere that people were taking used SU casings and sawing off 1/4" of the tube. They were using the pieces as thrust rings and epoxying them to the aft ends of the unused SU motors.

Thrown, welcome to TRF. For the purposes of this discussion, why not clarify what it is you are trying to accomplish? Interesting subject.

 

[rstaff3]

First, welcome to TRF!

Two points. First I remember hearing that NAR ruled that epoxying a thrust ring to a SU case is not a modification. I don't remember whether I heard this from the horse's mouth or second hand. They did not accept cutting the cases as far as I remember (?) Taking this forward, it makes sense, as this would not modify the performance of the motor. If you want to weld, grind or cut a metal case, this would not hold, as you are changing the physical characteristics and could weaken it.


Second, if it were to render the case uncertified, it would be uncertified, not 'EX'. I mention this because some poeple have wondered if you can modify a certified case to allow a motor to be used at EX events. The answer is no. EX has to do with home made propellants not cases, whether home made or modified.

 

[thrownthetowel]

Hi,
Thanks for the welcome. The weld (probably end up being epoxy) is for rention purposes. The fin can has very limited space aft of the motors, and I need a way of retaining the motor from the top. I saw posts about putting tape around the motor and pushing it down through the booster, but in my case this would not work. There will be a type of funnel above the motors to channel all of the ejection charges into one area.
I considered a weld because of its strength but the thought that welds (in fact any kind of heat treatment) can distort thin metal had crossed my mind and this is why I suggested epoxy. The case would in effect become a permanent part of the rocket, but the reload system would still work as intended. I understand what you're saying about the permanentness of any modification, so.........
If I epoxy bits to the case and place it in the booster, which can be taken apart at a later date (with a selection of power tools) and the bits pulled off.... Would that be legal? Thats my interpretation of what you are saying.

 

[rstaff3]

What I am saying is that I think epoxying things to the motor case is allowed in the safety code. This would have to be confirmed with an emal to the NAR. I am also saying the welding is not allowed since it is a mod to the motor design.

 

[shockwaveriderz]

https://nar.org/SandT/releases/r63.html

seems legal according to this

 

[thrownthetowel]

Ahh, brilliant. That clears things up nicely.
Thanks for your help.

TTT

 

[rstaff3]

Thanks shockie, I thought I'd seen that but couldn't find the reference. BTW it is not legal for contests.

 

[JDcluster]

Just as a general rule I'd go with: anything that has been changed internally; nozzle , liner or the propellant itself.



JD

 

[SwingWing]

I dunno how it relates to rules, but a casing is a pressure vessel and welding on a pressure vessel might not be a good idea.

 

[rstaff3]

Yup, a bad idea.

 

[rstaff3]

Regarding my second point in my initial post - I guess I was unclear. Of course you can make your own cases, etc for EX. However, you can not merely modify a case and use commercial or someone elses' propellant (team efforts not withstanding).

 

[thrownthetowel]

Hi all,
After thinking about the actual loads on the case, the epoxy wouldn't be string enough, and a weld is obviously out of the question, so it looks like I'll have to change my design a little.
Thanks for all your help though.

TTT

 

[efenton]

Maybe a clamp on ring?