Nose Cone Weight Why and When

Discussion in 'Beginners & Educational Programs' started by Chunker78, Feb 10, 2019 at 4:23 PM.

Help Support The Rocketry Forum by donating:

  1. Feb 10, 2019 at 4:23 PM #1

    Chunker78

    Chunker78

    Chunker78

    Active Member

    Joined:
    Dec 21, 2014
    Messages:
    39
    Likes Received:
    0
    Why do you add nose cone weight? How do you know that you need to add nose cone weight? Do you fly your rocket first and then determine it needs weight? Or will a simulation program give you a direct order to place weight? I hate to mess up a nice rocket not doing the right thing. Thanks
     
  2. Feb 10, 2019 at 4:30 PM #2

    Steve Shannon

    Steve Shannon

    Steve Shannon

    Well-Known Member

    Joined:
    Jul 23, 2011
    Messages:
    4,043
    Likes Received:
    661
    A simulation program (RockSim or OpenRocket) can estimate where your center of pressure and center of mass will be. Then, still within the simulator you can simulate adding a mass object, such as nose cone weight, to see how that effects the center of mass and your stability margin.

    But it doesn’t have to be that fancy. There’s also the cardboard cutout method to estimate center of pressure (Cp). With enough experience you’ll have a good feel for where Cp will end up.
     
  3. Feb 10, 2019 at 4:34 PM #3

    timbucktoo

    timbucktoo

    timbucktoo

    Well-Known Member Staff Member Global Mod

    Joined:
    Jun 13, 2014
    Messages:
    5,011
    Likes Received:
    276
    Gender:
    Male
    Location:
    Cocoa Beach
    Nose cone weight is used to increase the stability of rocket. By adding the weight to NC, it moves the CG (center of gravity) forward. You want a stability margin of at least 1.0 for most rockets. Many people prefer stability of 2.0 but there are exceptions to the rule. Also depends on rocket design.
    Some folks don’t use sims and just go by trial and error. Most use Openrocket or Rock sim. They won’t tell you how much to add, you have to play with it and see how it effects the CG.
     
    Jmhepworth and Steve Shannon like this.
  4. Feb 10, 2019 at 6:07 PM #4

    les

    les

    les

    Well-Known Member

    Joined:
    Jan 20, 2009
    Messages:
    2,380
    Likes Received:
    40
    Based on the OPS questions, I'm not sure they will understand this answer...

    For a rocket to be stable and fly correctly, the CG needs to be higher (closer to the nose cone) than the Center of Pressure (CP).
    An unstable rocket will "sky write", flying erratically and possibly flying back into the people or crashing to the ground.

    The CG is the balance point holding the rocket on its side fully prepped for flight, so with parachute/streamer, motor, etc.
    The CP is the point where the aerodynamic forces from the side balance. Think of a weather vane and moving the pivot point until when the wind blows it doesn't move.

    The CP can be moved down by making larger fins or having the fins swept back.
    The CG can be moved forward by adding nose weight.

    In terms of the stability margin mentioned above, the number refers to the rocket body diameter. A stability margin of 1 is the CG is forward of the CP by 1 body diameter. A margin of 2 would have the CG be 2 body diameters higher than the CP
     
  5. Feb 10, 2019 at 11:11 PM #5

    BABAR

    BABAR

    BABAR

    Builds Rockets for NASA TRF Supporter TRF Lifetime Supporter

    Joined:
    Aug 27, 2011
    Messages:
    3,300
    Likes Received:
    143
    Not sure of your (original poster) experience level. Sounds like you are concerned about crashing a rocket. Generally people just starting in Rocketry will have a kit for their first rocket. If it is a level 1 standard kit (3 or 4 fins and a nose cone) and you build it according to instructions and use recommended engines, it will be stable.
     
  6. Feb 10, 2019 at 11:29 PM #6

    GlenP

    GlenP

    GlenP

    Well-Known Member

    Joined:
    Oct 4, 2014
    Messages:
    1,252
    Likes Received:
    62
    Only on the end of a string. Load the rocket up with wadding, chute, and engine, as if ready to fly. Balance the rocket, and tie a string around the balance point. You may need to tape it in place so it does not shift. Make sure you can swing it around without hitting the ground or anything. Start slow, and if the rocket is stable it should point in the direction of travel. You can eventually go over your head like a cowboy as fast as you can. If it does not point in the direction of travel, then you can try adding a little weight until it does. This is called the "swing test."

    This works well for simple rocket designs with a long thin body. Oddball shaped rockets, or short fat rockets might not pass a swing test, but could still be stable in actual rocket flight.
     
  7. Feb 11, 2019 at 2:59 PM #7

    Bat-mite

    Bat-mite

    Bat-mite

    Rocketeer in MD

    Joined:
    Dec 5, 2013
    Messages:
    8,672
    Likes Received:
    385
    Gender:
    Male
    Location:
    Maryland
    Lots of good info here. Let me add a little depth.

    The center of pressure (CP) is the point where the rocket would spin in the wind. Picture a weather vane.

    The center of gravity (CG) is the point where the rocket would spin based on the force of gravity.

    Picture the CG a full caliber (i.e., the diameter of the body tube) forward (toward the nosecone) of the CP. You can see how to two forces will sort of cancel each other out.

    Now imagine the CP and CG sliding together until they are right on top of each other. Now you have a single point of rotation, and the rocket will spin around. This is bad.

    The CP is determined by the shape of the rocket, since it is the shape that catches the wind. Large fins move the CP back toward the nozzle because they catch more wind. The smaller the fins get in relationship to the body tube, the less they catch the wind, and the more the CP moves forward.

    The CG is simply the balance point of the loaded, ready to fly rocket. Load it up and balance it on a ruler or your finger or something.

    At this point you might think, well, I'll just add nose weight to be sure my rocket is stable. The problem with that is that the more the rocket weighs, the more thrust you need in order to safely launch the rocket. So a rocket that could launch safely on an A8-3 might need to move up to a B6-3 if you add more weight. Evenn a stable rocket will fly crazy if it doesn't have enough thrust.

    I hope this helps.
     
    Luzwingnut likes this.
  8. Feb 12, 2019 at 7:46 PM #8

    BABAR

    BABAR

    BABAR

    Builds Rockets for NASA TRF Supporter TRF Lifetime Supporter

    Joined:
    Aug 27, 2011
    Messages:
    3,300
    Likes Received:
    143
    A really great book (written well, entertaining as well as informative, answers this and a Lot of other questions) is Stine’s Handbook of model rocketry

    Can get a used copy at Amazon for under $8.00.

    https://www.amazon.com/gp/product/0471472425/?tag=skimlinks_replacement-20

    It will more than pay for itself in time and money saved for even the newest rocket builder.

    It will answer a lot of questions you don’t even have yet but will.
     
    Luzwingnut likes this.
  9. Feb 12, 2019 at 10:12 PM #9

    Jonathan 7

    Jonathan 7

    Jonathan 7

    Well-Known Member

    Joined:
    Jan 3, 2019
    Messages:
    56
    Likes Received:
    6
    Gender:
    Male
    I'm a believer!
     

Share This Page