Launch Lugs - How long is too long?

I have a 12in piece of 9/16" brass tubing that I'm going to use for a launch lug on my scratch-built Hermes rocket. The Hermes will be from 7 to 9 ft long when complete and 4" diameter for the most part. Is a 12" launch lug too long? I'd rather not have to cut it into 2 lugs because then you have to worry about getting them lined up just right. What does everyone think?

Give up, Mark.

On a rocket that may turn out to be 9 feet long you need two launch lugs separated by probably 3 or 4 feet. (I'm sure some expert will have a formula for figuring out exactly how much they need to be separated.) But I can tell you from simple experience a single LL isn't going to be enough for that rocket.

Now.... as far as aligning them... I have a method for that which I have used on a number of rockets. You take a solid, straight rod that just fits inside the LL's, and you tape each LL in place the proper distance apart on the rod. Then, you put epoxy on the rocket where the lugs will be glued. Put the rod with the lugs on it on the rocket, make sure it is straight, then tape the rod in place and let the epoxy set. VOILA !!! Perfectly aligned LL's.

Mark, I have a 4 foot piece of 1/2" launch rod that just fits inside of 9/16" LL's. You are welcome to borrow it to use to align your lugs.

Make it 3, on a rocket that size.

The idea of using a rod to align the lugs is perfect.

Space the lugs in a manner that will provide launch-rod stability ahead of and behind the rocket's center of pressure in order to
minimimize the effects of tip-off and rod-whip when the vehicle's force of acceleration tranfers torque to the launch rod.

A high impulse motor would produce better results than one with lower impulse.

Be sure to calculate the rocket's velocity at launch and the length of the rod needed for escape velocity.

Mark, would rail buttons be a better option for the Hermes?

Originally posted by Jase
Mark, would rail buttons be a better option for the Hermes?

Click to expand...

I'm already using rail buttons too. I've got rail buttons on one side and a launch lug on the other side. I want to be able to launch this rocket on either system. Right now, the Hypertek launch equipment at Whitakers hasn't been modified to allow it to be used with a rail. I intend to fix that in the future but, until then, I'll have to use the lugs with hybrids. I can use the buttons with solid propellant motors.

I may be a heretic, but I think a single 12" lug would work fine for that rocket. I have a 5' x 4" rocket (kit) that has a single 4" lug. Works fine. Scale that up and you'd have a 10' rocket with a 8" lug. The 1/2" lug should be Ok for a rocket up to ~15lbs (independent of the lenght).

Other opinions:

Make sure it spans the CG
Launch lugs are great
When using lugs, I think it looks much better to use two and bevel the ends. Alignment really isn't that hard.

Oops, I guess you have to consider the time on the rod also. If the rod is 6 feet long and the lug is 5 feet off the ground then you effectively have a 1 foot rod - not good. If the rod's 6' long and the lug is placed 2' from the base you may be OK, but you'd need to check the exit velocity etc.