Placement of Launch Lug

[ApolloMaker]

Howdy All --- have been working my way through Stien's great book and could not find any definitive method to the placement of single launch lugs. From what I could tell, the CG is key in this but could not verify since data there would suggest that the general geometry of the rocket has a large influence on it as well.

The particular application I am working on is a relatively heavy BT-55 kit bash that is 40 inches long and will be powered by as much as an E class motor. The fins will have some drag and the RockSim stability analysis gives a factor of 9. This rocket will carry payloads and speed is not the top priority. I want to launch on a 1/4" rod.

Any veterans have any general rules of thumb relating to optimal placement???

Thanks -- Bob --

 

[shreadvector]

https://www.rocketryforum.com/showthread.php?t=8525&highlight=location

 

[ApolloMaker]

Hi Fred --- thanks for the feedback. I read through the thread and did pick up some good info and reflection on experience. About the only solid evidence, which makes full sense, is keeping the launch lug(s) behind the CG. There still seems to be a broad opinion about the use of single versus multiple lugs and the pros and cons of each method.

It is certainly possible beyond this, I might be worrying too much about it. Looks like staying behind (below) the CG and keeping the friction minimized should provide for a stable, clean launch. But I will keep reading a bit and taking in some more opinion.

Many Thanks -- Bob --

 

[JRThro]

Bob,

On such a relatively long rocket, I'd probably use two launch lugs to keep any twisting/bending forces from hanging the rocket up on a single lug.

Also, if you're saying that Rocksim tells you that the rocket has 9 calibers of stability, that is extremely overstable. Your rocket will be *very* prone to weathercocking if there's much wind at all.

Ways to reduce the overstability would be to add nose weight, add another set of fins ahead of the CG, reduce the size and/or number of fins, or move the fins forward. I've used all 3 methods, but in your case, the last one probably won't help much unless you move the fins forward by quite a lot.

Generally speaking, long rockets will be overstable unless you reduce the stability somehow.

 

[MarkII]

John, I think that adding nose weight would accentuate the overstability, no reduce it. (Adding more to the stability margin.) A 40" long, 1.325" diameter rocket is going to be overstable no matter what Bob does, but plenty of popular rockets are designed like that and they fly just fine. The risk of weathercocking is greater with lower launch speeds, so Bob should use a motor that gives his rocket a good strong boost off the pad. I agree with you about using a pair of lugs. I believe that unless you use a very long bottom lug (and no one ever does that for quite understandable reasons), then the effective launch rod length is the distance from the bottom of the upper lug to the top of the rod. Once the upper lug clears the top end of the rod, nearly all of the rod's guidance is gone. You could go with a 3-lug setup, with two small lugs placed some distance apart aft of the CG and one lug up at the top, or else be sure to use a long rod. For practical reasons of stiffness, a rod that is, say, 6 feet in length needs to be at the very least 3/16" in diameter, with 1/4" being even better. If the rocket is very lightweight, it might be launching on a C motor, but because of the length of the rod, you will have to use larger lugs.

MarkII

 

[ApolloMaker]

Howdy Guys --- first, thanks for the comments and feedback.

Yes, the rocket as laid out presently is plenty overstable but should fly well as such with the caveat that it will be weathercocking sensitive as opposed to one that is "less stable" --- and I do agree with the assertion that a fast liftoff would be in line to minimize any wind effects.

Mark is right in that adding weight to the nose will move the CG forward and increase the stability factor. I doubt I would fly the rocket at this length and it is easily shortened -- it is a kitbashed obsolete Estes Converter which has sections which screw together to form its length. I built a longer bottom section to hold a larger (D or E) motor than was originally designed in the kit (maximum C size). Very much still in the tinkering stage and is really intended as a load lifter and test bed for deployment experiments and certainly not for high altitude competition This guy is very subject to changes before its first test flight!!

Regardless two launch lugs are already slated to be added for the reasons you guys stated. As you noted, the pitching moment on a longer, heavier rocket would be too much for a single lug.

Thanks much --- I think I have at least resolved the placement issue for a lug pair.

Thanks Again for the help --- Bob --