Bench Testing a Rocket....

Discussion in 'Low Power Rocketry (LPR)' started by lakeroadster, Jun 29, 2018.

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  1. Jun 29, 2018 #1

    lakeroadster

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    Anybody here bench test their LPR rockets?

    Seems like restraining the rocket on something like a "Workmate" and firing the engine would be a good test of the engine mount restraining rings / tape and the recovery system deployment.

    Would make for any interesting video too.

    Curious also what would be the best way to restrain the rocket?
     
  2. Jun 29, 2018 #2

    Flyfalcons

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    Nope.
     
  3. Jun 29, 2018 #3

    rharshberger

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    Nope, waste of motors.
     
  4. Jun 29, 2018 #4

    kuririn

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    I can see the benefit of that in testing your laundry deployment. My concern would be the force that an engine thrusting on the centering rings of a restrained rocket might be more than that of an unrestrained rocket in normal flight. Plus you would have to waste an engine to get the results. My bench tests are my launches. Also there is the aspect of a possible NAR rules violation. I seem to remember a rule about rockets being unrestrained when under power.
     
  5. Jun 30, 2018 #5

    lakeroadster

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    it seems prudent to "Waste" a motor that is less than $10 rather than a scratch built rocket..

    Thoughts...

    For a rocket with the engine sticking out of the engine mount it would be simple to use a hose clamp around the engine for restraint.Thus no thrust would be applied at all to the thrusting ring.
     
  6. Jun 30, 2018 #6

    Brainlord Mesomorph

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    I'm trying a new experimental rear-blow system in my new rocket, so yes I have an excuse for, I mean need to do static tests.

    because its rear blow (and a paint job I've been working on for weeks!), I'm going to fold foam rubber over the front of the rocket, fold cardboard over that, and use clamps to fasten that to a folding workbench..
     
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  7. Jun 30, 2018 #7

    lakeroadster

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    Cool! be sure to video that.

    Keep in mind that will try to buckle the air frame. Is the design of the body tube pretty substantial?

    Have you considered using foam with a hole in it such that the thrust is applied to the fins?
     
  8. Jun 30, 2018 #8

    kuririn

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    Hey guys,
    If all you're wanting to do is test your recovery deployment with an ejection charge, why not make an ejection charge canister with some cut off glove tips and 4F black powder? Video is here:
    ttps://www.youtube.com/watch?v=FgxdL8VBucM
    This is used for dual deployment for mid and high powered rockets. You would have to size the charge to match that for low power engines, like Estes. So you would have to do a little bit of research. And you need a long igniter, like an old Q2G2 or a Firstfire. Place this ahead of a spent engine casing in your rocket, feed the igniter leads through the nozzle, and light her up. Cheap, and you save a motor. Good stuff to learn for moving up to high power.
     
    Last edited: Jun 30, 2018
  9. Jun 30, 2018 #9

    blackjack2564

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    Why wouldn't you build a cheap unpainted test mule first, to try this new design. Instead of going all out with a full blown finished/painted rocket? Seems like a lot to risk for an untested idea.
    Or did you?




    Bad idea all the way around.

    What if the motor Cato's?
     
  10. Jun 30, 2018 #10

    Flyfalcons

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    What are you trying to test? That the nose will pop off? Given your very small parachute compartment volume, what reason do you have to believe it won't work?
     
  11. Jun 30, 2018 #11

    Brainlord Mesomorph

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    The rocket was an experiment in construction techniques. Half way through, I realized the (simple) rear blow system I had imagined wouldn't work. so I had to invent something. (details in my build thread)

    I'm pretty sure it'll work, but I'd like to see it work before I try it at 500 feet
     
    Last edited: Jun 30, 2018
  12. Jun 30, 2018 #12

    Steve Shannon

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    Your centering rings must be able to withstand the full thrust of the motor, but you don’t have to static fire a motor to test that. Just put a spent motor into the motor mount and then apply a weight to the aft end of the motor equal to the thrust (ideally add extra to identify a safety margin).
    If you do a static test of a design you have reason to doubt and the design fails, you now have part of a failed design flying (possibly unstable) and you’re responsible because you chose to take the risk. That’s okay if you come up with a way to mitigate the risk.
     
  13. Jun 30, 2018 #13

    Steve Shannon

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    In that case there’s really nothing learned from the test either. The motor mount would not be tested.
     
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  14. Jun 30, 2018 #14

    Brainlord Mesomorph

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    Good point, I should do a dead-weight pull test first.
    Edit: anyone know the "weight" of a typical Estes ejection charge?
    Edit: and of course I've been yanking this thing out with my fingers since I built it. It comes out.

    but that wouldn't simulate the pressure, speed and forces in an actual ejection.

    And by "static test" I mean not flying. (isn't that the appropriate NASA term?)

    I imagine "failure" would mean partial ejection of the rear blow section, burning the shock cord, breaking something.

    Nothing flies.
     
    Last edited: Jun 30, 2018
  15. Jun 30, 2018 #15

    Brainlord Mesomorph

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    Its sort of an oddrock. it has "wings" (big fins) and yes holding by those and the tube.

    and yes video (how else can we review our results? - science!)
     
  16. Jul 1, 2018 #16

    lakeroadster

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    The same thing that would happen if it were at the launch field. This is a LPR, utilizing C and D size motors. Statistically a CATO with one of those is unlikely.

    It'll work.. I just thought a bench test would be cool.
     
  17. Jul 1, 2018 #17

    lakeroadster

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    The static test, in the manner you are stating, isn't a true simulation of the actual forces. There is a shock / impact load to the engine mount when the motor fires, when the rocket is not in motion. Loading goes from no load, to full load, nearly instantaneously.

    This is especially true if the engine is restrained via a clip that allows it to slide upward when the engine fires.

    A bench test would be a max loading worse case scenario of the rockets motor mount components ability to restrain the motor.

    If it fails, you redesign and retest.... flying a failed design is a fools errand.
     
  18. Jul 1, 2018 #18

    Steve Shannon

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    I agree, onset of thrust more closely resembles a shock, but as long as you have strength tested your motor mount there’s little reason to believe that will cause failure.
    I wasn’t talking about flying a failed design in a later flight; I meant that if your experimental design fails and a portion of it flies off powered by the motor. I’ve seen a few motors fly through rocket bodies and then do some skywriting. I’ve also seen some motors fly in a stable manner, which always surprises me.
    Of course all of us flyers are ultimately responsible for whatever happens during our flights.
    You certainly are free to fly or static test your rocket. That’s your choice. I’ve never seen anyone intentionally static test a model rocket, but if there’s something you think you can learn from it, please post a video.
    Happy 4th!
     
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  19. Jul 1, 2018 #19

    Brainlord Mesomorph

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    Separate from my experiment, I think every rocketeer should have the opportunity to see an entire engine firing up close.

    Just in terms of being better able to visualize what's happening up there.

    I'm thinking about aiming fans at the rocket during the test, to simulate moving through air. (also to get the smoke out of the way so we can see what's happening)
     
  20. Jul 1, 2018 #20

    Flyfalcons

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    Do you feel your centering rings and motor retention are inadequate to handle the two pounds of thrust from a C motor? o_O
     
  21. Jul 1, 2018 #21

    lakeroadster

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    With the D12 it will have 6.67 lbs of thrust :D

    Trust Buy Verify... having spent the last 30 years as a Quality Assurance Manager I tend to like to verify things.
     
  22. Jul 1, 2018 #22

    Zeus-cat

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    If you do a bench test and the rocket fails can you modify the design? If not, why not just fly it?
     
  23. Jul 1, 2018 #23

    Flyfalcons

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    You didn't answer my question in earnest though. Do you have some reason to believe that the methods you have used to construct your rocket, which have been proven to be effective during millions of other LPR flights, will be inadequate and potentially fail?

    We've already agreed that your deployment system will work. And if you use a hose clamp to restrain the motor, then you will not be applying pressure to your centering rings. So what useful purpose is there to bench firing your rocket?
     
    Last edited: Jul 1, 2018
  24. Jul 1, 2018 #24

    lakeroadster

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    No reason at all.

    Useful purpose... Opinions vary. I'd like to try it simply because I never have bench tested a rocket. I personally think it will be interesting.

    Many people would say Model Rocketry has no useful purpose.. again, opinions vary.

    Do you understand the concept of "Trust but Verify"?

    The rocket is made from various components that are glued together. Any of the components, or the adhesive materials used to hold the components together, could fail.

    Lets assume the rocket is restrained by placing a compressible foam surface in front of the fins. Thus the fins would be subject to the forces resulting from the thrust of the engine.

    And also assume no hose clamp is used to restrain the engine, thus the rockets engine block would also be tested.

    Some people may find those tests useful.

    _____________________________________________________

    I messed around with some FEA today, looking at stresses on the rocket if it is constrained via the nose cone.. or restrained via the front of the X-wings.
     

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    Last edited: Jul 1, 2018
  25. Jul 6, 2018 #25

    milehigh

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    I agree with an earlier post about building a 'boilerplate' test model for such activities; one that just has the basic airframe and no time or money spent in applying a paint/detailing job. If the model survives the static test, it can then be flown (still nekid) in actual configuration. If it still survives, then it's maybe time for a good paint job, after which a second model should be built with all the full detailing. This is standard (or should be) procedure for building a scale model that may have marginal stability concerns, or some type of staging or cluster system that needs to be ironed out before any full-blown detailing is done.
     
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