75mm Composite Minimum Diameter Build

robopup

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Back from XPRS, a good time was had by all.

Unfortunately, my 75mm is... somewhere. At least watching from the pad, I believe it was a nominal flight. Some notes:
  • Final sim with correct weights was 43k
  • Motor worked
  • Held together through boost
  • I had one of my homebrew 70cm transmitters in the ebay. The estimated time of flight was 16 minutes. I had good tone for at least 15 minutes, but less than 20. I also had good direction. I have a bad habit where if things are going as expected at the pad, I'm not as intentional as I should be about monitoring tone for loss to get good time of flight. On this one, the tone was so good (I had it with the attenuator on at 15 minutes) I figured I didn't open my main and that I was somewhere close. Hopped in the car and when I got back out at 20 minutes the signal was gone.
  • I had a Merlin transmitter taped to the kevlar immediately above the fincan. I don't get tone until the apogee event because of the carbon fiber. Got good tone around estimated apogee time. The one weird thing I cannot explain about the flight is the buddy who was tracking on my Walston by at camp lost the Merlin at 3-4 minutes.
    • If the fincan had come in hot, I wouldn't expect a nominal time of flight from my other transmitter
    • Perhaps the antenna bent? I've been taping these trackers onto cords for years though, and have never had this problem to the point it causes a decisive loss of tone.
Shot on the pad:

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Still from the video:
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Video - our group was having a little trouble with our wireless box this launch...:
View attachment 75mm_trimmed.mp4

I flew Friday morning and spent the rest of that day and most of Saturday hunting. The black line is my best guess at direction when I had it in the air. I pinned a few other useful locations. EDIT: On second look at this image, a lot of the grid search close to the pad is from a separate hunt a year ago. Everything further out is from this weekend.
2022 search.jpg

In a fit of desperation, I went up Blackrock while my search companions were at the hot springs to see if I could get a tone. Nothin...
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Tony Alcocer was also super helpful with the search on his dirtbike, big time thank you!

I'm still hopeful that with Balls being where it is this year and Tony still poking around, it turns up. I've already begun noodling on the next version of this. Some other threads on this forum have done a little what would you do the same/different which I really enjoyed, so some brief thoughts off the top of my head:
  • Different
    • This is the last flight I think I'll do to 25k+ (other than the 54mm above I still have prepped but never flew) which doesn't have GPS tracking. I've been noodling on adding a GPS to my altimeter or as a separate board for a while and this is definitely the kick in the pants I need to take care of that.
    • I'm going to go with 4 fins for the next design and update my tower over the winter. I usually get pretty straight boosts with these 3 fin designs, but frequently get a bit of a wiggle out of the tower first, which you can see in the video here. I don't want to increase the span much more than I already have on the 3 fin version to keep bending down.
  • Same
    • Recovery+ebay configuration. This design packed super easy and super small. I was really pleased with how that part turned out. And no issues with the altimeter and baro holes at the nose, as far as I know. I am using the baro data for apogee detection, but kalman filtering with accelerometer data which makes all the difference.
    • No tip to tip on fins, continue to use 0.125" G10 with EA-60HP fillets. For reference, simmed max speed was just under mach 3.
 
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robopup

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Hey man, I've recently been reading all about the rocket you just launched, and unfortunately lost. I'm about to start building a 75mm min-diameter carbon fiber rocket to reach Mach 3 as an experiment through school. I've done a decent bit of research into the stability margins during supersonic flight, but I wanted to ask you directly since you had a similar launch to what we're going to experience. Typically the stability of lower-power rockets that we've built is in the 1.8-2.5 caliber range, but I'm curious what number your rocket was at. I understand that the CG moves as the propellant runs out, but in practice, I'm having a tough time figuring out what an appropriate fully-loaded number should be. One design that I have is calculated at about 3.3 cal with full propellant, and it increases to 4 cal at burnout. I would greatly appreciate some insight into this as it seems you've been around this for quite some time. Thanks in advance.

A DM which seemed more appropriate to answer publicly in this thread.

Here are my CG/CP/Stability margin plots in both Rasaero and Open Rocket using final as measured numbers. Historically, I've simply aimed for >2.0 calibers subsonic during design and been reasonably close with final numbers. On this project, I tried to pay a little more attention by plotting stability margin as a function of time.

In general, Cg is going to get better for stability as propellant is burned unless your fincan is super heavy and your nosecone super light. In my case, my Cg goes forward by about 2 in over the flight. This is potentially countered by Cp, which can go forward as speed increases. In my case, my supersonic Cp values never actually go forward of the subsonic in Rasaero, while Cp does move forward by about 4.5 in in Open Rocket. I don't have a great grasp on the assumptions that either program uses for their center of pressure models but watching the actual flight with the wiggle after coming out of the tower (Rasero predict this time to have lowest stability), other flights which Rasaero predicted to be stable but Open Rocket predicted to have marginal stability on and poking around online, I'm more inclined to believe Rasaero.

On my next fincan, I'll likely use a 4 fin version and target 2.0 calibers subsonic in Rasaero and then keep an eye on Cp at higher speeds. I'd like to get rid of that wiggle I've gotten lately coming out of my tower.
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LandonC

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Nosecone mold design:

I've gotten started on the tools for the mold. I'm going to use a similar process to what watheyak used in his 54mm build here. I'll use a male plug and a parting board to lay up the first half, then lay up the second half directly onto the first. The one major difference is that I'm going to use thin 3d printed inserts to apply pressure to the fabric and the seam, rather than a bladder. Here's my 54mm version as an example:
View attachment 527583
Do you think it's feasible to skip this process of making a plug and pouring the molds from scratch and instead 3D print the two halves of the mold? We would obviously smooth the molds and clean them up before the layup process, but is there any practicality in shortening the process and printing them?
 

robopup

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@LandonC - It might be possible. It's something I've considered and might actually do for something small and 1 off like a 24mm I've been noodling on, but I don't think it's something I would bother with for a 3 in nosecone. I have succesfully printed and used molds for nosetips as you describe. Some of the challenges I think you'll have:
  • strength and stiffness - This is the show stopper for me. Even these composite molds eventually get kinda beat up, even more so if you're not getting really clean, gentle pulls. I have a hard time imagining how a 3d printed mold that's not 100% density would last more than a few parts. And if you haven't done a lot of these, don't count on getting an awesome part on your first shot.
  • warping - These larger prints are kinda hard to do without warping. I've found it works better to keep densities quite low - I printed my plug at something like 10-15%, whatever I mentioned in my original post. Unfortunately, this is opposite of what you'd want to have any chance of a 3d printed mold robust enough to handle more than a couple pulls.
  • print time will be very long - especially if you manage to get your density higher per above
  • material cost - I've had much better luck with carbon PET w/r/t warping, but it's not cheap
  • seam lines - Seam lines are usually never as good when you go straight to a mold unless you're really intentional and have a plan on how you're going to do them.
  • keys for alignment - this could be tricky, especially because you'll need to finish the mold
This is all assuming you're running a cheap hobby printer like I am.
 

LandonC

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@robopup - Those are all great points, thank you. Thankfully we have large commercial-grade printers but they do have material costs attached to them. These pieces would be able to print whole without being chopped up, but the significant infill would make these pretty lengthy print times. This is what the draft of our mold looks like. We'll probably end up changing the simple connectors, but this is the general profile we want.
Mold.png
 

robopup

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What material are you planning to run on the fancier machines? If its still PLA or PET or similar plastic, all my points still apply. What other machines do you have access to? If I had unlimited access to a solid cnc'd mill or router and unlimited budget (or at least the budget to consider printing a usable mold) I would go straight to aluminum tooling and not bother with any of this.

Unrelated, if it were me I would leave the whole nose tip in the mold. The fabric lays down easier, and you keep your options open for pulling vacuum/applying pressure etc. You can always wack a tip off these parts which is what I do, but harder to reintroduce after you have your mold.
 

LandonC

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@robopup ,

About 3D printing: We can do SLA, PLA, ABS, PETG, etc. on the main printers (Craftbot XLs). We have smaller printers that can print carbon, but it's too small for this application. There's a handful of Raise3D Pro3s as well as some higher-end SLS and metal printers. There's a huge industrial printer we could use but there are a few hurdles to getting approval for it. We do have budget constraints, so machining a mold out of aluminum would take a significant amount of convincing to get approval for, although we do have the CNC capability. Now that I think about it, an SLS print of the mold would be a lot more practical than a standard PLA or PETG print, since it could give us a longer lifetime of use.

About the mold: I'll adjust the Solidworks part to reflect your comment. Seems obvious now that you mention it. Did you have any issues fastening the aluminum tip to the rest of the carbon cone?
 

LandonC

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The next design iteration for tomorrow is to remove excess material from the mold to cut down on print times.
 

robopup

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@DAllen - HA! Like any good RTF build thread...

@LandonC I have a couple other comments on your mold and images but probably makes sense to move over to your thread for those, I'll answer there. W/r/t the tip - no issues fastening. It's not permanently attached either - it simply threads onto a bolt coming out the front of the ebay. I did print 2 tools to get things just right though - one shown earlier in this thread which was used to cut the epoxy tip off and another to drill a straight hole for the step in the metal tip to insert.

Here are a few more pictures of my setup if they help. Shown with a 54mm which has the same design.
IMG_3061.jpg

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LandonC

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@robopup What does the joint look like between your main body tube and the nose cone? You mentioned that you had no shoulder on the NC, so I'm curious what your whole parachute/shock cord/e-Bay layout is. I tried to piece it together looking through your thread but I'm still a little unclear on it.
 

LandonC

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Also, what material is your e-bay made out of? You mentioned that there was a kevlar pass through but what is the kevlar attached to in the tip holding it all together? I haven't used a setup like this before.
 

robopup

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@LandonC - The motor acts at the coupler between the nosecone and fincan. The innards look(ed?) like this:
nosecone_joint.png

recovery.png
The relevant image of the recovery setup is reattached below - all this fits into the nosecone and the last ~4 in of the nosecone slide onto the top of the motor.
y_harness.JPG

The ebay material is PLA, but at least structurally, it doesn't matter what it is. It takes very little load. The nosecone+electronics+recovery is a) not very heavy and b) all its weight and load it causes are held by the kevlar.
nosetip_attachment.png

This is just the latest iteration I've landed on in years of trying and then tweaking various concepts. I'm attaching my L3 build here also for my bread and butter recovery setup I fly the most (it just doesn't pack quite as small which is why I've moved on). Adrian Adamson comes to mind as having a fairly unique design with his chute cannons he's been very successful with - I tried it and made it work but wasn't for me. Glad to see you're spending a ton of time reading the forums - that's half the battle, but I only use it for inspiration. The best thing you can do (and perhaps/hopefully you're already doing this) is to start building a few of these concepts - even if it's not on a full up flyable rocket, I would start building parts and seeing whether you like the design and assembly or not.
 

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LandonC

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That is extremely helpful to visualize everything, thank you. We're planning on starting fiberglass prototypes of stuff this weekend actually.
 

LandonC

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What type of rail buttons did you use? I've seen some teardrop-shaped ones but I wasn't sure if standard buttons were suitable for supersonic flight.
 

LandonC

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Not sure if you've mentioned this already, but did you run a GPS and/or wifi switches in your e-bay? It's my understanding that CF is good at blocking radio waves, so I'm currently trying to work out how to get those components to function inside our NC.
 

rharshberger

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Not sure if you've mentioned this already, but did you run a GPS and/or wifi switches in your e-bay? It's my understanding that CF is good at blocking radio waves, so I'm currently trying to work out how to get those components to function inside our NC.
Typically GPS and WiFi switches would be in either a fiberglass nosecone or altimeter bay section to prevent signal attenuation(?) caused by the carbon fiber.
 

robopup

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No GPS (unfortunately). All my new larger rockets like this will have one. No wifi switches either, just basic no frills PCB screw switches - see the picture of my ebay on the first page.

Rich is correct - carbon attenuates the signal significantly. This project had 2 beacon transmitters, a Merlin and one of my homebrews. The merlin was taped to the drogue shock cord. I have a very faint signal on the pad, lose signal on ascent due to the carbon fiber, then regain a solid signal if I deploy at apogee (at least enough to get the transmitter out). At least as beacons go, I consider this a feature and not a bug. I really like the positive confirmation that *something good* happened at apogee. I'm also keeping an eye on time of flight to see if the time I regain signal compares to the simmed apogee.

My homebrew is in the front of the nosecone. Here's the relevant build picture on how I get around the carbon fiber:
transmitter_location.png
 

plugger

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I don't know if it's true or not but I've been told that CF tubes start attenuating RF signals (in terms of electronics inside the tube) once the device is one caliber deep inside the tube. I normally fly with FG NCs and CF airframes and I've found that a GPS tracker/transmitter can sit slightly below the cf line (if that makes sense) without any rf attenuation occurring.
 

LandonC

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Here's my situation: We want to make the e-bay act as the coupler between the NC and airframe, both of which are CF. E-bay will be fiberglass most likely, G10 bulkheads etc. Is it practical if I wanted to package the GPS in the lowest point of the e-bay where it wouldn't be covered by CF AFTER separation at apogee? E-bay is probably eight inches long, so the bottom four inches would be exposed after separation while the upper 4in. remains in the NC.

We obviously wouldn't have a GPS signal at launch, which isn't too vital, but do you think we would encounter issues acquiring the signal once the rocket separates 50k feet in the air?
 

robopup

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I have a new project I'm working on where I'm considering taping a GPS tracker I'm working on to the shock cord, but inside a CF tube. No data at this point, I won't be making a decision until I've done a lot of bench testing.

Do you already have these parts? Why don't you stick your GPS in your CF airframe and see how things go?

And if there are problems... just use fiberglass instead. I've never had a structural failure due to fiberglass over carbon fiber use. I think in many cases it probably doesn't matter which you choose, you just have a little less margin. All else being equal, typical fiberglass+epoxy has a stiffness which is about 1/3 that of carbon+epoxy, but the max fiber strain is usually 1.5x or so that of carbon. You can further tip things in your favor by:
  • Reducing the length of your rocket, particularly the length and mass above the coupler. None of my min diameter rockets have more than about a caliber of airframe tubing which isn't supported by the motor.
  • No sloppy tubing connections. All my homebuilt tubes have the exact fit I'm going for. If I'm working with sloppy commercial fiberglass tubing I use aluminum tape to build up thickness where it needs it to get the fit I'm looking for. I also believe I help myself because I only ever use the motor as a coupler - I don't connect tubing sections with composite couplers (not that you can't do this, but it is a failure point I see let go in the field every so often).
 

waltr

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I would NOT do that since many GPS receivers needs several minutes to acquire enough satellites to obtain and good fix.
I have found that the EggFinder mini works best if it has 10 minutes to acquire satellites.
 

LandonC

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Thanks for the clarification. I've finalized the overall design and we're at the point now where little details like this need to be solved before we start building. In my eyes, the most likely solution is keeping the airframe CF but making the NC out of fiberglass instead. We'll have an aluminum tip still.
 

LandonC

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Do you think it's necessary to use a high-temp resin when laying up critical components like the NC or body tubes? I obviously don't want the NC to soften up or a tube to fold, but would using that type of resin just be over-building the rocket? We're still trying to figure out simulations for the heat the rocket will encounter going ~45k feet.
 

watheyak

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Do you think it's necessary to use a high-temp resin when laying up critical components like the NC or body tubes? I obviously don't want the NC to soften up or a tube to fold, but would using that type of resin just be over-building the rocket? We're still trying to figure out simulations for the heat the rocket will encounter going ~45k feet.

How fast will it go? In my experience, only above ~Mach 2.5 does heat start to be a real problem. Depending on the motor you're using, you may reach those speeds.

But lots of people go 45,000' without the added cost and complexity of high temp epoxy. And Mach rash is cool.
 

robopup

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TBH I've never calculated any stagnation temperatures. You're also at the limit of mach numbers I've been to, ~M3. I think as long as you're using a good quality laminating resin (with bonus points for a post cure), aerodynamic heating is pretty far down the list of things I worry about (much less aggressive, but I've had straight PLA 3d printed nosecones hold together at M2.5 as a reference).

Stagnation temperature is pretty straight forward to calculate:

But that's only half the story. What you really care about is heat transfer. If you want to go down the rabbit hole, it looks like there are quite a few hits for a string like "aerodynamic heating missile nose tip". Something like here looks like a good place to start:
In the half hour I poked around, I was in over my head pretty quickly but relevant terms for us are:
  • heating is inversely proportional to the square root of nose radius (ie if it matters for your nose tip material, don't make it sharp)
  • heating is proportional to the square root (I believe?) of air density (not good if you're flying a hard hitting motor like you've chosen)
  • heating is proportional to the cube (Again, I believe) of velocity (same as above)
  • time spent at high heat
I believe you mentioned your project is in preparation for a larger two stage. If you're trying to approximate the conditions that rocket will experience with this single stage, I would also pay close attention to max q. Have you compared the expected max q for this single stage to your other attempt?I wouldn't be surprised if this single stage is higher...

In general, doing M3 low to the ground and holding together is harder than doing it at altitude, so I wouldn't get too locked in specifically on mach number and temperature; there are other factors at play...
 
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