Launch lug location

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laming

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How important is the Launch lug location? I know that it can't interfere with the fins hitting the launch rod etc. . I also know that most are located so you see the belly of the rocket. but aside from that does it matter?
 
Close to the CG-CP area, so that there's less leverage trying to rip it off.
And that bit matters a lot.
As well as because of that leverage on the entire rocket, a not-straight lug might have one of its ends drag on the launch rod.
And until recovery time comes, drag is totally not your friend.
 
You will want the launch lug pretty close to the CG of the rocket. If the rocket only has one lug then I tend to place it between the GC and CP points. Longer rockets typically get two lugs. The upper lug will go at or just above the CG and the lower lug will go down toward the bottom. I normally pull it up off the bottom of the body tube about a 1/4 in.
 
How important is the Launch lug location?

Some people are going to say it's critical. I don't think so. As long as it doesn't cause drag or interferance with a body part, the forces acting on a low/mid powered rocket in the first few feet are small. Of course, there are exceptions to this (ie.; monocopter, etc.). I usually put two on, one at the top and one at the bottom of the body tube.
 
On the side, parallel with the long axis of the rocket, somewhat middle-ish. <<- as precise as you need to be 95% of the time for LPR

I have seen some off-set away from the rocket to support a wider section (HoJo, I'm looking at you!)

I have seen them off-set to the inside of the rocket but that is rare and required innovative recovery process (parachutes popped from side pods)

I have seen it once where someone build an internal rail support for a 4' section of 1010 rail for a SatV that launched from a scale pad.
 
On the side, parallel with the long axis of the rocket, somewhat middle-ish. <<- as precise as you need to be 95% of the time for LPR
👍 :bravo:👍
I have seen some off-set away from the rocket to support a wider section (HoJo, I'm looking at you!)
I saw in someone's build thread that he had placed a lug on the face of a fin instead of adding a dedicated stand-off. Genius.
 
👍 :bravo:👍I saw in someone's build thread that he had placed a lug on the face of a fin instead of adding a dedicated stand-off. Genius.

Nothing wrong with that. I've also put them along side a fin root. Would be interestin to use one *as* a fin root!
 
I usually put them alongside fin roots. If the fin is straight it guarantees the lug is straight, and the fillet partially hides it.
I am with Joe on this one. Unless it is a very long rocket, or it is going to be on the pad for a long time (happens at some club launches where the racks are loaded in shifts), I don’t think the CP Location matters much. For the above cases, breezes blowing while rocket is on the rod will cause a bending force if the lug is away from the CP. this can break off the lug while it is sitting on the rod on the pad.

assuming the fins are on straight (if not, you are already in the Kimchi!), putting the lug in the fin root assures it will align with the longitudinal axis of the rocket, will “tack“ quicker since it has more surface area, and will result in both a stronger fin joint AND a stronger lug joint. It’s also one less joint to fillet.

aerodynamically, not sure if it results in more or less drag than an isolated lug separate from the fin joint. maybe @BEC knows that one.

I also like the fin root because it keeps the lug near the tail end of the rocket. Remember, whatever length of rocket is BEHIND the lug is effectively reducing the equivalent “effective” length of the rod. For a long rocket, that can be a substantial fraction of rod length. This can be solved with two lugs, so for long rockets I go with on in The fin root and a second lined up with that one at or near the CP. This allows me to maximize use of the full length of the rod.
 
I don't remember which manuf, but one LPR kit maker had weak lugs. When I glued them on, the lugs flattened as the glue dried (Wood glue, etc.. tends to shrink when curing..) so I had non round lugs & wouldn't fit the launch rod.

I now use a rod to:
  • Ensure alignment between the upper & lower lugs And helps align them to the marked line for the lugs. Slip them onto the rod, add a bit of glue, then roll them into place. Secure [the rod] with a piece of tape.
  • Keeps them round / fit the intended rod.
  • (Also keeps glue out of the lug!)
 
aerodynamically, not sure if it results in more or less drag than an isolated lug separate from the fin joint. maybe @BEC knows that one.

Thanks for the thought, but no. It would take quite a bit to figure this out, and there are so many other variables in flight testing that it would be really difficult to isolate the effects of launch lug placement on performance. Intuitively, if the lug is filleted into the fin and body at that joint, it would seem to me to have a bit less drag, simply because there’s less exposed surface of the lug itself. I expect that in all but extreme cases the effect would be miniscule.

Estes designs tend to have a single lug near the CG, though some use two lugs, and Centuri was fond of the fin-root location. I’ve done so few original designs that weren’t competition models (and so were tower-launched — no lugs at all) that I’ve not formed a strong “this is what I would do” opinion on this. The main thing for me is to not have it binding on the rod, so I’ve been known to actually use longer-than-stock lugs on some of my most-flown models. The case that comes immediately to mind is my beloved Nova Payloaders. I’ve put the longer 2 3/8 inch long lug on recent builds rather than the 1 1/4 inch one that’s been on recent releases of the kit. It just feels less like binding up going onto the rod to me when done that way.
 
I now use a rod to:
  • Ensure alignment between the upper & lower lugs And helps align them to the marked line for the lugs. Slip them onto the rod, add a bit of glue, then roll them into place. Secure [the rod] with a piece of tape.
  • Keeps them round / fit the intended rod.
  • (Also keeps glue out of the lug!)
I use a rod as well when I have two lugs, but in a different process.
  • Attach the lower lug in at a fin root in the usual way.
  • Then insert a rod and use that to align the second lug.
Also, I like to use a slightly oversized rod, or add tape to a standard rod, so it's snugger in the lugs than the actual rod at launch time will be. That ensures even more accurate alignment, and is better at keeping the lug round if there's any need for that.
 
Also, I like to use a slightly oversized rod, or add tape to a standard rod, so it's snugger in the lugs than the actual rod at launch time will be. That ensures even more accurate alignment, and is better at keeping the lug round if there's any need for that.

Love it!! even better! :D
 
This internal lug needed to be slightly offset from the core motor tube to avoid the nose cone/droid dome. (Sputnik style rocket)

img_0675-jpg.330716
 
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I prefer splitting my lugs into 2 pieces, a larger portion near the tail, preferably at the base of fin below the CG and the other smaller portion above the CG, thus acting like one bigger longer lug. I just like the stability on launch - but that's me. As far as aerodynamics, next-to-the-fin is likely "cleaner", but I may be offsetting that small advantage with the other launch lug higher on the body tube sticking out, but altitude competition rockets in the past used similar systems. There will almost always be some turbulence coming off the nose-cone and nose-cone/body-tube interface and "root drag" at the base of the fin so, hard to say how much aero-messiness it's adding but I doubt much unless you're really into altitude advantages by any last foot or meter of altitude to be gained - counter to that the nicer straighter launch definately makes for nicer flights by my experience (a sloppy launch will definitely lose altitude and likely less-than-intended direction of flight) - but that is all my preference without wind-tunnel tests or exact tracking equipment. So, that's my Dos Centavos.
 
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