- This topic has 32 replies, 12 voices, and was last updated 14 years, 10 months ago by
Bruce R. Schaefer.
Bruce R. Schaefer.-
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Bret Packard
What method do you guys prefer for high performance designs of 4″ diameter or more? My preferred method is to use a kevlar strap and epoxy each end on opposite sides of the motor mount tube. The strap is long enough that it can extend a few feet out of the airframe, reducing the risk of zippers by increasing the surface area, then I can attach whatever type of shock cord I want.
For context, I am asking about methods that can be used where there is no more than 1/2″ between the motor tube and the airframe.
John A. Wilke
Epoxying Kevlar is NOT a good idea. It gets brittle, and it *will* break eventually.
I always tie into the motor’s forward closure, and I go out of my way to design my rockets to allow for that.
If that is not an option, I can show you some other options (much easier to show vs. tell in a post like this) but I would NOT use epoxy on Kevlar. it will eventually fail.
BTW, tying into the motor also is a good backup for not losing your motor…
Warren B. Musselman
I agree with John on this. ANY fabric webbing epoxied to the motor mount will eventually fail – usually sooner rather than later. At that point, the rocket becomes unflyable. I have a PML Io that I’ve been flying since before I got my L1 in 2003 and it has close to 60 flights on it all on motor ejection and the webbing below the piston is so brittle I don’t think it will take another flight.
Best way is to use a eyebolt on the motor casing for reloadables or have a completely zipperless design with an eyebolt or eyenut on the zipperless bulkhead.
Warren
Bret Packard
I planned to use an eyebolt threaded to the motor closure as a back up, but I didn’t think having a single attachment point was appropriate for a large rocket.
Warren B. Musselman
Do it all the time on rockets considerably heavier than you can get with a 4″ bird. The UprOar project (8″ O-powered) used a zipperless design with a 1/2″ U-bolt through the zipperless bulkhead.
Bret Packard
That’s good to know. Thanks Warren. Can you get threaded closures for the Pro54, Pro75 and Pro98 hardware?
Jeffrey Joe Hinton
Only the Long Burn Pro54 motors have a threaded forward closure (1/4-20) but that should not prevent you from a simple modification on your own providing you aren’t using the ejection charge. The Pro 75 and Pro98 forward closures are threaded for eyebolts – 3/8-16ths
Warren B. Musselman
For another point to add, I have epoxied 1/4-20 eyebolts into Pro-38 ejection wells after removing the ejection charge. Rough up the inside of the plastic well with 80 grit sandpaper and they hold very well. I’m not sure you could do the same with a Pro-54. Also, I’ve done that on some of my AT forward closures – 29mm (need 3/16″ eyebolts for that), 38mm and 54mm. I almost always fly with electronics, so it’s no loss to me. I do have regular and plugged ejection wells as well.
The epoxied-in eyebolts are removable from the aluminum forward closures if you heat the closure up enough for the epoxy to start bubbling.
Warren
Dave Smith
wow, this is an interesting thread! I have my L1 cert in my pocket and continue to feel a little gun shy on the L2.
What would really help me out is to find a willing experienced flyer who can let me pick his brain.
I am in Longmont and can bring the buttermilk, beer, or bourbon!I am getting the 4″ Nike Smoke from John and am anxious to do it by learning in advance from other people’s mistakes, not by my own.
Is there anyone in Longmont who would be fine with me invading your personal VAB?Warren B. Musselman
I’m in Longmont most days, be glad to shoot the shit – no need for the beer or bourbon.
Bret Packard
John A. Wilke
Warren B. Musselman
Jeffrey Joe Hinton
Dave Smith
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