CTI C-Star 5800 AKA "Shredder" with a composite airframe

[REK]

I don't think you can confidently say that without data to back it up. I'd be very concerned about single thickness, unsupported spans of hobby "grade" tubing or components under those types of loads. Maybe someone can provide data to help you figure it out, but it's unlikely.

This project is extreme for the hobby, it's not prudent to assume off-the-shelf components will work in a "big Estes rocket" type of configuration.

My apologies, I should have provided the data that mccordmw already provided.

Still with just pure analysis of already successful flights I can say with confidence that it will work.


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[bobkrech]

Can you give us an example of a MD flight that was successful without a protected leading edge and Nose cone point? If so how fast and high did it go?

Pretty sure you need to protect your LE and nose cone from erosion going Mack 3+

Taking on the N5800 in a MD is going to take you to the limits of ameture rockery, best place to start is by reading about those who have done it before.

https://www.ausrocketry.com/forum/viewtopic.php?f=10&t=3659&hilit=Mad+max

Temperature is the result of energy dissipation, not the cause of it. Your enemy in high Mach flight is energy dissipation and energy management, and 99% of the TRF members do not have a clue about the energy dissipation during and the materials and engineering required for high Mach flights.

The Patriot PAC-3 is an example of a military missile that does what many big amateur High Mach projects would like to do. Go look it up and see what make it tick.

To bring energy dissipation down to the hobby level, lets look a the highest energy dissipation process that hobbyists understand: motor nozzles. You will not see aluminum there, nor titanium nor steel. Rocket nozzles are made from phenolics, phenolic composites, graphite and Carbon-Carbon, not metals. These material pyrolize and ablate (vaporize) at high temperatures rather than melt. They retain strength at high temperature and do not have great thermal conductivity. The surfaces remain hot while the in-depth heating due to thermal conductivity is relatively low.

The motor nozzles, nose tip and leading edges of high Mach missiles are either phenolic, phenolic composites, graphite, carbon-carbon. The nose tips and leading edges can also be coated with ablative coating. For hobby use phenolic XX(X) is much better and lighter than an aluminum nose tip, and graphite is also better. Phenolic XX(X) would also make a much better leading edge for fins if require.

Not all epoxies are equivalent. The cheap aliphatic boat epoxies will melt and flow, and tip to tip coverings will delaminate and act as drag brakes. Real high temperature epoxies are highly cross-linked phenolics that pyrolize into a graphitic structure and slowly ablate. They do not melt and flow.

Real rocket folks do not cover CF composites with fiberglass. Fiberglass melts and peels backs long before a properly made graphite composite does anything!

A few amateurs may get lucky, but most will fail on high Mach flights without to doing the necessary homework. Remember the first routine high Mach rocket flights were conducted the late '40s and early '50s and were fully documented. All folks have to do it to go the Net and search for them......

 

[mccordmw]

Bob highlights exactly why I'm nervous about even attempting high mach flights. The complexity seems to scale exponentially with mach. I'm currently happy staying subsonic to transonic. My first mach busting attempt will be a smaller rocket not exceeding mach 1.25 range.

 

[mpitfield]

Temperature is the result of energy dissipation, not the cause of it. Your enemy in high Mach flight is energy dissipation and energy management, and 99% of the TRF members do not have a clue about the energy dissipation during and the materials and engineering required for high Mach flights.

The Patriot PAC-3 is an example of a military missile that does what many big amateur High Mach projects would like to do. Go look it up and see what make it tick.

To bring energy dissipation down to the hobby level, lets look a the highest energy dissipation process that hobbyists understand: motor nozzles. You will not see aluminum there, nor titanium nor steel. Rocket nozzles are made from phenolics, phenolic composites, graphite and Carbon-Carbon, not metals. These material pyrolize and ablate (vaporize) at high temperatures rather than melt. They retain strength at high temperature and do not have great thermal conductivity. The surfaces remain hot while the in-depth heating due to thermal conductivity is relatively low.

The motor nozzles, nose tip and leading edges of high Mach missiles are either phenolic, phenolic composites, graphite, carbon-carbon. The nose tips and leading edges can also be coated with ablative coating. For hobby use phenolic XX(X) is much better and lighter than an aluminum nose tip, and graphite is also better. Phenolic XX(X) would also make a much better leading edge for fins if require.

Not all epoxies are equivalent. The cheap aliphatic boat epoxies will melt and flow, and tip to tip coverings will delaminate and act as drag brakes. Real high temperature epoxies are highly cross-linked phenolics that pyrolize into a graphitic structure and slowly ablate. They do not melt and flow.

Real rocket folks do not cover CF composites with fiberglass. Fiberglass melts and peels backs long before a properly made graphite composite does anything!

A few amateurs may get lucky, but most will fail on high Mach flights without to doing the necessary homework. Remember the first routine high Mach rocket flights were conducted the late '40s and early '50s and were fully documented. All folks have to do it to go the Net and search for them......

Hi Bob,

Your posts are always interesting to read and insightful. If you are referring to something like this https://www.rocketryforum.com/showt...i-54-4000-Fins-Attached&p=1627474#post1627474 then yes the plan is to swap out to aluminum tip with this. I reached out to Charles Ogino from Carolina Composite Rocketry back in October for this and he is going to be making one for me. The ablative, or high temp epoxy discussion was coming but like I said I wanted this thread to be about the tube choice.

 

[markkoelsch]

So that's my point, it may not be fine.

As I stated: unsupported, single thickness spans of tubing should be kept to a minimum length or eliminated wherever possible.

That is true. It also does not take into consideration of the motor tube.

 

[dixontj93060]

The force calculation is for an angle of attack of 0 degrees. It can be updated to include AoA, but I was assuming if that deviated much, the build was screwed anyway. I do use FinSim separately to calculate if my fins will survive a wild flight.

I'm not understanding this statement. AoA is affected very little by build techniques (most people put their fins on straight). AoA most commonly arises from dynamic flight conditions like crosswinds/shear or arcing from pad launch angle, dynamic stability changes or other.

 

[mccordmw]

I'm not understanding this statement. AoA is affected very little by build techniques (most people put their fins on straight). AoA most commonly arises from dynamic flight conditions like crosswinds/shear or arcing from pad launch angle, dynamic stability changes or other.

AoA is relative to the motion of the rocket into the air. As the velocity increases, the vector of the rocket's forward motion massively outweighs the vector for crosswind velocity. Crosswind is only a factor at low velocities unless you go way up into the jetstream. AoA should be near zero unless something kicks the rocket into a new vector.

 

[mpitfield]

+1 to that. I found the buckling limits dropped very fast when the length increased. Axial compression is way way higher than buckling limits. Long tubes are like wet noodles. :-D

For example, a 4" x 4' CF tube can take 2200 lbs force before buckling.
A 4" x 3' tube goes up to 4000 lbs force. Cutting length by 25% doubled the resistance to buckling. While the numbers may be approximations, it hammers home the scientific theory behind what makes our models strong or weak.

A 4" x 6' tube will buckle at 1000 lbs force. That's about the same force load you'd get from the N5800. Shred likely.

Assuming the airframe is built straight and no kinks in a coupler, crooked nosecone, bend tubes, I'll second that the tubes will hold up fine. I've done a ton of research on axial compression and Euler's buckling, and I'm impressed at how robust both fiberglass and carbon fiber tubes really are. I put out a calculator showing how much force they can take, and you won't come near the limits of a carbon fiber tube. A 4' long fiberglass tube is almost beefy enough to take the force, but not quite.

Your carbon tube can take over 2x the force it will experience before it will buckle. (2,261 drag+inertia lbs force limit vs 993 lbs force from the flight)

View attachment 307449

Very cool and if the math is correct it looks like a great tool, how do I get my hands on it? One thing I noticed is that the airframe length is 48", my current model the booster is 59" with the motor extending 48+ of those inches and a 12" coupler/AV-Bay that extends 6" into the booster, leaving about 5" for recovery gear. The payload is 19" again with the AV bay extending 6" into it. I am sure that I can shave some lengths off of these tubes and adjust in other areas such as the fins to introduce a bit more drag.

I have thought of several models. One using the motor hardware as the coupler with no payload tube and my recovery gear and AV in the nosecone or just below it. While this seems to be the simplest and most survivable model it also tends to optimize the model for altitude and one of the challenges is to stay under 50,000' for Algoma and my L3 requirements. Realistically with this motor in a MD config it does not appear to be too difficult to get over that number. So the challenge is to create a model that is less than optimal and hit a specific target of 45K, with a focus on survivability. Quite frankly this is what attracts me the most to this project.

I have also looked at extending the AV bay from 12" to 16", but recessing the aft bulkhead so it is effectively double walled into the booster down to 49". However I am concerned about getting a clean separation at apogee with having to clear 10" of AV bay, as well as getting hung up on the recovery gear. Another option I looked at was double walling from the top of the motor to where the AV bay would be, however this obviously leaves a weak-point where the AV bay meets the double wall.

It looks like I am getting sucked into a deeper conversation here, so for the purpose of the tube strength discussion I think it would help to see a picture of the current model.

 

[mccordmw]

Very cool and if the math is correct it looks like a great tool, how do I get my hands on it?

Link was in the post on the word "calculator", but it's not that obvious to spot.

Tool: https://drive.google.com/open?id=1cGEeEXgq9ScOMsaF5nFxY3m08tIn-kY2F8sNV7NQd8k

The tube length should be for the longest unreinforced section of tubing. That is the weak point.

 

[jderimig]

Why are you using a Conical NC? Isn't a Haack series profile better for high mach flights?

 

[dixontj93060]

AoA is relative to the motion of the rocket into the air. As the velocity increases, the vector of the rocket's forward motion massively outweighs the vector for crosswind velocity. Crosswind is only a factor at low velocities unless you go way up into the jetstream. AoA should be near zero unless something kicks the rocket into a new vector.

Well maybe for an N5800 flight, but "near zero" is not true for many of our standard flights. Rocket velocity of 450fps hit with 10mph crosswind results in AoA of 2 degrees which could perturb the fins.

 

[mpitfield]

Why are you using a Conical NC? Isn't a Haack series profile better for high mach flights?

Hi John,

I also have a new 4” Von Karman nosecone sitting on the shelf, but using the 4” Conical is a matter of personal preference. Keep in mind maximum velocity and altitude is not my goal here so any CD advantages that the Von Karman or Haack series offers works against my goals. But yes as I understand it the Conical nosecone is at one end of the spectrum while the Von Karman and Haack are at the other end of the spectrum.

 

[jderimig]

Hi John,

I also have a new 4” Von Karman nosecone sitting on the shelf, but using the 4” Conical is a matter of personal preference. Keep in mind maximum velocity and altitude is not my goal here so any CD advantages that the Von Karman or Haack series offers works against my goals.

Ok, that's cool. Is there any particular reason you want to use CF for the airframe as opposed to plain thickwall FG?

 

[mpitfield]

Ok, that's cool. Is there any particular reason you want to use CF for the airframe as opposed to plain thickwall FG?

Actually no; to be honest I just picked CF because I had this tube in-stock already from another project I was considering. There was not a lot of consideration specific to this project, it was more an internal discussion that went something like, "hmm what am I going to do with this 4" CF airframe..." and here I am.

On the contrary the purpose of this thread is just that, is the tubing I have up to the task, if not what is.

Are you suggesting that thick-wall FW/FG is a better choice? If so to me that is counter intuitive so I very interested to know you thoughts.

 

[claytonbirchenough]

Subscribed. Will be following this one!

As far as the conical nosecone goes, I know it's a personal preference. I also know you wanted to keep this thread about the airframe but...

Would it be better to have a "blunt" tip on the nosecone to help better dissipate the heat?

 

[Zebedee]

I love the pointy nosecone. I'm also planning a 4" MD rocket but I am going for altitude so I'm planning for an N3301 to keep the speed down under M3 and a VK cone. 2nd flight might be a conical NC. or it might be the VK and an O3400.

 

[jderimig]

Are you suggesting that thick-wall FW/FG is a better choice? If so to me that is counter intuitive so I very interested to know you thoughts.

No I am not suggesting that. Now that I know you already have the tube it makes sense. When you indicated that speed or altitude is not the objective I wondered why you would incur the extra cost of CF when you can get the same strength for less money with FG albeit with more weight.

 

[bobkrech]

Bob highlights exactly why I'm nervous about even attempting high mach flights. The complexity seems to scale exponentially with mach. I'm currently happy staying subsonic to transonic. My first mach busting attempt will be a smaller rocket not exceeding mach 1.25 range.

"Exotic" materials are not required for simple quick rocket flights at velocities to ~Mach=2.5. A fiberboard airframe rocket with AC grade plywood fins and plastic NC assembled with TiteBond(R) (or equivalent) wood glue wiil survive just fine provide the fins are stiff (thick) enough not to flutter and the airframe and NC are stiff (thick) enough not to buckle at max V. If you make it stiff enough, your biggest problem will be finding it after the flight because it goes so high....

Hi Bob,

Your posts are always interesting to read and insightful. If you are referring to something like this https://www.rocketryforum.com/showt...i-54-4000-Fins-Attached&p=1627474#post1627474 then yes the plan is to swap out to aluminum tip with this. I reached out to Charles Ogino from Carolina Composite Rocketry back in October for this and he is going to be making one for me. The ablative, or high temp epoxy discussion was coming but like I said I wanted this thread to be about the tube choice.

The primary airframe requirement is stiffness to prevent column buckling, and it is not subject to much heating so material selection is not as important as the NC tip an the leading edges where peak aerodynamic heating can result in rapid and catastrophic material degradations above Mach=2.5.

Aerodynamic heating is proportional to the product of the Mach Number cubed and the atmospheric density. Going really fast at low altitudes for long times is where you have big thermal failure issues at the NC tip and fin leading edges.

• The aeroheating at Mach 2 is (2/1)^3 = 8 times higher than Mach 1, 16 times higher at Mach 2.5, 27 times higher at Mach 3, and 64 times higher at Mach 4!
• Altitude reduces the heating due to the density reduction: at 22 KFt the density is 50% of sea level so the heating is reduced by a factor of 2, and at 59KFt the density is 10% of sea level so the heating is reduced by a factor of 10.

Aerodynamic airframe heating due to conductive and convective heat transfer from the slipstream boundary layer is several orders of magnitude lower than on the leading edges and not an issue with hobby rockets. CF airframe is the lightest solution to make a sufficiently stiff tube for high Mach flight. A commercial tube made to industry specs and heat treated to 250F/350F should be fine.

 

[mpitfield]

"Exotic" materials are not required for simple quick rocket flights at velocities to ~Mach=2.5. A fiberboard airframe rocket with AC grade plywood fins and plastic NC assembled with TiteBond(R) (or equivalent) wood glue wiil survive just fine provide the fins are stiff (thick) enough not to flutter and the airframe and NC are stiff (thick) enough not to buckle at max V. If you make it stiff enough, your biggest problem will be finding it after the flight because it goes so high....


The primary airframe requirement is stiffness to prevent column buckling, and it is not subject to much heating so material selection is not as important as the NC tip an the leading edges where peak aerodynamic heating can result in rapid and catastrophic material degradations above Mach=2.5.

Aerodynamic heating is proportional to the product of the Mach Number cubed and the atmospheric density. Going really fast at low altitudes for long times is where you have big thermal failure issues at the NC tip and fin leading edges.

• The aeroheating at Mach 2 is (2/1)^3 = 8 times higher than Mach 1, 16 times higher at Mach 2.5, 27 times higher at Mach 3, and 64 times higher at Mach 4!
• Altitude reduces the heating due to the density reduction: at 22 KFt the density is 50% of sea level so the heating is reduced by a factor of 2, and at 59KFt the density is 10% of sea level so the heating is reduced by a factor of 10.

Aerodynamic airframe heating due to conductive and convective heat transfer from the slipstream boundary layer is several orders of magnitude lower than on the leading edges and not an issue with hobby rockets. CF airframe is the lightest solution to make a sufficiently stiff tube for high Mach flight. A commercial tube made to industry specs and heat treated to 250F/350F should be fine.

Some great math there showing the differences at Mach levels. I have read several threads where people have been discussing the differences when it comes to the aeroheating at Mach levels, but this is the first time I have seen some hard math and comparisons. I want to bookmark this one. Thanks Bob.

In regards to the commercial tube you reference do you have any recommendations that meet these specs?

 

[bobkrech]

Some great math there showing the differences at Mach levels. I have read several threads where people have been discussing the differences when it comes to the aeroheating at Mach levels, but this is the first time I have seen some hard math and comparisons. I want to bookmark this one. Thanks Bob.

In regards to the commercial tube you reference do you have any recommendations that meet these specs?

https://www.carbonfibertubeshop.com//index2.html and https://www.acpsales.com/home.html are 2 aerospace composite manufactures that I purchased CF tube, sheet and honeycomb panels for my past DoD work. (I had to pay for a third shift at ACP for one order because SpaceX was using them for 2-shifts a day for multiple weeks....) I won't give any quantitative data for ITAR reasons but properly processed CF composites are quite heat tolerant.

I gave a presentation on Aeroheating and other atmospheric effects at NARCON 2010. I would be surprised if it is not discussed in undergraduate aerospace and/or aerodynamics engineering classes.....

 

[bobkrech]

To add to what I wrote above. Today anyone can buy and X-Winder and make their own filament wound tubes. https://www.xwinder.com/

For $3K to $4K (or what 1 O-motor and L3 rocket costs) you can setup a filament winder that will make up to 8" OD tube by up to 20' long. All you need to get is a winding mandrel, high quality CF tow ($50 a pound) and a good quality phenolic epoxy resin (~$100-$120 per gallon). You will also have to make a heating tunnel to heat treat the composite tube to 250F/350F. It will then cost you about $65 per pound to make high quality CF tubing once you get the technique down. :wink:

 

[REK]

To add to what I wrote above. Today anyone can buy and X-Winder and make their own filament wound tubes. https://www.xwinder.com/

For $3K to $4K (or what 1 O-motor and L3 rocket costs) you can setup a filament winder that will make up to 8" OD tube by up to 20' long. All you need to get is a winding mandrel, high quality CF tow ($50 a pound) and a good quality phenolic epoxy resin (~$100-$120 per gallon). You will also have to make a heating tunnel to heat treat the composite tube to 250F/350F. It will then cost you about $65 per pound to make high quality CF tubing once you get the technique down. :wink:

I wish they had a budget friendly version, which I thought was the goal, but $3K to $4K is already way over.

When they started they had this down to $1.6K. That there is good enough to start off.

Where can I find phenolic epoxy resins? I have searched, but no dice.

Thanks


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[FredA]

We have an X-winder.
Let's just say they are not simple.

The inability to control the OD makes it hard to match the quality of vendors who grind the outside.
Plus - you still have a hard time making NC's and especially couplers - so plan on buying those regardless.

Unless you have a ton of tubes to make and a lot of time on your hands, I'd recommend buying instead of building.

 

[boatgeek]

Another thing to think about is that your weak point in your column is almost certainly not the tubing, even at the longest unsupported span. In buckling failures, it's almost always the connections that get you because they aren't as stiff as the tubing. A 4' 2x4 will theoretically hold several thousand pounds, but holding it dead straight under a load is nearly impossible. Same thing with your tubing. Your connections (ie couplers) will need to be really solid to make it all work.

 

[REK]

We have an X-winder.
Let's just say they are not simple.

The inability to control the OD makes it hard to match the quality of vendors who grind the outside.
Plus - you still have a hard time making NC's and especially couplers - so plan on buying those regardless.

Unless you have a ton of tubes to make and a lot of time on your hands, I'd recommend buying instead of building.

Yeah they made it too complex that I just lost interest.

I make my own roll wrapped and I am pleased with that.


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[Aksrockets]

Assuming the airframe is built straight and no kinks in a coupler, crooked nosecone, bend tubes, I'll second that the tubes will hold up fine. I've done a ton of research on axial compression and Euler's buckling, and I'm impressed at how robust both fiberglass and carbon fiber tubes really are. I put out a calculator showing how much force they can take, and you won't come near the limits of a carbon fiber tube. A 4' long fiberglass tube is almost beefy enough to take the force, but not quite.

Your carbon tube can take over 2x the force it will experience before it will buckle. (2,261 drag+inertia lbs force limit vs 993 lbs force from the flight)

View attachment 307449

How are you calculating fiber angles, fiber volumes and CF properties? Those contribute quite a bit to collum stability.
Here's 2 examples of different tubes, the first with a 0* fiber angle (along the length), the second with a 90* fiber angle. The 0* has a critical load about 15x that of the 90* tube.


BTW, this program is Autodesk Helius and has a free 30 day trial (or 3 years if you're a student).

Lotta blanket statements in this thread.

Alex

 

[mpitfield]

Lotta blanket statements in this thread.

I would say that a high 90% number of threads could also be characterized in the same way.

At this stage I am more inclined to try to put some realistic values on all of the loads and forces that the airframe could be subject to, and multiply those values by a reasonable margin. Then see if there is anything off the shelf that meets those specs, and if not, then compare roll your own viability vs. custom manufactured.

 

[jderimig]

Shouldn't a well designed MD rocket be mostly motor tube under the airframe? That part of the airframe is not going to fail under buckling. How much free length of tube past the motor is really needed? 20" for a well designed rocket especially if you consider NC mounted electronics and a single break recovery architecture?

 

[mpitfield]

for a well designed rocket

That is subjective and depends on the design intentions. If you re read my posts, the model you discuss was considered, as a matter of fact it was my first choice. However my goal isn't to go as high as I can, it is to target 45k. That being said this rocket design is arguably more challenging due to the issues discussed.

 

[jderimig]

That is subjective and depends on the design intentions. If you re read my posts, the model you discuss was considered, as a matter of fact it was my first choice. However my goal isn't to go as high as I can, it is to target 45k. That being said this rocket design is arguably more challenging due to the issues discussed.

Michael, just to clarify I wasn't implying any judgement on the design of the rocket. I was mainly responding to the recent series of posts on column buckling concerns on the airframe. The length of the column vulnerable to a buckling failure is that length that is forward of the motor tube. Regardless of the architecture its a good idea to minimize the length of that section where we can, this was mentioned in post #54.