Launch lug placement
 

I've done a little bit of looking around for this info, but I knew you guys would do as a 'last resort'... :D

Concerning launch lugs: What is the criteria for where to place (a) launch lug(s) on a particular rocket?

Since I've been building so many original designs lately (more are forthcoming), I figured I'd better get on the ball and at least try to knock that consideration off the 'WHOOPS!' list.

Thanks again!

You want two short lugs, placed reasonably far apart. I usually put one all the way aft, and another somewhere between half way up and 3/4 of the way up. Using two lugs avoids the chance of a single lug binding on the rod in a cross wind.

Plus, two lugs is better when using an undersized rod. For example, I've flown 3/16" and 1/4" lugs off smaller diameter rods. As long as the lugs are fairly far apart, the rocket can't get crossways with the rod - it'll stay perfectly parallel. That way, in a pinch, you can fly with a mismatched rod.

And, lastly, bevel the ends of the lugs. It looks so much better :D

Here's a shot showing the lugs on my Cherokee-3D. In this case, the top leg is shy of half way because the rocket separates just above the top lug. In such cases, I prefer to keep the lugs on the same section of rocket. That way, I only need to line them up one time, and not every time I fly it ;) Doug



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Thoretically, a single lug is placed at the CG. Double lugs, one at the rear and the other near the CG.

In reality, you place them where they won't get in the way of your decals. It doesn't matter a whole lot unless you do something crazy like using a single 1/2" lug at the top of the BT.

Sorry I didn't respond earlier. I was almost ready to post my response and got called out on a MVA with injuries. It's slicker'n owl doo doo here. :eek:

Kinda like this? :p

That's funny, Dave. But was that really due to wind induced torquing on the rod? Or was it just a tight fit? It almost looks like the clothes pin got caught in the motor hook.

Doug

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I concur, besides placing one as far aft as possible, the second one should be near the CG of the rocket or just above.

Top launch lugs intentionally placed at stability points for large popular motors would be a good end user trend to simplify knowing whether a rocket is stable with a particular other motor or not.

Jerry

The clothes pin wasn't caught, it was just along for the ride. Actually, I think it was a combination of the tight fit and a bit of a crosswind. Interesting flight nonetheless....

Not directly related to your question, but I wanted to post it as to see what everyone thinks and who knows it may help. This is an Interceptor that I am putting together now and is the first rocket with multiple lugs I have built. First of all, they call for 2 = ¾” lugs, but only gave me one 1 ¼” lug to work with. So, I cut at 5/8” and will duck the first few launches…

I did not feel confident that I could get the alignment of two lugs perfect, which in my mind adds friction and the potential to bind up upon liftoff. So, I had an old stainless dowel that saved from an accidental broken windshield on my boat. Anything that looks like usable hardware gets saved for a future unknown use, and I just found one. Anyway, I decided to use it as a straightedge as to align the lugs. I guess I could peer through them like one would a scope as to align, but again with my limited rookie-ism wasn’t satisfied and always looking to get jiggy-wit-it; I made it happen.

Is this a bit out of line, or has anyone botched dual lugs?

There are two ways to get them out of line. Assuming you mean they're not on a line parallel to the rocket's axis, if it's a little off, that won't matter much. As soon the rocket clears the rod, it will correct itself, with maybe only a slight drift having been imparted.

The other possible misalignment is the lugs not being coaxial. If they're a bit askew of each other, you'll pick up lots of drag on the rod which can kill your rod speed.

The dowel you used helps a lot. I similarly use a piece of launch rod. But I add some wraps of masking tape to the rod to make it fit snugly in the lugs. I am careful to not overlap these tape wraps. That is, I apply one helix of tape with no overlaps, then another, then another, etc, until the rod is very snug in the lugs. This way, the lugs are forced to be perfectly coaxial to the rod - all the slack is removed.

If you overlap the wraps, the built up section will tend to have a fat spot in the middle. The lug can still get misaligned on that. By avoiding the overlaps, you get a perfectly cylindrical build up.

Yours looks fine in the pic, so I wouldn't worry about it.



Doug

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That's a long popular notion. And hence the commonly accepted answer. But, in the most gentlemanly way, I question the benefit :)

If I'm loading a heavy HPR, with a single lug, on a horizontal rod, I see some upside. With the weight balanced on the CG, the rocket won't torque itself and possibly break the lug off. But once it's erected, there's very little load on the lug unless I get lots of crosswind. And in that case, I'd prefer to have the single lug at the CP. That way, the cross loading should be even on either end of the lug, and the rocket should stay vertical without the lug binding on the rod.

At least, when I think about, that's what I come up with.
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In the case of a single lug, I tend to favor making them longer to minimize binding. But users should keep in mind that the longer the lug, the easier it is to get a curve in it :) That can definitely add some undesirable friction.

Doug

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