Rear Ejection and Recovery

Discussion in 'Low Power Rocketry (LPR)' started by Dredger, Jul 15, 2013.

  1. Jul 15, 2013 #1

    Dredger

    Dredger

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    Regarding design of rear ejection, there are some tricky components I'm not sure how to design.

    1. How do you keep motor mount from sliding up to bulkhead, if not extending the motor mount tube to the bulkhead.

    2. How do you attach the kevlar string to the motor mount? Tie it around? Tie it to the rings? Gluing Kevlar ok y/n?

    3. Should both the parachute/streamer and BT both be tied to the engine housing? (Induced moment arm across mount?)

    For low powered rockets I think rear ejection is probably the best way to go for a couple of main reasons.

    A. The hot engine and mount is popped out from the body tube sooner. I suspect having the hot engine remain inside the tube is inherently bad for the rocket tube (and adjacent fin-tube bonds) and leads to material degradation and fatigue.

    B. Removing the engine and mount also reduces the weight in the tail of the rocket in recovery, so when it lands it's not as stressful on the fins.

    C. Engine housing (mounts) are usually what breaks down in low powered rockets I've noticed. With rear ejection, you can build a couple of engine housings and interchange on the rocket body without gluing...so you can keep firing.

    Only drawbacks other than 2-stages becomes trickier for black powder engines?

    I've been researching a number of sources and wonder if the Apogee Rockets isn't the leading authority? They have myriad articles and instructional videos..for free.

    This is the one on rear ejection:

    http://www.youtube.com/watch?v=TuhXsV7OIy0
     
    Last edited: Jul 16, 2013
  2. Jul 16, 2013 #2

    mwtoelle

    mwtoelle

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    Rear ejection is more difficult to set up properly. Airframes do wear out over time, it does not matter whether you are flying an Estes Alpha or a Boeing 787. Either vehicle will suffer wear and tear throughout its lifetime and will need appropriate maintenance to keep flying. Kevlar will degrade when exposed to the hot ejection gasses. So unlike what you might have heard, Kevlar is not a panacea for shock cords.

    Only true if every thing holds together. If the shock cord breaks, you have a lawn dart instead of a cracked or broken fin. Also, many rear ejection setups do not break the streamlining of the rocket itself. I absolutely hate lawn darts.
    I have noticed more shock cords breaking than bad motor mounts over the many years that I have been flying. If you look around eBay, you might be able to find the Estes Gemini-DC and Sizzler (2127). Please note that Estes has had three Sizzler kits available the years, only the 2127 version has rear ejection.
     
  3. Jul 16, 2013 #3

    bill_s

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    I made a rear ejection MPR, thread here: http://www.rocketryforum.com/showthread.php?42983-TriWarp-Cruiser-Fantasy-Geometric-structural

    Definitely made many mistakes and found ways it can go wrong. Should have made the engine mount about twice as long, parachute very tight fit. Also burned through the shock cord on first flight, lucky not to destroy rocket. Now use kevlar leader and carefully protect the elastic part. Composite ejection charges are very violent and I think something would break if I used all kevlar. I'm coming up with at least 80 PSI pressure at ejection, the rocket can clearly be seen to be pushed forward by the mount shooting out. I even went back to using some elastic between the mount and the parachute, after issues with strain levels there. I've even had issues with the outer tube distorting from the pressure. I have always achieved ejection though, despite a tight and leaky fit... In hindsight, I like the design of the little pyramids, with the shock cord stowed with the chute and coming out a notch in the back of the motor mount and attached to the back of the rocket. There is definitely room for improvements on my design!

    Fin damage is definitely reduced, especially considering rear ejection lands the rocket on its nose! Also a CATO was easier to repair, and a F30 premature incendiary ejection destroyed yet another motor mount but did not barbeque the rocket. Theoretically rear ejection is better in the case of high speed deployment, I had some successful ones and a problem one, and now try to avoid that.

    I used a coupler to block the outer edge of the front centering ring from sliding in further. I have a metal ring set into the front centering ring to attach cords on both sides. That Apogee video seems to show the shock cord to the rocket body passing through a little hole in the front ring. Usually the centering rings will not interfere with the shock cord load, but in some designs you might attach the parachute and cord to the body directly together, with a short line going to the motor mount.

    I don't know if there's any particular advantage to rear ejection specific to LPR. If the design makes things small, tight and fiddly, a small size rocket makes that worse. If that doesn't bother you, or the design is easier to prep, OK.
     
  4. Jul 16, 2013 #4

    boomtube

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    I built a mid-power rear deployment rocket I call “Backfire” which after a couple of perfect flights, followed by marginally successful recoveries; I finally worked out all the bugs.

    Reducing the supplied BP charge by one third along with drilling a vent hole in the airframe which vents some of the pressure just before the second (Forward) MMT centering ring exists the airframe tube. This stopped the motor mount assembly, along with the recovery package, from being blown out the back with the force of a cannon.

    There are two points along the length of the rocket where Kevlar cords are attached which lead back and into the parachute compartment where they are tied to one end of a long LOC-P style elastic shock-cord. This is tied to a D-ring attached to the MMT and on the opposite side of the MMT is another D-ring and shock cord which leads to the parachute. All of the above leads to the rocket coming down latterly, flat, as opposed to nose first.

    I have now flown and successfully recovered it twice using AeroTech F52 Ts. I’m hoping to graduate up to a G53 J at Airfest.

    Timing the deployment is tricky as you actually want the rocket to begin arching over before ejecting the chute. The last thing you want is for the rocket to be pointing straight up when the chute comes out the back as there would then be a pretty good chance the rocket will stall and fall straight back through the chute.

    As for how to prevent the motor mount assy. from traveling up the length of the airframe; a hollow tube coupler glued at the appropriate point will act as a thrust ring with the forward centering ring pressing against it.
     
  5. Jul 16, 2013 #5

    BABAR

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    1. How do you keep motor mount from sliding up to bulkhead, if not extending the motor mount tube to the bulkhead.

    Your central motor tube (I call it a pop pod) can extend all the way forward to the nose cone, so butts up right against it. Or you can plan ahead and put in "stops" on the inside of the outer rocket body that will hit your forward centering ring and keep it from sliding further forward.

    2. How do you attach the kevlar string to the motor mount? Tie it around? Tie it to the rings? Gluing Kevlar ok y/n?

    You can slide it between the forward centering ring and the motor mount tube and tie it around the pop pod behind the forward ring. Gluing helps. Don't use CA glue on kevlar, I'm told it makes it brittle.

    3. Should both the parachute/streamer and BT both be tied to the engine housing? (Induced moment arm across mount?)

    Keep one shock cord attached nose cone to pop pod. This will get some of the engine blast. Kind of disagree with Tim VM on this one, and concur with other poster here, Kevlar is not completely heat resistant and does degrade. Next to a wire leader however, probably best thing out there. Attach your or streamer to the mid portion of the pop pod between the forward and aft centering rings.

    For low powered rockets I think rear ejection is probably the best way to go for a couple of main reasons.

    A. The hot engine and mount is popped out from the body tube sooner. I suspect having the hot engine remain inside the tube is inherently bad for the rocket tube (and adjacent fin-tube bonds) and leads to material degradation and fatigue.

    I've had heat damage to MINIMUM diameter LPR rockets from engine heat--- seems more prominent with the Quest engines for me. If you have a motor mount (non-minimum diameter) you already have an effective insulator between your engine mount tube and your outer fuselage, I'd be surprised if you had heat damage to the fuselage. I've personally never had fin damage from the heat.

    B. Removing the engine and mount also reduces the weight in the tail of the rocket in recovery, so when it lands it's not as stressful on the fins.

    Absolutely true. On the other hand, now you are landing on your pointy nose cone (unless you rig an external mount for the shock cord so the rocket falls in a horizontal position. Tricky but can be done.)

    C. Engine housing (mounts) are usually what breaks down in low powered rockets I've noticed. With rear ejection, you can build a couple of engine housings and interchange on the rocket body without gluing...so you can keep firing.

    True. Or you can smear some epoxy in your pop pod motor mount interior just anterior to the engine block. This should take care of the heat stress of the ejection blast. Then keep using the same pop pod until it wears out.

    Only drawbacks other than 2-stages becomes trickier for black powder engines?

    I've been researching a number of sources and wonder if the Apogee Rockets isn't the leading authority? They have myriad articles and instructional videos..for free.

    This is the one on rear ejection:

    http://www.youtube.com/watch?v=TuhXsV7OIy0[/QUOTE]

    Good video (as always.) As mentioned, disagree regarding Kevlar, it DOES wear out over time. Also do NOT wrap/roll your streamer around the inner tube, it does NOT always conveniently unroll. I layer it back and forth flat to fit between the forward and aft centering rings and place it BESIDE the tube. Makes it a lot trickier to load but a lot more reliable. Same with parachutes.

    Rear eject is IDEAL for boost gliders. You can put a lot of weight on the forward end of your pop pod for stability on boost and eject it with the pop pod at apogee. You don't have ANY connection of the pop pod with the rocket/glider, so it ejects smoothly with no attachment.

    A DISADVANTAGE with rear eject is it takes up a lot of space and crowds your chute/streamer. One really good trick---- you can use a central tube that is SMALLER than your motor mount tube. For example, you are using an 18 mm engine with a BT-20 motor mount tube in your pop pod, with a BT-50 outer rocket fuselage/body. Give yourself an 4 or so centimeters extra BT-20 in front of the engine length, put TWO BT-20 to BT-5 centering rings (or just roll masking tape) to put a BT-5 motor tube just forward of your engine mount. Smear some epoxy liberally on the inside of the BT-5 (not enough to narrow it, but enought to protect it, as it will get a more concentrate engine blast) for 10 or so cm from the tail/engine end. Put two BT-50 to BT-20 centering rings on your BT-20, and one (or two) BT-50 to BT-5 centering rings on the front of you BT-5, which will run the length of the BT-50. The BT-5 will quite effectively "duct" the ejection blast/pressure forward to blow out your pop pod. The insertion of the BT-5 into the BT-20 forward end will work as your motor block. You will find the space for your recovery device between the BT-50 and the BT-5 is MUCH greater than between a BT-50 and BT-20.
     
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  6. Jul 17, 2013 #6

    Dredger

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    Thank you for the considered responses.

    I suspect two things. That having similar mass between the pods (ejection pod and body tube(bt)) will equalize separation velocity during ejection (M=mv1=mv2). (M)omentum will always be the same between pods(Newton's 2nd), but a disproportionate velocity (as created by higher mass on one side) might be detrimental to the roket (sic).

    I wonder if having some venting mechanism, or using larger tubes(inducing lower PSI and larger mass both sides), could also reduce the ejection velocities or force (?) mentioned above.

    Some ideas above might be well suited for images along with explanation. Much appreciated.
     
    Last edited: Jul 17, 2013
  7. Jul 17, 2013 #7

    BABAR

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    Not sure if the separation velocity is that big a deal. I have successfully (okay truth in advertising--once) done rear eject where i had two recovery devices, a chute for the rocket and a streamer for the pop pod. They intentionally separated and came down individually. Advantages where that the "nose weight" and the engine where actually on the pop pod, which was tougher and with streamer came down horizontally. The more delicate rocket proper was significantly lighter without the pop pod and came down lightly under the chute.
    Look up Uranus Explorer on RocketReviews.com
    Was a pain in the patootie to pack.
     
  8. Sep 29, 2018 #8

    BABAR

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  9. Sep 29, 2018 #9

    Micromeister

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    There are several ways to keep the motor mount from moving forward.
    As Babar just posted, the Motor tube can be extended so that the forward end reinforced with a tube coupler rests against the rear inside shoulder of the Nosecone.
    Another way is to use an externmal motor stop or 1/8" section of body tube on the aft end of the motor mount that rests against the outer airframe tube under thrust.
    If you are using centering rings, the shock-cord should be tied around the motor tube, if the motor mount is minimum diameter, a size up outer air-frame body tube should be used ie. BT-20 (18mm) motor tube, use a 19mm Airframe for the body of the rocket. then a Music Wire hook can be attached to the motor stop inside the forward end of the motor tube. I think I have a drawing of one of these rear ejection pods with hook.. If I can I'll upload it for your use.

    That said most of the time it is far easier to use centering rings with standard motor tubes to create a rear ejection pod.
     

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    Last edited: Sep 29, 2018
  10. Sep 29, 2018 #10

    matthewdlaudato

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  11. Sep 29, 2018 #11

    kuririn

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