Rod length

[jamjammer53150]

form time to time i hear "You need a Long rod for that " what would you conside long
My 3/16 i can get to 5 ish feet , and i have rockets longer than 3 or so , or with out the extension it will go 3 ft .
so any rule here ?

[garmtn]

I would say, it only gets critical on your larger rockets. I use a 4' rod on my Saturn V last Friday and it flew fine. A more critical question would be width and weight? Also, if I put my Saturn on a 3'x 1/8" rod, it would b "flexing" all over the place. Thats what u want to avoid. ;)

[Shreadvector]

form time to time i hear "You need a Long rod for that " what would you conside long
My 3/16 i can get to 5 ish feet , and i have rockets longer than 3 or so , or with out the extension it will go 3 ft .
so any rule here ?

It is quite simple. You need a launch rod that can suppor the rocket and guide it until it is flying fast enough for the fins to properly guide it through the air.

If the rocket is heavy and the motor does not produce a large enough lift-off thrust, you will need a long and stiff launch rod to guide it a long time until it build sup enough airspeed.

If the rocket is lightweight and the motor produces a huge lift-off thrust, then the rod can be fairly short.

I use:
1/8" x 4'
3/16" x 5'
1/4" x 6'
Steel launch rods.

I put the appropriate launch lugs on my models so that I can use the rod or rods appropriate for the rocket size and the motors I think I might use in them.

A simulation program can help you with the speed off the launch rod because you can try out different rockets and rocket weights and different motors and also vary the wind speed.

Ah yes, the wind speed. If there is a cross wind you will need more thrust and a longer and stiffer rod. Stiffer rod reduces the whipping effect - especially as the model slides up the rod. A higher thrust motor will allow the rocket to build up great speed before it leaves the rod and that helps prevent "weathercocking" where the rocket (if it's moving too slow relative to the cross wind) will rotate and head into the wind like a weathervane (which are often a rooster sitting on an arrow and that's why it's called weathercocking).

[tbzep]

Longer rods can help, but they can also cause problems because they are prone to more violent rod whip. The longer the rod, the larger the diameter needs to be to help counteract rod whip. I've got some old video from launches with 4' steel rods at 1/8" thick. The rods whipped back and forth wildly with medium size rockets. I think I'd want at least 3/16" steel at 4 feet, and anything longer than that I'd want 1/4". Steel is stiffer than aluminum, so I'd stick with steel on longer rods too.

To the best of my recollection, I have these rods at home:

1/8" aluminum 3 ft (2 piece, flimsy)
1/8" steel 3 ft (1 piece, decent)
3/16" aluminum 3 ft (2 piece, slightly better than 1/8" steel same length)
3/16" steel 5 ft (1 piece, whips like crazy and will be cut to 4 ft before it sees another rocket)
1/4" aluminum 4 ft (2 piece, decent)
1/4" steel 5 ft (2 piece, fair)
1/2" steel 6 ft (1 piece, solid)

[jamjammer53150]

Since I am now building some larger rockets , I got a 3 foot piece of brass 3/16 tube , then i got 2 bits of steel 3/32 ( i think) whatever size that fits right inside . I chopped 10inches off one and slid it inside the brass , which makes a 3 foot ish rod with a 10 inch sleeve on top , that i can slide the other part in making it around5 foot. so there is a small step down in diameter but i wouldthink it would do as i doubt I will ever do anything bigger than d engines , and i dont see me building a 4 foot rocket

[AFlyingMonkey]

all of my personal launch rods from 1/8th to 1/4 are all carbon fiber and 4ft long. Very little whip no matter which one i use. the 1/4 is hollow so i stuffed a piece of steel in the bottom of it for attaching it to the hub so it doesn't get crushed in the clamps . the 1/8th and 3/16th rods are solid, so no problems there.

other advantages are that they don't corrode like steel does, always very slick, esp after a bit of sanding (its graphite after all lol) and extremely strong. Yes its expensive up front but worth it over the long haul.

oohhh one other thing, you never ever have to worry about getting kinks in it either. :D ;)

[ghrocketman]

Acchhhh, rodd-whipp always gets such a bad-rap 'round here.
Nuttin' wrong with a nice decent amount of rod-whip.
Makes for a more EXCITING flight profile with some added scale 60's NASA randomness to assure lack of boredom.

Rod whip is akin to trailer-sway when towing on the freeway. You want a fair amount of each to keep the experience from being boring. :cool:

[Mark II]

Once you get to about 5 or 6 feet in rod length, it starts making more sense to switch over to a rail. Rails can be securely braced on all sides except the side that faces the rocket, so they can be made very steady and resistant to flexing.

Since our rockets use passive stabilization via fins, it is imperative that they get up to speed quickly via a good strong boost off the pad. Rods can be used up to a certain length, and rails can be made much longer, but there is a point beyond which neither method of rigid guidance is practical. If a rocket needs a very long rigid guide off the pad, it is a sign that it is either overweight, underpowered or both. Rather than seeking a longer rod or rail, the builder should first look at ways to reduce some of the rocket's lift-off weight. Once the rocket has been through The Biggest Loser routine, the builder should then evaluate how much power is needed to get the bird going at a fast enough speed to be stable and to get it up to at least the minimum altitude needed for it to be able to effectively deploy its recovery system. More powerful motors can be used if they are available and will fit into the design. One can also add parallel-staged boosters to the design if feasible. The builder needs to explore making these changes in the rocket itself rather than simply focusing on adding to the launch structure.

To answer the original question, a "normal" length of rod for LPR and MPR is 3 to 4 feet. A "long" rod is usually a 6 footer. For larger mid-powered rockets, an 8 foot rod is considered to be within the limits of a typical "long rod." These are the commonly used rod lengths. Remember that the relevant number is "effective rod length." This is the distance that the launch lug will travel on the rod when the rocket is boosted off the pad. It is the distance from the top of the lug to the upper tip of the rod when the rocket is placed on the pad and is ready to launch. When the rocket has dual launch lugs, one at the upper end and one at the aft end of the airframe, I am not sure whether you count the distance from the top of the upper lug or the top of the lower one. Someone else will have to answer that question for both of us.

Brass can be used for launch lugs, but I would never use any brass in the launch rod. It simply lacks sufficient rigidity. Steel is the preferred material for launch rods; since rocket exhaust is quite corrosive, type 315/316 (marine grade) stainless steel is even better, as is titanium. Launch rails are usually made from extruded aluminum.

MarkII

[tbzep]

Acchhhh, rodd-whipp always gets such a bad-rap 'round here.
Nuttin' wrong with a nice decent amount of rod-whip.
Makes for a more EXCITING flight profile with some added scale 60's NASA randomness to assure lack of boredom.

Rod whip is akin to trailer-sway when towing on the freeway. You want a fair amount of each to keep the experience from being boring. :cool:

I agree completely...if it's somebody else's rocket. :D

I normally launch in my pasture, which is surrounded by trees and isn't very wide in the direction the wind usually travels. As much as I love sanding, I don't particularly want to have to build a duplicate rocket because I got the original hung in a tree. :p If I'm at a large range like Shelby Farms with Don and Dave, I don't worry about rod whip. Escaped convicts and buffalo maybe, but not rod whip. :eek:

[Mark II]

Rod whip is akin to trailer-sway when towing on the freeway. You want a fair amount of each to keep the experience from being boring. :cool:Trailer sway when you are parked is an even better way to beat boredom. ;)

MarkII