Launch Lug Tip

 The Thrifty Rocketeer blog continues...

Here's a construction tip that was passed along to me.

This topic just recently popped up on one of the more public forums.



A father was puzzling over why there was a two inch launch lug included in his son's rocket kit, when his own personal experience was that instructions would tell the kit builder to slice it in two... producing two 1" launch lugs.   He mused, what is the difference and is it important?

Most of the responses suggested that it was a trade-off between the length of the lugs vs the friction that might occur.  How much torque could a  glued-on launch lug handle?

But I had a slightly different take upon the topic.  I assumed that the launch lug "straw" was going to be cut in half no matter what the lack of instructions might suggest.  And so, for me, the question was where and how to cut.

My recommendation was that the father or son measure the two inch straw and plan to cut it exactly in half with an Exacto or razor blade.  But the twist is this:

Cut the straw  (or launch lug) on a diagonal.... NOT a straight perpendicular cut at 90 degrees (like the ends of the lug currently look).


The reason for this is two fold.

First, the slight 45 degree angle extends the lug fractionally, and while this may not play a role in the slight amount of friction that is incurred, it DOES fractionally lengthen the amount of surface area along which the lug is glued.

Second, the bevel of 45 degrees lessens the air resistance to the leading edge of each lug, allowing just that much more thrust to be delivered to altitude.

Now, I know what you're going to say here... "How much difference can that make over all, if you've already got a streamlined rocket with straight fins, etc.?"

I'll tell you what I was told when someone first suggested feathering ALL fins on every kit that I built.  

When I asked, "How much difference will that make?"  The response was, "When you're striving to get your bird higher and higher, whether in a competition, or for your own satisfaction, there's any number of small tricks and modifications that you can make.  Feathering the lead edge of your fins may make the difference in a couple of feet or yards... and if that's your goal, why not?"

So, I submit to you a tip on how to lessen air resistance in your kit, if you're GOING to cut the launch lug in half.  Cut it midway on a 45 degree diagonal and turn both wedges to the front.

Also, feather those fins' leading edges on each fin.  It really doesn't take that much more effort, though it takes some patience at first.  (And remember to feather that edge BEFORE you seal the balsa or wood, as it is MUCH easier to sand to a bevel or feathered edge before it's hardened with CA, sealant, or paint. )

This has been the Thrifty Rocketeer saying " Get all the height you can."

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