Launch lug placement

[LV-426]

I have several rockets that call for the launch lug to be glued in the fin fillet. Would it hurt anything if I decided to glue the lug in between the fins instead of in the fillet? I think it looks better centered but if it leads to an unstable take-off I will follow the instructions.

 

[cavecentral]

I do it all the time. 1/8" through 1/2" lugs (back before rails became the standard).

 

[LV-426]

I do it all the time. 1/8" through 1/2" lugs (back before rails became the standard).

Do what all the time? Center the launch lug you mean?

 

[cavecentral]

Do what all the time? Center the launch lug you mean?

Yes - between the fins.

 

[LV-426]

Good to know, I wonder if Estes tells you to glue it in the fillet just to make it easy for the builder?

 

[cavecentral]

Beats me. Just glue it down and add a little more on each side after it dries.

 

[TopRamen]

Mine are always centered, or slightly off-centered if I butcher the Job, but I've never attached them to the Fin Fillets.

 

[JStarStar]

There is some school of thought that gluing the launch lug in one of the fin joints and then applying fillets to each side of the fin/lug joint reduces drag to some extent by reducing the number of 'drag objects' your rocket presents to the airflow.

But at best the effect of this would be minimal -- well below the allowable 10% variation in motor performance -- so any improvement in performance you might realize from this would be barely noticeable.

The most important thing about the launch lug is to be sure it is mounted absoutely parallel to the long axis of the body tube (through which the motor thrust is transmitted to the airframe) so when your motor ignites and starts pushing the rocket up the launch rod, the launch lug is not trying to get it to go in any other direction besides straight up.

Having the launch lug misaligned ("crooked") by only a degree or so can be barely noticeable to the naked eye, but can have disastrous effects by causing the rocket to travel at an angle to its thrust direction, which can result in friction, binding or snagging of the launch lug (especially on multi-piece launch rods with joints) or the launch rod itself bending or whipping, resulting in the rocket taking off at unforeseen and usually undesirable angles when it leaves the launch rod.

And if the launch lug is REALLY on crooked, once the rocket generates significant airspeed, that lug is going to act as a mini-tube fin/rudder/air vent causing the rocket to go even farther off course (or go into a spin).

(Although if you've managed to glue the lug on so far out of whack this effect became substantial, odds are real good you probably got one or more fins on cockeyed as well so the lug may be the least of your worries.)
:bang: :surprised:


So Reader's Digest condensed version: It is fine to mount the launch lug in other locations besides in the fin fillet joint, but wherever you put it, make sure it is on straight.

 

[JStarStar]

One brief addendum: When mounting a launch lug in the fin fillet joint, you need to be extra careful to make sure no bubbles or tiny glue drops form which might impede the lug sliding easily over the launch rod.

I remember building a rocket once in which a tiny bubble in the fin fillet (beneath the launch lug) produced a bump which rubbed on the launch rod when I tried to slide it onto the launch rod and made it a 'snug fit.' I had to scrape/sand down the fillet to get rid of the glue bump.

:gavel:

 

[hornet driver]

There are a few advantages to mounting them at the fin base--1-The fins are already aligned so you don't have to worry about it being straight--2-There is a litttle more surface area to grab on to--3-It does cause a bit less parasite drag in this location. All that said I mount mine centered between the fins, just looks more balanced to me. Of course most of the time I try to bury them inside the rocket or at least hide them since they bug me and most of my stuff is off the wall scratch anyway.I do try to get them as close to the centerline as possible. EXAMPLE----

 

[mwtoelle]

For most of my builds, I put the launch lug in a fin-body joint to reduce drag. Personally, I think the rocket looks cleaner with the lug in a fin-body joint. Another plus of this location is that the rocket can be flown from a tower launcher.

 

[Micromeister]

Ditto:
I almost always but the lugs in the fin/body joints for the same reasons already mentioned Just gives a much Cleaner look, particularly when cutting the launch lug pair down to minimal length size 1/4 to 1/2 inch max. length. Placing the smaller piece with the aft end of the lug on the lift-off GC Location and the longer with the forward edge just hidden by the leading edge of the fin.

 

[Larry Curcio]

I have several rockets that call for the launch lug to be glued in the fin fillet. Would it hurt anything if I decided to glue the lug in between the fins instead of in the fillet? I think it looks better centered but if it leads to an unstable take-off I will follow the instructions.

FWIW, this probably goes back to Mandell, Caporaso and Bengen's _Topics in Advanced Model Rocketry_. They put forth the notion that the launch lug was a lot more draggy than it appeared, and suggested it be put into the fillet because the fillet already causes turbulence. The marginal penalty was smaller.

 

[CRC]

Of course most of the time I try to bury them inside the rocket or at least hide them since they bug me and most of my stuff is off the wall scratch anyway.I do try to get them as close to the centerline as possible. EXAMPLE--View attachment 161325-View attachment 161326-View attachment 161327

That is guiness!! I'm going to "bury" the next rocket I build, great idea & job!