How Does A Motor Burn??

Discussion in 'Propulsion' started by deandome, Mar 28, 2006.

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  1. Mar 28, 2006 #1

    deandome

    deandome

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    [Insert cheap/easy joke here, i.e. "Hotly"]

    What I mean is, how does the combustion travel through the propellant.

    Does it work it's way from the bottom to the top (makes a LITTLE sense...as this would mean the ejection charge gets ignited last)? Does the whole middle 'core' (around the hole/c-slot) ignite at once & propogate outwards?

    And, god forbid, has anyone ever seen an animated/illustrated graphic that kinda shows what happens?

    Thanks!
     
  2. Mar 28, 2006 #2

    edwardw

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  3. Mar 28, 2006 #3

    deandome

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    THANKS!!!

    But that looks like combustion starts at the 'top' of the motor....is that where you put the ignitor in HPR motors?

    And I guess this means that the ejection delay starts burning right away. Have there been cases where the ejection charge goes off before the motor has completed firing?

    Dean
     
  4. Mar 28, 2006 #4

    RoyGreen

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    yes, you put the ignitor at the top of the motor, and yes, the delay starts burning right away.

    That's in the majority of composite motors.

    But, in certain endburning motors (Apogee E6 and F10 among them), and all black powder motors, the propellant burns first and ignites the delay when the flame front gets to it.

    As to ejection charges firing before burnout, yes, that happens, but it is a failure mode. I.E. the flame front has somehow gone where it shouldn't have, like around the delay grain. Or someone modified the delay incorrectly.
     
  5. Mar 28, 2006 #5

    MarkM

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    Also notice that the motor burns from the core outward toward the edge equally. This is for typical BATES grain achitecture, which is the majority of HPR motors. However, moonburners and star grains burn differently, although all start burning from the top.
     
  6. Mar 28, 2006 #6

    powderburner

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    Deandome,
    Composite motors generally start burning from the top, but they almost instantly begin burning over the entire length of the propellant grain.
    Once ignited and burning, it only takes a few hundredths of a second before the entire core is burning. Compared to a 'normal' burn time of up to several seconds, this can be considered instantaneous for most practical purposes.
    If you try to ignite the bottom end of the grain you usually get an internal instability in combustion. The motor 'chuffs' rapidly (very brief burn, pause, another very brief burn, pause, etc) and might go ahead and burn or might snuff itself out. Not good.

    Edward,
    VERY good graphic!
    Thanks
     
  7. Mar 29, 2006 #7

    trogdor

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    Here's a cool old DOS program that animates the whole thing and describes motors in general... may run fast even on glacial speed on a modern PC but worth a look.
     
  8. Mar 29, 2006 #8

    mrbusdriver

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    That's a fascinating video. It seems to show that the flame front somehow propagates between the propellant slugs (if I saw it correctly) Then kind of burns through them as individual hunks of fuel. Am I seeing this right? What sort of "issues" might this present to the design engineers? (make sure they are firmly secured to their liners for one, I might guess...)
    Thanks,
    Dave
     
  9. Mar 29, 2006 #9

    Ryan S.

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    Where a composite motor starts has nothing to do with the motor being composite. The grain geometry is what determines where a motor starts to burn. In any core burnning or slotted configuration (monolithic grain, BATES, moonburner, C-Slot, fin-o-cyl, etc) the motor generally starts burning at the top of the grain. Some motors do have combined geometries however where the motor may start burning in the middle. If you imagine an end burner that has a core drilled up it a certain percentage of its length, then the motor is started at the forward end of that core. Of course, an end burner starts burning at the aft end.

    In most circumstances once a motor begins burning, it will continue burning, but this is not the case for all motors, some (with old grains for example) will take a while to come up to pressure even though a single grain is burning.

    Some motors can be started at the bottom even in a BATES configuration, if I remember correctly the old Silver Streaks were started this way.

    Chuffing is caused by rapid pressure changes. When the pressure of a motor decreases rapidly the flame front moves away from the propellant surface (so less energy is transfered into the solid fuel to vaporize it) and less propellant is burning so pressure drops and the motor goes out. If enough energy is still present in the core it may restart and continue in the cycle causing chuffing.
     
  10. Mar 29, 2006 #10

    metalwizard

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    not all "composite" motors are the same. the 5"diam motors we use at work are x shaped propellent. and the nozzels have 8 holes around the outter edges. they ignite from the fromt of the motor, then burn in four places from the outside to the inside. but thats another story.
     
  11. Mar 29, 2006 #11

    Ryan S.

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    BATES motors burn on 3 surfaces on each grain. They DO NOT only burn outward from the core, they also burn on both ends. The theory behind this "slug" geometry is that as the cylinder (core) diameter increases the total length of the cylinder decreases (the ends are burning) and the total surface area of the grain as a hole stays the same giving a flat thrust curve. Bonding the casting tubes to the liner is a good idea because if you dont what tends to happen is, near the end of the burn casting tubes with propellant on them get shot out of the nozzle.

    BATES pose the least problems to design engineers. They are easy to cast and make (no complicated geometries like stars) and they are relatively easy to insulate (as opposed to end burners and moon burners where alot of insulation is needed.

    Stars also burn towards the edge - at first in a star shape which eventually turns into a cylinder because the star points burn off. This gives an interesting thrust curve.
     
  12. Mar 29, 2006 #12

    KermieD

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    This has got to be the most educational thread (for me) that I've read in a long time. Thanks for all the info guys!!!!
     
  13. Mar 29, 2006 #13

    11bravo

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    That vid is great to help folks understand the concept.
    Somewhere (a NASA link IIRC) there was an ani_gif of a BP motor, IIRC but I cannot find it right now.
    Maybe someone will know what I mean.
    Also, to demonstrate my ignance for the day, someone please explain "moonburner".
    I understand BATES grains and endburners.
    Moonburners I'm not familiar with.

    TIA,

    Greg
     
  14. Mar 29, 2006 #14

    Ryan S.

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    Moonburners are similiar to offset cores. They are single grain motors that have a peice remove (generally this peice is a core cast on one side) from one edge that make the grains sort of look like a moon. Imagine a long BATES grain with a core that is almost touching the edge of the casting tube. THe flame propogation increases the diameter of the core, but since the core is close to the casting tube the flame front hits the casting tube on one side and the surface area of the propellant decreases on that side and increases on the other until the flame front reaches the center of the casting tube (the point where the diameter of the grain is it's greatest) and then the overal surface area decreases and the thrust curve has a negative slope. The reason for doing this geometry is the web of the propellant (thickness from the edge of the casting tube to the core) is increased which results in a longer burn.

    The problems that arise in moons and similiar motors (d grains etc) is that first you need to cast the whole grain as one which is more of a problem in larger motors (where as, with BATES you can cast a grain at a time) and insulation. Since part of the wall of the case is exposed to a flame for a long period of the burn the motor needs to be well insulated to prevent burn through or case bubbling
     
  15. Mar 29, 2006 #15

    11bravo

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    Cool.
    Thanks.
     
  16. Mar 29, 2006 #16

    deandome

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    Actually....I never specified "HPR" or even composite, for that matter.

    Of course, that's MOSTLY what I mean, though I was also trying to get to the similarities/differences between mid/HP composites & estes-class powder motors.

    Mostly it was just plain curiosity....did the combustion path go top to bottom...center to sides...all at once?

    And how did this affect the ejection charge? I always assumed w/estes motors that the combustion path worked it's way from the bottom to the top where it lit the ejection charge & then it went 'poof'...it looks like that's actually correct!

    But with composites, where theres clearly an open path (hole/slot) going end-to-end through the grain, so this meant that the ejection charge process would need to be different, as the 'flame' would quickly reach the top of the motor where the ejection charge is.

    And this has been answered well enough for me...pretty early in this thread...basically the ejection 'charge' is actually a 'timer' that get's ignited instantaneously & burns throughout the main propellant burn plus 4-10 seconds of actual delay.

    Of course, I've enjoyed all the other banter here..I've learned a lot about stuff I didn't ask about, and it seems I've helped others understand all this as well (...UUUGHGH...I strained a muscle patting myself on the back :p ).

    Dean
     
  17. Mar 29, 2006 #17

    RoyGreen

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    Well, no. There are three separate flammable areas in the motor: the propellant, the delay, and the ejection charge.

    In an Estes motor, the propellant starts burning, then the delay, then the ejection charge.

    In a typical composite motor (but not all) the delay is ignited along with the propellant. The delay is the timer that then ignites the ejection charge when it burns out.
     
  18. Mar 30, 2006 #18

    Rock_It

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  19. Mar 30, 2006 #19

    daveyfire

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    thank u, next
    Just a thought on this. While it may be physically possible in very rare cases for the burn to propagate towards the top of the core in a BATES motor, it is incredibly hard on the motor due to the strange eddies and turbulent flow patterns that form on the way through the core. Towards the top, the gases have nowhere to go; as such gas has to make its way back down the core in the other direction, thus making hot spots where the flow slows and strange erosive patterns. The old Silver Streak motors were ignited from the bottom-middle because they were straight-cored black powder motors which burned so fast that there was little time for the propellant to interact; 0.8 seconds or less and it was gone. (The fact that they were black powder also explains why the "new" batch of F104SS motors in 1996ish, formerly F50, kept CATOing...)
     

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