Aerotech G8ST Motors at Research Launches

Discussion in 'Propulsion' started by Steve Shannon, Sep 10, 2019.

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  1. Sep 10, 2019 #1

    Steve Shannon

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    Although Aerotech G8 motors are not certified and may not be sold under NFPA 1125, those which were sold to consumers before the certification was revoked may still be used at Tripoli Research Launches in heat resistant motor mounts. Because of the high case temperature, users are advised to use caution.
     
  2. Sep 10, 2019 #2

    SecondRow

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    The fate of the G8 was a real bummer for me this year. I was looking forward to loading it in my turbo vortico and watching it fly for ~18 seconds. But I’m not TRA (or level 2) and there are no research launches I’m willing to drive to anyway. Ah, well. T:W was pretty marginal, so probably for the best.:(

    Maybe someone will video a launch on the motor and post it.
     
  3. Sep 17, 2019 #3

    Jacob Ritzke

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    I flew one in an apogee aspire with a custom deployment system. Never found the rocket but the long burn was awesome to hear!
     
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  4. Sep 17, 2019 #4

    Rocketjunkie

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    I use PML phenolic motor tubes where they may be exposed to high temperatures. An example is my Motoreater that has a central 54mm and 8 28mm outboards. A large White Lightning central motor and no outboards leaves the empty motor tubes exposed to the heat from the exhaust plume during the burn.
     
  5. Sep 17, 2019 #5

    Bat-mite

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    A moon burner that actually went to them moon?
     
  6. Sep 17, 2019 #6

    Rocketjunkie

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    A fast propellant (Super Thunder) end burner.
     
  7. Feb 16, 2020 #7

    Richard Dierking

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    What is the current status of the G8? Reported to be hot (as one would expect, because all composite motors are "hot" at the end of the burn), but what is the surface temperature? Better yet, is there a temperature curve for this motor? What is the maximum allowed case temperature for certification?
     
  8. Feb 16, 2020 #8

    dhbarr

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  9. Feb 16, 2020 #9

    Steve Shannon

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    The G8 was mistakenly announced as certified, but upon realizing that the case temperature was higher than allowed by NFPA 1125 7.4.1, TMT issued a correction. Unfortunately, the correction wasn’t issued before Aerotech manufactured some.

    NFPA 1125 7.4.1 requires that the exterior surfaces of motors under 160 N-sec not exceed 200°C. Motors above 160 N-sec are allowed to reach 220°C. In an apparent mistake of written code motors which measure exactly 160 N-sec are not mentioned.

    NFPA 1125 is currently in its review and revise cycle. Gary Rosenfield of Aerotech proposed that the surface temperature limit be removed for composite cases. We pushed back, thinking that no limit at all was probably not right either, but we would not be against revising the limit based on user data. So, we agreed to allow the remaining stock of G8 motors to be flown at Tripoli Research Launches and we would watch for any problems.

    I don’t remember what the actual temperature the G8 motors reach, but it’s well in excess of 200°C.
     
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  10. Feb 16, 2020 #10

    cwbullet

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    Hopefully, they find a solution site for this.
     
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  11. Feb 16, 2020 #11

    Richard Dierking

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    Thank you. You probably already know this, but ThrustCurve shows the motor as certified. I actually purchased a TVC kit from BPS Space because I thought the motor was "cool." Hopefully, Joe at BPS is not upset about how this went down.
    I agree about having a temperature limit. Is the NFPA limit intended to protect the rocket from damage, causing fires, or people getting hurt? That is, what is the reason for the limit?
    Honestly, I don't know much about the construction of HP motors. I'm taking a class in a couple months and frankly wasn't that interested until I saw this problem with heat.
    I've seen motors being static tested, and usually they are not in the kind of motor mount with a tube like you would typically see in a rocket. As most people know, paper or phenolic is a great insulator. So, heat would not dissipate rapidly after the burn. Also, I would think the temperature curve would rise abruptly at the end of the burn because the last of the propellant would provide some insulation. Well, unless it was an end-burner. What if the propellant grains were in a material that could be consumed but not as fervently? Or, 29mm motor enclosed in a 38 mm case?
    So, what I'm suggesting is rather than changing the standards dramatically, mitigate the problem.
     
  12. Feb 16, 2020 #12

    Steve Shannon

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    You make excellent points. Those comments should be made on the NFPA website. Anyone can comment by going to the standard being addressed.
     
  13. Feb 16, 2020 #13

    djs

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    Out of curiosity, do we know what the basis for the NFPA 1125 rule is? Is it to prevent ignition of surfaces (grass, paper, whatever) based on the heat? I mean- what made them choose those specific temperatures?

    Was also thinking about the impact of motor case heat on things like epoxy bonds in a motor mount.
     
  14. Feb 16, 2020 #14

    Richard Dierking

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    Exactly. I was a NFPA member many years ago, and I know how things are revised. Usually, just like state codes, there's a Statement of Reasons that precedes the code. Sometimes these statements can be difficult to locate or lost. But, you better find out why this temp was chosen before commenting to NFPA.
    Increasing safety would probably not be difficult to change. But, if there's a decrease in safety, good luck. Or for example, saying if someone could get injured by 220 C just as easily as say 250 C. Can you see the problem with that? Good way to get the temperature lowered. And, saying something like, I think a higher temperature would be OK because we want to use longer burn motors and higher temperatures are expected, would quickly be dismissed. If it's a material issue, how have the materials of amateur rockets changed since the adoption of these temperature standards? I'm guessing not much.
    I actually thanked AT at a launch a few years ago for helping Joe at BPS with developing the longer burn 29mm. It was good of them to do this. On the other hand, it should not be a surprise for a company tooled to make certain sized motor grains for single use or reloads to say we need to increase the temp limit to get this done. I don't blame them a bit. But, I'm an outsider and probably looking at this problem a bit differently.
     
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  15. Feb 16, 2020 #15

    Maxwelljets

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    As I understand it, those casing temperature restrictions are based on when 6061-T6 begins to lose its temper. If that's accurate, there's not really a reason it should be applied to single-use motors. Having different temp restrictions for reloadable vs single use motors makes a lot of sense IMO.
     
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  16. Feb 16, 2020 #16

    Richard Dierking

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    Good point Max and I agree. But, why the different temp standards based on the impulse? You probably wouldn't be surprised when asking how regs and particularly numbers in regs came about you sometimes hear, "well, we reached back..." ;-).
    If the current numbers were not substantiated or people don't really recall how they came about, it will make it much easier for the home team to win on this one.
     
  17. Feb 16, 2020 #17

    Steve Shannon

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    Those temperatures are also not far below the temperature where paper or dried grass burns.
     
  18. Feb 16, 2020 #18

    Richard Dierking

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    But, again, why the different maximum temperatures for impulse classes?
    Sounds to me like raising the 200 to 220 C might be possible but that alone will not get the G8 certified.
    Also, do TRA's certification specialists take temperatures during testing, or is this something the manufacturer's supply?
    Is this the tip of the iceberg?
    Personally, I still think there is an engineering solution to this. Something that perhaps could be applied to other long burn motors.
     
  19. Feb 16, 2020 #19

    0011001100

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    If Aerotech made the casing thicker (since they can for single use) would that help the OD temperature not get so high?

    220C does seem like a reasonable max temp based on aluminum casings, but at what point do fiberglass SU casings start to weaken? I feel like the temperature should be based on the casings type not impulse.
     
  20. Feb 16, 2020 #20

    Richard Dierking

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    Aluminum conducts heat very well and also cools quickly - if it can. But, the motor case is often in an insulating motor mount tube. Keeping in mind that I'm not a person with research motors, it seems to me the best way is to get the heat out at the end of the burn. Could a different kind of delay grain help decrease the temperature at the end of the burn? Would a thicker liner paper, treated paper, or another material help?
    I've never even taken a single use motor apart. Well, because you're not suppose to.
     
  21. Feb 16, 2020 #21

    Steve Shannon

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    I don’t know why for the temperatures.
    TMT marks the cases before test firings with a material that goes through an irreversible change at a specific temperature.
    I don’t think so.
    Yes, I think there’s a technical solution as well.
     
  22. Feb 16, 2020 #22

    Richard Dierking

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    Thank you Steve. Well, this thread is seriously affecting my message to likes received ratio (chuckle), so I'm going to stand by until there's some data available. The best solution is only going to be achieved with good data. If I could get my hands on a couple G8's I would see if John and some other knowledgeable people would be willing to test them at FAR. I asked around and I don't think any of the motors made it to California. (Yeah, I live in California, please stop smirking.)
    Hey, that's it, maybe TRA could require the motors to be tested in Montana during the month of January!
     
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  23. Feb 17, 2020 #23

    AeroTech

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    As Steve mentioned, RCS/AeroTech made a presentation at the fall 2019 NFPA Pyrotechnics Committee meeting to support a proposal to increase the temperature limit for non-metallic casings. This proposal is making its way through the NFPA process. I would love to post a PDF of the presentation but the file is very large.

    Our static and flight testing demonstrated that non-metallic (fiberglass-epoxy) casings will not ignite dried grass and common rocket materials even if their temperature is significantly above the ignition point of paper. Reasons include their low thermal conductivity, low thermal capacity and the limited time the casings are at the elevated temperature.

    The committee wanted independent verification of our test data as well as some additional tests which will be conducted by the NAR and/or Tripoli in the near future, before the next meeting.

    Yes, there >may be< technical solutions to mitigating the high casing temperature (which we are working on), but why not increase the current limits if they are unnecessarily restrictive? This is how we managed to get 'G' motors, high-power rocketry and reloadable metal-cased solid and hybrid motors approved for consumer use BTW.

    "Making the case thicker" does not necessarily solve the problem. It reduces performance significantly from an inert weight and mass fraction standpoint, and requires an even faster-burning propellant to generate the same thrust. Using thicker insulation with a thinner case solves this problem, but there are strength and porosity issues when using thinner cases. It also may not reduce the casing temperature below the current limit.
     
    Last edited: Feb 17, 2020
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  24. Feb 17, 2020 #24

    BEC

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    ....and it seems to me, as a complete outsider to this entire discussion (and NOT a motor designer), that it is unlikely anyone would be making a >79.99 N-s single use motor with a PAPER casing, where the increased temperature limit would be an issue. That at least suggests a rationale for raising the limit on SU motors of G impulse and up only.
     
  25. Feb 17, 2020 #25

    AeroTech

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    We are in agreement.

     
  26. Feb 17, 2020 #26

    Steve Shannon

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    I agree also, but that casing that isn’t paper could be in a paper motor mount tube. I don’t believe that the G8 will char it, but it would be interesting to know what temperature will in the exposure times rocket motors are likely to see.
    Anyway, that’s why we suggested a test period, to see what we actually observe from using the G8 motors.
     
  27. Feb 17, 2020 #27

    BEC

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    Oh, absolutely. I've heard one of those maxims that goes "one good test is worth 1000 expert opinions." That makes sense to me. And I don't even claim to be an expert here :). Far from it.
     
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  28. Feb 17, 2020 #28

    Richard Dierking

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    Great information! Thank you. So, can these motors be available again to test? Don't have to be certified. And, I'm not suggesting free. Or, are you satisfied that enough of them were distributed to enough people that will adequately test them and report their results?
     
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  29. Feb 17, 2020 #29

    Jacob Ritzke

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    I've used two G8 motors, I saw nothing on the rocket that would indicate that they were getting too hot inside. I just used regular kraft body tubes and they looked brand new afterwards (besides some dirt from landing)!
     
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  30. Feb 17, 2020 #30

    Richard Dierking

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    The website for the BPS Space Thrust Vector Control system recommends the AT G8 or G11 motor and the supplied TVC is 3D printed PLA. PLA has a low glass transition temp and melts at 155C. So, I'm curious to hear testimonials of flyers that have used these motors for rockets with TVC. Additionally, these rockets are not typically high-flyers and would probably be recovered pretty quickly.
    Also, please note that the files to print the TVC are also available and other materials for the control are possible. For example, I was going to try ABS and Nylon with CF (NylonX).
     

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