# proportional

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#### accooper

##### Well-Known Member
Anyone know where I can find a table or program that will show how to multiply to keep the proportions if say I want to build a rocket like one I have that has a BT-20 tube, but I wanted to make one like it but with a BT-60 tube.

I hope what I'm saying makes sense.

Andrew From Texas

search the archive...somebody posted a spreadsheet to scale up or down.

Is this it?

Is this it?
yeppers, I thought it was a spreadsheet. did you post it originally?

yea, I got that one, but it doesn't have the body tubes that I need, like from BT-5 thru BT-60.

Estes type stuff.

Andrew

yeppers, I thought it was a spreadsheet. did you post it originally?

No, it wasn't me. I've had that for so long I couldn't possibly remember where it came from.

To ACCooper,
The tubes you need are right there, just not under the BT (Estes) designations:
T-5 = BT-5 at .543 o.d.
T-20 = BT-20 at .736 o.d.
T-30 = BT-30 at .767 and so on . . .

The "T" is just the Totally Tubular prefix for the Estes BT tube equivalents.

This what you do to upscale a model rocket:
Lets say you have model that has a BT-30 sized tube and you want to upscale it to a BT-55.

The O.D. of the BT-55 is 1.325 in. and the O.D. of the BT-30 is 0.765 in.
( these measurments came off the chart the Sandman sent you).

Divide the larger tubes O.D. by the smaller tubes O.D.

1.325 divided by .765 = 1.7320261

Measure the length of the original tube, lets say it's 6.125 in.

Multiply the measurement of 6.124 in by the scaling factor of 1.7320261, you come up with 10.608659 inches (I would round it off to 10.5inches).

This is the length your BT-55 tube will be.

Use the same scaleing factor (1.7320261) for all the other measurements, like nose cone length and fin dimensions.

bradycros gave you the correct approach for scaling linear dimensions (lengths, diameters, spans) but just to be perfectly clear:

Angles do not scale

If the fin has a 45 degree sweepback angle on the original, it will still have a 45 degree sweepback angle on any upsize or downsize.

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You where right it is what I needed. I wasn't looking at the tube diameters, just the names, and I din't see to many Estes on there. Sorry for jumping to conclusions.

It will work for me.

Tanks for the help.

Andrew

RockSim 9 has scaling built in. Load a RockSim file of the original design, then select Rocket>Scale>By scale factor, then enter the scaling factor. It will scale up or down by any ratio that you want. You may then have to go in and tweak a few dimensions to match up with standard materials, but this will quickly give you a good starting point.

RockSim 9 has scaling built in. Load a RockSim file of the original design, then select Rocket>Scale>By scale factor, then enter the scaling factor. It will scale up or down by any ratio that you want. You may then have to go in and tweak a few dimensions to match up with standard materials, but this will quickly give you a good starting point.
Can you select stages, sections, fins or subassemblies to scale individually or do you have to scale the whole rocket?

It scales the entire rocket. I suppose you could scale the entire rocket in one file and then copy the scaled parts into another file.

I'm in Rocksim 9 and I have used scaling to make bigger rockets, but do not see how it's possible to scale them down.

Thanks again - I'm reposting to see if I have my time zone set correctly

The scale factor would be less than 1.

Once again you divide the size body tube you wish to end up with by the one you are stating out with. An easy example would be: If you want to half size a model the scale factor would be .5

OK-thanks
When I do (as an example) downscale 0.5 why do the fins seem to change location?

Because it is still using the original 'location' measurement from the original..If the fins are not flush with the end of the body tube in the original, for example are 1" forward, a downscale will still place the fins 1" from the end which wouldn't be correct for the downscale..You will find most of the time the default fin placement measurements are taken FROM the FRONT of the tube not the bottom..

Thanks my friend!!!!!!!

That doesn't match my experience. I just tried scaling a rocket and the fin 'location' scaled at the same rate as all the other dimensions. I scaled both up and down and both worked correctly. I also located the fin relative to the front and then relative to the rear and they both worked correctly.

This was with a fairly old version of RockSim 9 (version 9.0.5f80). What version are you using?

Lots of good info posted by others on how to scale up or down.

This may also be of some help to visualize or math-check once you have your scale factor worked out. A drawing of the Saturn-IB at 1/70 scale. The key thing in this case is that it lists the full size dimensions, and then the 1/70 model dimensions (underlined). You can do the math to divide the original by 70 and see how your result should match the model dimension listed.

The link below should be used to see the drawing at full size.

https://homepage.mac.com/georgegassaway/GRP/Scale/SATURN-IB/Sat-IB-70-100pct.gif

I will also upload it to appear below, but it will be automatically reduced in the process and the text will be hard (to impossible) to read.

- George Gassaway

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That doesn't match my experience. I just tried scaling a rocket and the fin 'location' scaled at the same rate as all the other dimensions. I scaled both up and down and both worked correctly. I also located the fin relative to the front and then relative to the rear and they both worked correctly.

This was with a fairly old version of RockSim 9 (version 9.0.5f80). What version are you using?

Well, I'll be ding banged! It does work correctly..Weird..I am using v9.04f78- yeah I need to install an update...I can't find the file I was messing with where the fins misbehaved when I tried the 'scale to' function..I remember down scaling it and it(RS) kept the same measurements from the front of the body tube leaving the fins 'floating' back behind the rocket:confused2: Just thought it would do that all the time(that was the only time I played around with the 'scale to' function)..So I sit corrected scratching my head..

To: George Gassaway

THANKS for sharing that drawing! That is really cool, and looks like a good reference to keep track of (for me at least, in case I ever build another one). I know a lot of work went into the drawing, and your willingness to share is greatly appreciated.

This what you do to upscale a model rocket:
Lets say you have model that has a BT-30 sized tube and you want to upscale it to a BT-55.

The O.D. of the BT-55 is 1.325 in. and the O.D. of the BT-30 is 0.765 in.
( these measurments came off the chart the Sandman sent you).

Divide the larger tubes O.D. by the smaller tubes O.D.

1.325 divided by .765 = 1.7320261

Measure the length of the original tube, lets say it's 6.125 in.

Multiply the measurement of 6.124 in by the scaling factor of 1.7320261, you come up with 10.608659 inches (I would round it off to 10.5inches).

This is the length your BT-55 tube will be.

Use the same scaleing factor (1.7320261) for all the other measurements, like nose cone length and fin dimensions.

1.7320 is the square root of 3, accurate to 4 digits past the decimal place - one part in 10000. There's no way that could be just coincidence...

To: George Gassaway

THANKS for sharing that drawing! That is really cool, and looks like a good reference to keep track of (for me at least, in case I ever build another one). I know a lot of work went into the drawing, and your willingness to share is greatly appreciated.

Let me second the thanks. I am getting ready to start on a Saturn 1B and that drawing will help me a lot.

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