High Power Launch Lugs?

[gary7]

Give me some reasons not to use launch lugs on an HPR burning an I218. I have an idea for what I think will be a very secure method of attaching the lugs. The loaded rocket will weigh in around 5 3/4 # on the pad. I know that Loc sells all their HPR kits with lugs stock.

 

[dixontj93060]

Your choice of launch guides should be based on what's readily available at your local club in addition to the size/weight of the rocket. If you pick a larger lug, just make sure they have a corresponding rod.

 

[cjl]

Rod whip.

It has nothing to do with how securely the lugs are attached, and everything to do with the stiffness of the guide mechanism. It's true that lugs can work on H and I motors, but they aren't as good as rails. Also, at club launches, there are usually far more rails available than rods.

 

[slogfilet]

To combat rod whip, you'll need to use thicker and thicker rods, which may not be available at your launch site.

Adding larger lugs will increase drag, if performance is a concern. They are also just plain bigger, if aesthetics are a concern.

To best match the size of the launch lugs to your rockets, you may need to use an assortment of rod sizes, which may not be available and/or changing out the rods could be a pain.

On the flip side... a standard launch button for 1010 rails is a one-size-fits-(almost)-all solution for anything smaller than HUGE. Low drag, unobtrusive, and rails are more available at launches than larger diameter rod.

 

[bobkrech]

A simple way of looking at the stiffness in bending (cantilever application)would be to consider the "second moment of area" of the rod. https://www.214m.com/html/Mechanical%20engineering%20other%20topics/20080415/10645.html

These can be calculated as follows: I(rod) = Pi x D^4 /64

In the method of looking at the problem, rod stiffness is proportional to the 4th power of the diameter. If you were to approximate a 1" rail as a 1" rod and then compare it to a 1/2" launch rod, the 1" rod would be stiffer than a 1/2" rod by a factor of 16!

The projection of a 1/4" rail button should be less than a 1" launch rod and is easier to mount, so the rail wins all around.

Bob

 

[MarkH]

To combat rod whip, you'll need to use thicker and thicker rods, which may not be available at your launch site.

Adding larger lugs will increase drag, if performance is a concern. They are also just plain bigger, if aesthetics are a concern.

To best match the size of the launch lugs to your rockets, you may need to use an assortment of rod sizes, which may not be available and/or changing out the rods could be a pain.

On the flip side... a standard launch button for 1010 rails is a one-size-fits-(almost)-all solution for anything smaller than HUGE. Low drag, unobtrusive, and rails are more available at launches than larger diameter rod.

I wouldn't consider a 1010 rail button to be low drag. They add a lot more frontal area than say a 1/4" lug. The point is moot though as someone really looking to reduce drag would would launch from a tower.

 

[cjl]

Mark: I don't know about that - I would guess that a 1010 button and a 1/4" lug add about the same amount of drag, with any bigger lugs (since you'd really want a 3/8 for an I) being higher drag.

 

[llickteig1]

Give me some reasons not to use launch lugs on an HPR burning an I218. I have an idea for what I think will be a very secure method of attaching the lugs. The loaded rocket will weigh in around 5 3/4 # on the pad. I know that Loc sells all their HPR kits with lugs stock.

Hundreds of rocketeers launching thousands of rockets can't be wrong - rail are better for HPR rockets.

In our storage trailer, we have several 3/4" and 1" (maybe even larger) rods from the "way back" days before the idea of using 80/20 rails was conceived. The big boys have been coming to Argonia to fly their rockets for almost 20 years. Fast forward from 1991 to 2010, those gigantic rods rarely if ever see daylight any more. We have something like 5 dozen rails, and they all get used. There's a reason for that - the rails are much more sturdy and less flexible than the rods and work better for rockets from 1# to 500#.

--Lance.
KLOUDBusters

 

[MarkII]

Another point in the rail's favor: you can bolt it to a sturdy mast or truss tower or stake it out with guy-wires to give it even more stability. You can't do either with a rod. Launch rods can only be free-standing.

 

[MattieShoes]

Out of curiosity, what are your opinions on the middle road -- rail lugs rather than buttons?

 

[MarkII]

Out of curiosity, what are your opinions on the middle road -- rail lugs rather than buttons?

Aluminum rail guides are likely to have more friction than Delrin buttons. Or so I've been told.

 

[llickteig1]

Out of curiosity, what are your opinions on the middle road -- rail lugs rather than buttons?

The lug-type rail guides are more difficult to get on straight. Rail buttons are a single pivot point and there are no worries about them being perfectly aligned with each other (if you use just 2 on a rocket). Even if they aren't perfectly alighed on the airframe, they won't cause big problems. Rail guides, on the other hand, have to be absolutely straight on the airframe and perfectly aligned with each other to prevent any binding on the rail.

Rail buttons are also cheaper and the screw connection more rerliable than surface mount, as with most rail guides I have seen.

--Lance.

 

[kramer714]

The size rocket you are building certainly doesn't require a rail, I launch a 5lb rocket on a J350 using a 3/8 rod. My lugs are brass tube bonded directly to the rocket. with PROPER PREP and selection of the right adhesive (DP190!)there isn't a problem.

Having said that, I added rail buttons to the same rocket since where I launch there are more rails available than 3/8 rods. That is the best reason to pick one or the other for a rocket this size.

 

[cjl]

The size rocket you are building certainly doesn't require a rail, I launch a 5lb rocket on a J350 using a 3/8 rod. My lugs are brass tube bonded directly to the rocket. with PROPER PREP and selection of the right adhesive (DP190!)there isn't a problem.

Having said that, I added rail buttons to the same rocket since where I launch there are more rails available than 3/8 rods. That is the best reason to pick one or the other for a rocket this size.

Oh, you can absolutely launch I and J motors off a 3/8" rod. You risk rod whip though (even with a 3/8), and it is certainly not as stiff as a 1010 rail. It's also not as common, meaning you'll have a harder time finding a pad at most club launches.

 

[ScrapDaddy]

Oh, you can absolutely launch I and J motors off a 3/8" rod. You risk rod whip though (even with a 3/8), and it is certainly not as stiff as a 1010 rail. It's also not as common, meaning you'll have a harder time finding a pad at most club launches.

Do HPR clubs generaly have a 15/16 rod or 1/2 rod on hand for that rare occasion?

 

[dave carver]

My stuff has been going off rails from when I started in High Power in '93. At first I used angle iron bolted together with styrene "I" beam modified to be the rail guide. When we went to the first Turkey Shoot in Las Vegas the owner of Black Sky Research, the first with the structural rails, spent some time looking over what we had. This was before Black Sky released the first commercially avaliable rails. On my larger projects I machined aluminum rail shoes after reading an artical in Space Modeling on Wallops Island rockets that used shoes. Friction was no problem, the area where the shoe slid on the steel angle I had rubbed with a wax candle, no gouging, no rubbing. I had also used the paraffin wax inside the shoe, just in case

 

[bobkrech]

At CMASS we have 1/4", 3/8", 1/2" and 1" rails at every high power pad.

While a rail is really the optimum in terms of directional stability, you can use a rod for a 5-3/4 pound rocket. I personally would use a 1" rail, but a 1/2" rod would be ok as well. If the rocket was a minimum diameter rocket, you probably could get away with a 3/8" diameter, but in anything more than a light breeze, the rockets going to be flopping around on a rod.

A stiff rod/rail insures that the ballistic impact zone will be where you want it to be. If there is significant rod whip and you have a non-deployment, your rocket can go almost anywhere if there's rod whip.

Bob

 

[gary7]

Ok, so I could probably use a 3/8 or 1/2 rod if I am sure to take one with me to club launches. Or, I could use rail buttons. My problem with buttons is available space. I have to mount the buttons on a tube that is within a ring tail. I have only 1 3/16 inch clearance to do that. The question then becomes, is there enough room for the rail button and a 1 inch rail to fit inside the ring tail? In other words, what is the absolute minimum distance necessary for the button and rail together from the surface of the rocket? Also, is there a rail smaller than 1 inch available? I can not find it if there is.

 

[bobkrech]

Ok, so I could probably use a 3/8 or 1/2 rod if I am sure to take one with me to club launches. Or, I could use rail buttons. My problem with buttons is available space. I have to mount the buttons on a tube that is within a ring tail. I have only 1 3/16 inch clearance to do that. The question then becomes, is there enough room for the rail button and a 1 inch rail to fit inside the ring tail? In other words, what is the absolute minimum distance necessary for the button and rail together from the surface of the rocket? Also, is there a rail smaller than 1 inch available? I can not find it if there is.

A 1" rail won't fit inside your ring-tail which is draggy anyway. I would mount both 3/8' and 1/2" launch lugs on the airframe on the support structure extending out to support the rings. You won't see them and they can be glue to both the airframe and the shortened fin/ring support. You probably have 3 fin/ring supports, so mount the lugs on opposites of the rocket.

Bob

 

[Handeman]

Ok, so I could probably use a 3/8 or 1/2 rod if I am sure to take one with me to club launches. Or, I could use rail buttons. My problem with buttons is available space. I have to mount the buttons on a tube that is within a ring tail. I have only 1 3/16 inch clearance to do that. The question then becomes, is there enough room for the rail button and a 1 inch rail to fit inside the ring tail? In other words, what is the absolute minimum distance necessary for the button and rail together from the surface of the rocket? Also, is there a rail smaller than 1 inch available? I can not find it if there is.

I have a Kraken upscale made from 3" tubes and there is plenty of room. I would check but my best guess is that any tube larger then about 1.75" dia. should be OK. 1.5" dia might work, but check the fit first.