Bertha's breech baby ...

Discussion in 'Low Power Rocketry (LPR)' started by beantownJPL, Aug 29, 2019.

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  1. Aug 29, 2019 #1

    beantownJPL

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    ... or how about Back Assward Baby Bertha?

    Ok ... so everyone's got (or has had) at least one member of the "Bertha Family". The Big Bertha and Baby Bertha are favorite smallish field flyers, and I've usually got one or the other, and sometimes both, in my fleet. My last Baby Bertha is on a nearby school roof, so I picked one up at a local shop recently.

    I usually like to put some kind of "personal touch" on my kit builds, and thought I'd take a shot at modding this kit for rear ejection. I figured that with an 18mm motor inside the BT-60, that there'd be plenty of room to wrap the 'chute around the stuffer. I also hate having to use recovery wadding (really sucks when "oops, forgot the wadding" happens...), so usually take steps to make it unnecessary when I can. I'm a big fan of ejection baffles for "conventional" recovery deploys...

    Peak of Flight issue #439 has an article on rear ejection that I used as thought fuel and hit the bench.

    The only materials I needed that aren't already included in the kit were a length of kevlar shock cord, a couple of centering rings, a motor clip retention ring and a length of BT-20.

    I tied off the shock cord to the nose cone, and cut off what looked like a sufficient length - I can always extend it later. I'll probably want to add a swivel anyway, and I don't have any at the moment.

    [​IMG]

    I had considered just gluing the nose cone to the body tube and letting the stuffer-tube nest on the end of the inner cone, but I was concerned about what ejection gasses fired right at the nose cone might do to it (and the paint) over time.

    I decided it'd be best to add a bulkhead, so I took a centering ring I had and glued a piece of cardstock over of the inner punch-out, and coated it in a light coat of 10-minute epoxy to harden it against the heat of the ejection charge.

    [​IMG]
    [​IMG]

    I measured the length of the shoulder and "inner cone" of the nose cone, and used a piece of BT-60 coupler to get the bulkhead in the right place and nice and square, and glued it in place with some titebond. The end of the stuffer will be right up against it, and the nose cone supporting it from the other side, so I think it'll survive ejection forces just fine. (Does anybody think I should drill the sides of the stuffer?)

    [​IMG]


    Now it was time to build up the motor pod. I cut a length of BT-20 to the same length as the distance from the bulkhead to the bottom of Bertha's body tube. I built this as you would have done for the "stock" body tube, with just a bit more separation on the centering rings to leave room to wrap the 'chute. I left the forward centering ring a bit aft to allow room to loosely coil the shock cord to avoid tangling during ejection.

    [​IMG]

    The "Peak of Flight" article mentioned that the forward centering ring can take a bit of a beating, so I added a motor clip retention ring on the forward side, feeding the kevlar underneath, and through a small hole I punched in the centering ring. I soaked some thin CA underneath to hold it all in place.

    [​IMG]

    I added a fillet of "Fix It" epoxy to the forward side, and a bit over the shock cord on the aft side to hold it all solidly in place. (Note the "blood knot" in the kevlar in the photo above, that should anchor the shock cord nicely in the putty when it hardens.) I left a loop on the end for recovery device attachment.

    [​IMG] [​IMG]

    Here it is, ready for "stuffing". One nice thing is that if there's any hitches I discover about the motor pod design is that it'll be pretty easy to just get a new motor mount kit and build a new one pretty quickly (assuming the airframe survives...).

    [​IMG]

    All that's left is fin attachment, and finishing! It's going to be a nice touch to have no seam at the nose cone to body tube junction.

    I plan to do a static test before proceeding too much further. I'll post results of that along with the finished build in due time.
     
    Last edited: Aug 29, 2019
  2. Aug 29, 2019 #2

    beantownJPL

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    Had a successful static fire test using an A8-3 before work this morning.

    I'll put a longer shock cord on before a real flight, this actually caused an almost zipper at the bottom of the body tube. Nothing that can't be easily smoothed out with a fingernail and hardened up with a bit of thin CA.

    Also, for testing purposes I just attached the 'chute directly to the kevlar. I'll use some flat elastic between the 'chute and the kevlar to give it a little more cushion. There's more than enough room in the "parachute bay" to handle a decent length of elastic shock cord.

     
    Last edited: Aug 29, 2019
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  3. Aug 29, 2019 #3

    BABAR

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    Looks like it should work great.

    Hope to see a successful flight report soon.
     
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  4. Aug 29, 2019 #4

    beantownJPL

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    Yeah, I'm pretty pleased with it. It's so easy to prep for flight, as compared to "conventional" recovery. I'm going to do more builds this way I think.

    I hope to write a successful flight report soon! Next realistic opportunity would be a week from this Saturday. I'll definitely be posting the completed build pics when it's ready.
     
  5. Aug 30, 2019 #5

    Huxter

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    Very interesting - thanks for the great write up! The Baby Bertha is kinda short for a baffle huh?

    So if you were to build another (regular) Big Bertha, would you put in a baffle or rear ejection?

    I like baffles too - for the ease of prepping for flight.
     
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  6. Aug 30, 2019 #6

    beantownJPL

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    My last Big Bertha got rod-whipped into a bad trajectory, and is now decorating a tree somewhere several hundred yards into a forest.

    That one had a baffle ... and a nice nylon chute.

    I'm definitely contemplating a breech stuffer setup for the next one if the little one behaves well.
     
    Last edited: Aug 30, 2019
  7. Aug 30, 2019 #7

    BEC

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    This is interesting. Thanks for posting it.

    I generally put a Semroc cup-style baffle (http://www.erockets.biz/semroc-ejection-baffle-kit-conical-fiber-bt-60-sem-eb-60/) in Big Berthas and other BT-60 models whenever i can. I just finished a Boosted Bertha today and have one in there with the top of the baffle about 7 inches down from the top of the tube. I will take it and a "regular" Big Bertha with me to Sod Blaster this weekend. I will also have a battered Baby Bertha and a Super Big Bertha with me as well.....
     
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  8. Aug 30, 2019 #8

    beantownJPL

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    So far I've only used the Apogee baffles. I also scratch-built one based on their design. A little more crude looking, but just as effective. Down there in the exhaust pipe, nobody can see it.

    Maybe I'll give the Semroc ones a try on my next front-shooter.
     
  9. Aug 30, 2019 #9

    beantownJPL

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    I just placed an order for a Boosted Bertha. Also a Black Brant II and a Sterling Silver (lost the sustainer for a prior SS, and also had a paint wrinkling issue on it - MKII. :D)
     
  10. Sep 6, 2019 #10

    beantownJPL

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    I had some build time this week. Fins are on, rough sanding done and first coat of primer sprayed. I think it's going to look awesome in paint with no nosecone seam.

    This is also the first time I tried epoxy fillets. By the time I got to the last few fins, I pulled them pretty clean. This is the worst side, so should clean up alright.

    I've got a rattle can of metal flake black that's going to look nice on it. I wish this model had water-slide decals.

    [​IMG]

    Some of you might notice my next build in the background.
     
    Last edited: Sep 6, 2019
  11. Sep 6, 2019 #11

    nosecone

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    Looking good, I am building one this weekend based on this. Thanks
     
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  12. Sep 6, 2019 #12

    beantownJPL

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    Nice, I'd like to see photos when you finish it. Also, if you do anything different, post it!
     
  13. Sep 6, 2019 #13

    Matt_The_RocketMan

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    I Dont know weather this impresses me or its something I haven't heard of yet. I feel like more problems than solutions are made from this. One would probably be, wouldn't the heat of the engine during flight melt the parachute, I know for a fact that the engine during flight is generating a lot of heat in and around the engine bay, so wouldn't this cause recovery problems in the long run? Also, how does the rocket react as to a massive weight shift? I feel like this is not worth hassle to make a rocket's recovery system backwards...
     
  14. Sep 6, 2019 #14

    ebruce1361

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    Rear ejection of motor tubes isn't a new idea, and has been done successfully for a long time. I think the only reason it isn't more popular is because it tends to ding up the paint job on nose cones, and also because deploying the nose slows the whole rocket down a bit before the parachute has to grab. As for the heat issue, I haven't seen a rear-eject melt a parachute, but if it is a concern, an insulating wrap can be put around the core tube.

    The one problem I HAVE seen with some rear ejections is a shroud line getting snagged between the forward centering ring and the body tube causing at best a tangled parachute, at worst a motor tube that only partially came out and the rocket comes down ballistic. Be careful on your packing!
     
  15. Sep 6, 2019 #15

    ebruce1361

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    I just got done with my first Big Bertha, but ramped things up with a payload bay and a gap staged booster. The maiden flight will be in about two weeks. Fingers crossed! 20190904_231202.jpg
     
  16. Sep 6, 2019 #16

    Matt_The_RocketMan

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    I would add that model kits do not come with the extra parts for this type of recovery, so I would see that it would unpopular. But as to the wadding paper yes it is annoying to put in, and expensive to buy but in all it does take at the fastest 2 mins to properly pack a rocket with the wadding and all, the older tech to me is probably the best and most effective if done right.
     
  17. Sep 6, 2019 #17

    beantownJPL

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    There was no issue with the heat during my static test, but I'll grant you that was only an A8. I'll use a larger motor on my next static test.

    As for changing the weight distribution, it's really not changing too much, and the added weight will be mostly going toward the nose anyway. That forward centering ring with the epoxy has moved up toward the front, so if anything it's adding some additional stability to an already over-stable rocket. It may be a bit more prone to weather cocking but it's a short, fat rocket, so shouldn't really be too much. Those big overhanging fins generate a lot of lift, this rocket should go pretty straight.

    Regarding "practicality" ... this particular build isn't about practicality, it's about messing around with things and trying something different "just because". I'm a tinkerer by nature. There's lots of rockets that go up, pop their nose cone and come down. There's lots of Berthas out there ... but none quite like this one. :D

    I'm not saying everyone should build their rockets this way, I'm just having fun putting a twist on this build.

    Rear-eject is not really something new ... there's lots of models out there that use it, or something similar. The Semroc Gyroc is one example of one I've recently built, but there are others.

    Right, I had to add some materials to build this. However, those materials are readily available, and cheap. The extra centering ring I used as a bulkhead came from an Estes Engine Mount Kit, the Kevlar from Apogee, and the BT-20 from Estes. Modding kits usually requires adding stuff not included in the bag.

    Yeah, the wadding isn't the end of the world. I have a big tupperware full of unopened packs of it, so it's not like I have a shortage of it either. I have models that require wadding, I don't bother with a baffle on anything smaller than a BT-60. However, I usually fly with my very enthusiastic 10 y.o. son, so rockets tend to go up fast and furious! Occasionally wadding has been omitted ... especially on a first flight of the day where maybe the 'chute had been stuffed in the tube for storage after a prior day of flying. My son grabs the rocket, stuffs a motor up it's butt and heads for the flightline, forgetting to check for wadding. Usually I'll check, but sometimes he's quicker than I am.

    "...it tends to ding up the paint job on nose cones..."

    Well, traditional ejection can be tough on fins and body tubes!

    "... and also because deploying the nose slows the whole rocket down a bit before the parachute has to grab ..."

    The motor pod should provide some drag before the 'chute pops. Also, with it wrapped around the tube, that'll give a little delay allowing the model to slow down a bit before it pops I'd think. Also since the drag is coming from the rear, there's virtually no chance of a zipper in the case of a high-speed deploy. Sure, I could shred a 'chute ... but nobody would accuse a Bertha of being a screamer.

    I don't think I'd try this on anything smaller than BT-60 unless I'm using a streamer instead of a 'chute. There's lots of room between the motor pod and body tube to comfortably wrap the chute around it, and it all fits loosely in the body tube. I don't anticipate any problems with ejection. Murphy will have the final say in the matter though!
     
    Last edited: Sep 6, 2019
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  18. Sep 6, 2019 #18

    beantownJPL

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    That's a gorgeous build, looks like Baby Bertha is trying to climb back up inside momma! I hope you'll post a video of the maiden!

    I've just received a shipment from acsupplyco.com that includes the new Boosted Bertha. I'll probably start on that build after I wrap this one.
     
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  19. Sep 6, 2019 #19

    Matt_The_RocketMan

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    I will add to the idea
    I think this concept should be used for HPR, this is would be so, because it leaves a lot of empty room in the upper half of the rocket, and because the rocket is going to naturally heavier at the bottom the ejection charge would probably not have to be used at all, this being you could add a lock mechanism for the engine / recovery system to stay in during apogee and flight, but as soon as the rocket stops coasting the lock mechanism could be programmed by the GPS altimeter system to release the motor and recovery system , in all you would let gravity do the work for you + saving in money due to no powerful ejection charge...

    If you would like to add to the idea be free to..
     
  20. Sep 6, 2019 #20

    beantownJPL

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    The problem I see is that everything is moving together at the same speed, without something to push the motor pod out the rear. If the rocket were hanging from something it'd fall out, but they're all in parabolic motion together unless one part or the other were accelerated by an additional force.

    I don't see them separating on their own. However ... if you were to add a spring of some sort, that would do it.
     
  21. Sep 6, 2019 #21

    ebruce1361

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    Matt_The_RocketMan, what you're describing sounds a lot like a drag separation; if the entire fin can were attached to the motor tube and recovery system. Even at apogee, unless the rocket went PERFECTLY straight up and back down, there is still motion forward relative to the rocket. As it crosses apogee, the drag exhibited on the fins can be used to pull that section out once unlocked as you describe.

    The problem with having the motor tube simply "fall out" like you said is that both the external portion of the rocket and the motor tube will be in freefall at the same rate. From the point of view of being inside the rocket, the motor tube will suddenly be floating free like passengers aboard the "vomit comet" (except in this case, the "passenger" almost perfectly fits inside the cabin with no room to move). When Newton said objects fall at the same rate regardless of mass, he meant it.

    Now, if you had a really good seal on the forward centering ring and the body/nose could withstand the pressure difference, you could rely on the internal air pressure ahead of the motor tube to push it out like an air cannon. But this would only work at sufficiently high altitudes, and there would have to be no leak whatsoever. A spring would be more reliable, but then that's a thing for the parachute and shock cord to snag on.
     
  22. Sep 6, 2019 #22

    Matt_The_RocketMan

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    Maybe if you could program the Gps/ tracking system to find a few second period where the engine mount is released at or near nearing apogee, this would allow the engine to move in the opposite direction of the rockets forward momentum, thus when the rocket slows down and starts falling the recovery system would be already out the back falling for 2 seconds depending on how long the shock cord is made, and this would all catch and unravel as the rocket is in falling position.

    I would say as the rocket is ending its climax apogee stage, then there should be enough backwards pull to let the engine bay fall out, maybe this could be achieved with a spring but the main idea is letting gravity do most of the work...
     
  23. Sep 6, 2019 #23

    beantownJPL

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    You'll need something to actively kick it out. Objects in motion stay in motion until acted upon by another force. Simply releasing the motor mount won't cause it to slide out, since it will have exactly the same momentum as there rest of the rocket. Gravity is acting on the whole thing equally and at the same time. Just because one part or another is "heavier" makes no difference.

    The"other force" could be a spring, aerodynamic drag (if it's full fin can separation), or an ejection charge.
     
  24. Sep 7, 2019 #24

    BABAR

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    Will the booster remain stable after separation? If so it may core sample
     
  25. Sep 7, 2019 #25

    BEC

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    I was wondering about that after looking at the picture. For comparison, here is what the newly released Boosted Bertha looks like. I can attest, after having flown this one four times at Sod Blaster last weekend, that its booster tumbles as it should. The fins on the booster are substantially larger than those of the sustainer (which have the same planform as regular Big Bertha fins).

    IMG_1601.JPG
     
  26. Sep 7, 2019 #26

    BABAR

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    Biggest problem I have encountered with rear eject (I have used it mainly for post gliders, Apogee Rockets DarkBird is a nice example) is SPACE. Should not be as big an issue with the Bertha. Lots of space between BT20 and BT60. A lot more challenging with BT5 to BT20 or BT20 to BT50!

    May want to layer a bit of epoxy on the inside forward edge of the nose cone, as it is going to get hit by flaming bits of propellant with every ejection.

    Do not wind the cord or the chute around the central tube, FOLD the chute and lay the cord carefully. Recommend packing JUST before flying and consider some talcum-powder if plastic chute. Packing is tricky, cuz you have to kind of eeek it in as you insert your pop pod. The pod MUST slide easily out. If you hold the rocket upright and shake it up and down, it should just fall out.

    I have never had problems with heat from ejection melting a chute or streamer.

    IF you need nose weight, here is a trick. Extend your inside motor mount tube all the way until it just touches the inside of the nose when full inserted. Get some flat lead or other material to put the weight on the outside of the tube, glued on firmly. This will provide nose weight on the upward flight, but will come out with the pod at ejection. Therefore the body of the rocket will have less mass at touch down, less damage. It is also a great help for pop pod boost gliders, gives you nose weight for stability on boost, takes it ALL out of the glider at separation



    If your landing field is anything but grass, consider putting a square of masking take on the tip of the nose cone at take off, since it will hit the ground tip first, the MAY reduce the ding you will get on landing

    Hope you have a straight trail and a soft landing!
     
  27. Sep 7, 2019 #27

    BABAR

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    You guys are losing me, the ejection gas will go forward through the tube. Since nose cone is glued it, will pressurize the forward end of the rocket. Pressure on the forward centering ring will force the pod backward (pressure is the same all around, but only the pod can move.).

    For this reason, the forward ring should be as far forward as possible, basically at the base of the nose cone shoulder. This does two things.

    1. Reduces the volume you need to pressurize
    2. Maximizes space for your chute. This allows much “looser” packing, which is a good thing.

    Rear ejection with black powder doesn’t require drag or spring or anything else.

    The forward centering ring protects the chute from the ejection gas and particles.
     
  28. Sep 7, 2019 #28

    beantownJPL

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    Yes, look at my photos on post #1. The forward centering ring is about ½" back from the end of the stuffer.

    I was responding to Matt_The_RocketMan who was proposing rear ejection with a gravity release for HPR without an ejection charge. I was explaining that separation won't happen without something to kick it out, since they'll all be in ballistic motion together. The pod won't just slide out when you mechanically release it with a servo or something.
     
    Last edited: Sep 7, 2019
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  29. Sep 7, 2019 #29

    beantownJPL

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    I did coat the forward bulkhead with epoxy.

    The Peak of Flight article I linked to in my first post suggested otherwise. I'll try both methods and see what works best in this model.
     
    Last edited: Sep 7, 2019
  30. Sep 7, 2019 #30

    wrad

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    My baby bertha has lost a nose cone on ejection twice and both times tumble recovered perfectly. Although with a stuffer tube for gap staging it might push the cg slightly forward compared to unmodified.
     
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