Launch rod length

So i am looking at a model that is 41 inches long but my launch rod is 36 inches what is the ratio for launch rod to model length

There is no ratio. You need to simply provide a launch rod that is long enough to allow the rocket to acellerate to a safe speed by the time it leaves the rod.

You need to consider the liftoff (fully loaded) mass of the rocket and the thrust level of the motor. If you run a computer simulation, you will see the speed as time goes by and you will also see the altitude, so you can see the speed it is movign at as it leaves the rod.

You also need a stiff enough rod that will not bend if there is a breeze or as the rocket motor ignites

See many, many other threads on this same subject using the search function.

Here is the way that I look at it- a 6" long rocket can accelerate at the same speed as a 36" long rocket, depending on the thrust to weight ratios of course. Once the rocket reaches the end of the rod, if it is going at a suitable speed for stability then that is all that is required.

But on the other hand- a longer rocket will have launch lugs farther apart and the forward launch lug will clear the end of the rod at less distance traveled than for the short rocket. So for this reason it would seem that a longer rod would be required.

In your example, the overall lengths of the rocket vs. the launch rod aren't so critical. The more important concept is "effective guide length" which is the distance that the launch lug will travel on the rod before it reaches the top. Because wherever the launch lug goes, the rocket will follow! Rockets that are taller than their guide rods are launched safely all the time, because their lugs stayed on the rod long enough for the rocket to reach the airspeed at which the fins begin to be effective as stabilizers. If the rocket is 41" long, and the lug is mounted 18" up from the bottom of it, then on a 36" rod the lug has 18" of travel. If the motor that is used in the rocket can get it up to the necessary speed within the first 18" of its journey, then 36" of rod will be enough. The correct length of launch rod to use for a given rocket is determined by the performance of the motor that is used to launch it. Obviously different motors have different performance curves, so the length of guidance needed for the rocket will be different for each motor used in it.

As others have mentioned, this is a matter of liftoff thrust vs. liftoff mass of the rocket. The unloaded mass (weight without motor) of the rocket stays the same regardless of the motor chosen, so the liftoff mass (the weight of the rocket + motor) will vary for the most part due to differences in masses of different motors. (At liftoff, the motor doesn't just have to lift the rocket, but also has to lift itself.) If a given motor has enough liftoff thrust (the amount of thrust it puts out in the first fraction of a second) to be able to get the rocket up to speed before the lugs run out of rod, then you're golden. If not, then you'll either need to switch to a higher-thrusting motor or else use a longer rod.

the motor you want to use, and the mass of the rocket are much more important than length. IE a mean machine is 6' or so, and the 3/16" launch rod that is used is definitely not 6', but it survives.

so yeah, what motor do you want to use, and how heavy is your rocket?

bjphoenix said:

But on the other hand- a longer rocket will have launch lugs farther apart and the forward launch lug will clear the end of the rod at less distance traveled than for the short rocket. So for this reason it would seem that a longer rod would be required.

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Well the launch lugs are as far apart as you choose to put them - separating them by a large distance on a long rocket may not be the best plan.

My rocket will be 44.9 inches and will be 3.7 oz or so the kit says .the launch lugs are one at the bottom and 1 10 inches up.the rod length id 36 inches

My 1/4 launch rod I use for E9's is 38 inches long. Standard hobby shop rod, K & S I believe is the manufacturer.

Rocket_Man said:

My 3/16 launch rod I use for E9's is 38 inches long. Standard hobby shop
rod, K & S I believe is the manufacturer.

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we are planning B6-4 for our first flight and c6-5 for the rest

Humma,

If you have any doubts about the launch rod being long enough, then it is probably time for you to get a 4 foot rod for your collection of range eqpt

You can find round rod (also called cold-rolled steel) at your local hardware store (Home Depot, Lowes, Sutherlands, etc) in a bin with short lengths of angle stock, all-thread, and other metal products. They will likely have it in 3/16th diam and 1/4 (and bigger), and in lengths like 3, 4, and 6 feet. You may only feel the need for 3/16 right now but I would encourage you to go ahead with the 1/4 inch for bigger projects in the future.

I specifically recommend against getting a 4 foot length of 1/8th diam. It just gets too "whippy" and will be more dangerous than beneficial.

powderburner said:

Humma,

If you have any doubts about the launch rod being long enough, then it is probably time for you to get a 4 foot rod for your collection of range eqpt

You can find round rod (also called cold-rolled steel) at your local hardware store (Home Depot, Lowes, Sutherlands, etc) in a bin with short lengths of angle stock, all-thread, and other metal products. They will likely have it in 3/16th diam and 1/4 (and bigger), and in lengths like 3, 4, and 6 feet. You may only feel the need for 3/16 right now but I would encourage you to go ahead with the 1/4 inch for bigger projects in the future.

I specifically recommend against getting a 4 foot length of 1/8th diam. It just gets too "whippy" and will be more dangerous than beneficial.

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This is excellent advice. I doubt that you can even find 1/8" rod in 48" lengths, though, so that probably won't be an issue.

Whoops, sorry about that! :y: I am using 1/4 inch launch rod! I definitely suggest 1/4 inch cause just like Mark said its not as "whippy" as the 3/16 stuff.

MarkII said:

This is excellent advice. I doubt that you can even find 1/8" rod in 48" lengths, though, so that probably won't be an issue.

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We buy them at Home Deopt for our standar club launch rack pads. 1/8" diameter by 4 foot long cold rolled steel. Not very whippy at all and they provide excellent guidance as the rockets build up speed so they can safely fly on their own.

We also use 3/16" dia by 5 foot long stainless steel and 1/4" dia by 6 foot long stainless steel rods.The 1/4 rods are cut in half and have threaded connections. Machined by one of our club senior members. He also made the alumimun tripod launchers for the 3/16" and 1/4" rods.

shreadvector said:

We buy them at Home Deopt for our standar club launch rack pads. 1/8" diameter by 4 foot long cold rolled steel. Not very whippy at all and they provide excellent guidance as the rockets build up speed so they can safely fly on their own.

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Oh that's right. Now that you mention it, I have seen them in that length. The cold-rolled type is the one to get. I must have been thinking about 1/8" rod in 6 foot (72") lengths. Awhile ago I was looking for 6' x 1/8" rods for some reason. Not easy to find, but with some digging you can find it. Even in stainless steel. It was powderburner, not me, who brought up the stiffness issue with longer lengths of thin rod. I think that would very likely be a problem with a 6' long, 1/8" diameter rod. Unless perhaps you get it in something like titanium.

I know that a 3-foot launch rod 1/8th-inch in diam is perfectly fine for many low power rockets.

I also know that a relatively large-sized low power rocket, with a big motor (D or E especially), can have problems on a 1/8th-inch launch rod and can often show pretty severe rod whip. Those modrocs (IMHO) need a larger diameter rod. Using a longer 1/8th-diam rod, when you already are seeing rod whip problems with a three-foot length, is not a good idea.

The whole subject, if I recall the OP correctly, was specifically a qstn about a large/heavy rocket on a standard-length 1/8th-diam rod and whether this rod was big enough. It was not a generic qstn about "average"-weight modrocs on 1/8th-diam rods. A large/heavy rocket that is already questionable on a three-foot rod is not the situation to recommend a longer rod unless the increased length comes with an increased rod diameter.

True, the discussion did drift into more general launch rod topics, but I think by then we had already answered the OP's question.

I can't remember why I was looking for a very long 1/8" rod, but it wasn't for a larger, heavier rocket. I agree with you; that diameter rod is appropriate for small lightweight rockets, but for anything more, and with motors that have more than C impulse, a larger rod is needed. One point of view is that it never hurts to use 3/16" lugs for all low power rockets regardless of mass.

MarkII said:

(snip)... that diameter rod is appropriate for small lightweight rockets, but for anything more, and with motors that have more than C impulse, a larger rod is needed. One point of view is that it never hurts to use 3/16" lugs for all low power rockets regardless of mass.

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I agree completely, and would even go a step further. I think all beginner rockets, especially "class-build" rockets, should be equipped with 3/16 or even 1/4 launch lugs.

The reason is that this class of rocket tends to get used under circumstances where school schedules are crucial and the flexibility is just not there to allow launch postponements on marginal weather days. Teachers just can't afford to mess up their whole week on the chance that tomorrow might be a better launch day. They have lesson plans to cover, they have their own schedules to keep, and most of them even have state and federal laws in their hair about completing a minimum set of subject units. Letting us come in and do a rocketry unit is disruptive enough, interrupting again and again for weather delays (when the weather looks fine to them) will quickly get you NOT invited back.

When I tell them that we will launch on Wednesday, and it is a little extra windy that day, I like it a whole bunch better if I am prepared with 1/4 inch launch lugs and 1/4 inch rods that are all four feet long, even for little rockets with 1/2A motors. I feel far safer proceeding with a class launch under such conditions. Otherwise, the launch might not happen at all.

powderburner said:

I agree completely, and would even go a step further. I think all beginner rockets, especially "class-build" rockets, should be equipped with 3/16 or even 1/4 launch lugs.

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I build ALL my birds with a minimum 3/16 launch lug and some of them are pretty small.

Admittedly I've launched it on E30's but that's just mostly showing off so I mostly fly it on D12's. Quick!

We use 1/8" diameter 4 foot long cold rolled steel rods for the basic launch pads because so many beginner models need to build up speed before leaving the rod for a safe flight. Ther is ZERO rod whip as these are not giant fat rockets with the motor 2 inches away from the rod and the rocket suspended 8 inches up from the anchor point of the rod.

With the new low thrust motors, even light rockets need extra long launch rods.

Bigger rockets get the larger diameter and even longer rods.

powderburner said:

I agree completely, and would even go a step further. I think all beginner rockets, especially "class-build" rockets, should be equipped with 3/16 or even 1/4 launch lugs.

The reason is that this class of rocket tends to get used under circumstances where school schedules are crucial and the flexibility is just not there to allow launch postponements on marginal weather days. Teachers just can't afford to mess up their whole week on the chance that tomorrow might be a better launch day. They have lesson plans to cover, they have their own schedules to keep, and most of them even have state and federal laws in their hair about completing a minimum set of subject units. Letting us come in and do a rocketry unit is disruptive enough, interrupting again and again for weather delays (when the weather looks fine to them) will quickly get you NOT invited back.

When I tell them that we will launch on Wednesday, and it is a little extra windy that day, I like it a whole bunch better if I am prepared with 1/4 inch launch lugs and 1/4 inch rods that are all four feet long, even for little rockets with 1/2A motors. I feel far safer proceeding with a class launch under such conditions. Otherwise, the launch might not happen at all.

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Standard launch rod diameter for micro rockets is 0.049". :wink: