Baffling question

Discussion in 'The Watering Hole' started by sooner.boomer, Nov 6, 2018.

Help Support The Rocketry Forum by donating:

  1. Nov 6, 2018 #1

    sooner.boomer

    sooner.boomer

    sooner.boomer

    Well-Known Member

    Joined:
    Mar 21, 2011
    Messages:
    2,272
    Likes Received:
    34
    I guess I'm getting old and lazy. I'd rather be flying rockets than preping them. It just takes a bit of extra time and materials to build in or add on an ejection baffle. I mostly use the 3-D (or 4-D) style; a series of (just over) half-round bulkheads aligned so that the successive baffle is rotated 180 degrees from the previous.

    So the question of the day is: "How far from the end of the motor should the first baffle be, and how far should the successive baffles be spaced?". I guess the best way would be to answer in body tube diameters (or motor mount diameters if you're building a rocket with an extended motor mount/stuffer tube).
     
  2. Nov 6, 2018 #2

    rharshberger

    rharshberger

    rharshberger

    Well-Known Member

    Joined:
    Oct 13, 2014
    Messages:
    7,082
    Likes Received:
    222
    Gender:
    Male
    Location:
    Pasco, WA
    The real question is how long to limit the motor by baffle placement. To protect my baffles I ditch the red cap (AT), and instead use masking tape to seal the ejection charge well, some of my baffles are less than 2 diameters off the top of the longest motor for that rocket and they seem to survive reasonably well (bottoms coated with thin layer of JB Weld). The other advantage to placing them forward of the motor as far as possible is to allow repairs to the fincan area in the event of damage, I can cut the area between the fincan and baffle out and use a coupler to splice back together if necessary.
     
  3. Nov 6, 2018 #3

    sooner.boomer

    sooner.boomer

    sooner.boomer

    Well-Known Member

    Joined:
    Mar 21, 2011
    Messages:
    2,272
    Likes Received:
    34
    I coat the bottom baffle, with yellow glue, epoxy, or JB Weld - whatever I have on hand. One thing I'm trying out is the addition of small triangular gussets on the side of the baffle away from the engine. I've never had one blow out, but this is cheap insurance.
     
  4. Nov 6, 2018 #4

    BABAR

    BABAR

    BABAR

    Builds Rockets for NASA TRF Supporter TRF Lifetime Supporter

    Joined:
    Aug 27, 2011
    Messages:
    3,077
    Likes Received:
    58
    As a non structural part, would a piece cut out of an aluminum can work?
     
  5. Nov 6, 2018 #5

    sooner.boomer

    sooner.boomer

    sooner.boomer

    Well-Known Member

    Joined:
    Mar 21, 2011
    Messages:
    2,272
    Likes Received:
    34
    In theory it would, because it would be (fairly) heat resistant, but I would be reluctant to use it. Paper and wood are porous, and a very strong bond can be made with yellow glue. You'd need (plain) epoxy or JB Weld to glue aluminum. Don't know if it would be worth the extra cost and weight (I'm thinking low/mid power rockets).
     
  6. Nov 6, 2018 #6

    solid_fuel

    solid_fuel

    solid_fuel

    Lifetime Supporter TRF Lifetime Supporter

    Joined:
    Aug 13, 2017
    Messages:
    1,173
    Likes Received:
    42
    Location:
    SEPA
    I’ve seen or heard of foil lined motor mounts so I would assume that a piece of aluminum foil glued to to bottom of the baffle might also give some protection against heat and small burning particulates. And be easier to work with than a piece from a soda can. Albeit the piece from the can would offer much better protection to the wood underneath.
     
  7. Nov 6, 2018 #7

    K'Tesh

    K'Tesh

    K'Tesh

    OpenRocket Chuck Norris

    Joined:
    Mar 27, 2013
    Messages:
    10,944
    Likes Received:
    72
    Generally speaking, I try to put my baffles as far forward as I can and still have enough space for the parachute and related hardware.
     
  8. Nov 6, 2018 #8

    sooner.boomer

    sooner.boomer

    sooner.boomer

    Well-Known Member

    Joined:
    Mar 21, 2011
    Messages:
    2,272
    Likes Received:
    34

    How would you do it on an 18" body tube? I'm thinking of making a motor mount and baffle as one unit (two pieces glued together) and slid in from the aft end. Have to use epoxy or other glue that would not grab as it was slid in.
     
  9. Nov 7, 2018 #9

    jqavins

    jqavins

    jqavins

    A True Homebert

    Joined:
    Sep 29, 2011
    Messages:
    877
    Likes Received:
    46
    Location:
    Hornell, NY
    Reading this has got me thinking, and it occurs to me that it might be a good idea to build the baffle into a coupler and not glue it, just friction fit or rivet, like a tiny ebay. The baffle, especially the first and second plates, will be taking a beating, so why not make it serviceable?
    I have a hunch that a piece of soda can might melt or burn through more easily than a piece of 1/8" inch balsa; that stuff is crazy thin. I'd stick with treating the wood. (I wonder about treating it with waterglass.) Unless...

    A piece of gorilla glass would do a great job, if you can get some and cut it (or get it cut) to shape. Any glass will take the heat easily, and gorilla glass is plenty strong.

    Well, or fiberglass.
     
  10. Nov 8, 2018 at 5:55 AM #10

    sooner.boomer

    sooner.boomer

    sooner.boomer

    Well-Known Member

    Joined:
    Mar 21, 2011
    Messages:
    2,272
    Likes Received:
    34
    As I mentioned, I've got a design with an 18" body tube. I think I'm making things harder than they need to be. I think I'll just order some coupler material from BMS, chop the body tube, and add the coupler with baffle disks to join the tube back together. I intend to offer this as a kit to the young son of a friend. I'll post plans here, too.
     
  11. Nov 8, 2018 at 8:33 PM #11

    gldknght

    gldknght

    gldknght

    Well-Known Member

    Joined:
    Jan 26, 2011
    Messages:
    558
    Likes Received:
    13
    Throw an aluminum soda can into a camp fire some time and watch what happens. Just make certain you don't breath the resulting smoke, it's toxic.

    Aluminum will burn, and doesn't need to get that hot to do so.
     
    Last edited: Nov 8, 2018 at 8:58 PM
  12. Nov 8, 2018 at 8:39 PM #12

    rharshberger

    rharshberger

    rharshberger

    Well-Known Member

    Joined:
    Oct 13, 2014
    Messages:
    7,082
    Likes Received:
    222
    Gender:
    Male
    Location:
    Pasco, WA
    However, due to the very short duration of an ejection charge flame a soda can aluminum shield should be fine.
     
  13. Nov 8, 2018 at 9:00 PM #13

    gldknght

    gldknght

    gldknght

    Well-Known Member

    Joined:
    Jan 26, 2011
    Messages:
    558
    Likes Received:
    13
    I wouldn't take the chance.
     
  14. Nov 8, 2018 at 9:17 PM #14

    solid_fuel

    solid_fuel

    solid_fuel

    Lifetime Supporter TRF Lifetime Supporter

    Joined:
    Aug 13, 2017
    Messages:
    1,173
    Likes Received:
    42
    Location:
    SEPA
    Balsa will burn from a lower intensity flame than aluminum
     
  15. Nov 8, 2018 at 10:07 PM #15

    jqavins

    jqavins

    jqavins

    A True Homebert

    Joined:
    Sep 29, 2011
    Messages:
    877
    Likes Received:
    46
    Location:
    Hornell, NY
    The first mention was of using just a piece of can, not soda can as a shield over wood. I wouldn't hesitate to use 1/16" aluminum, but those cans are only about 3.8 mils thick.
    • If the can material is glued flat to a piece of wood, heat is conducted very, very easily through it and into the glue. It can withstand higher temperatures than the wood, but will not see any higher temperature than the wood would without it. It's a fine shield.
    • If the can material stands alone, the heat is very poorly conducted along the metal so the peak temperature induced by a particle of burning BP may be a bunch higher. It may not, I don't know, but I i) wouldn't try it in flight without considerable testing, and ii) wouldn't bother testing it.
    • 1/8" balsa is about 30 times thicker than the can material, so the can can tolerate virtually no degradation before a hole appears.
    So, I'm not saying the can material will surely fail, your parachute will burn, and your rocket will crash; I'm just saying it seems like a really poor choice.
     

Share This Page