Upscaling calculations?

[Luv2launch]

How is it that you upscale a rocket?Is is divide the OD of the stock kit by the OD of the upscale tube then add that to your tube lengths or multiply?It's been a bit since I did the math for this and i have an upscale in mind that i might do.
Thanks

 

[MarkM]

How is it that you upscale a rocket?Is is divide the OD of the stock kit by the OD of the upscale tube

Divide the desired OD of the upscale by the OD of the original. This is your multiply factor. Then, multiply the lengths of all your tubes and all the fin dimensions by that multiply factor. You may have to do some tweaking on tube lengths to accommodate recovery gear, but the other than that it should all work out. Rocksim will calculate your CP; you have determine your CG once the rocket is complete with recovery gear loaded.

 

[RangerStl]

What MarkM said...

Your tubes are in select sizes, so that is the determining factor in what your scale multiplier is.

N

 

[gpoehlein]

Here's a quick conversion table for standard Estes body tube sizes. Multiply all dimensions of the rocket by the appropriate percentage.

View attachment Scale Factor.pdf

 

[MarkII]

I know that you didn't ask, but you follow the same procedure that MarkM outlined for downscaling as well: take the target tube diameter and divide it by the prototype's tube diameter to get the scale factor. The result will be a decimal number that is less than 1. You use this number to calculate the dimensions for scale models, for instance. Multiply the prototype's dimensions by that number to get the correct proportions for your model.

MarkII

 

[Luv2launch]

I know that you didn't ask, but you follow the same procedure that MarkM outlined for downscaling as well: take the target tube diameter and divide it by the prototype's tube diameter to get the scale factor. The result will be a decimal number that is less than 1. You use this number to calculate the dimensions for scale models, for instance. Multiply the prototype's dimensions by that number to get the correct proportions for your model.

MarkII

Thanks everyone I figured thats how it was done, it hit home when I looked in my copy of the handbook and saw how to downscale that it all made sense.I really need to keep that book where I can find it when I need it.
Technically if your upscaling a rocket that is stable as is wouldn't its upscale still be stable as well?

Well having thought about that it might not be true I upscvales a mosquito and it needed nose weight but i think that was more due to the larger selection of motors it could take being upscaled.

 

[MarkM]

What you said is basically true. Upscales of rockets that are stable as is will normally be stable in the upscale except ...

1) you drastically overbuild the fincan resulting in a very heavy aft end.
2) And for short rockets, especially those using large motors. You may need to add nose weight to give stability when flying certain motors.
3) Unique fin arrangements that push the CP far forward. A good example is the Jayhawk. In this instance, the CG is most likely going to be behind the CP for the larger motors so noseweight will be required

The longer the rocket, the more likely you'll be stable without adding noseweight - again depending on motor choice.

 

[Peartree]

The other thing that will vary with an upscale is materials. If you just made a rocket bigger using the same materials it would probably be okay, but that would mean that the "scale" of the wall thickness and nosecone thickness would also be the same and obviously there's no guarantee of that. Chances are often good that you will change nosecone or fin materials entirely, which will further change relative CG/CP placements so it still pays to be cautious.

 

[judo]

you drastically overbuild the fincan resulting in a very heavy aft end.

I can testify to that! (stoopid plywood!)

 

[MarkM]

The other thing that will vary with an upscale is materials. <snip> Chances are often good that you will change nosecone or fin materials entirely, which will further change relative CG/CP placements so it still pays to be cautious.

I don't believe this to be true...not if you hold your dimensions true to the original. My experience with upscales, which I've done several, is it depends more on the overall style of the rocket than the materials used. For example, I have built two different size upscales of the original Estes Bandit....4" (54mm MMT) and 6" (75mm MMT) diameter versions. While the original used balsa and thin-walled Estes BT-50, my upscales used fiberglassed phenolic tubing, thick plywood, multiple plywood CRs and quite a bit of epoxy - especially in the 6" version. Both upscales still have large margins of stability (>2.5) even with the largest motor they can accommodate with ZERO noseweight. Typically, standard 3 or 4 FNC designs of reasonable length will be perfectly stable in an upscale even for large motors without any noseweight regardless of materials. However, upscales of short, fat rockets like a Big Daddy or Fat Boy will require noseweight just like their smaller siblings do when designed to fly on AP motors.

Now, if I had used 1" thick plywood fins on my 4" diameter Bandit (which would have been silly) instead of the 1/4" I did use THEN I might have had a stability issue. As with any rocket construction you need to wise about material choice and what the reasonable material to use based on your flying.

 

[hardinlw]

Regarding whether the upscale (or downscale) will be stable, the center of pressure will be in the same relative location. For example, if the original had a CP 10" from the tip of the nose and your scale factor is 2, the upscale will have a CP 20" from the tip of the nose. Your materials, construction techniques, and choice of engine will determine the CG. If the CG is one diameter ahead of the CP, consider it stable. If you are upscaling an oddroc, you might want to put the CG in the same relative position even if that gives more than the normal one caliber stability. For example, if the original was stable with a CG 8" from the tip of the nose and the scale factor is 2, the upscale should have its CG no farther back than 16" from the tip of the nose.

 

[InFlight]

Here's some math for ya.

Example: The original is BT-5 (.544" OD) and it's 2" long.

If you're scaling up to a BT-80, which is 2.60" OD, the math is:

2.60 / 0.544 = 4.779 (scaling factor: BT-80 OD divided by BT-5 OD)

We now have this scaling factor: 4.779

2" x 4.779 = 9.56" (original BT length x scaling factor = up scaled body tube length)