Second Build - Minimum Diameter Cardboard

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I just got an idea:
I just measured the airframe, and the outer diameter is 1-9/16".

I could take 2" PVC pipe and make my own fly away rail guides.

The leftover ~3/16" space on each side could be filled with foam padding.

I would just have to make sure I measure everything thrice, because I don't have a rail to test this on.

Also, I didn't try to build my own fly away rail guide because the way I was thinking of building it was too dang heavy.
 
Where was the CG fully loaded, how far from the CP, and how deep into the transonic were you dipping?
 
The spiral damage to the airframe makes me think it’s related to the squirrelling.

If the tube failed a little and allowed the airframe to bend, I can see that starting a death spiral with canted thrust that then makes the spiral worse, bending the airframe more etc.

Also, for my 29mm MD on a g80 I only use 1/2 the black powder and even that broke the shock cord on the first flight and meant the next build revision saw me upgrade to 3mm Kevlar.

Flown twice since, successfully, with 3mm Kevlar and 1/2 the BP charge, which works out to be a little less than 0.7g (g80 ships with what looks like 1.5g)
 
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Did you balance it? My Mini Tomach doesn't have all the gear in it yet but I'm almost certain it is going to need some nose weight for bigger 29mm, let alone a 38mm motor (which concentrates its weight further aft than a comparable total impulse 29mm).
 
Hello Everyone,

I know it's been a while, I got really busy with work.

Where was the CG fully loaded, how far from the CP, and how deep into the transonic were you dipping?

Did you balance it? My Mini Tomach doesn't have all the gear in it yet but I'm almost certain it is going to need some nose weight for bigger 29mm, let alone a 38mm motor (which concentrates its weight further aft than a comparable total impulse 29mm).

I found a CG and CP before I launched it, but after reviewing my work I think I made a mistake when finding the CP.

Regardless, I did not expect an unstable rocket to spin out of control like it did. I thought poor stability would have resulted in an "S" or two drawn in the air.

Moving forward, I bought two books and plan on reading them thoroughly (especially the section on stability):

  • "Handbook of Model Rocketry" 7th Ed, by G. Harry and Bill Stine
  • "Modern High-Power Rocketry 2" by Mark Canepa

The spiral damage to the airframe makes me think it’s related to the squirrelling.

If the tube failed a little and allowed the airframe to bend, I can see that starting a death spiral with canted thrust that then makes the spiral worse, bending the airframe more etc.

Also, for my 29mm MD on a g80 I only use 1/2 the black powder and even that broke the shock cord on the first flight and meant the next build revision saw me upgrade to 3mm Kevlar.

Flown twice since, successfully, with 3mm Kevlar and 1/2 the BP charge, which works out to be a little less than 0.7g (g80 ships with what looks like 1.5g)

I agree, and the damage has made me think twice about using cardboard. I've realized what you gain (or loose) in weight will cost you durability when mishaps like this happen.

Regarding ejection charges, I realize now, I should have done some ground testing (in small incremental steps) before flight. Lots of lessons learned!

I'm in the bottom right hand corner of all those pictures setting up my green and grey Initiator on the new front row rails that Kurt had just put together. I missed the initial launch but witnessed the crash.

You had a front row seat! I'm glad I was set up on the pads further back.

Lessons Learned
  • Stability is highly crucial, and can lead to some very destructive flights if not taken seriously. This means you'll end up wasting your time and money.
  • Ground test ejection charges, starting with small increments.
  • Cardboard airframes might have strong crushing weights vertically, but not horizontally.

Conclusions
I have been considering laying down some fiberglass only over the bent section, but I feel that this would defeat the original purpose the rocket entirely. If I add more weight in the back, then I'm going to need to add more weight up at the nose cone in order to make sure CG is above CP.

I believe that this project is a good idea, but might be frustrating for a beginner like myself who will make mistakes. Making mistakes can lead to bent airframes, leaving you scratching your head on how to fix it (or worse, forcing you to re-do the entire lower booster section, fillets and all).

I'm glad I did this project, considering the cost of the kit ($41.95) and what I have learned.

I'm thinking of doing this same project, but with a fiberglass airframe instead. Maybe with the 1.6" Fiberglass Mini Screech Dual Deploy? If I do go that route, I'll continue posting here so there aren't too many threads.
 
If you do change it to minimum diameter [mini Screech]remember this......stock fins are 1/16 for use with 29mm motors.

Need to upgrade to thicker stock for flying with large 38mm motors .......3/32 or carbon plate 1/16.

Unless you do tip-tip with the 1/16 stock.
 
If you do change it to minimum diameter [mini Screech]remember this......stock fins are 1/16 for use with 29mm motors.

Need to upgrade to thicker stock for flying with large 38mm motors .......3/32 or carbon plate 1/16.

Unless you do tip-tip with the 1/16 stock.
You know, I didn't think of that.

Is there a risk on the fins snapping on landing?
 
It's not the landing although there is always that concern depending on where you launch...it's the flight stresses. The fins are out there finning away and having a great time at the speeds they are intended to fin. Once the speeds exceed that, the stresses on their little fin lives start to push and pull on the little guys until they no longer wish to put up with it and leave the party. That's the fun and challenging part of building light paper rockets that go fast. If the fins be it one or all give way, the body tube will not survive.
 
Lessons Learned
  • Stability is highly crucial, and can lead to some very destructive flights if not taken seriously. This means you'll end up wasting your time and money.
  • Ground test ejection charges, starting with small increments.
  • Cardboard airframes might have strong crushing weights vertically, but not horizontally.


-Stability does beg some thought (even on stock kits). What methods do you use for calculating CP? If you aren't using Openrocket or Rocsim, the Barrowman calculations are fine for simple geometries like this, and I'd personally steer away from the cardboard cutout method due to the fact that analytically more precise methods are easily available and useable by the average adult.
-Ground testing is the answer! (don't forget to plug the aft end of your tube)
-This is a pretty important concept. Compression and tensile stresses are directly proportional to the force on the member. Bending effects.....those are stress multipliers, and when a rocket flight goes abnormal, the airframe starts to see bending......
Your situation reminds me of a private launch company I interned with and their "Unwritten Rules of Rocketry" (that were written down in OneNote amusingly enough). The most applicable here went something like this: Have you ever tried to break a toothpick by pulling it from both ends? No, you fold it in half. DON'T Put Things Under Bending Loads!
 
It's not the landing although there is always that concern depending on where you launch...it's the flight stresses. The fins are out there finning away and having a great time at the speeds they are intended to fin. Once the speeds exceed that, the stresses on their little fin lives start to push and pull on the little guys until they no longer wish to put up with it and leave the party. That's the fun and challenging part of building light paper rockets that go fast. If the fins be it one or all give way, the body tube will not survive.

That makes a lot of sense, and is the reason why I put a layer of fiberglass over the wood fins on the Mini Tomach.

I was thinking that the G10 fins on the Mini Screech (that I'm thinking of buying) should still be able to hold up to the higher speeds of 38mm motors, but now I want to do more research.

The Mini Tomach was definitely was a lot of fun. The cardboard was easy to work with and I learned a lot. Part of me wants to buy the lower airframe and fins to try it again.

The thing is, I have some electronic payload ideas I want to experiment with, and I'm thinking that having to rebuild a paper rocket all the time might get in the way of that.
 
-Stability does beg some thought (even on stock kits). What methods do you use for calculating CP? If you aren't using Openrocket or Rocsim, the Barrowman calculations are fine for simple geometries like this, and I'd personally steer away from the cardboard cutout method due to the fact that analytically more precise methods are easily available and useable by the average adult.
-Ground testing is the answer! (don't forget to plug the aft end of your tube)
-This is a pretty important concept. Compression and tensile stresses are directly proportional to the force on the member. Bending effects.....those are stress multipliers, and when a rocket flight goes abnormal, the airframe starts to see bending......
Your situation reminds me of a private launch company I interned with and their "Unwritten Rules of Rocketry" (that were written down in OneNote amusingly enough). The most applicable here went something like this: Have you ever tried to break a toothpick by pulling it from both ends? No, you fold it in half. DON'T Put Things Under Bending Loads!

Aside from the software you can use, the Barroman method seems popular for finding CP. There is a lot of info online about it.

I love the toothpick analogy!
 
If you want a paper rocket that will stand up to dern near anything you can chuck at it and provides experimentation opportunity, look into the LOC Nuke Promax and the Vulcanite rockets. LOC's 54mm tube is seriously stout. Mine has taken J flights without blinking, no glass or other mods what so ever. Heck, I am thinking about building one of their Black Brants or IQSY (my favorite). It's the one in my avatar and served a great L2 platform but tracking is a must.

My Nuke has been an awesome test bed. I have tested out three different electronics bays and I am about to make another one. All I need to do is finish a new upper section and the sled. I actually plan to make another Nuke in the future for twin 24mm CTI 6 grain cases and dual deploy. Cheap rockets with a lot of personality and potential.
 
TheNewGuy said:
Regardless, I did not expect an unstable rocket to spin out of control like it did. I thought poor stability would have resulted in an "S" or two drawn in the air.

[*]"Handbook of Model Rocketry" 7th Ed, by G. Harry and Bill Stine

I agree, and the damage has made me think twice about using cardboard. I've realized what you gain (or loose) in weight will cost you durability when mishaps like this happen.

I have the Handbook of Model Rocketry 4th Edition. In the Stability and Shapes chapter you'll find a figure (in my edition it's figure 8-9) showing the flight path of a negatively stable rocket. It looks very similar to your smoke trails.

Here's an article by Barrowman himself illustrating the effect: https://www.rockets4schools.org/images/Rocket.Stability.Flight.pdf
 
After reading all you guys' replies, and doing some research, I am starting to see the wisdom behind you guys' comments.

The fins on the Mini-Screech are too thin (risking fin flutter if I went too fast), and the shape of the fins are weird also.

While doing some research, I found this interesting comment:

Fin flutter is hard to nail down. There seem to be no hard-and-fast rules of thumb about when it will occur. There has to be harmonic vibration along the fins induced by the air. This means (at least) that fin shape, material, stiffness and air speed all play a part.

Unless you're planning a high-performance rocket, I don't think G10 thickness will make that much of a difference with fin flutter. However, since your fins are going to be swept back, you may want to go with the thicker G10 to avoid damage on a rough landing.

If you plan a rocket which will break Mach, then fin shape and stiffness become very important. Fins should not be swept back and the length at the tip should be different from the length at the root. (The traditional clipped delta is the most common.)

Also, plywood laminated with fiberglass, or better yet carbon fiber, will be stiffer than G10 (and probably stronger). Plywood and fiberglass make an amazingly good fin material and are quite reasonably priced. You can also get a good shape on the fins (before 'glassing) without breathing fiberglass dust (from the G10).

I already fiberglassed the plywood fins on the Mini-Tomach! I'm thinking if I straighten out the airframe and apply a layer of fiberglass on it, I could make it a better vehicle to break mach than the Mini-Screech with it's 1/16" G10 fins.

I'm just loathing fiberglassing the airframe because it's a bit of a pain.
 
If you want a paper rocket that will stand up to dern near anything you can chuck at it and provides experimentation opportunity, look into the LOC Nuke Promax and the Vulcanite rockets. LOC's 54mm tube is seriously stout. Mine has taken J flights without blinking, no glass or other mods what so ever. Heck, I am thinking about building one of their Black Brants or IQSY (my favorite). It's the one in my avatar and served a great L2 platform but tracking is a must.

My Nuke has been an awesome test bed. I have tested out three different electronics bays and I am about to make another one. All I need to do is finish a new upper section and the sled. I actually plan to make another Nuke in the future for twin 24mm CTI 6 grain cases and dual deploy. Cheap rockets with a lot of personality and potential.

Those are great rockets, I'll keep them in mind!

Twin 24mm 6 grains? Do you have a build thread? Sounds awesome!

I have the Handbook of Model Rocketry 4th Edition. In the Stability and Shapes chapter you'll find a figure (in my edition it's figure 8-9) showing the flight path of a negatively stable rocket. It looks very similar to your smoke trails.

Here's an article by Barrowman himself illustrating the effect: https://www.rockets4schools.org/images/Rocket.Stability.Flight.pdf

Thank you for sharing! I looked up the figure, on the 7th edition, it's figure 9-10 on page 139.

It definitely looks a lot like what mine did.
 
I'm just loathing fiberglassing the airframe because it's a bit of a pain.

Can you get a coupler on the inside of the wrinkled area? It could reshape it and stiffen the area beyond original condition.

I also am putting off fiberglass until absolutely necessary lol
 
Can you get a coupler on the inside of the wrinkled area? It could reshape it and stiffen the area beyond original condition.

I also am putting off fiberglass until absolutely necessary lol
I was thinking about that, and realized if I did I would be limiting the length of motors I could put in the rocket.

I want to go crazy with the motors, which is another reason why fiberglassing the booster section of the rocket would help a lot.
 
I've repaired a busted tube with some fiberglass on the outside and simple Bob Smith epoxy. A repair over a small area will add very little weight and get you flying again.

This rocket flew again two weeks after this prang. I spent one night sanding the paint down in the affected area and applying the fiberglass, and a second night sanding the fiberglass smooth and repainting.

[video=youtube;9IUQYidnFSg]https://www.youtube.com/watch?v=9IUQYidnFSg[/video]
 
I've repaired a busted tube with some fiberglass on the outside and simple Bob Smith epoxy. A repair over a small area will add very little weight and get you flying again.

This rocket flew again two weeks after this prang. I spent one night sanding the paint down in the affected area and applying the fiberglass, and a second night sanding the fiberglass smooth and repainting.

[video=youtube;9IUQYidnFSg]https://www.youtube.com/watch?v=9IUQYidnFSg[/video]
That sounds like what I was thinking of doing. Three only thing I'm thinking of doing differently is to keep fiberglassing up until the switch band.
 
I was thinking about that, and realized if I did I would be limiting the length of motors I could put in the rocket.

I want to go crazy with the motors, which is another reason why fiberglassing the booster section of the rocket would help a lot.

Doh, topic memory fail.

Minimum diameter isn't my style, so I wasn't even considering that restricti on lol
 
I'll second the LOC NukeProMaxx. I've built most of them stock and broken mach and mile without too much work. Add a coupler and a section of BT and make it dual deploy. I do have 1 of mine that I did tip to tip for flights I know will break mach but good epoxy fillets is all that is really needed to keep the fins from giving up the ghost. I don't worry about fly away rail guides, rail guides don't add enough drag to be an issue for me. I've used acme rail guides, PML rail guides and rail buttons. If rail guide drag was an issue, I'd launch from a tower. LOC 38mm and 54mm tubing is extra strong when it comes to paper tubes. Shouldn't have issues with it, even with a non-nominal flight.
 
Since my last post, I have been trying to answer this question: How does the forces from the thrust of the rocket motor act on the airframe?

The reason I am asking this question is simple. If I know how the forces are acting on the rocket airframe, then I know where I need to reinforce with fiberglass. This prevents me from waiting time, materials, and resources. Additionally, I save myself from adding unnecessary weight by only reinforcing a single section.

I think I have found an answer, in a really old post from 2004. I have quoted the entire post, for the sake of context. The important parts are highlighted in bold:

I don’t know how comprehensively you want to go through all the different aspects of design, but even a shallow pass through this stuff might get quite involved. If there is part of this you don’t follow, please don’t be afraid to ask questions, and we will try our best to keep on answering.

Yes, the cross-sectional area (and I am going to use the abbreviation X-sectn) of a 5.5 inch diam body is 23.75 sq in, but this is the X-sectn you would use for aerodynamic purposes. For internal loads (the forces, stresses and strains acting inside the structural components), the X-sectn area you are interested in is that of the airframe tube shell, because the empty space inside the shell is not going to support any structural loads.

If you have a body of 5.5 inch diam, the circumference is 17.28 inches (pi times the diam). If we assume a wall thickness of 0.10 inches, the X-sectn area of the airframe tube is 1.73 sq in (circumf times thickness). And if there are any holes or openings in the side walls of the airframe, they represent a weak point because of the structural cutout (loads are slightly concentrated around the edges of the hole) and for the reduced shell X-sectn in that area.

I am guessing that your spreadsheet represents an aerodynamic drag load acting on the front end of your rocket design? Yes, there will be aerodynamic loads, if your rocket ever gets going fast enough. Before that happens, however, you have some significant accelerations and loads that occur during motor burn. You have the thrust force of the motor (most severe during peak thrust) acting against the inertia load of all components of the rocket that are located ahead of the motor mount. The highest levels of internal loads caused by motor thrust will be found in the airframe shell in the zone at, and immediately ahead of, the motor mounts. As you move forward through your rocket design, past the weight of the lower airframe, past the ejection baffles, past the altimeter bay, etc, the weight (actually, mass) of components that are yet further forward is progressively reduced. The upper sections of airframe do not require the structural ‘beef’ that the lower portions do in order to withstand internal loads due to motor thrust. So if you feel you must reinforce the airframe with fiberglass or carbon fiber, you are wasting your time (and adding lots of dead weight) to add reinforcement to any of the front half (or even two thirds?) of the structure.

Now if you want to build a rocket that will withstand a free-fall from 5,000 feet (ejection failure) and come up out of the ground unscathed, go ahead and reinforce everything. You can even build the front end out of titanium or something.

This ignores the whole matter of the stability (in a structural sense) of the airframe tube, and whether to expect local crippling, column buckling, or other failure modes. Usually something else gets ya long before you approach the raw shear strength limits of the material. Hopefully the manufacturer has performed some realistic tests of his airframe product; you should look for qualifying statements such as ‘1,000 pound load on a 60-inch-long section’ or something to that effect. If you want to test this yourself you can perform a simple test in your garage by standing the tube on end (on a cleanly cut, perpendicular end face) and stacking sandbags (of a known weight) on top. The tricky part will be balancing the whole mess, and applying each additional bag GENTLY. Stand clear, don’t perform this trick within ‘reach’ of anything breakable, and don’t let the kids play underneath the sandbags. And of course, after you test the tube to failure, it won’t be any good to build with.

As far as aerodynamic loads are concerned, an axial load of 1177 pounds acting on 1.73 square inches will give you a stress level of 680 psi. Even if you apply a safety factor of 1.25 to cover the ‘unknown unknowns,’ the stresses would still only be 850 psi, well within the advertised limit of 1000 psi, without any added airframe reinforcement. If your airframe tube has a wall thickness less than 0.10 inches, you will obviously need to re-run these numbers.

I would bet that some of the experienced HPR guys (sorry, I can’t help here because I ain’t one of ‘em) could give you some rules-of-thumb on which you could base your design. I don’t know if the database is really complete enough to try to analyze these designs from an engineering standpoint. It seems you pretty much have to build-n-fly HPR, and adjust the next one to your personal preferences based on experience.

Oh, yeah, almost forgot . . . welcome to TRF!!

el chubbo,

You got the entire idea and applied it correctly. The highest levels of internal structural loads will be at the spot between where the motor is pushing and the airframe above (and contents) is pushing back.

Whether you need to glass that spot, I can't begin to say because I don't have any numbers to begin a 'real' analysis. I have only built a few mid-power rockets, and no high power birds, so I am certainly no expert as far as practical experience. There are undoubtedly some very good reasons for glassing the entire body tube, such as protecting the tube during rough landings. My comments were pretty much limited to the design condition during powered flight.

You are thinking in the right direction, though, as far as alternate ways to beef up the airframe. There could well be ways to construct internal reinforcements (laminate a second piece of BT to the inside?) but unless you are working with some really BIG airframe tubing, you will have a tough time getting your hands inside there to do any work.

As far as in-flight aerodynamic forces, these loads will probably reach a maximum at the moment of burnout (assuming your thrust curve ends fairly sharply and doesn't dribble off to some low level?) when the rocket is moving at maximum velocity. For that condition, you will need to estimate the drag force acting on the nose cone. Keep in mind that this is only one component of the vehicle drag (drag forces on the fins, base drag, skin friction drag are all acting elsewhere). The nose drag becomes the main force acting on the airframe during coasting flight, and the mass of the airframe is pushing against it (inertial loads). So during the beginning of coasting flight the front end of the body tube needs to be strong enough (or, another way, stress levels need to be low enough) to withstand these loads, and the load levels are reduced as you get near the tail of the airframe.

One of the bits of information that is key to this whole question is this: what exactly are the strength characteristics of the body tube, especially under compressive loading? I have not ever seen numerical data for anything, from BT-5 to ten-inch airframe stock. Until we understand how strong the basic body tube is, it will be pretty hard to estimate how much reinforcement is really needed.

This makes sense to me, due to the fact that the airframe in my Mini-Tomach failed right above where the AeroTech 38/360 motor casing ended.

I now have a general idea on how far up from the fins I should be adding fiberglass to the airframe.

After I finish this fiberglassing process, I'll focus on stability and adding nose weight.
 
Have you used the motor spacer system from Cesaroni or Aerotech?

Perhaps you could use coupler segments similarly for shorter motors.
 
Hey Everyone,

After fiberglassing the bent portion of the airframe, and adding 1.5 oz of nose weight, I launched my rocket at LDRS.

It was a sunny day, with an occasional gust of wind right when you decided to launch your rocket.

The fight was a LOT better than my last one, and I'm estimating it probably went right under a mile. It still could have used an oz or two of additional nose weight because there was still a slight "s" in the flight path. I'll see if I can post the video soon.

The motor was an AeroTech H123W.

After launching the rocket, my search for the rocket lasted 6 hours. I searched everywhere around the wheat field, then searched in the wheat field itself (where I suspect it landed).

I had to eventually call it quits and go home, because I had completely ran out of water.

I'm really really really bummed I couldn't find it, but I'm also thankful for how much I learned from it.

In the rocket was my StratologgerCF, AeroTech 38/360 casing, AeroTech 38mm adapter, and 38mm Aft Closure. :(

Next time I know to use a tracker!

I'm already planning my next rocket, in order to continue my addiction.
 
Get an eggfinder mini... I’ve used it in my 29mm MD (after losing one) and I’ve always found it.

Twice it was only the nosecone with my altimeter and gps in it (single use motors with tape retention, so no great loss) but I’ve always got my electronics back...
 
So I'm pretty late to this party, but if you haven't already contacted me for a replacement, shoot me a PM and I will send you a new Fly Away Rail Guide. The latest version has much stiffer rods (better alloy and thicker aluminum for 8x strength) and thicker hinges. Holds up much better to hard landings.

~Andrew
 
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