The Big Ugly 8" build thread (and critiques)

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PhxRocketeer

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I have my L2 and I've built some high performance minimum designs as well as some just for fun types. Now I've decided to build an obnoxiously fat rocket for low flights. And I'm trying to do so relatively on the cheap and pretty custom. I also want to try some new techniques and push some boundaries to keep it relatively light. The goal is a simple to fly rocket that is dependable. I also would love to achieve a flying weight that can accommodate some 38mm motors so it's not always so $$. Aiming to get off a 6' rail with 30+ mph on smaller 350-500N motors (eg. I500, J425R DMS). The build plan currently looks like this (OR file at bottom), but I'm sure I'll end up modifying and missing some numbers.

Structural:
- 8" Sonotube (4ft)
- 3d printed Transition down to 5" (potential to do a FG version if I want to learn)
- 5" thick wall cardboard (~2ft)
- I may split this with a bulkhead if the BP charge is too large to pop the NC.

Fins: (still brainstorming)
- 3 fins because I'm lazy
- Complex sandwich: 2-3 layers of 6oz FG + 1/32" basswood + 3d printed skeleton + 1/32" basswood + 2-3 layers of 6oz FG
I need to calculate if this actually ends up lighter than a traditional plywood. The 3d print here just adds thickness, I may include spray foam as well if that is recommended.
- Will aim to do thru wall fins down to the MMT by using circular saw to cut out the bottom of the booster tube.

Nose:
- 12" 3d printed 'stubby' nose cone
- Will likely need some nose weight to add stability

Motor Mount: (expecting plenty of criticism here)
- 3d printer centering rings (Engineering analysis/testing in process)
- 54mm LOC MMT
- Retainer TBD
- BT-60 stuffer tube. I want to keep motor ejection an option for simple flights. I'll always have redundant ejection but prefer to avoid 2x electronics systems

Recovery:
- TBD 58-70" chute expected. Will be in the 5" section and eject the nose cone.
- Typically motor deploy + Eggtimer Quark for redundancy (2nd electronics if needed)
- Electronics will likely end up in a 3d printed case on the recovery line.

Paint:
- My friend recently vinyl wrapped a rocket for me and it turned out great and actually lighter than paint. So I'll probably do that again except for cone and fins


Currently I have the tubes and I'm planning to work on the booster section first. I've setup a test fixture so that I can optimize the design and weight of the 3d printed centering rings to keep the weight down. Any weight savings in the aft means additional savings in the nose weight I'll need to add. I have a nice test fixture setup for a static load test with very promising results. Aiming for ~1.5-2x margin for a K2050/ 500lb of thrust, which I would never actually fly in it anyway. I'll post some pics later if anyone is interested.

Looking forward to all of the negative feedback on 3d printed parts 😆. But I am looking for feedback on the fins, making an FG transition (thinking of using a 3d printed plug), or other areas of concern. usually I build quick, but I'm going to work on this one slowly and try to get it all right.

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Can you share the picture of the rocket that is vinyl wrapped?
Sure. It looks great from afar. Covers up minor imperfects, but these shipping tubes had deep and wide spirals that still show through. Obviously you can see the seams up close. My friend wraps cars for a living, so it went pretty quickly. Would probably take me much longer to do as well.

But it’s far more durable when bumping around and I weighed it and believe it’s lighter than similar paint jobs (depends on vinyls of course). And it’s ready instantly when you’re done.
 

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If I was RSO for this and you brought it to me for the first flight, I'd be asking a lot of questions around the 3D printed centering rings, and how you determined they're up to the task of the motor you're flying. We've seen a lot of 3D printed components in rockets flown at SLI - it can definitely be done in such a way as to be up to the task.

How thick walled is the construction form your using? Menard's and Home Depot sell stuff that's pretty thin, but the more commercial grade stuff is a lot thicker, and thus heavier. I'm curious about overall weight. The motors you gave as examples all hit pretty hard, so I suspect you've already thought about that.

-Kevin
 
But it’s far more durable when bumping around and I weighed it and believe it’s lighter than similar paint jobs (depends on vinyls of course). And it’s ready instantly when you’re done.
I was curious how he did the seams and the fin area and going up the tube? I know they make a tape that you put down and after you put the vinyl over top of the tape you can pull on one side of the tape and it cuts the vinyl real good and you do the same thing on the other side so that the two of them butt up almost perfectly.
 
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If I was RSO for this and you brought it to me for the first flight, I'd be asking a lot of questions around the 3D printed centering rings, and how you determined they're up to the task of the motor you're flying. We've seen a lot of 3D printed components in rockets flown at SLI - it can definitely be done in such a way as to be up to the task.

How thick walled is the construction form your using? Menard's and Home Depot sell stuff that's pretty thin, but the more commercial grade stuff is a lot thicker, and thus heavier. I'm curious about overall weight. The motors you gave as examples all hit pretty hard, so I suspect you've already thought about that.

-Kevin

Hey Kevin. For centering rings I'm testing them first using a static load test. My rings use a spoked design (shamelessly stolen from another poster who did some thorough analysis). The key is that these spokes are quite thick (currently 22.4mm) which adds far more strength than wide members. In order to do initial testing I've cut the ring into single spoke sections and then add features to do static load tests. It's quite remarkable how strong these materials are. I'm using an engineering grade high temp PLA+ that will be annealed for additional strength.

My math is as follows. The hardest hitting motor I could use (but won't) is a K2050 which has 470lb of thrust, we'll round up to 500lb. Using 2 rings and 10 spokes each then each must hold 500/2/10 = 25lb of thrust. I'm aiming for a 2-3x margin over the biggest motor I could put (though again I'll never actually fly this on a K2050). So 50lb.

50lb of thrust is is equivalent to hanging a 50lb weight in a static load. So that's what I've set up. Since I don't have an Instron tester, I fix those holes to a shelf and then hang the weight from the end, similar to how the motor would pull under thrust.

The tests so far on un-annealed samples have held over 100lb and have not yet failed (see picture). I'll continue to thin it out until I get closer to 75lb and then start doing annealing tests. Annealing should further strengthen the sample by 50-100%.

Once these small scale (and fast) tests complete I'll print the enter ring and do a similar style testing on the full scale ring. Honestly, this is more for fun than it is for anything else. I love a good engineering challenge.

Edit: The Sonotube is a Sakrete 1/8" wall. I (165lb) am able to stand on it on 1 foot. So quite strong.

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I was curious how he did the seams and the fin area and going up the tube? I know they make a tape that you put down and after you put the vinyl over top of the tape you can pull on one side of the tape and it cuts the vinyl real good and you do the same thing on the other side so that the two of them butt up almost perfectly.

Ah, we skipped the fin can and nose cone and I sprayed those. In this case I sprayed before, but I would probably spray after and tape a perfect line next time.
 
In case anyone is following, I performed several iterations of tests on the 3d printed sections. I've arrived at a solution that I think will be plenty strong with a weight of only 135g per ring. The 1/10th section failed at 70lb. The failures actually have less to do with the ribs themselves and more adhesion at the out ring. I may yet iterate further to improve that failure point by using 'via' style drills horizontally to cause the printer to drop some perpendicular lines in the outer ring, improving concentric adhesion. Not necessary, but who doesn't love further improving? 135g is super light so I'm pretty excited. Here are a few pictures of the failures. The testing allowed me to do some iterations on the design to reduce weight and be confident. And I found it super fun.

I'm pretty close to printing the full scale. But at an anticipated holding strength of >700lb I don't really have a means of testing it other than some standing/bouncing to give me some warm fuzzies.

Next challenge is making the 8" to 5" transition. My plan is to use the same spoked wheel design at the top/bottom and both interfaces then mostly 4-5 walls for the rest. I won't need nearly the same strength so I can make it much lighter. I had booked 600g for this transition but after the success here I'm hoping to get that to 300-400g instead.

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I should mention that I recognize that my test setup is imperfect because it uses a straight shelf. This may help or hurt the numbers. When I have time I plan to cut an arc out of a piece of wood to match the shape and start using that instead. I just didn't realize it until too late last night.
 
In case anyone is following, I performed several iterations of tests on the 3d printed sections. I've arrived at a solution that I think will be plenty strong with a weight of only 135g per ring. The 1/10th section failed at 70lb. The failures actually have less to do with the ribs themselves and more adhesion at the out ring. I may yet iterate further to improve that failure point by using 'via' style drills horizontally to cause the printer to drop some perpendicular lines in the outer ring, improving concentric adhesion. Not necessary, but who doesn't love further improving? 135g is super light so I'm pretty excited. Here are a few pictures of the failures. The testing allowed me to do some iterations on the design to reduce weight and be confident. And I found it super fun.

I love the destructive testing you're doing on this, as well as what you're showing. I'm confident this is going to give others ideas to run with, based on what you're doing.

Back to my earlier RSO comment - if you brought this to me, I asked questions and you walked me through the testing you've done, I'd be impressed. I might still give you a little bonus distance, based on motor/rocket size, but with this kind of testing? Odds are very high I'd sign that flight card (no card is guaranteed, I don't care who you are).

-Kevin
 
Don't worry, I love the feedback. I'm excited to take this build slowly and learn through it, rather than my usual rushed schedule. I'll post more updates once I start working on the transition and fins. The fins especially I need to do more research into what has worked well in the past to make light, strong fins for this kind of rocket without dropping $200 on G10 or carbon fiber sheets. I have all of the FG supplies I need for the outer layers, so I'm going to do my best to lighten the interior. I need to read up on how best to test fins to figure out if my stack will be strong enough for all of the loads and strains they'll feel over multiple flights.
 
Looks like one of Ron Shultz's original LOC rockets, but that was done in 7.5 and wood turned transition , Bruiser Style Fins those like as well.

That said, your rocket looks nice.

Is Sono tube really going to be less weight then LOC 7.5" tube?
Just asking as I don't know
Sonotube from Homedepot was $11. I think shipping a 7.5" tube from LOC would end up being considerably more after shipping. It was just a convenient choice and honestly it's a very sturdy and nice piece of tubing.
 
The fins especially I need to do more research into what has worked well in the past to make light, strong fins for this kind of rocket without dropping $200 on G10 or carbon fiber sheets.

Do some searching and reviewing of NASA Student Launch design documents - some teams have done some amazing things with 3D printed fins and fin cans. I can't tell you which teams off the top of my head, but we seem them every year and they keep getting better and better.

You should be able to find some solid info on those docs.

-Kevin
 
Or Build/Print a thrust ring, and you can put in balsa centering rings-ish at that point. 😉
You know my design does in fact have a thrust ring on the bottom centering ring. For whatever reason I hadn't really considered that as part of the strength. But now I'm realizing that may be a major oversight in my calculations.

Edit: Actually I'm not sure. I don't think it changes that the center core of the ring needs to be just as strong. It just improves the 'adhesion' to the wall.
 
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Do some searching and reviewing of NASA Student Launch design documents - some teams have done some amazing things with 3D printed fins and fin cans. I can't tell you which teams off the top of my head, but we seem them every year and they keep getting better and better.

You should be able to find some solid info on those docs.

-Kevin

This ^. The team I mentor has been using 3D printed centering rings for the last few years without issue. If you analyze the structure of a rocket and the transfer of thrust, you'll realize that maybe material selection for centering rings not as crucial as you might think. Of course, you need to look at your design and make your own decision... YMMV.
 
If you utilize a thrust plate the motor retainer thrusts on, from the thrust of the motor's thrust ring; the thrust goes into the body tube and fins, not the centering rings.

At that point the rest of the centering rings do not take the thrust, they center. But you have to consider that the recovery harness(s) needs something strong to connect to, unless your mounting them to the motor or something else.

The rocket in my Avatar has my custom 'motor adapter' centering rings screwed in with 3 screws into the body. I backed up my recovery harness not only to the motor adapter, but also the motor forward closure.

The reason for my custom 'motor adapter' was it was built to be able to take minimum diameter 98mm motors, but that flight was on a 75mm M motor. Put the motor adapter in it, thrust plate to body tube, and screw it in place. Motor retained with the 98 to 75mm boattail by Aeropack
 
If you utilize a thrust plate the motor retainer thrusts on, from the thrust of the motor's thrust ring; the thrust goes into the body tube and fins, not the centering rings.

At that point the rest of the centering rings do not take the thrust, they center. But you have to consider that the recovery harness(s) needs something strong to connect to, unless your mounting them to the motor or something else.

The rocket in my Avatar has my custom 'motor adapter' centering rings screwed in with 3 screws into the body. I backed up my recovery harness not only to the motor adapter, but also the motor forward closure.

The reason for my custom 'motor adapter' was it was built to be able to take minimum diameter 98mm motors, but that flight was on a 75mm M motor. Put the motor adapter in it, thrust plate to body tube, and screw it in place. Motor retained with the 98 to 75mm boattail by Aeropack
Art - Are we talking about the same thing? I'm talking about an outer ring on my aft centering ring. The centering rings are still supporting the force of the motor thrust from the inner tube to the outer tube if you draw the free body diagram. IMO all that does it reduce the reliance on the epoxy joint, but not the ring itself.

The rear ring will still flex under load, meaning the forward ring will pick up its share as well.
 
Art - Are we talking about the same thing? I'm talking about an outer ring on my aft centering ring. The centering rings are still supporting the force of the motor thrust from the inner tube to the outer tube if you draw the free body diagram. IMO all that does it reduce the reliance on the epoxy joint, but not the ring itself.

The rear ring will still flex under load, meaning the forward ring will pick up its share as well.

Sorta and not the same. I am talking about a Thrust Ring that takes the thrust from the motor retainer that puts it on the body tube. It would be made of aircraft plywood, as 8" is a bit large for aluminum thrust plates on many 4" rockets.

Your 3D printed rings would then not take any real load.
 
Sorta and not the same. I am talking about a Thrust Ring that takes the thrust from the motor retainer that puts it on the body tube. It would be made of aircraft plywood, as 8" is a bit large for aluminum thrust plates on many 4" rockets.

Your 3D printed rings would then not take any real load.
Ah. Nothing like that planned. Just a thrust ring on the first CR for a little insurance. All good. I’m confident in the rings holding power at this point.
 
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