Model Rocket Stabilisation Beginner Questions

Discussion in 'Beginners & Educational Programs' started by HStuart18, Apr 9, 2019.

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  1. Apr 9, 2019 #1

    HStuart18

    HStuart18

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    I am designing a model rocket and am using 3 B6-4 Engines. Inevitably there will be perturbation of its trajectory by wind and engine/thrust inconsistencies. The way that I currently understand the Centre of mass (CM) and Centre of pressure (CP) in terms of correcting an altered trajectory, is that the lift acts through the CP to create a torque about the CM in the opposite direction to the torque that initially caused the rotation of the rocket.

    So, say there is a constant clockwise torque of x Nm, I need to create a counterclockwise torque of (x + a) Nm. What should the value of a be? Is it a proportion of x? The value of a needs to be great enough that it sufficiently causes the rocket to right itself and overcome equilibrium, but it can't be large enough that it causes significant overcompensation (causing the rocket to pass vertical by its momentum).

    Also, what sort of torque values are likely to be induced to the rocket from wind and engine/thrust inconsistencies. Are the fins or other mechanical attributes of a rockeet realistically sufficient to create enough "counter-torque"? What about adjustable fins?

    Cheers, Harry
     
  2. Apr 9, 2019 #2
  3. Apr 9, 2019 #3

    neil_w

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    First of all: welcome to the forum!

    I'm not inclined to go into the weeds with the math questions you're asking here, so I'll just present some summary information for someone who is new to this.

    1) Your general statements about Center of Pressure (CP) and Center of Mass (CM) are correct. In rocketry we typically refer to the latter as Center of Gravity (CG), or if you prefer, "Centre of Gravity". :)

    2) To determine stability of a rocket, most around here use the rule of thumb that the CP should be at least 1-2 calibers behind the CG (where 1 caliber = max diameter of the rocket airframe). There are variations to this rule, and it's not always that simple, but remarkably often in it is. No one, to my knowledge, calculates torque values, at least not for models. A simulation program like OpenRocket (free, go grab it and start playing) can calculate the CG, CP, and hence the stability.

    3) Your concern about engine/thrust inconsistencies is misplaced, at least with regard to stability and model rockets. This is not generally an area we need to be worried about.

    4) Regarding this question: "Are the fins or other mechanical attributes of a rockeet realistically sufficient to create enough "counter-torque"? If not, then no one's rockets would fly, and I assure you they do. :)

    5) Do you have any background with rockets? If not, a 3-motor cluster is typically not the best way to start. Better to start with standard single-motor rockets before moving on to clusters.

    My suggestion: download OpenRocket (may I suggest one of the installers here: https://www.rocketryforum.com/threa...d-mac-to-solve-all-your-java-problems.143540/) and try playing around with some designs to get a feel for things. You can simulate the full 3-motor cluster. Post your files here for comment.

    Finally: I am not discouraging you from learning all the physics involved in keeping rockets stable, just suggesting that if you're goal is to build and fly a stable rocket, you don't generally need to go to this level of detail.

    That link is broken.
     
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  4. Apr 9, 2019 #4

    neil_w

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  5. Apr 9, 2019 #5

    BDB

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    Ditto to everything Neil said. Especially the use of OpenRocket.

    Rockets often spin on the way up. It actually stabiles the flight, so I wouldn't worry to much about it. Occasionally fins are even canted to induce a spin.

    Welcome to rocketry! Just start building and having fun.
     
  6. Apr 9, 2019 #6

    Bat-mite

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    Good video.
     
  7. Apr 10, 2019 #7

    DaveW6DPS

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    This may be available from your local public library. My county library has the 6th edition, but close enough.

    You can also find this book on Amazon for about $14 US.

    It has a lot of information and is a great place to start if you are interested in designing your own model rockets.

    I also suggest looking for a local rocketry club, where you can find folks with a variety of rocket related interests and we all like to show off our toys!
     
  8. Apr 10, 2019 #8

    HStuart18

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    Thanks for all the helpful responses!!! It seems like my best bet is to start familiarising myself with Open Rocket. I have attached a design that I spent today making. Disappointingly, it's only predicted to reach an altitude of <70m even with 4 B6-4 engines. With 3, it's significantly less. I find this surprising because this link shows a rocket using only 3 B6-4 engines yet it clearly exceeds well beyond 70m. Is there a blatant issue in my design? I tried to reach a stability level of 1 and have the parachute deploy closeish to the apogee.

    Cheers
     
  9. Apr 10, 2019 #9

    HStuart18

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  10. Apr 10, 2019 #10

    Rex R

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    first glance at your design, thoughts; it is heavy, roughly twice as heavy as it could be. if it were mine I would ditch the all plastic (lexan) design and use standard paper tubes with a balsa nose(btw a shoulder on the nose makes it a lot easier to hold the nose in place :)), with those simple mods I was able to bring the mass down to a bit over 200g (with engines) a an altitude of roughly 200m. and I hadn't begun to really work on it :). overall not a bad design for an early effort.
    Rex
     
  11. Apr 10, 2019 #11

    HStuart18

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    Hmmm, I see, so weight is the primary issue. Since I am doing this through a university club, I am actually required to 3D print the rocket with PLA (~1.25g/cm^3) and a minimum width of 2mm. Does this mean that I am doomed to exceeding 100m?
     
  12. Apr 10, 2019 #12

    OverTheTop

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    Lots of people are printing PLA rockets nowadays. If you have to stay under 100m and are exceeding that with the simulate rocket, you can print yourself a larger rocket. A larger rocket with the same mass as a lesser diameter rocket will always not go as high, due to the additional drag of the larger airframe.

    Have fun :).
     
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  13. Apr 10, 2019 #13

    HStuart18

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    Sorry I didn’t explain that well, I meant “am I doomed to staying under 100m” as in I’d like the rocket to exceed 100m. I am trying to figure out if there is any rocket design that would allow that with 3 or 4 B6-4 engines and PBA structure.
     
  14. Apr 10, 2019 #14

    Rex R

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    hmm, can you move the payload into the forward section? then extend that size diameter tube about 5" further aft, that might help. one problem you're facing (aside from weight) is that you're punching a fairly large hole in the air, so by reducing the 'wetted' surface area you'll reduce drag (and mass), and gain some altitude.
    Rex
     
  15. Apr 10, 2019 #15

    neil_w

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    Weight and drag are the two forces pushing your rocket down. Weight is going to be the one you need to fight most on this design. 2mm PLA is going to make for a heavy rocket, and 3xB6-4 is not a lot of impulse to try to push it upwards. This rocket isn't likely to go very high no matter what you do.

    Look for ways to reduce weight. I'd start with the motor mount area. Right now you have full tubes surrounding each motor. Perhaps they could be held in place by an outer shell that holds the cluster in place but doesn't surround each motor. Also, there's not need for a body tube surrounding the cluster. That's just more weight.

    After you minimize the weight of the motor mount, you can ripple through the rest of the design to minimize total weight while keeping stability.

    Seems like a good challenge. Good luck!
     
  16. Apr 10, 2019 #16

    HStuart18

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    Thanks for the help guys, I think I'm set on my way now! I've uploaded my revised model which has been improved significantly. Due to the fact that I am going to be modelling the rocket in a CAD system, I won't get too caught up in much more detail since I have no idea what design changes I'll have to make when it comes to unforseen challenges in 3D printing. As for now, I think it's a pretty decent guideline as to the general shape of the rocket. Thanks again!
     

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  17. Apr 10, 2019 #17

    neil_w

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    Good luck, and please keep us updated with your progress!
     
  18. Apr 11, 2019 #18

    BABAR

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    Welcome to forum!
    Is there some reason it “must” be a cluster and it “must” use B6-4 engines?
     
  19. Apr 23, 2019 #19

    The New Gagarine

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    How can I deploy a parachute on the top of my rocket ?
     
  20. Apr 23, 2019 #20

    Bat-mite

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    Buy a cheap rocket kit and all will be obvious.
     
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  21. Apr 24, 2019 #21

    The New Gagarine

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    Ok Thanks I will search a cheap rocket kit on the internet...
     
  22. Apr 24, 2019 #22

    The New Gagarine

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    Can I use a piece of jacket and some sewing thread instead of the rocket kit's parachute ?
     
  23. Apr 24, 2019 #23

    Steve Shannon

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    Not really, but all kits come with an appropriate parachute.
     
  24. Apr 24, 2019 #24

    neil_w

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    Why?

    Technically you can make one out of just about anything, but it'll probably be heavier, bulkier, and not perform as well as the one in the kit. It is of course possible to make good-quality chutes for yourself if you choose the correct materials, but I am sincerely interested in your motivation here.
     
  25. Apr 24, 2019 #25

    Nytrunner

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    My hunch is that New Gagarin isn't located in a region with a plentiful access to rocketry supplies. (if that is not true, I stand corrected)

    We are quite spoiled here in the states when it comes to availability (no matter how hard folks grumble about hardware supplies :rolleyes:)
     
  26. Apr 24, 2019 #26

    neil_w

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    I got that, but it sounded like the question was to replace a parachute in a kit with a home-made one. That's what I didn't get. If I misunderstand that then my apologies.
     

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