A few questions as I get started with my first fiblerglass build

Discussion in 'High Power Rocketry (HPR)' started by Idunno, Sep 19, 2019.

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  1. Sep 19, 2019 #1

    Idunno

    Idunno

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    1. I'm trying to select the shock cord i'm going to use for my first fiberglass rocket (scarab 54), and after reading through some of the threads on this forum i'm confused. The rocket will be somewhere in the 3lb range. It seems like a lot of people (at least in the threads I was browsing) are using 1/4" kevlar for rockets of similar size, but that seems like overkill to me. 1/8" kevlar has around 1200lb tensile strength. But then they use a 1/8" quick links, eye bolts and swivels, all of which are rated for 200lbs or so . Why get cord that is so much higher rated than the rest of the recovery hardware? Am I missing something? I think I read that a knot reduces the tensile strength by half (why?), but even then 1/4" seems like overkill at least for my rocket. Also i'm still not clear on how such large forces happen during ejection. I think i've read in another thread or two the worst case scenario would be where a motor with let's say 20g's of acceleration burns out instantly and you have rapid deceleration from +19g to -1g. But i'm not extrapolating the rest. The motor burns out, and all parts of the rocket undergo the same amount of rapid deceleration as gravity takes over. Then what?

    2. Is there any point in having a drogue chute for something this small? Would lower ejection at apogee with no drogue and then free fall with deploying the main chute at 700 feet be a bad idea?

    3. A question about openrocket: According to OR my CP and CG are pretty close, and so it seems I must put some weight in the nosecone to move CG forward. I tried putting a "mass component" object in the nosecone in OR (trying different mass types had no effect), but no matter how much mass I gave it the CG didn't change. What step am I missing?

    4. I was thinking about simply putting on a coat of colorshift for the finish. Does it show well over black fiberglass, or would I need a coat of solid black paint first?

    5. Is rocketpoxy pretty much just good just for fin fillets? Am I right that jb weld should be used for centering rings and motor retainer because it handle heat better?


    This is going to be my L1 cert rocket and while I want to make sure it gets built properly, I don't want to blindly add unnecessary weight. And for those who have one, can this thing seriously handle any 38mm motor without falling apart? Even the full size J's? OR has it breaking mach on those.
     
  2. Sep 19, 2019 #2

    Nytrunner

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    1, many folks like the fire-resistant properties of Kevlar and it's lightweight so they can pack lots of it in a rocket. You really need a long cord with Kevlar because it has no stretch and will jerk severely if the pieces reach the end of the cord . There is also tubular nylon as an option. It can char from ejection charges, but that's easily remedied with a cord protector nomex, or strip of duct tape near the ejection charge. It also has a a slight bit of stretch, and doesn't have to be as long as Kevlar.
    Forces on the recovery harness come from ejection events. If your motor let's go, or you deploy at speed for whatever unfortunate reason, ( horizontal flight, no apogee charge but the main comes out later, Etc) the shocks can be quite High. Experience rocketeers have given 50g as a design number

    2 I often fly drogue less on small high-power rocket like this. Be mindful of heavy nose cones that may drag the separated rocket back into a stable falling condition. A streamer is a decent drogue substitute also

    3, did you create this file from scratch? If you downloaded a file, there may be a CG and mass override in the rocket stage itself. Select the stage over in the tree of parts and hit edit or double click it and see if there's a mass override. ( I prefer making my own files)

    4, can't help you there

    5, J-B Weld is recommended for the retainer because that's at the nozzle end of the motor that gets the hottest. You can use the rocket poxy anywhere else you want. ( I mean, you could use JB if you really wanted to, but it's heavy and brittle. My level two uses wood glue everywhere except the metal retainer and plastic boattail)
     
  3. Sep 19, 2019 #3

    Cameron Anderson

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    Learn to tie a double figure 8 knot, it's like an 90% knot and useful for everything, I use rocketpoxy for everything, and go drogueless but with a swivel.
     
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  4. Sep 19, 2019 #4

    Andrew_ASC

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    Fiberglass airframe will handle Mach plus flights. Pushed other fiberglass kits over Mach 1.69. Working on an L2 project that predicts M2.44 and it’s canvas phenolic. Have done supersonic multistage for university. Fiberglass worked well. You start getting into flutter tendencies for specific fin combinations to really answer your question. Generally though the fins fail by vibrations before the body tube at extreme speeds.

    One thing on recovery you are over looking and it’s critical make sure you have a solid 20-25 ft of Kevlar cord there really isn’t any stretch in the material and you need slack or else the nosecone to cord or Kevlar cord to body tube will snap if too short and you’ll fail a cert. I ran about twenty feet of 3/16” Kevlar from OneBadhawk on my L1 which weighed about 3.3 pounds. It had a single fruity chute 24” Drogue as a main chute. It went below 2500 ft on a H motor and was kept within visual sight for recovery. Some people get a JOlly logic chute release or do dual deploy with a drogue then a separate main using a dual deploy altimeter and ejection charges. You technically don’t need those for an L1. You just need to fly the rocket and mark the CP and physically go get the intact rocket. Focus on the build. Make sure you sand the edges where epoxy is used people prefer anywhere from 60 to 220 grit for roughing the surface where epoxy is applied. Look up knots like bowline and alpine butterfly etc. When you sim rocket in OR make sure it’s going like 45 feet per second off the launch rail or faster. Typical club launch rails are 6-8 ft long.

    Safer to guess 1/4” quick link than have a 1/8” quick link fail. For stability try to keep it within the 1-2 stability margin. There’s excellent articles on apogee about sizing chutes for descent rates and also for stability margins. You can also measure all your components by hand then simply weigh and mass override into OR as you build it.

    You’ll want rocket poxy or other epoxy on the fin fillets and the Centering rings. Jb weld is fine cheaper epoxy for metal retainers high heat application. Really expensive epoxies like Cotronics I like, but the pricing gets expensive fast and isn’t needed on L1 in most cases especially for stuff below M1.4...

    When working with rocketpoxy it’s best to let it bubble out or warm it with a hair dryer on high or heat gun on low and sit for 30 minutes till it gets thicker. Then you spoon it over a sanded fin joint and pull a single pull across the joint with either the spoon tip or a popsicle stick dipped in isopropyl alcohol. If you mark fin and body tube with painters tape you can pull the tape up and have a clean fillet.

    Look up drilling/sizing vent holes. Look up ribbon folding shock cord. Fold it in zig zag Pattern so it doesn’t tangle when it comes out. You don’t want the internal pressure pushing the nosecone off before apogee and ejection charge. Lastly have fun. Why did I rant so long? Up is easy down is hard... Would rather see a guy like you not suffer from mistakes.
     
  5. Sep 19, 2019 #5

    Idunno

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    Swivel where? I assume one is always necessary for the main chute. You also use one for the lower air frame to ebay cord drogueless?
     
  6. Sep 19, 2019 #6

    Idunno

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    I downloaded it from a link on this forum (https://www.dropbox.com/s/ez3oejf78ouwhjh/scarab54-38.rkt?dl=0). There's no mass override on any individual component that I can see, but there is a message on each part that says something like "Component mass: 114g (overridden by stage)". What does overridden by stage mean?
     
  7. Sep 19, 2019 #7

    Cameron Anderson

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    I use a swivel where the chute attaches to the shock cord and then another at either end of line connecting the eBay to the lower air frame.
     
  8. Sep 19, 2019 #8

    Bat-mite

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    The mass of the entire rocket is overridden. Don't have OR with me, but I believe you click Stage, then Edit -> Properties.

    Since final mass includes things that are hard to measure, like weight of epoxy, lack fo weight due to drilled holes, etc., it is common practice to weight your finished rocket, then override the Stage mass to the actual value.
     
  9. Sep 19, 2019 #9

    Nytrunner

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    Bat's first line is correct. The whole rocket is overridden which is why you mass object does nothing yet.

    Alternative to overriding the whole stage when you finish the build, is to add a "paint and glue" object that makes up the difference and position it for balance
     
  10. Sep 19, 2019 #10

    Bat-mite

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    Curious, why would you prefer this over an override?
     
  11. Sep 19, 2019 #11

    Nytrunner

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    If I have a random rocket that I put together quick, or in the past, I'll do a real simple model of the exterior and then override the mass.

    But if I took the time to weigh and evaluate components as I design and build the thing, I want it as realistic as possible. Paint and glue wont change unless something unfortunate happens to the rocket. That way when I add or remove weight discs, I can change that mass object alone and be confident that the cg shift of the model is close to real life.

    This is important for me because I often use motors that the kit was never intended for
     
  12. Sep 19, 2019 #12

    Idunno

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    Just to make sure i'm understanding you properly, you use one swivel on the y harness and another at the ebay eye bolt? Why are two needed on the same line? And no swivel necessary on the forward ebay eye bolt?
     
  13. Sep 19, 2019 #13

    Bat-mite

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    No. He's saying one swivel to account for booster spin, and one to account for parachute spin.

    upload_2019-9-19_10-27-17.png
     
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  14. Sep 19, 2019 #14

    Cameron Anderson

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    Yep, what Bat-mite said. Helpful diagram too.
     
  15. Sep 19, 2019 #15

    Cameron Anderson

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    Back to shock chord though for a second - don't look at the excess strength of kevlar compared to the other components as the major determining factor. I use Kevlar as opposed to nylon webbing or tubular nylon because ejection charges will literally cook the nylon and ruin it after a few flights. Go with Kevlar for strength but KEEP Kevlar because it lasts and survived better.
     
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  16. Sep 19, 2019 #16

    Idunno

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    Thanks for the helpful diagram, that's what I would have expected. One more thing, what's the best way to keep the nosecone accessible? Epoxy the bulkhead to the coupler and then keep the nosecone attached to the coupler with rivets? I like how wildman kits have a threaded tip you can anchor the bulkhead to, but that's not an option for this kit.
     
  17. Sep 19, 2019 #17

    Jmhepworth

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    I like using the nosecone to hold a tracker, so having access to the nosecone is important. When I have a coupler that fits between the nosecone and the payload bay, I like putting the bulkhead on the nosecone side of the coupler (either epoxy it there or if it’s a stepped coupler, it doesn’t need epoxy). That puts the eye bolt inside of the coupler. I’m often pressed for space for a parachute, so that little extra space inside the coupler is useful for holding the shock cord. I attach the coupler to the payload bay with sheer pins and the coupler to the nosecone with metal screws that won’t sheer.
     
  18. Sep 19, 2019 #18

    boatgeek

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    I like to glue a centering ring into the nose cone, usually as far in as I can reasonably reach (or in front of a glue-in coupler if you have one). Before you glue it in, install a few T nuts in with the backs toward the tip of the nose and matching holes in a bulkhead. Then you can put your recovery harness anchor in the bulkhead and mount a tracker or altimeter in the middle on the front of the bulkhead. When you're prepping, assemble everything and screw the bulkhead to the centering ring through the T nuts. No doubt someone has better pictures of this.
     
  19. Sep 20, 2019 #19

    Bill Hanson

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    If you haven’t already, be sure to wash all your fiberglass parts in 1:3 iso alcohol and water with a couple of drops of dish soap. That will get rid of all the mold release and other coatings from the manufacturing process. If that stays on it can cause problems with epoxy adhesion and painting.

    As others have said, for you L1, keep things simple and straightforward. Above all, have fun with it!
     
  20. Sep 20, 2019 #20

    Wallace

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    20190920_180151.jpg Something you may not yet have considered with this being your 1st glass rocket, it's important to de-burr/round over the I.D. of the tube(s) anywhere whatever type of cord you end up going with comes in contact. A square edged fiberglass spinning rocket body tube makes a very effective rotary knife. If used, I prefer to place the quick link from Y harness at that juncture to not only eliminate the sawing potential but also spread the load across 2 points as additional zipper protection. And yes, RocketPoxy is fine for the entire build, it makes some people happier having the added heat resistance of JB weld (Original JB Weld, don't fall for JB Quick or other versions) for glue on metal motor retainers. Oh yea, good luck finding a reasonably sized/weight swivel that actually "swivels" rated for that recommended 50 G's, especially when you start building larger/heavier rockets. I made the mistake of asking in the past and it only served to stir up a whole bunch of crap, ending in no real answer other than "I don't use swivels". Those big heavy things that appear to be forged steel with what looks like a bolt or pin through the center will twist up your shock cord better than anything you could imagine. If it doesn't have actual bearings you'd be better off without it. I believe that steel one had a claimed rating of around 400 pounds (assumed that was failure not working since it wasn't specified), while that bulky yet somewhat light roller bearinged aluminum one is probably good to around 2000# working load. Very nice swivel but not cheap and since I've yet to use it I can't say how well the bearings'll hold up to corrosive as all hell B.P residue since they don't appear to be sealed. That's a 3" body tube Btw..
     
    Last edited: Sep 21, 2019
  21. Sep 20, 2019 #21

    solid_fuel

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    just to clarify this statement, I am assuming your rocket was made from wood and paper like a LOC kit and NOT a fiberglass kit?
     
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  22. Sep 21, 2019 #22

    Wallace

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    Safe assumption. Fiberglass and wood glue don't play well together. Pick an epoxy you like and go with it. Pretty much any will work with what you're currently building. Keeping in mind there are adhesive and laminating types. Laminating epoxies are much thinner/runnier so as to be able to easier wet out cloth. Plenty of people use them with additives such as fumed silica, micro beads, chopped glass/carbon, etc. etc. in order to thicken/strengthen 'em for use as glue and or fillets. That's all personal preference as long as your'e not doing extreme flights. RocketPoxy, while absolutely unusable for laminating makes a simple all around builders epoxy in a single pre-strengthened 2 part kit. Works as a fairly easy flowing adhesive when first mixed and after waiting the required time for it to thicken, makes nice fillets. Entirely up to you as far as how involved you want the process to be and how many individual products your'e willing to buy and experiment with ratio-wise.
     
    Last edited: Sep 21, 2019
  23. Sep 21, 2019 #23

    solid_fuel

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    I was pretty sure of that but the OP was asking about fiberglass kits.
     
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  24. Sep 21, 2019 #24

    Wallace

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    Oops, failed to check before my rant. ASS umed you were the O.P. Sorry.
     
  25. Sep 21, 2019 #25

    solid_fuel

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    No worries. There wasn’t clarification on that statement and the title of the post specifically was asking about FG so I wanted to make sure that OP and future readers didn’t misunderstand and think that wood glue was fine with FG. Also I posted that after reading only the first response.
     
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  26. Sep 21, 2019 #26

    Wallace

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    Well, here's hoping you found at least some usable info amongst all offered. No point in overthinking most of it, find what you're comfortable working with/spending money on. I have probably a dozen or more types of epoxies/fillers, etc, and can truthfully say most of them are an absolute pita to work with. All depend on ratios/weights/additives and all are weather dependent which make repeat ability questionable. If you're anywhere near Kenosha Wi. stop by and I'll happily give you plenty of whatever you care to try out since most of what I've fallen for will expire long before I can use 'em. Spend a bit of time searching for posts relating to specific questions you may have ( You'd be hard pressed to come up with something that hasn't been done), and most important of all, ENJOY the build.
     
  27. Sep 22, 2019 #27

    Nytrunner

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    Good clarified. My intent in mentioning the wood glue was to.illistrate that the PP doesnt need HT adhesive like JB weld on his motor mount.
     
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  28. Sep 22, 2019 #28

    Nytrunner

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    Good clarifier. My intent in mentioning the wood glue was to.illistrate that the PP doesnt need HT adhesive like JB weld on his motor mount.
     

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