Can someone tell me what the "Standard" size rod is for say, a Quest Harpoon rocket would be...I recently bought a Mantis for my 4" Gemini Titan, (1/4" rod) but also have smaller rockes and need another rod, but dont know what size to get....(I dont have any other launchers..)

1/8" and 3/16" are the two "standard" model rocket launch lug sizes.

The Quest Harpoon has a 1/8" launch lug.

You can grab a piece of 1/8" steel rod at your local hardware store and use it in the Mantis launch pad. However, it's a bit of a pain to swap out rods with the Mantis wheel design. Go to your local Wally World, Hobby Lobby (with 40% off coupon) or other box store and grab yourself an Estes or Quest starter set. For $15 or less you will have a cheap but very useable launch pad and rod to fly small stuff and not have to worry about swapping rods on the Mantis. As a bonus, you will have yourself a "wind tester" rocket to blast into oblivion at the beginning of your launch day.

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I love sanding.

Great idea...Im on it....

Thought I would ask this while I was thinking about it. And this seemed like a good thread to ask in!

Is there any standard to where a launch lug is mounted (at CG, CF, etc...)?!?

This is as much an art as it is a science question. But here goes (and this is just one guy's opinion).

One lug or two?

Small modrocs need only one, whereas the LMR's need two.

For one lug, near the CG is a good practice, since that is the focus of the rocket mass. Now where on the airframe does one place it? Obviously you have to miss the fins and any other obstruction. Beyond that the considerations are aerodynamics and aesthetics. My practice now is to keep my lugs small (but long enough that pitch and yaw while on the rod are minimized) and give them a 45 degree bevel cut.

For two lugs, it gets a little more complicated. The Estes practice is to have one up near the nose cone and one near the aft. On mine, I put the fore lug about 25% to 33% of the airframe length from airframe top. For example, on an airframe length of 18", I might put the fore lug 4.5" to 6" from the airframe top. The aft lug I place between 50% and 100% of the airframe diameter behind the CG. I can't tell you why I do it that way other than I "feel" that the rocket is in somehow "in balance". I am not an engineer (but I work for an engineering company!), but I think to get the true science on this you would need an ME to sketch out all of the moments (forces) being applied on the rod. I do think a couple of the biggest considerations for two lugs is that 1) they are in alignment, and 2) they slide on the rod easily. Keeping the rod helps with consideration 2. To help with consideration 1, take a section of launch rod with the two lugs and position them using painters tape (where necessary on both the rod and lugs). When they are in there proper position, tack them down with CA. Then fillet glue them with your adhesive of choice.

One could write an engineering thesis on strategies for lug placement to optimize launch speed and vehicle aerodynamics.

Of course you could ditch the lugs and use a tower!

My $0.02. HTH + YMMV.

Greg

I've found over the years that as long as you aren't launching in any significant wind, launch lug placement isn't all that big of a deal as long as you loosely follow Greg's advise. A single lug needs to be at least 1.5" long on small rockets and considerably longer if it's a long rocket. Deciding between one and two lugs for small rockets really depends on how it fits asthetically between roll patterns, decals, fins, etc. A perfect example is the Big Bertha. That single long lug isn't all that pretty, but works well if you have a roll pattern near the top. Two small lugs would be less conspicuous if the paint/decal scheme permitted.

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I love sanding.

Thanks for all the input.

I was asking for several reasons. First, because in designing and building Mary's Periwinkle rocket, she asked me that question.

Second, from time to time, on the tiny rockets they have bound up on the launch rod during boost.

Third, on my own design, the Ascendant (oval fins with cutouts), I cut two down and hid them in the fin fillet.

I have found it interesting that the two Semroc designs I have built (My Boid, Vega) both have the single launch lug down at the base of the rocket between two fins rather than further forward as in Estes or Quest rockets I've also built.

I can see arguments for several different placement approaches. I have no idea which one is "optimum". I suppose one would have to define what that means first....

I'd just point out that ole G. Harry suggested placing the lug in the fillet of a fin for aerodynamic reasons (probably area rule). I did this on my Sandman Big Sprint and it works fine.

I like to place a lug near the bottom of the rocket and a second lug between 1/3 and 1/2 of the way up from the bottom. My reasoning is that I want to keep the lug on the rod as long as possible to insure a straight launch, so I tend to keep the lugs toward the bottom.

But I do agree that aesthetics, paint scheme, decal placement and "looking about right" are all perfectly valid considerations.

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Awaiting First Launch: Too numerous to count
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