Homemade LPR-MPR launchpad - type of rod???
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sj_h1
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What launch rod is best for A to F motors? How long? Single or in pieces? If in pieces - how do I connect them? (Simple carving or something more sophisticated?)
Don't know about E and F motors but most A-D models use 1/8 inch rods. I would recommend a single piece rods to avoid models get caught in the splice and inadvertent rod separations.
powderburner
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Launch rod length is about giving your model rocket enough distance to accelerate to safe flying speed while it is still restrained by the launch rod.
If it is a day with no wind you can use a slightly shorter launch rod. If it is a gusty, windy day you will need a significantly longer launch rod. The NAR recently published a report on safe launch operations that explains how all this works under various cross-wind conditions.
Using the impulse-classification of the motor gives you a very general guide but what you really need to know is the rocket's speed at the end of the launch rod (or rail). The weight and drag of the rocket will obviously have an influence on this speed. You may need to use some flight simulation software, or do a few calculations of your own. This sort of thing is fairly easy to program on an EXCEL spreadsheet.
Launch rod diameter should be as big as you can tolerate. The larger diameter makes the launch rod more sturdy and stiff, and reduces the amount of sway and bending that you get on a windy day. The straight launch rod is just as important as the length. If you use a 1/8 inch launch rod with a heavy model rocket on a gusty day, the rocket might reach the end of the launch rod while it has bent all the way in one direction and is bouncing/swaying back, and the moving tip of the launch rod can turn the rocket (in unpredictable directions) as it separates.
No, I am not telling you to use a 25mm diameter launch rod for A- or B-powered rockets. The launch lug would be bigger than the rocket? Such a large launch rod would be over-kill.
Yes, 1/8 inch (approximately 4 mm) will work for small, light model rockets but it would be better to use 3/16 or 1/4 (approximately 8 mm) for heavier rockets. This would include almost all model rockets powered by D, E, or F motors, and possibly even some rockets powered by C motors. Remember, if you build using 1/4 inch launch lugs, they will always fit on a 1/8 inch rod, but if you build with 1/8 inch launch lugs and need a bigger launch rod on a windy day, you might not be able to launch at all.
And for heavier model rockets (probably some E-powered designs, and many F-powered rockets) you should probably use a rail system. These are much much more stiff and will give your rocket excellent directional control while it is on the launcher. The same length rules apply to rails; that is, the rail needs to be long enough to let the rocket reach safe flying speed.
I do not happen to like the Estes launch rods, for a couple reasons. They are good for beginners, and they usually work if you don't have any other rod material available, but that's about it. The two-piece construction has a joint in the middle that usually makes one half of the rod slightly mis-aligned with the other half (cheap parts) and can snag the rocket and slow it down in the middle of launch. The joint is also a weak point where these Estes rods have a tendency to break. When assembled, the Estes launch rod is only about 31 inches long. And if you try to purchase a replacement, they are ridiculously over-priced.
Go to your local hardware or building supply store and look for an assortment of round rod in various diameters and lengths. You can buy a one-piece rod made from solid steel that is 1/8 inch diameter and a full three feet long for a buck-n-a-half, and you can buy a four foot length for only a little more. (Yes, I know he's in Bulgaria, and they don't use inches or dollars.) Or, if the 1/8 is too flexible (and it is, in a four-foot length), you can usually find larger diameters that are still not expensive. If you clean off the motor residue at the end of the day, and keep the rod clean and lightly oiled, it will last a long time.
If it is a day with no wind you can use a slightly shorter launch rod. If it is a gusty, windy day you will need a significantly longer launch rod. The NAR recently published a report on safe launch operations that explains how all this works under various cross-wind conditions.
Using the impulse-classification of the motor gives you a very general guide but what you really need to know is the rocket's speed at the end of the launch rod (or rail). The weight and drag of the rocket will obviously have an influence on this speed. You may need to use some flight simulation software, or do a few calculations of your own. This sort of thing is fairly easy to program on an EXCEL spreadsheet.
Launch rod diameter should be as big as you can tolerate. The larger diameter makes the launch rod more sturdy and stiff, and reduces the amount of sway and bending that you get on a windy day. The straight launch rod is just as important as the length. If you use a 1/8 inch launch rod with a heavy model rocket on a gusty day, the rocket might reach the end of the launch rod while it has bent all the way in one direction and is bouncing/swaying back, and the moving tip of the launch rod can turn the rocket (in unpredictable directions) as it separates.
No, I am not telling you to use a 25mm diameter launch rod for A- or B-powered rockets. The launch lug would be bigger than the rocket? Such a large launch rod would be over-kill.
Yes, 1/8 inch (approximately 4 mm) will work for small, light model rockets but it would be better to use 3/16 or 1/4 (approximately 8 mm) for heavier rockets. This would include almost all model rockets powered by D, E, or F motors, and possibly even some rockets powered by C motors. Remember, if you build using 1/4 inch launch lugs, they will always fit on a 1/8 inch rod, but if you build with 1/8 inch launch lugs and need a bigger launch rod on a windy day, you might not be able to launch at all.
And for heavier model rockets (probably some E-powered designs, and many F-powered rockets) you should probably use a rail system. These are much much more stiff and will give your rocket excellent directional control while it is on the launcher. The same length rules apply to rails; that is, the rail needs to be long enough to let the rocket reach safe flying speed.
I do not happen to like the Estes launch rods, for a couple reasons. They are good for beginners, and they usually work if you don't have any other rod material available, but that's about it. The two-piece construction has a joint in the middle that usually makes one half of the rod slightly mis-aligned with the other half (cheap parts) and can snag the rocket and slow it down in the middle of launch. The joint is also a weak point where these Estes rods have a tendency to break. When assembled, the Estes launch rod is only about 31 inches long. And if you try to purchase a replacement, they are ridiculously over-priced.
Go to your local hardware or building supply store and look for an assortment of round rod in various diameters and lengths. You can buy a one-piece rod made from solid steel that is 1/8 inch diameter and a full three feet long for a buck-n-a-half, and you can buy a four foot length for only a little more. (Yes, I know he's in Bulgaria, and they don't use inches or dollars.) Or, if the 1/8 is too flexible (and it is, in a four-foot length), you can usually find larger diameters that are still not expensive. If you clean off the motor residue at the end of the day, and keep the rod clean and lightly oiled, it will last a long time.
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cjl
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Typically, you want 1/8" for A-C rockets, 3/16" for D through light F rockets, and 1/4" for heavier E through G rockets. You definitely want a 1 piece rod, and I usually use 3 feet for 1/8", and 4 feet for 3/16" and 1/4".
DexterLB
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I have 6mm lugs on all my rockets, and I think 5.4mm or 5mm launch rod will work. Rocksim does calculate the speed at the departure, and from what it shows 4feet will be OK. I also gave up the thought of having a several-part launch rod thanks to your post
I'm starting to get used to them
dedleytedley
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I made a serviceable launchpad from an old camera tripod. I selected a block of hardwood about 80mmx80mmx30mm and drilled holes for 3/16 and 1/8 rods most of the way through the block. On the other side I drilled a hole 1mm smaller than the diameter of the mounting bolt on the top of the tripod. I carefully turned the block onto the bolt cutting threads into the wood. A piece of sheet metal serves as the blast deflector and protects the wood. The leg adjustments allow for setting the base at an angle.
I've always found 3/16" and 1/4" (or 4/16" for comparison) similar enough that I just use 1/4" lugs on any kits that come with 3/16". That eliminates any need for a 3/16" rod.
If you order your one-piece rod(s) in the mail, I recommend keeping the mailing tube to store them in. You can keep them all in the same tube to save space. It will protect them from bending while transporting them, as well as protect your vehicle from upholstery tears and/or paint scratches. It can also serve as protection against poking out people's eyes if you don't have end caps.
Make sure you don't get aluminum rods. Get steel. Stainless steel is even better, but a little more expensive. I've heard that most 1/8" rods are steel already (but not stainless), but the stock 3/16" and 1/4" ones are all aluminum. If you're willing to pay for it, you can improve the rigidity a great deal by getting 1/4" tube instead of round rod, but tube costs about 4x as much. I can order a 6 foot/2 meter 1/4" stainless steel tube for US$19 before shipping. I suppose you could go even further and put a round rod inside the tube, but I haven't found a rod that would fit perfectly inside my tube. I haven't tried tube for 1/8" because I'm guessing it would be too easy to break.
Hope this helps.
If you order your one-piece rod(s) in the mail, I recommend keeping the mailing tube to store them in. You can keep them all in the same tube to save space. It will protect them from bending while transporting them, as well as protect your vehicle from upholstery tears and/or paint scratches. It can also serve as protection against poking out people's eyes if you don't have end caps.
Make sure you don't get aluminum rods. Get steel. Stainless steel is even better, but a little more expensive. I've heard that most 1/8" rods are steel already (but not stainless), but the stock 3/16" and 1/4" ones are all aluminum. If you're willing to pay for it, you can improve the rigidity a great deal by getting 1/4" tube instead of round rod, but tube costs about 4x as much. I can order a 6 foot/2 meter 1/4" stainless steel tube for US$19 before shipping. I suppose you could go even further and put a round rod inside the tube, but I haven't found a rod that would fit perfectly inside my tube. I haven't tried tube for 1/8" because I'm guessing it would be too easy to break.
Hope this helps.
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