# Building to scale

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#### Marlin523

##### Well-Known Member
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

if you have full scale measurements divide them by whatever scale you are. If you are building 1/10th then divide by 10, 1/30th divide by 30, etc.

Thats the way I do it. You can multiply by % and such but I find this the easiest.

Ben

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

-----------------------------------------------------------------

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

-----------------------------------------------------------------

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.

---------------------------------------------------------------

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?

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

-----------------------------------------------------------------

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

-----------------------------------------------------------------

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.

---------------------------------------------------------------

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?

Correct so far except to reach me go to https://excelsiorrocketry.com/
and contact me through the Excelsior web site.

Thanks guys. That gives me something to work with. I've seen Rocksim9 upscale rockets but I don't think you can downscale with it - True?

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

It's fairly simple and straightforward...

The "best" way to go about it is to first decide what size you want the model rocket to be, meaning choose what size tube you want to use. You're most limited on tube sizes, as they only come in SO many sizes and you don't really want to choose a scale that will end up somewhere neatly in between two tube sizes if you can possibly help it. Let's do a little example:

Say I want to build a Saturn V. The real Saturn V was 33 feet in diameter, or 396 inches*. I think I want a BT-80 based model, so the BT-80 based rocket will be 2.6 inches in diameter. If I divide 396 inches by 2.6 inches, I come up with 152.307, which means my rocket will be approximately 1:152 scale. Divide 2.6 inches by 396 inches, and you get 0.006565656565.... this is your "scaling factor" that you will multiply every other measurement by to find out what that part will measure on the model. NOTE: You have to put ALL your measurements into the SAME UNIT OF MEASURE (I'm using INCHES here so everything has to be in INCHES for this to work. Of course to get inches, just multiply the measurement in feet by 12... or if it's in meters, by 39.4 to get inches... you can easily look up "how many inches in a meter" on any search engine and it will instantly tell you (or any other unit of measure conversion). The REAL Saturn V was 363 feet tall* so multiplying that by 12 means it was 4,365 inches tall*, so if we multiply 4365 by 0.00656565 we get 28.5999 inches, so the model rocket would be about 28.6 inches tall.

Now you just convert every other measurement on the rocket into inches, multiply by the scale factor, and you'll get the length, diameter, etc. of the part in question. Say we want to know what size tube to use for the S-IVB third stage-- which was 260 inches in diameter (about 22 feet*) we take 260 multiplied by 0.006565656565 and we get 1.707070707 inches, which is a little larger than a BT-60, which is 1.6 inches in diameter-- adding a couple wraps of typing paper should make up for the 1/10 inch difference, or if it's just a sport rocket, that's close enough for most folks.

It's easiest to do if you have a picture of the rocket, or best yet a line drawing (maybe an engineering drawing with the original measurements on it?) then either copy that and write the model measurements on it as you sit with a calculator and figure them out, then you'll have a blueprint with all the measurements on it ready to construct your model.

Now, you CAN use metric on this, or even convert metric figures into inches if you prefer, but it's a bit more confusing and you have to check your numbers to make sure they 'make sense'. For instance, if you forgot to multiply the 363 foot tall height of the Saturn V* by 12 to put it in inches (4365) before you multiplied it by the scaling factor (0.0065656565) you'd get 2.383 inches... obviously there's a problem because the rocket would NOT be only a little over 2 inches tall! Once you do it a few times, (converting measurements) it will become second nature to you...

Also, angles DO NOT change with scale-- so don't multiply any angles by the scaling factor. The angles all stay the same, only the component size changes.

Good luck and hope this helps! OL JR

*= rough off the top of my head figures for the example. Yeah I know they're a bit off but I wasn't digging out ROTW for an example...

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The hard parts of building scale are:

1. getting as much information about the design of the "prototype" (the original rocket) as you can
2. creating all of the little detail parts, or at least as much of them as you want to add.
The first item takes research. The second one takes craftsmanship. You can get help with both of them by posing specific questions here. Enjoy!

MarkII

You mentioned information from a book.

Before I found this community here at TRF. I pciked up a copy of "Model Rocket Design & Construction" off of Apogee Components." I found it to be a fairly good resource of information. The chapter/s on scale model building were quite good, and it's not very exspensive.

A lot of good answers already posted here. See this sort of similar thread posted in &#8220;techniques&#8221; in the last week, which was based more on scaling rocket kits (like say a BT-80 version of a Baby Bertha)

Also see this Scale Tips page I created for NARAM-46, 50, and 51. This particular tips page is the version for &#8220;Scale&#8221; at NARAM-50, as opposed to &#8220;Sport Scale&#8221; held at N-46 and N-51 (Sport Scale models are not measured but proportions judged by eye, based on photos and/or drawings that do not need dimensions).

https://homepage.mac.com/georgegassaway/GRP/CONTEST/TIPS/N-50/scale.htm

A lot of useful info there including how to work out the scale factor, put dimensions on a drawing (for your case you only need to sketch out something really crude you can identify as to what dimensions go where for building the model), and various other tips for doing scale models.

- George Gassaway

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Thanks for the time all of you have spent giving me some great info. Much appreciated.

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