Body Tube Diameters

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Project_Gemini

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I am a BAR with 21 years of dwell time. I am interested in getting into scale scratch building, but I had a question. How do you keep the rockets true to scale when the manufactured body tube diameters do not equal the scale you would like to use? For example, I am looking to scratch build a PGM-11 Redstone in 1:35 scale. This means the diameter of the Missile Center Unit will be two inches, but there are no quality two inch body tubes. Does it come down to just saying 1:35 scaleish?
 
Try different manufacturers. You might come close to the diameter that you want. One possible source is Balsa Machine Services:

https://www.balsamachining.com/#

You can always choose a BT diameter that is close and then change the BT length so that the scale is correct. You can always cut the BT lengthwise twice and remove the long segment to be used as a joiner to reduce the diameter. This is a lot of work and the joiner will change the inside diameter, if you put it on the inside or leave a bump if you put the joiner on the outside. Some people make the scale approximate and call it "Sport Scale".
 
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I am a BAR with 21 years of dwell time. I am interested in getting into scale scratch building, but I had a question. How do you keep the rockets true to scale when the manufactured body tube diameters do not equal the scale you would like to use? For example, I am looking to scratch build a PGM-11 Redstone in 1:35 scale. This means the diameter of the Missile Center Unit will be two inches, but there are no quality two inch body tubes. Does it come down to just saying 1:35 scaleish?

In the FAI world we lay up our own airframe tubes using custom mandrels and fiberglass. These aren't thick-wall tubes like those found in HP rockets, though, although the fabrication techniques are very similar. For instance, I just did a set of 25mm and 30mm OD tubes for B and C impulse Scale Altitude models. They were made using a using a single layer of Japanese tissue as an underlayment, followed by two layers of .5 oz. glass. West 105/206 epoxy was used.

The resulting tubes are very light, very strong, and absolutely accurate for scale purposes. For your Redstone a 2" OD aluminum tube could be used as a mandrel. The wall thickness on these tubes is measured in fractions of a millimeter, so the scale error of the resulting tube would probably be less than 1%.

The basic technique can be found here: https://www.nar.org/contest-flying/...tion-techniques/lightweight-fiberglass-tubes/

James
 
It really depends on what you want as a final result.

If you are looking to do a real-deal, scale competition grade model you can either base the model's scale on available tube sizes, which gives you some odd scales (1/37.6, for example), or roll your own tubes.

If close enough is good enough, just use the tubes that are closest to the scale you want (1:35-ish, like you said).

I go back and forth myself. On some models I really go scale and build to a strange scale to match the tube diameter. Those end up with weld seams, proper rivet patterns and accurate fin airfoils and generally are long term pains in the neck. They are fun for me because, when I'm in the right frame of mind (or maybe out of it), I really like doing all of that fiddly little detail work. Other times if it looks like the original subject while out on the pad, that is good enough and I don't stress over that third decimal point.

Is there a particular reason you chose 1/35 scale? Do you want it to be the same scale as some other model or is it that it makes a really nice size model in that scale? (It does!)
 
It really depends on what you want as a final result.

If you are looking to do a real-deal, scale competition grade model you can either base the model's scale on available tube sizes, which gives you some odd scales (1/37.6, for example), or roll your own tubes.

If close enough is good enough, just use the tubes that are closest to the scale you want (1:35-ish, like you said).

I go back and forth myself. On some models I really go scale and build to a strange scale to match the tube diameter. Those end up with weld seams, proper rivet patterns and accurate fin airfoils and generally are long term pains in the neck. They are fun for me because, when I'm in the right frame of mind (or maybe out of it), I really like doing all of that fiddly little detail work. Other times if it looks like the original subject while out on the pad, that is good enough and I don't stress over that third decimal point.

Is there a particular reason you chose 1/35 scale? Do you want it to be the same scale as some other model or is it that it makes a really nice size model in that scale? (It does!)
 
erockets.biz has a good selection of tubing - including the Centuri ST-20 2.02" tubing for a 1/35 Redstone.

For a 1/35 Titan GLV, I ended up making a PVC mandrel and wrapping my own 3.44" paper tubing. It's turned out to be a handy size - it's couple hundredeths off of a 1/100 Shuttle external tank, and is the right size for a 1/35 Atlas Agena I have in mind.

That's the most work I've done to meet scale. Mostly I'm a '+/- 5% is good enough' kind of person.
 
Semroc ST-20 tubing is 2.04" OD. Available at erockets. Also, I've cut down slightly larger tubing to the right dimensions in the past for my LJ II clone and Mercury Atlas. This involves cutting a vertical strip from the larger tube and regluing the seams together making a smaller tube. Use the cut strip as a glue tab.

Edit: Also added a link for making your own spiral wound paper tubes from scratch:https://www.instructables.com/id/Make-your-own-Kraft-Paper-Tubes/
 
Pointing out that the Estes (formerly Centuri) Mercury Redstone is made from ST-20 tube, as is the old Estes Jupiter-C. So yes, that means those are slightly closer to 1/34 than to 1/35. So, is there a reason it needs to be exactly 1/35? If so, I'd use the split tube method described above, but I would start with a BT-70 tube for it, as the strip you'd have to take out of the ST-20 would be too tiny.
 
I would like to thank everyone for there recommendations and tips/tricks. There were several questions asking me why it had to be 1:35 scale. The reason for this is the popularity of 1:35 scale in military vehicles and figures in the hobby community. I plan on making a series of U.S. Army missile/rockets all in the same scale, and I came to the conclusion for 1:35 scale.

I think that I got the answer I was looking for. Either I need to do a 1:35 scaleish with existing body tubes or modify them to get the exact scale. I do not feel like I am ready to start creating my own using a custom mandrel.

 
erockets.biz has a good selection of tubing - including the Centuri ST-20 2.02" tubing for a 1/35 Redstone.

For a 1/35 Titan GLV, I ended up making a PVC mandrel and wrapping my own 3.44" paper tubing. It's turned out to be a handy size - it's couple hundredeths off of a 1/100 Shuttle external tank, and is the right size for a 1/35 Atlas Agena I have in mind.

That's the most work I've done to meet scale. Mostly I'm a '+/- 5% is good enough' kind of person.

Boy I wish one of the manufacturers would make 18" or 34" long 3.429 (call it 3.43) tubing...this would make the Atlas (and as you pointed out, Titans, SS ETs ) possible. Now that Estes brought back the 1/45 Apollo Little Joe II, a shorter version of the tube is there (can't order it separately though).
 
Boy I wish one of the manufacturers would make 18" or 34" long 3.429 (call it 3.43) tubing...this would make the Atlas (and as you pointed out, Titans, SS ETs ) possible. Now that Estes brought back the 1/45 Apollo Little Joe II, a shorter version of the tube is there (can't order it separately though).

I wound up making my own 3.42" tube by cutting down a BT-101 tube. Cut a vertical strip from the larger tube. The width of the strip should equal the difference in circumference between the larger and smaller tubes. Glue the cut seams together, using the cut strip as an internal glue tab. Try to be as precise as possible on the vertical cuts to minimize use of filler/putty on the glued seams. I think there was a page showing this technique back when I did it, but can't seem to find it now.
 
What type of glue did you use to glue it together, regular wood glue? Also, did the internal tab add to much extra weight on the seam side?
I wound up making my own 3.42" tube by cutting down a BT-101 tube. Cut a vertical strip from the larger tube. The width of the strip should equal the difference in circumference between the larger and smaller tubes. Glue the cut seams together, using the cut strip as an internal glue tab. Try to be as precise as possible on the vertical cuts to minimize use of filler/putty on the glued seams. I think there was a page showing this technique back when I did it, but can't seem to find it now.

Sent from my SM-G900V using Tapatalk
 
I wound up making my own 3.42" tube by cutting down a BT-101 tube. Cut a vertical strip from the larger tube. The width of the strip should equal the difference in circumference between the larger and smaller tubes. Glue the cut seams together, using the cut strip as an internal glue tab. Try to be as precise as possible on the vertical cuts to minimize use of filler/putty on the glued seams. I think there was a page showing this technique back when I did it, but can't seem to find it now.

I have used this method to make tubing couplers...I used Aileens glue (since it is really tacky and low shrink) and a typing paper tab to minimize thickness. Boy you'd need 3 hands to do this with a long tube (or clever fixturing ;) )

DSC_1769.jpg

DSC_1767.jpg

DSC_1766.jpg
 
What type of glue did you use to glue it together, regular wood glue? Also, did the internal tab add to much extra weight on the seam side?


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Regular wood glue. The strip is thin cardboard tubing, so asymmetrical weight distribution shouldn't be an issue. At the time, I decided to do a LJ II clone from the original Centuri plans because the Estes kit was kinda expensive. Cut down a BT-100 coupler. I'm in the process of stripping the home print decals and paint to apply John Pursley's Accur8 skins for the LJ II, but here's a couple of pics:

IMG_20180217_070925.jpgIMG_20180217_071145.jpg

And since you mentioned the Estes Mercury Atlas, here's my clone:

IMG_20180217_071649.jpg

Found a couple of 3.5" dia. circular cardboard boxes with caps for carrying accessories. Wish I could remember the company's name. When I deconstructed the ends I found it was double lined, so used the inner liner as a coupling, and the end caps as bulkhead/centering rings. Decals were from Sandman. No longer making decals, but I think Mark Hayes at Stickershock23 can do it. Display capsule and tower from Kevin Cespedes at Shapeways (Aerobotix?). Balsa flight capsule and bt-4 tubing and nose cone for the tower from erockets. Tube walls are thick, so rocket is pretty heavy. Did this before I learned about the split tube technique. It may just remain a shelf queen. If you are considering doing a project like this, suggest you build a Dr. Zooch Mercury Atlas first, to get some good spoofing ideas for the details on the rocket.
 
Thank you for the pictures, this really helped.
Regular wood glue. The strip is thin cardboard tubing, so asymmetrical weight distribution shouldn't be an issue. At the time, I decided to do a LJ II clone from the original Centuri plans because the Estes kit was kinda expensive. Cut down a BT-100 coupler. I'm in the process of stripping the home print decals and paint to apply John Pursley's Accur8 skins for the LJ II, but here's a couple of pics:

View attachment 338722View attachment 338723

And since you mentioned the Estes Mercury Atlas, here's my clone:

View attachment 338724

Found a couple of 3.5" dia. circular cardboard boxes with caps for carrying accessories. Wish I could remember the company's name. When I deconstructed the ends I found it was double lined, so used the inner liner as a coupling, and the end caps as bulkhead/centering rings. Decals were from Sandman. No longer making decals, but I think Mark Hayes at Stickershock23 can do it. Display capsule and tower from Kevin Cespedes at Shapeways (Aerobotix?). Balsa flight capsule and bt-4 tubing and nose cone for the tower from erockets. Tube walls are thick, so rocket is pretty heavy. Did this before I learned about the split tube technique. It may just remain a shelf queen. If you are considering doing a project like this, suggest you build a Dr. Zooch Mercury Atlas first, to get some good spoofing ideas for the details on the rocket.

Sent from my SM-G900V using Tapatalk
 
Get length to diameter ratio off of pictures. Solve for length by hobby tube diameter, algebra cross multiply by ratio.. It's not hard. You scale it to whatever tube diameter you can purchase that works for your budget. Done the cad files on HPR SAAB372 and BAMSE in 54mm airframe tubes plus two scratch design/build senior design mechanical engineering project HPR multistage competition rockets USRC SEDS, UTC 3rd place. Scale seems near trival after trying custom supersonic airfoils and even imploding first airframe interstage ever. Got a lesson of hard knocks on interstage design first trip to Utah. Haha.. Get nosecone equations they are invaluable if you can CAD. Personally I use solidworks. Many free options these days. Save as .STL or send to Xometry. Print many transitions and cones alike to scale rocket. You can slot own tubes with computer and get fins CNC'd. Or you can hand cut and draw templates just more time.
 
Then you can select motor tube centering rings off the shelf from Wildman or Loc/Madcow/PML/MAC etc and build. It helps if you have airframe outer Diameter when scaling to it. Swedish missiles like ton of lead weight in nose, like a pound for a SAAB RB-05 with 38mm Mmt. It's Wildman fiberglass kit airframe tube was 57.5mm OD and 54mm ID. It's length was scaled to its Od.
 
You could get OCD about tube sizes but if you just use hobby tube sizes it makes life so much easier. Have you seen quotes on new tube sizes? Don't bother and save some $$$$$$$$$$$$... And rolling own and a custom mandrel is a pain. I scale it all to a hobby tube diameter. Get fin info and rescale it's span to tube diameter by scalar and use scalar to adjust span, root, tip off of what was measured. Use excel it helps. Fins like BAMSE fins are harder use OR free form and some trig if you need to 3D fins in modeling program but I could airfoil those if I really wanted. It's best to not make stuff more expensive usually. Some parts may cost $5-200 to print depending on volume or precision and materials.
 
When you scale to a hobby tube diameter the only thing that you also scale is tube length which is so much more practical. There I'm gonna not try to rant anymore. You weren't wrong you found another way to scale that made it costs too much. Get ratio of real airframe body tube length and diameter that's all you need to start. Then get ratio of real nose length to diameter and scale nosecone for hobby uses. Then do same process on boat tail fin data. I likely couldn't model an N-1... But for many scale rockets that's plenty. Excel helps on fin datapoints if you need that level of detail.
 
If this is a flying HPR model you'll want quality HPR tube materials in adequate area with appropriate inside and outer diameter for a cross sectional tube area that is up to task of possible exposure to hundreds of pound force of thrust or more for stress reasons with a safety factor so it won't fail. You may get a sheared body tube when tube wall is too thin. This is why you won't see commercial scale kits brag about 1:35. They don't care if it's 1:33.456 as long as it flies safely and is economical. If you are making a museum grade static display to match a 1:35 astronaut then fine I won't judge, but a flying HPR model has more pressing issues than a exact artsy scale number. I think even some MPR motors could crack tubes given the right conditions.
 
How does Estes do it's body tube nomenclature? BT-50 that's about 1 inch.
How does 50 relate to an inch!?
 
How does 50 relate to an inch!?
Simple answer: It doesn't. The BT numbering is fundamentally arbitrary, although there is logic in it here and there. For example, a BT-60 can fit three BT-20s in a cluster. Higher numbers mean larger.

So, in general, don't try to look for significance in the numbers. Know the important facts, like BT-5, BT-20, and BT-50 are motor mount tubes for 13mm, 18mm, and 24mm motors, respectively. Committing diameters of the most frequently-used sizes to memory is also pretty useful when scratch-building.

And when in doubt, go to a reference table, such as https://www.ninfinger.org/rockets/body_tubes.html or https://www.erockets.biz/body-tube-sizes/.
 
Ugh! I was reading old Centauri literature and their system was simple. The digits was in tenths of inch. E.g. 13 meant simply 1.3 inches. So when they merged, the sensible system lost out!
 
I am a BAR with 21 years of dwell time. I am interested in getting into scale scratch building, but I had a question. How do you keep the rockets true to scale when the manufactured body tube diameters do not equal the scale you would like to use? For example, I am looking to scratch build a PGM-11 Redstone in 1:35 scale. This means the diameter of the Missile Center Unit will be two inches, but there are no quality two inch body tubes. Does it come down to just saying 1:35 scaleish?

The Redstone has a body diameter of 70" . . . So, for a true 1/35 model, it will be 2", as you said.

Here is a SIMPLE solution . . . Use a "full-length coupler" ( 1.997" O.D. ) for a 2.04" diameter tube and BUILD IT UP with a "Kraft Paper" wrap, then sand, until you get precisely 2.000".

https://www.balsamachining.com/#

1609004984004.png

Dave F.
 
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So when they merged, the sensible system lost out!
Up until recently Apogee used a metric designation for their body tubes.
So a BT-60 tube was a BT-41.6 (41.6 mm ID).
And a BT-80 was BT-66 (66 mm ID).
Very logical, but very confusing to their customers.
Sometimes you have to conform to a long established standard, no matter how illogical it is.
 
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