Marlin,
You can post your questions right here and we'll talk you through the process, or else you can PM me if you would rather take this offline.
Basically, the process revolves around finding the scale factor you want to work with. For example, if you are building a scale model of a Sidewinder (AIM-9), you start with the diameter of the full-size vehicle body--for a Sidewinder, that is 5 inches even. You also find the diameters of the model rocket body tube stock that you might want to use, such as BT5 (outside diam 0.54), BT20 (O.D. 0.736), BT50 (O.D. 0.976), BT60 (O.D. 1.637), BT70 (O.D. 2.217), and on and on. The diameters I just gave for BT are for "Estes" sizes but there are many more available. Check places like Totally Tubular, or Balsa Machining Service, and our TRF vendors for more choices in body tube size.
Next you do a little figuring and checking. If you want a small scale model, work out the math for a BT20 sized version. The scale factor you would be working with here is (BT20 diam)/(full-size diam), or with numbers in place,
Scale factor = (0.736)/(5.0) = 0.1472
You use this scale factor to multiply all the full-size dimensions to find the "model" dimensions. If a Sidewinder is 10 feet long (yeah, there are gobs of different versions and they all have slightly different lengths, but we are just using it for an example here) that converts to 120 inches for full scale. Multiply by the scale factor:
"Model" length = (scale factor) (full-size length)
= (0.1472) (120 inches) = 17.66 inches
Your completed model made from BT20 would be just about a foot-and-a-half long. This is a good size for working on and still big enough to be able to add some scale detail and markings, and would probably fly great on 18mm B and C motors. It might even fly OK (for small fields) on 13mm A motors. If the span of the fins on the full-size missile is 24.8 inches, your model would look like:
"Model" span overall = (scale factor) (full-size fin span)
= (0.1472 ) (24.8 inches) = 3.65 inches from tip to tip
To figure out the span of each fin you would need to subtract the body diameter (for the BT20) and then take the remaining length and divide it in half:
"Model" fin span per side = [(3.65 inches) - (0.736 BT20 diam)]/2
= 1.46 inches per fin
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If you repeat this same process to check how big a scale model would be using BT60, you start by figuring out the new scale factor. (These are a whole separate set of calculations that do not mix with the BT20 example above.) The equation form is the same but the number for the BT diameter is different:
Scale factor = (BT60 diam)/(full-size diam) = (1.637)/(5.0) = 0.3274
If the full-size missile is still 10 feet, or 120 inches, then the "model" length at the new scale factor will be:
"Model" length = (scale factor) (full-size length)
= (0.3274) (120 inches) = 39.29 inches
Now that's going to make a respectable-sized scale model! A model rocket that big is probably going to need a 24mm C11 motor, or a 24mm D12, or if it is light enough you might be able to use an Estes 24mm E motor (important note: Estes E motors may have a bit more impulse than Ds but they have less thrust and may not be safe for a heavy model rocket.) This size model rocket is beginning to "need" a composite E or F motor.
The corresponding overall fin span will be:
"Model" span overall = (scale factor) (full-size fin span)
= (0.3274 ) (24.8 inches) = 8.12 inches from tip to tip
"Model" fin span per side = [(8.12 inches) - (1.637 BT60 diam)]/2
= 3.24 inches per fin
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You are going to have to make the choice on how big your want your scale model to be. Bigger usually means more "WOW," bigger motors (and more expense per launch), more problems packing the car or storing it at home later. Smaller usually means fewer tears when you have the eventual lawn-dart, smaller motors (and more launches per $$), easy ground handling, better fit with most launchers, possible use of cardboard for fin material, etc. But you have to make the decision on scale factor.
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From here, I think you can see how to make a rough check on how big your scale model will turn out depending on what size body tube you want to use. Don't forget about nose cones....for a BT20-sized model there are several inexpensive balsa nose cones available from Balsa Machining Service, or from TRF's own "Sandman" (Gordon A.; PM him for more help), or from vendors like Uncle Mike's Rocket Shack, and many more. For a BT60-sized model you could use the plastic nose cone from an Estes Baby Bertha---these are also available from many places and would get you a good start for a scale-looking Sidewinder.
If your scale model has a more unique profile, you can get custom balsa nose cones made to order from Balsa Machining Company and Sandman's "Roachwerx" (did I get that right, Gordon?). You will need to be able to describe the exact lengths and diameters of your nose cone shape.
I hope that gets you started. Just for grins, do the math and see how long a scale Sidewinder is if you make it out of BT80 (outside diameter 2.60 inches). Then imagine mounting this thing on the roof of your car, pointed forward, and try to guess how far you would get before you got pulled over and questioned....
Come on back, Marlin, what's your next question?
Can you say what missile you would like to model?
Once I have measurements for a rocket, how do I "scale it" to a model. Seems rather confusing. Perhaps someone knows of a website or a book that explains how to build to scale. Thanks
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