How close to burning motor

Discussion in 'Propulsion' started by David Schwantz, Mar 25, 2020 at 1:34 PM.

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  1. Mar 25, 2020 at 1:34 PM #1

    David Schwantz

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    How close do you think you can get to a burning motor with PLA? Motor burn will be 2 seconds. Do you think it would survive? How far would the exhaust plume radiate out from the motor? If it melts, ok. But would not want it to catch on fire and land like that. Not to mention landing on it and breaking.
     
  2. Mar 25, 2020 at 2:28 PM #2

    ebruce1361

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    It all depends on the motor. The vacuum form polystyrene plastic that the Estes 1:100 Saturn V uses for the external fairings is a good two inches from the exhaust plume, and those fairings commonly melt on the bottom, but that kit is designed for a 24mm E motor. As a comparison, a lot of the Estes RTF rockets that fly on the 13mm mini motors don't have a whole lot of burning on the fins that sweep past the motor. Granted, those little motors don't burn very long, but the exposure seems limited as well since the exhaust plume is much smaller.

    My suggestion would be to look at photos of kit rockets with known part dimensions flying on the motor you are planning to use, and compare the diameter of the body tube to the exhaust plume. Even so, the plume itself isn't the only thing to consider, as heat will radiate out past where the plume is visible. Maybe protect the PLA part with a protective coating like JB Weld or some aluminum tape?
     
  3. Mar 25, 2020 at 2:48 PM #3

    FredA

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    How long is the rocket going to sit on the pad cooking before it gets up and goes?
    This is the period you need to worry about IMHO.
     
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  4. Mar 25, 2020 at 2:51 PM #4

    lakeroadster

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  5. Mar 25, 2020 at 2:55 PM #5

    David Schwantz

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    Hi Bruce, I built my Estes Saturn V with a 29mmt. It flies on G-74-6W, the fairings show no signs of melting at all. The new one will fly its first flight with a K550W. The dummy F-1s will sit far below the nozzle, burn time is short. I have thought about making them 1/2 engines and cutting away the inside portion. I have used BVM heat shield for years with turbines and it works great. I could paint that on and then cover it with Aluminum tape and it would look just like a batted F-1 ready for flight.
     
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  6. Mar 25, 2020 at 2:56 PM #6

    Zeus-cat

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    I have friends who have used 3-D printed parts near the motor mount and the problem isn't so much obvious melting, but distortion from a single or repeated exposure to heat. One trick is to leave the used motor in a 3-D printed motor mount until it has completely cooled so that the motor keeps the motor mount shaped properly. A high temp plastic is better if you insist on using 3-D printed parts near the motor.
     
  7. Mar 25, 2020 at 2:57 PM #7

    David Schwantz

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    Nope, no bench testing this one. Would not be safe. And would have to be at a launch anyway if I did.
     
  8. Mar 25, 2020 at 3:00 PM #8

    David Schwantz

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    Not using a 3D printed MMT. I am trying simulate the F-1 engines on a Saturn V for flight. Single K550W in the middle to start with. Also have to make them survive the landing.
     
  9. Mar 25, 2020 at 3:02 PM #9

    David Schwantz

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    With the K550W she will get up to speed fairly quick. I have flown this motor on 10 lb rockets before and after she has come to pressure, it's gone. Also good thought though, might be wise to use an angled blast deflector
     
  10. Mar 25, 2020 at 3:05 PM #10

    FredA

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    Or no blast deflector - not a fan of angled ones as it is a strong contributor to rod/rail whip.
    Remember, the blast plate takes the same force as the rocket while the rocket sits there.
     
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  11. Mar 25, 2020 at 3:11 PM #11

    David Schwantz

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    Cannot go without a deflector. We fly from a sod farm. Our pads are fairly stout, heavy and have a long leg span.
     
  12. Mar 25, 2020 at 3:28 PM #12

    FredA

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    Somewhere I have a photo of a 1010 rail being bent about 10-degrees off vertical due to a K motor and an angled blastplate that was attached about a foot above the rail base.
     
  13. Mar 25, 2020 at 3:46 PM #13

    David Schwantz

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    Fred, don't doubt that could happen. But hard to believe that the buttons did not snap off before the rail bent.
     
  14. Mar 25, 2020 at 3:47 PM #14

    FredA

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    The motor pushing on the angled plate doesn't really stress the rail buttons......
    The plate took all the torque
     
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  15. Mar 25, 2020 at 4:12 PM #15

    ebruce1361

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    Wow! I have seen several Saturns with melted fairings from a single 24mm motor, and my 5 D12 motor cluster definitely melted the ends. Maybe the higher impulse G74 pushes the rocket fast enough that the heat from the plume can't get through the air rushing down past the fairings.
     
  16. Mar 25, 2020 at 4:29 PM #16

    David Schwantz

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    My nozzle is 2.5" into the airframe also.
     

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  17. Mar 25, 2020 at 5:31 PM #17

    G_T

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    Now that is not a good plan. Sorry... The exhaust plume is very hot and will radiate/conduct/convect a lot of heat to the airframe in that configuration. You will melt/burn/destroy the lower airframe which is also your fin unit. That is a disaster waiting to happen. Additionally, in that configuration the lower airframe is also the rest of your nozzle. It makes for a severely overexpanded configuration, and you will be losing thrust. You will get hot incandescent exhaust circulating in there. It's expected lifetime will be measured in tenths of a second IMHO before significant failure begins. I would anticipate the bottom of the rocket may not even be there by the end of the burn.

    For what convection can do, here is an example. I used to have a 6" diameter roughly 80# rocket that I flew a number of times. On a research 114mm O for supersonic flights, the bottom of the thrust ring was fine. On an 88mm research N flying transonic, the diameter ratio was such that a convection cell established at the base of the rocket when it was moving. Now the motor tube stuck a half inch or an inch (don't recall) past the thrust plate. The thrust plate was an inch of plywood with a carrier plate of a half inch bolted onto it. That carrier plate was birch aircraft plywood, epoxy coated, and heavily painted with decently high temp engine paint. In about 5 seconds, the exhaust plume convection, which isn't naked eye visible, and the heat radiation, charred off 1/16" of the plywood from the bottom of the rocket. It did this every time I burned an 88mm N. Now these motors are a little hotter than your typical commercial motor, but still... That was a flat surface that the motor tube stuck through, and there was no tubing lip or anything like that at the base of the rocket.

    In your configuration, imagine the exhaust plume confined inside your rocket. That's what you've done.

    Were I the RSO, I wouldn't allow it to be lit. It will come off the pad much slower than you expect due to thrust loss. It will quickly burn out the bottom of the rocket removing the fins. In the process of burning out the bottom of the rocket there will be a time period with potentially severe asymmetric thrust. The rocket will veer off course. THEN it will have full thrust, and likely be unstable to boot. Sorry.

    Gerald
     
  18. Mar 25, 2020 at 6:04 PM #18

    David Schwantz

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    I'm sorry Gerald, but if you are referencing the above pics, that rocket has over 30 flights on it. With no signs of burning, charring or melting what so ever. The rocket that I am building, yet to fly will have the nozzle at the bottom of the thrust plate, in the open air, not in the tube. I am just surrounding it with fake engines that do not fully encompass the motor nozzle. And further, if you are familiar with a Saturn V you know the the fins are well above the engines so there is no chance of them being "burnt" off. I am just wondering if PLA might live or do I make stainless ones. So I believe that your assumptions are incorrect. Sorry.
     
    Last edited: Mar 25, 2020 at 7:42 PM
  19. Mar 25, 2020 at 6:42 PM #19

    FredA

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    Gerald - are you not sure this isn't IR heating or just damage from sitting on the pad???
     
  20. Mar 25, 2020 at 8:37 PM #20

    G_T

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    David, I was referencing those photos and your reference to a K motor. I presumed, incorrectly, that you were going to try the same thing. AP motors burn a lot hotter than little BP motors, and larger AP motors typically have higher metal content so they have some rather hot slag in the exhaust.

    FredA, yes, not due to pad or from IR heating as the difference is not all that much between a full N and a baby O. The main difference is the diameter of the motor tube vs the base diameter of the rocket. A 114mm motor in a 6" rocket isn't a large lip around the edge. An 88mm motor leaves a much larger lip. The 114mm has a little bigger exhaust plume and a bit more thrust (but also more weight) and yet it doesn't cause any damage. The 88mm causes considerable damage - But I just glued another layer of plywood on, recoated, repainted, and charred it off again. Propellants were of the same series and nearly identical in characteristics. Time on pad and acceleration were basically the same, just the 114mm accelerates for more time.

    https://www.rocketryforum.com/threads/sprite-6-and-a-baby-o.37382/page-4#post-473010 - rocket on pad
    https://www.rocketryforum.com/threads/sprite-6-and-a-baby-o.37382/page-4#post-518906 - pad video, including slow motion of motor startup

    No nearby blast deflector. Nothing impeding the exhaust plume. No long wait at ignition. Smoke from the pyrodex ignitor, a bit of flame as the propellant lights, then brief transparent subsonic exhaust as it is pressurizing but not yet burning metals, then gone.

    Gerald
     
  21. Mar 25, 2020 at 8:42 PM #21

    David Schwantz

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    HI Gerald, no problem. I was wanting others opinions. The Estes rocket in the pics does fly with AP motors, G74-6W. Thanks.
     
  22. Mar 27, 2020 at 5:13 AM #22

    Rob702Martinez

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    Does a flat blast deflector induce rail whip as well? Sorry to ask off topic questions.
     
  23. Mar 27, 2020 at 5:55 AM #23

    FredA

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    Actually, yes it can.
    The motor is offset from the rail so it does result in a small amount of torque.
     
  24. Mar 27, 2020 at 3:43 PM #24

    Rob702Martinez

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    So best to have a blast plate independent of the actual rail?
     
  25. Mar 27, 2020 at 4:13 PM #25

    FredA

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    Bingo
     

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