Max Thrust Advice for a Large BT-80 Rocket

[AtalaRockets]

Hello,

I have designed a slightly silly, large BT-80 rocket and am concerned about its strength as it seems that BT-80 can be weak due to its large diameter and thin wall. The body will be four 30" length ASP T-80H tubes which have a wall thickness of 0.031" or 0.775 mm according to their website, and the nose cone will be custom 3D printed out of a lightweight PLA and should weigh ~60 g. The parachute is 45" in diameter so it will only hit the ground at around 4 m/s or ~13 ft/s to avoid having the tube to kink on landing which I've read can be an issue with large BT-80 rockets. What is the maximum thrust this thing should be able to take?

Here is a photo of it modeled in OpenRocket (the file for it is also attached):


Note for the OpenRocket file: The engine mount is an Apogee BT-80 quick-change mount.

 

[Art Upton]

Can you upload the ORK file also ? the Photo is not going to give the Open Rocket wizards all they need to answer you.

 

[AtalaRockets]

Can you upload the ORK file also ? the Photo is not going to give the Open Rocket wizards all they need to answer you.

Thanks for the suggestion, I attached it to the original post.

 

[Back_at_it]

I don’t know where the failure point is but I saw someone launch a super big Bertha on an H motor last year and it survived

 

[SolarYellow]

AVOIDING THE BENDS!
Why Super Roc Models Buckle and How to Design for a Successful Flight
by Chris Flanigan (NAR 17540)

This link is set up to download automatically. Copy and paste into your browser bar if your browser is set to reject links like that.

https://www.nar.org/wp-content/uploads/2014/08/Avoiding_the_Bends.pdf

 

[BEC]

Chris would know. He does great analysis on all sorts of different types of models, not just Super Rocs.

I need to see if that program is still out there somewhere (since the Yahoo group he mentions in the report isn't) and run my two models for this year's NARAM through it. This year is C Super Roc duration. I have successfully test flown the second iteration of the smaller model on the intended motor (C12-6FJ) but it would be good to know what sort of margin it has....

Maybe I will just ping Chris....

 

[OzHybrid]

The largest bending force on a long rocket occurs in the exact middle. Glue the joiner in the middle so it's effectively a single section.

You should be good for 15+G loads as long as the rest of the construction is good.
Good luck with your flight.

Longest rocket to date 3500mm long 65mmOD

 

[AtalaRockets]

@SolarYellow, @BEC, thanks for the help! After a little bit of searching it seems like you can still download flexroc from what seems to be Chris’ Google site. I had only worried about thrust causing the rocket to fail, not aerodynamic forces, and I now feel a little foolish and interested in this program. Based on the paper about it, it seems like it does not simulate the effects of thrust from the motor on the airframe, is this true? Should I even be worried about the airframe failing due to thrust as long as I’m only using low-mid power motors?

 

[AtalaRockets]

The largest bending force on a long rocket occurs in the exact middle. Glue the joiner in the middle so it's effectively a single section.

You should be good for 15+G loads as long as the rest of the construction is good.
Good luck with your flight.

Longest rocket to date 3500mm long 65mmOD

Are you basing the G-load on the similar-size rocket you built?

 

[BEC]

Chris pointed me to that same site this morning. The later programs appear to have inputs for motors, so you can analyze. I haven't yet downloaded any of them to look more closely — this is just based on the descriptions Chris put up.

I probably won't be able to play with the programs for a bit. I'm off to observe some tARC flights, and I expect most of tomorrow will be similarly occupied. Since they are for Windows only I can't readily run them on the machine on which I'm typing this.

 

[Art Upton]

IF I Recall Correctly, BT-80H is Thick Wall, not thin wall and can fly on High Power motors just fine.

In the 80s older days I even flew a BT-80 thin wall rocket on 3 FSI F-100 clustered motors just fine. [making it a G motor]

With a rocket that long, it is not the thrust but the length to worry about and BEC has pointed you to the source of understanding that.

Edit: What is the underlying reason for a "Super ROC" length of BT-80H ?? I would have stopped at 2 tubes myself unless it is some kind of competition.

 

[AtalaRockets]

IF I Recall Correctly, BT-80H is Thick Wall, not thin wall and can fly on High Power motors just fine.

In the 80s older days I even flew a BT-80 thin wall rocket on 3 FSI F-100 clustered motors just fine. [making it a G motor]

With a rocket that long, it is not the thrust but the length to worry about and BEC has pointed you to the source of understanding that.

Edit: What is the underlying reason for a "Super ROC" length of BT-80H ?? I would have stopped at 2 tubes myself unless it is some kind of competition.

The ASP T-80H tube wall is about 50% thicker than standard BT-80 and only marginally thinner (0.031" vs 0.035") than LOC 2.56" tubing. BMS' T-80H has a wall thickness of 0.041". What's weird is that based on the weight of the LOC tube from Apogee's website, the ASP tube is 50 g lighter than the LOC tube for the same length, which makes me wonder if either the numbers are wrong or it's made of a less dense, possibly weaker material.

3 F-100s is impressive - that's about 285 N combined thrust assuming they all hit the peak at the same time.

For the reason, I just like building high-fineness ratio rockets. I have had small Estes rockets for a few years but really got into the hobby last summer with a Mean Machine. I then decided it would be a good idea to make another BT-60 rocket double its length, and did that by combining the bottom half of a Star Orbiter, Mean Machine, way too many Estes BT55/60 coupler packs (I wanted the long BT-60 ones for strength and only 2 are included per pack) and some spare BT-60s. In hindsight it wasn't a great idea, and the resulting rocket is highly dubious but hilarious to launch. The BT-80 rocket arose out of trying to make a similarly large, more rationally designed rocket that doesn't seem like it will snap in half every time it lifts off.

 

[SolarYellow]

did that by combining the bottom half of a Star Orbiter, Mean Machine, way too many Estes BT55/60 coupler packs (I wanted the long BT-60 ones for strength and only 2 are included per pack) and some spare BT-60s. In hindsight it wasn't a great idea, and the resulting rocket is highly dubious but hilarious to launch.

Could have used continuous, 34-inch lengths of BT-60 coupler. On the other hand, that basically gets you a chunk of 38mm motor mount tube.

 

[lr64]

Does this break into sections, or is it all glued together? The latter should hold up better. If sections are necessary, best if the middle section is as long as possible.

Seems like the pertinent factors are bending and buckling. 3 or 4 lengthwise stringers might be one way to deal with that, either on the inside or the outside. Maybe each stringer could be a strip of foam with a strip of carbon glued on one side. The other side would be flued to the body tube. These could be inside or outside. Any joints, unless quite close to the ends, should be as snug, straight, and secure as possible.

 

[mcderek]

Check out the Fliss kits Richter Recker if you haven’t. It’s a BT 80 based rocket with a 3X 24 mm cluster. It’s 92 inches long. I Chad staged mine with 3xE12-0 to 3 E12-8. It flew beautifully.

 

[Art Upton]

The ASP T-80H tube wall is about 50% thicker than standard BT-80 and only marginally thinner (0.031" vs 0.035") than LOC 2.56" tubing. BMS' T-80H has a wall thickness of 0.041". What's weird is that based on the weight of the LOC tube from Apogee's website, the ASP tube is 50 g lighter than the LOC tube for the same length, which makes me wonder if either the numbers are wrong or it's made of a less dense, possibly weaker material.

3 F-100s is impressive - that's about 285 N combined thrust assuming they all hit the peak at the same time.

For the reason, I just like building high-fineness ratio rockets. I have had small Estes rockets for a few years but really got into the hobby last summer with a Mean Machine. I then decided it would be a good idea to make another BT-60 rocket double its length, and did that by combining the bottom half of a Star Orbiter, Mean Machine, way too many Estes BT55/60 coupler packs (I wanted the long BT-60 ones for strength and only 2 are included per pack) and some spare BT-60s. In hindsight it wasn't a great idea, and the resulting rocket is highly dubious but hilarious to launch. The BT-80 rocket arose out of trying to make a similarly large, more rationally designed rocket that doesn't seem like it will snap in half every time it lifts off.

Just an FYI, the 70s and 80s FSI F100s were really an F50 AVG thrust and about 49ns total impulse. 3 of them would be about G150.

Now that I think of it, I would use BMS BT-80H and use inside full length couplers to reduce bending. This was also done to make Kraft tubing hold up better to higher thrust motors.

But now that adds weight and means you need to use higher thrust motors. The issue gets circular.

 

[Spacedog49Krell]

Thrust is not the issue; you need high thrust to achieve sufficient launch rod/rail velocity for stability. Keep the CG forward. The CTI motors moved the CG back, changing the bending moment and resonant frequency. Stiffen the empty sections with an extra central coupler or internal stringers.

 

[AtalaRockets]

Does this break into sections, or is it all glued together? The latter should hold up better. If sections are necessary, best if the middle section is as long as possible.

Seems like the pertinent factors are bending and buckling. 3 or 4 lengthwise stringers might be one way to deal with that, either on the inside or the outside. Maybe each stringer could be a strip of foam with a strip of carbon glued on one side. The other side would be flued to the body tube. These could be inside or outside. Any joints, unless quite close to the ends, should be as snug, straight, and secure as possible.

I have thought about using stringers, but wasn't sure what material I should use. The punchiest motor I would fly this on would most likely be an Aerotech G80T, so what size carbons strips do you think I will need? Would balsa/basswood also work, and if so, what size will I need the balsa/basswood ones to be? If I use stringers, could I also use thinner wall tube (like BMS standard BT-80)? I'm trying to keep the weight down as much as possible.

Edit: I attached the velocity and acceleration Vs. time graph from Openrocket for the G80T. The max acceleration is 101 m/s^2, and the maximum velocity is 112 m/s

Edit: Never mind about the balsa/basswood stringers, it seems like spruce would be the best material if I am going to make them out of wood.

 

[AtalaRockets]

Just an FYI, the 70s and 80s FSI F100s were really an F50 AVG thrust and about 49ns total impulse. 3 of them would be about G150.

Now that I think of it, I would use BMS BT-80H and use inside full length couplers to reduce bending. This was also done to make Kraft tubing hold up better to higher thrust motors.

But now that adds weight and means you need to use higher thrust motors. The issue gets circular.

Full length couplers and BMS BT-80H would make me a lot less worried about structural issues, but it seems like it would put this thing above 1500 g, and I don't have an L1 cert unfortunately.

 

[AtalaRockets]

Check out the Fliss kits Richter Recker if you haven’t. It’s a BT 80 based rocket with a 3X 24 mm cluster. It’s 92 inches long. I Chad staged mine with 3xE12-0 to 3 E12-8. It flew beautifully.

I found your post about staging it, that thing is awesome. FlisKits' website says it's BT-70, but it still gives me more confidence in my design. It am guessing it's thin wall based on the weight, as it's only 340 grams for that length.

 

[Alan15578]

I have thought about using stringers, but wasn't sure what material I should use. The punchiest motor I would fly this on would most likely be an Aerotech G80T, so what size carbons strips do you think I will need? Would balsa/basswood also work, and if so, what size will I need the balsa/basswood ones to be? If I use stringers, could I also use thinner wall tube (like BMS standard BT-80)? I'm trying to keep the weight down as much as possible.

Edit: I attached the velocity and acceleration Vs. time graph from Openrocket for the G80T. The max acceleration is 101 m/s^2, and the maximum velocity is 112 m/s

Edit: Never mind about the balsa/basswood stringers, it seems like spruce would be the best material if I am going to make them out of wood.

I would expect standard Estes BT-80 to work well without any reinforcement, but I don't know your building skills, and Estes tubes may not be as good as they used to be. In any event, it is max q that you need to worry about, not acceleration.

 

[lr64]

I have thought about using stringers, but wasn't sure what material I should use. The punchiest motor I would fly this on would most likely be an Aerotech G80T, so what size carbons strips do you think I will need? Would balsa/basswood also work, and if so, what size will I need the balsa/basswood ones to be? If I use stringers, could I also use thinner wall tube (like BMS standard BT-80)? I'm trying to keep the weight down as much as possible.

Edit: I attached the velocity and acceleration Vs. time graph from Openrocket for the G80T. The max acceleration is 101 m/s^2, and the maximum velocity is 112 m/s

Edit: Never mind about the balsa/basswood stringers, it seems like spruce would be the best material if I am going to make them out of wood.

I was suggesting that the the stringer could be a beam. Or you could call it a spar. One spar cap would be the cardboard tube, since it's so wide. The shear web would be the balsa, and then the carbon strip would be the other cap. This would be lighter and stiffer than spruce, except maybe where there are concentrated loads.

 

[SolarYellow]

Since we're still talking about reinforcing the tubes, my suggestion would be to exploit the I-beam characteristic of the tube itself. Place a goodly number of bulkheads (with holes for ventilation) along the length of the airframe to keep the cross section round, then you can rely on the skin strength and stiffness of the airframe itself. This is basically the same idea as "web" on a built-up I-beam or truss type structure. Don't need to be heavy. Estes-like cardstock or balsa should be fine if you have plenty of them.

If you take a wrapping paper tube and bend it, you'll notice it deforming so the cross-section is oval, rather than round. When the closer together sides of the oval get close enough together, the compression forces on the "inside" side are multiplied to the point where a buckling failure occurs. If you can keep the tube round, you'll go a long way toward delaying that failure mode.

 

[lr64]

Since we're still talking about reinforcing the tubes, my suggestion would be to exploit the I-beam characteristic of the tube itself. Place a goodly number of bulkheads (with holes for ventilation) along the length of the airframe to keep the cross section round, then you can rely on the skin strength and stiffness of the airframe itself. This is basically the same idea as "web" on a built-up I-beam or truss type structure. Don't need to be heavy. Estes-like cardstock or balsa should be fine if you have plenty of them.

If you take a wrapping paper tube and bend it, you'll notice it deforming so the cross-section is oval, rather than round. When the closer together sides of the oval get close enough together, the compression forces on the "inside" side are multiplied to the point where a buckling failure occurs. If you can keep the tube round, you'll go a long way toward delaying that failure mode.

The optimal strategy, except for the amount of work, is probably a bit of both, as seen in many airplanes. Or some kind of sandwich.

 

[AtalaRockets]

Sorry for the late response, but thank you for all of these building tips and techniques, I will keep them in mind! I finally got around to installing and running FlexRoc and according to the results this rocket, without reinforcement, should work with the motors I plan to fly it on with a decent margin of safety (20-30% minimum).