Does the ejection charge push out the parachute, or does the nose cone pull it out?

Discussion in 'Recovery' started by Zeus-cat, May 25, 2011.

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  1. May 25, 2011 #1

    Zeus-cat

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    There have been claims that ejection charges do not eject parachutes; rather, they push the nose cone off and the nose cone pulls out the parachute. I decided to test that theory.

    I used seven old dual deploy ejection charges for the test. I installed the ejection charges in a used D12 case and ran the wires out of the nozzle. I had to widen the nozzle slightly to make it easier to run the wires. I inserted the motor into my old beat up Estes Phoenix (BT-80) which is no longer flyable. I inserted a 12” x 12” Nomex chute protector, then the parachute and then installed the nose. None of these were connected to each other so if the chute was ejected it had to be from the ejection charge.

    I laid the rocket on the ground and put a brick under the forward section to elevate the nose slightly. I connected each charge to my launch controller and pushed the button.

    Two of the charges showed very high resistance and failed to fire.

    Two more fired, but were very weak and failed to dislodge the nose cone.

    Three charges fired and ejected the nose cone. Two of the ejections were weak and the nose cone only flew off about a foot or so. However, in both cases both the Nomex and chute were cleanly expelled from the rocket. The third charge was much more powerful and fired the nose about six feet. The chute was ejected about three feet, but the Nomex hung up in the top of the body tube.

    While certainly not an exhaustive series of tests it certainly shows that the ejection charge will eject the parachute without the nose cone pulling it out.
     
  2. May 25, 2011 #2

    Bazookadale

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    Never heard that before
     
  3. May 25, 2011 #3

    boomtube

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    I’ve had rockets blow off the nosecone without deploying the chute so my guess would be that if something allows enough of the ejection charge to bypass the wadding and chute, without actually blowing it out the open end, the flailing nosecone won’t by itself drag the chute out.

    This observation was on rockets with fairly light weight nosecones.
     
  4. May 25, 2011 #4

    MarkII

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    I have seen that statement (the nose cone pulls the parachute out) made a couple of times. It never made much sense to me. The issue was debated here sometime last year, I believe. In the end it doesn't matter how the chute comes out, as long as it gets out promptly.
     
  5. May 25, 2011 #5

    plano-doug

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    I believe this is a bigger contributor to getting the laundry out than it gets credit for. While it's possible, with enough ejection charge, to blow out just about anything, I've seen the after effects of enough ejection events to conclude the nosecone did most of the work. In too many cases, I find significant amounts of dog barf and wadding - lots, in other words - still in the tube despite the fact that the chute came and the rocket was safely recovered.

    The logical conclusion was that the pressure pulse from the ejection charge imparted enough velocity to the nosecone to cause it to pull the chute out.

    If the ejection blast had done the work of blowing the chute out, it would have blown all the wadding and dog barf out, too. Since that didn't happen, I concluded the nosecone did much of the work.

    Doug

    .
     
  6. May 25, 2011 #6

    scsager

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    I tend to believe the chute is "pulled out by the nose cone", or should be pulled out by the nose cone.

    The nose cone can come off and flail around in the wind without pulling the laundry out. I know this through personal experience.

    Wadding/nomex and the parachute are usually flexible enough that they may not form a tight seal depending on the airframe diameter involved.

    I think this is why the chute is usually attached to the nose cone or to the shock cord close to the nose cone... so that it is more likely to be pulled out by the nose cone.

    The video shows an example of a flailing nose cone on a 3" G powered rocket.

     
  7. May 25, 2011 #7

    georgegassaway

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    It depends on several factors.

    How TIGHT or LOOSE is the chute stored in the body?

    How much mass does the nose (or nose section) have, and how fast is the nose (section) ejected away from the main body?

    A classic Alpha type rocket, with the chute packed TIGHTLY inside the tube, when the nose ejects off the nose is so light and the chute so tight in the tube that the nose is not going to yank anything out.

    If the rocket body diameter is large enough and the packed chute packed so that it EASILY slides out of the tube, then the nose (section) has a good chance of pulling the chute out.

    And this segues over to a recovery system method that I like to use on some of my more important models, particularly scale models and larger models. I call it “semi-drogue”.

    Main chute is packed tightly enough into itself (shroud lines wrapped around to keep it from expanding inside the tube), so that it can slide very easily inside the tube.

    “Drogue” chute is a small chute attached to the nose cone, or nose section. That chute is loaded LAST, and is packed so small that if the nose comes off at all, it will definitely pull that drogue chute out. Literally the test for that is to test pack it, hold the rocket sideways, pull he nose off, let the nose cone fall down, and the nose should pull out drogue chute with it.

    And so, if for any reason the main chute does not get ejected out of the main body, the “drogue” chute will pull it out.

    This system has been 100% reliable, when the ejection got the nose off (can’t get the chute out if the nose stays on) and the ejection was not extremely late (deployment at extreme speeds either breaks the shock cord, rips the chutes, or both).

    I call it “semi-Drogue”, since true drogue chutes only pull out a chute, after which the drogue’s job is done (little or no significant pull). For this method, the “drogue” chute still helps slow the model. And most critically of all, if for any reason the main chute failed to deploy, the “drogue” chute would still help slow down the rocket somewhat. The rocket might still get damaged, but not nearly as badly as if the rocket came down with no chute at all.

    FWIW - the last photo is a nearly 4" diameter Little Joe-II model, which used a drogue on the Apollo BPC/tower nose section, and twin "Main" chutes.

    - George Gassaway

    ---Semi-Drogue-StarSpangledDBird-IMG_0473.jpg

    ---Semi-Drogue-Chute-Article.jpg
     
  8. May 25, 2011 #8

    cjl

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    George: I would call that a pilot chute, not a drogue, but it looks like a good idea. I'll have to try that sometime - a pilot chute will definitely extract the rest of the laundry if the ejection charge is unable to, so that should definitely increase the reliability of the recovery system.
     
  9. May 25, 2011 #9

    tbzep

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    George,

    Does your LJII capsule always stay pulled away like that? Seems like it might be a good way to keep the capsule from getting lost and also not get damaged banging up against the rest of the model.
     
  10. May 25, 2011 #10

    powderburner

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    I have never believed that the recovery system came out in any fashion other than forcible ejection by the charge in the motor (I'm talking Estes-style BP motors). If you have seen the vigorous levels of ejection force from the "modern" Estes motors (too much?) I don't think you could come to any other conclusion.

    (Zeus-cat: NICE work, BTW, on your "experiment" ---I never thought to actually test/demonstrate the systems like that but it sure sounds like it made the point)

    I do believe that you can have serious problems with the recovery system getting snagged on those stupid wads of rolled-up, glued-up shock cord anchors. The very last thing that I want to put inside the front end of my body tube is an obstacle like that. It's no wonder to me that some folks have a NC pop off but a chute wad stay put, my only question is why this doesn't happen more! If the anchor has to be internal then the recovery system should be tethered back at the thrust ring with a kevlar string. If you can stand the ugliness of an external anchor point, run the kevlar out the front, under the NC, and around the outside of the rocket back to an outside anchor point. Besides, with an external anchor, you can trim the rocket to hang sideways after ejection, and the fin rear tips don't get as banged up.
     
  11. May 25, 2011 #11

    MarkII

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    I'm not that concerned with which mechanism actually caused the parachute to leave the inside of the rocket, as long as it does in fact come out. The rest is kind of academic. But here's a question: would a spring-ejected nose cone pull out the laundry in the absence of any deployment charge from below? Has anyone ever tried that in a real-world test (in a typical launched hobby rocket)? Especially if the spring was just strong enough to disconnect the cone from the airframe, without propelling it outward any further? If the "dragged out by the nose cone" explanation was correct, then the parachute should come out and fully deploy in such a scenario.
     
    Last edited: May 25, 2011
  12. May 25, 2011 #12

    tbzep

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    There was a period of weak charges in certain Estes motors that often didn't have enough oomph to break the clay cap completely away from the motor. It only happened with D12's for me, but other motors may or may not have had the issue. In those cases, it always ejected the nosecone, but the chute would sometimes stop at the mouth of the BT beyond the shock cord mount. If the chute had been packed tighter, or the NC had a little more mass, it might have pulled it out.

    Of course, I've experienced the shotgun ejection you mention also...splitting body tubes down their spirals, ripping shock cords off, etc. These have almost always been with A8-3's and some 13mm motors where small body tube volume amplified the situation.
     
  13. May 25, 2011 #13

    MarkII

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    Another test of the "pulled out by nose cone" theory: suppose that the nose cone was not affixed to the top of the airframe via friction between a shoulder extending from its base and the inside wall of the airframe, as it is now, but instead, with an internal latch. If, at apogee, the latch released so that there was no longer a physical connection between the base of the nose cone and the top of the airframe, would the nose cone subsequently pull out the recovery system?

    These (this post and my previous one) constitute testable questions.
     
    Last edited: May 25, 2011
  14. May 25, 2011 #14

    cjl

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    Probably not, but that isn't really a good test. With an ejection charge, the nose is kicked off with a fairly substantial velocity, which is then able to drag out the chute (and I do think this is the case with most high power rockets, excepting those with pistons of course).
     
  15. May 25, 2011 #15

    bradycros

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    "Does the ejection charge push out the parachute, or does the nose cone pull it out?"


    Hmmm... Does the ejection charge of a cannon push out the connons contents, or does something pull out the contents?
     
  16. May 25, 2011 #16

    tbzep

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    Hmmm...Does the cannon ball have a shock cord tied to anything behind it? Bad analogy. ;)
     
  17. May 25, 2011 #17

    MrGneissGuy

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    Anecdotal at best, but all I know is that if the ejection charge is to push the parachute out, then I would expect that the parachute would need to be loosely wrapped with minimal open spaces between it and the inside wall of the body tube. It needs a decent seal, otherwise those forces follow the path of least resistance and travel through the gaps, not applying much force to the parachute. I would think, then, that a loosely wrapped parachute, which has less surface area in contact with the moving ejection gases, and has lots of gaps for the ejection gases to move around would be less likely to be pushed out.

    But, from my experience the opposite happens, every time that I've had a parachute not come out of the body tube, it was loosely packed and seemed to get stuck on the way out. While I may have had times when the parachute didn't open fully, whenever I've wrapped it tightly, leaving plenty of space between the chute and inside wall of the body tube, I've never had a problem with the parachute coming out.

    This leads me to think that the nose cone does do a fair amount of the work pulling the parachute out. But that's just based on my observations, not any scientific calculation or anything.
     
  18. May 25, 2011 #18

    bradycros

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    Hmmm... You could have a shock cord and a nose cone tied to a cannon ball. Seems pointless to do so, that cannon ball is coming out irreguardlessly. Analogy stands.
     
  19. May 25, 2011 #19

    tbzep

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    Nope. And where's shredvector when you say "irreguardlessly"? :p
    The nose cone is the part with the highest mass ejecting from the rocket. The cannon ball would be equivalent to the nose cone. The chute tied to the cannon ball could be yanked out by the inertia of the cannon ball. Of course, we'd have to severely retard the charge or the chute would be destroyed.

    I've played with this stuff in the past, testing HPR dual deployment. A large diameter rocket, sitting horizontally on saw horses during testing did not eject the drogue without the nosecone attached. The gases just went around the chute. However, once it was attached to a nose cone, a small charge kicked the the NC several yards away and pulled out the chute.

    When it comes to LPR, most of the time the chute would be ejected by the ejection charge whether the nose cone is there or not. However, once you move to larger diameters and more massive NC's, it can work the other way.
    :)
     
  20. May 25, 2011 #20

    bradycros

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    Plan A is to use the Ejection charge to push everything out.

    Backup to Plan A is Plan B, having the nose cone pull everything out.
     
  21. May 25, 2011 #21

    tbzep

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    Sounds good to me...for most LPR. :cheers:
     
  22. May 25, 2011 #22

    GregGleason

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    This is why I use pistons, even in most of my LPR. The piston will move it. However, like everything else in rocketry, how you implement a piston (as in short side of the CG toward expanding-gas end, diameter not too big or too small, etc.) is part of the process.

    Greg
     
  23. May 25, 2011 #23

    troj

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    Especially as parachutes get larger, trying to use a charge to push the parachute out of the tube is asking for problems. A small pilot chute, like George suggested, makes a lot more sense and is a lot more reliable.

    On our big projects, we put the ejection charges on top of the parachute. The charge does nothing more than separate the sections and allow an appropriate sized pilot chute to quite literally fall out of the airframe. The pilot chute then extracts the main.

    When I say "fall out," I mean that, quite literally. If necessary, we put a spacer in the bottom of the parachute tube, so that the top of the main is right at the top of the airframe. That way, the pilot chute and main have to be compressed a little bit to get things to fit together. Once the sections come apart, the pilot chute just falls out, and does its magic.

    -Kevin
     
  24. May 25, 2011 #24

    hardinlw

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    I first had to confront this question last year when the TARC team I was mentoring was planning to use the required streamer to recover the egg and a separate parachute to recover the booster. My initial concern was that the two recovery devices would tangle, so I recommended that they run ground tests to determine how best to pack the chute and streamer. I made a dummy engine and loaded it with BP for each test so they were not actually handling loose BP. The test article was an airframe tube with motor mount and launch lug but no fins mounted onto the launch rail and taped in place. Repeated tests failed to eject the parachute. Without fail, the payload section accompanied by the streamer would pop 10 feet into the air, but the parachute and nomex blanket stayed in the bottom of the airframe. The ejection gasses simply went around it. My conclusion was that this would be the case any time the parachute and wadding were loose. The team tried putting the chute on top of the streamer, but that did tangle everything up. Ultimately, they added a piston (I fly a lot of PML kits, so they'd seen pistons in use) with the bulkhead near the bottom. The streamer and parachute went into this side-by-side and were cupped by the piston body which kept them from unrolling until clear of the airframe. That worked like a charm.

    At this year's TARC finals, one of the teams I was helping as pad manager commented that they had loaded the booster chute on top of the payload chute because that was the only way they could get both to come out. They had considered a piston, but rejected it as being prone to jamming.

    Based on those observations, I'd say that when the parachute and wadding (nomex blanket) are loose in the airframe, the nose cone has to pull them out. If they fit tight, but completely fill the airframe, they act like a piston and are pushed out. If they are packed so as to be tight, but do not fill the airframe, AND you have a light nose cone, you may not get the laundry out.
     
  25. May 26, 2011 #25

    Zeus-cat

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    I did the experiment again with two smaller Nomex sheets; 9” X 9” and 6” X 6”. I built up six more charges using Pyrodex. I made the charges somewhat weak as I assumed energetic charges would eject everything with ease.

    I started with the 9” x 9” Nomex sheet. I loosely installed it, then a tightly wrapped parachute resting on top of the Nomex, and finally the nose cone. I used four charges with the 9x9 sheet. One charge fizzled and did not eject the nose cone. Two more charges ejected the nose, but failed to eject the parachute. The other charge was the most energetic and ejected the nose, chute and Nomex.

    I switched to the 6X6 for the final two charges. A 6X6 sheet should fit if carefully installed, but I installed the Nomex so that there was a gap on one side of the body tube. The first charge fizzled and ejected nothing. The last charge ejected the nose, but the chute stayed in the tube.

    Summarizing:

    9X9
    One bad charge
    Two charges ejected only the nose cone
    One charge ejected the nose, chute and Nomex

    6X6
    One bad charge
    One charge ejected only the nose cone

    In the cases where only the nose was ejected, the chute would have easily been pulled out of the body tube by the nose cone.

    It appears to me that the optimal method for packing a chute or streamer is to first form a reasonably good seal with your wadding or Nomex. Then add the chute or streamer, trying to keep it from being too tightly packed inside the body tube if possible. Finally, install the nose cone. When the ejection charge goes off you would like the nose, chute and wadding to all be ejected. If the charge fails to eject the chute then the momentum of the nose should pull it free if it is not too tightly packed in the tube.
     
  26. May 26, 2011 #26

    John Beans

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  27. May 26, 2011 #27

    MarkII

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    Right, but we're trying to control for that because it could confound the results. We want to take the whole pressurization part out of the scenario for the purpose of testing the hypothesis. Otherwise if we allow the nose cone to be shot off by a sudden increase in gas pressure in the tube behind it, then anything that was interposed between it and the source of the rapidly expanding gas would be shot off as well. And then we would be right back where we started. We want to test the hypothesis that the freed nose cone extracts the recovery system by making one or more flights during which the cone is simply disconnected from the airframe, and then seeing if it pulls out the parachute on its own without any push or propulsion from anything behind the parachute. We want to see if the hypothesis is correct that the nose cone is the sole agent that causes the extraction of the parachute from the rocket and that all the ejection charge does is break the connection between the nose cone and the rocket.
     
    Last edited: May 26, 2011
  28. May 26, 2011 #28

    MarkII

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    Data:
    Conclusion:
    I'm sorry, but your conclusion does not follow from your data. You had three trials in which the ejection charge popped off the nose cone, and yet the nose cone did not subsequently extract the parachute. In the only other trial in which the ejection charge successfully fired, you state that the force of the charge blew everything out. Your own data indicates that once it is knocked off the rocket, the nose cone by itself does not extract the parachute. The only time that the parachute came out was when the ejection charge blew it out.

    Conclusion based on your trials: disconnecting the nose cone does not result in the parachute being extracted from the rocket. The parachute is expelled by the force of the ejection charge. Other data from other experiments could lead to a different conclusion, but this is what your data shows.
     
    Last edited: May 26, 2011
  29. May 26, 2011 #29

    MarkII

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    Ground tests like those that have been reported in this thread are not completely accurate simulations of what occurs during recovery deployment when the rocket is in flight. During ground tests, the rocket is fixed and stationary, and only the nose cone is free to move and to respond to gravity. During actual flights, everything including the rocket and the disconnected nose cone are all in free fall, and are therefore nearly weightless in relationship to each other. This difference in conditions introduces confounding factors that could potentially compromise the validity of the ground tests.
     
    Last edited: May 26, 2011
  30. May 26, 2011 #30

    MarkII

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    I am also not so sure anymore that the question that is the subject of this thread is even valid. I am beginning to doubt that the deployment process can be reduced to an "either/or" question, because I think that the two factors that we are discussing act in concert simultaneously or nearly so. Therefore the question of which single action "causes" the recovery system to come out of the rocket may indeed be a false one, because in actual in-flight deployments, neither one of these two actions occurs alone and in isolation; the other action is also always present. In examples in which the parachute fails to deploy out of the rocket, it isn't due to the inadequacy of one action or the other, but rather due to the inadequacy of both actions together.

    I think that Zeus-Cat's data demonstrates that it requires the combined effect of of several actions under specific conditions to produce a successful deployment. No single action can produce the result alone.
     
    Last edited: May 26, 2011

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