When to go up a launch rod size

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lcorinth

lcorinth

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I'm about to build my second scratch rocket, and I'm not sure when it's appropriate to go from a 1/8 to a 3/16 inch launch rod.

The rocket will launch with a D12 motor - actually, it's two stages, with a D12 for the booster, and an E9 for the sustainer, but since the rocket will have cleared the rod when the second stage ignites, I feel like that might not matter. I have both sizes of launch lugs, and rods. I'd stick with the smaller size, but I don't know if it's recommended. When do you know you need to go to 3/16? When do you go up to 1/4?

Janus II With Camera Payload Bay Design 03.jpg
 
I would have to dig up my copy of the Model Rocketry handbook for the precise answer but the general rule of thumb I use is for up to D or small E power to use a 3/16 , then any thing with composite power a 1/4, or preferably a rail. The other thing to think about is the length of the rod, a heavier rocket with an e9 will do better with a longer rod, as they get off the pad slower, and need a longer rod to safely get up to speed before the fins can stabilize the rocket. If that was my rocket I would go for a 1/4. You can not go wrong if you too big. I even built a red max with rail buttons to fly 18mm composites. I am sure probably already know, but in case you did not, you will also need standoffs for the Launch lugs to clear your nose cone.

Looks like a neat rocket, keep us posted on your progress.
 
The handbook says that you can use an 1/8" rod for models up to 6oz.(pad weight). however, that also is dependent on lug placement. you're going to want at least a 48" long rod and that means a min of 3/16". from what I have seen, one could probably use a 3/16" rod for models up to 12 oz. and 1/4" up to 24 oz. after that a rail would be your best bet.
Rex
 
KurtH said:
I would have to dig up my copy of the Model Rocketry handbook for the precise answer but the general rule of thumb I use is for up to D or small E power to use a 3/16 , then any thing with composite power a 1/4, or preferably a rail. The other thing to think about is the length of the rod, a heavier rocket with an e9 will do better with a longer rod, as they get off the pad slower, and need a longer rod to safely get up to speed before the fins can stabilize the rocket. If that was my rocket I would go for a 1/4. You can not go wrong if you too big. I even built a red max with rail buttons to fly 18mm composites. I am sure probably already know, but in case you did not, you will also need standoffs for the Launch lugs to clear your nose cone.

Looks like a neat rocket, keep us posted on your progress.
Click to expand...

Yeah, OpenRocket doesn't let you create standoffs from what I can tell. It would be nice, because I'm sure they affect the rocket's performance, and I'd like it if you could print a template. I'd like to trim the lugs at a 45 degree angle, and I'd like it if the standoffs could follow that line down to the body tube.

What I'll probably do is create a new rocket with a fin set which is the size of the standoffs I need, then print the template for those off.
 
I don't build for performance or competition, so I tend to use 1/4" lugs on everything I build that doesn't have rail guides. That way I don't have to swap out my rod for different rockets when flying on my own --- just leave the 6' long 1/4" rod in place for every rocket. Also, I can use any pad I'm assigned to at a club launch without swapping.
 
My simple rule of thumb:
A - C engines, 1/8" rod.
D and E engines , 3/16" rod.
 
lcorinth said:
Yeah, OpenRocket doesn't let you create standoffs from what I can tell. It would be nice, because I'm sure they affect the rocket's performance, and I'd like it if you could print a template. I'd like to trim the lugs at a 45 degree angle, and I'd like it if the standoffs could follow that line down to the body tube.

What I'll probably do is create a new rocket with a fin set which is the size of the standoffs I need, then print the template for those off.
Click to expand...
In Rocksim you can create individual fins as standoffs and add tiny pods for the launch lugs. I don't know if OR allows this.
 
I now only use 1/4 on everything. I usually use plastistruct square tube. It has a little play in it. I think Afterburner does the same. On the heavier rockets my lugs are--long--like 3/4 the body length. I think it looks cleaner than a couple of small ones and seems to help with rod whip.Check out my album to see what I mean. If your using stand offs, 1/4 at least. Always best to keep the lugs close to the center line of the bt as you can.
 
hcmbanjo said:
My simple rule of thumb:
A - C engines, 1/8" rod.
D and E engines , 3/16" rod.
Click to expand...

Sounds good. That's what I suspected, but I wasn't sure.

I am also building a 3D Rocketry Nautilus II, which I thought had a 1/8" lug. Now that I look at it, it's a bit larger than my previous 1/8" lugs, but looks smaller than the 3/16" lugs I got from JonRocket.com (which look huge to me by comparison). I guess there's a little play in some of the lugs.
 
I don't have any rule of thumb, but I don't use a 1/8" rod any longer. If I buy a kit with a 1/8" lug, I just convert it to 3/16". I guess a 3/16" rod may look out of place on something really small, but it keeps me from having to swap out rods.

My latest scratch build I used 1/4" lugs, but I built it to fly on E and F motors exclusively. I may swap up to 1/4" lugs on my Vagabond as well, which would leave the 3/16" rod just to my A-C rockets.
 
hcmbanjo said:
My simple rule of thumb:
A - C engines, 1/8" rod.
D and E engines , 3/16" rod.
Click to expand...

This is pretty much the standard most use. :)

There is nothing wrong with using a bigger lug. In fact at the LPR club I go to, most folks are using the 1/8" pads, so if you have a 3/16" or 1/4", you can often skip the long line and use an open larger pad. Some folks will put both 1/8" and 3/16" lugs on their rockets for this purpose!


Jerome :)
 
It is really important for a two-stage rocket to stay vertical on the rod. You are using a D and an E motor so the rocket is technically a complex F rocket. You should use a 1/4" x 4' rod at a minimum for stiffness. You could even use a 10 mm or 15 mm rail for more stiffness but most clubs don't have them. A 6' long 1010 (1") rail would not be overkill either, and that's a common size.

Bob
 

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