Build Thread - US Rockets Dual 18mm Rear Eject

Discussion in 'Low Power Rocketry (LPR)' started by Antares JS, Mar 25, 2020 at 10:47 PM.

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  1. Mar 25, 2020 at 10:47 PM #1

    Antares JS

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    Hello and welcome to my build thread. It's a slightly unusual rocket, so I thought this would make for an interesting first build thread. I also found a couple of problems as I was examining the parts, so there are a few FYI's here for anyone thinking to build one of these. As the kit name implies, this rocket is a two-motor cluster with the motors being in a pair of side pods while the parachute ejects out the back of the main tube. I do plan to come up with an actual, cool name for the rocket at some point.

    First and biggest - the tube couplers that came with it had a pretty sloppy fit. When the stock couplers were pressed against one side of the inside of the body tubes, the other side would have a good half a millimeter of space or so between the tubes. I replaced them with a couple of BT-50 couplers from Apogee, and they fit much better. There are two couplers - one for gluing the two long tubes together, and one for making the plug that goes on the back where the parachute comes out.

    Second, this rocket includes no thrust rings or hooks. No motor retention at all. Now, the instructions say to wrap some tape around the aft ends of the motors for a thrust ring, and then tape them into place when you fly. This is fine, but me being the clod that I can sometimes be, I probably would have forgotten there was no thrust ring at some point and shoved a motor up into the rocket too far and gotten it stuck. I am adding a couple of thrust rings to make sure that never happens and then will secure the motors with a wrap of tape for flight.

    Third, the instructions are not always as clear as they could be. The writers seem to be assuming that you have some previous rocket experience, although that may be fair since this is an unusual configuration that is probably not going to be someone's' first rocket.

    Parts.jpg Instructions.jpg
     
  2. Mar 25, 2020 at 11:58 PM #2

    Speaknoevil

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    I always like Jerry's rockets.
     
  3. Mar 26, 2020 at 1:18 AM #3

    BABAR

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    Interesting. What keeps intertidal from pulling out the plug when the rocket launches?
     
  4. Mar 26, 2020 at 1:23 AM #4

    Antares JS

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    Would Jerry be the person behind US Rockets?
     
  5. Mar 26, 2020 at 1:25 AM #5

    Speaknoevil

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    Yes, Jerry Irvine.
     
  6. Mar 26, 2020 at 1:26 AM #6

    Antares JS

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    Friction fit + light weight. The instructions specify about 3 pounds of force to pull the plug off.
     
  7. Mar 26, 2020 at 1:51 AM #7

    Antares JS

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    Motor mounts. Nothing special. BT-20's with centering rings and thrust rings (again, thrust rings not included).
     

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  8. Mar 26, 2020 at 1:58 AM #8

    Antares JS

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    The interesting part - cutting the ejection charge vents and gluing the tubes together.

    Marked Up Tubes.jpg

    Cut Tubes.jpg

    Glued Tubes.jpg

    Filleted Tubes.jpg

    Inside Tubes.jpg
     
  9. Mar 26, 2020 at 2:00 AM #9

    Antares JS

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    One more sub-assembly for tonight - the plug. This goes on the back of the middle tube and is ejected in place of the nose to release the parachute. It consists of a BT-50 coupler, a 1/2" long piece of BT-50 tube, a plywood plate, a screw eye, and kevlar string. The other end of the string gets tied to the center nose.

    Plug 1.jpg

    Plug 2.jpg
     
  10. Mar 26, 2020 at 2:17 AM #10

    Off Grid Gecko

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    I don't understand the dual rockets. I expect clusters to be closed off with a single nozzle for the gasses to escape from. I'm a noob in this department, but how do you ensure that they light and eject at the same time without causing insanity if one of them flames out first? Luck? Or are estes rockets that precise these days?
     
  11. Mar 26, 2020 at 3:16 AM #11

    Antares JS

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    It is pretty easy to get black powder motors to light simultaneously. All you have to do is wire the igniters in parallel and use a 12-volt power source. With Estes igniters, since the wires are short, when lighting a cluster, I usually first twist some extra wire onto them and tape it in place. Then you can twist these extensions from multiple igniters together so that you have two leads to clip your controller to. For an extra bit of reliability, I also use igniters dipped in quickdip to make sure they are extra energetic and WILL light the motors. I’ve lit up to three motors simultaneously this way.

    As far as ejecting at the same time, Estes motors are remarkably consistent. Motors of the same type will reliably work exactly the same, so if you start them simultaneously, you’re golden.
     
  12. Mar 26, 2020 at 3:43 AM #12

    Off Grid Gecko

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    cool to know. I was thinking about a rear eject parachute config today so your title sparked my interest.
     
  13. Mar 26, 2020 at 4:04 AM #13

    BABAR

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    Does it really matter if one ejection charge is a bit late? I suspect one would be sufficient to eject the laundry.

    It’s and interesting design, seems a bit overcomplicated. You could just about build it almost the same way except glue in the tail cork and eject the chute forward out the middle Tube.

    Hope it doesn’t land on rock or pavement, gonna do a number on the central nose cone tip. On the other hand, is it lands on soft grass or a plowed field on a windless day, gonna make a nice picture of it “sticks” the landing!
     
  14. Mar 26, 2020 at 4:08 AM #14

    Off Grid Gecko

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    Flame-outs have a tendency of inducing flat spins. Not something you want to happen at speed. I suppose with a well enough distance between the CP and CG, it wouldn't matter much. But I lack experience on this front so it would bother me.
     
  15. Mar 26, 2020 at 4:21 AM #15

    BABAR

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    While ejections from the two engines may certainly be at different times, when the first blows, the other will likely be burning the delay grain so not contributing much if anything to the propulsion. You’d have to be RaEeeeaaaallly off to have one deploying while other us under thrust. So unlikely to significantly torque the rocket, although I guess the smoke plume might have some effect on the deploying chute.
     
  16. Mar 26, 2020 at 4:32 AM #16

    Nytrunner

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    The trick to clusters is having sufficient current to ignite them both. +/- 1/10th of a second isnt going to affect the flight perceptibly.

    As for delay precision, heheheh.
    Whats the NFPA ruling? Up to 10% variation or 2 seconds, whichever is smaller?
     
    Last edited: Mar 26, 2020 at 12:15 PM
  17. Mar 26, 2020 at 11:22 AM #17

    Antares JS

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    According to the instructions, the idea behind the rear ejection is that doesn't have to do a "flip-over" when it ejects the parachute, which I guess makes sense if you put a slightly too-long delay in.

    What I consider to be the advantage to rear ejection is that you're a lot less likely to break a fin off if you come down on a hard surface.
     
  18. Mar 26, 2020 at 12:56 PM #18

    neil_w

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    Noted for the record: on March 26, 2020, BABAR argued *against* using rear ejection in a rocket. :D

    Kidding aside, I assume that the rear ejection is part of the point of this rocket. "Just because".
     
  19. Mar 26, 2020 at 6:10 PM #19

    kuririn

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    Yeah, it's a marketing gimmick. Could have easily and more cheaply made this a standard eject.
    Antares, maybe you could name it "Dub'L Backfire".
    Or "Dual Butt Kick'R".
    Built mine stock except for two mods:
    1. Added engine hooks. Don't like friction fit or end wrap if I can avoid it.
    2. Reinforced the ejection port glue seams with heavy cardstock, then coated with thin CA.*
    Also, the distance from the ejection duct to the aft end of the rocket is short, not much room for the recovery laundry. You don't want it to extend past the port or your chute and shroud lines will get singed. Also need some room for wadding. The chute supplied is VERY thin, but for a reason. It will pack very small and tight. Try to fit it into the plug first.
    *This is based on prior experience with ported ejection gasses on my Hydra 7 and Trident.
    No matter how heavy I fillet the gasses always seem to find a weak spot in the glue seam.
    Probably overbuilding OCD.:D
    0326200808[1].jpg
     
    Last edited: Mar 26, 2020 at 6:19 PM
  20. Mar 26, 2020 at 6:48 PM #20

    Antares JS

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    I kind of like "Double Backfire," but not random apostrophes. :p I'll definitely consider that one.

    I refuse to fly with only friction fit unless it's a multistager and I have no choice, but flying a LOC Viper 3 a few times taught me to be okay with end wrap.

    I'm interested to see if I can get the glue to hold by itself... if it doesn't, I can repair it later. Not like this is a high power rocket I've sunk hundreds of dollars into. The last time I built a rocket like this that vented into other tubes was an Estes Gemini DC back in the 90's, and that one came with the holes already made and a couple of tiny pieces of 1/4" launch lug that served as a little "pipe" that went through the holes in the tubes and sealed the opening. Of course, that one was the opposite of this one with a single motor in the middle and two rear-ejecting parachutes in the side pods.

    I was actually thinking about this today. I was also thinking about installing the shock cord mount into the back with a paper trifold instead of tying it to the center nose. That way, I can keep it out of the way of the ejection charges more easily since I've read on this forum now that Kevlar string will break if exposed to ejection charges repeatedly. Do you have any tips for making sure the parachute actually comes out of the plug and doesn't just get stuck in it? I was thinking I could use the extra length of kevlar I will have available to make sure the attach point of the parachute is far enough away from the end that the plug will yank itself off the parachute with its momentum, but I'd be interested in any other ideas.
     
  21. Mar 26, 2020 at 7:16 PM #21

    kuririn

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    Sorry, by "into the plug" I meant halfway in the plug and halfway in the aft of the rocket to save space.
    Can't comment on best method since I've yet to maiden her.
    Perhaps someone who has flown this can elucidate.
     
  22. Mar 26, 2020 at 8:02 PM #22

    BABAR

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    If I was to scratch build something like this, I would use much longer side boosters, put the ports much further forward (also brings CG forward). System would then work kind of like a baffle (I think this is how the Trident worked), and you could go with little or no wadding.

    If I had the KIT, I'd probably stick a motor mount in the middle tail sections, put the laundry up front, and work it as a 3 motor in-line cluster.
     
  23. Mar 27, 2020 at 1:18 AM #23

    Antares JS

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    I didn't have much time tonight so had to settle for just sanding the fins. Note that the fins are NOT isoceles triangles. If you build one, be sure to round the correct edges.

    20200326_211648.jpg
     
  24. Mar 27, 2020 at 4:47 AM #24

    BABAR

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    If you name it “Double Backfire” I hope you fly the crap out of it!

    You are right on about Kevlar not being flameproof. Any chance you can use a length of wire fishing leader that will extend to just below the vents, and attach your Kevlar there?

    Not exactly sure how you make sure you don’t push the parachute ABOVE the vents when packing? Seems like this would benefit from a bulkhead just forward of the vents, less tube to pressurize.
     

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