CTI 75mm O-ring Grain Spacers Mis-sized - anyone else?

Discussion in 'Propulsion' started by farsidius, Aug 7, 2018.

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  1. Aug 7, 2018 #1

    farsidius

    farsidius

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    Didn't see a post about this (not that the search function would have found it) so I thought I would ask.

    Went to assemble a CTI M2045 Blue Streak (6 gr) at our club launch two weeks ago and all the grain spacer O-rings were the wrong size. Someone on the assembly line had mistakenly packaged all 6 grains with the closure O-rings instead, which are too fat and too big in diameter to fit in the liner. I can't even begin to express the frustration with this. You drive five hours, camp three days only to have your biggest launch of the year frustrated by O-rings.

    This was one of a handful of 75mm motors I ordered over the Black Friday weekend so, when I got home I looked through all the other 75mm motors I have on hand. I found a 3 gr L1050 Blue that also had the wrong size. Both motors were manufactured Nov 22 2017. I have other 75mm from other earlier dates that were all good (some whites, classics, skids and another blue)

    I'm curious if anyone else has run across this problem, or if it's isolated to this manufacturing date/lot? One more thing to add to my check list before heading out to launch.

    Also, I've contacted the vendor I purchased from and he's sending me replacement O-rings.

    And, for the curious, yes, I asked a lot of other attendees if they had extra (sometimes you don't need the last o-ring) but I couldn't scrounge up enough. I had one extra in my box.
     
  2. Aug 7, 2018 #2

    ericm541

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    I would of been really mad, thats also why i keep my spare o-rings never know what you'll find when you open everything up. I have an L1395, when i get home i'm going to check out the o-rings.
     
  3. Aug 7, 2018 #3

    Bat-mite

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    That's awful. CTI has had so many problems over the last couple of years.
     
  4. Aug 7, 2018 #4

    ericm541

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    Just checked mine, i have a L1395 and M1770 and both spacers are identical, i did compare to the extra 98mm spacer i have and the thickness is significantly different. The L1395 has a nice ziplock bag i did not open the M1770 it is sealed however i did compare the o-ring and they are the same. Second photo is a 98MM grain spacer next to the 75MM spacer, i'm sure there's a reason they are different. Got any photos or dimension of the spacer O-ring?






    IMG_3026[1].JPG IMG_3027[1].JPG
     
    Last edited: Aug 8, 2018
  5. Aug 8, 2018 #5

    Maxwelljets

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    My club bought a CTI 75 reload and opened it to find that it had come with 98mm O-rings. Fortunately, we had another test launch planned for the next month, so we just bought another reload and used that set of o-rings, and bought replacement o-rings.

    Several months later, we bought a 98mm reload that was missing all o-rings (possibly not CTI's fault, it looked like the reload kit had been in storage for a long time so there's a chance they just got lost). Fortunately we still had the set of 98mm o-rings that came with the very first reload, so it all worked out.

    It's unfortunate that this was not a localized incident.
     
  6. Aug 8, 2018 #6

    ericm541

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    Every 75MM cti reload i've purchased the instructions and o-rings are inside the liner with the nozzle and forward seal disk taped on the ends, all of that in a sealed bag. Spacer rings are of course in with each grain, first thing i do is check the liner for cracks then o-rings.
     
  7. Aug 8, 2018 #7

    Bat-mite

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    Sounds like a massive recall is coming.
     
  8. Aug 8, 2018 #8

    farsidius

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    From the photo, your spacer O-rings look right. It's the same size as the grain. The mis-sized ones I got were bigger around than the grain. They were the same size as the forward/aft closure O-rings which are in the zip-lock bag inside the liner. I'll snap a photo and share when I get back home tonight
     
  9. Aug 8, 2018 #9

    ericm541

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    I see alot of differences in the CTI instructions compared to what you get sometimes. I went and assembled the grains and spacers in the liner last night just to satisfy my curiosity. Instructions show thinner grain spacer and the forward insulator disk i got is plastic, i plan on using the L motor in an RMS case i've heard people recommend using RMS forward seal disk yet theres no o-ring for it unless a grain spacer is used which i didnt think about trying.
     
  10. Aug 8, 2018 #10

    jd2cylman

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    I just got done building a CTI L1355SS for Airfest. The grain spacers included in the kit were the correct diameter, but they were the same thickness as the AT forward seal disc o-ring(3/32 instead of 1/16). This particular reload uses 2 full grains and four 1/2 grains. By the time I got it all assembled, the top grain stuck out of the liner 1/16". So, since my L900 was missing spacer o-rings, I ordered a buttload of o-rings from McMaster Carr. Using 1/16" o-rings really shortened up the grain stack. Enough so that I could use the AT forward seal disc.
     
    Last edited: Aug 9, 2018 at 6:15 PM
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  11. Aug 8, 2018 #11

    ericm541

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    interesting, when i assembled the grains/spacers in the liner there was space even with the RMS seal disk, thats a good idea though having extra o-rings on hand, always thought the o-rings provided were special.
     
  12. Aug 9, 2018 at 2:01 PM #12

    n3tjm

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    If those o rings are the ones used for spacing the grains, i would think they could be used anyway unless the extra thickness will prevent you from getting the closures on. Just bend the o rings so they fit in the liner. Once the motor fires, the grains will keep themselves separated.
     
  13. Aug 9, 2018 at 10:54 PM #13

    farsidius

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    Left side is what was in with the grains as spacers; right side is what was supposed to be with the grains. almost a full cm difference in diameter

    O-rings.jpg
     
  14. Aug 10, 2018 at 12:42 PM #14

    jimzcatz

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    Diameter is irrelevant for spacer rings, but thickness might be a problem. I would have cut the ring since it's not providing a seal. Left one out if I had to to make it fit.
     
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  15. Aug 10, 2018 at 7:05 PM #15

    farsidius

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    I thought about that but you'd end up having to leave more than one out with the 6 grain. And, I don't know enough to know how leaving out space or doubling the space between grains would affect motor performance.
     
  16. Aug 13, 2018 at 9:38 PM #16

    Dave A

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    Are the grain spacer o-rings for a better seal where the grains join so less burning to the liner? Just curious...
     
  17. Aug 13, 2018 at 10:00 PM #17

    eggplant

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    They don't seal anything. They're there so the faces of the grain are exposed and can ignite at the same time, which is necessary for the motor to burn with the desired thrustcurve.
     
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  18. Aug 13, 2018 at 11:25 PM #18

    ericm541

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    how much would thickness matter? The grain spacers for my 98mm M2505 were half the thickness of both the 75MM i have, post #4.
     

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