Do motors such as Estes A8-0, B4-0, C6-0 kick backwards or not?

Discussion in 'Propulsion' started by Funkworks, Jun 24, 2019.

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  1. Jun 24, 2019 #1

    Funkworks

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    I'm getting conflicting information with regards to booster motors, such as the Estes C6-0. Do they have no ejection charge at all? Or do they have an ejection charge that will ignite immediately after the propulsion is over with? Specifically, does the C6-0 motor kick backwards at the end, or not?

    My goal here is to add 4 motors around a central one in a 1:100 Saturn V with a custom cluster mount. If the C6-0's do not have an ejection charge and kickback, then it should be ok for the central one to burn for a longer time. If however, there's a pop opposing the central thrust, I don't know what's going to happen, and I'd rather my rocket not be the Guinea pig.
     
  2. Jun 24, 2019 #2

    lakeroadster

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    I asked a similar question in regard to the canted cluster set-up on my lifting rocket.

    My understanding is that these do experience burn through. They are not capped. So therefore the answer to your question is there is no ejection charge but yes if they are installed in a closed cavity there will be some amount of kick back.

    I emailed Estes and asked them how much kickback force and got a generic non-answer.

    I'd suggest you perform a static test to see what happens with your with your specific configuration .

    Be sure to videotape it, so we can live vicariously through your experimentation.

    You could also plug the engine yourself but according to most RSO's that is deemed "modifying" the motor and is "Verbotten" if you are launching at a NAR event.

    Here's a link to the other thread...
    Plugging Booster Motors For Cluster Applications
     
    Last edited: Jun 24, 2019
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  3. Jun 24, 2019 #3

    Funkworks

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    Excellent. I finally have a good reason to do a static test!
     
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  4. Jun 24, 2019 #4

    GlenP

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    Anyone who has flown an x-24 bug can attest, a C6-0 engine gets expelled pretty forcefully.

    Maybe you could use a plugged and vented engine mount tube like on the Estes astron scout. It lets the engine kick back a short distance in a plugged tube to expose a vent hole. Might require some additional creative venting in a cluster arrangement.

    http://www.spacemodeling.org/jimz/k-01.htm
     
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  5. Jun 24, 2019 #5

    lakeroadster

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    It makes sense that the force expelling the motor is significant. Think about the motor burning and the force it takes to push the gasses out the little hole in the nozzle at the base of the engine. Then the burning in the motor breaches the propellant at the top of the motor, now that internal force takes the path of least resistance and fills the void in the top end of the motor case.

    I'm looking forward to seeing that static test Euclid
     
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  6. Jun 24, 2019 #6

    heada

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    There is no delay, and no ejection charge, just the exposed top of the grain. Since the motor chamber is under pressure when the top of the grain burns through, you'll see some pressure out of the top but it quickly dies as the opening gets larger. If you have nowhere for that initial pressure spike to go, it can eject an unsecured motor. If the pressure spike has a place to go, then it will act like a very, very weak ejection charge.

    If you want it to have a kick, seal the top of the MMT. If you don't, then vent the top of the MMT.
     
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  7. Jun 24, 2019 #7

    Funkworks

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    Yes me too. I set up a static test last winter to test my launch controller. Fortunately, it confirmed I had bad contacts and allowed me to make the repair before launch day. I have both a C6-0 and C6-5 eagerly awaiting at home. Should be able to do it sometime this week.

    Properly sizing the vent is the issue here.

    I agree that "creative" is a very appropriate word.
     
  8. Jun 24, 2019 #8

    lakeroadster

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    Pretty significant force.. looks like Macklin Missile Works has already done all the testig for you Euclid.....

    "A 1/2" wooden dowel was inserted into the motor cavity without glue. At burnout motor casing ejected from dowel. Friction alone is not sufficient to secure plug and prevent ejection."



    ---------------------------------------------------------------------------------------------------
    And the previously discussed somewhat controversial method of plugging the motor.....

    "A 1/2" wooden dowel was inserted into the motor cavity to which wood glue was applied and left to cure for about 20 hours. The motor was fired and remained attached to the dowel. Gluing a short dowel piece into a motor cavity should prevent motor ejection from the motor mount at burnout."

     
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  9. Jun 24, 2019 #9

    Funkworks

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    Well well. Still planning on it. If it feels like too much, I'll go with (1x D12-0) + (4x C6-0) in the rocket and enjoy the show.
     
    Last edited: Jun 24, 2019
  10. Jun 24, 2019 #10

    SDramstad

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    What happens if the center motor doesnt light? Rhetorical question........
     
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  11. Jun 24, 2019 #11

    lakeroadster

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    Better slip at least one motor in there with an ejection charge... :p

    Belt and Suspenders approach is to run ejection charges in all the motors.... that way you pretty much know the laundry will be deployed if any or all of the motors light off ...
     
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  12. Jun 24, 2019 #12

    Funkworks

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    Here we go:
    The ejection charge from a C6-0 seems too much, especially if using 4 or 5 units. The chute will pop at high speed.

    The only 5-motor cluster that can be done with my current set up is [4x C6-5] + [1xD12-5]. This will ensure a 5-second coast before ejection. I'm just worried 5 ejection charges, igniting simultaneously, will be too much for the build.

    I still have a few days to think about this before launch.
     
    Last edited: Jun 24, 2019
  13. Jun 24, 2019 #13

    Nytrunner

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    As stated above, The -0 (booster) motors don't have actual ejection charges, they just release the pressure from the chamber when the last bit of propellant is too thin to contain it.

    If you can work it, venting out the back would be my vote
     
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  14. Jun 24, 2019 #14

    lakeroadster

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    Since you are worried, use C6-7's instead of C6-5's.

    With the allowable variances in burn time as well as delay duration the odds of them all ejecting at the same time is unlikely.

    Some research on the ThrustCurve's site shows that NFPA 1125, Code for the Manufacture of Model Rocket and High Power Rocket Motors, allows an alarming amount of motor deviation from stated values....
    • The total impulse must not have a standard deviation greater than 6.7%.
    • The ejection delay must not vary more than 1.5 second or 20% (whichever is greater, up to 3s) from average.
    • The average thrust must not vary more than 20% (or 1N for model rocket motors, 10N for high-power motors, whichever is greater) from average.
     
    Last edited: Jun 24, 2019
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  15. Jun 24, 2019 #15

    Funkworks

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    According to OpenRockets, 4x C6-5 motors still gets me 178 feet, and an ejection at 100 ft!
     
  16. Jun 24, 2019 #16

    caveduck

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    You can epoxy plug 18/24mm booster motors and it works just fine, but goes into a gray area on the safety code. It is arguably better than gluing in a section of dowel though, since the piece of dowel could become a projectile.

    A small side effect is that a plugged booster will have slightly more total impulse because thrust continues a little past the time where the front of the propellant grain would normally rupture and depressurize the chamber.
     
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  17. Jun 24, 2019 #17

    Funkworks

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    Because it qualifies as a motor modification? I'd use the steel reinforced JB Weld ("3960 PSI") variety if I knew association officials were ok with it.
     
  18. Jun 24, 2019 #18

    lakeroadster

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    Arguably indeed.

    Whatever reasoning you have for thinking the wooden dowel is a projectile can also be used for the epoxy plug, or the JB weld plug becoming a projectile.

    Regardless of the strength of the plug material, it's the fact that it is in some way bonded with the motor tube and thus deemed "a modification" that seems to be the bone of contention.
     
  19. Jun 24, 2019 #19

    BEC

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    Plugging is clearly a grey area and can be interpreted as a motor modification, yes.

    I like Lakeroadster's suggestion to use C6-7s. Even if you stay with -5s there is enough variation in delays that it is really unlikely they would all go off simultaneously. More likely they will go off in quick succession kind of like a string of Black Cats or such. (I have heard this effect flying B Cluster Altitude models - which use 5 motors). Or even two C6-5s and two C6-7s.

    Where would these gases go in your modification of the Saturn V? In the stock build, the 24mm mount goes into the stuffer tube and from there hopefully will eject your 'chutes as it should. The 18s would vent where - just into the larger cavity of the S-I/S-II section? If so, the idea of putting some vent holes in the bottom plate to let the gases out that way would also work, I expect.
     
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  20. Jun 24, 2019 #20

    Funkworks

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    I just simulated [1x D12-5] + [2x C6-5] + [2x B6-4], and unless I somehow goofed up, this gives ejection at apogee with the [2x B6-4]. The 3 others go off just a little later. I lose about 100 ft, but I'll take it until I get my hands on C6-7's.

    Yes, gases go through motor mount baffles and into the stuffer tube to the 3 chutes. I'd put off vents for now if I can, since a slew of questions come to mind, and I'll probably need a week to study this carefully. Placement, size, dependency on motor type, number, checking out those kits and customs that have some, etc. Interesting but enough for today!
     
  21. Jun 24, 2019 #21

    BABAR

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    All zero delay Motors would be an extraordinary bad choice for two reasons.

    First, with no delay you are going to pop your laundry at full speed which will likely rip your Chute right off the rocket.

    Second it is possible if one or more Motors doesn't light at the pad that unless you pack wadding in each motortube separately the forward ejection charge from the engines that do ignite on the pad could potentially ignite the zero delay motors that did not ignite on the pad from the forward end backward since they have no clay cap.

    Safest bet is a D12 5 in the middle and C6 7s on the outside. Most likely your D 12-5 will perform appropriately and eject your laundry with the other Motors just under 2 seconds later firing into a completely empty body tube chamber with no significant effect. If on the other hand the D 12-5 doesn't ignite or for some other reason doesn't deploy the laundry you have the others as backup.
     
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  22. Jun 24, 2019 #22

    caveduck

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    Not quite the same reasoning actually. The epoxy plugs (at one time frequently seen in international competition BTW) will either stay in place or disintegrate, while a wooden plug will hold together, though in fairness it's unlikely to get spit out. I wouldn't use JB weld either - it has rather poor strength properties due to all the filler - check the threads on epoxy specs. 5-minute epoxy will work fine.
     
    Last edited: Jun 25, 2019
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  23. Jun 25, 2019 #23

    Rocketjunkie

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    1. Plugging booster motors with epoxy works well. Unless you are flying at a NAR launch, you can get away with this.
    2. Plugging motors with glue is considered a motor modification.
    3. Have you built the rocket yet? Allowing spent motor casings to eject is allowed by the safety code but not the contest code. Just friction fit well enough they don't fall out from acceleration of the other motors and allow them to eject. Still use booster motors and have bulkheads in the motor tubes above the motor. If for a contest you will have to vent the tubes. If using motor ejection, I would plug 2 outboard motor tubes and have the other 3 go thru. If using electronic ejection, plug all the motor tubes (not the motors).
     
  24. Jun 25, 2019 #24

    Tom

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    I am kind of surprised by this! I always thought it was the second stage motor igniting that spit the booster off.
     
  25. Jun 25, 2019 #25

    Nytrunner

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    By design. The ignition of the sustainer Should be the separator.

    That's why you need to tape motors together (direct stage) or vent the booster (gap stage) so the puff of the booster gasses dont cause separation w/o ignition
     
  26. Jun 26, 2019 #26

    RoyAtl

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    yes, they do kick back as gasses expand forward upon breach of the forward wall of propellant. However, this is not necessarily the same as an ejection charge in a delayed motor.

    As the delay element burns it leaves a lot of gunk in the combustion chamber, and this acts as a partial seal against large amounts of gas going out through the nozzle. The ejection charge itself, while pressed, is not as pressed as the propellant, and has a lot of surface area that will burn quickly forward. Therefore, while I have no evidence, I would think that a delayed motor's ejection charge will have more of a kick than a spent booster motor's breakthrough, but both will still have a kick.
     
  27. Jun 27, 2019 #27

    vcp

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    From that plugging motors thread:

    On 18mm motors, I 3D printed little 'top hat' shaped plugs that friction-fit into the motors. Besides the friction-fit, the brim of the hat is clamped between the motor and the motor mount. A small tab hanging 'below' the hat allows fingertip removal.
     
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  28. Jun 30, 2019 #28

    Funkworks

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    Yes, I figured that out since. As I said a little later, that idea was superseded with [1x D12-5] + [2x C6-5] + [2x B6-4]. The 2x B's are going off first, which is great since the total power won't be excessive, and the second one acts as backup to the first (while simultaneous).

    And since then, I found out that [1x E12-4] + [2x C6-5] + [2x B6-4] was even better! Eagerly waiting to launch this next weekend!

    In the future, I think my best bet for altitude will be [1x E12-4] + [2x C6-5] + [2x C6-7].

    Technically, I like the idea, but regulation-wise, I'll stay in line.

    Yes I have! It's a beautiful 1:100 Saturn V. It looks just like any other, but on the inside, there's that custom motor mount (5 hooks), and in the 3rd stage: 150 grams of added weight in the form of a bundle of dowels, nails, epoxy, bubble wrap and velcro. I added removable rivets to the 3rd stage and service module to allow for the weight distribution to be changed. So many motor configurations are possible.

    I created evidence specifically for post #12 of this thread! Enjoy!
     
    Last edited: Jun 30, 2019
  29. Jun 30, 2019 #29

    BABAR

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  30. Jun 30, 2019 #30

    Funkworks

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    double post, again
     
    Last edited: Jun 30, 2019

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