Rear Ejection... when, where, why?

Discussion in 'Techniques' started by Cody Webster, Sep 17, 2018.

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  1. Sep 17, 2018 #1

    Cody Webster

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    I find rear ejection interesting. I dont have any models using rear ejection but I am thinking about building a rocket and using rear ejection as an experiment/learning oppurtunity. I have read some threads about it and I assume the main reasons for using RE would be space constraints. Anyone use it just for the heck of it? I dont see it used very often, so I am wondering why and when to use or not to use rear ejection? Are there benefits and drawbacks to rear ejection? If its simply easier not to use rear ejection I am always up for a challenge and like to think outside of the box when building my rockets. Thoughts?
     
  2. Sep 17, 2018 #2

    Gary Byrum

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    When I built my upscale Astron Gyroc, I decided on rear ejection to cut down on the weight for the helicopter recovery the Gyroc uses. There was a streamer attached between the CR's in the motor mount and "flap tabs" installed on it to hold the flaps in place until ejection. This worked quite well. The Gyroc was built with a BT 55 and the MM was 24mm. Rear ejection is necessary on this model because the nose cone is glued in place.

    DSCF3222.JPG DSCF3217.JPG
     
  3. Sep 17, 2018 #3

    kuririn

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    To me, the biggest advantage to using rear ejection is in having the rocket come down front end first, thus lessening the chances of landing damage to the fins.
     
  4. Sep 17, 2018 #4

    Eric

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    I have only used rear ejection with my pyramid. But it worked great.

    But I can see the advantage of nose first landing to protect more extreme fin designs.
    0330181649.jpg
     
  5. Sep 17, 2018 #5

    Gary Byrum

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    Depending on the size of your rocket, you may need some kind of recovery. Larger models would certainly need it. Something small that can rely on tumble recovery probably wouldn't. Fin damage is likely to happen any rocket at some point. Even the best efforts in making them sturdy is no guarantee. It just a given that it will and usually can happen.
     
  6. Sep 17, 2018 #6

    Lugnut56

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    I built a US Rockets "Duel 18mm Rear Eject" (yes, that is it's offical name) awhile back because it looked interesting. The biggest issue with this one is the distance between the pod gas pass-thru vents and the rear of the main body is fairly close. This does not leave a lot of room for wadding, chute and shock cord. I was also concerned about shoving the wadding in past the pass-thru vents, so I installed a CR with a small round section of balsa glued across it to act as a stop for the wadding, but still allows the ejection gases to pass thru. Only flown it once so far (way under powered with A8-3), but as kuririn stated, it comes in nose first, so no damage to the pods/fins.



    Duel 18 mm (1).jpg Duel 18 mm (2).jpg
     
  7. Sep 17, 2018 #7

    MoeB

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    When? End of coast phase. Where? Apogee. Why? Get the laundry out.
     
  8. Sep 17, 2018 #8

    BABAR

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    Reasons for Rear Ejection
    1. Delicate rear fin structure, so you want it to come down nose first, so the initial impact between rocket and asphalt is the nose cone.
    2. Boost Glider models that use a central "pop pod". A big advantage here is you can put some extra mass/weight in the forward edge of the pod that will eject with the pod. This gives you better stability on boost, but both takes that weight OFF in glide AND moves CG back on glide. Good example of this is the Apogee SR-71 DarkBird
    3. Sometimes it just fits with the concept of the model, example is the Pyramid mentioned above, also the Uranus Explorer
    https://www.rocketreviews.com/umm-me-uranus-explorer-2-scratch-by-tom-markel.html

    there may be some other reasons I haven't thought of.

    Rear ejection is definitely more complicated and less reliable, as the central tube has to extend forward of the recovery device (has to push everything BACK) and therefore you have MUCH less space for your streamer or chute. We are talking really tight. Advantage (and it is a minor advantage) is you don't need wadding. The forward SLIDING centering rings (fixed to the pod, slide relative to rocket) will prevent exhaust from hitting your recovery device.



    If you want to use this method just for it's "cool factor", recommend you use a LARGE body tube for your rocket (say at least BT-60 for a BT-20 motor mount), and make it a fairly LONG rocket, don't try this with short and stubby. If you read the Uranus Explorer review above, you will understand how I know this.

    I also recommend if you have the space for it that you use separate recovery devices for the pop pod and the main rocket. That will take the weight of the pod and engine off the main rocket on descent, which I think is an advantage. For an 18 mm engine pop pod, that can recovery easily on just a streamer.

    When you pack it for launch, the pop pod should be so loose it almost slides out on its own. If it seems tight at all, don't launch. High chance the pod won't eject (and then your rocket will DEFINITELY come down nose first!)

    Good luck and straight trails

    And soft Landings!!!
     
    Last edited: Sep 18, 2018
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  9. Sep 17, 2018 #9

    kuririn

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    l to r: Sunward Khufu's Pyramid, Estes Gemini DC, US Rockets Dual 18mm …..yada yada.

    IMG_20180917_075743.jpg

    I've found that the space constraints are greater, not less with rear ejection. As lugnut stated you have a very small recovery compartment with the US Rockets model. Consequently, the supplied chute's plastic is extremely thin (like the kind you get covering your dry cleaning). The Gemini DC's two chutes have to be stuffed into two side boosters. With the exception of the pyramid design, no reason that these models HAVE to be rearward ejection. I suspect it's more of a marketing gimmick rather than operational necessity. BTW Eric, nice scratch pyramid!
     
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  10. Sep 18, 2018 #10

    Cody Webster

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    Thanks for the replies!
     
  11. Oct 10, 2018 #11

    Brainlord Mesomorph

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    I did rear blow on the "Spirit of Cydonia" (sig pic). Did it b/c I was afraid of the nosecone breaking those fin-points. makes for nice smooth front end.
    There's a build thread with static tests of rear blow videos.
     
  12. Oct 19, 2018 #12

    BABAR

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    Saw the test stand success. Did this fly yet?
     
  13. Oct 19, 2018 #13

    GlenP

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    more typical with rotor or glider recovery, like the Centuri X-24 Bug, the Estes Skydart, check out the Apogee Darkbird for a 13mm, since you can eject the nose weight required for rocket stability out the rear and this re-balances for the glider configuration.
     
  14. Nov 18, 2018 #14

    bjphoenix

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    I see some photos that seem to have a big assembly sort of like a motor mount eject out the back. Is it necessary for these to be a tight fit so they don't come out prematurely or do they always stay inside due to inertia?
     
  15. Nov 19, 2018 #15

    BABAR

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    If my physics is still right given the forward force of the motor is IN the motor mount from launch until ejection fires there is no net force that would dislodge the motor mount pod. So by force during propellant burn and by inertia during delay burn no reason to have premature separation.

    Generally better to be fairly "loose". Perhaps not so loose that it falls out as you carry the rocket to the pad

    As mentioned this is one of the downsides of rear eject. Not nearly as much space for a chute or streamer.
     
  16. Nov 19, 2018 #16

    Gary Byrum

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    Now that, can depend on how long your MM is and or how big your airframe is. When I built my upscale Gyroc, I used a 24mm motor mount in a 55. There's probably 3.5-4" of space between the centering rings where I put the streamer. This gives me ample space for an 8' (or so) streamer. And it provides just a little bit of snugness in the 55. Had it been a BT 60, well, that's a mile of room for the streamer material I use.

    upload_2018-11-18_20-3-8.jpeg
     
  17. Nov 19, 2018 #17

    BABAR

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    Good point, the LENGTH of usable space for rear eject can be similar to or greater than that of forward eject.

    But the DIAMETER of usable space and more importantly the usable room for “bunchiness” of a folded chute or streamer goes waaay down. Certainly do-able, but much more challenging to get a folded up chute to loosely conform to the space between TWO cylinders than to just stick it into the central cavity of a SINGLE cylinder, even if that SINGLE cylinder is smaller than your outer of the TWO cylinders. For pyramids and cone type rockets however rear eject not just do-able but may be ideal. Also may be good for short stubby rockets. Hmmmm, thanks, you may have solved my problems for my Squirrel Rocket!

    And for streamers , as I am sure you already know, I had to learn the hard way that the logical “just roll the streamer around the central tube” like toilet paper on a roll doesn’t work, it won’t just “unroll itself” on descent. IMO streamer has to be folded.

    For me, anyway, both streamers and chutes are much more challenging to pack up and insert into the rocket for flight for rear eject compared to forward eject. But I am thinkino BT-5 to BT-20 or BT-20 to BT-50. Maybe I just have thick fingerss, but even BT-50 To 60 is a challenge for me to pack a descent size chute.

    Also, I believe for your GYROC your recovery streamer was only used to safely regard descent of the pod. If you ALSO needed room for enough laundry to recover the body of the rocket itself AND shock cord from the pod TO the rocket itself, it may be a bit more challenging. Pods don’t need nearly as much laundry to recover safely by themselves. For my birds can almost use tumble recovery on the pod, streamer is as much to help visibility as it is retard descent.
     
  18. Nov 19, 2018 #18

    Gary Byrum

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    The thinner the chute material, the better off you are with space. Yes, I do fold 1/2 my streamer up on one side and the other half on the other side. I didn't even attempt rolling it at all. Sorta knew better. This Gyroc required real ejection since I wanted to lighten the load for the helicopter descent. If I were to design a rocket where everything had to be tethered, I'd certainly make sure I had enough room between the motor mount and the nose cone to store a shock cord. Actually not that challenging at all. Chances are, if there's enough room in the rocket, the ejection pod could be slightly longer so I could barf the end of it.
     
  19. Nov 19, 2018 #19

    snrkl

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  20. Nov 19, 2018 #20

    BABAR

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    Very cool. Violates my "spider sense" that you shouldn't be able to do two things with the gas of one ejection charge, but you have proven it works.

    My fear looking at the design is that the booster stage ejection gas would light the sustainer engine (good) and that sustainer would separate from booster BEFORE the gas blew out the rear eject pod. Once the sustainer separates, ALL the pressure in the system would likely vent FORWARD out the now gaping hole in the front of the sustainer.

    Still seems to me that if that motor mount doesn't slide out REALLY easily that even a slight delay would result in failure to deploy, but foolish to argue with success.

    I think this is essentially the same thing as one of the new Apogee Rockets two stage designs uses.
     

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