Launch Rod Length?

[Cajunman06]

My question pertains mainly to the Sport division, but I was wondering what would be the optimum launch rod length for 1/8th and 3/16th diameter rods? I have the old standard Estes two piece rods AND a 4' length of one piece in both sizes. Seems the Estes rods aren't long enough to accomodate some of the longer kit birds. My concern is that at the longer lengths, especially the 1/8th rod, they get sort of "whippy". I know I could "experiment" by cutting short lengths off, but I was wondering about anyone's opinions here.

Thanks

 

[Micromeister]

Rod length, really doesn't have a lot to do with model length. It has to do with guideing the model until it has sufficent speed for the fins to direct the models path.
Most of the smaller lighter weight models fly from a 1/8" rod, and as you noticed, if the rod is over 3 feet long is gets really whippy so I'd loop of a foot from your 4 foot piece. 3/16" rods will handle models up to about a pound or a little over, they can be 3 feet long or as long as 6', I'd say for "general use" leave your 4 foot piece as is. Larger Model Rockets (up to 1500g 3.3lbs) will need a 1/4" x 6 foot long rod or even 3/8" or 1/2" dia. depending on the model diameter and weight.
An easy way to think about it is the larger and more important heavier the model the longer and thicker the rod will need to be.

 

[dluders]

The general rule of thumb for the rocket's minimum velocity (at the top of the launch rod/rail/tower) is 44 feet per second (or 30 MPH). I've seen lots of heavy rockets being launched with motors having low initial thrust, and they just arc over and crash because they didn't achieve 30 MPH at the top of the launch rod. If you want to launch the LONG-burn motors, you need a longer launch rod!

Launch RAILS are a LOT stiffer than any rod, and they come in longer standard lengths. If your rocket club has rails, consider using them. For your Sport Division launches, you'll achieve slightly higher altitudes just be shedding the launch lugs. Better yet, use a launch TOWER!

 

[Cajunman06]

Originally posted by Micromister
Rod length, really doesn't have a lot to do with model length. It has to do with guideing the model until it has sufficent speed for the fins to direct the models path.

I understand that the length NORMALLY doesn't have anything to do with the rocket length, but my step-son built the Quest Super Eagle and the standard Estes two piece rod was not long enough to engage both lugs.

I'm thinking that if I could find small diameter steel tubing instead of rod, it would be stiffer. Guess I need to start searching the web.

 

[airforce]

I just finished a Big Daddy. Man, with it just sitting there on it's fins it looks like it could take off without any rod. Now everybody don't start yelling at me at once.
I wasn't raised in rocketry but it sure looks like it would be a neat thing to try.
Ok, I'm waiting for all the feedback as to why this should not be attempted.
Thanks in advance.
Randy

 

[loopy]

Airforce, it all comes down to stability. The fins will not stabilize the rocket until they have sufficient air flowing over their surfaces. Kind of like an airplane - it can't get lift until there's enough air flowing over the wings. You can try launching it without a rod, but stand behind something...

Mark

 

[airforce]

Wouldn't ya know it. It's always that stabililty thing. Why can't they design a rocket that has ports to be used for stability alignment and pipe some of the thurst through them to keep it all lined up.
Now there's a project for you scratch builders...
Thanks Loopy.
Randy

 

[rstaff3]

What loopy said. There is no way you should try this. The suggestion you had wouldn't work either. Outside pertubations due to minor (undetectable) mis-alignments during construction and ambient conditions would still make the rocket unstable until it was at speed. Unless of course you had gimballed engines and the like. Not likely on Big Daddy.

 

[rstaff3]

Originally posted by Cajunman06
I understand that the length NORMALLY doesn't have anything to do with the rocket length, but my step-son built the Quest Super Eagle and the standard Estes two piece rod was not long enough to engage both lugs.

I'm thinking that if I could find small diameter steel tubing instead of rod, it would be stiffer. Guess I need to start searching the web.

The longer/larger diameter rod wouldn't hurt but I am surprised that this is the case. The Estes Mean Machine is pretty long and still worls with Estes rods (although I usually fly mine from a BlackSky raul using buttons from raibuttons.com). maybe all you need is another lug in between the current ones?

 

[jflis]

well, since no one else mentioned it...

launching it without a rod, rail, tower or some other means of controlling it during its first few feet of flight is a violation of the Safety Code (paragraph 6), which reads:

* Launcher. I will launch my rocket from a launch rod, tower, or rail that is pointed to within 30 degrees of the vertical to ensure that the rocket flies nearly straight up, and I will use a blast deflector to prevent the motor's exhaust from hitting the ground. To prevent accidental eye injury, I will place launchers so that the end of the launch rod is above eye level or will cap the end of the rod when it is not in use.

 

[airforce]

Thanks guys, ya know I wasn't going to launch without a pad.....I influence young'uns, must set a good example as a 4H advisor.
What would be lowest priced rail launch system out there. I ask because I always want to try to introduce things that would be new to my group. They know of no other launch method then the standard Estes, Quest pads.
That rail/button system sounds very interesting.
Randy

 

[rstaff3]

railbuttons.com has plans for a rail that costs about $25. It uses an aluminum extrusion available from McMaster Carr and other places.

 

[airforce]

thank you very much.
Randy

 

[MarkABrown]

Originally posted by airforce
Thanks guys, ya know I wasn't going to launch without a pad.....I influence young'uns, must set a good example as a 4H advisor.
What would be lowest priced rail launch system out there. I ask because I always want to try to introduce things that would be new to my group. They know of no other launch method then the standard Estes, Quest pads.
That rail/button system sounds very interesting.
Randy

Below is a link to some pictures of a launch rail I'm building. I've got about $60-70 stuck into it but, I may have gone a little overboard. I've got a tilt mechanism, an integrated blast plate that tilts with the rail, and a mounting bracket to hold the Hypertek launch stem. You could probably build one as cheap as $30.

My Launch Rail Pictures

 

[airforce]

Great pictures Mark, thanks for providing this info.
I'm going to print it and show my 4H buddy, we just might try to build one.
Randy