Mean Machine - Mid separation

Discussion in 'Low Power Rocketry (LPR)' started by beantownJPL, Sep 18, 2019.

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  1. Sep 18, 2019 #1

    beantownJPL

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    I've got a (mostly) stock built Mean Machine that I've got probably 10 flights on D12 and E12. I like this model a lot and it's a great flyer. The modifications I made to the original build were to use the longer Apogee tube couplers (3" instead of 1.5"), an Apogee ejection baffle and rail guides as well as lugs (which I upgraded to 1/4").

    While it's a great flyer, it is just a bit delicate. After 4 or 5 flights on its first day, it was taking a beating. I had to re-glue fins, and on the last landing, it lightly kinked the lower-most body tube. I repaired the tube by cutting at the kink, and adding a coupler. Flew it again this last weekend, and it snapped off a fin on landing again, but no other damage.

    It's a big long lever, and 6' of BT-60 hitting the ground tail first is a fair bit of energy. The kink occurred just where you'd expect a 6' stick to break if you hit it on the ground. I could use a bigger 'chute, but that just means longer walks, and a greater chance of decorating a tree.

    Long story short, Estes has warrantied me a new one because (unrelated to any overall design issue), I had a "shotgun ejection" that tore out the motor hook and ejected the motor. So, I'm going to build this one a little different.

    I plan to discard the plastic coupler and add an Apogee coupler in the middle with a bulkhead and screw eye, to which I will attach one 15" 'chute. The lower half receives an ejection baffle at the lower coupler location, also with a screw-eye to which I'll attach a second 15" 'chute. On deploy, the two sections completely separate, and come down independently.

    My only concern is ensuring both 'chutes deploy. While I'm fairly sure the ejection charge will be sufficient to blow out the laundry, not having the nose cone pulling the lower 'chute out is a lost redundancy.

    Options I've considered:
    1. Attach a bit of thread to the peak of the lower parachute, and tape it to the inside of the upper section of the airframe.
    2. Add velcro to the tops of both 'chutes and stick 'em together as I load the bundle into the airframe.
    3. Decided --> Piston ejection. I have the parts ... might be fun. The ejection baffle should keep most of the gunk in the lower part of the rocket, so the "piston tube" should stay relatively clean. Decision made on post #8.
    4. Umm ... something better? -> Interesting option for future build: "Semi-drogue" described by @georgegassaway in post #16 & #23
    I've drawn it up in OpenRocket ... so no worries about stability.

    Screen Shot 2019-09-18 at 2.05.27 AM.png

    My posts on this build for those who wish to follow along:
    1. Aft-section completed on post #26
    2. Piston ejection & separation test (airframe complete) on post #27
     

    Attached Files:

    Last edited: Sep 23, 2019
  2. Sep 18, 2019 #2

    MALBAR 70

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    I have a Custom Rockets Equinox that I built this way.The rocket is supposed to have two 18" chutes, but I usually use one 24" X type chute and a very long shock cord.
     
  3. Sep 18, 2019 #3

    ebruce1361

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    Maybe you could just load the nose section parachute in first, then the tail section chute. Keep all the shroud lines bundled up inside each parachute and have the individual shock cords on opposing sides to minimize tangling. When the rocket separates at ejection, the nose half will drag its parachute out, and in doing so, that parachute will push the tail chute out first.
     
  4. Sep 18, 2019 #4

    beantownJPL

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    That's a thought. Certainly simple enough. What if you wrapped a rubber band around the two chutes? The flailing about post-ejection might be enough to yank them free as long as it wasn't wound too tight. If I was ready to drop $130, a JLCR would be cool in this application.
    IMG_20190918_102129.jpg
     
  5. Sep 18, 2019 #5

    ebruce1361

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    I'd be wary of bundling the parachutes side-by-side given the diameter of the Mean Machine. Since the motor tube itself is pretty short, that ejection charge has to pressurize quite a big volume even if you build it with the separation halfway up, so having the parachutes bundled like that could produce enough friction to hamper the deployment since the ejection charge can only push so much. Another potential issue I see is the parachute bundle can spin at the ends of the shock cords after deployment and tangle tightly enough to prevent them from letting go of each other.

    If you want to physically connect the parachutes so one drags the other out, your idea of a small (and I mean SMALL) dot of velcro on the tops of the canopies or maybe a piece of masking tape rolled sticky side out might be your best bet. Even so, when I have multiple parachutes in a rocket, I try to have them in contact with each other as little as possible to prevent tangles. Once they're out of the fuselage, the further they are from each other, the better.
     
    Last edited: Sep 18, 2019
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  6. Sep 18, 2019 #6

    beantownJPL

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    All good food for thought. What are your thoughts on using a piston? I'm leaning in that direction since it physically shoves the whole bundle out when the charge fires. The 'chutes can be bundled independently, and stacked on top of each other with the shock cords on opposite ends, should reduce the possibility of fouling each other.

    I figure I can also keep the shock cords on the shorter side no matter which strategy I go with, since I don't need to protect the model from getting slapped by the NC. I'm planning on using 1/8" round elastic cord, since it should all be well protected from ejection temperatures.
     
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  7. Sep 18, 2019 #7

    ebruce1361

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    I have personally never used piston ejection, but I can definitely see how that would improve things. The motor will only have to pressurize the area between the motor tube and the piston base, so you'll get better use of the energy providing your piston isn't too heavy. It would also take the place of the ejection baffle, so you'll have one less component to worry about. Just don't forget to include a vent hole in the body just below where the bottom of the piston ends up when moved fully forward; you want as much pressure as you can get to deploy the recovery system, but when that piston stops moving the gas has to go somewhere. I've seen piston rockets split open on the side or with the motor tube blown out the back because the ejection gasses had nowhere to go after moving the piston.

    Like you said, the parachutes can be independently bundled and stacked, so once they come out, they are already moving away from each other, thus reducing risk of tangles.
     
  8. Sep 18, 2019 #8

    beantownJPL

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    I was thinking I'd make the piston from a BT-60 coupler with a bulkhead. Instead of having a captive piston that stops at the end of the BT, I'd allow it to fully eject - a couple knots in the elastic shock cord on either side of the bulkhead should keep it in place a fair distance down from the 'chute, and then anchor the other end to the ejection baffle with enough length of shock cord that it'll be well clear of the model by the time the 'chute pops.

    You're correct that with the piston I can dispense with the ejection baffle, but I think I'll keep it in the design. I've read that one of the failure modes for piston ejection is residue from the ejection charge fouling the inside of the body tube and causing the piston to either not slide well or at all. The ejection baffle should keep all (or at least most) of the dirty stuff down in the engine room.

    I've currently got over 10 calibers of stability, so not worried about a few grams shifting the CG a little further aft. Also not going for altitude records; if I want to go higher I can use a composite E or F.
     
  9. Sep 18, 2019 #9

    ebruce1361

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    I hadn't even thought about an ejectable (is that even a word?) piston. Yeah, that would facilitate the separation as well as eliminate the need for an exhaust port. And you raise a good point about the fouling of the inside of the body without the baffle. I suppose you could run a bottle brush through the body after each flight to clean up the mess and maybe even lubricate the whole system with graphite dust, but that might just be superfluous with the baffle.

    Even though you aren't going for altitude records, flying an altimeter can give you some fun data. With a bulkhead at the rear of the upper half and the nose not glued in place, you've got a payload bay about three feet long. Might as well put it to use! I have an old Estes Longshot (2128) that I made the separation about midpoint with the upper section a payload bay. I can fit a whole bag of Jolly Rancher's candies in there that I pass out to kids at our club launches after it flies them. Something about G-forces makes candy taste better!
     
  10. Sep 18, 2019 #10

    beantownJPL

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    I don't currently own an altimeter, but I'm definitely putting an AltimeterThree on my Christmas list. I hope I've been a good-enough boy!

    I was thinking it'd be nice to have a blended nose cone like my Breech Baby, but the idea of being able to use the forward section as a payload bay is intriguing. Decisions, decisions... Maybe I could move the twist-lock connector up to the first joint. I could hide the joint with some sort of paint detail and still have a sleek, smooth nose section and a payload bay.
     
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  11. Sep 18, 2019 #11

    ebruce1361

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    I got a JL Altimeter3 last year and I love that thing! Beware of water hazards as it doesn't like getting wet and make sure you have at least a tiny vent hole because the calculation for altitude is based on changes in air pressure, but I HIGHLY recommend getting one!
     
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  12. Sep 18, 2019 #12

    OC-Patrick

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    I made my MM mid-separation using an upper body bulkhead, but using the same parachute - works fine, did the same thing with the Star Orbiter. The only problem I have with this rocket is the relatively thin wall tubes and the paint scheme, black. It's first flight was on a sunny (but not hot) day. The rocket literally bowed up due to the sun heating up the sunny side as I was prepping. No more long, thin-walled dark-colored rocket for me...
     
  13. Sep 18, 2019 #13

    ebruce1361

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    There's a reason nearly every rocket built to go to space is painted white! Bummer on the warped Mean Machine. Were you able to straighten it out and still fly it?
     
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  14. Sep 18, 2019 #14

    beantownJPL

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    I have read about the issues with warping in the sun, and definitely kept that in mind. The current Mean Machine sports Blue and White livery, possibly in response to that issue.. I painted mine similarly.
    IMG_20190824_115142.jpg

    I agree with you on the thin-walled tubes, part of the reason I've decided to do a mid-body separation. I did suffer a kink in the lower body tube on one landing. The dual chute, separated descent idea is just for fun, and because I like to do things a little different when I do a repeat build.

    I can understand why they are using thin-wall tubing though. The added weight of heavier tubing would definitely push this rocket solidly into the mid-power realm.

    The other thing I'm not crazy about is the 3/16" launch lugs. This TALL rocket really needs some support on the pad ... in my original build I added 1/4" lugs AND rail guides. I love launching this thing off the rail when it's available. On the rod it bobs and sways in the slightest breeze, who knows where it'll be pointing when the motor lights off. It also looks a heck of a lot better on the rail ... looks a little silly on the rod.

    Here's a rod launch:


    ...followed by a rail launch:
     
  15. Sep 19, 2019 #15

    OC-Patrick

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    It was launched slightly crooked but flew well, no issues. It straightened itself out.
     
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  16. Sep 19, 2019 #16

    georgegassaway

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    You guys are over-complicating this. I have seen too many people do models with two separate sections that they have a chute on one, and a chute on the other.... and something screws up so one of those sections crashes with no chute deployed (or they tangle together and NONE deploy, that rubber-band-together and HOPE they pull apart idea risks that). Often the main body doesn't get its chute out, while the upper section drifts down slowly with its chute. I saw that happen one time too many with one of my team's scale models and figured screw that!

    In 1980 I came up with what I call "semi-drogue". Use a LONG shock cord to attach both sections. Use two chutes, a big main at about 2/3 of the long shock cord's length from the main body, and a smaller chute near the upper section. If ONE chute works, the whole model comes down safely. Load the main in first, packed so it slides loosely inside the tube. Load in the smaller (upper) chute LAST, and packed small enough it can almost "fall" out of the tube. So when the front section is ejected off, it takes the small chute with it, and the small chute deploys every time. And if the main chute did not get blown out of the tube by ejection, that small chute "yanks" the main chute out, like a drogue chute would.

    With both deployed, the shock cord between the main chute and the drogue is usually laying near-horizontal, as the airflow between the two chutes causes a bit of leaning away from each other.

    Number of times this system has failed me since 1980, if the nose section came off as planned at ejection? ZERO. Yet I still see people having models crash with separate chute systems, where the main body didn't get its chute out and the upper section drifts down safely on the chute that could have kept the main body from crashing, if not also yank out the chute that didn't eject.

    FWIW - The model below, BT-80 (2.6"), 6 feet tall.
    [​IMG]
    Ryan Stenberg's A Div winning D-Region Tomahawk (OOP Estes 1.8" kit) scale model at NARAM-61, using Semi-Drogue.
    [​IMG]
    Here is an extreme example of semi-drogue. A HUGE Pumpkin Leaf Bag converted into a tall parachute. It needed help to get it out of the 4" tube. So a 12" chute (barely visible) was used literally as a drogue. Not exactly like the examples I mention above, no big long upper section, but still the principle.
    [​IMG]
     
    Last edited: Sep 19, 2019
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  17. Sep 19, 2019 #17

    beantownJPL

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    That's brilliantly simple. I'm definitely going to try this in a future build.

    I'm going forward with the piston ejection because I really want to play with the idea. However, there's nothing about how I'm going to build it that would preclude me trying your system later.

    That looks really cool coming down. I think two separate parts coming down (if successful) will be fun too.

    Let's not forget why we do this. Rocket parts are cheap (at least at this scale). Build time is fun. Play with stuff and see what work/is cool.

    Every build I do I try to make a learning experience. It gives me confidence as I move onto higher power and/or more complex builds. I'm a tinkerer ... failures just show me the stuff I didn't think of.
     
  18. Sep 19, 2019 #18

    BBowmaster

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    I am absolutely doing my Mean Machine this way. I always fear losing one piece in a dual eject rocket. I think I’m going to try this with my 1/100 Saturn V as well.
     
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  19. Sep 19, 2019 #19

    beantownJPL

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    Please feel free to add your build to this thread. I think the Mean Machine is a great rocket that is also ripe for mod ideas. It'd be cool if this thread became a repository for Mean Machine mods. I'd even change the title!
     
  20. Sep 19, 2019 #20

    beantownJPL

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    I also have a 1/100 Saturn V. There is definitely no need to modify the recovery system on this model. It works perfectly as a stock build. Just don't try to launch it on anything less than an E30. Every launch I've seen on less than this ended up ... not successfully.

    The "laundry room" on this model is very shallow, and all three chutes deploy every time. I'm not saying this to discourage you from implementing @georgegassaway's method, but it's definitely not necessary.

    Edit: @georgegassaway - I hereby dub your deploy design a "gassaway deploy". Feel free to correct me if it's been otherwise named.
     
  21. Sep 19, 2019 #21

    ebruce1361

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    I did my 1/100 Saturn V with a 5X cluster of 24mm mounts (first flight will be all D12s, then I plan on feeding it Aerotech E15s). As for the recovery, I agree the parachutes practically fall out on their own because the body diameter is so forgiving. I made four 21" nylon chutes with the classic orange and white gores to look like the real Apollo chutes. Three bring the booster down, and the fourth is for the nose. I'm using a nomex blanket that will be bundled behind and around all four chutes, so the only thing that could prevent deployment is if the nose refuses to deploy. To account for that possibility, I'm staggering the delays so I have two fire at three seconds and one more at five seconds (the other two motors are D12-0s in reinforced sealed tubes).

    I was hoping to have this in the air for the 50th anniversary, but the club we fly with cancelled their event in July due to muddy field conditions, and then last month was their annual bowling day. So the maiden flight is due for this Saturday. 20190818_190652.jpg 20190818_190442.jpg
     
  22. Sep 19, 2019 #22

    beantownJPL

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    Very nice build. I built mine stock, but did upgrade the chutes to nylon ones from Apogee, it seemed blasphemous to use the plastic ones. I also added a Nomex blanket in mine.

    I maidened it on 7/20 at my club on Estes an E30-4 while still in primer (that was sprayed the night before the launch). I haven't gotten around to finishing the paint job yet ... leaving that for a winter project, as I've been focusing my free time more on "every day" fliers.
     
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  23. Sep 20, 2019 #23

    georgegassaway

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    I call it "Semi-Drogue" recovery. Not a total drogue since the smaller (but not THAT small) chute is usually doing its fair share of slowing the model down, not just a "token" chute, or a chute that is almost insignificant once the main chute opens (except for that huge pumpkin bag chute and its relatively teeny 12" chute). So, this method is a "Semi" drogue, not total drogue.

    Had an article about it in the "SNOAR News" newsletter in the mid-late 1980's.

    EDIT - I searched TRF to find various times when I have mentioned this before. Here's one from 2011:
    https://www.rocketryforum.com/threa...-the-nose-cone-pull-it-out.22519/#post-199952

    Here's a poor quality image from part of the SNOAR News article.
    [​IMG][​IMG]
     
    Last edited: Sep 20, 2019
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  24. Sep 20, 2019 #24

    nosecone

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    I made mine middle separation and had no issues, but it was launched only about 10 times back in the 80s. Back then I didn't have access to bulkheads so I made one with brown craft paper jammed in the tube with a lot of Elmer's Glue.
     
  25. Sep 20, 2019 #25

    BABAR

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    Given the length of the model, isn't there sufficient space even WITH mid point separation for the normal size chute, packed in the tail section?

    Also, for the non-purists, putting the fins on swept forward rather than aft will result in the motor casing taking the brunt of the initial impact rather than the find. Effect on CG is minimal, likely negligible in a rocket of this size.
     
  26. Sep 20, 2019 #26

    beantownJPL

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    Yes, there's plenty of space for the normal-sized chute. I'm doing the dual-chute, separated recovery for novelty.

    Good idea on the fins, but I already installed them last night.
    IMG_20190920_141437 (1).jpg
     
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  27. Sep 23, 2019 #27

    beantownJPL

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    Made some more progress. All major construction is completed, the airframe feels pretty solid when all mated together.

    Fins are filleted, piston is built, moved the twist lock connector to the forward tube junction to act as a payload bay.

    A "blow test" shows that the piston can provide an energetic separation and ejection just with the pressure I can generate with my lungs. Ejection was forceful and had a satisfying "pop" with very little back pressure - shot the forward section clear off the end of the table. It felt about like firing a dart from a blowgun.

    Should be interesting in the air with the more energetic ejection charge from a D or E motor! Should get the two parts of the airframe well away from each other, especially if I pack the forward chute in such a way to encourage a slow deploy. I'd do a static test with a motor, but I don't want to waste a 24mm motor on the ground...

    (edit: the "puffing" sound you hear in the background is some game my kid was playing on the iPad just out of frame. The ejection shown below was a single, moderately strong, puff of breath from my mouth on the motor tube.)


    Below, you can see the piston assembly fashioned from a 1.5" long (standard Estes size) BT-60 coupler with a plywood bulkhead. I had considered using a 3" Apogee BT-60 coupler, but I figured that I wanted to minimize friction, and it only really needs to be long enough to avoid twisting and getting stuck in the bore.

    I drilled a 3/16" hole for the 1/8" round elastic shock cord to pass through. I tied stopper knots on each side of the bulkhead to lock it in place. It just took a little bit of sanding of the piston to ensure it slid smoothly in the bore without binding. I also installed a shoulder (1/4" section from a BT-60 coupler) about half-way down the body tube to prevent the piston from sliding too far down, yet leave plenty of room above the piston for recovery gear.

    You can also see the coupler for attaching the upper and lower airframe sections. I had originally left a 3/4" shoulder, but this didn't end up feeling very secure. It wiggled easily when the full rocket was assembled, and I could totally envision an unintended separation happening during boost (or worse, at high speed at motor burnout). To provide a more positive connection, I spliced another 1.5" coupler to the end, and this feels very strong now. I thought I might have to trim the length down a bit to avoid too much friction but, as the test above shows, it works just fine as is, so I'll keep the extra length for strength and a more positive fit.
    IMG_20190922_200301.jpg

    Here's all the pieces of the airframe. Nose cone is glued in and the joint will be filled and blended smooth. The twist-lock connector joins the top two tubes, followed by the mid-body junction with the piston.
    IMG_20190922_201506.jpg
     
    Last edited: Sep 23, 2019
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  28. Sep 23, 2019 #28

    BBowmaster

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    That may say more for your lungs than the rocket design. Looking very good. Hope you get a chance to launch before winter. I’d love to see how it turns out before I begin mine.
     
    beantownJPL likes this.
  29. Sep 23, 2019 #29

    beantownJPL

    beantownJPL

    beantownJPL

    Up up and away... Wait! Come back! TRF Supporter

    Joined:
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    Gender:
    Male
    Location:
    Massachusetts
    Next club launch is October 5th. Should be ready well before then. Just needs finishing work now.

    Edit: Wait, did you just call me a blow hard??? o_O
     
    Last edited: Sep 23, 2019
  30. Sep 23, 2019 #30

    ebruce1361

    ebruce1361

    ebruce1361

    The man with the plan. And some duct tape.

    Joined:
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    Location:
    Pinellas County, Florida
    Looks like a solid and simple design! Anxiously awaiting report of that thing punching a hole in the sky!
     
    beantownJPL likes this.

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