First attempt at an upscale scratch build, need help

RobPodlaski · Feb 10, 2014

Greetings,

I have had a design for a rocket that I want to build using a plastic model at 1/350 scale as a template. I have several questions, and more than a few roadblocks in my way. The plastic model is 30mm diameter, so I first want to upscale it to an Estes BT60 body tube which is 12mm larger in diameter than the diameter of the plastic model. That is simple to figure, add 12mm to all diameters to get the correct scale. What I don't know, and this is my first roadblock, is does this also mean adding 12mm to the length, fin measurements, ect. to get the correct scale for the rest of the rocket design? The design itself will eventually be up scaled several times, but I first want to test flight stability on a small scale, using a c6-5 motor. I tried a trial version of ROCSIM and found I am lacking the degree in engineering needed to understand how to utilize the program, so I am going old fashioned, graph paper and rulers. I know someone out there has done an upscale, so help me get this design to lift off.

blackbrandt · Feb 10, 2014

You are adding the dimension. You want to make the scale factor be a multiple, AKA, you multiply all dimensions by 1.4. The Handbook of Model Rocketry has a good section on how to scale.

You have an original dimension of 30mm, and you want to get to 42mm, so multiply every dimension except for angles by 1.4.

Also, look at Openrocket (openrocket.sourceforge.net), it is an opensource program that does almost the same stuff as rocksim.

Welcome aboard!

Matt

Cookie the Dog's Owner · Feb 10, 2014

Your scale factor is 42/30, or 1.4. Multiply all dimensions by 1.4, and that will give you the correct size,

kjohnson · Feb 10, 2014

Just apply that 1.4 to the dimensions, not any angles you have in the original. The angles will be the same.

kj

K'Tesh · Feb 10, 2014

30mm is original (100%)
42mm is goal (X%)

mathematically the ratios should be the same

30mm/100%= 42mm/X%

so

42*100=30*X

4200=30X

4200/30=X

X=140

So like everyone says to do, multiply all your numbers (except angles) by 1.4 and you've got your new numbers (except for angles which are a constant).

Now that this is all cleared up...

What's the rocket you're building?

chris m · Feb 10, 2014

And if you use open rocket ,we all will recommend a sim program . Helps with motor choice and cp and cg . Just take that scale factor of 1.4 and click scale and input 140% which is your 1.4 and the program will scale for you . Scratch that you can't scale a model that not there . But make a sim as you go and can up scale when your ready I.e bt 60- 4" or even 10" what ever you want

RobPodlaski · Feb 10, 2014

Ok, thanks for the help with scale, after checking my design, and some quick math, I realized at 1.4 scale a C6-5 will not fit. Next logical choice would be to upscale to a BT80 tube, which if the formula is correct would be upgrading from 30mm to 66mm or an aspect ratio of measurement times 2.2 (correct me if wrong). Now, I know my original plan was for a BT60 body which i know can handle a C engine. Anyone ever launch a BT80 on a C engine? The tail of my design on paper right now is 9mm at 1.4 ratio gives me a diameter of 12.6mm (too small for a C engine which is 18mm). So I have no choice but to go to a BT80, and if the measurements are correct, a C will fit using the BT80 which would give me a diameter to scale 2.2 of just shy of 20mm (perfect), the length of the rocket would then be 722mm. I am not sure if the thrust of a C will lift it now. Will build it using BT80 tubes and see how it handles a "C". I am alot close to starting the build than I was earlier, thanks to all who assisted, and replied, if you have any insight before i break out the glue please pass it along.

mikemech · Feb 10, 2014

66mm (2.6") diameter and 722mm (~29") long is a large rocket. I would think that an 18mm motor will be very marginal. Do you have an initial weight estimate? If we knew what you were building it would help us help you.

blackbrandt · Feb 10, 2014

I don't see why you wouldn't want a 24mm motor mount in it... More choices of motors.

powderburner · Feb 10, 2014

RobPodlaski said:
Anyone ever launch a BT80 on a C engine?

You mean, like an Estes Fat Boy? One or two of those have been launched, and fly just fine on 18mm C motors.

You will have to watch out for excess weight. Keep it light.

So, why do you think an 18mm C motor will not fit into a BT60 tube?

Remember, too, that you have other options. BT70 is another size that might work for you, as well as other standard tube sizes. Last resort: roll your own tubes to the size you need.

Zebedee · Feb 10, 2014

RobPodlaski said:
Ok, thanks for the help with scale, after checking my design, and some quick math, I realized at 1.4 scale a C6-5 will not fit.

Why do you think a C6-5 won't fit?

kjohnson · Feb 10, 2014

powderburner said:
So, why do you think an 18mm C motor will not fit into a BT60 tube?

The OP said there is a taper at the tail that when built to scale with the BT60 main diameter is too small.

1/350 is a common ship building scale- my guess is a flying sub.

kj

RobPodlaski · Feb 10, 2014

Well, in order to help clear the air, and let the cat out of the bag so to speak this is the design of the rocket

as you can see from the photo, the rear tapers down dramatically, which is why, from my measurements, using a bt60 tube and the ratio of 1.4 would leave the back end diameter 12.6mm (which is too small for an 18mm "C"). Someone mentioned rolling my own tubes... in order to get the taper in the back that is what i was planning to do, but again, have never attempted anything this complex, and have no idea how to roll my own tubes. I wanted to keep the design a secret, so at my next club launch I could get the cool factor, but I am fast learning the limits of my abilities, hence I am reaching out. Learned quite a bit so far, the plan is to eventually upscale this to 8" diameter, possibly bigger, but first I want to see if this will fly. Was going to call the rocket SUB-SONIC. hope this helps with advice.

RobPodlaski · Feb 10, 2014

kjohnson said:
The OP said there is a taper at the tail that when built to scale with the BT60 main diameter is too small.

1/350 is a common ship building scale- my guess is a flying sub.
kj

pretty good detective work!!

kjohnson · Feb 10, 2014

RobPodlaski said:
Someone mentioned rolling my own tubes... in order to get the taper in the back that is what i was planning to do, but again, have never attempted anything this complex, and have no idea how to roll my own tubes. the plan is to eventually upscale this to 8" diameter, possibly bigger, but first I want to see if this will fly.

For a boilerplate model, you can do a simple cone transition from the larger diameter to the smaller. There are online transition tools available, I'm sure someone will chime in with a link. If you get good results with the small model, you can think about making a more complex shape. One way is to carve a master shape out of foam, and then use fiberglass and epoxy to get a skin.

You could also cheat a little and make the main tube BT60 and transition down to a BT5 to use a 13mm A motor. BT70 as mentioned is 56.3mm in diameter, and Balsa Machining Service sells a really nice ellipsoidal balsa cone for it.

kj

T-Rex · Feb 10, 2014

Given the the original has the propeller shroud, you could adjust the position of that shroud forward a small amount thus hiding the slightly larger rear opening. I seriously doubt that anyone will notice. Just a thought.

K'Tesh · Feb 10, 2014

RobPodlaski said:
Well, in order to help clear the air, and let the cat out of the bag so to speak this is the design of the rocket

View attachment 161683

as you can see from the photo, the rear tapers down dramatically, which is why, from my measurements, using a bt60 tube and the ratio of 1.4 would leave the back end diameter 12.6mm (which is too small for an 18mm "C"). Someone mentioned rolling my own tubes... in order to get the taper in the back that is what i was planning to do, but again, have never attempted anything this complex, and have no idea how to roll my own tubes. I wanted to keep the design a secret, so at my next club launch I could get the cool factor, but I am fast learning the limits of my abilities, hence I am reaching out. Learned quite a bit so far, the plan is to eventually upscale this to 8" diameter, possibly bigger, but first I want to see if this will fly. Was going to call the rocket SUB-SONIC. hope this helps with advice.

Just an idea...

On my Binder Design Velociraptor downscale build, I had to "fudge" the numbers to get a tail cone that would match the 24mm diameter engines (ideally, the tail cone and the nose cone would have been the same shape, I had to settle for a PNC-60RL to get the needed diameter). If you're happy with the BT-60, then why not fudge it a little? You could always make the tail cone swappable so that there's one that would work for your scale needs, and one for flying needs. Perhaps the flying version could be a rear ejection system?

For even more "fun" make it so it can launch missiles/torpedos :wink:

RobPodlaski · Feb 16, 2014

I have started working on the build, anyone have advice on how to build the boat tail transition without using a template from rocksim or other CAD programs which elude me on how to get this design into them? I am sure it would be easier to do this in a CAD program, but I cannot grasp how to put the design I have traced into it. Not to mention, it would solve the question of will this actually fly! I have seen a few videos on youtube from Apogee on building transitions, where he reccomends using rocksim to print off a template. I dont have access to the book he says has the guide for building it, was wondering if someone out there knows a simple way to do this. here are the dimensions i have calculated:

starting at 66mm centering ring > 56mm going aft transition to 61mm centering ring
then from the 61mm ring 56mm going aft transition to 50mm centering ring
from the 50mm rign, 56mm going aft transition to 30mm centering ring
from the 30mm ring, 27mm going aft to a 20mm tube (bt20)

so basically:

66mm ring <-----56mm----> 61mm ring <-----56mm-----> 50mm ring <-----56mm-----> 30 mm ring <----- 27 mm -----> 20mm body tube

advice greatly appreciated!

K'Tesh · Feb 16, 2014

RobPodlaski said:
I have started working on the build, anyone have advice on how to build the boat tail transition without using a template from rocksim or other CAD programs which elude me on how to get this design into them? I am sure it would be easier to do this in a CAD program, but I cannot grasp how to put the design I have traced into it. Not to mention, it would solve the question of will this actually fly! I have seen a few videos on youtube from Apogee on building transitions, where he reccomends using rocksim to print off a template. I dont have access to the book he says has the guide for building it, was wondering if someone out there knows a simple way to do this. here are the dimensions i have calculated:

starting at 66mm centering ring > 56mm going aft transition to 61mm centering ring
then from the 61mm ring 56mm going aft transition to 50mm centering ring
from the 50mm rign, 56mm going aft transition to 30mm centering ring
from the 30mm ring, 27mm going aft to a 20mm tube (bt20)

so basically:

66mm ring <-----56mm----> 61mm ring <-----56mm-----> 50mm ring <-----56mm-----> 30 mm ring <----- 27 mm -----> 20mm body tube

advice greatly appreciated!

Its not that difficult with Open Rocket, however it may be a series of transitions... You can look at my tutorial on nosecones, and that may give you some ideas. Below is an .ork that I whipped up real quick, but you'll need to check your thicknesses as I don't know what you're planning on using for it.

View attachment sub-tail.ork

astronwolf · Feb 16, 2014

There are commercially available tubes between BT-60 and BT-80 for you to consider.

1.8" diameter Aerotech or Semroc tubing
2.04" diameter Centuri ST-20 tubing available from BMS or Semroc
Estes BT-70 tubing available from BMS/Semroc/Aerospace Speciality Products

You don't have to make the leap to BT-80. There are other options.

-Wolf

bradycros · Feb 16, 2014

RobPodlaski said:
I have started working on the build, anyone have advice on how to build the boat tail transition without using a template from rocksim or other CAD programs which elude me on how to get this design into them? I am sure it would be easier to do this in a CAD program, but I cannot grasp how to put the design I have traced into it. Not to mention, it would solve the question of will this actually fly! I have seen a few videos on youtube from Apogee on building transitions, where he reccomends using rocksim to print off a template. I dont have access to the book he says has the guide for building it, was wondering if someone out there knows a simple way to do this. here are the dimensions i have calculated:

starting at 66mm centering ring > 56mm going aft transition to 61mm centering ring
then from the 61mm ring 56mm going aft transition to 50mm centering ring
from the 50mm rign, 56mm going aft transition to 30mm centering ring
from the 30mm ring, 27mm going aft to a 20mm tube (bt20)

so basically:

66mm ring <-----56mm----> 61mm ring <-----56mm-----> 50mm ring <-----56mm-----> 30 mm ring <----- 27 mm -----> 20mm body tube

advice greatly appreciated!

Give this tool a try. You'll find it to be both handy and dandy.

You can print the transitions directly onto the card stock.

www.payloadbay.com

Select the 'TOOLS' button in the upper left hand corner of the home page-
Select 'Transition Tool' when the next page comes up-
Enter your measurements.

RobPodlaski · Feb 18, 2014

Thought you would like to see the progress so far... have completed the motor mount, attached the "fins" inside the prop shroud, put the centering rings on in preparation for the transition for the boat tail, and carved the coning tower "fin".

RobPodlaski · Feb 18, 2014

few more pics of the coning tower

kjohnson · Feb 18, 2014

Good progress. A technique I use when making a paper transition is to have a thicker ring at the large diameter section that sticks out from the body tube about 1-2mm so you have a place to glue on the front of the transition.

kj

Zeroignite · Feb 18, 2014

Veeeery interesting build. I'm gonna follow this for sure.

RobPodlaski · Feb 20, 2014

Transitions completed, sanded, and glued into main tube. A little more sanding to even it out, then on to attaching the fins, and covering the prop shroud.

Airobert · Feb 20, 2014

It's a bit late now, but here is a ratio calculator I use/built in excel.
Simply put the original scale dimension value (Original BT diameter??) and the new value for the same part (New BT diameter??) It then calculates the scale value.
Then just name and measure the original parts and it shows the new measurement.

View attachment Ratio calculator for model rockets.xls

qquake2k · Feb 21, 2014

Okay, now this is one of the coolest designs I've seen in a very long time! A flying submarine! Outstanding! Are you or were you in the Navy?

RobPodlaski · Feb 22, 2014

qquake2k said:
Okay, now this is one of the coolest designs I've seen in a very long time! A flying submarine! Outstanding! Are you or were you in the Navy?

thanks, yeah served 84-94

RobPodlaski · Feb 24, 2014

rear fins attached

this is the pic of the sub for comparison:

First attempt at an upscale scratch build, need help

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